RT Journal Article
SR Electronic
T1 Presynaptic Muscarinic M<sub>2</sub>-Receptor-Mediated Inhibition of N-Type Ca<sup>2+</sup> Channels in Cultured Sphenopalatine Ganglion: Direct Evidence for Acetylcholine Inhibition of Cerebral Nitrergic Neurogenic Vasodilation
JF Journal of Pharmacology and Experimental Therapeutics
JO J Pharmacol Exp Ther
FD American Society for Pharmacology and Experimental Therapeutics
SP 397
OP 405
DO 10.1124/jpet.302.1.397
VO 302
IS 1
A1 J. Liu
A1 M. S. Evans
A1 T. J.-F. Lee
YR 2002
UL http://jpet.aspetjournals.org/content/302/1/397.abstract
AB Results of previous pharmacological studies suggested that presynaptic muscarinic M2 receptors on cerebral perivascular nitric oxidergic (nitrergic) nerves mediated inhibition of nitric oxide release from these nerves. The inhibition was thought to be primarily attributable to a decreased Ca2+ influx through N-type Ca2+ channels on nitrergic nerves, but direct evidence supporting this hypothesis was not presented. In the present study, we used cultured rat sphenopalatine ganglion (SPG), a major source of nitrergic nerves to cerebral blood vessels, to investigate the role of muscarinic M2 receptors in modulating voltage-dependent Ca2+ channels. SPG neuronal soma and dendrites were immunoreactive for both N-type Ca2+ channels and muscarinic M2 receptors, indicating that muscarinic M2receptors were colocalized with N-type Ca2+ channels. Using the whole-cell voltage-clamp technique, we found that voltage-dependent Ca2+ currents in cultured SPG were largely blocked by ω-conotoxin, an N-type calcium channel antagonist, but were not affected by nifedipine, an L-type calcium antagonist. The Ca2+ current was inhibited by acetylcholine (ACh) and arecaidine but-2-ynyl ester tosylate (ABET), a preferential muscarinic M2-receptor agonist, in a concentration-dependent manner. The inhibition was reversed by atropine and methoctramine (a muscarinic M2-receptor antagonist), but was not affected by muscarinic M1-, M3-, or M4-receptor antagonists. Consistent with this, preferential muscarinic M1-receptor agonists McN-A-343 and oxotremorine did not affect the Ca2+ current. Furthermore, pretreatment with pertussis toxin and guanosine 5′-O-(3-thio)triphosphate prevented ACh and ABET inhibition of Ca2+ currents. These results are consistent with pharmacological findings in the pig basilar arteries and provide direct evidence supporting our hypothesis that M2-receptor-mediated inhibition of cerebral nitrergic neurogenic vasodilation is due to a Gi-protein-mediated suppression of Ca2+ influx via voltage-dependent N-type Ca2+ channels on perivascular nerves. The American Society for Pharmacology and Experimental Therapeutics