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Lack of evidence for increased descending inhibition on the dorsal horn of the rat following periaqueductal grey morphine microinjections
Author(s) -
Dickenson Anthony H.,
Bars Daniel
Publication year - 1987
Publication title -
british journal of pharmacology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.432
H-Index - 211
eISSN - 1476-5381
pISSN - 0007-1188
DOI - 10.1111/j.1476-5381.1987.tb11321.x
Subject(s) - microinjections , microinjection , morphine , periaqueductal gray , nociception , noxious stimulus , medicine , neuroscience , dorsum , chemistry , anesthesia , excitatory postsynaptic potential , central nervous system , midbrain , anatomy , endocrinology , inhibitory postsynaptic potential , biology , receptor
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.