Alterations in hypothalamic-pituitary-adrenal axis activity and in levels of proopiomelanocortin and corticotropin-releasing hormone-receptor 1 mRNAs in the pituitary and hypothalamus of the rat during chronic 'binge' cocaine and withdrawal.

Tolerance to the stimulatory effects of cocaine on the hypothalamic-pituitary-adrenal (HPA) axis develops after chronic 'binge' cocaine exposure in the rat. This blunting of HPA axis activity in response to cocaine is associated with a cocaine-induced reduction of corticotropin-releasing hormone (CRH) mRNA level in the hypothalamus. There is limited information about the effects of withdrawal from chronic cocaine on HPA activity. The present studies were undertaken to determine levels of the HPA hormones adrenocorticotropic hormone (ACTH) and corticosterone across 10 days of withdrawal following chronic 'binge' pattern cocaine administration (3 x 15 mg/kg/day at hourly intervals) for 14 days. Male Fischer rats showed a significantly attenuated HPA axis response to chronic 'binge' pattern cocaine administration 30 min after the last injection on the 14th day, as measured by both plasma ACTH and corticosterone levels at the nadir time point. Twenty-four hours following the final administration of 'binge' cocaine (the 1st day of withdrawal), a significant elevation of plasma ACTH levels and a modest, but significant, elevation of plasma corticosterone levels were found at the nadir time point. This acute withdrawal-related activation of the hormones of the HPA axis was no longer found on the 10th day of withdrawal. In the anterior pituitary, levels of both proopiomelanocortin (POMC) and CRH-receptor 1 (R1) mRNAs were significantly higher than saline controls on the 14th day of chronic 'binge' cocaine and were at control levels on the 4th day of withdrawal. In the neurointermediate lobe of the pituitary, a sustained reduction in POMC mRNA levels was observed on the 3rd, 7th and 14th day of chronic 'binge' cocaine, but POMC mRNA was at control levels by the 4th day of withdrawal. In the hypothalamus, POMC mRNA levels showed a transient decrease on the 1st day of 'binge' cocaine with no change during chronic 'binge' cocaine or its withdrawal. CRH mRNA levels in the hypothalamus were not different from saline controls on the 1st and 4th days of withdrawal. Taken together, the present results show that after development of adaptation or tolerance to chronic 'binge' cocaine there is an increase in HPA activity during acute cocaine withdrawal. In addition to being associated with CRH input from the hypothalamus, the activation of the HPA axis by cocaine withdrawal may be, at least in part, due to the increased POMC and/or CRH-R1 gene expression observed in the anterior pituitary after chronic 'binge' cocaine.