Effect of fentanyl and naloxone on human somatic and auditory-evoked potential components

The effect of fentanyl (a morphine agonist) and naloxone (a morphine antagonist) on early and late components of somatic (SEP)- and auditory (AEP)-evoked potentials was studied in patients undergoing minor surgical procedures, in which these compounds were used in producing and regulating a state of neuroleptanalgesia. Fentanyl (2.5, 5.0 and 10.0 micrograms/kg), naloxone (1.5 and 3.0 micrograms/kg) and isotonic saline (for comparative purposes) were injected just before surgery, intravenously through a catheter and following a single blind procedure and three different pharmacological paradigms with four consecutive conditions each: (1) Initial baseline (C), first saline (S), second saline (S') and late baseline (C'). (2) C, First fentanyl (F), second naloxone (N') and C. (3) C, First naloxone (N), second fentanyl (F') and C'. Special care was taken in controlling the constancy of the muscular and cochlear receptor activation concomitant to somatic-evoked potentials and auditory-evoked potentials, determined by the amplitude of the muscular response at the tenar muscles (MP) and component I of the brain stem potentials ( ABSP ). Evaluation by the patients pain, topognoses and hearing and other somatic and autonomic indicators of the level of the analgesic response were also controlled. Fentanyl significantly reduced, while naloxone increased, the amplitude of late components P150 of somatic-evoked potentials and auditory-evoked potentials. Concomitantly, fentanyl increased, while naloxone decreased, the spatial threshold (two point discrimination test) at finger tip and arm. These effects were observed in patients taking various doses, although they were more consistent with larger doses of these compounds.(ABSTRACT TRUNCATED AT 250 WORDS)