A reciprocal adrenergic-cholinergic axoaxonic synapse in the mammalian gut.

Simultaneous stimulation of perivascular nerves inhibited the release of acetylcholine from stimulated cholinergic nerves of the rabbit jejunum. Adrenergic nerves were responsible for this inhibition because it did not occur in animals previously injected with 6-hydroxydopamine. Acetylcholine inhibited the release of transmitter from stimulated adrenergic axons; this effect was blocked by atropine. Since atropine enhanced the release of adrenergic transmitter when both adrenergic and cholinergic nerves were activated simultaneously (at 4.0 Hz), it seems likely that cholinergic nerves also inhibit release of norepinephrine (NE). Radioautographic examination of the myenteric plexus, incubated with tritiated NE, revealed a striking marginal distribution of adrenergic axons around the periphery of the myenteric plexus. Ultrastructural studies, with KMnO4 used to identify adrenergic terminal varicosities, confirmed this distribution and also revealed complexes formed between the terminal varicosities of adrenergic and probable cholinergic axons. The component varicosities forming these complexes contacted one another with no intervening Schwann cell elements. It is concluded that there is a reciprocal axoaxonic synapse between adrenergic and cholinergic neurons in the mammalian myenteric plexus.