Presynaptic muscarinic receptors in guinea pig superior cervical ganglion

This study characterizes the presynaptic muscarinic cholinergic receptors associated with the modulation of the electrically-evoked acetylcholine output from guinea pig superior cervical ganglion preincubated with [3H]choline. The M1-selective agonist pilocarpine had no effect while carbachol and oxotremorine strongly decreased the evoked outflow of tritium. Atropine increased such evoked release of [3H]acetylcholine whereas the M1-selective antagonist pirenzepine was ineffective. Moreover, atropine but not pirenzepine antagonized the inhibitory effect of carbachol. These results suggest that the guinea-pig superior cervical ganglion is equipped with presynaptic inhibitory muscarinic receptors of the M2 subtype.