Naloxone partial reversal of the antinociception produced by dipyrone microinjected into the periaqueductal gray of rats. Possible involvement of medullary off- and on-cells.
Medullary off- and on-cells have been proposed to inhibit and facilitate, respectively, nociceptive transmission. Upon heating the tail in lightly anesthetized rats, the tail flick (TF) reflex occurs only after off-cells decrease and on-cells increase their activity. Dipyrone (DIP) microinjection (100 micrograms/0.5 microliter) into the periaqueductal gray (PAG) caused retardation in the off-cell pause, on-cell burst and corresponding TF. This effect was partly reverted by naloxone given i.v. (l mg/kg) or microinjected into PAG (5 micrograms/0.5 microliter). These results suggest that endogenous opioids are partly responsible for the central antinociceptive action of DIP, and that such action involves medullary off- and on-cells.
Tortorici
V
V
Centro de Biofisica y Bioquímica, Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas, Venezuela.
Vásquez
E
E
Vanegas
H
H
D000818:Animals; D004177:Dipyrone / Q000494:pharmacology*; D008297:Male; D008845:Microinjections; D009270:Naloxone / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D010487:Periaqueductal Gray / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
D009270:Naloxone; D004177:Dipyrone