THE EXCITATION AND DEPRESSION OF MAMMALIAN CORTICAL NEURONES BY AMINO ACIDS

Effect of electrical vs. chemical deep brain stimulation at midbrain sites on micturition in anaesthetized rats

AIM

To understand how deep brain stimulation of the midbrain influences control of the urinary bladder.

METHODS

In urethane-anaesthetized male rats, saline was infused continuously into the bladder to evoke cycles of filling and voiding. The effect of electrical (0.1-2.0 ms pulses, 5-180 Hz, 0.5-2.5 V) compared to chemical stimulation (microinjection of D,L-homocysteic acid, 50 nL 0.1 M solution) at the same midbrain sites was tested.

RESULTS

Electrical stimulation of the periaqueductal grey matter and surrounding midbrain disrupted normal coordinated voiding activity in detrusor and sphincters muscles and suppressed urine output. The effect occurred within seconds was reversible and not secondary to cardiorespiratory changes. Bladder compliance remained unchanged. Chemical stimulation over the same area using microinjection of D,L-homocysteic acid (DLH) to preferentially activate somatodendritic receptors decreased the frequency of micturition but did not disrupt the coordinated pattern of voiding. In contrast, chemical stimulation within the caudal ventrolateral periaqueductal grey, in the area where critical synapses in the micturition reflex pathway are located, increased the frequency of micturition.

CONCLUSION

Electrical deep brain stimulation within the midbrain can inhibit reflex micturition. We suggest that the applied stimulus entrained activity in the neural circuitry locally, thereby imposing an unphysiological pattern of activity. In a way similar to the use of electrical signals to 'jam' radio transmission, this may prevent a synchronized pattern of efferent activity being transmitted to the spinal outflows to orchestrate a coordinated voiding response. Further experiments to record neuronal firing in the midbrain during the deep brain stimulation will be necessary to test this hypothesis.

Probing presynaptic regulation of extracellular dopamine with iontophoresis

Iontophoresis allows for localized drug ejections directly into brain regions of interest driven by the application of current. Our lab has previously adapted a method to quantitatively monitor iontophoretic ejections. Here those principles have been applied in vivo to modulate electrically evoked release of dopamine in anesthetized rats. A neutral, electroactive marker molecule that is ejected purely by electroosmotic flow (EOF) was used to monitor indirectly the ejection of electroinactive dopaminergic drugs (raclopride, quinpirole, and nomifensine). Electrode placements were marked with an iontophoretically ejected dye, pontamine sky blue. We show that EOF marker molecules, acetaminophen (AP) and 2-(4-nitrophenoxy) ethanol, have no effect on electrically evoked dopamine release in the striatum or the sensitivity of electrode. Additionally, we establish that a short, 30 second ejection of raclopride, quinpirole, or nomifensine with iontophoresis is sufficient to affect autoreceptor regulation and the re-uptake of dopamine. These effects vary in lifetime, indicating that this technique can be used to study receptor kinetics.

The role of taurine in neuronal protection following transient global forebrain ischemia

Osmoregulation and post ischemic glutamate surge suppression (PIGSS) are important mechanisms in the neuroprotective properties of taurine. We studied the role of taurine in PIGSS following transient global forebrain ischemia (TGFI). A group of gerbils received a high dose of continuous intracerebral taurine during the peri-ischemic period. Beta-alanine was given similarly to a negative control group. The control group consisted of animals undergoing only TGFI. On the fourth day following commencement of drug administration, TGFI was induced. Concurrently, half the animals from each group receiving an agent had intracerebral microdialysis. All animals underwent histological assessment at day 7. The microdialysis and histological data was analyzed. Our results showed that taurine treatment did not cause PIGSS. The histological difference between the three groups was statistically insignificant. We conclude that intracerebral taurine in the dosage administered during peri-ischemic period, does not result in PIGSS or histologically evident neuroprotection.

Mesencephalic stimulation elicits inhibition of phrenic nerve activity in cat

1. Previous work from this laboratory has indicated that the mesencephalon is the anatomical substrate for a mechanism capable of inhibiting central respiratory drive in glomectomized cats for periods of up to 1 h or more following brief exposure to systemic hypoxia; phrenic nerve activity was used as an index of central respiratory drive. 2. The present study was undertaken to further localize the region responsible for the observed post-hypoxic inhibition of respiratory drive. We studied the phrenic nerve response to stimulations of the mesencephalon in anaesthetized, paralysed peripherally chemo-denervated cats with end-expired PCO2 and body temperature servo-controlled. 3. Stimulations of two types were employed. Electrical stimulation allowed rapid determination of sites from which phrenic inhibition could be elicited. Microinjections of excitatory amino acids were used subsequently in order to confine excitation to neuronal cell bodies and not axons of passage. 4. Stimulation of discrete regions of the ventromedial aspect of the mesencephalon in the vicinity of the red nucleus produced substantial inhibition of phrenic activity which lasted up to 45 min. Stimulation of other areas of the mesencephalon either produced no phrenic inhibition or resulted in a slight stimulation of phrenic activity. 5. The results are discussed in the context of the central respiratory response to hypoxia.

Nucleus tractus solitarius as mediator of evoked parabrachial cardiovascular responses in the decerebrate rabbit

1. The present study has assessed the importance of neurones within the nucleus tractus solitarius (NTS) in mediating the cardiovascular response evoked from the parabrachial nucleus (PBN) in the decerebrate rabbit. Microinjection techniques were employed so that the magnitude of the circulatory responses elicited from the PBN could be compared before, and after, kainic acid or bicuculline were microinjected into restricted regions of the NTS. 2. Electrical stimulation of the PBN (both medial and lateral regions) evoked variable changes in heart rate, a pressor response, vasoconstriction in the hindlimb and an increase in renal sympathetic nerve activity. Glutamate injected into these regions of the PBN elicited a similar pattern of response except that a tachycardia was observed consistently. 3. Both electrical and chemical stimulation of restricted regions of the NTS evoked bradycardia and a depressor response together with an increase in femoral vascular conductance and an inhibition of activity in the renal nerve. 4. Chemical lesions placed in these regions of the NTS by microinjecting kainic acid were found to attenuate both the heart rate and arterial blood pressure responses elicited from sites in the medial and lateral PBN using either electrical or chemical stimulation. Equivalent effects were produced on microinjecting the GABAa receptor antagonist bicuculline into the NTS. 5. These data indicate that NTS neurones play a part in mediating the cardiovascular responses that are evoked from the PBN and suggest that the action of the PBN at the level of the NTS is mediated via a GABAergic mechanism.

Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat

1. Simultaneous recordings were made from postganglionic sympathetic fibres supplying hindlimb skin and skeletal muscle in chloralose-anaesthetized, artificially ventilated cats. Single-fibre activity was either isolated by dissection or discriminated from few-fibre preparations of fascicles in the left superficial peroneal or sural nerve (innervating hairy skin) and common peroneal nerve (innervating muscle). Vasoconstrictor fibres were identified by their spontaneous activity as well as their responses to stimulation of the lumbar sympathetic chain and to changes in baroreceptor activity. The baroreceptors were then denervated by bilateral section of the vagi, carotid sinus and aortic nerves. 2. In five cats, neurones in the region of the subretrofacial nucleus were activated chemically by microinjections of 2-10 nl 0.5 M-sodium glutamate from a micropipette inserted into the ventral surface of the medulla. Both skin and muscle vasoconstrictor fibres were activated by glutamate injections into this region on either side of the medulla. Arterial pressure also rose. 3. Glutamate injections at forty-two sites evoked a positive response, defined as an increase in cutaneous and/or muscle vasoconstrictor fibre activity of at least 25%. This response was evoked only in the cutaneous fibre at sixteen of these sites ('skin points'), only in the muscle fibre at seven sites ('muscle points'), and in both fibres in the remainder ('mixed points'). The largest percentage increases in activity of either type of fibre were obtained from mixed points. 4. The blood pressure rises following glutamate stimulation of muscle points were significantly greater than those produced by stimulation of skin points. Analysis of all positive responses showed that the evoked rise in blood pressure was significantly correlated with muscle sympathetic activity but not with cutaneous sympathetic activity. 5. Glutamate stimulation at different sites could evoke differential responses in skin and muscle vasoconstrictor fibres without any detectable change in the pattern of phrenic nerve discharge. 6. Skin points were grouped in the medial part of the subretrofacial region, and muscle points in the lateral part. In addition, for all positive responses there was a highly significant correlation between the ratio of muscle to cutaneous sympathetic activity evoked, and the distance from the mid-line of the corresponding injection site. 7. These results demonstrate a functional differentiation among subretrofacial neurones in their relative control of the sympathetic vasoconstrictor supply to skin and skeletal muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Quantitative studies on some antagonists of N-methyl D-aspartate in slices of rat cerebral cortex

Coronal sections of rat brain (500 micron thick) were trimmed to form 'wedges' of tissue consisting of cerebral cortex and corpus callosum. When these slices were placed in a two-compartment bath, the cortical tissue could be depolarized, relative to the corpus callosum, by superfusions of high K+, or by amino acids such as L-glutamate, L-aspartate, quisqualate, kainate and N-methyl D-aspartate (NMDA). Responses to NMDA were reduced by magnesium ions, by the organic antagonists (-)-2-amino 5-phosphonovalerate (APV) and 2-amino 7-phosphonoheptanoate (APH), and by the dissociative anaesthetic ketamine. In this preparation, all these antagonists shifted the NMDA dose-response curve to the right in a parallel manner. A Schild plot for Mg2+ had a slope significantly less than unity, indicative of a non-competitive action, whilst Schild plots for (-)-APV, APH and ketamine appeared linear and had slopes of approximately 1. Analysis of the results of combination experiments suggested that the presumed competitive antagonists, (-)-APV and APH, share a common site of action as NMDA antagonists, and that this site is distinct from that at which ketamine exerts its action. The action of Mg2+ is clearly different from that of either (-)-APV or ketamine. It is concluded that ketamine is a non-competitive antagonist of NMDA and may act at an allosteric site on the NMDA receptor complex to influence its function.

Retrograde transport of gamma-amino[3H]butyric acid reveals specific interlaminar connections in the striate cortex of monkey

Several lines of evidence suggest that gamma-aminobutyric acid is an inhibitory neurotransmitter in the cerebral cortex. To study the intracortical projection of neurons that selectively accumulate this amino acid, we injected radioactive gamma-aminobutyric acid into the upper layers of the striate cortex of monkeys along tracks at an oblique angle to the pia. Sections from the injected area were then processed by a combination of autoradiography and Golgi impregnation to reveal the distribution of labeled neurons and their morphological characteristics. Labeled neurons always occurred around the injection site in each layer. In addition, a consistent radial pattern of perikaryal labeling was observed in layers IVc-VI below the injection track in layers I-IVa. The closer the injection track was to the pia the deeper the peak density of labeled cells appeared. After injection in layers IVa and the lower part of III, the highest number of labeled neurons was in layer IVc; after injection in the upper part of layer III, most labeled neurons were in layer V; and, after injection in layers I and II, the proportion of labeled neurons increased in the lower part of layer V and in layer VI. All these neurons in the infragranular layers are presumably labeled by retrograde axonal transport via the labeled fiber bundles that extended from upper to lower layers. Thirty-four Golgi-stained neurons of various types were also examined for retrograde labeling. Two were labeled, and both were aspiny stellate cells in layer V. The arrangement of these putative GABAergic neurones, with axons that ascend from lower to upper layers in a regular pattern and arborize locally, would enable them to mediate inhibition within cortical columns and between neighboring columns.

The excitation and depression of spinal neurones by ibotenic acid

1. The firing of spinal interneurones and Renshaw cells by microelectrophoretic (+/-)-ibotenate, which was approximately eight times more active as an excitant than L-glutamate, was followed by prolonged depression of the sensitivity of the neurones to excitant amino acids and acetylcholine. 2. The depression, which lasted for 15--30 min when ibotenate was ejected for 3--6 min, was blocked by the GABA-antagonist bicuculline methochloride, and was independent of prior firing since it occurred with subthreshold concentrations of ibotenate and when ibotenate firing had been blocked by DL-alpha-aminoadipate. 3. When administered electrophoretically for 5 min, muscimol, a potent GABA agonist, reduced neuronal excitability for prolonged periods and this effect was also prevented by bicuculline methochloride. 4. The depression of neuronal excitability produced by GABA, taurine, isoguvacine or 3-aminopropane sulphonate, ejected for periods of 5--6 min, recovered rapidly. 5. It is suggested that ibotenate is converted in vivo to muscimol or a related compound which has a prolonged, bicuculline-sensitive depressant action on the excitability of neurones.

The actions of excitatory amino acids on motoneurones in the feline spinal cord

1. Combined recording or ionophoretic electrodes of the concentric type were used to investigate the depolarizing responses of DL-homocysteate (DLH) and L-glutamate in cat lumbar motoneurones. 2. Typically, DLH responses were slow both in onset and recovery, while glutamate responses were fast in onset and recovery and were frequently accompanied by a post-response hyperpolarization. 3. DLH responses (smaller than those necessary to evoke firing) were accompanied by a stable decrease in GM. This decrease was usually more than could be accounted for by anomalous rectification of the membrane. 4. Small glutamate responses were accompanied by either a small decrease, no change or a small increase in GM. There was a biphasic change in GM during large responses: GM decreased during the rising phase and early part of the response plateau and thereafter increased as the depolarization was maintained. It is proposed that the high conductance state during glutamate application (but not the depolarization itself) is a manifestation of glutamate uptake. 5. Firing evoked by DLH was stable during very long applications of the drug. Firing evoked by glutamate was usually of short duration, despite the maintained depolarization. 6. No reversal potential for the DLH responses could be demonstrated, but the responses decreased in size both with hyperpolarization and depolarization of the membrane. A 'null point' of the response in the negative direction was found to be approximately -95 mV. 7. DLH resonses were insensitive to changes in the internal Cl concentration. When the external K concentration was increased by K+ ionophoresis, the DLH responses became smaller. It is concluded that the DLH response is probably mediated via a decrease in K+ conductance and that the availability of this conductance channel is potential dependent. 8. Changes in the sizes of evoked potentials (e.p.s.p.s, i.p.s.p.s and a.h.p.s) with DLH and glutamate responses were investigated. The size of each of these evoked potentials was inversely related to GM during the responses; thus they all showed stable increases during DLH responses. E.p.s.p.s recorded during DLH were of longer half-width and time-to-peak than the control, but there was no change in the maximum slope (V.sec-1). When e.p.s.p.s decreased in size with glutamate the time-to-peak remained constant. 9. Acidic amino acids have been implicated as natural excitatory transmitters. The consequence of our results for the mechanism of excitatory transmission is therefore discussed.

Effects of excitatory and inhibitory amino acids on phasic respiratory neurons

The responsiveness of phasically active brainstem respiratory neurons to several amino acids was investigated in cats under Dial anesthesia. Four-barreled microelectrodes were used to extrude iontophoretically the putative neurotransmitters L-glutamate, L-asparatate, glycine, and gamma-aminobutyric acid (GABA), L-glutamate and L-aspartate caused increased activity when applied to either inspiratory or expiratory neurons and appeared to be equal in efficacy. Likewise, GABA and glycine depressed ongoing phasic neural activity of both inspiratory and expiratory units. In this case, however, the dosage of GABA required to produce a given depression was significantly less than the required dosage of glycine. These findings support the hypothesis that L-glutamate and/or L-aspartate may act as excitatory neurotransmitter agents at the synapses of brainstem respiratory neurons and conversely, GABA may act as the natural inhibitory neurotransmitter.

The effects of dopamine, noradrenaline and serotonin in the visual cortex of the cat

The predominant effect of dopamine, norepinephrine and serotonin on the photically-evoked urinary activity was a prolonged inhibition of firing. These amines were also able to block acetylcholine-induced excitations and for longer periods of time than GABA.

Structure-activity relations of excitatory amino acids on frog and rat spinal neurones

1 A series of compounds structurally related to glutamic acid has been tested on frog and rat spinal neurones. The substances were added to procaine-containing medium bathing the isolated hemiscected spinal cord of the frog, and their potencies in depolarizing motoneurones were assessed by the magnitude of the potential produced in the ventral root. The electrophoretic technique was used to administer the substances around single interneurones of the rat spinal cord and the relative potencies of the compounds as excitants assessed by the magnitude of the currents required to produce similar rates of neuronal firing. 2 Parallel structure-activity relations were observed in the two series of experiments, suggesting that the receptors for excitatory amino acids on frog and rat spinal neurones are similar. 3 Quisqualate, domoate and kainate were the strongest excitants in both animals, with potencies around two orders of magnitude higher than that of L-glutamate. 4 2,4,5-Trihydroxyphenylalanine (6-OH-DOPA) was a stronger excitant and L-3,4-dihydroxyphenylalanine (L-dopa) a weaker excotamt than L-glutamate on frog spinal motoneurones. The former compounds was also a more potent convulsant than L-glutamate on intraventricular injection into mouse brain. The lack of activity of 6-OH-DOPA on electrophoretic administration was attributed to oxidation. 5 Unlike the majority of amino acid excitants, several of the compounds shown in the present work to have moderate excitatory activity are not anionic at physiological pH. This indicates either that two negatively charged groups are not essential for interaction with a common excitatory receptor, or that more than one type of receptor is involved in the actions demonstrated.

The in vivo inactivation of GABA and other inhibitory amino acids in the cat nervous system

In cats anaesthetised with pentobarbitone, the effect of inhibitors of the in vitro cellular uptake of GABA were tested on the responses of single central neurones to GABA and other depressant amino acids. (4)- AND (-)-nepecotic acid, (4)-2,4-diaminobutyric acid (DABA) and 2,2-dimethyl-beta-alanine, enhanced the action of GABA on spinal, cerebellar and cerebral cortical neurones. In the spinal cord DABA, and to a less estent (-)-nipecotic acid, enhanced the action of beta-alanine, whereas the actions of glycine and taurine were unaffected by DABA and reduced by (-)-nipecotic acid. In the cerebellum and cerebral cortex, these two substances enhanced the action of GABA, usually to a greater extent than that of beta-alanine and taurine, although this specificity was not marked. The GABA-mediated basket cell inhibition of Purkinje cells in the cerebellum was unaffected by DABA and (-)-nipecotic acid, and neither substance appears suitable for determining the role of uptake processes in the inactivation of synapitcally released GABA. Quantitatively these in vivo results agree more closely with recent vitro uptake studies in cat tissue than the previously published data on rat cerebral cortex and dorsal root ganglia, and the observations provide further evidence for the importance of cellular uptake in maintaining low extraneuronal concentrations of inhibitory amino acid transmitters.

Sensitivity of Purkinje cell dendrites to glutamic acid

The distribution of glutamate sensitive site was studied in vitro in thin cerebellar sections from guinea-pigs, in which Purkinje cell bodies and some of the principal dendrites were identified microscopically. Glutamate administered near the cell body induced firing. Stronger excitation, however, was produced when glutamate was administered to the molecular layer along a strip of tissue extending from the soma of the cell under study towards the pial surface of the slice. Excitation induced by glutamate slowly declined in some cells during prolonged administration. D-Glutamate was a weaker excitant than the L-isomer. These results suggest that the dendrite of the Purkinje cell is more sensitive to glutamate that the cell soma.

Suppression of conditioned drinking by taurine and related compounds

Mice were conditioned to respond for water reinforcements on a FR-5 schedule. Taurine, injected intraperitoneally at doses of 9.0, 13.8, and 21.3 mmole/kg 30 min prior to the experimental session, produced a dose-related decrease in both the initial response rate and total number of reinforcements received by mice deprived of water for 24 hr. The structural analogues of taurine (aminomethanesulfonic acid, 3-aminopropanesulfonic acid, beta-alanine, cysteamine, and glycine) also produced a hypodipsia. Doses of taurine which produced depression of responding for water reinforcements were used which produced no suppression of spontaneous motor activity, rotarod performance, Sidman avoidance, or shuttle-box avoidance. After intraperitoneal injection, the concentration of taurine increased in the hypothalamus and medulla, but not in other brain areas. We suggest that taurine might be acting by specifically depressing areas of the hypothalamus which stimulate drinking.

Gamma-aminobutyric acid binding to receptor sites in the rat central nervous system

[(3)H]Gamma-aminobutyric acid (GABA) binds to synaptic membrane fractions of rat brain in a selective fashion representing an interaction with postsynaptic GABA receptors. Inhibition of [(3)H]GABA binding by a variety of amino acids closely parallels their ability to mimic the synaptic inhibitory actions of GABA and does not correlate with their relative affinity for the presynaptic synaptosomal GABA uptake system. [(3)H]GABA binding is saturable with an affinity constant of about 0.1 muM. The GABA antagonist bicuculline inhibits [(3)H]GABA binding with half maximal effects at 5 muM, whereas it requires a concentration of 0.5 mM to reduce synaptosomal GABA uptake by 50%. In subcellular fractionation experiments [(3)H]GABA binding is most enriched in crude synaptic membranes. [(3)H]GABA binding is greatest in the cerebellum, least in the spinal cord and medulla oblongatapons, with intermediate values in the thalamus, hippocampus, hypothalamus, cerebral cortex, midbrain, and corpus striatum.

Cortical pyramidal tract interneurones and their sensitivity to L-glutamic acid

1. Pyramidal tract interneurones, defined as neurones which are activated synaptically as a result of pyramidal tract stimulation, have been identified in the rat cerebral cortex. The number of evoked spikes depended upon stimulus strength, and stimulation in a specific thalamic nucleus produced a burst of activity lasting for up to 1 sec.2. These cells are readily excited by a brief (50 msec) pulse of glutamate applied by micro-iontophoresis. Other, unidentified cells are not so responsive.3. Synaptically evoked spikes resulting from pyramidal tract stimulation can be blocked by the iontophoretic and I.P. administration of substances shown to antagonize glutamate excitation of cells.4. The results support suggestions that glutamic acid is a neurotransmitter in the cerebral cortex. The evidence presented further indicates that glutamic acid could be the transmitter released by the pyramidal tract.

Distribution and localization of sites of gamma aminobutyric acid metabolism in the adult rat brain

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The action of nerveside, a peptide extracted from brain, on myenteric ganglia in Auerbach's plexus

1. In the rabbit jejunum nerveside causes alternating contractions of the longitudinal and circular muscles, which are similar to the peristaltic reflex produced by distension of the gut. These effects are abolished by dibucaine (Nupercaine) and tetrodotoxin. The site of action is located in Auerbach's plexus.2. Circular muscle contractions due to nerveside are blocked by hexamethonium, nicotine, morphine, atropine and hyoscine. There is no circular response to nerveside after removal of Auerbach's plexus. It is suggested that nerveside acts by stimulating presynaptic nerve terminals which are cholinergic and that the circular muscle is innervated by cholinergic nerve fibres.3. Longitudinal muscle contractions due to nerveside are not affected by hexamethonium but can be blocked by paralysis of myenteric ganglia with nicotine. They are not abolished by morphine or by cholinolytic drugs. It is suggested that nerveside acts by stimulating either receptors insensitive to acetylcholine in ganglion cells or non-cholinergic presynaptic nerve terminals. It is further suggested that the longitudinal muscle of the rabbit gut is supplied by non-cholinergic as well as cholinergic nerve fibres.

Micro-iontophoretic studies on neurones in the cuneate nucleus

1. Cuneate cells in anaesthetized cats were strongly excited by L-glutamate, and somewhat less by D-glutamate; cells which receive afferents from hair receptors were particularly sensitive.2. Glutamate could be used to demonstrate post-synaptic inhibitory inputs from the dorsal column, the medial lemniscus and the frontal cortex.3. Many cuneate cells were also strongly excited by adenosinetriphosphate (ATP); this was probably due to the chelating action of ATP, as citric acid was also quite effective.4. gamma-Aminobutyric acid (GABA) readily blocked all forms of spontaneous and evoked activity, except antidromic invasion of cuneothalamic neurones; cells which receive proprioceptive afferents were particularly sensitive to GABA. Glycine had a comparable effect.5. Acetylcholine (ACh), catecholamines, histamine, 5-hydroxytryptamine (5-HT) and an extract containing substance P mostly had only weak depressant actions. Cholinergic and mono-aminergic mechanisms are probably not very significant in the cuneate.6. These results are consistent with the possibility that glutamate and GABA (or glycine), or some closely related compounds, are the main excitatory and inhibitory transmitters in the cuneate nucleus.7. If ATP is released from afferent nerve endings, it could also play a significant role in excitation.

Pharmacological studies on feline Betz cells

1. A study has been made of the sensitivity of single neurones in the pericruciate cortex of anaesthetized and unanaesthetized cats to cholinomimetics administered electrophoretically from multibarrel micropipettes.2. A high proportion of deep pyramidal cells, including Betz cells, were excited by these substances, and the receptors involved have muscarinic characteristics.3. Atropine specifically reduced the sensitivity of cortical neurones to acetylcholine, but no such action could be demonstrated for dihydro-beta-erythroidine, gallamine or general anaesthetics.4. The significance of these results is discussed in relation to the possible synaptic or non-synaptic action of acetylcholine upon cortical neurones.

Acetylcholine sensitivity of cerebellar neurones in the cat

1. Cholinomimetics, acetylcholine antagonists and some other compounds of pharmacological interest were administered electrophoretically near neurones within the vermal cerebellar cortex of anaesthetized (pentobarbitone) and unanaesthetized (cerveau isolé) cats.2. The neurones were identified by position within the cortex, spontaneous activity, and the responses to afferent and antidromic stimulation.3. Purkinje cells, but neither granule nor basket cells, were excited by cholinomimetics, and the acetylcholine receptors had muscarinic properties. Excitation was often preceded by depression of the spontaneous firing.4. Intravenously administered atropine and dihydro-beta-erythroidine did not depress the synaptic excitation of cerebellar neurones evoked by impulses in mossy, climbing or parallel fibres.5. Acetylcholine is thus unlikely to be an excitatory transmitter within the feline cerebellum, particularly at mossy fibre-granule cell synapses, despite the presence of relatively high levels of acetylcholinesterase within mossy fibre terminals.

Micro-electrophoretic studies of neurones in the cat hippocampus

1. Drugs have been applied micro-electrophoretically to units in the hippocampal cortex of the anaesthetized cat, and their effects on cell firing were recorded simultaneously.2. L-Glutamate rapidly and powerfully excited hippocampal units, an effect which was quickly reversed on stopping the expelling current. The local application of L-glutamate also excited a fast seizure discharge at 15-50/sec. Both these effects of L-glutamate were strongly depressed by fimbrial stimulation.3. gamma-Aminobutyric acid had a strong depressant action on all the units on which it was tested; the time course of this effect was rapid.4. ACh excited half the units to which it was applied. Characteristically this excitation developed slowly over many seconds and persisted after stopping the expelling current. Most cholinoceptive units were found to be concentrated in the superficial layer of the cortex corresponding to the hippocampal pyramidal cells and their main dendritic processes.5. Atropine selectively blocked the excitation of cholinoceptive units by ACh, but not the excitation by L-glutamate. No cholinoceptive units were blocked by dihydro-beta-erythroidine, though several were selectively blocked by dimethyl (+)-tubocurarine.6. The most usual effect seen with 5-HT was depression, though several units were found to be excited. Some of the units tested with 3-hydroxytyramine (dopamine) or noradrenaline were found to be depressed.

The distribution of free amino acids in subcellular fractions of guinea-pig brain

1. The free amino acids of homogenates of guinea-pig brain in 0.32m-sucrose and of subcellular fractions derived therefrom have been estimated by the method of Moore & Stein. 2. Seven amino acids together accounted for over 80% of the free amino compounds; these are, in decreasing order of abundance: glutamate, aspartate, gamma-aminobutyrate, glycine, serine, alanine and threonine. In addition, there are appreciable quantities of amide (presumably glutamine). 3. Control experiments showed that the pattern of free amino acid occurrence in sucrose homogenates was similar to that of brains of animals killed by freezing in liquid nitrogen and extracted immediately without thawing. 4. The subcellular distribution of the amino acids resembled that of soluble cytoplasmic markers; there was no specific localization in a fraction rich in isolated presynaptic nerve terminals of amino acids capable of exciting or depressing central neurones. 5. The significance of the results is discussed in relation to the possible role of centrally active amino acids as transmitters.