Effects of epidural bupivacaine and morphine on spinal cord and cerebral metabolism in rats

Despite the wide use of epidural administration of local anesthetics and opioids for surgical anesthesia and pain management, little is known about their merabolic effects on the spinal cord and brain. The present study was undertaken to examine the effects of epidural bupivacaine and morphine on spinal cord and cerebral merabolism by measuring local glucose utilization as a reflection of functional change. In 18 rats given bupivacaune either epidurally (n＝5) or intramuscularly (n＝5) as well as morphine epidurally (n＝5) or intravenously (n＝3), the duration of analgesia was eamined with the tail flick and hot plate tests. Local glucose utilization was measured, using 2-[^<14>C] deoxyglucose method, in 28 rats equally divided into four groups (7 in each) depending on the drug used : epidural saline (40μl), epidural bupivacaine (300μg/40μl), epidural morphine (15μg/40μl) and intramuscular bupivacaine (300μg/40μl). Epidural bupivacaine and morphine produced analgesia for approximately 45 and 60 minutes, respectively, while neither intramusclar bupivacaine nor intravenous morphine produced analgesia. Glucose utilization in the brain and in the thoracic and lumber spinal cord was decreased by epidural bupivacaine in amounts adequate to produce analgesia. On the other hands, with epidural morphine in amounts adequate to produce analgesia glucose utilization was unchanged both in the spinal cord and brain. The results suggest that epidural bupivacaine decreases the spinal cord glucose utilization by blocking sensory input to the spinal cord and through its direct effect on the spinal cord, while epidural morphine dose not affect the spontaneous neuronal activity in the spinal cprd even though it is sufficient to inhibit neuronal transmission of noxious stimuli. Since the decreases in cerebral glucose utilization with epidural bupivacaine were similar to those in intramusclar bupivacaine, decreases in cerebral glucose utilization in many structures with epidural bupivacaine might be due to systemic abosorption of the drug. However, the effects of the reduction in sensory input to the brain can not be entirely excluded. In summary, epidural bupivacaine and morphine in amounts adequate to produce analgesia has different metabolic effects on the spinal cord and brain possibly due to the different mechanisms of analgesia.