The Antinociceptive Action of Etorphine in the Dorsal Horn Is Due to a Direct Spinal Action and Not to Activation of Descending Inhibition


Etorphine, microinjected into the brainstem or administered intravenously, inhibited the firing of dorsal horn neurones to noxious heat in spinal or non-spinal anaesthetized cats and in decerebrate, non-anaesthetized cats with intact spinal cords. 2Small doses of etorphine sometimes caused facilitation, especially when the cord was intact, but this was invariably followed by inhibition at higher doses. 3The ED50 for inhibition (mean 3.9 μg/kg) after microinjection into nucleus raphe magnus, nucleus reticularis magnocellularis or the lateral tegmental field was similar at all sites in anaesthetized, non-spinal cats. 4The ED50 for microinjection was not increased by spinal transection in anaesthetized cats (mean ED50, 2.6 μg/kg) and was similar to the ED50 in decerebrate, non-anaesthetized cats. 5Intravenous administration was 2 to 3 times more effective than microinjection and the time course of inhibition was faster after intravenous administration than after microinjection. 6It is concluded that etorphine inhibits dorsal horn neurones after microinjection or intravenous administration by a direct action on the spinal cord and not by activating a descending inhibition. After microinjection it rapidly enters the general circulation and subsequently distributes into the spinal cord. 7It is also concluded that naloxone readily gains entry to the circulation from the brain because microinjection antagonized the effects of systemic etorphine on dorsal horn neurones in spinal cats.


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© 1983 British Pharmacological Society, All rights reserved.

Publication Date

01 Jan 1983