Microinjection of CART (cocaine- and amphetamine-regulated transcript) peptide into the nucleus accumbens inhibits the cocaine-induced upregulation of dopamine receptors and locomotor sensitization.

Repeated exposure to addictive drugs enhances dopamine receptor (DR) signaling and the ultimate phosphorylation of the cyclic adenosine 5'-monophosphate (cAMP)-response element-binding protein (CREB)-regulated cocaine- and amphetamine-regulated transcript (CART) expression in the nucleus accumbens (NAcc). These effects are known to contribute to the expression of behavioral sensitization. CART peptides are neuropeptides that modulate drug reward and reinforcement. The present experiments investigated the effects of CART 55-102 microinjection into the NAcc on (1) the phosphorylation of CREB, (2) cAMP/protein kinase A (PKA) signaling and (3) extracellular signal-regulated kinase (ERK) phosphorylated kinase signaling. Here, we show that repeated microinjections into the NAcc of CART 55-102 peptides (1.0 or 2.5μg, 0.5μl/side) attenuates cocaine-induced enhancements of D1R, D2R and D3R phosphorylation in this sites. Furthermore, the microinjection of CART 55-102 followed by repeated injections of cocaine (15mg/kg) dose-dependently blocked the enhancement of cAMP levels, PKA activity and pERK and pCREB levels on the fifth day of cocaine administration. The cocaine-induced locomotor activity and behavioral sensitization in rats were also inhibited by the 5-day-microinjection of CART peptides. These results suggest that the phosphorylation of CREB by cocaine in the NAcc was blocked by the CART 55-102 peptide via the inhibition of D1R and D2R stimulation, D3R phosphorylation, cAMP/PKA signaling and ERK phosphorylated kinase signaling. These effects may have played a compensatory inhibitory role in the behavioral sensitization of rats that received microinjections of CART 55-102.