Monoclonal antibody to GABA binding protein, a possible GABAB receptor

Immunohistochemical demonstration of GABAB receptors in the rat gastrointestinal tract

Immunohistochemical localization of GABA(B)-receptors was demonstrated in the rat gastrointestinal tract using a monoclonal antibody (GB-1) raised against the purified GABA(B)-receptor. Immunoreactive staining for GABA(B)-receptors was found in some populations of endocrine, muscular and neuronal components in the stomach and gut wall. Positive mucosal epithelial, probably endocrine, cells were distributed throughout the stomach and intestine. Double immunostaining indicated that such positive cells for GABA(B)-receptors often co-possessed serotonin in the small intestine but not in the gastric body. In the muscular layer of the digestive canal, positive staining was seen as dotty granules punctuated on the surface of muscle fibers. In the enteric nervous system, positive neuronal somata were found in both submucosal and myenteric ganglia throughout the entire canal extending from the stomach to the rectum. This is the first report to visualize the cellular localization of GABA(B)-receptors in the gastrointestinal system of the rat, and should provide a fundamental basis for future studies on gastrointestinal functions regulated by GABA(B)-receptors.

GABAB receptors

GABAB receptors are a distinct subclass of receptors for the major inhibitory transmitter 4-aminobutanoic acid (GABA) that mediate depression of synaptic transmission and contribute to the inhibition controlling neuronal excitability. The development of specific agonists and antagonists for these receptors has led to a better understanding of their physiology and pharmacology, highlighting their diverse coupling to different intracellular effectors through Gi/G(o) proteins. This review emphasises our current knowledge of the neurophysiology and neurochemistry of GABAB receptors, including their heterogeneity, as well as the therapeutic potential of drugs acting at these sites.

Structure and function of cerebral GABAA and GABAB receptors

The receptor for GABA (gamma-aminobutyric acid), an inhibitory neurotransmitter in the brain, has been classified into GABAA and GABAB types. The GABAA receptor was purified by means of affinity column chromatography using benzodiazepine as an immobilized ligand. The results indicated that the GABAA receptor consists of several subunits and forms a GABA-gated Cl- channel, which is coupled with the benzodiazepine receptor. The molecular weight of the GABAA receptor complex was estimated to be approximately 300 kDa. Furthermore, cDNA cloning of GABAA receptor subunits was performed and the primary structure of these subunits was deduced. The results suggested that these subunits possess four transmembrane domains in their structure which are important for the formation of the Cl- channel. On the other hand, activation of GABAB receptors induced the inhibition of adenylyl cyclase activity and phosphatidylinositol turnover via inhibitory GTP-binding proteins such as G(i) and/or G(o). The GABAB receptor was purified using baclofen affinity and immunoaffinity column chromatographies. It was confirmed that the purified GABAB receptor protein is about 80 kDa in its molecular weight. This protein is capable of inducing the inhibition of adenylyl cyclase when it is reconstituted with G(i)/G(o) protein in the phospholipid vesicle system. Currently available data indicate that GABAA and GABAB receptors in the central nervous system are distinct not only in terms of their molecules but also their signal transduction systems. However, the primary structure and synaptic localization of GABAB receptor molecules in the brain remain to be clarified.

Cerebral GABAB receptor: proposed mechanisms of action and purification procedures

The GABAB receptor in brain is one of the GABA receptor subtypes, and has been found to be negatively coupled to adenylate cyclase and phosphatidylinositide turnover. This receptor easily solubilizes from cerebral synaptic membrane preparations by 3-[(3-cholamidopropyl) dimethylammonio]-1-propanesulfonate (CHAPS) in the presence of asolectin. GABAB receptor solubilized from bovine cerebral cortex was purified using baclofen-coupled affinity beads (baclofen-coupled Toyopearl beads). Using these procedures, almost pure GABAB receptor (80 KDa protein) was obtained in the affinity eluate. A monoclonal antibody has been also raised against the purified GABAB receptor. The antibody recognized a protein of about 80 KDa in bovine brain synaptic membrane. Immunoabsorbent agarose beads conjugated with the antibody were able to remove more than 90% of the baclofen suppressive GABA binding activity in the solubilized synaptic membrane, and this system was found to be useful for the immunoaffinity column chromatographic separation of GABAB receptor. Preliminary studies of immunohistochemical visualization of GABAB receptor in the rat cerebellum suggested that this receptor may be exclusively localized at the presynaptic site of GABAergic neurons.