Lack of habituation of pain evoked potentials after naloxone

Pain evoked potentials (EPs) were recorded in nucleus caudatus (NC) of rabbits after electrical stimulation of the dental pulp. After 50-60 noxious stimuli the decrease in amplitude of evoked potentials occurred. Naloxone, 1 mg/kg, temporarily abolished this effect and even after 350 noxious stimuli the habituation was not observed. On the other hand, naloxone did not affect the diminution of monosynaptic transcallosal potentials (TC-EP) evoked by repeated stimulation. It may be suggested that the habituation of EPs elicited by pain stimulation is due to endorphin release.