Do GnRH neurons express the gene for the NMDA receptor?

Previous studies have revealed that in several animal models, N-methyl-d,l-Aspartate (NMA) stimulates LH secretion by acting at a suprapituitary site. In addition, NMDA receptor antagonists appear to block GnRH neuronal activation on the afternoon of proestrous as evidenced by the lack of c-Fos expression in the neurons and by the absence of an ovulatory LH surge. However, administration of NMA does not induce c-Fos or c-Jun expression in GnRH neurons. To better understand the effects of NMDA receptor activation on GnRH neuronal function, we examined whether GnRH neurons express the NMDA receptor in male rats, and in female rats during diestrus and proestrus, by performing double label in situ hybridization. An ³⁵S-labeled cRNA probe for the NMDA receptor subunit (NMDAR1) was used to quantify NMDAR1 mRNA and a digoxigenin-labeled cRNA probe for GnRH was used to identify GnRH neurons. The data were quantified and expressed as grains/average cell area. In male and female rats, less than 5% of GnRH neurons expressed grain levels twice the minimum detectable level and were considered double-labeled. However, many non-GnRH neurons in the same areas as GnRH neurons expressed high levels of NMDAR1 mRNA. These results suggest that the effects of NMA on GnRH secretion are unlikely to be mediated solely by the activation of NMDA receptors on GnRH neurons. Given the widespread expression of NMDAR1 mRNA in the hypothalamus, it is possible that the stimulatory effects of NMA on GnRH neurons are indirect through activation of other neurons.