An Interaction of Oxytocin Receptors with Metabotropic Glutamate Receptors in Hypothalamic Astrocytes

Hypothalamic astrocytes play a critical role in the regulation and support of many different neuroendocrine events, and are affected by oestradiol. Both nuclear and membrane oestrogen receptors (ERs) are expressed in astrocytes. Upon oestradiol activation, membrane‐associated ER signals through the type 1a metabotropic glutamate receptor (mGluR1a) to induce an increase of free cytoplasmic calcium concentration ([Ca²⁺]_i). Because the expression of oxytocin receptors (OTRs) is modulated by oestradiol, we tested whether oestradiol also influences oxytocin signalling. Oxytocin at 1, 10, and 100 nm induced a [Ca²⁺]_i flux measured as a change in relative fluorescence [ΔF Ca²⁺ = 330 ± 17 relative fluorescent units (RFU), ΔF Ca²⁺ = 331 ± 22 RFU, and ΔF Ca²⁺ = 347 ± 13 RFU, respectively] in primary cultures of female post‐pubertal hypothalamic astrocytes. Interestingly, OTRs interacted with mGluRs. The mGluR1a antagonist, LY 367385 (20 nm ), blocked the oxytocin (1 nm )‐induced [Ca²⁺]_i flux (ΔF Ca²⁺ = 344 ± 19 versus 127 ± 11 RFU, P < 0.001). Conversely, the mGluR1a receptor agonist, (RS)‐3,5‐dihydroxyphenyl‐glycine (100 nm ), increased the oxytocin (1 nm )‐induced [Ca²⁺]_i response (ΔF Ca²⁺ = 670 ± 31 RFU) compared to either compound alone (P < 0.001). Because both oxytocin and oestradiol rapidly signal through the mGluR1a, we treated hypothalamic astrocytes sequentially with oxytocin and oestradiol to determine whether stimulation with one hormone affected the subsequent [Ca²⁺]_i response to the second hormone. Oestradiol treatment did not change the subsequent [Ca²⁺]_i flux to oxytocin (P > 0.05) and previous oxytocin exposure did not affect the [Ca²⁺]_i response to oestradiol (P > 0.05). Furthermore, simultaneous oestradiol and oxytocin stimulation failed to yield a synergistic [Ca²⁺]_i response. These results suggest that the OTR signals through the mGluR1a to release Ca²⁺ from intracellular stores and rapid, nongenomic oestradiol stimulation does not influence OTR signalling in astrocytes.     

Comparison of relative binding affinities to fish and mammalian estrogen receptors: The regulatory implications

Screening and testing of chemicals binding to estrogen receptors (ERs) emerge as an important issue in several regulatory programs or frameworks. Discrepancies exist, however, whether fish ERs should be included in the regulatory programs. In view of the differences in binding affinities to ERα and ERβ and the significant contribution of ERβ to biological effects of chemicals, it remains unknown whether both types of ERs are needed for the regulatory purposes. This study collected publications on binding affinities to both mammalian and fish ERs for 65 chemicals, covering a wide range of strong, moderate, weak and non-ER binders. Systematic evaluation of the data was performed in order to compare the difference in binding affinity of chemicals to fish and mammalian ERs and to subtypes of ERs. Except the reference estrogen 17β-estradiol, all 64 chemicals have differential values of relative binding affinity (RBA), which result mostly from the inter-laboratory tests other than interspecies differences. It is concluded that ER binding in one vertebrate species or one subtype of ERs could be extrapolated to other species or subtypes of ERs for most of chemicals for the regulatory purpose. Fish ERs are likely more sensitive to some chemicals of weak binders than mammalian ERs, suggesting the importance of including fish ERs in the regulatory programs. Issues on data interpretation and testing strategy for the regulatory purpose have been discussed.     

Estrogen: A master regulator of bioenergetic systems in the brain and body

Estrogen is a fundamental regulator of the metabolic system of the female brain and body. Within the brain, estrogen regulates glucose transport, aerobic glycolysis, and mitochondrial function to generate ATP. In the body, estrogen protects against adiposity, insulin resistance, and type II diabetes, and regulates energy intake and expenditure. During menopause, decline in circulating estrogen is coincident with decline in brain bioenergetics and shift towards a metabolically compromised phenotype. Compensatory bioenergetic adaptations, or lack thereof, to estrogen loss could determine risk of late-onset Alzheimer’s disease. Estrogen coordinates brain and body metabolism, such that peripheral metabolic state can indicate bioenergetic status of the brain. By generating biomarker profiles that encompass peripheral metabolic changes occurring with menopause, individual risk profiles for decreased brain bioenergetics and cognitive decline can be created. Biomarker profiles could identify women at risk while also serving as indicators of efficacy of hormone therapy or other preventative interventions.