ALPHA-BUNGAROTOXIN BINDING TO HIPPOCAMPAL INTERNEURONS - IMMUNOCYTOCHEMICAL CHARACTERIZATION AND EFFECTS ON GROWTH-FACTOR EXPRESSION

The nicotinic cholinergic antagonist alpha-bungarotoxin (alpha-BT) bin ds throughout the rat hippocampal formation. The binding is displaceab le by d-tubocurarine. The most heavily labeled cells are GABA-containi ng interneurons in the dentate and in Ammon's horn. These neurons have several different morphologies and contain several neuropeptides. Alp ha-BT-labeled interneurons in the dentate are small cells between the granular and molecular layers that often contain neuropeptide Y. Alpha -BT-labeled interneurons in CA1 are medium-sized interneurons, occasio nally found in stratum pyramidale, but more often found in stratum rad iatum and stratum lacunosum moleculare. These neurons often contain ch olecystokinin. The largest alpha-BT-labeled interneurons are found in CA3, in both stratum radiatum and stratum lucidum. These neurons are m ultipolar and frequently are autofluorescent. They often contain somat ostatin or cholecystokinin. These large interneurons have been found t o receive medial septal innervation and may also have projections that provide inhibitory feedback directly to the medial septal nucleus. Th e cholinergic innervation of the hippocampus from the medial septal nu cleus is under the trophic regulation of NGF and brain-derived neurotr ophic factor, even in adult life. Expression of mRNA for both these fa ctors is increased in CA3 and the dentate after intraventricular admin istration of alpha-BT, but not after administration of the muscarinic antagonist atropine. Alpha-BT-sensitive cholinergic receptors on inhib itory interneurons may be critical to medial septal regulation of the hippocampal activity, including the habituation of response to sensory input.