Behavioral and biological adaptations underlying neonatal opioid withdrawal syndrome

Opioid-linked overdose death rates have reached unprecedented levels in the United States. The growing incidence of Opioid Use Disorder (OUD) is concomitant with elevated rates of OUD in women during pregnancy and through parturition. Neonates born to mothers with active OUD can develop opioid dependence in utero and display various signs of postnatal withdrawal, a condition termed Neonatal Opioid Withdrawal Syndrome (NOWS). Common symptomatic features of NOWS include sleep disturbances, low birth weight, altered heart and respiratory rates, increased irritability, high-pitched crying, feeding difficulties. Many of these symptomatic presentations are driven by dysregulated function of the autonomic nervous system and hyperirritability of the sympathetic nervous system. Given the increasing incidence of NOWS, there is an alarming lack of knowledge regarding the long-term effects of perinatal opioid exposure on behavioral and neurodevelopmental outcomes. Murine models provide efficient means to understand the neurobiological adaptations impacted by opioid exposure during perinatal neurodevelopment that drive long-term effects on cognitive, social, affective, and reward-related behaviors. We describe a rodent model of third-trimester-equivalent opioid exposure which produces replicable, opioid withdrawal-related phenotypes including robust thermal hyperalgesia and altered ultrasonic vocalization (USV) profiles. We present results from two drug regimens of the model, differing in the schedule of opioid administration (once or twice daily injections of morphine from postnatal day (P) 1-14; 15.0 mg/kg). Beyond hyperalgesia and altered USV profiles, both drug regimens lead to weight loss. Furthermore, both models resulted in transcriptional adaptations within brain regions relevant to opioid dependence and withdrawal. Twice-daily exposure resulted in sex-specific changes in metabolic gene expression in the brainstem, while once-daily exposure resulted in down-regulation of genes related to myelin and dopaminergic circuitry development in the nucleus accumbens. We found minimal evidence for behavioral consequences associated with once-daily morphine exposure during adulthood; there were no significant effects of perinatal morphine on cognitive, reward-related, or fear learning tasks. This could potentially indicate compensatory mechanisms that mitigate the adverse effects of third trimester-equivalent morphine exposure over time. Lastly, we identified epigenetic mechanisms potentially driving dysregulation of normal development within the central nervous system following pre-natal opioid exposure in humans. We analyzed placental samples from pregnancies with opioid exposure and identified gene networks containing altered DNA methylation patterns. Notably, we found enrichment within the ‘integral component of the plasma membrane’ and ‘synapse assembly’ functional networks, indicating potential effects of prenatal opioid exposure on neural connectivity and transmission. Together, the transcriptional adaptations identified in rodent brain tissue and the epigenetic modifications identified in human placental tissue provide novel mechanistic insight as to how perinatal opioid exposure impacts neural and fetal development