Hormonal contraceptives in adolescence impact microglia in the rat medial prefrontal cortex and hippocampus
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Date
2024-05
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The Ohio State University
Abstract
Adolescence is a critical period of brain and behavioral maturation which, for females, is mediated in part by ovarian hormones. It is also a time when many individuals begin to use hormonal contraceptives (HCs), synthetic hormones that act to suppress endogenous ovarian hormone levels. Little is known about the extent to which neurodevelopmental processes in maturing brain regions like the prefrontal cortex (PFC) may be impacted by adolescent HC use. Our lab has shown that adolescent HC exposure alters measures of synaptic refinement and myelination in the female rat medial PFC (mPFC). These are two critical aspects of cortical development mediated by microglia, the brain’s resident innate immune cells. Thus, we hypothesize that HCs may affect mPFC maturation through alterations in microglia and the neuroimmune environment of the mPFC. Because the hippocampus (HPC) is another brain region that matures throughout adolescence, is highly populated with microglia, and is sensitive to ovarian hormones, we also sought to determine if microglia in the HPC would be sensitive to HCs.
Intact female Sprague-Dawley rats were randomly assigned to receive daily subcutaneous injections of vehicle or 10 ug ethinyl estradiol + 20 ug levonorgestrel (HCs) throughout adolescence from postnatal day (PND) 35-56. Daily vaginal lavage was performed beginning on PND 40. On PND 57-58, blood was collected for ELISA (n = 7-9/group) and ovarian weight measured. Brains were also collected for immunohistochemistry and microscopy (n = 10-12/group).
HC treatment was effective, with lavage indicating disrupted estrous cycling and ELISA quantification showing diminished luteinizing hormone concentration in HC-treated rats along with reduced relative ovarian weight. Less mPFC Iba1 (microglia) immunolabeling and fewer Iba1+ cells were observed in HC-treated rats. Additionally, HC treatment affected mPFC Iba1+ cell morphology, as indicated by shorter branches, fewer branch junctions, and larger soma sizes. The spatial relationship of microglia was also affected in the mPFC of HC-treated rats such that there was a reduction in the spacing index as well as a trend toward greater distance between nearest neighbors. In the HPC, microglial alterations following HC administration was observed in both the dorsal and ventral HPC, with HC-treated rats showing reductions in branch length, junction number, and branch number. These effects were largely restricted to the CA1 subregion. In addition, HC-treated rats displayed reduced Iba1 immunolabeling in only the CA1 region of the dorsal hippocampus.
Altogether, these data provide some of the first evidence demonstrating that one of the most widely used pharmaceuticals (HCs) given during a vulnerable developmental period (adolescence) affects microglia in the PFC and HPC to possibly influence the neurodevelopment of these brain regions.
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Keywords
Hormonal contraceptives, Sex hormones, Microglia, Adolescence, Neurodevelopment