Perceptual and motor effects of morphine and buprenorphine in baboons

The effects of morphine and buprenorphine on auditory perceptual discriminations and response latency ("reaction time") in baboons are compared. The task employed synthetic human vowel sounds that are readily generated in the laboratory, and closely approximate natural baboon "grunt" vocalizations [J. Acoust. Soc. Am. 101 (1997) 2951]. Baboons pressed a lever to produce one repeating "standard" vowel, and released the lever only when one of four other "comparison" vowels occasionally occurred in place of the standard vowel. The percentage of correct detections and median reaction time for each comparison were measured following intramuscular drug administrations of morphine (0.01-1.8 mg/kg) and buprenorphine (0.00032-0.032 mg/kg). Both morphine and buprenorphine impaired vowel discriminability, and greater impairments occurred for those comparison vowels that were more similar in formant structure to the standard vowel. Morphine increased reaction time in all baboons, and buprenorphine increased reaction time in two of three baboons. Morphine's perceptual effects occurred within 20-40 min following drug administration; buprenorphine's perceptual effects occurred 50-100 min following drug administration. Morphine and buprenorphine did not differ in the time course of their maximal reaction time effects. The results demonstrate that both morphine and buprenorphine can impair auditory discriminations involving human vowel sounds in baboons, as well as lengthen reaction times to the stimuli.