In vivo voltammetric studies of the effect of morphine on the serotonergic system in the cat spinal cord.

The purpose of the present paper is to investigate the effect of morphine on the serotonergic system through an analysis of electrochemical signals in the spinal cord of the anesthetized cat. Electrochemical signals showed a peak of 5-hydroxyindole at 280-300 mV, and were highest at a depth of 1500-2500 microns in the dorsal horn of the spinal cord. Electrical stimulation of the nucleus raphe magnus (NRM) produced an increase in the electrochemical signal. Five-hydroxytryptophan (25 mg/kg, i.v.) also enhanced the signal remarkably. Both systemic administration of morphine (1-2 mg/kg, i.v.) and microinjection (5 and 10 micrograms/microliter) of morphine into the NRM increased the signal. In conclusion, it is suggested that morphine-induced enhancement of the peak of 5-hydroxyindole contributes to the activity of the serotonergic system in the cat spinal cord.