Regional distribution of postsynaptic receptor binding for gamma-aminobutyric acid (GABA) in monkey brain

A Mechanistic Link from GABA to Cortical Architecture and Perception

Understanding both the organization of the human cortex and its relation to the performance of distinct functions is fundamental in neuroscience. The primary sensory cortices display topographic organization, whereby receptive fields follow a characteristic pattern, from tonotopy to retinotopy to somatotopy [1]. GABAergic signaling is vital to the maintenance of cortical receptive fields [2]; however, it is unclear how this fine-grain inhibition relates to measurable patterns of perception [3, 4]. Based on perceptual changes following perturbation of the GABAergic system, it is conceivable that the resting level of cortical GABAergic tone directly relates to the spatial specificity of activation in response to a given input [5, 6, 7]. The specificity of cortical activation can be considered in terms of cortical tuning: greater cortical tuning yields more localized recruitment of cortical territory in response to a given input. We applied a combination of fMRI, MR spectroscopy, and psychophysics to substantiate the link between the cortical neurochemical milieu, the tuning of cortical activity, and variability in perceptual acuity, using human somatosensory cortex as a model. We provide data that explain human perceptual acuity in terms of both the underlying cellular and metabolic processes. Specifically, higher concentrations of sensorimotor GABA are associated with more selective cortical tuning, which in turn is associated with enhanced perception. These results show anatomical and neurochemical specificity and are replicated in an independent cohort. The mechanistic link from neurochemistry to perception provides a vital step in understanding population variability in sensory behavior, informing metabolic therapeutic interventions to restore perceptual abilities clinically.

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GABAergic tone correlates with perceptual acuity in the human somatosensory system

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This relationship is mediated by the tuning of activity in somatosensory cortex

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We explain perceptual acuity via the underlying cellular and metabolic processes

A legacy of discovery: from monoamines to GABA

Seldom does a single individual have such a profound effect on the development of a scientific discipline as Erminio Costa had on neuropharmacology. During nearly sixty years of research, Costa and his collaborators helped established many of the basic principles of the pharmacodynamic actions of psychotherapeutics. His contributions range from defining basic neurochemical, physiological and behavioral properties of neurotransmitters and their receptors, to the development of novel theories for drug discovery. Outlined in this report is a portion of his work relating to the involvement of monoamines and GABA in mediating the symptoms of neuropsychiatric disorders and as targets for drug therapies. These studies were selected for review because of their influence on my own work and as an illustration of his logical and insightful approach to research and his clever use of techniques and technologies. Given the significance of his work, the legions of scientist who collaborated with him, and those inspired by his reports, his research will continue to have an impact as long as there is a search for new therapeutics to alleviate the pain and suffering associated with neurological and psychiatric disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

GABA receptor subunit mRNA expression in brain of conflict, yoked control and control rats

Animal conflict models have been used for years as a preclinical screen for predicting anxiolytic therapeutic efficacy. Anxiolytics, including benzodiazepines, increase punished responding. This suggests that the punished behavior may be mediated by the GABA receptor. To evaluate this hypothesis, we performed in situ hybridization histochemistry studies of GABA receptor subunits (alpha1-alpha4) and synthetic enzymes glutamic acid decarboxylase (GAD65 and GAD67) in four groups of rats: conflict (punishment), yoked controls (rats shocked without conflict training history), fixed interval only controls (rats that worked for food but were not shocked) and untreated controls. With conflict behavioral training, bilateral reduction of mRNA for the GABAA alpha1 subunit was seen relative to controls in the cortex, thalamus and hippocampus. In contrast, alteration of alpha2 mRNA levels appeared only in the yoked control group, with increased levels seen in the thalamus and cortex and decreased levels in the hippocampus. There were no differences in the alpha2 mRNA level between the control and the conflict behavioral trained animals. Further, no significant differences were found between groups in the mRNA levels for the alpha3 subunit, alpha4 subunit, GAD65, and GAD67. These results suggest that the behaviors related to conflict and uncontrollable aversive stimuli (yoked control group) are accompanied and perhaps mediated by selective changes in the GABAA alpha1 or alpha2 subunits, respectively. These findings highlight the potential usefulness of the conflict model as a means of elucidating the biological underpinnings of anxiety disorder. Published by Elsevier Science B.V. All rights reserved.

GABAA and GABAB receptors differentially regulate synaptic transmission in the auditory thalamo-amygdala pathway: an in vivo microiontophoretic study and a model

Stimulation of the medical geniculate body elicits extracellular single unit responses in the lateral nucleus of the amygdala that are dependent upon glutamatergic neurotransmission [Li et al. (1995) Exp. Brain Res., 105-87-100]. In the present study, we examined the contribution of inhibitory amino acid transmission to these excitatory responses. Antagonists of GABAA or GABAB receptors were delivered microiontophoretically to cells activated by stimulation of the medial geniculate body. Blockade of GABAA receptors with bicuculline resulted in a pronounced increase in evoked short latency unit responses (4-8 ms). In some cases, cells that were not responsive to the stimulation became responsive in the presence of bicuculline. In contrast, delivery of GABAB antagonists, Phaclofen or 2-OH-saclofen, did not affect these short-latency responses. Using paired-pulse stimulation, both short (< 30 ms) and longer (> 50 ms) latency inhibitory processes were revealed. GABAA blockade eliminated the short latency inhibition and GABAB blockade eliminated the longer latency inhibition in most cells. These results suggest that the activation of GABAA and GABAB receptors differentially regulate glutamatergic synaptic transmission in the auditory thalamo-amygdala pathway. Moreover, our findings suggest that at least part of this regulation is via a feedforward mechanism. We tested the sufficiency of feedforward inhibition to account for the data using a simple computational model that incorporates the results presented here.

Distribution of GABA immunoreactivity in the amygdaloid complex of the cat

This study describes the distribution of GABA immunoreactivity in the amygdaloid complex of cats. At the light microscopic level, immunopositive structures consisted of morphologically diverse somata and numerous small punctate elements. The latter accounted for most of the staining at low magnification and, at the electron microscopic level, were found to be axon terminals establishing symmetric synaptic contacts with a variety of postsynaptic profiles. Deep and superficial amygdaloid nuclei could be assigned to one of four groups according to (i) the intensity of immunolabeling they displayed, (ii) their density in reactive somata, and (iii) the size of the immunopositive somata they contained. Intercalated cell masses displayed the highest density of strongly immunoreactive cell bodies and presumed GABAergic terminals. However, electron microscope observations showed that intercalated somata were almost devoid of synaptic contacts. In contrast, central and medial nuclei were characterized by a low density of intensely immunoreactive somata and an elevated concentration for GABAergic terminals which contacted somatic and dendritic profiles. In addition, central and medial nuclei contained numerous neurons displaying low to moderate immunoreactivity. Superficial amygdaloid nuclei and nuclei of the basolateral complex displayed an intermediate density of immunoreactive somata and a low to moderate concentration of presumed terminals. Analysis of the distribution of soma areas within these nuclei revealed that the basolateral complex contains a distinct subpopulation of larger immunoreactive neurons. In light of recent electrophysiological findings, these results suggest that the intra-amygdaloid GABAergic system plays a major role in controlling the synaptic responsiveness and spontaneous activity of amygdaloid neurons.

Localization of GABA-like immunoreactivity in the monkey amygdala

Neurons exhibiting GABA-like immunoreactivity were identified in the monkey amygdala using an avidin-biotin immunohistochemical technique. The pattern of GABA immunoreactivity was very similar in the basolateral and superficial amygdaloid nuclei. In these regions GABA-positive cells were nonpyramidal neurons that were often arranged in clusters or curvilinear rows. These GABA-positive nonpyramidal neurons constituted about 25% of the total neuronal population of the basolateral and superficial amygdaloid nuclei. Numerous GABA-positive puncta resembling axon terminals were observed both in the neuropil and encapsulating the perikarya of GABA-negative pyramidal cells. The pattern of GABA-like immunoreactivity was different in the central and medial amygdaloid nuclei. These regions contained a very dense array of GABA-positive puncta. There were numerous GABA-positive neurons in the lateral subdivision of the central nucleus and fewer cells in the medial nucleus and medial subdivision of the central nucleus. Many immunoreactive puncta were observed contacting the perikarya and dendrites of GABA-positive cells in these regions. The intercalated nuclei consisted of numerous, small, GABA-positive neurons and a few, larger, GABA-negative cells. Both cell types were contacted by GABA-positive puncta. This study indicates that neuronal subpopulations in each of the amygdaloid nuclei of the monkey are GABAergic. The pattern of immunoreactivity varies in different amygdaloid regions and is very similar to that described in the rat. Certain aspects of the functional organization of this rich GABAergic circuitry can be elucidated by correlating the findings of the present investigation with previous anatomical, physiological, and pharmacological studies of the amygdala.

Decreased histamine H1 receptors in the frontal cortex of brains from patients with chronic schizophrenia

Involvement of histamine H1 receptor in the brains of schizophrenic patients was investigated using 3H-mepyramine as a ligand. The specific 3H-mepyramine binding in the frontal cortex was saturable with the dissociation constant (Kd) of about 0.6 nM and the maximum number of binding sites (Bmax) of 64 fmol/mg protein. Specific H1 antagonists, mepyramine (Ki = 1.4 nM), promethazine (Ki = 1.4 nM), diphenylpyraline (Ki = 4.1 nM), triprolidine (Ki = 5.3 nM), diphenylhydramine (Ki = 35 nM), but not the specific H2 antagonist, cimetidine (Ki greater than 10(5) nM), strongly inhibited the 3H-mepyramine binding. Regional distribution of the specific 3H-mepyramine binding was in the order of: frontal cortex greater than hippocampus greater than cerebellum greater than hypothalamus greater than thalamus, putamen, and pallidum. The specific 3H-mepyramine binding in schizophrenic brains was reduced by 56% in the frontal cortex. Representative Scatchard analyses of the specific 3H-mepyramine binding revealed changes resulting from a decrease in receptor density but not in receptor affinity. Down-regulation of the histamine H1 receptor in the frontal cortex may be involved in the pathophysiology of schizophrenia.

Distribution and binding parameters of GABA and benzodiazepine receptors in the cat motor thalamus and adjacent nuclear groups

Quantitative receptor binding autoradiography technique was utilized to study GABA and benzodiazepine receptors in the cat motor thalamus (ventral anterior, ventral medial and ventral lateral nuclei) and adjacent thalamic subdivisions. Binding parameters (Bmax and Kd) and distribution pattern of the binding sites for 3 tritiated ligands [3H]muscimol ([3H]MUS), [3H]flunitrazepam ([3H]FLU) and [3H]baclofen ([3H]BAC) were analyzed and compared using measurements from discrete and anatomically well-defined thalamic regions. There was little correlation in the regional distribution of the 3 binding sites. The concentration of [3H]BAC binding sites in thalamic nuclei of interest was very low, practically at the limit of resolution of the quantitative autoradiographic technique; whereas appreciable quantities of [3H]MUS and [3H]FLU binding sites were present in the motor and adjacent limbic nuclei of the thalamus. There was more difference between the nuclei in regard to the number of high affinity GABA receptors than benzodiazepine receptors. Moreover, the ratio of Bmax[3H]MUS/Bmax[3H]FLU varied from 2.2 to 4.4 in different thalamic regions suggesting the presence of a diverse population of GABAA and benzodiazepine receptors. The distribution pattern of the 3 binding sites was compared to the topography of GABAergic afferents of the basal ganglia origin and the frequency of GABAergic synapses formed by thalamic local circuit neurons (LCN) in the motor thalamus that were established earlier. It was concluded that in the cat motor thalamus: (1) none of the ligands studied appear to reveal the receptors associated with nigro- or pallidothalamic synapses; (2) [3H]MUS binding sites may be associated with the dendrodendritic contacts formed by LCN; and (3) the [3H]FLU binding sites are physically unrelated to [3H]MUS binding sites. The concentration of [3H]FLU and [3H]MUS binding sites in the midline nuclei and of [3H]MUS binding sites in the limbic nuclei was remarkably high. It was concluded that in addition to previously suggested limbic structures, the midline nuclei with their very high content of benzodiazepine receptors may be considered as a neuroanatomical substrate of certain forms of anxiety.

Specific binding of a phenyl-pyridazinium derivative endowed with GABAA receptor antagonist activity to rat brain

SR 95531 has been shown to be a potent, selective, reversible and competitive GABAA antagonist. In the present study we report that (3H)SR 95531 binds with high affinity and in a specific and saturable manner to rat brain membranes. Scatchard analysis revealed two binding sites (KD: 6 nM; Bmax: 0.24 pmol/mg protein and KD: 38 nM; Bmax: 0.66 pmol/mg protein). Only GABA ligands were effective displacers of (3H)SR 95531. The respective IC50 values obtained with these compounds suggests that (3H)SR 95531 labels the GABA receptor in its antagonist conformation.

Preincubation of homogenates from rat brain thalamus and striatum affects the rate of [3H]GABA uptake

The effect of preincubation of brain homogenates on the subsequent uptake of [3H]GABA was studied in rat thalamus and corpus striatum. The results show that in both brain regions preincubation causes a decrease in the initial rate of [3H]GABA influx, the phenomenon being most evident in the thalamus.

Spontaneous release of gamma-aminobutyric acid formed from putrescine and its enhanced Ca2+-dependent release by high K+ stimulation in the brains of freely moving rats

The spontaneous release of [3H] gamma-aminobutyric acid ([3H]GABA) in various areas of rat brain injected with [3H]putrescine was examined using a push-pull perfusion technique. The release in a 25-min perfusate was highest in the caudate-putamen. The effect of high K+ stimulation on the release of [3H]GABA formed from [3H]putrescine was examined in the caudate-putamen. The release was enhanced by high K+ solution in a Ca2+-dependent manner.

Activation of GABA receptors in the hypothalamus stimulates secretion of growth hormone and prolactin

Localized intracerebral microinjections of GABA, muscimol, picrotoxin and bicuculline were made in the anterior and basal hypothalamus to determine possible sites of action of GABA in the regulation of prolactin and growth hormone (GH) secretion. Studies were carried out in unanesthetized male rats with chronic indwelling atrial cannulae and intracerebral guide cannulae which permitted stress free blood sampling and intrahypothalamic injections, respectively. Preoptic/anterior hypothalamic area. (PO/AHA) injection of muscimol (0.16 nmol) stimulated both prolactin and GH secretion. GABA (1600 nmol) stimulated prolactin. Bicuculline (0.016 and 0.16 nmol) inhibited GH secretion. Medial basal hypothalamic (MBH) injection of muscimol (0.1 and 1.0 nmol) and GABA (1000 nmol) stimulated prolactin but had no effect on GH secretion. Picrotoxin and bicuculline did not stimulate GH. These findings indicate that activation of PO/AHA GABAergic receptors facilitates secretion of GH and prolactin and activation of MBH GABAergic receptors stimulates secretion of prolactin. It is proposed that GABA inhibits somatostatin neurons in the PO/AHA to facilitate GH and inhibits tuberoinfundibular dopamine or GABA neurons in the MBH to stimulate prolactin.

Changes in gamma-aminobutyrate, glutamate, aspartate, glycine, and taurine contents in the striatum after unilateral nigrostriatal lesions in rats

Contents of five amino acids (gamma-aminobutyrate (GABA), glutamate, aspartate, glycine, and taurine) were assayed in the rat striatum in which nigrostriatal pathways were unilaterally destroyed by 6-hydroxydopamine. GABA content in the operated side was significantly increased 1 month after the surgery compared with that in the nonoperated side. Contents of four other amino acids assayed did not change significantly. Among possible mechanisms for this elevation we postulated that sprouting of the GABAergic terminals in the striatum would be most probable.

Specific [3H]gamma-aminobutyric acid binding to vestibular membranes of the chick inner ear

To support a postulated neurotransmitter character of gamma-aminobutyric acid (GABA) in the vertebrate vestibule, [3H]GABA binding was measured in a crude membrane preparation of chick inner ear ampullary cristae. In the absence of divalent cations bound [3H]GABA was displaced by unlabeled GABA, muscimol or bicuculline, but it was not displaced by (+/-)-baclofen. A single population of [3H]GABA binding sites with an equilibrium constant of 19.4 nM and a maximum binding capacity of 0.58 pmol/mg protein was found. These results suggest the possible existence of a synaptic GABAA receptor in the chick inner ear membranes and sustain the neurotransmitter role of GABA in the chick vestibule.

Autoradiography of benzodiazepine receptor binding in the central nervous system of the normal C57BL6J mouse

[3H]flunitrazepam has been used as a photoaffinity label for the specific, clonazepam-displaceable 1,4-benzodiazepine binding sites in sections of normal C57BL6J mouse brain and spinal cord. Binding was visualized by light microscope autoradiography and quantified by a simple microdensitometric procedure. Specific flunitrazepam binding was seen to be highest in the colliculi, cerebral cortex, hippocampal formation, interpeduncular nucleus, mamillary body, hypothalamus, olfactory tubercle, and in the molecular layer and deep nuclei of the cerebellum. The distribution of specific flunitrazepam binding sites in mouse brain and spinal cord is discussed in terms of the known actions of the benzodiazepines.

Saturable binding of 35S-t-butylbicyclophosphorothionate to the sites linked to the GABA receptor and the interaction with GABAergic agents

35S-t-Butylbicyclophosphorothionate (35S-TBPS) binds in a concentration-saturable manner to specific sites on membranes from rat cerebral cortex. Using a filtration assay at 25 degrees C, in 250 mM NaCl, specific binding of 35S-TBPS constitutes about 84 to 94 percent of total binding, depending on radioligand concentrations. 35S-TBPS binding is optimal in the presence of NaCl or NaBr and substantially less in the presence of NaI or NaF. It is sensitive to the treatment with 0.05 percent Triton X-100 but not to repeated freezing and thawing, procedures which increase 3H-GABA binding. Pharmacological studies show that 35S-TBPS binding is strongly inhibited by GABA-A receptor agonists (e.g., GABA and muscimol) and by the noncompetitive antagonist, picrotoxin, but not the competitive antagonist, bicuculline. Compounds which enhance binding of radioactive GABA and benzodiazepines, such as the pyrazolopyridines, cartazolate and tracazolate, and a diaryltriazine, LY81067, are also potent inhibitors of 35S-TBPS binding, with LY81067 being the most effective. The effects of GABA, picrotoxin and LY81067 on the saturable binding of 35S-TBPS in cortical membranes are compared. The present findings are consistent with the interpretation that 35S-TBPS binds at or near the picrotoxin-sensitive anion recognition sites of the GABA/benzodiazepine/picrotoxin receptor complex.

Actions of benzodiazepines on the neuroendocrine system

The administration of benzodiazepines to rats is associated with alterations in the secretion of anterior pituitary hormones. Although basal secretion of prolactin and thyroid stimulating hormone (TSH) was not changed by benzodiazepines, the secretory response to physiological and pharmacological stimuli was attenuated. Diazepam reduced the rise in secretion of prolactin during proestrus and the increase in release induced by stress, administration of fluoxetine plus 5-hydroxytryptophan and haloperidol. Diazepam also blocked the rise in release of TSH induced by exposure to cold. To establish haloperidol. Diazepam also blocked the rise in release of TSH induced by exposure to cold. To establish the site at which benzodiazepines act, anterior pituitary binding for [3H]diazepam was characterized. Scatchard analysis indicated a Kd of 11 nM and a Bmax of 27 fmol/10(6) cells. In cultures of anterior pituitary cells diazepam enhanced the inhibition of prolactin secretion produced by gamma-aminobutyric acid (GABA) and dopamine. The potency of benzodiazepine analogues established that the benzodiazepine binding site in the pituitary belonged to the peripheral subtype of benzodiazepine receptors. However, the potency of analogues for inhibition of the release of prolactin and TSH following systemic administration indicates that benzodiazepines with affinity for the central subtype of receptors were the most potent. These data are interpreted to suggest that the actions of benzodiazepines on the neuroendocrine system result from their actions within the CNS on the neurons involved in the regulation of pituitary function. In addition, peripheral benzodiazepine receptors are present on cells of the anterior pituitary and these binding sites can modulate secretion of prolactin in response to its inhibitory regulators.

Biochemical and electrophysiological characteristics of mammalian GABA receptors

The concept that GABA is a neurotransmitter in the mammalian CNS is supported by both electrophysiological and biochemical data. Whereas the electrophysiological studies are essential for demonstrating a specific functional response to GABA, the biochemical approach is useful for characterizing the molecular properties of this site. As a result of these studies the concept of the GABA receptor has progressed from a simple model of a single recognition site associated with a chloride channel to a more complex structure having a variety of interacting components. Thus, both electrophysiological and biochemical data support the existence of at least two pharmacologically distinct types of GABA receptors, based on the sensitivity to bicuculline. Also, anatomically, there appear to be two different types of receptors, those located postsynaptically on the soma or dendrites of a neighboring cell and those found presynaptically on GABAergic and other neurotransmitter terminals. From biochemical studies it appears that the GABA receptor may be composed of at least three distinct interacting components. One of these, the recognition site, may exist in two conformations, with one preferring agonists and the other having a higher affinity for antagonists. Ion channels may be considered a second component, with some of these regulating the passage of chloride ion, whereas others may be associated with calcium transport. The third major element of GABA receptors appears to be a benzodiazepine recognition site, although only a certain population of GABA receptors may be endowed with this property. In addition to these, the GABA receptor complex appears to contain substances that modulate the recognition site by influencing the availability of higher affinity binding proteins. It would appear therefore that changes affecting any one of these constituents can influence the characteristics of the others. While increasing the complexity of the system, this arrangement makes for a more sensitive and adaptable receptor mechanism. Thus the GABA receptor can be envisioned as a supramolecular complex of interacting sites, all of which contribute to the functional expression of receptor activation. Because of this complexity, GABA receptors can theoretically be modified in a variety of ways by drug treatment or disease. Accordingly, it may be possible to develop selective agonists and antagonists that may act at one of the basic components, as well as agents that may alter the receptor modulators. Conversely, a disorder of any of these entities may result in an alteration of GABA receptor function, which in turn could contribute to the symptoms of a variety of neuropsychiatric disorders.(ABSTRACT TRUNCATED AT 400 WORDS)

High specific binding of [3H]GABA and [3H]muscimol to membranes from dendrodendritic synaptosomes of the rat olfactory bulb

Olfactory bulbs contain dendrodendritic synapses, which occur between granule cells and mitral cells, and gamma-aminobutyric acid (GABA) is thought to act as an inhibitory neurotransmitter at these synapses. Synaptosomes derived from the dendrodendritic synapses of the olfactory bulb were shown previously to contain considerable L-glutamate decarboxylase activity. The subcellular distribution and binding parameters of [3H]GABA and [3H]muscimol binding sites have now been determined in the rat olfactory bulb. Of all fractions examined, crude synaptic membranes (CSM) prepared from the dendrodendritic synaptosomes were shown to have the highest specific binding activity and accounted for nearly all of the total binding activity for both ligands. The specific binding activities for [3H]GABA and for [3H]muscimol were greatly increased after treating the CSM with 0.05% Triton X-100. Binding was shown to be Na+-independent, reversible, pharmacologically specific, and saturable. High- and low-affinity sites were detected for both ligands, and both classes of sites had appreciably lower KD values for muscimol (KD1 = 3.1 nM, KD2 = 25.1 nM) than for GABA (KD1 = 8.6 nM; KD2 = 63.7 nM). The amounts of the high-affinity binding sites for muscimol and GABA were similar (Bmax = 1.7 and 1.5 pmol/mg protein, respectively). The results of the present experiments indicate that the GABA and muscimol binding sites represent the GABA postsynaptic receptor, presumably on mitral cell dendrites, and provide further support for the hypothesis that GABA functions as a neurotransmitter at the dendrodendritic synapses in the olfactory bulb.

Neurotransmitter receptor ligand binding and enzyme regional distribution in the pigeon visual system

The relative importance of acetylcholine, dopamine, endogenous opiates, gamma-aminobutyric acid (GABA), glutamate, glycine, noradrenaline, and serotonin as transmitters in the pigeon visual system was estimated by measuring the activity of choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD), and aromatic amino acid decarboxylase (AAD) as well as the binding of dihydroalprenolol, etorphine, kainic acid, muscimol, serotonin, spiroperidol, strychnine, and quinuclidinyl benzilate (QNB) in the tectum opticum, nucleus rotundus, ectostriatum, dorsolateral thalamus, and hyperstriatum (Wulst). As a nonvisual reference structure, the paleostriatal complex was included in the examination. The regional distribution of most of these parameters was very similar to data reported in the mammalian CNS supporting the hypothesis that the avian tectofugal and thalamofugal visual systems are homologous to the mammalian tecto-thalamo-cortical and retino-geniculo-striate pathways, respectively. On the basis of the low values of their parameters, some transmitters can be excluded as significant contributors in a number of structures. As a hypothesis for further investigations, the presence of cholinergic and serotoninergic systems in the Wulst, possibly originating in the dorsolateral thalamus and nucleus raphe, respectively, and of glycinergic and dopaminergic terminals in the paleostriatal complex is proposed.

The effect of intraperitoneal injection of GABA or GHBA on monoamine histofluorescence in the rat brain

Using the Falck-Hillarp histofluorescence method we studied the effect of gamma-aminobutyric acid (GABA) or gamma-hydroxybutyric acid (GHBA) on catecholaminergic (CA) and serotoninergic (5-HT) neurons in the rat brain. Both GABA as well as GHBA enhanced the CA fluorescence in noradrenergic (NA) terminals in the cerebellar cortex, which suggested that the NA content increased in these terminals. GHBA but not GABA also enhanced the fluorescence in dopaminergic (DA) terminals in the neostriatum and limbic structures. No significant changes of fluorescence were found in 5-HT neurons after both compounds. The results obtained suggest that, apart from the known GABA-DA interaction, there exists also a GABA-NA interaction in the cerebellar cortex.

Neurotransmitter, receptor and biochemical changes in monkey cortical epileptic foci

Epileptic and normal Macaca mulatta monkey cortex was investigated using ligand binding techniques. Subpial injections of aluminum hydroxide gel into the left sensorimotor cortex produced stable seizure frequencies over a two year period and resulted in specific biochemical and receptor abnormalities. Pair matched CSF samples comparing epileptic and non-epileptic hemispheres showed a significant decreased GABA concentration over the epileptic side. The epileptic cortex demonstrated markedly reduced GABA receptor binding and diminished tissue GABA concentration and GAD activity. Two patterns of receptor loss were observed: nonspecific local cellular drop out involving multiple neurotransmitter receptors; and distal receptor loss which was specific for the neurotransmitter intervention pattern of the cortex. GABAergic receptor loss was more marked than receptor losses for the other neurotransmitter and was more widespread. Scatchard plot analysis demonstrated that the diminished GABAergic receptors within the focus were due to receptor loss and not affinity changes. Spearman rank correlations showed a significant correlation only between the degree of GABAergic receptor loss or decrease in GAD activity and the seizure frequency. Epilepsy appears to be a multifactoral disorder with multiple neuroreceptor abnormalities, the most notable of which are the destruction of GABAergic neurons and GABA receptors.

Glutamic acid decarboxylase in sea lamprey (Petromyzon marinus): characterization, localization, and developmental changes

We have carried out assays for glutamic acid decarboxylase (GAD) in homogenates of brain and spinal cord from larval and adult sea lamprey (Petromyzon marinus). The enzyme had similar characteristics in both stages. Optimal pH was 6.8; optimal temperature was 27-30 degrees C; Km at 27 degrees C was 5 mM. GAD activity was distributed uniformly along the length of the spinal cord. Specific activities for the larval cord and brain were 26 and 63 nm CO2/mg protein/h, respectively. The specific activities for the adult cord and brain were 29 and 236 nm CO2/mg protein/h, respectively. Thus, the activity of cord homogenates did not change significantly between larval and adult stages, but that of the brain increased about fourfold.

Distribution of GABA receptors in the rat cerebellum

The distribution of GABA receptors in the cerebellum is not homogeneous. In comparison with detergent-treated membranes from the whole tissue the number of [3H]muscimol binding sites per mg protein (Bmax) is about doubled in preparations enriched in large fragments of the cerebellar glomeruli, and it is about one-third in the dissected deep nuclei. On the other hand, the apparent affinity (Kd) is similar in the different preparations. Comparison of the results with earlier studies suggests a heterogeneity in cerebellar GABA receptors and/or their control.

Discrete regional analysis of norepinephrine, dopamine, choline acetyltransferase and glutamic acid decarboxylase in the brain of the newborn and pubescent monkey

A method is described for the removal of discrete areas of the monkey brain. A detailed mapping of norepinephrine, dopamine, choline acetyltransferase and glutamic acid decarboxylase in the newborn and pubescent monkey brain is presented.

Phylogenetic distribution of bicuculline-sensitive gamma-amino-butyric acid (GABA) receptor binding

Bicuculline-sensitive [3H]GABA receptor binding was studied in membrane fractions prepared from vertebrate whole brain or invertebrate cephalic ganglia. In tissue not treated with Triton X-100, a significant amount of bicuculline-displaceable [3H]GABA binding was detected in the brains of all vertebrates studied, with the hagfish brain binding over twice as much [3H]GABA as the spiny dogfish, the next oldest species. All other vertebrates bound similar amounts of [3H]GABA, being one-third to one-fourth that observed in the hagfish. In contrast, after Triton treatment, the hagfish displayed the least amount of bicuculline-sensitive [3H]GABA binding and, under those conditions, the amount of binding observed increased in an evolutionary fashion. No measurable bicuculline-sensitive GABA receptor binding was noted in any invertebrate studied. These results suggest that bicuculline-sensitive GABA receptors are present in the brains of all vertebrates and that during the course of evolution there developed a Triton-sensitive substance(s) whose presence modifies the kinetic properties of this receptor site.

An in vivo method for testing putative GABA-like compounds

A technique is described that enables compounds with GABA-ergic properties to be rapidly identified in vivo. Electrical stimulation of the neostriatum in the conscious rat evoked a contralateral head-turn. Evidence is presented that this easily timed motor response involves, at least in part, GABA-ergic mechanisms in the globus pallidus. GABA drugs were injected through a cannula into the ipsilateral globus pallidus and their effects on head-turning observed. Known GABA agonists including muscimol slowed the head-turn, whereas the GABA antagonist picrotoxin facilitated it. A number of drugs such as baclofen, diazepam, and pentobarbitone which have been attributed with GABA-like properties behaved like GABA agonists in the head-turn model following either intrapallidal or intraperitoneal injection. Other drugs, e.g. tranquillizers, with no known GABA-like properties, did not effect the head-turn time.

Characterization of 3H-GABA receptor binding to rat brain synaptosomal membranes: effect of non GABAergic compounds

Characteristics of 3H-GABA binding to rat brain synaptic membranes in vitro have been investigated. The specific binding of 3H-GABA displays saturation kinetics. Only one single population of receptor sites was found (Km = 31.3 nM) with a concentration of 2.09 pmol/mg protein. Only GABA agonists show inhibitory effect on the binding, whereas GABA antagonists, GABA-uptake inhibitors, and inhibitors of GAD and GABA-T are without effect. The order of potencies for GABA agonists are: Muscimol greater than GABA greater than or equal to 4,5-dihydromuscimol greater than 3-aminoproprane sulphonic acid greater than isoguvacine greater than THIP greater than 3-hydroxy-GABA greater than imidazol-4-acetic acid. Agonists and antagonists from other neurone systems as well as neuroleptics and benzodiazepines had no or only a very slight potency in the binding test.

In vivo release of endogenous GABA in the cat hypothalamus

The posterior hypothalamus of anaesthetized cats was superfused with artificial cerebrospinal fluid through a push-pull cannula and the release of endogenous GABA from the hypothalamus into the superfusate was studied. The resting release of GABA varied rhythmically, since phases of high rate of release were separated from each other by phases of low rate of release. The time interval between two adjacent phases of high rate of release was about 70 min. Electrical stimulation of the posterior hypothalamus with the tip of the cannula enhanced the rate of release of GABA in a frequency-dependent way. Superfusion of the hypothalamus with CSF which contained a high concentration of potassium and a low concentration of sodium increased the rate of release of GABA; this effect was dependent on the presence of calcium ions in the superfusing fluid. Pretreatment of the cats with reserpine reduced the levels of GABA in hypothalamus and rest of brain and the concentration of GABA in the superfusate as well. Stimulation of the locus coeruleus with a bipolar electrode elicited an increased release of GABA in the hypothalamus.