Adaptive changes in the proenkephalin and D2 dopamine receptor mRNA expression after chronic cocaine in the nucleus accumbens and striatum of the rat.

The effects of single and repeated cocaine administration on proenkephalin (PENK) and D2 dopamine receptor mRNA expression in the nucleus accumbens and striatum of the rat were studied. Acute cocaine administration increased PENK expression of mRNA in the striatum and decreased it in both those structures after 24 and 48 h. D2 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 injection. 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 D2 expression of mRNA was found after 3 h in those structures, but no changes were found after 24 and 48 h following withdrawal. The obtained results suggest that in the nucleus accumbens and striatum there is an opposite regulation between PENK and D2 receptor gene expression a short time after single and chronic cocaine administration. Hence, these data provide further evidence for the significance of the PENK and dopamine systems in the neurochemical mechanism of cocaine.