Lack of evidence for increased descending inhibition on the dorsal horn of the rat following periaqueductal grey morphine microinjections.
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 3.2 T) 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 micrograms) of morphine in 0.5 microliter 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.
Dickenson
A H
AH
Dept. of Pharmacology, University College, London.
Le Bars
D
D
D000818:Animals; D001345:Autoradiography; D005071:Evoked Potentials / Q000187:drug effects; D008297:Male; D008845:Microinjections; D009020:Morphine / Q000008:administration & dosage / Q000494:pharmacology*; D009412:Nerve Fibers / Q000187:drug effects; D009474:Neurons / Q000187:drug effects*; D010487:Periaqueductal Gray; D051381:Rats; D011919:Rats, Inbred Strains; D013997:Time Factors
D009020:Morphine