Molecular biology of inhibitory amino acid receptors

Septal co-infusions of glucose with the benzodiazepine agonist chlordiazepoxide impair memory, but co-infusions of glucose with the opiate morphine do not

We have found repeatedly that medial septal (MS) infusions of glucose impair memory when co-infused with the gamma-amino butyric acid (GABA) agonist muscimol. The present experiments sought to determine whether the memory-impairing effects of this concentration of glucose would generalize to another GABA(A) receptor agonist and to an agonist from another neurotransmitter system that is known to impair memory. Specifically, we determined whether the dose of glucose that produces memory deficits when combined with muscimol in the MS would also impair memory when co-infused with the GABA(A) receptor modulator chlordiazepoxide (CDP) or the opiate morphine. Male Sprague-Dawley rats were given MS co-infusions and then 15 min later tested for spontaneous alternation or given shock avoidance training (retention tested 48 h later). The results showed that MS infusions of the higher dose of glucose with morphine did not produce memory deficits, whereas, the performance of rats given MS co-infusions of CDP with glucose was impaired. These findings suggest that the memory-impairing effects of brain glucose administration may involve an interaction with the GABA(A) receptor.

Immunohistochemical localization of the beta 2 and beta 3 subunits of the GABAA receptor in the basolateral amygdala of the rat and monkey

The basolateral amygdala has a strong intrinsic inhibitory system mediated by GABAA receptors and is the main site of the anxiolytic actions of benzodiazepines. In an effort to identify the anatomical substrates for these transmitter and drug actions, immunohistochemical techniques were used to analyse the neuronal localization of the beta 2 and beta 3 receptor subunits of the GABAA-benzodiazepine receptor complex in the rat and monkey basolateral amygdala. The overall pattern of GABAA-benzodiazepine receptor immunoreactivity was very similar in both species. The density of the immunoreactivity in the neuropil varied in different nuclei of the basolateral amygdaloid complex. In both species the neuropil of the lateral nucleus exhibited the most robust staining. Immunoreactivity was also seen in neuronal perikarya and dendrites where it was localized to the cytoplasm and/or surface membrane. The cell type with the strongest immunoreactivity was a subpopulation of small non-pyramidal neurons that had numerous thin dendrites. Other larger non-pyramidal neurons were also stained. Pyramidal neurons in the rat and monkey basolateral amygdala exhibited light to moderate perikaryal staining that varied in different nuclei. The results of this study indicate that the pattern of GABAA-benzodiazepine receptor immunoreactivity in the neuropil of the rat and monkey basolateral amygdala closely resembled the distribution of benzodiazepine receptors localized in previous radioligand autoradiographic studies. The finding of intense immunoreactivity in subpopulations of non-pyramidal neurons suggests the existence of disinhibitory mechanisms which may be important for the activation of basolateral amygdaloid projection neurons.

GABA receptor molecules of insects

Receptors for 4-aminobutyric acid (GABA) have been identified in both central and peripheral nervous systems of several invertebrate phyla. To date, much of the information derived from physiological and biochemical studies on insect GABA receptors relates to GABA-gated chloride channels that show some similarities with vertebrate GABAA receptors. Like their vertebrate central nervous system (CNS) counterparts, agonist activation of such insect GABA receptors leads to a rapid, picrotoxin-sensitive increase in chloride ion conductance across the cell membrane. In insects, responses to GABA can be modulated by certain benzodiazepines and barbiturates. However, recent studies have detected a number of striking pharmacological differences between GABA-gated chloride channels of insects and vertebrates. Receptor binding, electrophysiological and 36Cl- flux assays have indicated that many insect receptors of this type are insensitive to the vertebrate GABAA antagonists bicuculline and pitrazepin. Benzodiazepine binding sites coupled to insect GABA receptors display a pharmacological profile distinct from that of corresponding sites in vertebrate CNS. Receptor binding studies have also demonstrated differences between convulsant binding sites of insect and vertebrate receptors. Insect GABA receptor molecules are important target sites for several chemically-distinct classes of insecticidally-active molecules. By characterizing these pharmacological properties in detail, it may prove possible to exploit differences between vertebrate and insect GABA receptors in the rational design of novel, more selective pest control agents. The recent application of the powerful techniques of molecular biology has revealed a diversity of vertebrate GABAA receptor subunits and their respective isoforms that can assemble in vivo to form a multiplicity of receptor subtypes. Molecular cloning of insect GABA receptor subunits will not only enhance our understanding of invertebrate neurotransmitter receptor diversity but will also permit the precise identification of the sites of action of pest control agents.

Intraseptal injection of GABA and benzodiazepine receptor ligands alters high-affinity choline transport in the hippocampus

Injection of GABA and benzodiazepine (BDZ) agonists and antagonists into the medial septum produced bidirectional alterations in hippocampal high-affinity choline transport (HAChT). Male Sprague-Dawley rats were injected in the medial septum with either drug vehicle, a BDZ agonist, antagonist, or inverse agonist, or with a GABA-A or GABA-B agonist or antagonist and sacrificed 1 h later for assessment of HAChT in hippocampal synaptosomes. The GABA-A agonist muscimol, the GABA-B agonist baclofen, and the BDZ agonist chlordiazepoxide (CDP) produced dose-related decreases in HAChT 1 h following injection into the septum. The muscimol-induced decrease in HAChT was prevented by prior intraseptal injection of the GABA-A antagonist, bicuculline. Intraseptal injection of GABA-A (bicuculline) or GABA-B (2-hydroxysaclofen) antagonists did not alter HAChT, whereas the BDZ antagonist flumazenil (RO15,1788) and the BDZ inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) increased this measure up to 30% in a dose-dependent manner. These results demonstrate that cholinergic neurons in the medial septum can be modulated in a bidirectional way through the pharmacological manipulation of GABA-A, GABA-B, and BDZ receptors. The potential functional and therapeutic consequences of these interactions are discussed.

Chlordiazepoxide-induced working memory impairments: site specificity and reversal by flumazenil (RO15-1788)

The following studies examined the dose and time dependence, site specificity, and reversibility of chlordiazepoxide (CDP)-induced working memory impairments in adult male Sprague-Dawley rats. The rats were tested in a delayed non-match-to-sample radial-arm maze task in which a 1-h delay was imposed between the first four (predelay) and all subsequent (postdelay) arm choices. Intraperitoneal (ip) injection of 2.5 or 5.0 but not 1.25 mg/kg CDP immediately following the predelay session impaired performance in the task. CDP increased the number of errors and decreased the number of correct choices during the postdelay session. The observed working memory impairments also appeared to be site specific since injection of CDP into the medial septum, but not into the anterior amygdala nuclei, immediately following the predelay session also impaired working memory in a dose-related manner. Furthermore, there was a time window for CDP-induced working memory impairments since intraseptal injection of the drug immediately but not 15 min following the predelay session disrupted memory. This observation suggests that the performance deficits reflect disrupted working memory and not proactive effects on performance or the induction of state-dependent learning. In the final experiment, rats were injected ip with either saline or an amnestic dose of CDP (5.0 mg/kg) following the predelay session and then were immediately infused with 10 nmol flumazenil (RO15-1788), a benzodiazepine receptor antagonist or vehicle, into either the medial septum or anterior nuclei of the amygdala. Intraseptal injection of flumazenil prevented the working memory impairments produced by ip injection of CDP. In contrast, intra-amygdala injection of flumazenil did not attenuate, enhance, or modify the CDP-induced working memory impairment. These observations suggest that CDP disrupts working memory by interacting with benzodiazepine receptors in the medial septum.

Involvement of tryptophan residue(s) in the specific binding of agonists/antagonists to 5-HT3 receptors in NG108-15 clonal cells

Chemical modification of the 5-HT3 receptors in membranes from NG108-15 hybridoma cells was achieved using protein modifying reagents specific for various amino acid residues: N-bromosuccinimide for tryptophan, dithiothreitol for cystine, sodium tetrathionate for cysteine, 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline for aspartic and glutamic acids, diethylpyrocarbonate for histidine, tetranitromethane for tyrosine and 2,3-butanedione for arginine. Among all the reagents tested, N-bromosuccinimide produced the largest alteration in the specific binding of [3H]zacopride onto 5-HT3 receptors. A significant reduction in Bmax (approximately 50%) with no change in Kd were noted on [3H]zacopride specific binding to membranes which were incubated with 40 microM N-bromosuccinimide for 60 min at 25 degrees. The occupancy of 5-HT3 receptor binding sites by various 5-HT3 agonists and antagonists (phenylbiguanide, ondansetron, granisetron, MDL 72222) prevented, at least partially, any subsequent reduction in [3H]zacopride specific binding by N-bromosuccinimide treatment. However, neither m-chloro-phenylbiguanide, among the agonists, nor zacopride, among the antagonists, were able to prevent the effect of N-bromosuccinimide, suggesting that variations might exist in the molecular mechanisms implicated in the binding of 5-HT3 ligands to the recognition site on 5-HT3 receptors. Nevertheless, these data support the suggestion that tryptophan residue(s) are probably involved in the binding of agonists and antagonists onto 5-HT3 receptors in NG108-15 cell membranes.

Pharmacological and molecular basis for dopamine D-2 receptor diversity

This review will focus on the main lines of evidence that suggest the existence of multiple types of dopamine D-2 receptors. Dopamine D-2 receptors share structural elements suggesting that they belong to a gene superfamily classified as G-protein-coupled receptors and show an archetypical topology predicted to consist of seven putative transmembrane domains. Activation of D-2 receptors results in a variety of responses, including inhibition of cyclic AMP formation, inhibition of phosphoinositol turnover, increase of K-channel activity, and inhibition of Ca influx. The G protein(s) linking the D-2 receptors to these responses have not been completely identified, nor has the possible hierarchy of these regulatory proteins in transforming the incoming signal into a change of second-messenger levels. A lot of experimental data support the hypothesis that there are multiple signal-processing pathways activated by dopamine through D-2-receptor stimulation. Recently, the identification of dopaminergic drugs that discriminate among the different transduction pathways and the isolation of distinct cDNAs encoding proteins that share binding profile indicative of D-2 receptors clearly indicate multiple forms of D-2 receptors. Pharmacologically, at least two distinct categories of dopamine D-2 receptors exist in rat pituitary. The first (D-2a) is insensitive to BHT 920 and coupled to inhibition of adenylyl cyclase activity; the second (D-2b) is activated by BHT 920 and linked to voltage-dependent K channels. The two types of dopamine D-2 receptors differ in their structure, G-protein-coupled and effector. Each of the three basic receptor units shows a certain degree of heterogeneity, which may affect the quality and the kinetic of the response. This variety may represent the molecular basis for the diversity in pharmacological and functional profiles of different dopamine D-2 receptors located in various brain areas and peripheral tissues.

Beta-alanine potentiation of [3H]flunitrazepam binding to rat spinal cord homogenates

The effect of beta-alanine, gamma-aminobutyric acid (GABA), and other functionally related amino acids on [3H]flunitrazepam binding to rat spinal cord homogenates was studied. beta-Alanine potentiated [3H]flunitrazepam binding by 40% and GABA by 88%. Taurine increased the binding by 19%. Hypotaurine produced an 11% increase. No significant effect was seen in glycine, alanine, serine, valine or the dipeptide carnosine. The beta-alanine increase in [3H]flunitrazepam binding was completely inhibited by 10 microM strychnine, whereas the GABA increase required 0.1 mM strychnine to be fully suppressed. Results suggest that beta-alanine specifically potentiates binding of [3H]flunitrazepam in rat spinal cord homogenates.

Differential effect of zinc on the vertebrate GABAA-receptor complex

1. gamma-Aminobutyric acid (GABA) responses were recorded from rat superior cervical ganglia (SCG) in culture using the whole cell recording technique. 2. Zinc (50-300 microM) reversibly antagonized the GABA response in embryonic and young post-natal neurones, while neurones cultured from adult animals were far less sensitive and occasionally resistant to zinc blockade. Cadmium (100-300 microM) also antagonised the GABA response, while barium (100 microM-2 mM) was ineffective. 3. The differential blocking effect of zinc on cultured neurones of different ages also occurred in intact SCG tissue. 4. The GABA log dose-response curve constructed with foetal or adult cultured neurones was reduced in a non-competitive manner by zinc. This inhibition was minimally affected by the membrane potential. 5. The GABA response recorded intracellularly from guinea-pig pyriform cortical slices was enhanced by zinc (300-500 microM), which occurred concurrently with a decrease in the input conductance of the cell. The enhancement was unaffected by prior blockade of the GABA uptake carrier by 1 mM nipecotic acid. This phenomenon could be reproduced by barium (300 microM) and cadmium (300 microM). 6. We conclude that the vertebrate neuronal GABAA-receptor becomes less sensitive to zinc with neural (GABAA-receptor?) development, and the enhanced GABA response recorded in the CNS is a consequence of the reduction in the input conductance and not due to a direct effect on the receptor complex.

Intraseptal administration of muscimol produces dose-dependent memory impairments in the rat

The present study examined the effects of intraseptal administration of the GABAergic agonist muscimol on performance of a radial-arm maze (RAM) task. Male Long-Evans rats were trained to perform a RAM task in which a 1-h delay was imposed between the sample and the test session. In this task rats have access to four out of eight maze arms during a predelay session. Following a 1-h delay, rats are returned to the maze and allowed to freely choose among all eight arms. Arms not blocked during the predelay session are baited, and entry into an arm chosen during the predelay session or a repeated entry into a postdelay chosen arm constitutes an error. Following acquisition, animals were implanted with a single cannula aimed at the medial septum. A within-subjects design was utilized to examine the effects of intraseptal administration of muscimol (0.0, 0.75, 1.5 or 3.0 nmol) on performance in this task. All drugs or artificial cerebrospinal fluid were administered immediately following the predelay session. Muscimol, a GABA-A agonist, produced a dose-dependent impairment in maze performance as evidenced by fewer correct choices in the first four postdelay choices and an increase in the number of errors. Intraseptal administration of muscimol did not significantly alter latency per choice on the RAM task nor did it affect locomotor activity levels. Muscimol-induced impairments were also observed when a 4-h delay was imposed between the fourth and the fifth maze selection, suggesting that the behavioral deficit represents an inability to store or retain spatial working memories rather than a general performance deficit. These data indicated that pharmacological manipulation of GABA-A receptors within the medial septum modifies working memory processes. The potential interaction of GABAergic and cholinergic mechanisms in the modulation of working memory processes is discussed.

n-[3H]butyl-beta-carboline-3-carboxylate, a putative endogenous ligand, binds preferentially to subtype 1 of central benzodiazepine receptors

Synthetic n-butyl beta-carboline-3-carboxylate, an endogenous central benzodiazepine receptor inhibitor found in brain, was tritium-labeled from the butenyl ester. Binding of this [3H]beta-carboline was concentrated particularly in the synaptosomal membrane fraction of the cerebral cortex; this fraction showed a single type of high-affinity site (KD = 2.7 +/- 0.1 nM) with a Bmax of 1.16 +/- 0.08 pmol/mg of protein. The number of sites labeled was about half of that obtained with [3H]flunitrazepam binding (Bmax = 2.36 +/- 0.06 pmol/mg of protein). On the other hand, in the cerebellum, both ligands bound to practically the same number of sites. When [3H]flunitrazepam binding was done in the presence of 10(-11)-10(-5) M butyl beta-carboline, the differences between the two brain regions were more apparent. In cerebellar membranes the data fitted a straight line in the Eadie-Hofstee plot; this finding and a Hill number near unity suggest a single type of binding site. In the cortical membranes the data of binding fitted a concave curve, and the Hill number was 0.6. These are characteristics of two types of binding sites with different affinities (KD1 = 0.6-1.5 nM and KD2 = 12-18 nM). The differentiation of a high- and low-affinity site in the cerebral cortex was corroborated by experiments in which [3H]butyl beta-carboline binding was displaced by the triazolopyridazine CL 218,872. These results demonstrate that in the cerebral cortex there are two subtypes of sites (1 and 2) of central benzodiazepine receptors and that CL 218,872 binds preferentially to subtype 1.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Steroid modulation of the GABA/benzodiazepine receptor-linked chloride ionophore

Recent findings suggest that steroids with sedative-hypnotic properties interact specifically with the gamma-aminobutyric acidA/benzodiazepine receptor-chloride ionophore complex (GBRC). They show positive heterotropic cooperativity by allosterically enhancing the binding of GABA agonists and the clinically useful benzodiazepines (BZs) to their respective recognition sites. These steroids have stringent structural requirements for activity at the GBRC, with the essential requirements for high potency being a 3 alpha-hydroxyl group and a 5 alpha-reduced A-ring. Some of these steroids are naturally occurring metabolites of progesterone and deoxycorticosterone and have nanomolar potencies as potentiators of chloride channel conductance. These 3 alpha-hydroxylated, 5 alpha-reduced steroids do not act through any known sites on the GBRC. Thus, the exact site and mechanism of action remain to be determined. Together with the observation that physiological levels of these metabolites are sufficient to influence the function of the GBRC, the evidence clearly suggests a role for these steroids in the normal regulation of brain excitability by potentiating the postsynaptic effects of gamma-aminobutyric acid (GABA). Pharmacological studies of the GBRC-active steroids show that they possess anxiolytic and anticonvulsant activities. The potential therapeutic application of these steroids in the treatment of mood disorders and catamenial exacerbation of seizures associated with the menstrual cycle is discussed. Collectively, the evidence from the studies of these steroids imply that another mechanism by which the endocrine system influences brain function has been identified. Its characterization will provide important insight into how steroids modulate brain excitability under normal and pathophysiological states.

Isolation of a cDNA clone for the alpha subunit of the human GABA-A receptor

A cDNA clone of an alpha subunit of the human GABA-A receptor has been isolated. The human clone (pCLL800) contains 1055 nucleotides in an open reading frame and 260 nucleotides in the 5' non-coding region. The 351 amino acid sequence of this human alpha subunit shows 97% homology with its bovine counterpart. Hybridization of pCLL800 to Northern blots shows a 3.9/4.3 Kb RNA doublet in human cortex, rat whole brain, cortex, hippocampus, midbrain, olfactory bulb and cerebellum. Developmental studies show that the levels of the rat alpha mRNA increase between one and three weeks of age in a manner similar to the development of the benzodiazepine binding sites.

Comparison of the molecular structure of GABA/benzodiazepine receptors purified from rat and human cerebellum

The use of improved affinity chromatographic techniques has allowed for the substantial copurification of both the benzodiazepine and the GABA receptor from brain. These preparations have been used to begin characterization of the benzodiazepine receptor at a molecular level. We have recently purified benzodiazepine receptor from human and rat cerebellum, and SDS-PAGE has revealed that both preparations consist of a major protein of 50 kDa and a minor protein of 55 kDa. These proteins are recognized by a series of monoclonal antibodies prepared against the rat benzodiazepine receptor suggesting the rat and human receptors share several common antigenic domains. Several other approaches have been employed to further investigate possible homology between the rat and human receptors. Proteolytic degradation studies have shown that the major limiting photolabeled peptide fragment generated in rat and human is similar as determined by HPLC analysis. Isoelectric focusing and SDS (two-dimensional) electrophoresis have revealed that the immunoreactive, photolabeled 50 kDa protein, and the purified receptor have identical PI values. The receptor from both human and rat are glycoproteins as determined by lectin binding studies. However, exposure of these proteins to neuraminidase fails to alter the pharmacology of the receptors indicating possible similarities in their posttranslational glycosylation. Thus, it appears that some degree of structural homology exists between the rat and human benzodiazepine receptors.