receptores farmacologico

7/23/2019 receptores farmacologico

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D iv isio n o f P ha rm ac olo gy G la xo R es ea rc h In stitu te R es ea rc hTriangle Park N orth Carolina; 2Baylor College of M edicine C enter for

E xp er im e nt al T he ra pe uti cs H o us to n Tex as ;a nd 3 La bo ra to ry of Cell Biology Na tional Institute of M ental H ealth N ational Institutes of HBethesda Maryland

Introduction

R eceptors: an operational definitionFunctional tissue system s

Pharm acological criteria for classification

A . R eco gn itio n1. Antagonists2 . A gon ists

Transduction

M olecular b iology relevant to receptor characterizationB asis for a m olecular nom enclature

Parallel pharm acological and m olecular nom enclaturesSuggested ru les for nam ing receptors

ConclusionsReferences

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0031.6997/92/4403-0351$03.00/0PH A R M A C O L O G IC A L R E V IE W S

Copyright 1992 by The A merican Society for Pharm acology and Experimental Therapeutics

I.

II .III.

IV .

V .V I.

VII.VIII.

IXX

II D efin ition of P harm acological R ecep tors

TER RY P. K ENA K IN , RIC HA RD A. BO ND ,2 A N D TO M I. BO NN ER3

Vol. 44, N o. 3Printed in U S A

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I Introduction

TRADITIONALLY, different receptors have been distin-

guished by drugs (chem icals) for w hich the receptorsvary in their responsiveness. This has led to the gradual

recognition of additional receptors for the sam e ligands.

A s the structures of the ligands have been refined , thishas allow ed the distinguishing of m ultiple subtypes of

receptors. The central problem in identify ing new recep-

tors, thus far, has been in defining the significance of

variations in pharm acological properties, i.e., w hat is the

m inim um difference that is necessary to establish a new

subtype. The recent cloning of num erous receptor m ole-

cules for m any of the m ajor neurotransm itters has m ade

it clear that there are often m ore clearly distinct receptor

m olecules expressed in a single m am m alian species than

can be easily distinguished w ith availab le drugs. A l-

though these m olecules can be expected to eventuallyprovide the basis for identifying the corresponding recep-tors and m ay allow the developm ent of m ore selective

drugs, the current lack of sufficiently selective drugs

m eans that it is d ifficu lt to estab lish the physio logical

functions that m ore selective drugs m ight target.

N evertheless, it is clear that m any m ore such receptorsw ill be identified in the near future and that to m inim ize

confusion in the literature, it w ill be necessary to arriveat a generally accepted defin ition of w hat constitu tes

new receptors, as w ell as a m eans of nam ing them . These

issues have been central to the deliberations of the In-

ternational U nion of Pharm acology C om m ittee on R e-

ceptor N om enclature and D rug C lassification. Inpaper, w e sum m arize the current pharm acological

m olecular approaches to the definition of the criteriaw hich alterations in cellular function resulting frompresence of a ligand can be ascribed to specific recep

for that ligand and present suggestions for the nam

of these receptors.

II R ecep tors A n O perational D efinition

W e w ill discuss som e criteria by w hich alterations

cellular function resulting from the presence of a lcan be ascribed to specific receptors for that ligand

the bio logical system . It w ill be axiom atic that thecriteria for qualification for the operational term recare the functions ofrecognition an d t ransduct ion B y thisdefinition, a receptor m ust recognize a distinct chementity and translate inform ation from that entity i

form that the cell can read to alter its state according

e.g., by a change in m em brane perm eability, activationof a guanine nucleotide regulatory protein, an alterain the transcription of D N A .

Langley, a pioneer in the field of receptor researdefined a receptor (in his w ords, receptive substanc

as being the site of com petition for agonists and aonists (recognition) and the vehicle for the transm issio

of the stim ulus of agonist interaction to the cell (trduction) for the production of ap hysio lo gica l respo nseThe criterion of transduction m ay require furthercussion for processes know n to produce a biochem i

change in a cell w ith an as yet unknow n physio logic


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Am plif ica tion of St imulus

r s s u r Drug

L Affini ty

35 2 K EN A K IN ET A L .

resu ltant. T hese criteria w ould ex clude sites that are

concerned w ith the bio logical disposition of m olecu les

bu t not cellu lar m etabolism (i.e., silent recep tors). In-

cluded in this category w ould be the neuronal up tak e site

for catecho lam ines (targets for tricy clic antidepressants)

and sites such as the clearance recep tors fo r atrial natri-

uretic factor (M aack et al., 1987). For these to be in -

cluded , m odif ications to the criteria of transductionw ould need to be m ade. T his def inition also distinguishes

bind ing sites w ith no k now n phy sio log ical sign if icance

(i.e ., accep tors; Furchgott, 1972 ; A riens, 1984; G reen

and M aay an i, 1987 ; L aduron , 1987) f rom recep tors. T h is

def in ition does not dif ferentiate entities curren tly

thought to be pharm aco log ical recepto rs f rom enz y m es.

T o dif feren tiate these, w e w ould stipu late that the rec-

ognition unit should not chem ically alter the endogenous

ligand. T his w ould separate enz y m e substrates from hor-

m ones, neurotransm itters, and other agon ists for drug

receptors.

I I I . F u n ct ion a l T issu e System s

T he relativ e m erits, strengths, and w eak nesses of d if -

feren t approaches to recep tor pharm aco logy hav e been

debated. In general, m ulticellular sy stem s such as iso-lated tissues hav e the adv an tages resu lting f rom stim ulus

am plif ication m echan ism s, because these allow the quan-

tif ication of agon ist ef f icacy . A lso, they are better p red ic-

to rs of drug response in hum ans. H ow ev er, by their

natu re, these sy stem s necessitate indirect approaches to

receptors that binding does not requ ire. A schem atic of

the relativ e m erits of v arious sy stem s used to study drugs

is g iv en in f igure 1.A relativ ely new dev elopm ent in th is debate is the

discov ery that recepto rs m ay in teract w ith each other on

the lev el o f the cell m em brane by sharing ef fector cou-

p lers. For ex am ple, a com m on coupling G protein m ay

be shared by opioid and a2-adrenoceptors in N G 1O 8-15

cells (L ee et al., 1988), rabb it locus coeruleus neurons

(A ghajanian and W ang, 1987; C hristie et al., 1987), and

rabb it cortex (L im berger et al., 1988). U nder these cir-

cum stances, isolated tissues w ould be better predicto rs

of organ responses to drugs than subcellu lar or reconsti-

tu ted sy stem s. H ow ev er, f rom the point o f v iew of recep-

M em brane S ing le C ell Iso lated In Vivo In Vivo

Fragm ents C ells C ulture T issues (im paired R ef lex es) (Intact)

[I C on tro l D ru g C on ce nt rat io nNo R ef lex O bserv ed

Integratedii I1

M easu re R elativ eEff icacy

M easu re Prim aryResponse

F I G . 1. R ep r esen ta t ion of d iffer en t system s forth e pharmaco log ica l

stu d y of drug r ecep tor s. Som e th eor et ica l ad van ta ges of va r iou s test

sys tems (fr om K en ak in , 1987a).

tor classif ication, these interactions m ay prov e to b

se rio us c om p licatio n.

Perhaps m ore relev an t is the potential fo r isom eriza-

tion of receptors in d if feren t sy stem s (both phy siologica

and artif icial such as ex pression sy stem s) by com pl

form ation w ith m em brane-bound pro teins (i.e ., G

teins). T h is is a w ell-k now n phenom enon for m any

nists, m ak ing them unreliab le tools fo r recep tor classication. T ernary com plex form ation and recep tor iso

eriz ation are assumed not to occur w ith an tagonists.

H ow ev er, antagon ism is a pharm aco log ical def initio

bestow ed upon a drug because it block s the ef fects o

agonist and does no t produce a m easurab le response.

com pletely ru le out recepto r isom eriz ation , it m ust

show n that the ex perim ental sy stem possesses the

ported properties and am plif ication characteristics

dem onstrate low lev els of agon ism . In general, there

no reason a prio ri to assum e that ternary com plex

m ation am ong a ligand, receptor, and G protein w

sum m arily prom ote dissociation of thea j3 and y sub-

un its o f the G pro tein and, thus, rev eal agonism . R ecdata concerning an tagon ists that activ ely destab iliz e

nary com plex form ation (C osta and H erz , 1989 ; Costa

a ., 1992) show that the observ ed af f inity can be grea

dependen t on G proteins in m em brane sy stem s. U n

these circum stances, the env ironm ent of the receptor

determ ine m easured af f in ity , thereby in troducing th

retical and practical p rob lem s w ith the use of such

tagonist af f in ity data for recepto r classif ication . T h

ideas are d iscussed further in relation to the concep ts

negativ e ef f icacy and recep tor precoup ling (v ide inf ra

IV . P h a r m a cologica l C r iter ia for C la ss ifica t ion

A p r er eq u isit e fo r a m ea n in gfu l d iscu ssion of p ossibcriteria for receptor def inition should be the presen tation

of the lim itations of the m ethods used to estim ate

param eters on w hich the criteria are based.

A Recogni t ion

D iscussion of the def in ition of receptors inev itab l

centers on the criteria fo r classif ication of recep tors

M uch has been w ritten about the pharm acological

teria for recep tors, bo th f rom the point o f v iew of bind

and iso lated tissue studies (Furchgo tt, 1972 ; B lack , 1

M olinof f et al., 1981; L aduron , 1984; L aduron, 19

G reen and M aay an i, 1987 ; K enak in , 1987a; N ahorsk i

1987; B raestrup and A ndersen, 1989 ; G reen, 1990).term s of receptor binding stud ies, it is generally agre

that ligands m ust hav e selectiv ity , bind in a satu rab l

m anner, and, w here appropriate , dem onstrate stereose

lectiv ity for the proposed recep tor. It is ex pected t

there be a class of chem icals that bind selectiv elyto

recep tors, in a satu rab le m anner, that can be d isplaced

com petitiv ely by other chem icals or rem ov ed by w ash

w ith drug-f ree m edia. Often added to th is list is the need

for an endogenous agonist in additionto selectiv e b ind ing

w ith foreign ligands. T h is w ould ex clude classif ications


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F I G . 2. Potent ia l Schild regression s an d possible experimental con-

di t ions that w ou ld cau se d ev iat ion from sim p le com p et it iv e b eh av iou r(from Kenakin, 1987b).

PH A RM AC OLO G ICA L R ECEPTO RS 35 3

such as cannabinoid receptors for w hich, currently, there

is no know n endogenous agonist. In such cases, this

condition has less to do w ith definition of the receptorthan w ith its proper identification . A lthough the princi-

p le of nam ing a receptor after its endogenous agonist

should be m aintained w herever possib le, the w ithholdingof receptor status from an entity, w hich otherw ise satis-

fies all criteria for defining a receptor, could be counter-productive. Such receptors should be accepted as re-

ceptors w ith the understanding that the nam e should bechanged w hen the identity of the endogenous ligand

becom es clear. O ne potential error in doing so is that w e

m ight be giv ing tw o nam es to the sam e receptor, if itsendogenous ligand turns out to be one for receptors that

already have been identified. A related error, e.g ., theidentification of subtypes of substance P receptors before

the identification of the related peptides, neurokin in Aand neurokinin B , is that w e incorrectly give one agonist

nam e to a group of receptors that actually has a group

of closely related endogenous agonists. O ccasional errors

of both types are unavoidable in a useful nom enclaturesystem.

The traditional m ethod of receptor classification has

been by the relative potencies and selectivities of antag-onists and agonists. Specifically , the follow ing criteriahave been used: a affin ity of antagonists, b potency

ratio of agonists, c affinity of agonists, and d relative

in trinsic efficacy of agonists. It is w orth considering thesem ethods and how they are used in the receptor classifi-

cation process.1 . Antagon i st s. The m easurem ent of antagonist po-

tency in functional assays by the Schild technique (A run-

lakshana and Schild , 1959) has yielded an abundance ofdata for receptor classification . U nder these conditions,the abscissa in tercept of a Schild regression (under

equilibrium conditions w ith com pounds interacting at asingle site) w ith the antagonist tested over a concentra-

tion range yields an im portant param eter for receptorclassification . If the regression is linear w ith unit slope,th is param eter is defined as an estim ate of the equilib-

rium dissociation constant of the antagonist-receptor

com plex. If one assum es that antagonists bind com peti-

tively to the agonist-binding site, d ifferences in the equi-

librium dissociation constants of antagonist-receptor

com plexes m easured w ith this m ethod can be used todefine receptor subtypes. A suggested standard for theproposal of a novel receptor subtype is for the dissocia-tion constant K B of a com pound (or ideally tw o corn-

pounds w ith chem ically diverse structures) to exhib it a1 log unit or greater d ifference from its value(s) at know n

receptors. In general, the m inim al requirem ents for ac-curate estim ation ofKB values by th is m ethod are as

follows.1 . M easurem ents m ust be m ade under equilibrium

conditions (Furchgott, 1972, 1978). There is m uch data

to show how nonequilibrium conditions can give the

appearance of equilibria and sim ple com petitive antagnism (K enakin, 1987b) yet y ield erroneous estim ates

KB (fig . 2). T hree m ajor objectives in estab lish inglibrium conditions are: a elim ination of sites of losincluding bio logical processes such as uptake sys

and degradative enzym es, as w ell as pro tectionphysicochem ical breakdow n (i.e., au tooxidation of

chols) of the ligands interacting w ith the receptor; bthe elim ination of the possible in terference from en

enously released ligands that interact w ith the recepan d c the achievem ent of tem poral equilibrium .

general, these conditions are of m uch greater concernagonists than antagonists, particu larly w hen using

lated tissue preparations.2 . The Schild regression is based on the assum ptio

that the observed antagonism is of the sim ple com petit

type. U nder these conditions, a slope that is d ifferfrom unity has no m eaning in term s of the chem i

constant of interaction betw een an antagonist areceptor. T herefore, if the slope is not statistically

ferent from unity , it should be constrained to u(M acK ay, 1978) and the in tercept u tilized as an estim

of the KB. This is based on the assum ption thatinfin ite num ber of assays w ould yield a slope exaequal to unity.

3 . The regression should be linear w ith unit slopea considerable concentration range (m inim ally 3100-fo ld w here possib le). There are instances in w

physio logical antagonism (e.g., $-adrenoceptor-m ediatedinhibition of cholinergic contraction in guinea pig

chea; K enakin, 1982) can produce linear Schild re

sions of unit slope. This usually cannot be dem onstrate

over large ranges of concentration.

4 . O ne possible criterion for defin ing receptor diences via Schild analysis is a difference in the eleva

of regression lines, i.e., a difference in location along

elope

= i(Spurious

a . C he mi ca l In te rf er en ce

tk bm p oral lneuilsbriumC A ntag on isl R em ov alS a t u r a t i o n

a. C onco m itant R eceptor an dU pt ak e B lo ck ad e

t Te mp or al l ne gu il sb rs um( di ff us io n l im it in g )

0 123


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354 K EN A K IN ET A L .

log an tagon ist ax is that w ould indicate a dif ference in

KB (S nedecor and C ochran, 1967). B ecause regression

lines are used to characteriz e antagonist po tency , a sta-

tistical com parison w ould rem ov e sub jectiv ity (K enak in

and B lack , 1978). T h is also w ould elim inate character-

iz ations such as receptor sub ty pe X -lik e w hen tw o

regressions are sim ilar but no t co inciden t.

A potential com plicating facto r in S child analy sis isthe case in w hich recep tors are substan tially precoupled

to G proteins in the absence of agon ists (C osta et al.,

1990, 1992). U nder these circum stances, there w ould be

a heterogeneity of b ind ing sites av ailable to the antago-

nist, if the precoupled recep tor has a d if feren t confor-

m ation than the uncoupled one. T his also raises the

specter of negativ e ef f icacy for an tagonists that m ay

stabilize the guanosine diphosphate-G pro tein com plex .

T he potency of such an tagon ists then w ould be sub ject

to the relativ e proportions of recep tor and G protein and,

thus, w ould be tissue dependent (W regget and D eL ean ,

1984; C osta and H erz , 1989 ; C osta et a ., 1992). T he

degree of precoup ling of recep tors can produce a sin istral

shif t o f the S child regression but no detectab le non line-

arity (C osta et a ., 1992). T herefore, no obv ious ind ica-

tion of coupling pro tein ef fects w ould be seen by S child

analy sis. S uch drugs w ould v ary in ef fectiv eness in d if -

feren t sy stem s of v ary ing degrees of receptor precoupling ,

w hich, in tu rn, has been show n to v ary w ith ex perim ental

cond itions such as cellu lar integrity (C osta et a ., 1992).

Presen tly , it is unclear to w hat ex tent th is is a sign if icant

haz ard in classif ication of recep tors w ith an tagonists, but

dif ferences in observ ed antagonist af f inity w ith cellular

integrity and /or cy clic nucleo tide lev els w ould be m di-

cators of a po tential p rob lem .

A lthough a case cou ld be m ade for a purely pharm a-

co log ical def inition of receptors based on the quantitativ e

scale of potency of antagonists, a po int to be considered

is the possibility that dif feren tbinding doma ins o f re ce p-

tors then m igh t be classif ied as un ique receptors. It is

k now n from deletion m utation stud ies that dif ferent

binding dom ains for agonists and antagonists can be

dif feren tiated for agonists and antagonists. T hese data

and that o f others open the possibility that m any drugs

thought to be com petitiv e w ith agonists for com m on

bind ing sites m ay , in fact, be allosteric effectors binding

to sites phy sically rem ov ed f rom the agonist-binding site.

A possib ly usefu l techn ique to dif ferentiate true com -petitiv eness at a com m on site on the receptor and allo-

steric interaction isresultant analysis w ith w hich the

an tagonism of a test antagonist is m easured in con-

junction w ith that of a reference antagonist (B lack et

a ., 1986; K enak in and B eek , 1987; L eff and M orse, 1987).

T h is m ethod is sim ilar to the additiv e dose ratio m ethod

to determ ine com petitiv eness (Paton and R ang, 1965)

but has the added adv an tage of com pensating for sec-

ondary ef fects of the test antagon ist. T his is because the

contro l dose-response curv e is determ ined in the pres-

ence of the test antagon ist, thereby canceling secondar

ef fects on the b lock ade produced by the reference an t

onist thereaf ter. R esultan t analy sis can be used to

ferentiate apparen t sim ple com petitiv e an tagon ism f

allosteric in teraction (K enak in and B oselli, 1989).

T here are data to suggest that the agonist b ind ing

at least som e G pro tein-link ed receptors is con tained

w ithin the sev en m em brane-spanning portions ofreceptors and that these regions are the m ost high

conserv ed in term s of am ino acid hom ology across spe

and tissues. If allosteric ef fectors are used for recepto

classif ication and these drugs b ind to the heterogeneous

outer portions of recepto rs rem ov ed f rom the endogenou

agonist-binding site , then it w ould be possible that

erogeneous b ind ing across species and tissues cou ld

su it not f rom dif ferences in the endogenous agonis

binding dom ain but, rather, f rom dif ferences in alloster

m odulation sites. U nder these circum stances, it w ould

possib le that recep tor heterogeneity w ould not be

tected by the in teraction of the recep tor w ith endogenouagonists but on ly w ith foreign ligands. T hus, it cou ld

conceiv ed that the classif ication of recep tors on the b

of ailosteric sites m igh t open a Pandoras box of b

erogeneity because, conceiv ab ly , recepto rs constructed

v arious cells f rom dif feren t species w ould be d if feren

due to the av ailability of dif feren t m aterials for recept

construction . In practical term s, such a Pandoras b

m ight translate into a cornucopia of po ten tial pharm a

cological selectiv ity . H ow ev er, the danger fo r therapeutic

pharm acology w ould be that structure-activ ity relation

ships, based on such regions in one test sy stem used

screening m olecu les for bio logical activ ity , m ight not

hav e a counterpart in the hum an receptor.

T here now is m ounting ev idence that, ev en w hen

k now n nonequilibrium conditions are ef fectiv ely

traliz ed or allow ed for, there still is considerable heter

geneity in estim ated KB v alues as m easured in a func-

tional sy stem . O ne daunting prospect is the possibility

of v ariable af f inity of ligands for recepto rs resulting f

d if ferences in m em brane com ponen ts (B ev an et

1989). It is w ell k now n that dif feren tial purif ication

receptors can produce dif ferences in binding prof iles

agonists and an tagon ists. O f po tentially greater conce

are data to suggest thatflexibility is a characteristic of

recep tors required for their function w ith respecttransduction. U nder these circum stances, the

arrangem ent of subunit structure w ould be a m ajor

term inan t of ligand af f inity and this, in turn , w ould

sensitiv e to the env ironm ent for that recep tor (W ei

stein, 1987). T his pro tean nature of receptors sugges

that un ifo rm classif ication , based so lely upon liga

recogn ition, w ould be m ore v ariab le than assum ed , if

recep tor w ere sim ply a recognition un it w ith no tran

ducing properties. T he poten tial for negativ e ef f icacy

recep tor precoup ling should introduce caution in assum


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PH A R M A C O L O G IC A L R EC EPT O R S 355

in g that ligand af f inity fo r recepto rs w ill prov ide the go ld

standard for classif ication .

2 . Ag on is ts . R ecep tors also hav e been classif ied phar-

m acologically w ith agonists, but the possib le problem s

with this appr oach ar e com pounded by the addeddrugproperty of intrinsic ef f icacy . S pecif ically , agon ists pos-

sess the properties of af f inity (lik e an tagonists) and also

efficacy, the ability to im part a change of state in therecep tor. H isto rically , agonist po tency ratios hav e been

the estab lished m ethod of receptor classif ication by ag-

onists, the m ost w ell-k now n ex am ple being the classif i-

cation of adrenoceptors in to a and /3 s ub ty pes by A hlqu ist

(1948). T he prem ise is that the potency of an agonist

depends upon the drug factors of intrinsic eff icacy and

af f in ity and, also, the tissue facto rs of recep tor density

and the ef f iciency w ith w hich the receptors are coupled

to ef fecto r m echan ism s. In a giv en tissue in w hich tw o

agonists are com pared directly , the tissue facto rs cancel

and their relativ e potency depends only upon the drug

param eters ef f icacy and af f in ity . U nder these circum -

stances, the relative potency of agon ists becom es a recep-

tor f ingerprint w hich can be utiliz ed in receptor clas-

sif ication. S om etim es, this m ethod is degraded to the use

of the relativ e order of po tency for classif ication, but this

can lead to serious errors. G iv en the prerequisite that

the relativ e potencies of agon ists depend only upon drug-

related param eters, agonist potency ratios should be

useful m easures of recep tor sub ty pes. H ow ev er, recent

ev idence indicates that the m agnitude ofbo th the af f inity

and ef f icacy of agon ists m ay not be independent of tissue

factors.

In term s of the af f inity of agon ists fo r recepto rs, ev en

w hen care is tak en to attain equilibrium conditions iniso lated tissue preparations, theoretical m odeling and

ex perim ental data hav e show n that the scales of potency

and selectiv ity of agonists all can be obfuscated in intact

systems (M acK ay , 1987; K enak in, 1989). If the agonist

induces a change in the recep tor to w hich it is b ind ing

(e.g ., as in an ion channel f rom a closed to an open state,

the form ation of a ternary com plex w ith a m em brane-

coup ling protein ), then the ex isting m ethods for the

estim ation of the af f in ity of agonists fo r recep tors y ield

an apparent af f in ity that m ay characteriz e the com plete

m echan ism of receptor activ ation (C olquhoun, 1987).

T his w ould include m em brane com ponents in add ition

to the receptor, and, therefore, these data w ould not be

useful for recep tor classif ication (M acK ay , 1978; L ef f

and H arper, 1989; L ef f et a ., 1990 ; M acK ay , 1990; K en-

ak in et al., 1990). Errors in the estim ation of agonist

aff inity w ould lead to concom itant errors in agon ist

potency ratios.

B. Tr an sd uctio n

B y def inition , the transduction aspect o f recep to r

m echanism s suggests selectiv e interaction of the recep tor

w ith unique m em brane or cy tosolic com ponents of the

cell that then carry the m essage im parted by the dr

T heoretically , part of the receptor classif ication proce

could include in form ation about the transducing rec

nition sites on the receptor and ev en to the ov erall

observ ed phy sio log ical response. In term s of agon ist

tiv ity , there are tw o possibilities for recepto r classif ica

tion : the type or the magnitude of the response.

T he ty pe of response clearly is inadequate as a toolclassification. D ata f rom a num ber of sources dem on-

strate considerable cross-talk betw een receptors a

ef fecto r sy stem s. It has been show n in reconstitu tion

studies that m any recep tors are capab le of in teracting

w ith m ore than one ty pe of G protein , and there

suggestive ev idence that this m ay occur phy siolog ically

in cells (K enak in, 1988, 1990). C lassif ication by phy sio-

logical function is not practical f rom the poin t o f v

that a g iv en biochem ical p rocess m ay hav e d if feren

ef fects in d if ferent cells depend ing on the processing

the b iochem ical signal. A lso , som e agonists are k now n

be pleiotropic, producing m any biochem ical ef fects

cells that m ay also be d if ferentially coupled to cy toso

processes. T herefore, it is conceiv ab le that agonists

low intrinsic ef f icacy w ould activ ate only the m ost

ciently coupled of these processes and produce a prof i

o f activ ity d if ferent f rom that of a pow erfu l high-ef f icac

agonist. U nder these conditions, agonists o f d if ferin

intrinsic ef f icacy could produce d if ferent pharm acologi-

cal ef fects by activ ating thesame recep tor. For ex am ple,

opioid receptors m ediate the inhib ition of adeny late

clase and the stim ulation of G T Pase in N G 1O 8-15 c

but the latter ef fect is m ore sensitiv e to dim inution

recep tor density (sm aller ef fectiv e receptor reserv

(C osta et al., 1988). Presum ably , the stim ulation oG T Pase also w ould be less sensitiv e to activ ation by

ef f icacy agonists. G iv en these constrain ts, classif ication

by ef fector w ould seem to be unsu itab le.

T he other possib ility is to use the m agnitude of

sponse to a giv en agonist f or classif ication. It is clear

that the in trinsic activ ity of agon ists is not a usef

param eter for recepto r ty p ing , because its m agnitude

varies w ith the ef f iciency of stim ulus-response couplin

T here are num erous ex am ples of low -ef f icacy agoni

that p roduce direct responses on ly in highly ef f icien tl

coup led tissues and act on ly as an tagon ists in oth

sy stem s. T herefore, nu ll m ethods m ust be used to fa

ou t the tissue effects and , hopefu lly , y ield param eters

relating only to the agonist-recep tor pairs. T heoretically ,

the m easurem ent of relativ e ef f icacy of agon ists fu l

this function , bu t again, the com plex ity of m em bran

dynam ics needs tobe considered. M easurem ents of ef f

cacy and relativ e potency w ould be inv alid m easures

recep tor selectiv ity in cases in w hich recep tors are p

m iscuous w ith respect to the ef fector coup ling pro tei

w ith w hich they interact (K enak in, 1988; K enak in and

M organ, 1989). For ex am ple, in IM R -32 tum or cells,

M 3 m uscarinic acety lcholine receptor sub ty pe selectiv e


6/12


couples to phosphoinositide hy droly sis and the M 4 sub-

type to inhibition of adeny late cy clase, but in cells con-

tam ing M 2 subty pe m R N A , the ex pressed M 2 recep tor

couples to both responses (Pink as-K ram ask i et al., 1990).

A n ex am ple of w here recep tor prom iscuity , w ith respect

to receptor-ef fector coupling m echan ism s, m ay occur in

nativ e m em branes is in rat pancreatic acinar cells. In

th is sy stem , cho lecy stok inin functionally interacts w ithsix G 5 pro teins and w ith G i, G 12 , and G 13 pro teins as

w ell (S chnefel et a ., 1990). A nother m ay be adenosine

A 1 receptors f rom bov ine brain w hich copurify w ith G 11,

G 2, and G 0 w hen activ ated by an agonist (M unsh i et al.,

1991). T he ability of th is receptor to in teract w ith all

th ree G pro teins w ith agonist activ ation w as conf irm ed

in reconstitution studies. U nder these circum stances,

dif f erent agonists cou ld pred ispose the form ation of dif -

feren t ternary com plex es, m ak ing the relativ e av ailab ility

of G pro teins in v arious m em branes a determ inant of

ef f icacy . In these sy stem s, the relativ e agonist po tency

and/or intrinsic ef f icacy w ould not be usefu l scales for

re ce pto r c las sif ic atio n.

G iv en these po ten tial pitfalls to the use of agonists for

recep to r classif ication, the follow ing w ould be a m inim al

list o f requirem ents fo r their use. a A s f or an tagon ists,

equilibrium conditions should be attained. For agonists,

this m ay be m ore d if f icu lt in v iew of b iochem ical m ech-

an ism s in tissues that are designed to rem ov e endogenous

agonists from the receptor com partm ent (i.e., degrada-

tiv e enz y m es, uptak e processes). It is a prerequisite for

recep to r classif ication stud ies that the concen tration of

drug at the receptor be k now n accurately . b Potency

ratios should be used and not rank order of potency . T he

correct use of agonist potency ratios alw ay s ensurescorrect rank orders, bu t the conv erse is not alw ay s true.

c If a recepto r sub ty pe is being classif ied , the endoge-

nous agonist fo r that receptor sub ty pe m ust be included

in the analy sis. d A gonists shou ld dem onstrate stereo-

selectiv ity at the receptor w here appropriate .

V M olecular B iology R elevant to R eceptorCharacterization

T w o prob lem s hav e plagued a purely pharm acological

def in ition of recep tor subty pes. T he f irstis th e q uestio n

of w hether observ ed pharm aco log ical dif ferences repre-

sen t d if ferent recep tor m olecules or sim ply cell-specif ic

dif ferences in the env ironm ent of a com m on recep torm olecu le. T his question has arisen in sev eral fo rm s in

the preceding d iscussion . T he second is the prob lem of

k now ing w hether there is a pure receptor population in

a giv en tissue, a prob lem that m ay generally hav e been

underestim ated. T he cloning of recep tors has rev ealed

that there are of ten m ore d if ferent recep tor m olecules

for a giv en ligand than generally recognized pharm aco-

log ical subty pes. A lthough one m igh t be inclined to ex -

pect that, therefo re, there w ill be a d istinct recep tor

m olecu le ev ery w here there is a reasonable suggestion of

a pharm acological sub ty pe, one should ex pect that

som e cases there w ill be few er d istinct m olecules th

there are apparent pharm acolog ical sub ty pes, because

cell o r species-specif ic v ariations in the properties

w hat should be considered a sing le receptor m olecule

section IV for a discussion of species v ariation). A

ollary of the ex istence of prev iously unrecognized

ty pes is the observ ation that the occurrence of m ultipsub ty pes in a tissue, or ev en a cell line, is m ore com m o

than had been generally recogn iz ed . T he introduction

m olecular b iology in the form of cloned receptors p

v ides a basis for allev iating these prob lem s.

T he characteriz ation of cloned recep tors shou ld ev

tually prov ide the inform ation on w hich to base a

m enclatu re that truly ref lects the m olecular identity

the receptors so that a particular subty pe of receptor w

refer to a specif ic pro tein (or set o f proteins for m ultip l

subunit receptors). In particular, the ex pression of clon

receptors in cell lines w ill prov ide pure populations

recep tors in identical cellular env ironm ents so that

factors that allow the dif ferent recep tors to be operation

ally distinguished can be iden tif ied. T o prov ide a p

population of receptors, the untransfected host cell line

in to w hich the cloned receptors are in troduced m ust

produce m easurable am ounts of any ofthe receptor sub-

ty pes in question . A lthough it is relativ ely easy to

such cell lines, it is no t y et clear how m any or w h

specif ic cell lines should be used to hav e an adequat

representation ofthe cellu lar env ironm ents that the re

ceptor experiences in v iv o.For ex am ple, the f ib rob last

cell lines w hich m olecular bio log ists hav e used for in i

characteriz ation of G protein -coup led recep tors m ay

contain signif icant am ounts of specif icG proteins thatin teract w ith the receptors in their natu ral cellu larv ironm ent. B ecause w e do not k now w ith w hich spec

G pro teins indiv idual recepto rs in teract or ev en w hat

fu ll repertoire of G proteins is, the cho ice of the m

appropriate host cell lines is not im m ediately obv iou

H ow ev er, som e pharm aco logical properties of these

ceptors, such as antagonist af f inities, m ay not depend

w hich G proteins are present so that the choice of h

cell line w ill no t be critical.

T he cloned receptors also prov ide new m ethods

determ in ing w hich recep tor m olecules are presen t i

g iv en cell o r tissue. T he D N A sequences of the clon

can prov ide probes of h igh sub ty pe specif icity fordetection of receptor m R N A s by a v ariety of m ethod

such as northern blots and in situ hy bridiz ation . T hese

m ethods vary in the sensitiv ity for detecting low leve

of m R N A , as w ell as in their spatial resolution. F

ex am ple, in situ hy brid ization is capab le of identify ing

ind iv idual cells in tissue sections, w hereas northern bl

ty p ically use R N A ex tracted f rom a w hole tissue. T h

if there is a low lev el o f m R N A ex pressed in m any c

of a g iv en tissue, a northern b lo t that sum s the sign

from al l of the cells w ill be m ore sensitiv e than in sit


7/12

PH A RM A CO L O G IC A L R ECEPT OR S 357

hy bridiz ation , w hereas in situ hy brid iz ation w ill be m ore

sensitiv e if there are only a few cells ex pressing m ore

substantial levels ofm R N A . O ther m ethods such as

solution hy bridization coupled w ith probe protection as-

says or rev erse transcription coupled w ith polym erase

chain reaction am plif ication of fer po tentially greater

sensitiv ity , although generally not w ith the single-cell

resolu tion of in situ hy brid ization. U nfortunately , therelativ e am ounts of tw o m R N A s m ay not ref lect the

relativ e am ounts of recep tor protein in a giv en tissue for

tw o reasons. First, there can be large d if ferences in the

ef f iciency w ith w hich dif feren t m essages are conv erted

in to pro tein. S econd , at least fo r tissues con tain ing neu-

ronal cells, in w hich the m R N A is located in the cell

body but the recep tors are located predom inan tly on

ax onal and dendritic p rojections, there m ay be a sub-

stantial dif ference betw een the location of the m R N A

and the receptor pro tein . A lthough not y et w idely av ail-

able, subty pe-specif ic an tibod ies against portions of the

cloned receptor sequences shou ld allow m ore accurate

determ ination of recep tor distribu tions and heterogene-

ity . For ex am ple, antibodies specif ic f or the dif f erent

m uscarinic acety lcho line receptors hav e been m ade using

as antigens proteins deriv ed f rom ex pression inEsche-

r ichia coli of m ost of the dom ain connecting the f if th and

six th transm em brane dom ains (L ev ey et a ., 1991).

A ll o f the receptors that hav e been cloned and se-

quenced belong to one of a sm all num ber of fam ilies of

structurally sim ilar p roteins, such as the ligand-gated

ion channels, w hich hav e sev eral dif ferent but related

subunits, and the G pro tein-coupled recep tors, w hich are

sing le-subunit recep to rs characteristically hav ing sev en

m em brane-spanning dom ains. For som e ligands, such as

acety lcho line, there are recep tors, nicotinic and m usca-

rinic, that belong to m ore than one structu ral fam ily .

O ther ex am ples are G A B A A * v ersus G A B A B and 5-H T 3

v ersus 5-H T 1 and 5-H T 2. T he dif ferent structures gen-

erally correspond to distinctly dif feren t m odes of signal

transduction and can be used as a prim ary m eans of

subd iv id ing receptors for a giv en ligand . T here is, how -

ev er, a second structural class of recepto rs that can

interact w ith G proteins. T hese are the receptors for

peptides such as insulin and insu lin-lik e grow th factors

(O k am oto et a ., 1990). T hey consist o f tw o identical

subunits, each of w hich has only a sing le m em brane-

spanning dom ain .V I B asis for a M olecular N om enclatur e

T he m ost im portant con tribution of m olecu lar b iology

to receptor characterization is that it allow s the identi-

f ication of the actual m olecu les that underlie the phar-

m acology and prov ides an ev olu tional perspectiv e w ith

w hich to iden tify species-specif ic v ariation in recep tor

pharm acology , w hich is essential fo r ex trapolating re-

su lts in an im al m odels to hum an therapeutics.* Abbreviat ions G A B A , y -am inobuty ric acid; 5-H T , 5-hy drox y -

tryptamine.

T he m ajor questions concern ing a m olecu lar def initio

of recep to r sub ty pes are how dif ferent does the sequen

of a m olecule have to be to justif y a new nam e, and h

can one establish the m olecular identity of the receptor

that m ediates a phy siological response. If w e iden tify

recep tors by their pro tein sequences, w hich seem s to

the m ost appropriate m olecular identif ier, w e shoul

attach the sam e nam e to all m inor variants in sequencesuch as naturally occurring alleles or in v itro created

m utants. Ifsuch variants dif fer in any im portant recepto

properties, then they w ould be referred to as specif

v ariants, e.g., the A la197 allele or m utant of a giv e

recepto r. D if ferences in g ly cosy lation cou ld be referr

to as dif f erent glycosy lation states if glycosy lation prov

to be im portant. H ow ev er, ev idence based on m utating

the glycosy lation sites of the f2-adrenoceptor (R ands et

a ., 1990) and the m 2 m uscarinic acety lcholine recep t

(v an K oppen and N athanson, 1990) indicates that

of any or all gly cosy lation sites has little ef f ect on

pharm acology . L ik ew ise, v arian ts resulting f rom al

nativ e sp licing , as in the case of the dopam ine D 2 rec

tor, should be considered as length v ariants of a single

recepto r sub ty pe. T he tw o D 2 sequences dif fer by

insertion or deletion of 29 am ino acids near the m iddl

of the cy toplasm ic dom ain connecting the f if th and s

transm em brane dom ains. B ecause this dom ain has be

im plicated in the specif icity of G protein binding

other G protein-linked receptors, it has been suggested

that the tw o form s m ay bind to dif ferent G proteins

(E idne et a ., 1989; G iros et al., 1989). If th is w ere to

the case, variant nam es should be used to distinguish

tw o m olecules. H ow ev er, the sequence d if ference oc

in the portion of the cy top lasm ic dom ain that candeleted f rom both adrenoceptors and m uscarinic acety

choline receptors w ithout af fecting functional respon

(S trader et a ., 1987; S hapiro and N athanson, 1989

T hus, the biological signif icance of this dif f erence

m ains to be determ ined. O n the other hand, biologically

signif icant alternative splicing does occur in the glut

m ate receptor genes G lu-A , -B , -C , and -D (S om m er

a ., 1990). T he alternativ ely spliced form s of these rec

tors, w hich prov ide dif f erent sequences for a 38-am ino

acid region preceding the fourth transm em brane dom a

hav e been referred to as the f lip and f lop form

A lthough these m ay not be the m ost suitable nam es

these alternativ e fo rm s of the proteins, it is clear th

they should be giv en dif ferent nam es because they im pa

dif f erent channel properties.

M ore substantial sequence variations, in the approx i-

m ate range of 1 to 10% , such as those that occur betw een

m am m alian species during the course of evolution

receptor m olecules, also should be included under a sing

nam e. T his assum es that it is clear that the m olecules

are true hom ologs, i.e., the sole descendants of the sam

m olecule in the m ost recent com m on ancestor of the

species being considered. T he rationale fo r do ing s


8/12


that function is generally conserved during ev olution so

that subty pes hav ing the sam e nam e could be ex pected

to hav e the sam e physiological functions in dif f erent

species. T he ev olution of protein sequences for fam ilies

of related proteins, such as the hem oglobins, has been

the subject of study for m ore than 20 y ears and prov ides

the foundation for identif y ing w hich sequences are true

hom ologs. If there are the sam e num ber of functionalgenes for dif ferent subty pes of a given receptor in the

tw o species, it is generally not dif f icult to identif y w hich

pairs of receptors, one from each species, are the true

hom ologs. T hus, assum ing that all of the subty pe genes

ex isted in the ancestor of all m am m als, one can expect

that the sequences for a single subtype in all m am m als

w ill be m ore closely related to each other than the various

subty pes in any one species are to each other. How ev er,

if there hav e been gene duplications since the evolution-

ary div ergence of the tw o species and if the duplicated

gene has acquired a function, then the correspondence

m ay not be clear because there are now tw o genes derived

f rom the ancestral gene in one species but only one in

the other species. O ne can expect that the tw o genes in

the f irst species w ill be m ore sim ilar to each other than

they are to the single gene in the second species. T he

question of gene duplication w ill have to be resolved for

each gene fam ily , i.e ., each set of receptors for the sam e

ligand and hav ing the sam e general receptor structure.

In general, this com plication can be ex pected to be rela-

tiv ely infrequent am ong m am m als but m ore serious w hen

com paring m am m alian recep tors to recep tors f rom other

vertebrates or ev en inv ertebrates.

In princip le, if one has, for ex am ple, a rat recepto r

sequence and w ants to k now w hether it is the hom olog

of one of the k now n hum an subtype sequences, one w ould

need the sequences of all the hum an subty pes for that

ligand to determ ine w ith certainty w hich w as the m ost

closely related to the rat sequence. In practice, one prob-

ably w ould have a potentially incom plete set of hum an

subty pe sequences to com pare and, therefore, w ould w ant

to know w hether the rat sequence is the hom olog of one

of the know n sequences or represents a new subtype. If

the rat sequence is not signif icantly m ore closely related

to one of the hum an sequences than to the others, it

alm ost certain ly represen ts a new subty pe. If it d if fers

by 1 to 10% from one of the sequences, it is probably the

hom olog of that sequence. For G protein-coupled recep-tors for w hich the sam e receptor has been cloned from

sev eral species such as cow , pig, rat, m ouse, ham ster, and

hum an, the hom ologous genes typically hav e 85 to 95%

am ino acid sequence identity . T he m em brane-spanning

dom ains of these receptors are quite conserv ed betw een

subtypes, but there are regions (the am ino term inal to

the beginning of the f irst transm em brane dom ain, be-

tween the f if th and six th transm em brane dom ains, and

follow ing the seventh transm em brane to the carbox y l

term inal) that show little conservation not only in se-

quence but in their leng th. Ex am ination of such reg io

can of ten be m ore usefu l than the ov erall sequence w

getting an indication of w hether tw o sequences are sp

cies hom ologs. W hen com paring sequences, one shoul

be alert to the possibility of sequencing erro rs. T he m

troublesom e error is a f ram e-shif t error, w hich causes

incorrect translation of an ex tensiv e portion the D N

sequence into am ino acid sequence w hen a sing le nucltide is erroneously deleted or inserted. S uch errors a

not uncom m on. If there is a second com pensating err

that resu lts in resto ration of the correct read ing f ram

the errors are of ten not uncovered until sequences a

com pared betw een dif ferent laboratories or betw een

cies (G uyer et a ., 1990).

In som e cases, it m ay be dif f icu lt to decide w hethe

tw o sequences are species hom ologs w ithou t supp lem en

tary inform ation. T he m ost useful inform ation is pr

v ided by a D N A hy brid ization test, w hereby one de

m ines w hat hum an gene is m ost closely related by

bridiz ing a probe deriv ed f rom the rat sequence to

S outhern b lo t of restriction enz y m e digests of hum

genom ic D N A and ask ing w hether the m ost stable (w

respect to tem perature) bands of hy bridiz ation corr

spond to those characteristic of one of the k now n hum

genes. T his test requires that the probes used to def in

the characteristic patterns of the k now n hum an gen

represen t as closely as possible the sam e reg ion of

am ino acid sequence as the rat p robe so that the hom o

ogous rat and hum an probes w ould detect p recisely

sam e restriction f ragm ents. Just as one shou ld use 5

era d rugs to characteriz e a recep tor, one should u

sev eral restriction enz y m es for such an analy sis to avthe possibility that tw o dif ferent genes m ight g iv e

sam e restriction f ragm ents fo r a single random ly chos

enz y m e. A nother fo rm of corroborating ev idence w o

be w hether the receptor (either protein or m R N A )

the sam e tissue distribution in the tw o species, assum ing,

of course, that the distributions are suff iciently distinct

to distinguish am ong subtypes.

B ecause the recep tors hav e not ev olv ed to d istinguish

am ong v arious sy n thetic ligands and changing ev en

single am ino acid in the receptor can cause substantial

changes in ligand af f inities (S ury anaray ana et a ., 199

there is reason to ex pect that there w ill be species d if f

ences for som e synthetic ligands (ev en though the aff i

ity for the endogenous ligand m ay be unchanged)w hat w e w ould identify as a sing le m olecular sub ty p

S uch dif ferences w ill need to be noted in the pharm acological def initions attached to the m olecular nam es a

are clearly of m ajor im portance in selecting anim a

m odels for testing drugs intended for hum an use. T he

H T 1 B and 5-H T 1D receptors prov ide a clear case of suc

species variation. Even before the recent cloning of the

recep to rs, it appeared p lausible that they m ight represen

such species dif ferences of a single m olecule based

their sim ilar distributions and functional properties b


9/12

PH A R M A C O L O G IC A L R EC EPT O R S 359

apparen t m utual ex clusion in d if ferent species. From the

cloning of these receptors, it is now clear that there are

tw o receptors in hum ans w ith 5-H T 1D pharm acological

p roperties, w hereas the rat and m ouse hom ologs of one

of these tw o genes has 5-H T 1B pharm aco log ical p roper-

ties (H artig et al., 1992). T he adv antage of identify ing

such species v arian ts as the sam e recep tor is that func-

tion can be ex pected to be largely conserv ed betw eenspecies, ev en if there are changes in structu re that alter

its in teraction w ith sy nthetic ligands.

Ind iv idual sub ty pes of m ultiple-subunit recepto rs can

be m olecularly def ined by identify ing each of the subunits

that are present, e.g., the R i receptor cou ld be def ined

as con taining the al, b3 , c2 and d5 subunits. A m ajor

unresolv ed question is w hich of the m ultitude of possib le

com binations of subunits actually occurs to form in v iv o

receptors.

T he question of identify ing w hich m olecule m ediates

a particular phy sio log ical response w ill largely depend

on hav ing d iscrim inating drugs av ailab le, bu t in som e

cases, such as cell lines or tissue sam ples, m olecu lar too ls

such as specif ic D N A probes for hy brid iz ation to m R N A

or subty pe-specif ic antibod ies generated f rom clone-de-

riv ed an tigens m ay be helpfu l in iden tify ing w hich sub-

ty pes are present or absent. H ow ev er, absence is alw ay s

diff icu lt to estab lish and m R N A distribution in tissue

m ay not coincide w ith the recep tor distribution. It m ay

also be possible to iden tify the phy siolog ical function of

specif ic subty pes by purely genetic m eans, such as gene

inactiv ation or replacem ent in transgen ic an im als or the

inh ibition of translation of specif ic m R N A s by the use

of antisense R N A s. T o the ex ten t that there are k now n

m olecu lar sub ty pes for w hich discrim inating drugs hav eno t y et been identif ied , caution w ould dictate that the

recep tor be identif ied as, for ex am ple, m olecu lel o r

possibly m olecu le3. A tten tion shou ld be g iv en to the

possibility that m ore than one recep tor ty pe is inv olv ed

w hich necessitates using m ore than one or tw o drugs to

m ak e the identif ication.

V I I . P a r a llel P h a r m a cologica l a n d M olecu la r

N o m e n c l a t u r e s

T he cloned receptors that are presen tly av ailable hav e

no t y et been stud ied thoroughly enough to allow a full

def inition for any set o f recep tors that w ould allow theunam biguous assignm ent of a m olecular nam e to an in

v iv o receptor, especially in the case in w hich m ultiple

related receptors m ay be present. N ev ertheless, the char-

acterization of cloned adrenoceptors and m uscarin ic ace-

ty lcholine receptors has adv anced far enough that w e

can predict w ith a reasonab le am ount of conf idence the

basic facto rs that w ill be im portan t in def in ing sub ty pes

of G pro tein-coupled recep tors. S im ilarly , the cloning of

a substantial num ber of subunits of n ico tin ic acety lcho-

line recep tors and G A B A A recep tors has def ined the

problem s but has no t y et prov ided the solu tions for

def ining m olecular sub ty pes of ligand-gated ion chann

recep tors. T he task of the In ternational U nion of Ph

m acology C om m ittee on R ecep tor N om enclatu re

D rug C lassif ication is to f ind a unifo rm fram ew ork

def ining receptor nam es that w ill allow all recepto rs

be def ined accord ing to the sam e princip les so that

subcom m ittees dev oted to particu lar receptor ty pes

form ulate nom enclature proposals w ith a com m onm at. T o m inim iz e changes in the nom enclature and

the associated def in itions w ith the adv ent of new in

m ation, especially new subty pes, w e should try to pred

w hich factors are essen tial to the def in ition of sub ty p

and w hich factors are suf f icien tly v ariab le (e.g., w

cellular env ironm ent) that they shou ld no t be considere

as parts of the prim ary def in ition. T he latter facto

m igh t or m ight no t be included as secondary inform ation

in m uch the sam e w ay as a dictionary f requently includ

in form ation distinguishing the usage of w ords w ith

ilar m ean ings. T o the ex ten t that one cannot accurately

pred ict all the factors, a f ram ew ork m ust be designe

that allow s the incorporation of new inform ation

the def in itions w ithout drastically altering the nom e

clatu re. H ow ev er, substantial changes in def inition w

retain ing the sam e nam es m ay generate som e confusio

G iv en our present state of k now ledge, w e need a tr

sitional sy stem such as the form at suggested by J.

G reen (1990), o r as used in theTr ends i n Phar macol ogi cal

Science nom enclature supp lem ent (v o l. 12 , 1991), w h

can incorporate m ost of the observ ationally relev a

inform ation to prov ide parallel pharm aco log ical

structu ral (or m olecu lar) def initions. S uch a form at

low s the correspondence betw een these tw o def in ition

to be indicated . H ow ev er, to av oid m isinterpretation,there should be an ex p licit assessm ent of the reliab ilit

of the correspondence betw een m olecular and pharm

cological subty pes. O ne should not ind icate that a part

ular m olecular sub ty pe corresponds to a giv en pharm

cological sub ty pe sim ply because its pharm acological

prof ile m ost closely m atches that of the pharm acological

subty pe, w hen , in fact, sev eral m olecu lar subty pes

no t be distingu ished by the pharm acological def initio

O ne w ould hope that, as add itional data becom e av ailab

concern ing the pharm aco log ical p roperties of cloned

ceptors such that the pharm acological def initions can

ref ined , the in form ation that does no t allow unam biguous distinction of the m olecular identities w ill be m ov e

to foo tno te status, and the pharm aco log ical and m ol

ular def initions w ill coalesce. T here is general agreem en

that recepto rs shou ld be def ined as a ligand-binding

w ith a ligand-m ediated functional response, the respon

being biochem ical, electrical, o r m echanical in its m

festation . T here is also general agreem ent that the

m ary label in the nam es shou ld be the endogenous lig

(o r, p rov isionally , ano ther ligand w hen the endogenou

ligand is unk now n). W hether the com pendium of re

tors should list the ligands alphabetically or alphabeti


10/12

360 K EN A K IN ET A L .

cally w ithin chem ical ty pes (such as am ines, am ino acids,

pep tides, etc.) is open for discussion. T hey could easily

be listed one w ay w ith an index listing them the o ther

w ay . A secondary classif ication , w hich is w ell g rounded

in m olecu lar structure and is observ ationally distingu ish-

ab le, w ould be on the basis o f structural ty pe of recep tor,

e.g., G pro tein-coupled or ligand-gated ion channel. S uch

a distinction has already been m ade in som e cases, suchas nico tinic v ersus m uscarin ic acety lcholine receptors or

G A B A A v ersus G A B A B recep tors, but not in other cases,

such as the 5-H T 3 receptor w hich is not giv en a nam e

distinctly d if feren t f rom the 5-H T 1, 5-H T 2, and 5-H T 4

recep tors. T h is is one subgroup ing of recep tors for the

sam e ligand that is not lik ely to change w ith add itional

info rm ation and , therefore, could safely and usefully be

incorporated in to the nom enclature.

V I I I . Su ggested R u les for N a m in g R ecep tor s

W e are faced w ith the tw in prob lem s of receptor no-

m enclatu re and classif ication. T o the ex ten t that they

can be d istinguished , nom enclature im plies a set of

nam es w ith def in itions that allow s a one-to -one corre-

spondence betw een nam es and objects (such as receptors)

w ith specif ic properties, w hereas classif ication im plies a

hierarch ical o rdering of a group of objects based on their

degree of sim ilarity to each other. T here is clearly a

pressing need for an agreed upon set o f nam es and a set

o f criteria fo r def ining w hat constitu tes a new recep tor

sub ty pe, but it is no t clear that there is a m ajor need for

these nam es to ref lect an ex tensiv e classif ication schem e.

T he ex isting tendency tow ard classif ication in recep tor

nom enclatu re is a natu ral consequence of the process inw hich new subty pes generally hav e been identif ied

th rough the use of new drugs, w hich could distingu ish

d if ferent recep to rs w ith in a prev iously unreso lv ed sub-

ty pe. In contrast, w e can ex pect that in the future m any

new subty pes w ill be identif ied as d istinct m olecules

th rough cloning and, in m any cases, the drugs capable of

d istinguish ing the subty pes w ill hav e to be dev eloped

subsequently . G iv en our relativ ely incom plete k now ledge

of the num ber of recep tors and of the fu ll range of the

properties of those receptors that hav e already been

iden tif ied, it w ould probably be a m istak e in term s of

stability of the nom enclature to incorporate m ore than

the m ost rudim entary classif ication, i.e ., the ligand and

the structu ral ty pe (G protein -coup led, ligand-gated ionchannel, etc.), into the nom enclature.

C lassif ication neu tral labels such as 1, 2, 3,... are

preferred to labels such as 1A , 1B , 2A , 2B ,..., w hich

im ply d if fering degrees of sim ilarity , because the per-

ceiv ed relationsh ips betw een sub ty pes can change w ith

the adv ent of new subty pes or new drugs. It is inev itable

that such changes w ill occur if the relationships are based

on a few properties of the receptors. T his is clearly the

case in the tax onom y of p lants and anim als in w hich, if

one considers on ly a few specif ic characteristics, one can

deriv e dif feren t phy logenetic trees depend ing on w h

characteristics are used . T he case of the 5-H T 1c recep to

is ev idence that the sam e th ing can easily happen w

receptors. It w as originally classif ied as a 5-H T 1 subty p

based on a sing le drug but subsequently appeared to

m ore sim ilar to 5-H T 2 both pharm acologically and

com parison of the cloned sequences. If relationships

not im plied by the nam es, the relatedness of d if ferensubty pes cou ld be included as supplem entary inform

tion so that it can be m odif ied w ithou t m odify ing

nom enclature. A s a rule, the m ore subty pes there a

the m ore d if f icu lt it w ill be to def ine m eaningfu l r

tionships. C learly , there w ill be cases, such as adrenocep-

tors, in w hich a nom enclatu re that is not classif ication

neu tral is so w e estab lished that it w ould no t m ak

sense to change the w hole sy stem unless it w ere

becom e unw ieldy because of the iden tif ication of e

m ore subty pes.

A lthough it w as ex ped ient fo r the a2-adrenocep tors,

the nam ing of m olecular sub ty pes af ter their chrom o

som al location is no t desirable. G enerally , the hum a

chrom osom al location is not estab lished for som e ti

af ter a cloned receptor is published , especially if it is

a hum an clone. Furtherm ore, there m ay be m ore th

one sub ty pe on the sam e chrom osom e as in the case

the m 1 and m 4 m uscarin ic acety icho line recep tors.

I X. C on clu sion s

For the presen t, tw o ongoing sy stem s of recep tor d

inition , structural and pharm acological, appear to

necessary . O ne w ould be based upon receptor structure

w ith po ten tially incom plete pharm acological criteriA f ter pharm acological criteria hav e been fulf illed, th

recep tors w ould be def ined in bo th sy stem s. T he o t

w ould def ine recep tors by pharm acolog ical criteria (

endogenous agonist, antagonists). A gain , it w ould

conceiv able that there w ould be incom plete data for so

recep tors def ined in this category , therefo re, preclud ing

def in ition in the o ther. A com pletely def ined recepto

w ould possess a un ique pharm acological p rof ile based

agonist and an tagonist data, a k now n endogenous ligan

and a d istinct am ino acid sequence. It m ay be th

recep tor identif ication by structure w ill p rov e to b

technically m ore sim ple task than identif ication by

gand pharm aco logy if recep tor env ironm ent (i.e ., lip

coup ling proteins) p lay s a signif icant ro le in def iningpharm aco logy of som e recep tors. In these cases, recep to

pharm aco logy w ould v ary w ith the tissue or cellul

sy stem but no t be ind icativ e of d if ferent recepto r ty p

T he reso lution of the question of w hether recepto r

v ironm ent is a signif icant factor in receptor identif ica-

tion and/or classif ication w ill require the use of sy stem

in w hich the structu ral iden tity of the recep tor is w

defined.

T he nom enclature sy stem w ill clearly ev olv e at a r

p roportional to the diligence of all concerned in reco


11/12

P H A R M A C O L O G IC A L R E C E P T O R S 361

ciling the structural and pharm acological definitions. A san initial fram ew ork for im plem enting these parallel sets

of definitions in a form that facilitates their reconcilia-tion , the In ternational U nion of Pharm acology C om m it-tee on R eceptor N om enclature and D rug C lassification

has adopted a tabular form at closely resem bling theTrends in Pharmacological Sciences receptor nom encla-

tu re su pp le me nt.

Acknowledgments. W e ack n ow led ge t h e in -d ep th com m en ts con cern -ing this paper by D r. D on Jenkinson, University College, London,

U n it ed K in gd om .

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