REGULATION OF C-FOS EXPRESSION BY ACUTE AND CHRONIC MORPHINE ADMINISTRATION IN RAT-BRAIN

Effects of acute and chronic morphine administration on the expression of immediate early gene c-fos in different regions of the rat brain w as studied by Northern-blot hybridization. It was found that acute tre atment of rats with morphine (10 mg/kg) produced on average a 2-fold i ncrease in the levels of c-fos mRNA in cerebral cortex, hippocampus an d cerebellum as compared to saline injected animals. A maximal peak of c-fos mRNA induction was observed within 15-30 minutes after morphine injection. No increase of c-fos mRNA was detected in the hypothalamus , striatum, midbrain and brain steam where levels of c-fos induction a fter acute morphine administration were not substantially different fr om c-fos expression caused by injection of saline. Chronic treatment o f rats with morphine for 9 days abolished the ability of subsequent mo rphine administration to induce c-fos expression in the cerebral corte x. It is hypothesized that the observed regulation of c-fos mRNA level s by morphine represents a part of the long-term changes underlying th e development of opiate addiction.