Roles of endogenous cholinergic neurons in the induction of long-term potentiation at hippocampal mossy fiber synapses

Abstract

To evaluate the functional role of endogenous acetylcholine (ACh) in the induction of long-term potentiation (LTP) at mossy fiber-CA3 synapses, the influence of cholinergic hypofunction on it was investigated. Administration of a cholinergic neurotoxin, ethylcholine mustard aziridinium ion (AF64A; 5 nmol, i.c.v.), to guinea pigs one week prior to preparing slices resulted in a significant decrease in the magnitude of LTP, associated with a significant decrease in cholineacetyltransferase (ChAT) activity and the number of ChAT immunoreactive cells in the hippocampal slices. Bath-application of a cholinesterase inhibitor, physostigmine at 0.1 μM and 10 μM, attenuated and augmented, respectively, the magnitude of LTP in slices prepared from vehicle-treated animals (naive slices), whereas that in slices prepared from AF64A-treated animals (lesioned slices) was not significantly affected by physostigmine at any concentration tested. The induction of LTP in naive slices was inhibited or facilitated by a muscarinic M 1 antagonist pirenzepine (1 μM) and by an M 2 antagonist AF-DX 116 (1 μM) alone, respectively, whereas that in lesioned slices was not significantly changed by either of them. Furthermore, bath-applied carbachol (CCh) at 0.01–10 μM augmented the magnitude of LTP in lesioned slices, whereas the induction of LTP in naive slices was inhibited and facilitated by CCh at 0.01–0.1 μM and 1–10 μM, respectively, as reported previously. Such an augmentation of LTP by CCh was reversed by pirenzepine, but not by AF-DX 116. These observations suggest that AF64A induces the defect in ACh release and the hypofunction of M 2 receptors, but not of M 1, at least during the induction of LTP at mossy fiber-CA3 synapses. Thus, it is conceivable that endogenous ACh plays a facilitatory and inhibitory role through muscarinic M 1 and M 2 receptors in the induction of LTP in the mossy fiber-CA3 pathway.