Abstract
Repeated noncontingent cocaine injections, which lead to behavioral sensitization, increase AMPA receptor (AMPAR) transmission in the rodent nucleus accumbens (NAc) in a withdrawal-dependent manner. On withdrawal days (WD) 10-21, this is attributable to upregulation of GluA1A2-containing AMPARs. However, synaptic incorporation of GluA2-lacking/Ca(2+)-permeable AMPARs (CP-AMPARs) was observed after longer withdrawal (WD35) from repeated noncontingent cocaine injections in young mice (Mameli et al., 2009). CP-AMPARs had previously been observed in NAc synapses only after prolonged (WD30-WD47) withdrawal from extended-access cocaine self-administration. Our goal was to determine whether rats receiving repeated noncontingent cocaine injections during adulthood similarly exhibit CP-AMPARs in the NAc after prolonged withdrawal. For comparison, we began by evaluating CP-AMPARs on WD35-WD49 after extended-access cocaine self-administration. Confirming our previous results, whole-cell recordings revealed inwardly rectifying AMPAR EPSCs, a hallmark of CP-AMPARs. This was observed in both core and shell. Next, we conducted the same analysis in adult rats treated with eight daily noncontingent cocaine injections and recorded on WD35-WD49. AMPAR EPSCs in core and shell did not show inward rectification and were insensitive to 1-naphthylacetylspermine (a selective antagonist of CP-AMPARs). Locomotor sensitization could still be demonstrated after this long withdrawal period, although the upregulation of GluA1A2-containing AMPARs observed at earlier withdrawal times was no longer detected. In conclusion, in adult rats, accumulation of synaptic CP-AMPARs in the NAc occurs after prolonged withdrawal from extended-access cocaine self-administration but not after prolonged withdrawal from noncontingent cocaine injections.