Melatonin Partially Attenuates Oxycodone-Induced Placental Stress Signaling and Fetal Brain Apoptosis in a Sex-Specific Manner

BACKGROUND: Maternal oxycodone (oxy) exposure can disrupt placental function and fetal neurodevelopment, but the molecular mechanisms remain unclear. We investigated whether prenatal oxy exposure activates inflammation and stress response pathways in the placenta and fetal brain, and if maternal melatonin supplementation attenuates these effects. METHODS: Female Sprague-Dawley rats received either saline or oxy via oral gavage for 15 days before mating (10-15mg/kg/day dose escalation) and throughout pregnancy (15mg/kg/day). From gestational day (GD) 12.5, half of the dams received melatonin (10mg/kg/day). On GD 19.5, placental and fetal brain tissues were collected. Changes in expression of markers of oxidative stress, antioxidant defense signaling, inflammation, ER stress, and apoptosis were assessed by western blotting. Data were analyzed by two-way ANOVA with Tukey's post hoc test. RESULTS: Neither oxy exposure nor melatonin treatment increased markers of oxidative stress or antioxidant defenses in the placenta and fetal brain. Oxy exposure increased placental IL-1β expression but did not alter expression of the other inflammatory markers examined. Oxy increased phosphorylation of eIF2α and increased the phospho-eIF2α:eIF2α ratio in the placentas of male fetuses, and fetal brains of both sexes. CHOP expression was increased in the placentas and brains of female, but not male fetuses after oxy exposure. Oxy exposure increased levels of cleaved caspase-3 and cleaved caspase-9 in the fetal brain, but not the placenta; melatonin treatment attenuated the oxy-induced increase in cleaved caspase-9, but not cleaved caspase-3. CONCLUSION: Prenatal oxy exposure induced a modest inflammatory response in the placenta and activated the integrated stress response and intrinsic apoptotic signaling in the fetal brain. Maternal melatonin supplementation partially mitigated the oxy-induced upregulation of caspase-9 but did not prevent stress signaling in either tissue. These findings demonstrate the presence of sex-specific placental and fetal brain responses to prenatal oxy exposure but suggest that melatonin may not provide complete protection against oxy-induced neurodevelopmental impairment.