Presence of NMDA Receptor mRNA in the anterior pituitary of the female rat: Steroid modulation and changes during gonadotropin surge induction

N-methyl-D-aspartate (NMDA) receptors have been implicated in the regulation of anterior pituitary hormone secretion. A part of the regulatory effect of NMDA on pituitary hormone release could be due to a direct regulatory effect at the pituitary. The present study examined whether mRNA for the NMDA receptor is expressed in the anterior pituitary of female rats. Additionally, the effect of different steroids on pituitary NMDA receptor mRNA levels was also examined. Using Northern analysis, hybridization of the NMDA R1 cDNA probe to female rat anterior pituitary RNA yielded a 4.0-kb band, demonstrating the presence of the mRNA for the NMDA receptor in the pituitary. In the ovariectomized immature rat, estradiol treatment was found to significantly suppress pituitary NMDA receptor mRNA levels. Conversely, pituitary NMDA receptor mRNA levels were elevated by the treatment of estrogen-primed ovariectomized rats with progesterone. The increase of pituitary NMDA receptor mRNA levels by progesterone occurred at a time when progesterone induces an LH surge. Using the pregnant mare serum gonadotropin-primed immature rat, pituitary NMDA receptor mRNA levels were found to increase in a progressive manner throughout the afternoon of proestrus to reach a peak at 2000 h - the time of peak preovulatory LH surge levels. In conclusion, the present study demonstrates that the mRNA for the NMDA receptor is expressed in the anterior pituitary. The NMDA receptor mRNA in the pituitary is regulated by steroids with peak levels paralleling gonadotropin surge expression. As a whole, the present study provides further evidence that the pituitary could be an important site of action for neuroendocrine effects of excitatory amino acids.