Hypoalgesia in response to sensitization during acute noise stress.

Three experiments examined the antinociceptive response shown by rats during exposure to loud noise. Noise exposure resulted in a time-dependent elevation of radiant heat tail flick latency that varied as a function of stimulus intensity. Noise stress hypoalgesia in response to a 90-dB stimulus was blocked by pretreatment with the opioid antagonist naltrexone (0.1-7.0 mg/kg). Systemic administration of midazolam (2 mg/kg) prior to exposure to the stressor attenuated the elevation in tail flick latency. Because topographically similar antinociceptive responses may be elicited with a low intensity noise stimulus that has served as a Pavlovian conditional stimulus for shock, the use of this paradigm may permit direct comparisons of associative and nonassociative fear responses using qualitatively similar auditory stimuli.