Morphine decreases cerebral glucose utilization in limbic and forebrain regions while pain has no effect

Morphine alters affective states in humans and produces reinforcement in humans and animal subjects. Therefore, the present study was designed to determine whether morphine affected the functional activity in brain structures involved in emotion and motivation. Since opioid-induced analgesia largely reflects an influence on the emotional component of pain, the effect of pain on activity in these structures was also studied. Local rates of glucose utilization were measured by the 2-deoxy-D[1-14C]glucose method in the brains of rats, that received morphine or saline, with or without a painful stimulus. Many rostral, and particularly diencephalic, nuclei showed decreased glucose utilization in morphine-treated rats, although caudal regions were mostly unaffected. There were dose-dependent decreases in glucose utilization with doses between 1 and 10 mg/kg morphine. However, nociceptive stimulation, with either formalin or tail-immersion, produced no significant effect on cerebral glucose utilization. The 2-deoxy-D-[1-14C]-glucose technique, as used here, may lack adequate sensitivity to delineate areas of the brain mediating nociception in rats.