Basal and cocaine-induced opioid receptor gene expression in the rat CNS analyzed by competitive reverse...

The basal mRNA levels of kappa and mu opioid receptors, as well as their regulation after 'binge' cocaine administration, were determined with competitive reverse transcription polymerase chain reaction (RT-PCR) in brain regions of male Sprague-Dawley rats. The procedure proved to be a reliable method to quantify the relative opioid receptor gene expression. The highest basal mRNA levels for the kappa opioid receptor were found in the nucleus accumbens and hypothalamus, whereas the highest basal mRNA expression for the mu opioid receptor was observed in the hypothalamus. Rats were separately treated with 'binge' (three hourly injections) cocaine HCl (45 mg/kg/day i.p.) or saline (1 ml/kg i.p.) for 2 days. A significant down-regulation of the kappa opioid receptor mRNA was detected in the nucleus accumbens. The mu opioid receptor mRNA was not affected. The data suggest a selective effect on kappa receptor expression in the nucleus accumbens as a consequence of 'binge' cocaine use.