The Inhibition of Sympathetic Preganglionic Neurons by Somatic Aff erents

Bssr~ ~VYSZOGKCJDSK.;I A N D CANIO P O L ~ S A Dcprirttr!clt~t o/ PIry~ io lo~ j~ , Afc'Grll Linisersity, 1Wontrc)nl, Q~rehec'

Recei~led May 8, 1972

WYSZOGRODSKII, H ., arid H30~os.+j, C . 1973. 'The itahibision of syrslpatkctic prcganglionic neurons by sonlatic afTerents. Can. J . Physiol. Pharrnacnl. 51, 29-38.

The inhibitory effect of sciatic and 11lnar nerve afferent stitnulaiion on the firing frequency of sympathetic preganglionic neurons was studied in :incsthetized or unanesthetized deccrebrate cats, with intact spinal cords or. with spinal cords sectioned at C?. The spontaneous firing and the firing evoked by antidronaic stimulation, by iontolphoretic glutamate, and by ~l~echanical in-jury could bc depressed, in the preparations both with intact and sectioned spinal cord. The depression was not preceded by excitation. The minimun~ stimulus strength required for the inhibition was, on Liverlxge, 15 times the nerve threshold. TIlc inhibition elicited by single shocks lasted several hundred milliseconds and was longer in the intact than in the spinal preparation. The results show that the neural pathway used by high threshold so~rlatic afferents for inhibition of the syn~pathetic: preganglion- ic neurons is coinplcte within the spinal cord and suggest that the inhibition is probabiy acting on the preganglionic neuron membrane.

k'Bi~szsc;reo~sttr, I. , et POLOSA, C. 897.3. The inhibition of sympathetic p~ega~g l ion ic neurons by somatic afferents. Can. .I. Physiol. Phar~nacol. 51, 29-38.

Nous nvons Ctudii l'efTet inhibitrur de la stimulation des nerfs sciatiqt~e e l ulnaire sur Ha frkryiience dc dCcharge de ncalrolies syrnpathiqucs pr@garsglisnairej. Cette Ctude a kt6 m e d e chez le chat anes- thCsi0; le chat non anesthdsik; Ha 1noeHlc kpinikre Ctant irltactc OIL sectionnke en Cz. L'activitC spontanke de ces neurones de mCme qtne i'activite kvoq~de par stimulation anti-dromique, l'appliication ionto- phorktiq~ie de glutamate c t par lesions mkcaniques peuvent etre dirninuees par la stimulation du nerf sciatique ou ulnaire, et cequelque soit 1'Ctat de la moelle Cpinikre (intacte ou sectionnee). Ccttc diminution tie l'activite la'est pas precedkc par imc augmentation. L'intensitk minimale de stimula- tion nCcessaire pour obtenir urn effet inhibiteur est, en moyenne, 15 fois plus ilcv6c que l'intensite sellis de stim~ilation du nerf. L'effet inhibitcur produit par l'application de stimuli isolCs dure plm- sieurs centaines dc msec., cette durCe Ctant plus petite chez l'animal spinal. Ces rksultats indiquent quc les voies nerveuses ernpruntees par ces affkrences somatiques h seuil &lev6 sont entikrement localist2es drnns la mcselle kpinikre et q i ~ c B'inhibition produite rksulte d'a~ne action sur la membrane ctes ncurcmes sympatlaic~ues preganglionaires. [Trad~ut par Be journal]

Studies of multifiber preparations of sym- pathetic rlervcs have shown that stimulation of high threshold afkrents in limb nerves can evokz, in addition to the already known excitatory effects, depression of background and reflex discharges (Sell et nl. 1958; Bca- cham and Pcrl 11964; Fedin? et rrl. 1966; Koizumi e t c r l . 1968: Alanis and Defill6 1968; Ikvarnura cJt rrl. 1969; Coote and Perez-Gonzalez 1970). With this type of pre- paration, however, it is difficult to establish with certainty whether the depression is due to true inhibition or is a sequel of a pre- ceding reflex excitation, i.e. postcxcitatory depression.

Recently, single-unit studies of sympa- thetic prcganglionic neurons QS.P.N.s) have provided definite evidence for inhibition by

showing that depression of background ac- tivity. as a result of somatic afkrent stimula- tion, can occur without prior excitation (Wyszogrodski 19'70: Jiinig and Schmidt 1970). The properties of the ncural circuit(s) n-nediating this inhibition are not known, however.

This paper describes an investigation of some properties oC the inhibitory action of somatic afferents on S.P.N.s, based on data obtained by extracellular recording from single S.P.N.s in the spinal cord, and dis- cusses its possible mechanisms and central organization. The present results suggest that the mechanism of inhibition is, at least in part, postsynaptic (i.6~. on the S.P.N. membrane) and that the inlaibitory pathway is complete within the spinal cord.

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Scveniy-six cats were uscd. Sixty-seven were ancsthet- izcd with sodiirln pentobarbital (40 111gikg i.p, supple- mented by itstravenous doses as rcq~rired). Nine others wcre decerebratect under ether by senaoving the fore- brain, anterior to the tc~ltori i~m, via a craniotorny. In this group, ether was discontinared after completing al! surgery. Spinal anitm~als were preparcd by a comp!etc cord transection at C2, after lar-llincctorny and infiltra- tion of the site with 2:); novocaine. All animals wcre paralyzed with galia~niile trielhiodidc (3 n?g/kg every hourj, tkoracotomized, and artificially ventilated. Rcceai temperature was n~aintaincd at 37 "C with an infrared lamp and a hcating blankct. Arterial pressure was continuously anonitored. The cervical sympathetic (used for antictron~ic identification), sciatic, and ulnas nerves were prepared for stimulation, and mounted either on buried bipolar silver elcctrodes or on pairs of sisver electrodes in a p o d of warm paraffin oil. Stimuli were rectangular jslrlses of 0.3-1 .0 ms duration, delivered by a Grass S88 stilnulator via an S I U 4678 isolation unit. Threshold strengths ( T ) for electrical stimulation of limb ncrvcs were determined by recording the volley with electrodes on a small L7 dorsal root fi!ament o r on the nerve proxilmal to the stimulating point. Thresholds so determined appeared to coincide with those detcr~nincd by the appearance of a just detectable muscle twitch, bcfore paralysis (Johansson 1962)- Threshold values ranged, in different experiments, between 100 and 200 I-nV. Limb nerve stimulations with singlc maximal shocks or repetitive stimulation at repetition rates below 0 .5 Hz evoked no changes in arterial prcssurc. The rneth- ods used for identification of and recording single

S.P.N..; of the first thoracic: segment with glass micro- pipettes Miere dessr-lbed previously (Polosa 1967). Argu- ments in se~pport of the asserrnption~ that recording was from the soma-dendritic regitrn of the ncurcpr-as were also pr escnted previoiesly (Pola9sa 1966). In some experiment<, double-barrel glass micropipetees were used, filled with 0 . 5 hf Na glutamate and with saturated NaCI (Curti5 v t c r l . 196411; JCr&ievii and Philiis 8963). Poststimulirc histogrsrns were obt:lincd with the computer dcscribcd by Burns ct rrl. (1965). The data prcsentcd below %were oblairlcd fron-n a sample of 250 S.P.N.s from 7 h cats.

Results

Inhihirior~ in h'crrs ~virir Ifrraci Sj9irzul ( ' 0 ~ 6 1

Background and evoked discharge of S.P.N.s could be inhibited by limb nerve ctisnuki- tion. Forthy-seven o u t of 142 antidromically identified units 11ad background activity a t the low mean rate and with the large variabil- ity of intcrspike interval previously reported 3s typical for S.P.N.s (Polosa 1968; Manna rd 1970). The mean rate of this sannple was 2.1 spikesis (S.E.M. 0.1, rangc 0.5-6.0). A single shock of appropr ia te intensity (see p. 32) to khe ~ c i a t i c or tllnrtr nerve de- pressed background firing of I3 units in the absence of any prcvious excitation. This effect is shown in Fig. I A and H. Silent S.P.N.s could be made to discl~arge repsti-

25yV I 1 s

FIG. I . (A, B) Inhibition of S.P.N, spontaneous firing by srngle shocks to ulnar nerve. (A) Control, 35 superimposed swecps, trrggered by any spontaneous splke. Mean firing rate of the arnat 0 . 5 spikesi's. (B) SBlocks to ulnlmr nerve ( 5 V, 0.5 ins), 25 se~perimpcrscd sweeps triggered by stirnulbas pulse. Note abscnce of sprkcs for 2 s after stimulus. ( C , 89) Inhib~tion by single sllocks to ultlar nerve of S.P.N. firing evoked by steady release of glutamate. (6') Control single sweeps. Avcrngc number of spikes pcr sweep is I I . (D) Shocks to u!nar nerve (10 V, 0 . 5 ms). Swceps triggered by stirnulii~ artcfiict. Average number of spikes pea- sweeps 15 2.

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tively by releasing glutamate from the pipette with long current pulses (seconds to minutes). For small ejecting currents, the glutamate- evoked discharge was of low repetition rate (e.g. 1-4 sgikes/s) armd either regular or irregular. The latter mode of firing resentbled somewhat the spontaneous firing of these neurons. For larger currents, the discharge was at a higher frequency (e.g. 10-15 spikes/s) and always regular, but thc spikes often became progressively smaller and the firing stopped, probably as a result of inactivation block, whish was reversed by interrupting the glutamate ejection. Similar observations have been made with DL-homocysteic acid on S.P.N.s by Hor~go and Ryall (1966) and DeGroat and Wyall (1967). The glutamate- evoked activity, like the background activity, could be depressed by single shocks to the limb nerves in 117 units (Fig. 1 C and D). The depression could be partly overcome by in- creasing the rate of release of glutamate. For one additional unit, inhibition of glutamate- evoked firing, with latency similar to that of the inhibition obtained in other units with

single shocks, could only be evoked by tetanic stimulation of the nerves. Firing evoked by electrode movement, and due probably to injury to the cell, could also be inhibited (3 units, two shown in Fig. 2). Antidromic firing, evoked by single shocks to the cervi- cal nerve, was never blocked by single or repetitive stimulation of limb nerves. However, when pairs s f antidromic shocks were given, a decreased probability of anti- dromic invasion during limb nerve stimulation was detected in four cells. The individual stimuli in the pair (of intensity three to five times the unit threshold) had to be spaced so that the testing antidrornic spike would be reaching the cell body during the early re- covery phase following the conditioning firing (conditioning-testing interval of the order of 6-8 ms) and would be on the verge of or actually failing to invade the cell body in a proportion of the trials. These failures were not due to absolute refractoriness of the axon, because the latter could still be excited at shorter conditioning-testing intervals than those at which the soma-dendritic spike began

FIG. 3. Inhibition of injury discharge. (A, B, E) Quiescent S.B.N. At beginning of A, unit starts to fire as a result of small electrode movement. Discharge is inhibited by shocks to the ulnar nerve (15 V, 0 .5 ms) given at 11s (stimulus artefacts marked by dots). Towards end of B, unit gives burst of high frequency discharge and stops firing. In E, 20 superimposed sweeps triggered by stimulus to show absence of excitation. (C, 8, F) Another S.P.N., firing as a result of electrode movement. Discharge inhibited by shocks to the ulnar nerve (8 V, B ms) at 11s (stimulus artefacts marked by dot)- In F 1.5 superimposed sweeps triggered by stimulus artefact, to show absence of excitation.

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to fail (c$ Polosa 1966, p . 30; Brock et a/. 1953) and because tlae proportion of success- ful invasions could be increased or, less fre- quently, decreased by tetanic stimulation of limb afferents (see Discussion ; also Wyszo- grodski 7978, Fig. 14). Fig. 3 shows an example of a quiescent unit in which tlse probability of antidrornic invasion decreased during tetanic stimulation of the uInar nerve.

The stimulus strength required for evoking a just detectable inhibition ranged in different units from 4 to 55 times T, on average 15 times T (1 1 units). From these figures it cara be inferred that group I I I afPerents, and possibly son-ae of group I1 (Brock et OH. I951 ; Eccles and Lundberg 1959; Rosenberg 1970), were responsible for the inhibition. The same fiber groups are known to evoke hypotensive responses in aiaestlnetized cats when stimulated repetitively at low frequency (Eaporte and Montastruc 1957; Laporte ct a/ . 1940; Skoglund 1960 q Coote and Perez-Gonzalez 1970). With the exception of the experiments in which the relation of afferent fiber types to the inhibition was specifically investigated, the stimulus intensity used was that which gave most pronokinced inhibitory effects.

200yV

20 ms FIG. 3. lnhi bition of antidromic invasion. Antidrsrnic

shock pairs given every 5 s. Sweep shows events follcdwjng test shock only. (A, Cb Controls, before and after B. Spike response is present after each test antidrornic stimulus. (Bb During ulnar nerve stimulation at 101s. Spike response t o testing antidromic s t i~ ; t ius is absent. There was a slight blood pressure drop ( 5 mrn Hg) during afferent stimulation.

The latency of tlae inhibition was estimated from poststimulus histograms in units witln spontaneous or glutamate-evoked firing as the interval between the stirnulus artefact and the beginning of the response, i.e. the time when the firing of tine unit, as judged by eye, started to deviate from the tanstimu- lated baseline. Mean latency was 96 rns (S.E.M. 12, n -- 19, range 40-240) with sciatic nerve stimulation and 48 ms (S.E.M. 7, n - 18, range 10-128) with ulnar nerve stimulation. The diEerence in latency between sciatic and ulnar nerve stimulation is statis- tically significant ( B -< 0.0018. The shortest latencies, with tlne d n a r nerve, suggest pathways with only few synapses. In several units, due to short duration of contact, low frequency and high variability of firing, and long duration or inhibition, latency could not be estimated with any degree of certainty. For tlae few units in which satis- Fdctory threshold determinations were made an estimate of central delay was made. Values for peripheral condalction velocities were obtained from fiber threshold using the plots published by Eccles atad Lundberg (1 959) and Rosenberg (1 970). Peripheral con- duction distance was taken between cathode and either L, dorsal soot entry zone (sciatic nerve) or T, dorsal root entry zone (ulnar nerve). After subtracting the estimated peri - pheral conduction time, central delay was 58 ms (S.E.M. 15, n - 4, range 30-93) for sciatic nerve stimulation and d l rns (S.E.M. 8, n - 7, range 16-77) with ulnar nerve stimul- ation.

The duration of the inhibition, evoked by single maximal stimuli, was on average 1108 n ~ s (S.E.M. 155, f.1 - 19. range 180- 2400) with sciatic nerve stimulation, and 720 ms (S.E.M. 104, n - 19, range 140-2000) with ulnar nerve stimulation. This difference in duration could be attributed to a greater temporal dispersion of' the sciatic ;alley, due to its longer peripheral and central path. The inhibition was of several hundred milliseconds duration even witin just tlsreshold stimulus iratensity. N o systematic difference ira tlae duration of inl-aibition was observed whether spontaneous activity or glutamatc- evoked activity was used as a background for detecting inhibition. For instance, 'in the group of cells inhibited by sciatic nerve

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stimulation, the duration of nnhibitit)al was 1190 ms (S.E.M. 235. r l 10, range 180- 2400) i t 1 the spoa~tataeously active units and 1043 rns (S.E.M. 337, i l -- t1> range 240 -2208) in the glutai-iaate-excited units. The difference in duration between the two groups was not significant ( p -- 0.2). The proportion of the neuron population that could be inhibited by .;omatic afrerent stimulation varied con- siderably from one experinlent to tlsc other. On the basis of data from glutamate ex- periments in which nearly every cell could be made to hire, the proportion was estimated to be between one-third and one-half. The ulnar nerve appeared a more powerful inhibitory nerve for the '%', S.P.N.s than the sciatic nerve, as shown by the fact that when the cf'fects of stimulating both nerves were tested on the same cell, the likelihood

of obtaining inhibition from the ulnar was higher than from tlze sciaiic. Out of 18 units. in which either input or both had an effect, nine were inhibited by ulnar nerve only, one by sciatic nerve only, and tlze remaining eight by botlz nerves.

All the data presented so far refer to 'pure' inhibition, i . ~ . depression of firing in the absence of prior excitation. In 22 other units, a depression of firing was pre- ceded by excitation. For 16 of these, the depression of firing, following the early ex- citation. could be reasonably attributed to post-firing depression because it was no longer i n duration than the depression that follows antidromic or spontaneous firing in the same cell (Fig. 4). The latter appears to be due to subnormality (Polosa 1967, 1968). For the other six units, which were

Frc;. 4. Con~parison of duration of silent pcriod for an S.P.N. following reflex (A). antidroniic (B), and spontane- ous (C) firing. Forty superimposed sweeps in each record. Sweeps are triggercd by stimulus to ulnar nerve in A. by antidronmic stimulus in B, and by any spontaneous spike in C. The reflex in A had a minimum latency of 72 ms and an average firing index of 0 .8 spikes per stin~ulus. The silent period i~ longcst in R probably bccausc some of the antidrornic firings fell close to a preceding spontaneous firing, thus adding to the level of post-firing excitability dc- pression. When the antidromic stin~ulus was subthreshold, the interval between stimulus and subsequent spontaneous firing approached zero (not shown).

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excited with a low firing index, the depression was clearly due to inhibition, because the depression was present in trials which failed to excite the cell.

I~lhihitisn in the Acute Spinal Cut All the observations on spinal animals

were made between 2 and 8 h after spinal cord section. Arterial pressure, as expected, was lower (P < 0.001) in this.group (average 70 mm Hg, S.E.Pc4. 9 .7, n - 28, range 40-1 10) than in the group with intact spinal cord (average 120 mm Hg, S.E.M. 4.7, 1.2 = 48, range 75-1 60). Spontaneous activity was present in this preparation in 25 out of 108 antidrornically identified units, at the mean rate of 1 - 3 spikes/s (S.E.M. 0.04, range 0.3- 4.3). These mean rates were lower (P < 0.008) than in the animals with intact spinal

cord. Single shock stimulation of sciatic or ulnar nerve inhibited the background activity of five units and the glutamate-evoked activity of nine other units. For seven addi- tional units a detectable inhibition of latency, similar to that evoked by single shocks (see below), could only be evoked by tetranic stimulation of the nerves. By contrast, in the group with intact spinal cord tetanic stirnulatioil recruited only one additional unit (see p. 31). Examples of inhibition in spinal animals are given in Fig. 5. Anti- dromic invasion, tested with double shocks as described before, was inhibited in two cases. Average latency of inhibition with sciatic nerve stimulation was 65 ms (S.E.M. 8.4, n - 8, range 40-100); with ulnar nervc stimulation it was 40 rns (S.E.M. 9.2, m - 7,

150 ms 150 rns E

IOOJPV 50 rrV

30 rns 30 rres

FIG. 5. Inhibition of glutamateevoked (A-F) and spontaneous (G) activity of S.P.N.s in the acute spinal cat. (A, D) Controls for two units. Sixty sweeps triggered by any spike. (B, E) Sixty sweeps triggered by shocks to ulnar nerve. (C, F) Same as in B and E, but at faster sweep speed to show absence of excitation. (G) Inhibition by a tetanus (marked by horizontal line) to sciatic nerve (18/s). Ns arterial pressure changes associated with the stimulation. Upper- most calibrations apply to A,B and D,E, respectively.

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range 30-50). At least 20 ms of this 25 ms diRerence between sciatic and ulnar nerve latencies was due to intraspinal coilduction time between sciatic and ujnar nerve entry zones. Conduction velocity between these twb points in the cord would then be 10 m/s. Average duration of inhibition was 280 rns (S.E.M. 54, n = 9, range 100-480) with sciatic nerve stimulation, and 266 ms (S.E.M. 42, FT = 7 , range 140-450) with ulnar nerve stimulation. ~ G e n these data are compared with those of the group with intact cord, it - . appears that the average duration s f the inhibition evoked bv a single shock

J U

is greatly reduced after spinal cord section (P < 0.001). The average latency of ulnar inhibition is slightly decreased (P < 0.05), while that of sciatic inhibition is more drasti- cally decreased (P .::. 0.001). The intensity of peripheral nerve stimulation, necessary for evoking a just detectable inhibition, was comparable to that required i n the cat with in- tact spinal cord. The proportion or inhibited units to total S.P.N. population also appeared similar to shat found in cats with intact spinal cord.

& f ~ r t s of Sorjzatic Aflerents o1.s Other Units in and near the Lateral Horn

Occasionally, units were recorded at elec- trode locations where antidromic responses were also recorded which did not respond to antidromic stimulation, had spikes of larger size and shorter duration than usual for %.P.N.s, and fired spontaneously at frequencies higher than those usually observed for S.P.N.S. These unidentified units were probably inter- neurons of the lateral horn. Some of these units were excited by high threshold afferents in the sciatic nerve, with latencies of 20-30 ms and duration of excitation of 20-40 ms (several units responded with bursts of spikes). Other units were inhibited, with latencies of 20-$0 ms and durations of 200-500 ms. These neurons could be connected with and pre- synaptic to the S.P.N. The excited units do not qualify as inhibitory interneurons, without additional stipulations, because their dischar- ge was of short latency and duration, while the inhibition of the S.P.N.s was of long duration. The inhibited units could, on the other hand, be units whose depression could cause disfacilitation of S.P.W .s, because the

duration of their inhibition was comparable to that of the S.P.N.s. However, there is no evidence, so far, that a process of disfacilita- tion is involved in 111s depression of S.P.N. activity caused by somatic afferents. Thus these observatioi~s are inconclusive.

Two possibilities can bc considered con- cerning the site of action of the inhibitory mechanism responsible for the effects just described. One possibility is that the inhibitory mechanism operates directly on thc S.P.N. itself, i.e. by postsynaptic inhibition. Al- ternatively, the inhibitory ~necl~anism could operate 011 neurons presynaptic to the S.P.N. and contributing to the excitatory processes underlying S.P.N. background activity, thus depressing the S.P.N. by a1 process of dis- facilitation. Tlae fact that glutamate-evoked discharges and, in fewer cases, antidromic invasion and in~ury discharge were blocked by the inhibitory mechanism suggests that the first possibility is more probable, i .c. the inhibitory effects obscrved must have been, at least in part, postsynaptic on the S.P.N., although a disfacilitation cannot be entirely excluded at present. This sonclusion is based, for the observations on glutamate-evoked firing, on the assumption that glutamate, released by an extracellular pipette, fires the S.P.N.s by a direct action oil their soma- dendritic membrane. This assumption is based on arguments presented by KrnjeviC and Phillis (1963) and by Krnjevid t.t a / , (1966). It seems unlikely that the observed suppression of glutamate-evoked activity was in fact due to the removal of a subthreshold tonic facilitation (i.e. a disf'acilitation) which fired the cell by summating with the glutamate- evoked depolarization, because often the glutamate-evoked discharge was at rates several times higher than those of any spon- taneous discharges observed in these neurons (Polosa 1968; Mannard 1970), yet it could be completely suppressed. Moreover, in these cases the glutamate-evoked discharge was, unlike the spontaneous, regular. If the spon- taneous synaptic activity, with its typical irregularity (Mannard 1 97(3), could not in- fluence the discharge pattern of the neuron, it must have been minimal (relative to the

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glutamate-evoked depolarization). For sacral parasympathetic neurons, it has been shown (DeGroa t 194 1) t l ~ a t a disfacilitatory process, evoked by cutaneous stimuli, does not depress DL-homocysteic acid evoked activity, while it de- presses reflexly evoked firing. The inhibition of injury discharges (cf. Fig. 2) alsc~ suggests that the inhibition acts on the membrane of the S.P.N. The block of antidrc~mic invasion, observed in a few silent cells, can be inter- preted similarly. The argument would be that since the units were silent, any background facilitatory synaptic activity, if present, must havc keen minimal. The fact, however, that the block of antidromic invasion was very seldom observed deserves discussion. The rationale of using antidromic shock pairs was to lower the safety fdctor for soma invasion by the antidromic test spike to a point where the process should become sensitive to varia- tions in membrane potential of the soma. An indication that this objective was attained was the relative ease with which, in these conditions, facilitation of antidromic invasion during stimulation of limb afferents was observed (Wyszogrodski 1970). The chance of observing an inhibition of antidromic invasion, however, was much lower (perhaps one-tenth or less). A possible explanation for the rare observation of inhibition of antidro- inic invasion is that the conditioning anti- dromic spike is followed by an after-hyper- polarization similar in mechanism (i.e. due to an increased g,) and time course to that of the alpha-motoneurons (Baldissera and Gustafsson 1970; but L$ Fernandez de Molina et al. 1965). With the C - 7' intervals used in the present experiments (e.g. 6-8 ms), the test antidromic spike would appear at the axon hillock at a time when soma membrane potential is at or near the peak of the after- hyperpolarization. Assuming at this time the membrane potential to be drifting towards E,, it is possible that an inhibitory process, based on an ionic mechanism with similar equilibrium potential, may not add much to the already present hyperpolarization.

Concerning the properties of the circuitry connecting the afferents with the S.B.N.s, one remarkable property of the inhibition is the existence, as a rule, of very long central delays (qfi p. 32). These could be accounted for in terms of slow conduction in fine axons

and/or mediation through long chains of interneurons. Synapses with ultralong syn- aptic delays (cJ Libet 1967) are another possibility. However, even a relatively simple synaptic pathway could generate such long delays if, far instance. the hypothetical inhibitory interneurons were excited by slowly rising E:P.s.P.s.~ Or, 1.P.S.P.s could be initially evoked in the hypotlxtical inhibitory interneurons, followed by a late E.P.S.P.; the long central delays would then be a meas- ure of the duration of the early inhibition of" the interneurons.

Previous studies of the excitation of S.P.N.s by high threshold somatic afferents (Sato et al. 8965; Coote and Downmail 1966; Sato and Schmidt 1971) have revealed the existence of a discrete segmental and inter- segmental organization which gives origin to short-latency and long-latency reflexes, respectively. The size of the short-latency reflex decreases sharply with distance from the segmental level of the illput, while size (and delay) of the long-latency reflex is inde- pendent of the segmental level of the input. The latter reflex is greatly attenuated in the spinal animal, thus suggesting the possible existence of a supraspinal relay in its path- way (Sato and Schmidt 1971).

Whether a similar organization exists for the inhibitory reflex cannot be stated on the basis of our data. The observation, reported above, that the ulnar nerve (closely related segmental- ly to the recording site) appeared to be more effective than the sciatic nerve (whose seg- mental level is at least 15 segments away from the recording site) in inhibiting the TI S.P.N.s, can be taken as an indication of the existence of neural networks with different properties for mediation of inhibition within the same (or the adjacent) spinal segment and for the intersegmental inhibition. The large difference in central delay between ulnar- and sciatic- evoked inhibition could also be interpreted in such terms. However, a simpler explana- tion for the difference in delay could be in terms of the longer intraspinal path, with coilsequent greater temporal dispersion, for the sciatic volley. Alternatively, the difference in central delay could be explained if the

IE.P.S.P., excitatory postsynaptic potential; I.P.S.P., inhibitory postsynaptic potential.

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inhibitsrv reflex were mediated bv a smirso- BROCK, L. G., E c c r ~ s , 3 . C:., and K a r , ~ , W. 1951. Ex- J J 1

bulbs-spinal circuit, as suggested by some aut l~ors (Koizumi ct nl. 1968; lwamura cpt ua'. 2 969 ; Coote and Perez-Gon;lale~ 1970) ; the shorter distance between ulnar afferents entry levcl and medulla cc~uld then account for the direrence. However-, the persistence of TI S.P.N. inhibition by both ulnar and sciatic nerve in the acute spinal cat naakes it clear that entirely spinal inhikltory pathways exist, for both the segmental and the interseg- mental effects. It might be worth mentioning that the inhibition of sacral parasympathetic neurons by high threshold sonlatic atrerents also persists in spinal preparations (DeGroat and Rya%l 1969). The duration of inhibition in the spinal animals was, however, markedly reduced and repetitive stimulation of the afrerents was o f ~ e n rcquired i n order to evoke the inlsibitory effect. Thus, the inhibition appeared somewhat impaired by the removal of supraspi1i:ll structures. Several pcpssible explanations can be c~Kered to accoumlt for this fact: ( ( I ) a fraction of the inhibitory pathways connecting somatic affcrents and S.P.N.s runs through supraspinrzl structures ; (h ) removal of tonic facilitation of supraspinal origin impairs transmission in inhibitory pathways which are entirely spinal; (c) the relatively low blood pressure of the spinal preparation may result in a reduced blood flow through the cord, with consequent hypoxia, which may block synaptic trans- mission in spinal pathways. Since there was no apparent relt~tionship, in the spinal cats, between blood pressure levels and incidence of inhibition, the latter possibility cc~eald probably be ruled out.

We wish to thank Drs. R. W. Ryall and A. Mannard for reading this nsanuscript. Financial support fi-om the Medical Research Council is gratef~~lly acknowledged.

AH- AN^. J., and DEFPLL~, B. 1968. The etTerent discharge of the sympathetic and parasympathetic cardiac fibers in response to the simultaneot~s activation of saphenous nerve and carotid receptors. Arch. Int. Physiol. Biochem. 76, 236- '952.

BALDISSERA. F., and GUSTAFSSON, B. 1930. Time course and potential dependence of the mcnlbrane con- ductance change during the after hyperpolarization in the cat's alpha motoneurones. Brain Res. 17, 365-368.

Bf~c'tn,z~, W. S., and PFRL, E. R. 1963. Background and reflex discharge of sympathetic preganglionic neu- rones in the spinal cat. 3. Physiol. 172, 400416.

perimental investigations on the afferent fibres in musclc nerves. Proc. R . Soc. B, 138, 453-475.

BROCK, L. G., Coohfos, J . S., and ECTLES, .F. C . 1953. Tntraccllular recording from antidromically activated motoneurones. J. Physiol. 122, 429-461.

BURNS, B. D., FERCH, W., and MANDL, G. 1965. A neuro- physiological computer. Electron. Eng. 37, 20-24.

COOTE, -8. H . , and DOWNMAN, C. B. Ig. 1966. Central pathways of some autollsrnic reflex dischargcs. J . khysiol. 183, 714-729.

COOTE, J. H., and PFREZ-SONZAL~L, J. F. 1970. The re- sponse of some sympathetic neurones to volleys in various afferent nerves. J. Physiol. 208. 261-278.

CURTIS, h). R., PIIILLIS, J. W.. and W,za-~rhs, J. C . 1960. The chemical excitation of spinal neurones by certain acidic amino acids. J. l'hysiol. 158, 656-682.

D ~ G R O K I , W. C. 1971. Inhibition and excitation of sacral parasympathetic neurons by visceral and c~ataneous .itimuli in the cat. Brain Res. 33, 499-503.

DLGROAT, W. c., and KYALL, M. FV. 1967. An excitatory action of 5-hydroxytryptamine on sympathetic preganglionlc neurones. Exp . Brain Res, 3, 299-305.

DEGROA'F, W. C . , and RYALL. R. W. 1969. Reflexes to sacral parasympathetic neurones concerned with nnicturition in the cat. J. Physiol. 200, 87-108.

Eccr,~s, R. M., and LUNBBERG, A. 1959. Synaptic actions in motoneurones by afferents which may evoke the flexion reflex. Arch. Ital. Biol. 97, 199-221.

FFDINA, k., KATUKSKIH, A. Y . A.., KBIAYUTI~, V. M., and MITS~NYI, A. 1966. Responses of renal sym- pathetic nerves to stimulation of afferent A and C fibres of tibia1 and mesenterial nerves. Acta Physiol. Acad. Sci. Hung. 29. 157-176.

B;ERN,~NDEZ DE MOLINA, A., KUNO, M.. and PER^,, E. R . 1965. Antidromically evoked responses from sym- pathetic preganglionic neurones. J . Physiol. 188, 321-335.

HONCO, T., and RYALI,, R. W. 1966. EIectrophysiological and ms'croeBectrophoretic studies on sympathetic preganglionic neurones in the spinal cord. Acta Physiol. Scand. 58, 96-109.

HWA'MURA, Y.. UCHINO, PI., OZAWP,. S.. and KUDO. N. 1969. Excltatorj~ and inhibitory components of somalo-sympathetic reflex. Brain Kes. 16, 351--358.

JANIG, W., and SCHMIDT, R. F. 1970. Single unit responses in the cervical sympathetic trunk upon somatic nerve stimulation. Pfliigers Arch. 317. 199-216.

.JOIIAYSSON, B. 1962. Circulatory responses to stimulation of somatic afferents. Acta IBhysiol. Scand. 57, Suppl. 198.

KOIZURIH~ K., SAW, A., KAUFMAN, A,, and BROOKS, C. M. 1968. Studies of sympathetic neuron discharges modified by central and peripheral excitation. Brain Kes. 11, 212-224.

K W N J E V I ~ , K., and PHILLIS, J. W. 1963. Iontrophoretic studies of neurones In the mamma8ian cerebral cortex. J. Physiol. 165, 274-304.

KRNJEVI~ , K.. R A Y D I ~ , M., and !;TRAUC;UAN, D. \jr. 1966. An inhibitory process in the cerebral sortex. 3 . Physiol. 184, 1-8.

LAPORTI., Y. , BESSOU, la., and B o u r s s ~ ~ , S. 1960. Action rdflexc des differents types de fibres affkrentes d'ori- gine musculaire sur la pression sanguine. Arch. Ital. Biol. 98, 206-221.

Can

. J. P

hysi

ol. P

harm

acol

. Dow

nloa

ded

from

ww

w.n

rcre

sear

chpr

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38 CiiN. J. PHYSIOL. PHAIRIL%ACOL. VOL. 51, 1973

LAPORTF, Y., and MON BASTMUC, P. 195'7. R6le des diR6- rents types de fibres afT6rentes daa~s les reflexes circulatoires gknkrauw d'origine cutanee. J. Fhysiol. Paris, 49, 1039-16349.

LIEET, 5. 1967. Long larer~t periods and hsther analysis of slow synaptic responses in sympathetic ganglia. J . Weurophysiol. 90, 494-5 14.

MANNARD, A. @. 1970. Patterns of background Biring of singlc sympathetic preganglionlc units in the cat spinal cord. Ph.D. Thesis, McGilI University, Montreal: Quebec.

P o ~ o s a , C. 1966. The silent period of sympathetic pre- ganglionic neurones. P11.D. Thesis, McGill Universi- ty, Montreal, Quebec.

- 1967. Tlae silent period of sympathetic pre-

ganglionic neurons. Can. J. Physiol. Pharmacol. 45, 1033-1045.

. -- 1468. Spontaneous activity s f sympathetic

preganglionic neurons. Can. 5 . Physiol. Plaarrnacol. 96, 887-896.

ROSFNBERQ;, M. E. 1970. Synaptic connexions of alpha extensor motoneurones with ipsilateral and contra- lateral cutaneous nerves. J . Physiol. 207, 231-255.

SATO, A., and Scsr~ras~r. R. F. 1971. Spinal and supra- spinal compomcnts of the reflew discharges into lumbar and thoracic white rarnl. 9 . Physiol. 212, 839-850.

S A ' ~ O , A., TSBJSHIMA, N., and FWIMCIKI, B. 1965. Reflex potentials sf lumbar sympathetic trunk with sciatic nerve stimulation in cats. Jap. J. Physiol. 15, 5325 539.

S k u , R., ERDELIY, .As, and SCHAEF~R, H. 1958. Unter- sarch~lngen Eber den Einfl~lss peripherer Nerven- reizung auf die sympathische Aktivitat. Pfliigers Arch. 26'7, 566-581.

SKOGLUNI>, C. R. 1940. Vasomotor reflexes from muscle. Acta Physiol. Scand. 50, 31 1-327.

WYSZ~GK~DSKI , I. 1970. Properties of somato-sympa- thetic reflexes. M.Sc. Thesis, McGill University, Montreal. Q~iebec.

Can

. J. P

hysi

ol. P

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