Effect of morphine on some sympathetically innervated effectors.

Morphine, in doses of 0.5 to 55 mg/kg, inhibited contraction of the nictitating membrane of the cat following stimulation of the postganglionic sympathetic nerve fibres. Morphine was more effective at low than at high frequencies of stimulation, independently of the size of contraction of the membrane; the speed of contraction was reduced at all frequencies. Cocaine potentiated the contraction of the nictitating membrane following nerve stimulation more at low than at high frequencies, and antagonized the action of morphine. These findings, and the absence of an effect of morphine on the action of injected noradrenaline, make it likely that morphine interferes with the release of noradrenaline from the postganglionic nerve endings in the nictitating membrane. Morphine had no effect on the cardioaccelerator action of the cardiac nerves and inconsistent results were obtained on the emptying of the spleen after stimulation of the splenic nerves.