INHIBITION OF SPINAL NOCICEPTIVE NEURONS BY MICROINJECTIONS OF SOMATOSTATIN INTO THE NUCLEUS RAPHE MAGNUS AND THE MIDBRAIN PERIAQUEDUCTAL GRAY OF THE ANESTHETIZED CAT

The effects of somatostatin (SOM) after intravenous application and in tracerebral microinjection into the medullary nucleus raphe magnus (NR M) or into the periaqueductal gray (FAG) on the spinal nociceptive tra nsmission was quantitatively studied in the anesthetized cat. Noxious heat-evoked responses of multireceptive lumbar spinal dorsal horn neur ons were reversibly depressed to 56.6 +/- 9.7% of the control after sy stemically applied SOM (7 mu g/kg i.v.; 7 mu g/kg per h infusion rate) . At 11 of 14 brainstem microinjection sites in the NRM and PAG, SOM ( 2.5 mu g/mu l) attenuated the heat-evoked responses to 58.9 +/- 6.2% ( n = 5) (NRM) and 64.4 +/- 6.3% (n = 6) (PAG) of the control. After mic roinjection, maximal inhibition was reached within 8-14 min (NRM) or 2 3-29 min (PAG), respectively. Inhibition was reversible within 60 min after the injection. Thus, SOM has an antinociceptive potency by activ ating descending inhibition of nociceptive dorsal horn neurons from th e NRM and FAG.