Hippocampal interneurons are highly diverse. So far, attempts to define subgroups based on characteristic combinations of morphology, electrophysiology and transmitter sensitivity have failed. Using whole-cell recordings from visually identified CA1 interneurons, we examined the effect of the cholinergic agonist, carbachol (CCh, 20 μM), on EPSP-like waveforms evoked by brief glutamate pulses onto their proximal dendrites in the presence of bicuculline, D-APV and TTX. In interneurons of stratum oriens, CCh consistently reduced these glutamate-induced postsynaptic potentials (GPSPs) by 20 - 60% in an atropine-sensitive manner. The voltage dependence as well as the low Ba2+ (200 μM) sensitivity of this effect suggested that it resulted from the muscarinic activation of a GIRK conductance, as we have recently reported from CA1 pyramidal neurons (SFN Abstr. 25: 899.1, 1999). By contrast, the vast majority of interneurons (86%) recorded in stratum radiatum displayed no increase (43%) or a substantial enhancement...