INTRATHECAL 5-HYDROXYTRYPTAMINE AND ELECTRICAL-STIMULATION OF THE NUCLEUS RAPHE MAGNUS IN RATS BOTH REDUCE THE ANTINOCICEPTIVE POTENCY OF INTRATHECALLY ADMINISTERED NORADRENALINE

The antinociceptive potency of noradrenaline (NA), as assessed by suppression of the spinal nocifensive tail flick and paw with drawal reflexes was examined. The tail flick latency (TFL) was increased to the cut-off-value for a period of approximately 120 min following the intrathecal microinjection of a standard 15 nmol dose of NA. A similar intrathecal dose of NA produced an increase in threshold to elicit the paw withdrawal reflex for a period of approximately 20 min. When preceded by a standard intrathecal microinjection of 260 nmol 5-hydroxytryptamine, the antinociceptive potency of NA was significantly reduced as reflected by both the tail flick and paw withdrawal tests. In addition, electrical stimulation of the posterior raphe complex immediately before and during the NA-induced increase in TFL, significantly reduced the antinociceptive potency of NA. It is concluded that spinal tryptaminergic activity can reduce the duration of the antinociceptive action of intrathecally applied NA.