Lack of evidence for increased descending inhibition on the dorsal horn of the rat following periaqueductal grey morphine microinjections

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Abstract
1 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. 2 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. 3 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. 4 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.