Modulation of carbachol-induced antinociception from the rat periaqueductal gray

The tail-flick latency (TFL) and the vocalisation test (VT) thresholds were all increased by microinjecting CCh into the dorsal periaqueductal gray (d PAG) of rats. The effects on the TFL were mimicked by dimethyl-phenylpipera zinium, and inhibited by local mecamylamine or intraperitoneal (i.p.) pheno xybenzamine, The effects on the VT were mimicked by bethanechol and inhibit ed by local mecamylamine, atropine or naloxone. The effects on the threshol ds for motor defence reaction were inhibited by i.p. methysergide or naloxo ne, and prolonged by i.p. phenoxybenzamine, The effects on the threshold fo r vocalisation during the stimulation were blocked by i.p. methysergide and shortened by i.p. phenoxybenzamine or naloxone. No significant effect of C Ch was found on open arm exploration of rats in the elevated plus maze para digm. We conclude that the effects of CCh from the dPAG is not due to an an xiolytic effect, and depends on the activation of local cholinergic and opi oid sites for the supraspinal modulation of "affective" component of pain r esponse, and nicotinic sites for the activation of descending pain pathways , (C) 2000 Elsevier Science Inc.