Influence of chronic hypoxia on optical density of μ-opioid receptors in fetal rat brain

Fetal brain is considered to be the major body organ, critical for the future quality of human life. Offspring exposed to prenatal hypoxia has been evidenced to experience behavioral abnormalities as a result of the injury sustained by neuronal cells in the brain. The relatively early appearance of opioid receptors proved susceptible to endogenous and exogenous factors. Increased concentrations of neurotransmitters in the maternal circulation and amniotic fluid induced by hypoxic exposure imply their role in the regulation of cellular division and differentiation processes. Endogenous neuropeptides and specific opioid receptors are distributed in those brain structures that are associated with behavior and reproduction. Fetuses exposed to the adverse effects of increased opioid level incur structural brain tissue abnormalities. Objectives: The present study seeks to determine the effects of long-term hypoxic exposure during gestation on the expression of opioid receptors in specific brain regions in both sexes. Material and methods: The study was conducted on pregnant Sprague-Dawley rats, (120 days old, body weight between 250 and 300 g). Experiments were carried out in order to determine the effect of long-term hypoxia on mu-opioid receptor density in selected structures of fetal central nervous system: caudate- putamen (CPu), zona germinata (ZG), nucleus accumbens (NA), olfactory tubercle (OT), Median Part Medial Preoptic Area (MMPoA) and Lateral Part Medial Preoptic Area (LMPoA). Pregnant female rats were assigned to two research groups: the control group (N=6) and the experimental group subject to prolonged hypoxia for 24 hours from the gestational day 15 to gestational day 20 (E-15-E20). At E-21 rats were sacrificed, their fetuses were removed and their brains were incubated with radioligands. The mu-opioid receptor incubation in selected brain structures was performed with a specific radioisotope [3H]DAMGO [tyrosyl-3,5,-3H(N)-D-Ala-Gly-N-methyl-Phe-Gly-enkephalin]. Optical density of mu-opioid receptors was determined at E-21 of gestation during long-term exposure to chronic hypoxia induced from E-15 to E-21 of gestation. Experimental model coupled with an innovative autoradiography allowed for a precise assessment of the lesions sustained by fetal brain tissues due to hypoxia and the adaptive mechanisms of the central nervous system in reaction to hypoxic exposure. Results: Statistically significant chronic hypoxia (p<0.05) downregulated the values of mu-opioid receptors optical density in relation to control group in CPu and ZG. Chronic hypoxia in ZG substantially reduces the values of mu-opioid receptors optical density in males (p<0.05). The differences among remaining groups did not show to be statistically significant. Conclusions: The obtained results of mu-opioid receptor expression can be detected in specific fetal brain regions that mediate sexual behavior and may be attributable to behavioral changes of experimental animals due to hypoxic exposure during gestation.