Involvement of M(2) muscarinic receptors in relaxant response of circular muscle of mouse gastric antrum.

Our previous study showed that atropine significantly inhibited the sustained relaxation induced by electrical field stimulation (EFS) in the circular muscle strips prepared from the mouse antrum, and that pituitary adenylate cyclase activating peptide (PACAP) partially mediated the sustained relaxation. The muscarinic receptor subtype associated with the sustained relaxation was examined in the present study by using each muscarinic receptor subtype of knockout (KO) mice. EFS-induced sustained relaxation in the antrum prepared from M(2) receptor KO mice was significantly less than that of wild-type mice. Atropine failed to inhibit this relaxation. On the other hand, similar sustained relaxation and inhibitory effects of atropine to those of wild-type mice were observed in M(1), M(3) and M(4) receptor KO mice. Exogenously added PACAP-27 relaxed the antral strips of wild-type and M(2) receptor KO mice to a similar extent. Immunohistochemical study revealed that M(2) receptor immunoreactivity was localized with PACAP-immunoreactivity in enteric neurons within the antrum of wild-type mice. In contrast, atropine did not affect the EFS-induced sustained relaxation in the gastric fundus. These results suggest that M(2) receptors modulate the sustained relaxation, probably through the regulation of PACAP release, in the mouse antrum.