Selective effect of fentanyl on group III and IV somatosympathetic reflexes

The effect of fentanyl on sympathetic reflexes evoked by supramaximal electrical stimulation of the radial nerve, and the subsequent reversal of its effects by naloxone, have been observed in 10 dogs anaesthetized with alpha-chloralose, paralysed with suxamethonium and artificially ventilated. During infusions of 5 micrograms kg-1 min-1 the late, long-latency, sympathetic response evoked by unmyelinated fibres was abolished at a mean dose of 27 micrograms kg-1 (SD 12.6 micrograms kg-1) after which the early, short-latency response evoked by small myelinated fibres was eliminated at a mean dose of 90.3 micrograms kg-1 (SD 54.6 micrograms kg-1) so that there was no longer any response to stimulation of the radial nerve. During a subsequent infusion of naloxone (200 micrograms min-1) the late response returned to control values at a mean dose of 0.5 mg and subsequently the early response reappeared to return to control values at a total dose of 1.6 mg. In 2 preparations phrenic nerve activity was abolished after 6.1 and 17.4 micrograms kg-1 of fentanyl and returned immediately before the late response, during the infusion of naloxone. In 2 preparations, induced tolerance occurred so that the early response could not be eliminated.