Gamma-aminobutyric Acid Receptor Sites Research Articles

A biochemical rationale for the interaction between gastrointestinal yeast and autism

Autism is a disorder characterized by difficulty with social interactions, difficulty expressing empathy and intimacy and, in many cases, mild to severe language and learning deficits. Current estimates suggest autism now affects approximately one in 88 children, with rates increasing rapidly, making autism one of the most common and devastating developmental disorders. This trend is especially alarming considering that a cause for this disorder has yet to be discovered, nor are there successful biological treatments. Here a possible biochemical etiology is proposed for a certain spectrum of autism based on a reaction between propionic acid and ammonia released by Candida albicans in the gastrointestinal tract. A reaction between ammonia and propionic acid should result in the production of beta-alanine, a chemical similar in composition to gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter which has been shown to be present in higher quantities in autistic patients. Assuming beta-alanine is able to cross the blood–brain barrier, beta-alanine would be used in the brain as a partial antagonist, blocking the receptor sites for GABA, thus facilitating the production of more GABA to achieve equilibrium. An excess of GABA has been proposed as a possible contributor to autism. Further research should be conducted with this hypothesis to determine whether the chemical reaction in the human body between propionic acid and ammonia does in fact produce a chemical structurally and functionally similar to beta-alanine, as well as how this product affects the brain. Positive conclusions from this follow-on research could result in a preventative screening test for sensitivity to propionic acid and gastrointestinal yeast, thus slowing the progression of this type of autism. A more targeted treatment for children already diagnosed with autism could also result.

Benzodiazepine receptor sites in the chick optic lobe: Development and pharmacological characterization

To investigate the interaction between gamma-aminobutyric acid (GABA) and benzodiazepine (BZD) receptor sites during development, the time-course of appearance of flunitrazepam (FNZ) binding sites and their pharmacological characterization were studied in developing chick optic lobe. At the earliest stage examined, embryonic day (Ed) 12, the receptor density was 30.9% (0.05 +/- 0.01 pmol/mg protein) of that found in the chick optic lobes of adult chicks. The adult value was achieved on Ed 16 (0.16 +/- 0.01 pmol/mg protein). After this stage there was a sharp and transient increase in specific [3H]FNZ binding of about two-fold reaching a maximal value between hatching and the postnatal day (pnd) 2 (0.33 +/- 0.01 pmol/mg protein). Scatchard analysis at different stages of development revealed the presence of a single population of specific FNZ binding sites. The increase in [3H]FNZ binding during development was due to a large number of binding sites while their affinity remained unchanged. Competition experiments in the chick optic lobe revealed that the order of potency for displacement of specific [3H]FNZ binding paralleled the pharmacological potency of the BZDs tested. The IC50s for clonazepam, flunitrazepam, Ro 15-1788 and chlordiazepoxide were 3.02, 4.30, 0.32, and 4778.64 nM respectively. Ro 5-4864, a potent inhibitor of BZD binding to peripheral tissues, had no effect on specific [3H]FNZ binding indicating that only central BZD binding sites are present in the chick optic lobe. The peak of maximal expression of BZD receptor sites precedes in 5-6 days the peak of GABA receptor sites indicating a precocious development of BZD receptor sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Predominant Epithelial Localization of Type A Gamma-Aminobutyric Acid Receptor Sites within Rat Seminal Vesicles and Prostate Glands

In order to understand the role of the gamma-aminobutyric acid (GABA)-ergic system in the reproductive organs of the male, the biochemical characteristics and the autoradiographic localization of GABA type A (GABAA) receptor sites were studied in sections of the rat epididymis, seminal vesicles and prostate gland (lateral lobe) using 3H-muscimol as a ligand. No specific 3H-muscimol binding occurred in the epididymis. 3H-muscimol was bound to sections of seminal vesicles and prostate glands in a manner consistent with the labeling of specific GABAA receptors. Binding was saturable, reversible, with Kd values of 27.5 +/- 2.0 and 26.0 +/- 1.9 nmol/l and Bmax values of 230 +/- 21 and 160 +/- 13 fmol/mg protein for seminal vesicles and prostate glands, respectively. The rank order of potency of GABAergic drugs in inhibiting 3H-muscimol binding to sections of seminal vesicles and prostate glands was also consistent with the labeling of GABAA receptors. Autoradiography demonstrated the predominant accumulation of silver grains in the epithelium of seminal vesicles and prostatic glandular tissue. These findings further support the suggestion of a possible role of GABA in the male reproductive function. The predominant epithelial localization of GABAA receptors in the seminal vesicles and in the prostate gland allows us to hypothesize that GABA may be involved in the secretory activities of these glands.

Structure-epileptogenicity relationship of quinolones with special reference to their interaction with gamma-aminobutyric acid receptor sites

The relationship between the chemical structure and epileptogenic activity of quinolones was investigated. When the quinolones were administered intravenously to mice concomitantly with oral biphenylacetic acid, a major metabolite of the nonsteroidal antiinflammatory drug fenbufen, enoxacin, norfloxacin, ciprofloxacin, and pipemidic acid, which have an unsubstituted piperazine moiety at the 7 position of their parent nuclei, provoked clonic convulsions and subsequent death at doses of 6.25 mg/kg or more in a dose-dependent manner. AM-1091 and T-3262, which have an unsubstituted aminopyrrolidine moiety at their 7 positions, were less epileptogenic than the compounds listed above were. In contrast, ofloxacin, AT-4140, and nalidixic acid, which have piperazine substituted with methyl group(s) or no piperazine moiety at their 7 positions, never induced convulsions, even at doses of 100 mg/kg. Lomefloxacin, which has a 3-methyl piperazine, however, provoked convulsions at doses of 6.25 mg/kg or more. In the presence of biphenylacetic acid, all the test quinolones except nalidixic acid competitively inhibited [3H]muscimol binding to receptor sites for gamma-aminobutyric acid (GABA) in vitro. Nalidixic acid did not inhibit the binding at all, even at the highest concentration tested, i.e., 10(-4) M. The 50% inhibition doses for [3H]muscimol binding varied within 4 orders of magnitude or more, between 10(-8) to more than 10(-4) M for various compounds, and there was a close correlation between the epileptogenic activities of quinolones and their inhibitory potencies for [3H]muscimol binding to GABA receptor sites. These results indicate that the epileptogenic activity of quinolones possibly relates to the GABA-like structures of substituents at their 7 positions, which act as antagonists of GABA receptors.

Fungal tremorgens: the mechanism of action of single nitrogen containing toxins--a hypothesis.

Tremorgenic mycotoxins are fungal secondary metabolites with a specific effect on the central nervous system (CNS). Except for a few toxins that are produced by Claviceps paspali, a plant parasitic fungus, most tremorgenic mycotoxins are synthesized by common saprophytic moulds of the genera Penicillium and Aspergillus. Since these compounds produce sustained tremor in the abscence of other neurotoxic effects, several authors have suggested that they are the causative agents of a number of naturally occurring incoordination syndromes in ruminants. The nature of the tremor produced by these compounds in laboratory animals is clinically indistinguishable from that occurring naturally. In particular, the most implicated tremorgenic mycotoxins are those that contain a single nitrogen atom in their molecules. Although individual compounds within this group are produced by unrelated fungal species, they all contain a similar biologically active chemical moiety. To date, their mechanism of action is unknown, and their role in neuropharmacology has not yet been defined. However, the presence of a GABA-like conformation within their active nucleus and the limited torsional flexibility of this moiety suggest that they are partial agonists of GABA (gamma-aminobutyric acid). A hypothetical mode of action of these toxins at the GABA receptor sites is presented and discussed.

GABAA receptor sites modulating catecholamine secretion in the rat adrenal gland: evidence from 3H-muscimol autoradiography and in vivo functional studies.

The occurrence and distribution of specific 3H-muscimol binding sites, most probably identical with A type gamma-aminobutyric acid (GABA) receptors, were studied in sections of the rat adrenal gland by light microscope autoradiography. Specific binding was found primarily in the adrenal medulla, in association with chromaffin cells. A limited number of binding sites was also observed within the adrenal cortex. In urethane-anaesthetized hexamethonium-pretreated rats, intravenous GABA produced a set of 'excitatory' cardiovascular effects (increase in heart rate, force of contraction and blood pressure) which were mimicked by intravenous muscimol but not by intravenous baclofen, and were antagonized by pretreatment with bicuculline. The cardiovascular excitatory effects of intravenous GABA were unaffected by reserpine pretreatment, markedly reduced by administration of phentolamine plus propranolol, and almost completely abolished by adrenalectomy. Our findings indicate the presence of GABA receptor sites on adrenal chromaffin cells, whose excitation can produce changes in cardiovascular function.

Sodium-independent, bicuculline-sensitive [3H]GABA binding to isolated rat hepatocytes.

To determine whether hepatocytes possess specific receptor sites for gamma-aminobutyric acid (GABA), a potent amino acid neurotransmitter, [3H]GABA, was added to sodium-free suspensions of Percoll-purified hepatocytes derived from collagenase-perfused rat livers under various experimental conditions and in the presence or absence of specific GABA receptor agonists (muscimol) and antagonists (bicuculline). The effects of GABA, muscimol, and bicuculline on hepatocyte resting membrane potentials were also determined. Specific binding of [3H]GABA to hepatocytes was a consistent finding. GABA-hepatocyte interactions were reversible and temperature dependent. Muscimol and bicuculline inhibited binding in a dose-dependent manner (IC50, 30 nM and 50 microM, respectively), whereas strychnine (1.0-100 microM), a nonspecific central nervous system stimulant, had no appreciable effect. Both GABA and muscimol (100 microM) caused significant hyperpolarization of hepatocyte resting membrane potential (delta PD 5.4 +/- 3.1 and 22.2 +/- 16.2 mV, respectively, means +/- SD, P less than 0.0005). Bicuculline (100 microM) inhibited the effect of muscimol (P less than 0.05). The results of this study suggest that specific GABA receptor sites exist on the surface of isolated rat hepatocytes. The presence of such sites raises the possibility that, in addition to adrenergic and cholinergic innervation, hepatic function may be influenced by GABA-ergic neurotransmitter mechanisms.

Isothiouronium compounds as gamma-aminobutyric acid agonists.

Analogues of gamma-aminobutyric acid (GABA) incorporating an isothiouronium salt as a replacement for a protonated amino functional group have been investigated for activity on: GABA receptors in the guinea-pig ileum; [3H]-GABA and [3H]-diazepam binding to rat brain membranes; and GABA uptake and transamination. For the homologous series of omega-isothiouronium alkanoic acids, maximum GABA-mimetic activity was found at 3-[(aminoiminomethyl)thio]propanoic acid. Introduction of unsaturation into this compound gave two isomeric conformationally restricted analogues. The trans isomer was inactive at GABA receptors while the cis compound ((Z)-3-[(aminoiminomethyl)thio]prop-2-enoic acid (ZAPA)) was more potent than muscimol and GABA as a GABA agonist with respect to low affinity GABA receptor sites. Both isomers were moderately potent at inhibiting the uptake of [3H]-GABA into rat brain slices. Comparison of possible conformations of the two unsaturated isomers by interactive computer graphics modelling and comparison with muscimol has led to a plausible active conformation of ZAPA, which may be a selective and potent agonist for low affinity GABA binding sites.

Characterization of gamma-aminobutyric acid-benzodiazepine receptor complexes by protection against inactivation by group-specific reagents.

The chemical topography of the gamma-aminobutyric acid (GABA) and benzodiazepine (BZ) receptors was investigated in a thoroughly washed cortical membrane preparation of the rat. Chemical modification by several amino- and tyrosyl-selective reagents and the protection from it by direct and allosteric ligands of the GABA-BZ receptor complex were used to identify the residues at the binding sites. Inhibition of specific GABA binding by p-diazobenzenesulfonic acid (DSA), tetranitromethane (TNM), and N-acetylimidazole and the selective and complete protection from it by GABA and muscimol suggest the presence of a tyrosine residue at the GABAA site. TNM, like DSA, selectively decreased the number of the low-affinity GABA receptors, and this could be completely protected only by GABA concentrations that can saturate the low-affinity sites. TNM pretreatment also abolished the muscimol enhancement of [3H]diazepam binding, which suggests that the low-affinity GABA receptor sites are responsible for this enhancement. Inhibition of GABA binding by pyridoxal-5-phosphate (PLP) and the selective protection by GABA and muscimol support the presence of a lysine residue at the GABAA receptor site. Complete and selective protection from diethylpyrocarbonate (DEP) inhibition of [3H]diazepam binding by flurazepam suggests the presence of a histidine residue at the BZ site. Flurazepam selectively protected from inhibition of [3H]diazepam binding by N-bromosuccinimide and N-acetylimidazole, but not that by DSA and TNM, which does not allow a unanimous conclusion regarding the presence of tyrosine or tryptophan residues at the BZ site.

The effects of benzodiazepine and non-benzodiazepine anxiolytics on locus coeruleus unit activity.

Two theories have been put forth concerning the anxiolytic actions of the anti-anxiety drugs. One theory maintains that these drugs decrease locus coeruleus output, and the other maintains that they facilitate gamma-aminobutyric acid (GABA) neurotransmission at benzodiazepine (BZ)-linked GABA receptors. The BZ-anxiolytic diazepam does decrease locus coeruleus neuronal impulse flow. However, this decrease is not due to effects on BZ-linked GABA receptors in the locus coeruleus. Furthermore, the non-BZ anxiolytic buspirone, its metabolite and its analog all slightly increase locus coeruleus neuronal impulse flow. This increase, in the case of the metabolite, appears to be due, in part, to blockade of alpha 2-adrenoceptors. Finally, buspirone, unlike diazepam, did not potentiate GABA inhibition at BZ-linked GABA receptor sites (i.e. cerebellar Purkinje cells). These data suggest that the non-BZ anxiolytic buspirone produces its anti-anxiety effects by unconventional mechanisms.

Biochemical identification of pharmacologically and functionally distinct GABA receptors in rat brain

Receptor binding studies were undertaken in an attempt to identify and characterize pharmacologically and functionally distinct receptor sites for gamma-aminobutyric acid (GABA) in rat brain. The results indicated that the potency of bicuculline, a GABA receptor antagonist, to displace membrane-bound [3H]GABA varies significantly among different brain regions, with the greatest potency found in the cerebral cortex and midbrain. In addition, in the presence of 50 mM ammonium thiocyanate, the potency of bicuculline to displace specifically bound [3H]GABA was increased significantly, with the magnitude of this increase being greater in some brain areas than others. The biological relevance of this thiocyanate-induced shift in the potency of bicuculline to inhibit [3H]GABA binding was indicated by the finding that ammonium thiocyanate also increased the potency of bicuculline to inhibit GABA-activated benzodiazepine receptor binding, a biochemical measure of GABA receptor function. Receptor site saturation analysis revealed that ammonium thiocyanate selectively abolished the high affinity GABA binding site without affecting either the low affinity component or GABA-activated benzodiazepine receptor binding. These findings provide further evidence for the existence of pharmacologically distinct GABA receptor sites, with some being more sensitive to the blocking action of bicuculline than others. Furthermore, the data provide direct evidence to support the hypothesis that only low affinity GABA receptor sites are linked to the benzodiazepine receptor, indicating that the kinetically different GABA binding sites are also functionally distinct. The discovery that ammonium thiocyanate selectively destroys high affinity GABA receptor binding may be useful for further defining the pharmacological, biochemical, and functional differences between GABA receptors in brain.