©Dr. Anwar Siddiqui

PowerPoint® Seminar Slide Presentation prepared by Dr. Anwar Hasan Siddiqui, Senior Resident, Dep't of Physiology, JNMC,AMU,ALIGARH

Physiology Seminar11/08/2012

NEUROTRANSMITTER

INTRODUCTION• Neurotransmitters are endogenous chemicals that transmit

signals from a neuron to a target cell across a synapse

• Synapses are the junctions where neurons release a chemical neurotransmitter that acts on a postsynaptic target cell, which can be another neuron or a muscle or gland cell

• Some chemicals released by neurons have little or no direct effects on their own but can modify the effects of neurotransmitters. These chemicals are called neuromodulators.

Criteria That Define a Neurotransmitter

THE FATHER OF NEUROSCIENCE• Otto Loewi (German

pharmacologist)

• Discovered the chemical nature of neurotransmission (Acetylcholine) across synapse

• Loewi was awarded the Nobel Prize in Physiology (1936)

The Experiment :

Identified neurotransmitters and neuromodulators can be divided into two major categories:

SMALL-MOLECULE TRANSMITTERS Monoamines (eg, Acetylcholine, Serotonin, Histamine), Catecholamines (Dopamine, Norepinephrine Epinephrine)Amino Acids (eg, Glutamate, GABA, Glycine).

LARGE-MOLECULE TRANSMITTERS. Include a large number of peptides called neuropeptides including substance P, enkephalin, vasopressin, and a host of others.

There are also other substances thought to be released into the synaptic cleft to act as either a transmitter or modulator of synaptic transmission. These include purine derivatives like Adenosine, Adenosine Triphosphate (ATP) and Nitric Oxide (NO).

Neurotransmitter receptors

Two broad classes: LIGAND-GATED ION CHANNELS Open immediately upon neurotransmitter binding

G PROTEIN–COUPLED RECEPTORS.Neurotransmitter binding to a G protein–coupled receptor induces the opening or closing of a separate ion channel protein over a period of seconds to minutes. These are “slow” neurotransmitter receptors.

Each ligand has many subtypes of receptors : selective effect at different sitesPresynaptic receptors, or Autoreceptors : provide feedback control

• Receptors are concentrated in clusters in postsynaptic structures close to the endings of neurons that secrete the neurotransmitters specific for them. This is generally due to the presence of specific binding proteins for them.

In the case of nicotinic acetylcholine receptors at the neuromuscular junction, the protein is rapsyn

In the case of excitatory glutamatergic receptors, a family of PB2-binding proteins is involved.

GABAA receptors are associated with the protein gephyrin, which also binds glycine receptors, and

GABAC receptors are bound to the cytoskeleton in the retina by the protein MAP-1B.

DESENSITIZATION

Prolonged exposure to their ligands causes most receptors to become unresponsive. This can be of two types:

Homologous desensitization, with loss of responsiveness only to the particular ligand and maintained responsiveness of the cell to other ligands

Heterologous desensitization, in which the cell becomes unresponsive to other ligands as well.

Reuptake• From the synaptic cleft back into the cytoplasm of the neuron

The reuptake systems employ two families of transporter proteins:

Members include transporters for norepinephrine, dopamine, serotonin, GABA, and glycine, as well as transporters for proline, taurine, and the acetylcholine precursor choline. In addition, there may be an epinephrine transporter.

The other family is made up of at least three transporters that mediate glutamate uptake by neurons and two that transport glutamate into astrocytes.

• There are in addition two vesicular monoamine transporters, VMAT1 and VMAT2, that transport neurotransmitters from the cytoplasm to synaptic vesicles. They are coded by different genes but have extensive homology:

Both have a broad specificity, moving dopamine, norepinephrine, epinephrine, serotonin, and histamine from the cytoplasm into secretory granules.

Both are inhibited by reserpine, which accounts for the marked monoamine depletion produced by this drug.

Like the neurotransmitter membrane transporter family, they have 12 transmembrane domains, but they have little homology to the other transporters.

• There is also a vesicular GABA transporter (VGAT) that moves GABA and glycine into vesicles and a vesicular acetylcholine transporter.

Monoamine is synthesized in the cytoplasm and the secretory granules (1) and its concentration in secretory granules is maintained (2) by the two vesicular monoamine transporters (VMAT). The monoamine is secreted by exocytosis of the granules

(3), and it acts (4) on receptors (Y-shaped structures labeled R). Many of these receptors are postsynaptic, but some are presynaptic and some are located on glia. In addition, there is extensive reuptake into the cytoplasm of the presynaptic terminal (5) via the monoamine neurotransmitter transporter (NTT) for the monoamine that is synthesized in the neuron.

Reuptake is a major factor in terminating the action of transmitters, when inhibited, the effects of transmitter release are increased and prolonged. This has clinical consequences.

Several effective antidepressant drugs are inhibitors of the reuptake of amine transmitters.

Cocaine is believed to inhibit dopamine reuptake.

Glutamate uptake into neurons and glia is important because glutamate is an excitotoxin that can kill cells by overstimulating them. There is evidence that during ischemia and anoxia, loss of neurons is increased because glutamate reuptake is inhibited.

Acetylcholine

Acetylcholine, which is the acetyl ester of choline, is largely enclosed in small, clear synaptic vesicles in high concentration in the terminal boutons of cholinergic neurons

• Acetylcholine is the transmitter at the neuromuscular junction, in autonomic ganglia, and in postganglionic parasympathetic nerve-target organ junctions and some postganglionic sympathetic nerve-target junctions. It is also found within the brain, including the basal forebrain complex and pontomesencephalic cholinergic complex . These systems may be involved in regulation of sleep-wake states, learning, and memory.

• Cholinergic neurons actively take up choline via a transporter. Choline is also synthesized in neurons.

• The enzyme choline acetyltransferase is found in high concentration in the cytoplasm of cholinergic nerve endings. Acetylcholine is then taken up into synaptic vesicles by a vesicular transporter (VAChT).

• Removed via Hydrolysis to choline and acetate, a reaction catalyzed by the enzyme ACETYLCHOLINESTERASE.

Acetylcholine Receptors

MuscarinicNicotinic

Muscarinic receptors• Muscarine, the alkaloid responsible for the toxicity of toadstools, has

little effect on the receptors in autonomic ganglia but mimics the stimulatory action of acetylcholine on smooth muscle and glands.

• These actions of acetylcholine are therefore called muscarinic actions, and the receptors involved are muscarinic cholinergic receptors.

• They are blocked by the drug atropine.• Five types, encoded by five separate genes, have been cloned. • The exact status of M5 is uncertain, but the remaining four receptors are

coupled via G proteins to adenylyl cyclase, K+ channels, and/or phospholipase C .

• M1 is abundant in the brain.

• The M2 receptor is found in the heart.

• The M4 receptor is found in pancreatic acinar and islet tissue.

• The M3 and M4 receptors are associated with smooth muscle.

Nicotinic receptors• In Sympathetic Ganglia, the actions of Ach are unaffected by

atropine but MIMICKED BY NICOTINE. Consequently, these actions of Ach are nicotinic actions and the receptors are nicotinic cholinergic receptors.

• Nicotinic receptors are subdivided into those at neuromuscular junctions and those found in autonomic ganglia and the central nervous system

• Both muscarinic and nicotinic acetylcholine receptors are found in large numbers in the brain.

• The nicotinic acetylcholine receptors are members of a superfamily of ligand-gated ion channels

• Each nicotinic cholinergic receptor is made up of five subunits that form a central channel which, when the receptor is activated, permits the passage of Na+ and other cations. A prominent feature of neuronal nicotinic cholinergic receptors is their high permeability to Ca2+.

• The 5 subunits come from a menu of 16 known subunits, α1–α9, β1–β5, γ , δ

and ε , coded by 16 different genes.

• THE MUSCLE TYPE NICOTINIC RECEPTOR found in the fetus is made up of two α1 subunits, a β1 subunit, a γ subunit, and a δ subunit . In adult,the γ subunit is replaced by a δ subunit, which decreases the channel open time but increases its conductance.

• The nicotinic cholinergic RECEPTORS IN AUTONOMIC GANGLIA usually contain α3 subunits in combination with others.

Many of the nicotinic cholinergic receptors in the brain are located presynaptically on glutamate-secreting axon terminals, and they facilitate the release of this transmitter. However, others are postsynaptic. Some are located on structures other than neurons, and some seem to be free in the interstitial fluid, that is, they are perisynaptic in location.

Serotonin• Serotonin is formed in the

body by hydroxylation and decarboxylation of the essential amino acid TRYPTOPHAN

• Tryptophan hydroxylase in the human CNS is slightly different from the tryptophan hydroxylase in peripheral tissues, and is coded by a different gene.

SEROTONIN (5-HYDROXYTRYPTAMINE; 5-HT) is present in highest concentration in blood platelets and in the gastrointestinal tract, where it is found in the enterochromaffin cells and the myenteric plexus.

It is also found within the brain stem in the midline raphé nuclei which project to portions of the hypothalamus, the limbic system, the neocortex, the cerebellum, and the spinal cord There is evidence for a relationship between

behavior and brain serotonin content.

• After release from serotonergic neurons, much of the released serotonin is recaptured by an active reuptake mechanism and inactivated by MONOAMINE OXIDASE (MAO) to form 5-hydroxyindoleacetic acid (5-HIAA).

• This substance is the principal urinary metabolite of serotonin, and its urinary output is used as an index of the rate of serotonin metabolism in the body

Serotonergic Receptors• 5-HT1 - 5-HT7 receptors• Most of these are G protein-coupled receptors • 5-HT1 => 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, & 5-HT1F

• 5-HT2 => 5-HT2A, 5-HT2B, & 5-HT2C

• 5-HT2A receptors mediate platelet aggregation and smooth muscle contraction.

• 5-HT3 receptors are ligand-gated ion channels present in the GIT & the area postrema & are related to vomiting.

• 5-HT4 receptors are also present in the GIT, where they facilitate secretion and peristalsis, & in the brain.

• 5-HT5 => 5-HT5A & 5-HT5B

• 5-HT6 & 5-HT7 are distributed throughout the limbic system, and the 5-HT6 receptors have a high affinity for antidepressant drugs.

Histamine• Histamine is formed by decarboxylation of the amino acid

histidine .

• Histaminergic neurons have their cell bodies in the tuberomammillary nucleus of the posterior hypothalamus, and their axons project to all parts of the brain, including the cerebral cortex and the spinal cord.

• Histamine is also found in cells in the gastric mucosa and in heparin-containing cells called mast cells that are plentiful in the anterior and posterior lobes of the pituitary gland as well as at body surfaces.

• The three known types of histamine receptors— H1, H2, and H3—are all found in both peripheral tissues and the brain.

• Mostof the H3 receptors are presynaptic, and they mediate inhibition of the release of histamine and other transmitters via a G protein. H1 receptors activate phospholipase C, and H2 receptors increase the intracellular cAMP concentration.

• Evidence links brain histamine to arousal, sexual behavior, blood pressure, drinking, pain thresholds, and regulation of the secretion of several anterior pituitary hormones.

Catecholamines• Norepinephrine, Epinephrine, & Dopamine• The chemical transmitter present at most sympathetic

postganglionic endings is norepinephrine. It is stored in the synaptic knobs of the neurons that secrete it in characteristic small vesicles that have a dense core.

• NOREPINEPHRINE and its methyl derivative, EPINEPHRINE, are secreted by the adrenal medulla

• Tyrosine hydroxylase, which catalyzes the RATE LIMITING step, is subject to feedback inhibition by dopamine and norepinephrine, thus providing internal control of the synthetic process.

• The cell bodies of the norepinephrine-containing neurons are located in the locus ceruleus and other medullary and pontine nuclei .

RATE LIM STEP

Catabolism of Catecholamines• Removed from the synaptic cleft by binding to postsynaptic receptors,

binding to presynaptic receptors , reuptake into the presynaptic neurons, or catabolism. Reuptake is a major mechanism in the case of norepinephrine.

• Epinephrine and norepinephrine are metabolized to biologically inactive products by oxidation and methylation. The former reaction is catalyzed by MAO and the latter by catechol -O –methyltransferase (COMT).

• EXTRACELLULAR epinephrine and norepinephrine are for the most part O-methylated, and measurement of the concentrations of the O-methylated derivatives normetanephrine and metanephrine in the urine is a good index of the rate of secretion of norepinephrine and epinephrine.

• The O-methylated derivatives that are not excreted are largely oxidized, and 3-methoxy-4-hydroxymandelic acid (vanillylmandelic acid, VMA) is the most plentiful catecholamine metabolite in the urine. Small amounts of the O-methylated derivatives are also conjugated to sulfate and glucuronide.

• In the NORADRENERGIC NERVE TERMINALS, on the other hand, some of the norepinephrine is constantly being converted by intracellular MAO to the physiologically inactive deaminated derivatives, 3,4-dihydroxymandelic acid (DOMA) and its corresponding glycol (DHPG). These are subsequently converted to their corresponding O-methyl derivatives, VMA and 3-methoxy-4-hydroxyphenylglycol (MHPG).

α & β Receptors• Epinephrine and norepinephrine both act on and receptors,

with norepinephrine having a greater affinity for α-adrenergic receptors and epinephrine for β-adrenergic receptors.

• G protein-coupled receptors, and each has multiple forms

Dopamine• In certain parts of the brain, catecholamine synthesis stops at dopamine• Active reuptake of dopamine occurs via a Na+- and Cl–-dependent

dopamine transporter. • Dopamine is metabolized to inactive compounds by MAO and COMT in

a manner analogous to the inactivation of norepinephrine• Dopaminergic neurons are located in several brain regions including the

nigrostriatal system, which projects from the substantia nigra to the striatum and is involved in motor control, and the mesocortical system.

• The mesocortical system projects to the nucleus accumbens and limbic subcortical areas, and it is involved in reward behavior and addiction.

• Studies by PET scanning in normal humans show that a steady loss of dopamine receptors occurs in the basal ganglia with age. The loss is greater in men than in women.

Dopamine Receptors• Five different dopamine receptors have been cloned, and several of

these exist in multiple forms.• Most, but perhaps not all, of the responses to these receptors are

mediated by heterotrimeric G proteins.• Overstimulation of D2 receptors is thought to be related to

schizophrenia. • D3 receptors are highly localized, especially to the nucleus accumbens

Glutamate • The amino acid glutamate is the main excitatory transmitter in the brain

and spinal cord( 75% of the excitatory transmission in the brain. )• Glutamate is formed by reductive amination of the Krebs cycle

intermediate α-ketoglutarate in the cytoplasm. • The reaction is reversible, but in glutaminergic neurons, glutamate is

concentrated in synaptic vesicles by the vesicle-bound transporter BPN1. • The cytoplasmic store of glutamine is enriched by three transporters

that import glutamate from the interstitial fluid, and two additional transporters carry glutamate into astrocytes, where it is converted to glutamine and passed on to glutaminergic neurons.

• Released glutamate is taken up by astrocytes and converted to glutamine, which passes back to the neurons and is converted back to glutamate, which is released as the synaptic transmitter.

• Uptake into neurons and astrocytes is the main mechanism for removal of glutamate from synapses

The glutamate–glutamine cycle through glutaminergic neurons and astrocytes.

Glutamate ReceptorsTwo types: METABOTROPIC RECEPTORS IONOTROPIC RECEPTORS

THE METABOTROPIC RECEPTORS • G protein-coupled receptors that increase intracellular IP3 and DAG

levels or decrease intracellular cAMP levels. • Eleven subtypes• Presynaptic & postsynaptic, and widely distributed in the brain. • They appear to be involved in the production of synaptic plasticity,

particularly in the hippocampus and the cerebellum. • Knockout of the gene for one of these receptors, one of the forms of

mGluR1, causes severe motor incoordination and deficits in spatial learning.

THE IONOTROPIC RECEPTORS

• Ligand-gated ion channels.• There are three general types, each named for the congeners of

glutamate to which they respond in maximum fashion. Kainate receptors (kainate is an acid isolated from seaweed)Simple ion channels that, when open, permit Na+ influx and K+ efflux4 AMPA subunits have been identified

AMPA receptors ( amino-3-hydroxy-5-methylisoxazole-4-propionate)

Two populations : one is a simple Na+ channel and one also passes Ca2+.5 kainate subunits have been identified

NMDA receptors (N-methyl-D-aspartate).

NMDA receptors• A cation channel: permits passage of relatively large amounts of

Ca2+

• Glycine facilitates its function by binding to it, & appears to be essential for its normal response to glutamate.

• When glutamate binds to it, it opens, but at normal membrane potentials, its channel is blocked by a Mg2+ ion.

• Phencyclidine and ketamine, which produce amnesia and a feeling of dissociation from the environment, bind to another site inside the channel. Most target neurons for glutamate have both AMPA and NMDA receptors.

• Kainate receptors are located presynaptically on GABA-secreting nerve endings and postsynaptically at various localized sites in the brain. Kainate and AMPA receptors are found in glia as well as neurons, but it appears that NMDA receptors occur only in neurons

• The concentration of NMDA receptors in the hippocampus is high, and blockade of these receptors prevents long-term potentiation, a long-lasting facilitation of transmission in neural pathways following a brief period of high-frequency stimulation. Thus, these receptors may well be involved in MEMORY AND LEARNING.

NMDA receptor

GABA• Major inhibitory mediator in the brain, including being

responsible for presynaptic inhibition.

• Formed by decarboxylation of glutamate . The enzyme glutamate decarboxylase (GAD), is present in nerve endings in many parts of the brain.

• Metabolized primarily by transamination to succinic semialdehyde and thence to succinate in the citric acid cycle. GABA transaminase (GABA-T) catalyzes the transamination.

• In addition, there is an active reuptake of GABA via the GABA transporter. A vesicular GABA transporter (VGAT) transports GABA and glycine into secretory vesicles

GABA Receptors• Three subtypes of GABA receptors have been identified: GABAA,

GABAB, and GABAC

• The GABAA and GABAB receptors are widely distributed in the CNS, whereas in adult vertebrates the GABAC receptors are found almost exclusively in the retina.

• The GABAA and GABAC receptors are ion channels made up of five subunits surrounding a pore . In this case, the ion is Cl– .

• The GABAB receptors are metabotropic ,coupled to heterotrimeric G proteins that increase conductance in K+ channels, inhibit adenylyl cyclase, and inhibit Ca2+ influx.

• Increases in Cl– influx and K+ efflux and decreases in Ca2+ influx all hyperpolarize neurons, producing an IPSP. The G protein mediation of GABAB receptor effects is unique in that a G protein heterodimer, rather than a single protein, is involved.

• There is a chronic low-level stimulation of GABAA receptors in the CNS that is aided by GABA in the interstitial fluid. This background stimulation cuts down on the "noise" caused by incidental discharge of the billions of neural units and greatly IMPROVES THE SIGNAL-TO-NOISE RATIO in the brain. It may be that this GABA discharge declines with advancing age, resulting in a loss of specificity of responses of visual neurons.

• The increase in Cl– conductance produced by GABAA receptors is potentiated by benzodiazepines, drugs that have marked anti-anxiety activity and are also effective muscle relaxants, anticonvulsants, and sedatives. Benzodiazepines bind to the α subunits.

• At least in part, barbiturates and alcohol also act by facilitating Cl– conductance.

• Metabolites of the steroid hormones progesterone and deoxycorticosterone bind to GABAA receptors and increase Cl– conductance.

• It has been known for many years that progesterone and deoxycorticosterone are sleep-inducing and anesthetic in large doses, and these effects are due to their action on GABAA receptors.

Glycine

• Glycine has both excitatory and inhibitory effects in the CNS. • When it binds to NMDA receptors, it makes them more sensitive.It

appears to spill over from synaptic junctions into the interstitial fluid, and in the spinal cord, for example, this glycine may facilitate pain transmission by NMDA receptors in the dorsal horn.

• Glycine is also responsible in part for direct inhibition, primarily in the brain stem and spinal cord. Like GABA, it acts by increasing Cl– conductance. Its action is antagonized by strychnine.

• The clinical picture of convulsions and muscular hyperactivity produced by strychnine emphasizes the importance of postsynaptic inhibition in normal neural function.

RECEPTOR• The glycine receptor responsible for inhibition is a Cl– channel. • It is a pentamer made up of two subunits: The ligand-binding α subunit The structural β subunit.

• Recently, solid evidence has been presented that three kinds of neurons are responsible for direct inhibition in the spinal cord:

neurons that secrete glycine, neurons that secrete GABA, and neurons that secrete both. Presumably, neurons that secrete only glycine have the glycine transporter GLYT2, those that secrete only GABA have GAD, and those that secrete glycine and GABA have both. This third type of neuron is of special interest because the neurons seem to have glycine and GABA in the same vesicles.

Anesthesia• Alcohols, barbiturates, and many volatile inhaled anesthetics

as well act on ion channel receptors and specifically on GABAA and glycine receptors to increase Cl– conductance. Regional variation in anesthetic actions in the CNS seems to parallel the variation in subtypes of GABAA receptors.

• Other inhaled anesthetics do not act by increasing GABA receptor activity, but appear to act by inhibiting NMDA and AMPA receptors instead.

• Local anesthetics produce anesthesia by blocking conduction in peripheral nerves via reversibly binding to and inactivating Na+ channels. When depolarization and propagation are interrupted, the individual loses sensation in the area supplied by the nerve.

Large-Molecule Transmitters: Neuropeptides• Substance P & Other Tachykinins: • Substance P is a polypeptide containing 11 amino acid residues that is

found in the intestine, various peripheral nerves, and many parts of the CNS.

• It is one of a family of 6 mammalian polypeptides called tachykinins that differ at the amino terminal end but have in common the carboxyl terminal sequence.

• Substance P is found in high concentration in the endings of primary afferent neurons in the spinal cord, and it is probably the mediator at the first synapse in the pathways for pain transmission in the dorsal horn.

• It is also found in high concentrations in the nigrostriatal system, where its concentration is proportional to that of dopamine, and in the hypothalamus, where it may play a role in neuroendocrine regulation

• In the intestine, it is involved in peristalsis.

Opioid PeptidesPeptides that bind to opioid receptors are called opioid peptides. The ENKEPHALINS are found in nerve endings in the gastrointestinal tract and many different parts of the brain, and they appear to function as synaptic transmitters. They are found in the substantia gelatinosa and have analgesic activity when injected into the brain stem. They also decrease intestinal motility.

METABOLISMEnkephalins are metabolized primarily by two peptidases• Enkephalinase A, which splits the Gly-Phe bond, and • Enkephalinase B, which splits the Gly-Gly bond. • Aminopeptidase, which splits the Tyr-Gly bond, also contributes to

their metabolism.

RECEPTORS

• µ , κ , δ• All three are G protein-coupled receptors, and all inhibit adenylyl

cyclase.• Activation of µ receptors increases K+ conductance, hyperpolarizing

central neurons and primary afferents. Activation of κ and δ receptors closes Ca2+ channels.

Physiologic effects of opiate receptor stimulation

Receptor affinity

Other PolypeptidesSOMATOSTATIN is found in various parts of the brain, where it apparently functions as a neurotransmitter with effects on sensory input, locomotor activity, and cognitive function. • In the endocrine pancreas, it inhibits insulin secretion and the

secretion of other pancreatic hormones• In the gastrointestinal tract, it is an important inhibitory

gastrointestinal regulator.

A family of five different SOMATOSTATIN RECEPTORS have been identified (SSTR1 through SSTR5). • All are G protein-coupled receptors. They inhibit adenylyl cyclase

and exert various other effects on intracellular messenger systems.• It appears that SSTR2 mediates cognitive effects and inhibition of

growth hormone secretion, whereas SSTR5 mediates the inhibition of insulin secretion.

• VASOPRESSIN & OXYTOCIN are not only secreted as hormones but also are present in neurons that project to the brain stem and spinal cord

• The brain contains BRADYKININ, ANGIOTENSIN II & ENDOTHELIN

• The gastrointestinal hormones VIP, CCK-4, and CCK-8 are also found in the brain. There are two kinds of CCK receptors in the brain, CCK-A and CCK-B.

• GASTRIN, NEUROTENSIN, GALANIN, AND GASTRIN-RELEASING PEPTIDE are also found in the gastrointestinal tract and brain

CALCITONIN GENE-RELATED PEPTIDE (CGRP)

• Is a polypeptide that exists in two forms : CGRPα and CGRPβ

• CGRP is present in the gastrointestinal tract, found in primary afferent neurons, neurons that project impulses to the thalamus, and neurons in the medial forebrain bundle. It is also present along with substance P in the branches of primary afferent neurons that end near blood vessels.

• Its injection causes vasodilation.

• CGRPα and the calcium-lowering hormone calcitonin are both products of the calcitonin gene.

NEUROPEPTIDE Y

• Is a polypeptide containing 36 amino acid residues that acts on at least two of the 4 known G protein-coupled receptors: Y1, Y2, Y4, and Y5.

• Neuropeptide Y is found throughout the brain and the autonomic nervous system.

• When injected into the hypothalamus, this polypeptide increases food intake, and inhibitors of neuropeptide Y synthesis decrease food intake.

• Neuropeptide Y-containing neurons have their cell bodies in the arcuate nuclei and project to the paraventricular nuclei

Other Chemical Transmitters

PURINE & PYRIMIDINE TRANSMITTERS• ATP, URIDINE, ADENOSINE, AND ADENOSINE METABOLITES

are neurotransmitters or neuromodulators.

ADENOSINE is a neuromodulator that acts as a general CNS depressant and has additional widespread effects throughout the body. • 4 receptors: A1, A2A, A2B, and A3. G protein-coupled & increase

(A2A and A2B) or decrease (A1 and A3) cAMP concentrations. • The stimulatory effects of coffee and tea are due to blockade of

adenosine receptors by caffeine and theophylline.

ATP• ATP has now been shown to mediate rapid synaptic responses in the

autonomic nervous system and a fast response in the habenula. • ATP binds to P2X receptors which are ligand-gated ion channel

receptors; seven subtypes (P2X1–P2X7) have been identified. • P2X receptors have widespread distributions throughout the body;

for example, P2X1 and P2X2 receptors are present in the dorsal horn, indicating a role for ATP in sensory transmission.

• ATP also binds to P2Y receptors which are G protein-coupled receptors. There are eight subtypes of P2Y receptors: P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14

• It appears that soluble nucleotidases are released with ATP, and these accelerate its removal after it has produced its effects.

CANNABINOIDS• Two receptors with a high affinity for tetrahydrocannabinol (THC),

the psychoactive ingredient in marijuana, have been cloned. • The CB1 receptor triggers a G protein-mediated decrease in

intracellular cAMP levels and is common in central pain pathways as well as in parts of the cerebellum, hippocampus, and cerebral cortex.

• The endogenous ligand for the receptor is ANANDAMIDE, a derivative of arachidonic acid. This compound mimics the euphoria, calmness, dream states, drowsiness, and analgesia produced by marijuana.

• There are also CB1 receptors in peripheral tissues, and blockade of these receptors reduces the vasodilator effect of anandamide.

• A CB2 receptor has also been cloned, and its endogenous ligand may be palmitoylethanolamide (PEA). However, the physiologic role of this compound is unsettled.

Gases

NITRIC OXIDE (NO)

• The compound released by the endothelium of blood vessels as EDRF, is also produced in the brain.

• It is synthesized from arginine, a reaction catalyzed in the brain by one of the three forms of NO synthase.

• NO synthase requires NADPH• It activates guanylyl cyclase and, unlike other transmitters, it is a

gas, which crosses cell membranes with ease and binds directly to guanylyl cyclase.

• It may be the signal by which postsynaptic neurons communicate with presynaptic endings in long-term potentiation and long-term depression.

Other SubstancesPROSTAGLANDINS • Are derivatives of arachidonic acid found in the nervous system, present in

nerve-ending fractions of brain homogenates and are released from neural tissue in vitro. A putative prostaglandin transporter with 12 membrane-spanning domains has been described.

• However, prostaglandins appear to exert their effects by modulating reactions mediated by cAMP rather than by functioning as synaptic transmitters.

NEUROACTIVE STEROIDS• They are not neurotransmitters in the usual sense. • Evidence has now accumulated that the brain can produce some

hormonally active steroids from simpler steroid precursors, and the term neurosteroids has been coined to refer to these products. Progesterone facilitates the formation of myelin, but the exact role of most steroids in the regulation of brain function remains to be determined.

Thank you…