Opioid use during gestation may result in neonatal opioid withdrawal syndrome (NOWS), a postpartum withdrawal syndrome that typically manifests 24–72 h after birth. NOWS is commonly characterized by incessant high-pitched crying, irritability, gastrointestinal issues, and sleep disturbances (Grossman and Berkwitt, 2019). Nationally, the incidence of NOWS increased by 82% between 2010 and 2017, and by 2018 approximately 80 newborns a day were diagnosed with NOWS (HCUP, 2021; Hirai et al., 2021). These infants have longer hospital stays and are readmitted to the hospital at higher rates than their healthy counterparts, further contributing to the economic consequences of NOWS (Shrestha et al., 2021).

Both pharmacologic and nonpharmacologic interventions are currently in use for the treatment of NOWS. The primary pharmacologic strategy for managing opioid withdrawal in neonates is replacement therapy with either morphine or methadone (Patrick et al., 2016). Recent evidence suggests that buprenorphine (BPN), a partial mu-opioid receptor (MOR) agonist and kappa-opioid receptor (KOR) antagonist commonly prescribed to adults to treat opioid use disorder (Shulman et al., 2019), may also be effective in reducing NOWS symptoms. Infants born to opioid-dependent women treated with BPN have shorter hospital stays, less severe withdrawal, and fewer neurobehavioral issues compared to those whose mothers were treated with methadone (Jones et al., 2010, Brogly et al., 2014, Coyle et al., 2012). Furthermore, opioid-exposed infants treated postnatally with BPN have shorter hospital stays and require a shorter treatment course than those treated with methadone (Hall et al., 2016) or morphine (Hall et al., 2018, Kraft et al., 2008, Kraft et al., 2017, Kraft et al., 2011). These findings warrant further investigation into the potential therapeutic effects of BPN in neonatal populations.

Chronic opioid exposure is known to induce MOR-mediated changes in signal transduction mechanisms (including, but not limited to receptor desensitization and internalization), which can alter opioid tone and ultimately lead to the development of tolerance, dependence, and hyperalgesia (Keith et al., 1998, Roeckel et al., 2016, Sternini et al., 1996, Whistler et al., 1999, Arden et al., 1995). The periaqueductal gray (PAG) is a brain region highly implicated in both opioid dependence and nociception (Jones and Barr, 2001, McPhie and Barr, 2009, van den Hoogen et al., 2021, Vazquez-Leon et al., 2021, Bozarth and Wise, 1984). Previous studies have shown that neonatal morphine-dependent rodents exhibit increased cFos expression in the PAG following naloxone or naltrexone-precipitated withdrawal (McPhie and Barr, 2009, van den Hoogen et al., 2021), and microinjection of an opiate antagonist directly into the PAG is sufficient to precipitate withdrawal (Jones and Barr, 2001). Furthermore, in adult rats, naloxone administration following repeated microinjections of morphine into the PAG produces thermal hyperalgesia that is associated with increased neuronal activity in the PAG (Lane et al., 2004).

Prolonged opioid exposure is thought to initiate the recruitment of brain stress and antireward systems, contributing to negative affective states during withdrawal (Koob and Le Moal, 2008). Opioids activate the hypothalamic-pituitary-adrenal axis by stimulating the elevated release of corticotropin-releasing hormone (CRH, also known as corticotropin-releasing factor) (Buckingham, 1982). Drug withdrawal is associated with the overactivation of the CRH system (Logrip et al., 2011). CRH has also been shown to promote dynorphin release and subsequent activation of the KOR (Land et al., 2008), which may further contribute to the withdrawal syndrome. Indeed, upregulation of the dynorphin/KOR system appears to underlie the aversive components of drug withdrawal (Koob, 2013).

The short and long-term neurodevelopmental effects of chronic early-life opioid exposure are well-characterized in the rodent literature (Harder and Murphy, 2019, Simmons et al., 2023, Byrnes and Vassoler, 2018, Fodor et al., 2014). This work has been paramount in enhancing our understanding of the neurobiological mechanisms underlying neonatal opioid exposure and withdrawal. However, the majority of these studies compare opioid-exposed offspring to drug-naive controls. Furthermore, very few preclinical studies have investigated whether BPN treatment is effective in reducing withdrawal severity following chronic exposure to a full agonist.

Our lab and others have demonstrated in mice that treatment with morphine during the first two postnatal weeks, a developmental period analogous to the third trimester of a human pregnancy (Semple et al., 2013), recapitulates several NOWS-related symptoms, including blunted growth, developmental delays, and elevated distress during withdrawal (Robinson et al., 2020, Borrelli et al., 2021). Using this model, we sought to test the hypothesis that acute BPN treatment can attenuate naloxone-precipitated withdrawal and hyperalgesia in morphine-dependent neonates. To further elucidate the neuroadaptive processes occurring after chronic opioid exposure and withdrawal, we investigated the effects of combined morphine and BPN exposure on stress and withdrawal-related gene expression in the PAG. Overall, we found that neonatal morphine exposure induced a robust naloxone-precipitated withdrawal phenotype that was alleviated by acute treatment with BPN. Although we did not observe morphine-induced hyperalgesia, BPN treatment significantly reduced thermal sensitivity in morphine-exposed mice. Lastly, regardless of BPN treatment, morphine-treated mice exhibited altered mRNA expression in the PAG, which may have important implications for the neurodevelopmental consequences of early-life opioid exposure.