This report includes work arising from the following clinical protocols: NCT00026832, NCT00100360, NCT00001177, and NCT00001322. Previous studies have identified the presence of abnormalities in resting state activity and reward processing in mood disorders. Indeed, altered resting state function represents an intriguing possible explanation for the generation of ovarian hormone-related state changes characterized by predictable alterations in social, cognitive (e.g., autobiographical memory), and affective function in women with PMDD. Results from our resting state H215O PET studies in healthy premenopausal women (n = 40) demonstrate effects of both hypogonadism and ovarian steroids on resting rCBF in several brain regions, including BA25, insula, and medial PFC (mPFC). Evidence also suggests that genomic variation in sex-steroid receptors or in the systems regulated by sex steroids may contribute to the ovarian hormone-sensitivity observed in PMDD. Thus, as a first step, we examined the effects of common allelic variations in women during each of the hormonal conditions established within the GnRH agonist-induced hypogonadism and ovarian steroid addback protocol. As a first pass, we have tested specific, common polymorphisms (BDNF Val66Met and COMT Val158Met) that are molecularly functional, regulated by sex steroids and that are known to alter brain function, affect and cognition. In these studies, therefore, we employ these polymorphisms as probes for the involvement of these genes and the molecules they modulate in ovarian steroid behavioral/cognitive sensitivity. First, we examined the interaction between BDNF Val66Met polymorphism and ovarian hormones on PET measurements of rCBF and fMRI BOLD signal while women performed a well-established working memory (WM) task (i.e. the n-back). Preclinical studies have documented multiple sites of interaction between estradiol and BDNF: estradiol increases BDNF function by binding to a putative estrogen response element (ERE) on the BDNF gene and/or by inducing the BDNF receptor, TrkB, and estradiol and BDNF activate similar signaling cascades and pathways through estrogen receptors and TrkB; respectively. Additionally, both BDNF and estradiol alone facilitate neural activity in the hippocampus and conjointly influence neural growth, survival, and plasticity in the hippocampus. In contrast, the BDNF Met protein has been shown to impair intracellular processing and activity-dependent modulation of BDNF in transfected hippocampal neuronal cultures. Finally, the humanized BDNF mouse displays atypical estrous cycle-related variations in both anxiety-like behaviors (i.e., open field, elevated-plus maze) and object placement memory compared with wild type mice. Moreover, these ovarian steroid-regulated behaviors in the humanized mouse are accompanied by alterations in hippocampal plasticity, relevant to both our understanding of behavioral sensitivity to ovarian steroids and stress-related disorders. We focused on neural recruitment in the DLPFC and hippocampus. In addition to the well-established n-back activation of the prefrontal cortex (PFC) during WM, there is also a well-documented de-activation of the hippocampus observed in healthy controls at higher cognitive loads (possibly reflecting an uncoupling of hippocampal input to optimize prefrontal involvement in the increasing working memory load). For the PET analyses, 29 women were Val homozygotes and 10 were Met carriers; for the fMRI analysis there were 20 Val homozygotes and seven Met carriers. Participants were matched across genotypes for age, racial distribution, and handedness. Additionally, neither genotype nor hormone effects were observed in either depressive symptom ratings or working memory performance. In only Met carriers (i.e., women with the BDNF met allele in replace of the normal Val allele), the hippocampus was abnormally activated (not deactivated) during estradiol but not during the hypogonadal state or progesterone replacement. Separate analyses of the 0-back and 2-back indicate that these changes were due to neural activity during the working memory (2-back) condition and not the sensorimotor (0-back) control task. This study is the first in women to demonstrate a BDNF genotype by estradiol interaction on a cognitively-related neurophysiological response. These data suggest that the Met allele of the BDNF gene conveys an abnormal sensitivity to the presence of estradiol on hippocampal function, similar to that reported in the Met knock-in mice compared with wild type mice. In a companion collaboration with the Rockefeller University, we examine the effects of ovariectomy and estradiol replacement on both behavior and hippocampal gene expression in the female humanized BDNF heterozygous Met mouse. Preliminary data from these translational studies suggest that heterozygous Met mice show a differential behavioral sensitivity to estradiol in several hippocampal tasks and in anxiety-like behaviors. Preliminary data from whole transcriptome RNA sequencing in the BDNF heterozygous Met mouse (hippocampus) and in women with PMDD (lymphoblastoid cell lines) show a significant overlap in the expression of several gene networks stimulated by estradiol but a similar overlap in gene expression was observed with estradiol stimulation in neither wild type mice nor asymptomatic control women. This cross-species overlap in gene expression suggests the potential physiologic (and behavioral) relevance of the BDNF heterozygote met mouse to our understanding of PMDD. In a second study, we employed similar methods to examine the impact of the COMT Val158Met genotype on PET measured rCBF in the PFC. COMT plays an important role in the regulation of intra-synaptic dopamine levels in the PFC and also has a high affinity for the hydroxylated metabolites of estradiol/estrone (i.e., catecholestrogens). Moreover, the activity of COMT is reported to be both sexually dimorphic and modulated by estradiol (possibly by a putative ERE on the COMT gene in some human tissues). Consistent with a recent meta-analysis, whole-brain voxel-wise analysis revealed a significant COMT genotype by hormone interaction in the DLPFC, a finding that reflected estradiol-related changes in both Val and Met homozygotes. Specifically, the Met homozygotes (n = 11) showed an increase in DLPFC activation during estradiol compared with hypogonadism, whereas Val homozygotes (n = 11) showed the opposite pattern with lower activation during estradiol. DLPFC activation in heterozygotes (n = 13) was intermediate between the two homozygous groups. Significant effects of genotype were observed only during estradiol conditions, with Met homozygotes having significantly increased DLPFC activation compared with Val homozygotes. These data are consistent with observations that both COMT genotype and the presence or absence of estradiol influence dopaminergic function in the PFC. The mechanisms underlying this observation remain to be defined but are consistent with estradiol impacting PFC dopamine tuning through effects on DA synthesis and/or COMT enzyme activity, with attendant alterations in DLPFC efficiency. Overall, the findings from these studies demonstrate that in healthy women, variations in genes relevant to ovarian steroids can impact functional neurocircuitry and network level gene expression in regionally- and hormonally-specific ways here, specifically, interactions between BDNF and COMT genotypes and estradiol on the activation of functional neurocircuitry in the hippocampus (where BDNF is highly expressed) and the DLPFC (where COMT has primacy for dopamine trafficking), respectively. In studies currently underway, we will examine these gene-hormone interactions in reward-responsive and resting state neural circuits.
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