Naloxone alters pain perception and somatosensory evoked potentials in normal subjects

Abstract

THE discovery of opiate-like peptides (endorphins) in brain, blood and cerebrospinal fluid^1–4 raises questions as to the physiological role of these substances. Given the prominent analgesic effects of opiates, the regulation of pain perception is a candidate for endorphin action. Endorphins microinjected into the periacqueductal grey matter of rats induce analgesia which is reversed by naloxone, a pure narcotic antagonist⁵. If endorphins play an active part in the regulation of pain, then naloxone administered to man should alter pain appreciation. Naloxone has not been shown to have analgesic, respiratory, euphoric, pupillary or electroencephalogram (EEG) effects in man⁶. Studies in rats^7,8 and one early study in man⁹ have suggested slight hyperalgesic effects with naloxone. El-Sobky et al.¹⁰ failed to demonstrate naloxone effects on electric shock pain judgments in five subjects. In the experiments reported here, subjects were divided into pain sensitive and pain insensitive subgroups. The insensitive subjects found shocks significantly more painful after naloxone administration while the sensitive group experienced them as less painful. Evoked potentials showed similar significant group differences. These results suggest that individual differences in pain sensitivity may relate to differences in an endorphin system.