Glucose Utilization in the Rat Brain during Chronic Morphine Treatment and Naloxone-Precipitated Morphine Withdrawal

Rates of local cerebral glucose utilization (LCGU) were measured in morphine-dependent and morphine-abstinent rats. Morphine pellets were implanted s.c. (one pellet 7 days before glucose utilization measurement and two pellets 4 days before the meas-urement) to produce opiold dependence. LCGU rates in 85 brain regions of placebo- and morphine-pelleted rats were similar. In contrast, LCGU rates, 5 hr after implantation of one morphine pellet, were decreased significantly in six areas. The lack of a chronic morphine effect on LCGU suggests tolerance to mor-phine. Additional support for this view was that an additional dose of morphine (8 mg/kg s.c.) in morphine-dependent rats was also ineffective in altering LCGU and in an antinociception (hot plate) test. Furthermore, plasma morphine levels in chronically treated animals were greater than those in acutely treated ani