Differential effects of buprenorphine and pentazocine on the regional cerebral metabolic rate for glucose in the conscious rat.

The effects of buprenorphine (BNP, 10-200 micrograms/kg, i.v.) and pentazocine (PTZ, 2.5-10 mg/kg, i.v.) on the regional cerebral metabolic rate for glucose (rCMRglc) were analyzed in nine anatomically discrete areas of the conscious rat brain by the simultaneous use of [14C]2-deoxyglucose, the glucose analogue that can be phosphorylated in the brain, and [3H]3-O-methylglucose, a nonmetabolizable glucose analogue. Originally, this method was developed by Gjedde and Diemer in the rat and in humans. The rCMRglc was significantly decreased by BNP (100 or 200 micrograms/kg) in most of the brain regions investigated, except the cerebellum. In contrast, PTZ (10 mg/kg) significantly increased rCMRglc in the cerebral cortex and medulla. In the cerebral cortex and medulla, the direction of the effect on rCMRglc was opposite for BNP (22% decrease at the dose of 200 micrograms/kg) and PTZ (22% increase at the dose of 10 mg/kg). These findings strongly suggest that the discrepancies between the marked effects of BNP (a partial mu agonist and kappa antagonist) and PTZ (a mu antagonist and kappa agonist) on rCMRglc reflect the selectivity of agonist action at the different types of opioid receptors, mu and kappa receptors, in the rat brain.