NMDA receptor subunit NR2b: Effects on LH release and GnRH gene expression in young and middle-aged female rats, with modulation by estradiol

Background/Aims: The loss of reproductive capacity during aging involves changes in the neural regulation of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons controlling reproduction. This neuronal circuitry includes glutamate receptors on GnRH neurons. Previously, we reported an increase in the expression of the NR2b subunit protein of the NMDA receptor on GnRH neurons in middle-aged compared to young female rats. Here, we examined the functional implications of the NR2b subunit on the onset of reproductive aging, using an NR2b-specific antagonist ifenprodil. Methods: Young (3-5 months) and middle-aged (10-13 months) female rats were ovariectomized (OVX), 17β-estradiol (E₂) or vehicle (cholesterol) treated, and implanted with a jugular catheter. Serial blood sampling was undertaken every 10 min for 4 h, with ifenprodil (10 mg/kg) or vehicle injected (i.p.) after 1 h of baseline sampling. The pulsatile release of pituitary LH and levels of GnRH mRNA in hypothalamus were quantified as indices of the reproductive axis. Results: Our results showed effects of ifenprodil on both endpoints. In OVX rats given cholesterol, neither age nor ifenprodil had any effects on LH release. In E₂-treated rats, aging was associated with significant decreases in pulsatile LH release. Additionally, ifenprodil stimulated parameters of pulsatile LH release in both young and middle-aged animals. Ifenprodil had few effects on GnRH mRNA; the only significant effect of ifenprodil was found in the middle-aged, cholesterol group. Conclusion: Together, these findings support a role for the NR2b subunit of the NMDAR in GnRH/LH regulation. Because most of these effects were exhibited on pituitary LH release in the absence of a concomitant change in GnRH gene expression, it is likely that NMDA receptors containing the NR2b subunit play a role in GnRH-induced LH release, independent of de novo GnRH gene expression.