[Changes in CNS catecholamine levels in tolerance to morphine-induced central excitation in mice]

Effects of a morphine single injection on catecholaminergic system in the brain were studied in mice treated with morphine-admixed food (MAF). The ambulatory activity was determined with the tilting cage method for 60 min after single morphine injection (10 mg/kg s.c.). After measurement of the ambulatory activity, mice were sacrificed and each brain was dissected out. Catecholamine levels were determined by HPLC-ECD system. Tolerance to morphine-induced ambulatory activity was developed by MAF (2 mg/g food) treatment for 5 days. The activating effect of a single morphine injection on dopamine metabolic rate in striatum and cortex in naive mice was reduced by MAF treatment. In midbrain and hippocampus, there was no influence of MAF treatment on the activation of dopaminergic neurons. It is suggested that dopaminergic neurons in striatum and cortex were hardly activated by morphine injection in MAF-treated mice, which might lead to tolerance to morphine-induced ambulatory activation.