Effects of Morphine and Naloxone on Dorsal Horn Neurones in the Cat

Abstract

Morphine, applied by microiontophoresis to functionally identified dorsal horn neurones in segments L5–L7 of cats (chloralose anaesthetized, decerebrated or high spinal), produced primarily a depression of the discharge of neurones responding to noxious radiant heat applied to the skin. It depressed on-going activity (12 out of 20 neurones), glutamate-evoked excitation (8/8) and the response to the noxious stimulus (13/21). The response of two additional neurones to heat was potentiated. The effects began 10–30 s from the onset of application, reached a maximum in up to 8 min and outlasted application by up to 10 min. Morphine had relatively little effect on on-going activity and glutamate-evoked excitation of neurones responding to non-noxious stimuli (n = 18). Naloxone (intravenously and iontophoretic) reversed these depressions (4/11). It is suggested that morphine may produce analgesia, at least in part, by a direct action on a specific morphine receptor in the spinal cord.