Muscarinic autoreceptors are differentially affected by selective muscarinic antagonists in rat hippocampus.

The effects of atropine, pirenzepine and AF-DX 116 on the high K(+)-evoked release of endogenous ACh from rat hippocampal slices were compared. As expected, atropine in concentrations of 10(?5) and 10(?6)M increased the release of acetylcholine, to 128 and 157% of the control value, respectively. While AF-DX 116 was also able to increase the release to 148 and 156% of the control value at the above concentrations, pirenzepine had no effect on the release. This may indicate that no M(1) receptors are involved in the control of presynaptic acetylcholine release, probably because of their postsynaptic localization, and that the presynaptic muscarinic cholinergic autoreceptors are equivalent to the M(2) receptors in rat hippocampus.