Comparative studies of the effects of 5-hydroxytryptamine and noradrenaline on the rat anococcygeus muscle

The effects of 5-hydroxytryptamine (5HT) and noradrenaline (NA) have been studied on rat anococcygeus muscle.

1. 5HT and NA produced a dose-dependent contraction of rat anococcygeus muscle. Cyproheptadine (1.0×10^?6 M), a specific 5HT receptor blocker, failed to inhibit the responses to either 5HT or NA.
2. However, phentolamine, a specificalpha receptor antagonist competitively blocked the responses to 5HT and NA.
3. The responses to 5HT were inhibited in the reserpinized (5 mg/kg i.p. 24 h) and 6-hydroxydopamine (6-OHDA) pre-treated preparations. 6-OHDA produced a leftward shift of the dose-response curve of NA. Reserpine pre-treatment potentiated lower doses of NA and the threshold dose of NA was significantly decreased.
4. Nialamide (2.2×10^?6 M), the mono-amine oxidase inhibitor produced a significant leftward shift of the dose-response curve of both 5HT and NA. Pyrogallol (2.3×10^?5 M), the catechol-o-methyl transferase inhibitor also potentiated the responses to both 5HT and NA, but the potentiation was significant at lower doses of 5HT and NA.
5. Our data suggest that 5HT- and NA-induced contractions in rat anococcygeus muscle are mediated through commonalpha adrenoceptors. 5HT actions are probably indirect, mediated through the release of NA.