Structural, developmental and functional heterogeneity of rat GABAA receptors

Expression of gamma-aminobutyric acid(A) receptor subunits in the vestibular system

OBJECTIVES

Profile the expression of genes encoding GABAA receptor subunits in the vestibular end organs of a rat.

MATERIALS AND METHODS

Using a combination of reverse transcription followed by polymerase chain reaction (PCR) with gene-specific primers, expression of mRNAs encoding 13 individual GABA(A) receptor subunits was examined.

RESULTS

PCR amplification products representing subunit gene expression for alpha1-6, beta1-3, and gamma1-3, but not for delta, subunits were amplified, suggesting multiple molecular levels of regulation of vestibular GABA(A) receptor expression. Nucleotide sequencing confirmed the identity of rat vestibular end-organs subunit cDNAs.

CONCLUSION

These results give the most direct evidence to date that GABAA receptors composed of the detected subunits are expressed in the mammalian vestibular system, giving new support to previous investigations implicating GABA as a vestibular neurotransmitter.

Ion channels in epilepsy

There are specific alterations in the structure or function of ion channels in the epileptic brain. Some of these alterations may promote hyperexcitability, whereas others may protect neurons from the deleterious effects of epileptic discharges. With the use of human tissue resected from epilepsy patients and the comparison of cellular properties to those found in well-defined experimental models, we will continue to gain insight into the specific ion channel changes associated with epilepsies. Further genetic studies will help to elucidate the altered molecular mechanisms underlying ion channel changes in this devastating neurological disorder (Noebels, 1996). Whether it is a change in structure, function, or both, the study of ion channels in epilepsies will soon reveal specific characteristics of ion channels found only in epileptic tissue. If the altered properties of such ion channels cannot be found in control (nonepileptic) neurons, these channels might be called "epileptic" ion channels. An understanding of the specific structure, function, and pharmacology of these "epileptic" channels will yield important clues for future therapeutical approaches aimed at preventing epileptogenesis, and insight into the processes whereby ion channels become "epileptic" may finally open the way to prophylactic treatments of the epilepsies.

From ion currents to genomic analysis: recent advances in GABAA receptor research

The gamma-aminobutyric acid type A (GABAA) receptor represents an elementary switching mechanism integral to the functioning of the central nervous system and a locus for the action of many mood- and emotion-altering agents such as benzodiazepines, barbiturates, steroids, and alcohol. Anxiety, sleep disorders, and convulsive disorders have been effectively treated with therapeutic agents that enhance the action of GABA at the GABAA receptor or increase the concentration of GABA in nervous tissue. The GABAA receptor is a multimeric membrane-spanning ligand-gated ion channel that admits chloride upon binding of the neurotransmitter GABA and is modulated by many endogenous and therapeutically important agents. Since GABA is the major inhibitory neurotransmitter in the CNS, modulation of its response has profound implications for brain functioning. The GABAA receptor is virtually the only site of action for the centrally acting benzodiazepines, the most widely prescribed of the anti-anxiety medications. Increasing evidence points to an important role for GABA in epilepsy and various neuropsychiatric disorders. Recent advances in molecular biology and complementary information derived from pharmacology, biochemistry, electrophysiology, anatomy and cell biology, and behavior have led to a phenomenal growth in our understanding of the structure, function, regulation, and evolution of the GABAA receptor. Benzodiazepines, barbiturates, steroids, polyvalent cations, and ethanol act as positive or negative modulators of receptor function. The description of a receptor gene superfamily comprising the subunits of the GABAA, nicotinic acetylcholine, and glycine receptors has led to a new way of thinking about gene expression and receptor assembly in the nervous system. Seventeen genetically distinct subunit subtypes (alpha 1-alpha 6, beta 1-beta 4, gamma 1-gamma 4, delta, p1-p2) and alternatively spliced variants contribute to the molecular architecture of the GABAA receptor. Mysteriously, certain preferred combinations of subunits, most notably the alpha 1 beta 2 gamma 2 arrangement, are widely codistributed, while the expression of other subunits, such as beta 1 or alpha 6, is severely restricted to specific neurons in the hippocampal formation or cerebellar cortex. Nervous tissue has the capacity to exert control over receptor number, allosteric uncoupling, subunit mRNA levels, and posttranslational modifications through cellular signal transduction mechanisms under active investigation. The genomic organization of the GABAA receptor genes suggests that the present abundance of subtypes arose during evolution through the duplication and translocations of a primordial alpha-beta-gamma gene cluster. This review describes these varied aspects of GABAA receptor research with special emphasis on contemporary cellular and molecular discoveries.

Astroglia-released factor shows similar effects as benzodiazepine inverse agonists

Media conditioned by cultured neonatal cerebral cortex microexplants (CCM) or astrocytes (ACM) contain low molecular weight (< 1,000 Da) substance(s) which inhibits the gamma aminobutyric acid (GABA)-induced inward current recorded in cerebellar granule cells and hippocampal neurons in culture using the whole-cell patch-clamp technique. This effect is specific for CCM and ACM, as medium conditioned by PC12 cells (PC12CM) does not affect the GABA response of these cells. It is also specific for GABA-induced currents because glutamate-induced currents do not change either in amplitude or in shape in the presence of CCM or ACM. The inhibitory effect on the GABA response in cerebellar granule cells of both ACM and CCM could be suppressed by flumazenil, a specific benzodiazepine (BZD) antagonist and could be mimicked by two BZD inverse agonists. These data thus demonstrate the presence of a BZD inverse agonist-like activity in CCM and ACM. This effect of ACM on different neuronal cell types was heterogenous since no detectable effect could be observed on the GABA-induced current in GABA-responsive dorsal root ganglion (DRG) neurons, presumably reflecting a functional heterogeneity of the GABAA receptors present in these different neuronal subsets. By the release of such an endogenous BZD inverse agonist-like activity, glia cells could possibly modulate GABAA receptor-mediated responses.

Heterogeneity and differential expression of the gamma-aminobutyric acidA (GABAA)/benzodiazepine receptor in the avian brain during development

1. The changes in the GABAA/benzodiazepine receptor in chicken brain during development has been studied by using 3H-flunitrazepam as the probe for the benzodiazepine modulator site and the antibodies recognizing the receptor protein. In the telencephalon and optic tectum, the proteins of 48, 50, and 51 kD were markedly labeled by 3H-flunitrazepam from embryonic day 18 to postnatal days, as revealed by photoaffinity labeling and SDS-PAGE of the brain membranes; the 51-kD protein appeared to be the predominant one in labeling intensity except at embryonic day 18 and postnatal days 14 and 28, whereas the 47- and 50-kD proteins were dominant in the cerebellum. However, the 47- and 48-kD proteins were faintly seen after postnatal day 28 in the three regions examined. 2. Immunoblotting using a monoclonal antibody against the 50- and 51-kD proteins showed that the straining pattern in the developing telecephalon or optic tectum was similar to the 50 kD/51 kD pattern obtained from fluorography. The antibody also stained the 50- and 51-kD proteins in the cerebellum despite the fact that the 51-kD protein was barely seen in the fluorogram. Moreover, the 50-kD protein was recognized by an antiserum raised against a partial sequence of the alpha 1 subunit of the receptor expressed in bacteria. The staining levels for the 50-kd protein by the antiserum on immunoblots of the brain regions were low in embryonic animals but higher during postnatal stages, consistent with that seen in fluorograms. 3. Receptor binding autoradiography using 3H-flunitrazepam exhibited that varying degrees of labeling intensity occurred among various brain areas at different ages. High densities of binding were present in the olfactory bulb, paleostriatum, optic tectum, and midbrain. These results support the diversity of the GABAA/benzodiazepine receptor in the vertebrate CNS.