Abused drugs modulate RGS4 mRNA levels in rat brain: comparison between acute drug treatment and a drug challenge after chronic treatment.

While different classes of abused drugs interact with distinct signaling substrates, it appears that all utilize receptors in the mesolimbic dopamine system to mediate their reinforcing effects. The regulator of G-protein signaling (RGS) proteins modulate G-protein coupled receptor (GPCR) signaling by increasing the rate of GTP hydrolysis of G proteins. This study was undertaken to determine whether morphine, cocaine, or amphetamine would modulate RGS4 mRNA levels in relevant brain regions. Acute administration of morphine and cocaine decreased levels of RGS4 mRNA in the reticulotegmental pontine nucleus (RtTg) and locus coeruleus (LC). Increases in RGS 4 mRNA levels were observed in the nucleus accumbens (NAc) and dorsal central gray (CGD). Acute drug challenge after chronic drug administration increased RGS4 mRNA in the CGD and decreased RGS4 levels in the red nucleus and RtTg. Interestingly, the LC exhibited biphasic modulation, with decreased RGS4 mRNA levels after acute administration and increased levels after chronic administration. These findings indicate that RGS4 mRNA levels are modulated in a similar manner by different drugs of abuse and imply that a common substrate could mediate some effects of abused drugs.