Regulation of the activity of hippocampal stratum oriens interneurons by alpha 7 nicotinic acetylcholine receptors

GABAergic interneurons have been shown to be a major target of cholinergic inputs to the hippocampus. Because these interneurons project to pyramidal neurons as well as other interneurons, activation of the cholinergic system is likely to produce a complex modulation of local inhibitory activity. To better understand the role of postsynaptic alpha7 nicotinic acetylcholine receptors in the hippocampus, we have characterized the effects of nicotini c agents on local interneurons of the rat CAI stratum oriens in terms of ac tivation, desensitization, and region of axonal termination. Fast applicati on of acetylcholine onto stratum oriens interneurons during whole-cell reco rdings from hippocampal slices activated the majority of cells tested, and these responses were mediated almost entirely by alpha7 nicotinic acetylcho line receptors. Anatomical reconstructions showed no clear relationship bet ween the acetylcholine responsivity of interneurons and the regions to whic h their axons project. Currents mediated by a7 receptors declined markedly during repetitive activation in the theta rhythm range (4-12 Hz) when activ ated by either pressure application or synaptic release of acetylcholine. H owever, the decay of alpha7 receptor-mediated currents was unaffected by tr eatment with the cholinesterase, inhibitor neostigmine (10 nM-10 muM), sugg esting that hydrolysis of acetylcholine is not a rate-limiting step in the termination of these responses. From these findings we suggest that nicotinic receptor activity in this reg ion has an extensive and complex impact on local inhibitory circuits that i s mediated by activation of several classes of intrinsic GABAergic cells. I n addition, desensitization of the a7 nicotinic acetylcholine receptor is l ikely to contribute to the decay of individual responses to pressure applic ation of agonist, and may also act in a cumulative fashion to impair the ab ility of these receptors to support repetitive activity during trains of ac tivation. If applicable to alpha7 receptor responses in vivo, we suggest it may be difficult to enhance these responses for therapeutic purposes with cholinesterase inhibitors. (C) 2001 IBRO. Published by Elsevier Science Ltd . All rights reserved.