Exposure to a nonfunctional hot plate as a factor in the assessment of morphine-induced analgesia and analgesic tolerance in rats.

Rats not exposed to a hot plate with or without morphine and later tested on the functional hot plate with or without morphine, displayed increased paw lick latency relative to same-injected rats given pretest hot plate exposure. This analgesic effect, was termed behavioral analgesia since it, unlike morphine-induced analgesia, was not reversed by naloxone (Experiment 2). Behavioral tolerance was evident in animals exposed to the nonfunctional hot plate regardless of drug treatment and was dissociated from pharmacological tolerance. Behavioral analgesia and tolerance reported here may involve habituation to novel distractive stimuli associated with the hot plate test environment.