Effects of Phenobarbital-induced Ovulatory Delay on the Follicular Population and Serum Levels of Stei-oids and Gonadotropins in the Hamster: A Model for Atresia1

Abstract

A model was developed for the study of atresia of preovulatory follicles in the hamster. The effects of ovulatory delay on the ovarian follicular population, number of ova spontaneously ovulated and serum concentrations of steroids and gonadotropins were assessed. This model was based on earlier observations where atresia of rat preovulatory follicles was reported after 3 or 4 days of continued postponement of ovulation with sodium pentobarbital. Cyclic hamsters weighing 90-110 g were maintained on a 14 h of light: 10 h of dark schedule (lights on at 0500 h). Hamsters injected s.c. with 6.5 mg phenobarbital (Phen)/100 g body wt 1300 h on proestrus exhibited 1 day of ovulatory delay. Hamsters injected with 6.5 mg Phen at 1300 h on proestrus and also with 13 mg Phen at 1200 h the next day exhibited a 2 day delay; an additional injection of 19.5 mg Phen at 1200 h on the following day was required for 3 days of ovulatory delay. Histological examination of the ovary revealed that the diameter of antral follicles on proestrus (561 .+-. 12 .mu.m) increased significantly after 1 day of ovulatory delay (680 .+-. 11 .mu.m). After 2 days of ovulatory delay, no significant alteration of antral follicular diameter was observed; on Day 3 of ovulatory delay, the original set of preovulatory follicles exhibited early signs of atresia as evidenced by degenerating granulosa cells and by pyknotic nuclei within the oocytes. On Day 3 of delay a new set of small-diameter antral follicles (527 .+-. 22 .mu.m) was observed; as the original set of preovulatory follicles became atretic, they were gradually replaced by a new set of antral follicles. Unlike the rat after 3 days of barbiturate administration, the hamster ovulated 1.5 days later. This was due to the rapid recruitment of antral follicles as the original set became atretic. The number of ova shed after 2 (17.9 .+-. 0.4 ova) and 3 (19.1 .+-. 0.6 ova) days of delay (but not after 1-day delay, 12.5 .+-. 0.3 ova) was significantly higher than in untreated cyclic hamsters (11.3 .+-. 0.7 ova). The higher ovulation rate may have been the result of recruitment of stage V follicles observed only in delayed hamsters. Injections of Phen were required at 1200 h (1 h earlier than the 1st injection) to cause 2 and 3 days of ovulatory delay. Determination of the serum concentrations of LH [lutropin] and FSH [follitropin] during the preovulatory period in delayed hamsters revealed that the rising phase of the LH surge occurred .apprx. 1 h prior to that in the normal cyclic hamster. Successively larger doses of Phen, 13 and 19.5 mg, were required to block ovulation for 2 and 3 days, respectively; this may be casually related to the (classical) induction of tolerance to Phen. During the 3-day period of ovulatory delay, the serum estradiol concentrations were .apprx. 50 pg/ml (correlating well with the normal number of antral follicles). The serum concentrations of progesterone (P) and androstenedione (A) at 1000 h were higher in delayed animals than in untreated proestrous controls. On the morning of day 1 of delay, the serum FSH concentration was higher than in proestrous controls. During delay, the serum LH level was not different from that in proestrous controls. Apparently the Phen-treated proestrous hamster is a useful model for the study of atresia of preovulatory follicles. The delayed induced atresia may be due to an alteration of the response of preovulatory follicle to gonadotropins since the atresia occurred concomitantly with the recruitment of a new set of antral follicles. The increase in the serum concentrations of P and A may be related to changes in steroidogenesis of the delayed preovulatory follicles.