CANNABINOID RECEPTOR-MEDIATED INHIBITION OF THE RAT TAIL-FLICK REFLEXAFTER MICROINJECTION INTO THE ROSTRAL VENTROMEDIAL MEDULLA

Systemic administration of cannabinoids produce profound antinocicepti on in rodents. The purpose of this study was to examine the contributi on of the rostral ventromedial medulla (RVM) to cannabinoid-mediated i nhibition of the tail-flick reflex. Rats received direct injections of two selective cannabinoid agonists, WIN55,212-2 and HU210, into the R VM. Both compounds significantly elevated tail-flick latencies by over 50%. WIN55,212-3, the inactive enantiomer, was without effect. Furthe rmore, co-administration of the selective cannabinoid receptor antagon ist, SR141716A greatly attenuated the antinociception produced by HU21 0. Finally, injections of WIN55,212-2 outside the region of the RVM di d not affect tail-flick latencies. These results demonstrate that the cannabinoid receptor system participates in the descending control of nociception and raise the possibility that actions of endogenous canna binoids in the RVM may modulate nociceptive responsiveness. (C) 1998 E lsevier Science Ireland Ltd.