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.