MUSCARINIC AMPLIFICATION OF FAST EXCITATION IN HILAR NEURONS AND INHIBITION IN GRANULE CELLS IN THE GUINEA-PIG HIPPOCAMPUS

1. Effects of the cholinergic agonist, carbachol (CCh), or the acetylc holinesterase inhibitor, eserine, on presumed inhibitory hilar neurone s and on inhibition in granule cells were studied by intracellular rec ording in guinea-pig hippocampal slices. 2. CCh (1-5 mu M) strongly en hanced the discharge activity of hilar neurones and spontaneous and ev oked IPSPs in granule cells. 3. Eserine, in an atropine-sensitive mann er, increased the excitability of hilar neurones through effects on me mbrane properties and on excitatory synaptic barrage. EPSPs readily tr iggered long-duration burst discharges. In granule cells, the amplitud es of evoked GABA(A), and GABA(B) receptor-mediated IPSPs were enhance d. 4. In the presence of eserine and antagonists for glutamatergic and GABAergic synaptic transmission, train stimulation evoked atropine-se nsitive slow EPSPs. In contrast to those in granule cells, slow EPSPs in hilar neurones were invariably preceded by a strong burst-after-hyp erpolarization. 5. We suggest that acetylcholine, released from septo- hippocampal fibres, amplifies fast synaptic excitation of inhibitory h ilar neurones and inhibition of granule cells. In the dentate area, mu scarinic receptor-mediated effects are faster than anticipated from th e time course of the slow EPSP.