Neuropeptide-induced contraction and relaxation of the mouse anococcygeus muscle

Isometric tension responses to neuropeptides were recorded from anococcygeus muscles isolated from male mice. This smooth muscle tissue is innervated by inhibitory nonadrenergic, noncholinergic nerves that resemble, ultrastructurally, the peptidergic neurons of the gastrointestinal tract; the physiological function of the anococcygeus is not known. Slow sustained contractions were produced by oxytocin (0.2-20 nM), [Arg8]vasopressin (0.4-200 nM), and [Arg]-vasotocin (0.4-100 nM); the mouse anococcygeus is, therefore, one of the few examples of nonvascular smooth muscle from male mammals to respond to low concentrations of oxytocin and related peptides. Substance P (0.5-8 microM) caused distinctive, biphasic increases in muscle tone of some, but not all, preparations. Other neuropeptides producing contractions were neurotensin (2-100 microM) and thyrotropin-releasing hormone (2-100 microM); the responses were of similar time course and displayed selective cross-desensitization, suggesting that these two peptides act through a common distinct mechanism. Tetradecapeptide somatostatin (10-80 microM) and its analog urotensin II (0.1-5 microM), a dodecapeptide from the urophysis of the teleost fish Gillichthys mirabilis, produced similar slowly developing relaxations of carbachol-induced tone. Piscine urotensin II, of which there are no reported effects on nonvascular mammalian systems, was 20-50 times more potent than somatostatin, a well-established mammalian hormone. Of the peptides studied, only vasoactive intestinal polypeptide (0.05-1 microM) caused rapid powerful relaxations in low concentrations; this is consistent with its proposed involvement in nonadrenergic, noncholinergic neurotransmission in the mouse anococcygeus.