Regulators of G-protein signaling (RGS) in rat striatum: Response to acute and repeated amphetamine administration

Amphetamine induces the desensitization of receptors within the rat striatum through a poorly understood process. The recently identified family of negative regulators of G-protein signaling (RGS) prompted us to identify and examine the expression of RGS mRNAs in response to acute and repeated amphetamine. A subset of RGS were cloned from the rat striatum. We discovered that acute amphetamine dynamically regulates RGS expression levels. For RGS2 this pattern strongly resembled that of the immediate early gene c-fos; RGS9 was uniquely depressed by amphetamine. The RGS9 protein is expressed as two isoforms: RGS9S and RGS9L. RGS9L is exclusively expressed within the basal ganglia, while RGS9S is exclusively expressed in pineal and retinal. This pattern of tissue specificity is conserved between rat and human. In contrast, RGS2 is widely expressed in brain and somatic tissues of rat and human. RGS9L contains a carboxy-terminal polyproline domain of unknown function. We found both RGS2 and RGS9 to be expressed within 1D-substance-P neurons of the striatum and a population of D1-substance-P negative neurons that are presumably D2-enkephlin neurons. We also examined the pharmacology of the RGS2 response in response to acute amphetamine using select dopamine D1 and D2 receptor antagonists. The D1 antagonist SCH23390 blocked the induction of RGS2 by amphetamine, while the D2 antagonist raclopride by itself induced RGS2 mRNA. Amphetamine in combination with raclopride enhanced---in an additive manner---the induction of RGS2 mRNA by amphetamine. Together these results suggests that the D1 receptor pathway, expressed within substance-P neurons, is involved in the induction of RGS2 by amphetamine. Because of the many similarities between RGS2 and c-fos, we examined the effects of repeated amphetamine on RGS2 (and RGS3, RGS4 and RGS5) expression. Repeated amphetamine desensitizes the capacity of c-fos to be induced by a challenge injection of amphetamine. RGS5 was also profoundly desensitized by repeated amphetamine, while RGS2 and RGS3 were not desensitized. RGS4 was not effected by either acute or repeated amphetamine. This result suggests that RGS2 and RGS3 may continue to regulate Galphai- and Galphaq-coupled signal transduction pathways within striatal neurons during the course of repeated amphetamine treatment.