Somatostatin inhibits cAMP-mediated cholinergic transmission in the myenteric plexus.

The mechanism by which somatostatin acts to modulate cholinergic transmission is not clear. In this study we investigated the role of the adenosine 3',5'-cyclic monophosphate (cAMP) system in mediating cholinergic transmission in the guinea pig myenteric plexus and examined the ability of somatostatin to alter acetylcholine (ACh) release stimulated by various cAMP agonists. Forskolin, 8-bromo-cAMP, vasoactive intestinal peptide (VIP), and cholera toxin each stimulated the release of [3H]ACh in a dose-related manner. Addition of theophylline enhanced the release of [3H]ACh stimulated by these cAMP agonists. In contrast 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase, antagonized the action of forskolin, VIP, and cholera toxin but had no effect on that evoked by 8-bromo-cAMP. These observations suggest that cAMP may serve as a physiological mediator for ACh release from myenteric neurons. Somatostatin inhibited release of [3H]ACh evoked by various cAMP agonists in a dose-related manner. Maximal inhibition, observed in the presence of 10(-6) M somatostatin was 48 +/- 5, 47 +/- 9, and 43 +/- 12% of control for forskolin-, VIP-, and cholera toxin-evoked release of [3H]ACh. In contrast somatostatin at 10(-6) M inhibited only 20 +/- 5% of the release of [3H]ACh stimulated by 8-bromo-cAMP. Pretreatment with pertussis toxin antagonized the inhibitory effect of somatostatin on the release of [3H]ACh evoked by forskolin, VIP, or cholera toxin but had no effect on the inhibitory action of somatostatin on the release of [3H]ACh evoked by 8-bromo-cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)