Shared and divergent transcriptomic regulation in nucleus accumbens D1 and D2 medium spiny neurons by morphine
Substance use disorders (SUDs) induce widespread molecular dysregulation in nucleus accumbens (NAc), a brain region pivotal for coordinating motivation and reward, which is linked to neural and behavioral disturbances promoting addiction. Despite the overlapping symptomatology of SUDs, different drug classes exert partly unique influences on neural circuits, cell types, physiology, and gene expression. To better understand common and divergent molecular mechanisms governing SUD pathology, we characterized the cell-type-specific restructuring of the NAc transcriptional landscape after psychostimulant or opioid exposure. We combined fluorescence-activated nuclei sorting and RNA sequencing to profile NAc D1 and D2 medium spiny neurons (MSNs) across morphine exposure paradigms, including initial exposure, prolonged withdrawal after repeated exposure, and re-exposure post-withdrawal. Our analyses reveal that D1 MSNs display many convergent transcriptional responses, whereas D2 MSNs manifest highly divergent responses, with morphine causing far more adaptations in this cell type. Overall design: Male double-transgenic mice expressing D1- or D2-specific nuclear-tagged GFP were generated by crossing LSL-eGFP::L10a (IMSR_JAX:022367) mice with either D1-Cre (MGI:3836633) or D2-Cre (MGI:3836635) mice. Morphine sulphate (from the National Institute on Drug Abuse) were dissolved in 0.9% saline and administered intraperitoneally at doses of 20 mg/kg or 10 mg/kg, respectively. These doses were chosen based on their roughly equivalent rewarding effects, as established in prior conditioned place preference experiments. In the cocaine cohort, mice received saline or morphine each day for 15 days prior to withdrawal. As depicted in Figure 1A, after 30 days of homecage withdrawal, mice received challenge injections of saline, or morphine and were sacrificed 1 h later. Treatments: SS = Saline exposure during acquisition, with Saline bolus during the challenge SM = Saline exposure during acquisition, with Morphine bolus during the challenge MS = Morphine exposure during acquisition, with Saline bolus during the challenge MM = Morphine exposure during acquisition, with  Morphine bolus during the challenge