Long-term abstinence from developmental cocaine exposure alters Arc/Arg3.1 modulation in the rat medial prefrontal cortex

Cocaine is a psychostimulant whose abuse causes a social and economic burden for our society. Most of the published literature deals with acute effects of cocaine or short-term abstinence in adult animals but much less information exists on neuroplastic changes following long-term abstinence. We have recently shown that the long-term abstinence following developmental exposure to cocaine results in increased Activity-Regulated Cytoskeletal- associated protein (Arc/Arg3.1) expression in the crude synaptosomal fraction (Giannotti et al. Int J Neuropsychopharmacology 7(4):625-634, 2014). Given that Arc/Arg3.1 localizes not only at active synapse but also in the nucleus (Okuno et al. Cell 149:886-898, 2012; Korb et al. Nat Neurosci 16:874-883 2013; Bloomer et al. Brain Res 1153:20-33 2007), we investigated Arc/Arg3.1 protein levels in the whole homogenate and the nuclear fraction of animals exposed to cocaine during adolescence. We observed the increased expression of Arc/Arg3.1 in both the fractions, suggesting that up-regulation of Arc/Arg3.1 protein may be partly due to the increased nuclear expression of Arc/Arg3.1 in the medial prefrontal cortex (mPFC) of rats sacrificed at postnatal day 90, following 48 days of abstinence. This effect seems to cause reduced Gria1 transcription. We also found reduced expression of fragile X mental retardation gene (FMR1) which normally inhibits Arc/Arg3.1 translation together with reduced expression of Ubiquitin-protein ligase E3A (Ube3a) that normally causes Arc/Arg3.1 protein degradation via ubiquitination. Further, we found increased expression of metabotropic glutamate receptor 5 (GRM5) which is also involved in the regulation of Arc/Arg3.1 expression. Taken together, our findings show that abstinence from developmental exposure to cocaine is associated with alterations in the finely tuned mechanisms that regulate Arc/Arg3.1 expression.