Lack of effects of tetracaine in rat nucleus paragigantocellularis (PGC) on analgesia from periqueductal gray (PAG) met-enkephalin (ME) injections (in raphe magnus-blocking doses), and on baseline tail-flick latency

We showed previously that (1μg) tetracaine block of PAG prevents the analgesic effect of PGC-ME injections, and also produces hyperalgesia regarding baseline tail-flick latencies. In the present study we injected PGC (unilaterally and bilaterally) with tetracaine so as to study its possible role in tonic descending inhibition of pain, as well as its necessity for analgesia due to PAG opiate injection. Two μg of tetracaine in PGC unilaterally or bilaterally failed to affect baseline tail-flick latencies and failed to attenuate at all the distinct analgesic effect of 2μg ME injected into PAG. This analgesic effect was seen as a change in mean tail-flick latency from about 5.5 to 7.0 seconds 8-12 minutes post-injection, recovering to 5.5 second baseline by 25 minutes. Two μg tetracaine in nucleus raphe magnus (RM) was however sufficient to block the analgesic effect of RM stimulation. Thus PGC does not appear to participate in tonic descending pain inhibition (as PAG appears to do) nor is its functional integrity necessary for PAG ME analgesia (though PAG is necessary for PGC analgesia).