The effects of drugs on the sensitivity of the rat anococcygeus muscle to agonists.

1. The effects of cocaine, 6-hydroxydopamine (6-OHDA), reserpine and 17-beta-oestradiol on the sensitivity of the rat anococcygeus muscle to noradrenaline (NA), acetylcholine (ACh) and KCl were investigated.2. Cocaine (10(-5)M) increased the sensitivity of the anococcygeus to NA (100-fold) but not to ACh or KCl.3. 6-OHDA treatment, whether discontinuous over 6 days or continuous for 12 days, also produced a specific increase in sensitivity to NA (50-fold).4. Reserpine ((1 mg/kg)/day) produced a small increase in sensitivity of the muscle to NA and ACh but not to KCl, after 6 days and 12 days treatment.5. 17-beta-Oestradiol (10(-5)M) had no effect on the sensitivity of the anococcygeus to NA or ACh, but reduced the sensitivity to KCl and the possible mechanism of this effect is discussed.6. Cocaine increased, while reserpine, 6-OHDA and 17-beta-oestradiol decreased, the maximum response of the muscle to KCl. The response to KCl was shown to consist of at least three components; a direct action on the muscle, an effect due to release of NA, and an inhibitory action, probably due to release of the unknown inhibitory transmitter.