Possible role of non-NMDA receptor-mediated neurotransmission in steroid-induced and preovulatory gonadotropin surges in the rat

The purpose of the present study was to determine the role of non-NMDA receptor-mediated neurotransmission in the expression and production of the progesterone-induced gonadotropin surge in the estrogen-primed ovariectomized adult rat and the preovulatory gonadotropin and prolactin (PRL) surges in the PMSG-primed immature rat. Adult female rats (45 days old) ovariectomized for 2 weeks were implanted with a third ventricular cannulae 1 week before the experiment. Estradiol (5 μg) was injected subcutaneously (sc) at 1700 h on Days 59 and 60 and either vehicle or progesterone (0.5 mg/rat) injected at 0900 h on Day 61. Vehicle or the non-NMDA antagonist DNQX (20 nmol/10 μl) was administered via the third ventricle cannula at 0800 and 1030 h on Day 61. From 1200 to 1800 h on Day 61 blood samples were withdrawn hourly from jugular cannulas for serum LH and FSH measurements. DNQX treatment completely blocked the progesterone-induced LH surge. FSH secretion, on the other hand, was unaffected by DNQX treatment. To study the effect of DNQX on preovulatory gonadotropin and PRL secretion, 27-day-old female rats were implanted with a third ventricle cannulae followed by sc injection of PMSG (8 IU) at 28 days of age (0800 h). On Day 30, either vehicle or DNQX (15 nmol/5 μl) were administered via the third ventricle cannula at 1100 and 1500 h. Groups of rats were sacrificed at 1600, 1800, and 2000 h on Day 30 for serum LH, FSH, and PRL measurements. DNQX treatment significantly attenuated the preovulatory LH surge at 1600 and 1800 h with no effects at 2000 h as the values began to return to baseline. As was the case in the steroid-treated rat, DNQX had no effect on the preovulatory FSH surge. DNQX treatment did, however, significantly attenuate the preovulatory surge of PRL at 1600 and 1800 h. Taken as a whole, the present studies support a role for non-NMDA receptor neurotransmission in the regulation of steroid-induced LH secretion as well as preovulatory LH and PRL surges. In contrast, non-NMDA receptors appear to play little to no role in regulating FSH secretion.