Vol. 295, Issue 2, 607-613, November 2000

Signaling Mechanisms Coupled to Presynaptic A₁- and H₃-Receptors in the Inhibition of Cholinergic Contractile Responses of the Guinea Pig Ileum

John J. Lee¹ and Michael E. Parsons

Biosciences Department, University of Hertfordshire, Hatfield, Hertfordshire, United Kingdom

The mechanisms coupled to adenosine A₁- and histamine H₃-receptors have been examined in the presynaptic inhibition of acetylcholine (ACh) release from the guinea pig ileum. Electrically evoked twitch contractions were used as a measure of neuronal ACh release. A₁- and H₃-receptors were activated by adenosine and R-()-methylhistamine (RAMH), respectively. The neuroinhibitory effect of adenosine and RAMH was augmented in the presence of the N-type Ca²⁺ channel blocker, -conotoxin GVIA but unaffected by the L-type Ca²⁺ channel blocker, nifedipine. The irreversible adenylyl cyclase inhibitor, MDL-12330A, potentiated the action of both adenosine and RAMH. Conversely, neither agonist was affected by the cAMP phosphodiesterase III and IV inhibitors, SKF-95654 and Ro-20-1724, respectively, or the cAMP antagonist, (R_p)-adenosine 3',5'-cyclic monophosphorothioate triethylamine. The neuromodulatory effect of adenosine, only, was potentiated by the cGMP phosphodiesterase V inhibitors, SKF-96231 and 1,3-dimethyl-6-(2-propoxy-5-methanesulfonylamidophenyl)- pyrazolo[3,4-d]pyrimidin-4-(5H)-one but was unmodified by the cGMP analog, 8-bromo-cGMP or the guanylyl cyclase inhibitors, N-methylhydroxylamine and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxaline-1-one (ODQ). N-Methylhydroxylamine reduced, and ODQ potentiated, the inhibitory action of H₃-receptor activation, but 8-bromo-cGMP was without effect. The study suggests that presynaptic A₁- and H₃-receptors inhibit cholinergic neurotransmission in the guinea pig ileum by limiting the availability of intraneuronal Ca²⁺ via inhibition of N-type Ca²⁺ channels. The balance of evidence does not support the involvement of the adenylyl cyclase/cAMP or guanylyl cyclase/cGMP systems.