Intra-medial prefrontal cortex injection of quinpirole, but not SKF 38393, blocks the acute motor-stimulant response to cocaine in the rat.

RATIONALE

Considerable evidence suggests that the medial prefrontal cortex (mPFC) is an important region in mediating certain behavioral and neurochemical responses to cocaine. However, a role for cortical dopamine (DA) receptor subtypes in modulating these responses has yet to be elucidated.

OBJECTIVES

This study investigated the effects of intra-mPFC administration of DA agonists on the acute motor-stimulant response to cocaine. In addition, in vivo microdialysis techniques were employed to determine the effects of intracortical injection on cocaine-induced extracellular DA concentrations in the nucleus accumbens (NAC).

METHODS

One week following bilateral cannulae implantation over the mPFC and the NAC (for dialysis experiments), male Sprague-Dawley rats received an intra-mPFC injection of saline, the DA D2-like agonist quinpirole (0.015, 0.05, 0.15, 0.5, 1.5, or 5.0 nmol per side) or the partial DA D1-like agonist SKF 38393 (0.5, 1.5, or 5.0 nmol per side) approximately 5 min before peripheral administration of saline or cocaine (15 mg/kg, i.p.). For dialysis experiments, only the highest dose of quinpirole was examined.

RESULTS

Pretreatment with quinpirole produced a dose-dependent decrease in cocaine-induced motor activity, with the highest doses resulting in a complete abolition of the acute motor-stimulant response to cocaine. In contrast, intra-mPFC administration of SKF 38393 was not shown, at the doses tested, to alter cocaine-induced motor activity. In agreement with the behavioral effects, intra-mPFC quinpirole injection (5 nmol per side) significantly blocked cocaine-induced DA overflow in the NAC.

CONCLUSIONS

The results of the present study provide additional support that the mPFC is a neural substrate through which cocaine, in part, produces its motor-stimulant effects. In addition, these data suggest that modulation of cortical DA D2 receptors can block acute cocaine-induced behavioral (locomotor activity) and neurochemical (DA concentrations in the NAC) responses in the rat.