ADAPTIVE-CHANGES IN THE PROENKEPHALIN AND D-2 DOPAMINE-RECEPTOR MESSENGER-RNA EXPRESSION AFTER CHRONIC COCAINE IN THE NUCLEUS-ACCUMBENS ANDSTRIATUM OF THE RAT

The effects of single and repeated cocaine administration on proenkeph alin (PENK) and D-2 dopamine receptor mRNA expression in the nucleus a ccumbens and striatum of the rat were studied. Acute cocaine administr ation increased PENK expression of mRNA in the striatum and decreased it in both those structures after 24 and 48 h. D-2 receptor expression of mRNA fell after 3 h, returned to the control value after 24 h and rose after 48 h in both those brain regions following single cocaine i njection. Repeated cocaine increased PENK expression of mRNA after 3 h , but after 24 and 48 h depletion of mRNA expression was observed. On the other hand, a decrease in the D-2 expression of mRNA was found aft er 3 h in those structures, but no changes were found after 24 and 48 h following withdrawal. The obtained results suggest that in the nucle us accumbens and striatum there is an opposite regulation between PENK and D-2 receptor gene expression a short time after single and chroni c cocaine administration. Hence, these data provide further evidence f or the significance of the PENK and dopamine systems in the neurochemi cal mechanism of cocaine.