Reversible tetracaine block of rat periaqueductal gray (PAG) decreases baseline tail-flick latency and prevents analgesic effects of met-enkephalin injections in nucleus paragigantocellularis (PGC)

One μg of tetracaine in the rat periaqueductal gray (PAG) produced a decline in baseline tail-flick latencies (hyperalgesia) from about 5 to 3.5 s over the course of 9 min, after which the latencies increased to about 4.5 s. One μg of Met-enkephalin in PGC caused an expected increase in latencies (analgesia) from about 4.25 to 6.2 s in 9 min, with recovery to 4.7 s after 15 min post-injection. Giving the preceding 2 nanoinjections simultaneously led to an essentially total block of the PGC analgesia. A control injection in PAG simultaneous with a Met-enkephalin injection in PGC did not block the latter's analgesic effect. Single control (artificial cerebrospinal fluid) injections in PAG or PGC were without effect. The hyperalgesic effect of PAG tetracain supports the involvement of PAG in normally occuring, tonic descending pain inhibition. The block of PGC met-enkephalin analgesia by distant injection of tetracaine in PAG supports the necessity of PAG integrity for PGC analgesic function.