Increased brainstem microglia is associated with an attenuated hypercarbic ventilatory response in a rat model of neonatal opioid withdrawal syndrome

Abstract

Background Infants born with neonatal opioid withdrawal syndrome (NOWS) display abnormal respiratory patterns including tachypnea, apnea and impaired ventilatory responses to hypoxia and hypercarbia challenges. Infants with NOWS are also at an increased risk of sudden infant death syndrome (SIDS), which is associated with impairments in respiratory control and increased gliosis in key brainstem respiratory control regions including the nucleus tractus solitarius (nTS). Objective Use a rat model of neonatal opioid withdrawal syndrome to assess whether pre- and postnatal morphine exposure attenuates the ventilatory responses to hypoxia and hypercarbia challenges in the offspring and whether they are associated with increased nTS microglia expression. Design/Methods Pregnant female rats (dams) received twice-daily subcutaneous injections of morphine (5mg/kg) during the third (last) week of pregnancy to simulate 3rd trimester in utero opioid exposure. After birth, the pups received once-daily subcutaneous injections of morphine (1mg/kg), through postnatal day 10 to simulate postnatal morphine therapy. Whole-body plethysmography was used precisely 24 hours later to assess ventilatory defense responses to acute hypoxia (HVR) and hypercapnia (HCVR) challenge (5 minutes at 10% O2 and 5% CO2, respectively). Brainstems were removed to assess nTS microglia expression using immunohistochemistry and a glial (Iba-1) marker. Results Compared to saline-treated pups, morphine exposure attenuated the HCVR, but not the HVR (Fig. 1A). Morphine-exposed offspring also expressed increased number of microglia in the nTS (Fig. 1B). Conclusion The disturbances in neonatal hypercarbia challenge are associated with increased brainstem microglia expression, which could contribute to the etiology underlying the increased risk of SIDS in infants with NOWS.