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 [Delta F Ca2+ = 330 +/- 17 relative fluorescent units (RFU), Delta F Ca2+ = 331 +/- 22 RFU, and Delta F 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 (Delta F 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 (Delta F 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.