THETA-ACTIVITY-DEPENDENT AND THETA-ACTIVITY-INDEPENDENT MUSCARINIC FACILITATION OF LONG-TERM POTENTIATION IN GUINEA-PIG HIPPOCAMPAL SLICES

Cholinergic input to the hippocampus originates in the septum and diag onal band. Guinea pig hippocampal slices in a bath of carbachol, a cho linergic agonist, displayed different patterns of rhythmical activitie s depending on the carbachol concentration. Exposure to 50 or 100 mu M led to intermittent induction of theta-like activities (TLAs). Long-t erm potentiation (LTP), induced by tetanus, was facilitated at concent rations within the optimum range for generating TLAs. This facilitatio n was blocked by the muscarinic receptor antagonist atropine. Augmenta tion of LTP during TLAs was greater than that during the rest period o f TLAs which, in turn, was greater than that induced without activatio n of the muscarinic receptors. These results suggest that there are tw o muscarinic facilitation processes of LTP, one dependent on and the o ther independent of TLAs, with the former being more easily facilitate d than the latter. (C) 1997 Elsevier Science Ireland Ltd.