Fig. 1. Deregulated expression of MSN marker genes in the CPu, NAc and CeA allows to best discriminate between morphine and cocaine abstinence.

A We used qRT-PCR to compare the expression of 30 candidate genes, among which 14 previously identified HTT-related genes (purple characters), 13 additional MSN marker genes and Oxt as a marker transcript of social behavior, in the CPu, NAc, CeA and BNST under morphine and cocaine abstinence conditions compared to their saline counterparts. Hierarchical clustering organized gene expression data in three to six clusters per region. In a majority of these clusters, HTT-related and MSN marker genes displayed down-regulated expression under morphine conditions contrasting with up-regulated or unchanged expression under cocaine conditions. This pattern of gene expression was observed in the CPu (clusters a and c), NAc (cluster c), CeA (cluster c) and, to a lesser extent, in the BNST (cluster a). B Seven MSN marker genes displayed consistent down-regulated expression in the CPu, NAc and CeA under morphine conditions contrasting with no regulation or upregulation under cocaine abstinence, among which Grm4, coding for the mGlu4 receptor. Gene expression data are expressed as fold change versus vehicle abstinent for each condition (mean ± SEM, n = 5 per group). Comparison to vehicle group: p < 0.05 (two-tailed t-test, p value adjusted using Benjamin–Hochberg correction for multiple testing). qRT-PCR data used for clustering are displayed in Table S2.