Urethane-anesthetized rats were prepared for stimulation of the perforant path and for recording from the granule cell region of the hippocampal dentate gyrus. Subjects were administered varying doses of allethrin (a prototype type I pyrethroid) or deltamethrin (a prototype type II pyrethroid), and the excitability of the perforant path and granule cells was tested. Both pyrethroids produced a dose-dependent decrease in the responsiveness of granule cells, following stimulation of the perforant path, that lasted up to 100 msec. Analysis suggested that the pyrethroid-induced effects were associated with an increase in interneuronally mediated inhibition. Neither the perforant path axon or terminal nor the granule cell was affected by doses which appeared to affect interneurons. Basal excitability of the granule cells was also decreased by deltamethrin. This effect may have been secondary to an increase in tonic inhibition evoked by the same mechanisms responsible for the increase in phasic inhibition.
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