The effect of the muscarinic antagonist atropine on associative long-term potentiation (aLTP) of the CA1 population EPSPs was studied in rat hippocampal slices. Local application of atropine (10(-4) M) significantly suppressed aLTP in a 'weak' radiatum input, measured 40-60 min after the tetanization (128 +/- 10% vs. 168 +/- 9% during control, P < 0.03) provided that a 'strong' supporting tetanization was applied to the stratum oriens. LTP in the 'strong' stratum oriens input itself was not suppressed and even tended to be enhanced by atropine (158 +/- 4% vs. 137 +/- 13 during control). The results suggest that synaptically released endogenous acetylcholine supports induction of aLTP in the 'weak' input and does not support induction of homosynaptic LTP in the 'strong' input. A possible physiological role of the cholinergic neuromodulatory system in the hippocampal CA1 region hence could consist in 'equalising' LTP-like changes of synaptic conductivity in pathways with high and low level of activity.
