Hot-plate learning in mice is unaltered by immediate post-training administration of naloxone, naltrexone or morphine

Repeated exposure to the hot-plate assay results in shorter response latencies in both rats and mice. In view of the postulated role of endorphins in memory processes, the present study examined the effects of immediate post-trial opiate-induced manipulations on this phenomenon of hot-plate learning. In experiment 1, adult male mice were injected with naloxone (0.1-10.0 mg/kg, i.p.) immediately after initial hot-plate testing and were reassessed 24 and 48 hr later. In experiment 2, mice were similarly treated with either naltrexone (0.1-10.0 mg/kg) or morphine (0.5-5.0 mg/kg) and retested 48 and 72 hr later. Results indicated a potent training effect in all groups but failed to reveal any significant effects of drugs. These data are inconsistent with a role for endorphins in hot-plate learning and, further, may question the generality of the involvement of opioids in memory processes.