Cocaine facilitation of prefrontal cortex self-stimulation: a microstructural and pharmacological analysis

A novel self-stimulation methodology involving a fixed-interval (FI-5 s) schedule of reinforcement, microanalysis and threshold evaluation was used to investigate the effects of cocaine on rats lever pressing for electrical stimulation of the prefrontal cortex. Cocaine (15 mg/kg) increased medial prefrontal cortex (MPC) self-stimulation rates under FI-5 by a mean of 269% and reduced current thresholds for self-stimulation. A similar facilitation was evident with self-stimulation of the sulcal prefrontal cortex. Microanalysis showed that cocaine decreased inter-response times and post-reinforcement pauses, increased responding in the second and third quartiles of the inter-reinforcement interval (IRI) and decreased responding in the fourth IRI quartile. Schedule control of responding was still evident following cocaine despite the profound facilitation of response rates. Increased response rates were seen up to 48 h following a single dose of cocaine, suggesting sensitization of the PFC reinforcement substrate. The acute effects of cocaine on MPC self-stimulation were completely reversed by the dopamine (DA) D1 antagonist SCH 23390 0.02 mg/kg) and the D2 antagonist raclopride (0.3 mg/kg) but not by naloxone (0.5 mg/kg). These results are consistent with previous studies demonstrating the PFC as part of the neural substrate mediating cocaine reward. Further, these results implicate DA receptors in the reinforcing properties of both cocaine and MPC self-stimulation.