Alpha-adrenoceptor-mediated inhibition of acetylcholine release in guinea-pig superior cervical ganglion.

The effect of different substances on the electrically evoked acetylcholine (ACh) output from guinea-pig superior cervical ganglion (SCG) preincubated with [3H]choline was investigated. Forskolin significantly reduced neurotransmitter release and dose-dependently enhanced cyclic AMP (cAMP) levels. Phenylephrine and clonidine, alpha 1-and alpha 2-adrenergic agonists respectively, caused a significant inhibition of ACh output from presynaptic nerve terminals. In the presence of RMI 12330A, phenylephrine but not clonidine failed to reduce neurotransmitter release. These results suggest that cAMP and different alpha-adrenergic receptors are involved in the regulation of cholinergic transmission.