Effect of adrenalectomy on cocaine facilitation of medial prefrontal cortex self-stimulation

Abstract

Adrenalectomy (ADX) is known to block the acquisition of intravenous cocaine self-administration. A previous study therefore examined whether ADX decreases sensitivity of the `brain reward system' in general, or its response to cocaine in particular, by measuring thresholds for intracranial self-stimulation with and without concurrent cocaine administration. ADX had no effect on thresholds for lateral hypothalamic self-stimulation (LHSS) and did not alter the cocaine dose–response curve for lowering the LHSS threshold. This result suggested that ADX does not affect sensitivity of the brain reward system. However, medial prefrontal cortex (MPFC) appears to be an important site in the mediation of cocaine reinforcing effects, and MPFC self-stimulation (MPFCSS) is mediated by a neural substrate that is largely independent of that which mediates LHSS. The present study therefore assessed whether ADX diminishes cocaine facilitation of MPFCSS. It was found that the threshold-lowering effect of cocaine (5.0, 10.0 and 20.0 mg/kg, i.p.) did not differ between ADX rats maintained on 0.7% saline, ADX rats maintained on corticosterone (50 μg/ml) in 0.7% saline, and sham-operated controls. However, there was a trend toward desensitization of MPFCSS, itself, following ADX in the group that did not receive corticosterone supplementation. Based on this observation, and the similar responses of MPFCSS and cocaine self-administration to noncontingent priming stimulation, stress, and NMDA receptor antagonism, it is speculated that acquisition of MPFCSS and cocaine self-administration may be dependent upon a common sensitization process that is regulated by corticosterone.