ACTIONS OF DRUGS ON SINGLE NEURONES IN THE CEREBRAL CORTEX

From cAMP to adenosine: an illuminating shift of focus

In a remarkable career, straddling five decades, John Phillis pursued with fierce determination and exceptional energy the main goal of his scientific life, to throw light on the chemical agents that control brain function. Starting in Australia, he settled in North America, first in Canada, then in the USA, where his long tenure at Wayne State brought his career to its culmination.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

Regional differences in the distribution of muscarinic cholinergic receptors in the macaque cerebral cortex

The in vitro autoradiographic technique was used to characterize the density and laminar distribution of muscarinic cholinergic receptors in 12 cytoarchitectonic areas in the frontal, parietal, and occipital lobes of the rhesus monkey. The entire population of muscarinic receptors was labeled with [3H]quinuclidinyl-benzilate; the M1 receptor subtype was labeled with [3H]pirenzepine; and the density of the M2 receptor subtype was estimated by subtracting the density of M1 receptors from the total population. The overall density of M1 and M2 receptor subtypes was similar throughout the cerebral cortex. However, their laminar distribution varied regionally. In cortical regions of the parietal and occipital lobes and in the primary motor cortex of the frontal lobe, both M1 and M2 receptor subtypes were concentrated in the supragranular layers. By contrast, in prefrontal cortical areas, the combined population of M1 and M2 receptors was evenly distributed across the cortical layers, though M1 receptors were most dense and M2 receptors least dense in layer IV. The difference in the distribution of cholinergic receptors in the prefrontal cortex compared to other neocortical areas reveals a degree of chemoarchitectural specificity of this region with respect to cholinergic markers that has escaped immunohistochemical and other anatomical and functional techniques.

Pharmacological profile of nicotinic acetylcholine receptors in the rat prefrontal cortex: an electrophysiological study in a slice preparation

The specificity of nicotinic receptors in the neocortex has been questioned previously because: (i) electrophysiological responses to nicotine could not be blocked by nicotinic antagonists, and (ii) the effect of nicotine was not mimicked by acetylcholine. In the present study, the presence of functional nicotinic receptors in rat neocortex has been assessed in a slice preparation of prefrontal cortex, using evoked field potential and unit recordings. Nicotine and the nicotinic agonists, dimethylphenylpiperazinium, cytisine, acetylthiocholine, applied by iontophoresis, produced an increase in the negative wave of field potentials, reflecting an increased excitability of cortical neurons. This effect was blocked by the selective probe for neuronal nicotinic receptors Toxin F (1.4 microM in the perfusion medium) and by dihydro-beta-erythroidine (100 microM). Alpha-bungarotoxin, the blocker of skeletal muscle acetylcholine receptor had no effect. Iontophoretically applied acetylcholine, muscarine and pilocarpine, on the other hand, produced a decrease in the field potential amplitude, which was blocked by atropine and scopolamine (1-10 microM). In the presence of eserine (10 microM), the muscarinic effect of acetylcholine was dramatically altered, leading to the development of a nicotinic response sensitive to Toxin F. Thus, the physiological activation of nicotinic receptors in rat prefrontal cortex appears to require higher concentrations of acetylcholine than do muscarinic receptors. Our results show that: (i) the rat prefrontal cortex possesses functional nicotinic receptors with a pharmacological profile clearly distinct from muscle receptors, and (ii) a nicotinic effect of acetylcholine can be revealed when its degradation by acetylcholinesterase is inhibited.

Effect of zinc on neuronal activity in the rat forebrain

Zinc ions, which are unevenly distributed in the CNS and can be released from nerve terminals, have been implicated as causative agents in epileptogenesis. The present study has shown that intraventricular administration to anesthetized rats causes seizure activity of the ECOG and convulsions. Since the manner in which zinc influences neuronal activity and triggers convulsions is unclear, studies were also made of its effect on spontaneous and evoked activity in the rat forebrain. It was found that iontophoretic application of zinc to cortical neurons causes slow and often prolonged increases in firing rate, usually accompanied by bursts of high frequency discharge in just under half the studies. Another cation, barium, evoked excitatory responses of a similar type and a reduction in potassium permeability may underlie the effects of both cations. In contrast, calcium, magnesium, manganese and cerium caused short duration depressant effects. The depression induced by calcium, but not by the other cations, could be blocked by zinc. Similarly, in the hippocampus zinc depressed calcium-dependent potentiation in subfield CA3 evoked by paired-pulse stimulation of mossy fibers; excitatory effects (namely an increase in spike amplitude and appearance of multiple population spikes) were seen at higher zinc concentrations. The depressant effects of an enkephalin analog on cortical firing rate were also blocked by zinc, consistent with studies from another laboratory suggesting enkephalin/zinc interactions. In contrast, the depressant effect of GABA could not be blocked by zinc, although an antagonism has been reported in the lobster muscle. Firm conclusions regarding the mechanism(s) underlying the triggering of seizure activity by zinc cannot yet be drawn, but the results of these studies would be consistent with an interference with calcium and/or potassium ion activity rather than with GABA binding sites.

The gamma-aminobutyric acid-mimetic action of omega-aminocaprylic acid in rat cortex

The 8-carbon chain analogue of gamma-aminobutyric acid (GABA), omega-aminocaprylic acid (omega-AC), was administered to single cortical neurones of rat by microiontophoresis and its effect on cell firing compared to the effect elicited by GABA. The spontaneous and glutamate-elicited firing rates of all cells examined were decreased by omega-AC, the molecule having an apparent potency approximately one-fifth that of GABA. Inhibitions induced by omega-AC were antagonized by bicuculline and enhanced by flurazepam. These data favour the view that extending chain length per se of the GABA molecule decreases the inhibitory potency of postsynaptic GABA receptor agonists. The results do not support the view that such structural changes alter the mode of action of GABA analogues, from inhibition to excitation, as has been suggested previously.

Pressor response to electrical and chemical stimulation of nucleus raphe dorsalis in the cat

The dorsal raphe nucleus was stimulated electrically and chemically in 57 cats with blood pressure being measured by an arterial catheter. Blood pressure rose by a mean of 119.5 +/- 8.9% on stimulation of the dorsal raphe nucleus at a frequency of 200 sec-1. This pressor response was frequency-dependent over the range 0.2 to 200 sec-1 and the site of origin was localized to the dorsal raphe nucleus. The pressor response could also be obtained by the injection of D,L-homocysteic acid into the nucleus, and is therefore likely to be due to activation of cell bodies rather than axons of passage. The response was blocked by high spinal cord section but was not affected by bilateral clamping of the adrenal glands or supracollicular decerebration. The pressor response was significantly reduced by pretreatment of the cats with parachlorophenylalanine (PCPA) and abolished by the alpha receptor blocker phentolamine, suggesting that both serotonergic and noradrenergic synapses are involved.

Zinc: effect and interaction with other cations in the cortex of the rat

Zinc and other cations were iontophoretically applied to neurones in the frontoparietal cortex of urethane-anaesthetised rats and their effect on the activity of these cells was examined. Slow and often prolonged increases in firing rate occurred in 45.2% of neurones after zinc application and 72.7% after barium. Calcium caused short duration decreases in firing rate in 89.3% of neurones as did manganese (78.6%) cerium (21.9%) and magnesium (33.3%). When applied simultaneously zinc blocked the depressant effect of calcium (in 79.3% of neurones) but not that of manganese or GABA; other ions were ineffective. These results suggest that zinc, which is known to be present in nerve terminals, may modulate neuronal activity.

Immunocytochemical characterization of cloned cells from normal adult rat brain

A cell clone, isolated from normal adult rat brain and maintained in culture for many passages, has been previously characterized by electron microscopy. Immunohistochemistry has now been shown that it possesses glutamine synthetase (GS) which supports its astrocytic origin. Cells failed, however, to express glial fibrillary acidic protein (GFAP) but did express the intermediate filament protein vimentin. The presence of fibronectin confirms the normal derivation of these cells and excludes the possibility of malignant transformation.

Neurochemical correlates of the kindling model of epilepsy

Repeated electrical stimulation of the brain can produce many epileptogenic effects including those which characterize the kindling model. Kindling stimulation, by definition, changes the brain is such a way that formerly subconvulsive stimuli can elicit electrographic and convulsive seizure activity. In addition, the kindled animal becomes more susceptible to many, but not all, other types of seizures. These facts suggest that kindling produces brain changes which may selectively model some types of epileptiform excitability. In order to understand the basis for such changes numerous neurochemical studies have been attempted in the last few years. Although many changes have been demonstrated to be produced by kindling, few studies have been designed to specifically examine the long-lasting (permanent) neurochemical correlates of kindling stimulation. In this review, neurochemical data relevant to kindling are presented and discussed in terms of their possible significance to the seizure susceptibility changes produced by kindling.

The role of glutamine synthetase in the diagnosis of cerebral tumours

Glutamine synthetase (GS) was studied in a range of human brain tumours and compared with results obtained by staining for glial fibrillary acidic protein (GFAP) using the indirect immunoperoxidase and peroxidase-anti-peroxidase methods. All astrocytomas showed positive staining with GS, the degree of which was related to the extent of differentiation and the amount of cytoplasm of the constituent cells. Ependymomas were only weakly positive, although a dense GFAP reaction was detected around blood vessels; possibly due to the presence of astrocyte foot processes. Medulloblastomas showed signs of astrocytic differentiation by the presence of clusters of positively staining cells amongst undifferentiated neoplastic cells. Oligodendrocytomas and meningiomas were generally negative apart from occasional astrocytes within the former neoplasms. GS and GFAP staining corresponded in virtually all cases; however, GS demonstrated poorly-fibrillated cells better than GFAP, but did not show cell processes so clearly. Although GS appears to be cell-specific in the central nervous system for normal, reactive and neoplastic astrocytes, it is not organ-specific and therefore also stains cells of other tissues, e.g. liver. GS offers a practical alternative to GFAP as an astrocyte specific marker in human cerebral tumours and may be of diagnostic value in cases where poorly-differentiated astrocytic tumours have been found negative to conventional histological stains for glial fibres or demonstrate a weak positive reaction to GFAP.

Tryptamine: a neuromodulator or neurotransmitter in mammalian brain?

Tryptamine synthesized by decarboxylation of L-tryptophan occurs as an endogenous constituent of mammalian brain albeit at very low concentrations (low ng/g range). It is primarily metabolized by oxidative deamination by MAO and possesses an extremely rapid turnover and half-life. Subcellular localization appears to be in nerve terminals and it is releasable by electrical or potassium evoked depolarization. Neuropharmacological and electrophysiological data strongly suggest the existence of post-synaptic receptors for tryptamine independent of those for 5HT. There may exist a rostrally projecting neuronal tryptamine containing system arising from cell bodies in or close to the nucleus raphé medianus. The demonstration of specific receptors for tryptamine in the CNS strongly indicates a transmitter role, although a strong case can be made for a role as a modifier of central 5HT systems. The possibility also exists that 5HT and tryptamine may be mediators of functionally opposite neuronal pathways. Whatever the role of tryptamine in the CNS it is clear that it not simply present as an accident of metabolism or a "biological artefact." The indications are that it possesses important functions in central neurotransmission.

Effects of prostaglandin F2alpha on cerebral cortical evoked potentials

The cerebral cortical action of prostaglandin F2alpha (PGF2alpha) has been determined by recording the effects of intracarotid injections of PGF2alpha on cerebral evoked potentials. PGF2alpha differentially reduced cortical evoked potentials. The cortical action of PGF2alpha appeared to be qualitatively identical with that of norepinephrine (NE) but weaker. A protection of the cortex from the inhibitory action of NE by a preceding dose of PGF2alpha was demonstrated. The actions of both PGF2alpha and NE appear to be on the same or related postsynaptic receptors. The actions described were at doses that did not reduced oxygen availability. PGF2alpha may act as a modulator of adrenergic transmission in the cortex. The intracellular recording in the companion paper supplies the further critical evidence that PGF2alpha has a synaptic inhibitory action.

The effect of phospholipases and proteases on the binding of gamma-aminobutyric acid to junctional complexes of rat cerebellum

A preparation enriched in junctional complexes, as judged by marker enzymes and electron microscopy, was prepared from rat cerebellum. The junctional complexes were incubated with gamma-amino [14C]butyric acid at 25degreesC for 10 min, using [3H]sucrose as a marker for entrapped space, Total binding was determined in the absence of, and non-specific binding in the presence of, and excess of unlabelled gamma-aminobutyric acid. The difference bewteen the two binding values, i.e. the specific binding, was saturable and reversible, and showed positive cooperativity with a Hill number of about 2. The specific binding was inhibited by N-methylbicuculline, picrotoxinine and imidazole-4-acetic acid, but not by curare, strychnine or L-2,4-diaminobutyric acid. The above compounds had little effect on the non-specipic binding, but addition of ethylenediaminetetraacetic acid decreased non-specific binding by 80%. Trypsin, pronase, phospholipase A2 (EC 3.1.1.4), lysolecithin and sodium dodecyl sulfate decreased binding. Phospholipase C (EC 3.1.4.3) increased the specific binding by 260%. Phospholipids competed with gamma-aminobutyric acid for binding, with phosphatidylethanolamine being more potent than phosphatidylcholine. These results lend support for Watkins' hypothesis that phosphatidylethanolamine competes with gamma-aminobutyric acid for binding to the receptor protein.

Effects of glutamic acid relatives on the electrical activity of an identified molluscan giant neurone (Achatina fulica Férussac)

Effects of glutamic acid and its relatives were examined on the electrical activity of two kinds of neurones (the PON, periodically oscillating neurone, and the TAN, tonically autoactive neurone) identified in the suboesophageal ganglia of an African giant snail, Achatina fulica Férussac. L- and D-Glu, L-Asp, Gly and beta-Ala did not show any effect on the two neurones. However, beta-hydroxyglutamic acid (BHGA) showed a remarkable inhibitory effect on the PON. Erythro-L-BHGA had the strongest effect of these stereoisomers, and the critical concentration of this substance to produce the effect was 10(-6)-3 X 10(-5) g/ml (6-18 muM) when administered in the bath application. We confirmed by the microdrop application that erythro-L-BHGA directly hyperpolarized the PON neuromembrane. When the curve of current-voltage relationships (I-V curve) of the PON neuromembrane measured under erythro-L-BHGA at 10(-5) g/ml (61 muM) was superimposed on the curve of the normal state using the firing level as the common standard, these two curves showed concordance in a wide range of membrane polarization level. This concordance implies that the membrane resistance was maintained normally under erythro-L-BHGA at this concentration. A higher concentration of 10(-4) g/ml (0.61 mM) of this substance caused a decrease of PON membrane resistance and a remarkable elevation of its firing level. The TAN was not sensitive to BHGA, but sensitive to GABA and its derivatives.

Size and duration of inhibition in the medial geniculate body in unanesthetized cats

A method which allows repeated micro-electrode recordings from subcortical structures without using any drugs is described. This method was adopted in combination with convential implantation techniques to study click-evoked potentials and inhibitory processes in the auditory system of the cat. The click-evoked potentials in MG were hardly affected by moderate doses of barbiturate and only to a minor degree in the auditory cortex. In the unanaesthetized animal the most significant contribution to the click-evoked inhibition in the auditory system was due to mechanisms in the MG. The inhibition was diminished both in size and duration as compared with the situation in anaesthetized cats. The MG cells showed a tendency to cyclic inhibition in the unanaesthetized cat, but not so regularly as following administration of sodium pentobarbital. The action of barbiturates on the auditory system is discussed.

Feeding following intraventricular injection of CA++, MG++ or pentobarbital in pigs

The effects on food intake, of injections into the lateral ventricle of Ca++, Mg++ or equimolar mixtures of the 2 ions have been studied in pigs. The chloride salts were made up in normal saline which together with 1.35 percent NaC1 was used as a control injection. Dosages of 12.5 mumoles, 25 mumoles, or 50 mumoles either Ca++ or Mg++ elicited increased food intake in the 30 min following the injection and there was a predominantly linear feeding response to increasing concentration of the two ions. Ca++ was more effective in eliciting increased food intake than Mg++. Equimolar mixtures of Ca++ and Mg++ of 6.5 + 6.5 mumoles, 12.5 + 12.5 mumoles or 25 + 25 mumoles also increased food intake linearly and the mixture of ions was more effective than equivalent concentrations of Ca++ or Mg++ alone. No antagonism between Ca++ and Mg++ was seen with respect to food intake. Intraventricular injections of 16 mumoles of sodium pentobarbital also elicited increased food intake in the 30 min following injection. It is suggested that the ions and the barbiturate probably act by deression of periventricular neurons, particularly in the medial hypothalamus which is adjacent to the third ventricle.

The action of 5-hydroxytryptamine on chemoreceptor discharges of the cat's carotid body

1 Chemoreceptor discharges were recorded in vivo from fine filaments of the carotid sinus nerve containing a single or several active units; their frequency was used as an index of receptor activity. The effects of 5-hydroxytryptamine (5-HT) on chemoreceptors were studied in 26 adult cats. At times, sinus baroreceptor discharges were recorded from the carotid nerve and the effect of 5-HT on the discharges was examined. 2 Intra-carotid injections of 5-HT (2-20 mug) induced a sharp and brief increase in chemoreceptor discharges, followed by depression or block which lasted for several seconds. Repeated injections at short intervals, and a small dose after a large dose of 5-HT resulted in depressed or blocked response to 5-HT. 3 5-HT in high doses (10-20 mug, i.a.) slightly depressed the chemoreceptor discharges induced by either acetylcholine (ACh) or NaCN, when these substances were applied within 20 s after 5-HT. 5-HT (5-20 mug, i.a.) applied during asphyxia induced a further increase in chemoreceptor discharges, soon followed by block of the discharges lasting for several seconds. 4 Atropine or hexamethonium in high doses did not change the chemoreceptor response to 5-HT, while that to ACh was markedly depressed. 5 (+)-Lysergic diethylamide (LSD), methysergide or gramine did not alter the response to 5-HT, while LSD in low doses produced a marked increase in chemoreceptor discharges. 6 Acute and chronic treatment with reserpine (5-10 mg/kg, i.v.) of the animals did not change the sensitivity and the reactivity of the chemoreceptor to ACh and NaCN, while the chemoreceptor response to 5-HT was augmented, indicating an increase in the sensitivity of chemoreceptors to 5-HT. 7 5-HT in small doses (2-10 mug, i.a.) induced a marked increase in sinus baroreceptor discharges; subsequently discharges were depressed or blocked for several seconds. 8 The results are discussed in relation to possible mechanism of action of 5-HT on the chemoreceptors. It is concluded that the exogenous 5-HT probably acts directly on the chemosensory nerve endings and depolarizes them, but 5-HT contained in the carotid body does not play a significant role in the generation of chemoreceptor discharges.

Divalent cations: effects on post-synaptic pharmacology of invertebrate synapses

(1) The effects of divalent cations (Ca++, Mg++, Sr++ and Co++) were studied on the post-synaptic responses of crustacean neuromuscular junctions and identified molluscan neurons to bath and iontophoretic application of putative transmitters. (2) The glutamate response of the crustacean muscle was parabolically dependent on [Ca++]0, while the ACh response of an identified molluscan neuron was inversely dependent on[Ca++]0. Elevated [Ca++]0 depressed both glutamate and ACh depolarizations in a concentration-dependent, reversible manner. Low concentrations of Co++ also depressed both depolarizations in a concentration-dependent, reversible manner. (3) Double-reciprocal plot analyses of the Ca++ and Co++ depressions indicate that these agents were apparently not acting to reduce the affinity of the receptor for the agonist. Elevated concentrations of both Ca++ and Co++ shifted the inversion potential of the ACh response in a hyperpolarizing direction, suggesting a preferential block of the receptor-coupled Na+ conductance. (4) Neither Ca++ nor Co++ depressed Cl- or K+-dependent responses coupled to the putative transmitters GABA, glutamate, dopamine or ACh. (5) The selective inhibition of the ACh and glutamate responses by the general anesthetic pentobarbital was examined as a function of[Ca++]0. Decreasing [Ca++]0 by 5-fold decreased the pentobarbital inhibition by about 50% while increasing [Ca++]0 by 5-fold produced an insignificant increase in the inhibition. (6) The data indicate that divalent cations, like general anesthetics, selectively depress post-synaptic excitatory responses that are primarily Na+-dependent. This selective depression by Ca++ could contribute to its anesthetic and anticonvulsant properties when present in elevated concentrations in the ventricular fluid. The mechanism by which divalent ions and general anesthetics selectively depress receptor-coupled conductances appear to be different: divalent ions preferentially attack the Na+ component while anesthetics block Na+ and K+ conductance equally (possibly by affecting the kinetics of the mechanism).

Binding of gamma-aminobutyric acid and taurine to synaptosomal fractions of regions of the feline CNS

The "binding" of 3H-gamma-aminobutyric acid (3H-GABA) and 14C-taurine (7 X 10(-5) M) to synaptosomal fractions of the feline CNS was studied using differential centrifugation and double-isotope methods. Of the regions examined, the "binding" of GABA (in nmole/mg protein) occurred to the greatest extent, in cerebral cortex, olfactory bulb, and colliculi and, to a lesser extent, in caudate nucleus, corona radiata, and fornix. Distribution ratios revealed that GABA binding was greater in the cerebral cortex than in caudate nucleus, brain stem, spinal cord, and white matter regions. The binding of taurine (in nmole/mg protein) was lowest in cerebral cortex and highest in brain stem and spinal gray regions. Distribution ratios for taurine indicated that its accumulation was similar in all CNS regions studied, except for slightly higher values for cerebellar cortex and fornix. These results indicated that preferential binding of both GABA and taurine exists among regions of the feline CNS.

Morphine and neurotransmitter substances: Microiontophoretic study in the rat brain stem

1 The effects of microiontophoretically applied morphine and its interactions with the effects of microiontophoretic applications of either acetylcholine, (-)-noradrenaline or 5-hydroxytryptamine have been studied on single neurones in the brain stem of rats anaesthetized with urethane.2 Morphine excited or inhibited most neurones tested and the effects, especially excitation, were often extremely powerful. However, the time course of the excitatory and inhibitory effects were somewhat different.3 Desensitization to the excitation produced by morphine was seen after repeated or prolonged applications and it is suggested that this phenomenon may be related to the tolerance which develops after chronic administration of morphine. No desensitization was observed to inhibition of neuronal activity by morphine.4 Morphine usually reduced the excitation of neurones by acetylcholine, noradrenaline or 5-hydroxytryptamine but sometimes potentiated the effect, although not always on the same neurones. Inhibition of neuronal activity by these compounds was never modified by morphine and neither were the effects of glutamate or D,L-homocysteic acid when used as control agonists.5 The in vitro release of morphine from six micropipettes was determined and the transport number was calculated to be 0.051 (s.d. 0.021).6 The implications of these observations in explaining the pharmacological actions of morphine are discussed.

Glutamic acid mimicking of synaptic inhibition on the giant serotonin neurone of the snail

Stimulation of the snail's tentacle nerves results in hyperpolarization and inhibition of the 'giant serotonin-containing neurone'located in the brain. Glutamic acid mimics this effect. Available data indicate that the reversal potential for the glutamic acid effect is very similar to that of the synaptic action being about 5 to 8 mV more negative than the resting potential in each case.