Maternal opioids decrease mu‐opioid receptor expression in the neonatal preBötzinger Complex

Abstract

An understudied population in the opioid crisis are infants exposed to in utero opioids experiencing severe respiratory deficits. Our recent work demonstrated maternal opioids destabilized neonatal breathing, increased apneas, and blunted opioid‐induced respiratory frequency depression; however, the mechanism(s) underlying these neonatal breathing deficits remains unclear. We hypothesized deficits in neonatal breathing after maternal opioids are due to downregulation of mu‐opioid receptors on key cell types (e.g. neurons and astrocytes), essential for respiratory rhythm generation and modulation in the preBötzinger Complex. While opioid receptor expression is downregulated in the whole brain after maternal opioids, it remains unclear if opioid receptor expression changes in the preBötzinger Complex after in utero opioids to induce neonatal breathing deficits. To assess the typical developmental changes in opioid receptors in the preBötzinger Complex, mu‐opioid receptor expression was first assessed using immunohistochemistry and confocal microscopy on neonatal neurons and astrocytes in the preBötzinger Complex at three ages: the day of birth (postnatal day 0, P0) when neonates must begin robust rhythmic breathing, an early neonatal age (postnatal day 4, P4), and during a critical developmental window (postnatal day 11, P11). At all ages, mu‐opioid receptors were expressed on both neurons and astrocytes in the preBötzinger Complex; however, mu‐opioid receptor expression decreased from P0 (n=6) to P4 (n=6) and was lowest at P11 (n=6). Preliminary data after maternal opioids suggest mu‐opioid receptor expression was lower in preBötzinger Complexes at P0 (n=2) and P4 (n=2), but returned to control levels by P11 (n=2). These results are further supported by in vitro brainstem‐spinal cord recordings of the cervical rootlets (C4) from neonates after maternal opioids, demonstrating decreased opioid sensitivity of isolated respiratory control circuitry after maternal opioids (n=2). Results from these studies will contribute to our understanding of the mechanisms contributing to impaired neonatal breathing after maternal opioids and may lead to better treatments for infants suffering breathing deficits after maternal opioids.