Abstract
Our previous studies have demonstrated that repeated cocaine (COC) administration reduces voltage-sensitive sodium and calcium currents (I(Na) or VSSCs and I(Ca) or VSCCs, respectively) in medium spiny nucleus accumbens (NAc) neurons of rats. The present findings further indicate that chronic COC-induced I(Na) 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 I(Na), while inhibition of protein phosphatase 1 (PP1) by phosphorylated dopamine- and cAMP-regulated phosphoprotein (M(r)=32 kDa) (DARPP-32) at the site of threonine 34 (p-Thr.34-DARPP-32) suppressed I(Na), in freshly dissociated NAc neurons of saline-pretreated rats. However, the effects of CaN on enhancing I(Na) were significantly attenuated, and the action of p-Thr.34-DARPP-32 to decrease I(Na) was mimicked, although not potentiated, by repeated COC pretreatment. Dephosphorylation of Na(+) channels by PP1 also enhanced I(Na), but this effect of PP1 on I(Na) 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and DARPP-32 were significantly decreased and increased, respectively, while the PP1 levels were unchanged, in the COC-withdrawn NAc as compared to saline-pretreated controls.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 PP1 modulate the increase in I(Na) via enhancing dephosphorylation, while p-Thr.34-DARPP-32 reduces I(Na) by inhibiting PP1-induced dephosphorylation, thereby stabilizing the phosphorylation state, of Na(+) channels in NAc neurons. They also suggest that chronic COC-induced I(Na) reduction may be attributed to a reduction in Ca(2+) signaling, which disrupts the physiological balance of phosphorylation and dephosphorylation of Na(+) channels.
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