EFFECTS OF MORPHINE ON NEURONAL AND BEHAVIORAL-RESPONSES TO VISCERAL AND SOMATIC NOCICEPTION AT THE LEVEL OF SPINAL-CORD

This study was undertaken to examine the role of morphine in modulatio n of nociception in visceral and somatic pain tests at the level of th e spinal cord, using neurophysiological and behavioural reflex assays. In the neurophysiological study we recorded extracellularly the activ ity of the single viscero-somatic convergent neurons of the spinal dor sal horn, which was evoked by the colorectal distension (80 mmHg) of n oxious visceral stimulation and the radiant heat (51 degrees C) of nox ious somatic stimulation, in decerebrated, spinally transected cats. S pinally administered morphine (200 mu g) produced significant suppress ion of noxiously evoked activity by both stimuli in a time-dependent m anner. In addition, intravenously administered naloxone reversed the s uppressive effects of morphine. In the behavioral reflex study, colore ctal distension threshold and tail-flick latency were measured in rats chronically implanted with lumbar intrathecal catheter. Intrathecally administered morphine significantly elevated the colorectal distensio n threshold and prolonged the tail-flick latency in a time- and dose-d ependent manner. The results of the present study demonstrated that sp inal morphine was capable of suppressing the evoked activity of the vi scero-somatic convergent neurons, resulting in suppression visceral an d somatic pain behavioural reflexes.