Ethylketocyclazocine and bremazocine analgesia in neonatal rats.

In three experiments we examined the analgesic potency of kappa opioid receptor agonists in 2- and 16-day-old rats. Ethylketocyclazocine (1-50 mg/kg) produced similar dose- and time-dependent increases in the latency to retract a hind paw from a noxious thermal stimulus in rats of both ages. Bremazocine (0.001-10 mg/kg), a kappa agonist with reported antagonist activity at mu receptors, was also effective in producing analgesia in 2-day-old rats. The dose-effect relationship for bremazocine was nonmonotonic. Bremazocine analgesia (0.1 mg/kg) was reversed by both naltrexone and MR2266, a putative kappa opioid antagonist. These results are discussed in terms of the functional integrity of a kappa analgesic system in the developing rat.