LACK OF EVIDENCE FOR INCREASED DESCENDING INHIBITION ON THE DORSAL HORN OF THE RAT FOLLOWING PERIAQUEDUCTAL GRAY MORPHINE MICROINJECTIONS

Recordings were made from 18 neurones in the dorsal horn of the rat, anaesthetized with halothane. All cells received A‐and C‐fibre inputs and responded to innocuous and noxious stimuli applied to their excitatory receptive fields located on the extremity of the ipsilateral hindpaw. Transcutaneous application of suprathreshold (mean 3.2T) 2 ms square‐wave pulses to the centre of the receptive fields resulted in responses to A‐and C‐fibre activation being observed; a mean 32.4 ± 6.0 C‐fibre latency spikes were evoked per stimulus. A high dose (20 μg) of morphine in 0.5 μl sterile saline, microinjected into the periaqueductal grey matter (PAG) had no effect on the C‐fibre‐evoked activity of thirteen cells (72%) and facilitated 5 neurones (28%). Microinjection sites covered most of the PAG particularly the caudal medioventral zone. A relatively high dose (6 mg kg−1, i.v.) of systemic morphine chloride, sufficient to elicit the direct spinal action of the opiate, inhibited all 5 cells tested. We conclude that there is little evidence that the supraspinal action of morphine includes increased descending controls and depression of dorsal horn neurones in the rat. 1987 British Pharmacological Society