The presence and the effects of neuropeptide Y in rat anococcygeus muscle.

Isolated anococcygeus muscle from male rats was examined for the presence of neuropeptide Y-immunoreactive nerves and for the effects of neuropeptide Y on its tone and its contractile/relaxant responses to electrical field stimulation, acetylcholine, guanethidine and noradrenaline. Using peroxidase anti-peroxidase immunohistochemistry in stretch preparation of the anococcygeus, neuropeptide Y-immunoreactive nerve fibres were observed, in abundance, running along both vascular as well as non-vascular smooth muscle cells. Neuropeptide Y (> 250 nM) evoked phentolamine and tetrodotoxin-resistant contractile response. Neuropeptide Y, even in subspasmogenic concentrations, potentiated contractions evoked by acetylcholine, guanethidine and noradrenaline. Electrical field stimulation (trains of 3-4 pulses, 0.1 ms, 10 Hz) of the isolated anococcygeus preparation produced robust, phentolamine and tetrodotoxin sensitive contractions. Neuropeptide Y (< 10 nM) exerted a biphasic effect on the electrical field stimulation-evoked contractions; an early potentiation was followed by a delayed and progressive inhibition. Neuropeptide Y (> 10 nM) caused a concentration-dependent potentiation of electrical field stimulation-evoked contraction alone, matching its potentiation of noradrenaline-evoked contraction. Electrical field stimulation (5 pulses, 0.1 ms, 10 Hz) of guanethidine (50 microM)-contracted anococcygeus induced a relaxant response and neuropeptide Y (1-100 nM) exerted a concentration-related slight and variable effect on the electrical field stimulation-evoked relaxant response (1 nM, augmentation; 10 nM, no effect; 100 nM, reduction). It is concluded that rat anococcygeus muscle has a rich neuropeptide Y-containing innervation and neuropeptide Y is mostly stored within adrenergic nerves. The main functional roles of neuropeptide Y in the anococcygeus muscle are likely to be post-junctionally mediated facilitation and prejunctionally mediated inhibition of adrenergic motor transmission.