ABSTRACT: Opioid use during pregnancy can lead to negative infant health outcomes, including neonatal opioid withdrawal syndrome (NOWS). NOWS comprises gastrointestinal, autonomic nervous system, and neurological dysfunction that manifest during spontaneous withdrawal. Current treatments involve non-pharmacological and pharmacological interventions, however, there is no one standardized approach, in part because of variability in NOWS severity. To effectively model NOWS traits in mice, we used a third trimester-approximate opioid exposure paradigm, where neonatal inbred FVB/NJ and outbred Carworth Farms White (CFW) pups were injected twice-daily with morphine (10-15 mg/kg, s.c.) or saline (0.9%, 20 ul/g, s.c.) from postnatal day (P) one to P14. We observed reduced body weight gain, hypothermia, thermal hyperalgesia, and increased ultrasonic vocalizations (USVs). Neonatal USVs are emitted exclusively in isolation to communicate distress and thus serve as a model behavior for affective states. On P14, we observed altered USV syllable profiles during spontaneous morphine withdrawal, including an increase in Complex 3 syllables in FVB/NJ females (but not males) and in CFW mice of both sexes. Brainstem transcriptomics revealed an upregulation of the kappa opioid receptor (Oprk1), whose activation has previously been shown to contribute to withdrawal-induced dysphoria. Treatment with the kappa opioid receptor (KOR) antagonist, nor-BNI (30 mg/kg, s.c.), significantly reduced USV emission in FVB/NJ females, but not FVB/NJ males during spontaneous morphine withdrawal. Furthermore, treatment with the KOR agonist, U50,488h (0.625 mg/kg, s.c.), was sufficient to increase USV emission on P10 (both sexes) and on P14 (females only) in FVB/NJ mice. Together, these results indicate a female-specific recruitment of the dynorphin/KOR system in neonatal opioid withdrawal symptom severity.
