Cocaine treatment- and withdrawal-induced alterations in the expression and serine phosphorylation of the NR1 NMDA receptor subunit.

Abstract

Repeated administration of cocaine alters the expression of the NMDA receptor subunits, NR1 and NR2B in a region- and withdrawal time-dependent manner. The present experiments extend these findings by characterizing the effects of cocaine withdrawal on specific NR1 splice variant expression. In addition, changes in the serine phosphorylation of NR1 and in the expression of postsynaptic density protein, PSD-95, were measured as potential molecular mechanisms for the cocaine-induced alterations in receptor subunit expression. Rats were injected with either saline or cocaine for 7 consecutive days and were sacrificed 24 h or 14 days following the last injection. Brain regions putatively identified in mediating the behavioral and neurochemical effects of cocaine, such as the frontal cortex, neostriatum, and hippocampus, were microdissected and used for immunoblotting experiments. In addition, drug-induced changes in NR1 phosphorylation in frontal cortex were investigated using immunohistochemistry. After 2 weeks of withdrawal from cocaine, NR1 subunit expression in the neostriatum was down-regulated approximately 27%, as compared to saline-treated control rats. Further, at 24 h but not 14 days of withdrawal, phosphorylation of serine residues 896 and 897 was reduced approximately 34% in the frontal cortex of rats treated with cocaine, as compared to controls. Results suggest that early changes in kinase or phosphatase activity may contribute to prolonged cocaine-induced alterations in NR1 expression. Thus, NR1 phosphorylation could be important for the activation of pathways that are substrates for the effects of cocaine exposure and withdrawal.