Repeated cocaine administration calcineurin (PP2B) but enhances DARPP-32 modulation of sodium currents in rat nucleus accumbens neurons

Abstract

Our previous studies have demonstrated that repeated cocaine (COC) administration reduces voltage-sensitive sodium and calcium currents (I Na or VSSCs and ICa or VSCCs, respectively) in medium spiny nucleus accumbens (NAc) neurons of rats. The present findings further indicate that chronic COC-induced INa reduction in NAc neurons is regulated by decreased dephosphorylation and enhanced phosphorylation of Na + channels. Whole-cell voltage-clamp recordings revealed that dephosphorylation of Na+ channels by calcineurin (CaN) enhanced INa, while inhibition of protein phosphatase I (PPI) by phosphorylated dopamine- and cAMP-regulated phosphoprotein (Mr = 32 kDa) (DARPP-32) at the site of threonine 34 (p-Thr.34-DARPP-32) suppressed INa, in freshly dissociated NAc neurons of saline-pretreated rats. However, the effects of CaN on enhancing INa were significantly attenuated, and the action of p-Thr.34-DARPP-32 to decrease INa was mimicked, although not potentiated, by repeated COC pretreatment. Dephosphorylation of Na+ channels by PPI also enhanced I Na, but this effect of PPI on INa was not apparently affected by repeated COC administration. Western blot analysis indicates that the protein levels of CaN and DARPP-32 were significantly decreased and increased, respectively, while the PPI levels were unchanged, in the COC-withdrawn NAc as compared to saline-pretreated controls. Combined with previous findings, our results indicate that both CaN and PPI modulate the increase in INa via enhancing dephosphorylation, while p-Thr.34-DARPP-32 reduces INa by inhibiting PPI-induced dephosphorylation, thereby stabilizing the phosphorylation state, of Na + channels in NAc neurons. They also suggest that chronic COC-induced INa reduction may be attributed to a reduction in Ca2+ signaling, which disrupts the physiological balance of phosphorylation and dephosphorylation of Na+ channels. © 2005 Nature Publishing Group All rights reserved.