A pharmacological characterization of chloride- and potassium-dependent inhibitions in the CA3 region of the rat hippocampus in vitro.

Population spikes evoked in CA3 pyramidal cells of rat hippocampal slices by stimulation of the fimbria are subject to an early and a late inhibition following activation of the perforant path or the mossy fibres. The early inhibition is known to be GABA-mediated, and is blocked by addition of bicuculline to the superfusing medium; however the late inhibition is bicuculline-insensitive. Both inhibitions are reduced by the addition of (+/-)-baclofen or noradrenaline to the medium; the early inhibition only is blocked by D-Ala-D-Leu-enkephalinamide while the late inhibition is preferentially reduced by kainate. These data together with the results in the preceding paper suggest that both inhibitions are synaptically mediated, possibly by two distinct types of interneurone, one GABAergic and a second which may release an unidentified transmitter.