Withdrawal from cocaine self-administration alters nmda receptor-mediated Ca²⁺ entry in nucleus accumbens dendritic spines

We previously showed that the time-dependent intensification ("incubation") of cue-induced cocaine seeking after withdrawal from extended-access cocaine self-administration is accompanied by accumulation of Ca²⁺-permeable AMPA receptors (CP-AMPARs) in the rat nucleus accumbens (NAc). These results suggest an enduring change in Ca²⁺ signaling in NAc dendritic spines. The purpose of the present study was to determine if Ca²⁺ signaling via NMDA receptors (NMDARs) is also altered after incubation. Rats self-administered cocaine or saline for 10 days (6 h/day). After 45-47 days of withdrawal, NMDAR-mediated Ca²⁺ entry elicited by glutamate uncaging was monitored in individual NAc dendritic spines. NMDAR currents were simultaneously recorded using whole cell patch clamp recordings. We also measured NMDAR subunit levels in a postsynaptic density (PSD) fraction prepared from the NAc of identically treated rats. NMDAR currents did not differ between groups, but a smaller percentage of spines in the cocaine group responded to glutamate uncaging with NMDAR-mediated Ca²⁺ entry. No significant group differences in NMDAR subunit protein levels were found. The decrease in the proportion of spines showing NMDAR-mediated Ca²⁺ entry suggests that NAc neurons in the cocaine group contain more spines which lack NMDARs (non-responding spines). The fact that cocaine and saline groups did not differ in NMDAR currents or NMDAR subunit levels suggests that the number of NMDARs on responding spines is not significantly altered by cocaine exposure. These findings are discussed in light of increases in dendritic spine density in the NAc observed after withdrawal from repeated cocaine exposure.