Administration of cyanoke-tone, an inhibitor of progesterone synthesis, just prior to the proestrous critical period significantly increased both the magnitude and duration of the proestrous LH surge and uterine weight without inhibiting ovulation in the 4-day cycling rat.

The purpose of this study was to assess the relative roles of the neural and endocrine feed-back systems in the regulation of gonadotropin secretion and behavior in the female rat. Retrochiasmatic knife cuts were placed in animals which were tested for mating behavior on five consecutive days, subsequent to the development of cornified vaginal smears. A knife-cut control group was autopsied immediately after behavioral tests. A second knife-cut group and a group of cycling controls were acutely ovariectomized, and their behavior was assessed until mating ceased. Some of each of the two groups were administered small daily amounts of estradiol cypionate (estrogen) under two-dose regimens. Mating behavior and vaginal cytology were followed, and autopsies were performed 70 days after ovariectomy. At autopsy, serum was collected for LH, FSH, estradiol, and progesterone radioimmunoassays. The knife cuts caused high levels of mating on five consecutive days of testing in all animals. After ovariectomy, the surgical group mated at higher rates for longer time periods than did the controls, and the vaginal smears were predominately leukocytic. Knife cut controls had significantly elevated serum estrogen titers over cycling controls except for proestrous morning values. Exogenous estrogen was retained longer in the serum in the surgical than in the control group. Progesterone was relatively low in the retrochiasmatic cut groups. Serum LH was not elevated in the surgical groups except following ovariectomy; exogenous estrogen not only stimulated more mating sooner in the surgical group, but inhibited high levels of LH caused by ovariectomy. FSH was elevated in the surgical and control ovariectomized groups, and exogenous estrogen lowered FSH in the surgical but not the control group. Estrogen and LH levels were compared between the control and knife cut groups, and it was determined that the set-point for negative feedback effects of estrogen on LH was comparable for the two groups. It was concluded that retrochiasmatic knife cuts cause high estrogen titers, which may be responsible for the continuous behavior in animals with intact ovaries, whereas, in the absence of ovaries, the surgical treatment may reduce fore-brain inhibition on behavior. The data also suggest that the mediobasal hypothalamus in animals with retrochiasmatic knife cuts responds normally to exogenous estrogen by reducing LH secretion in the ovariectomized animal.

Regulation of the ovulatory cycle involves the interdependence of the adenohypophysis, the ovaries, the CNS, and the external environment. Several distinctions in brain functions and the overlapping control of several neuro-endocrine systems are pointed out. Important neuro-anatomical relationships between basomedial hypothalamus and other brain regions are noted. The anatomical structure is correlated with the control of gonadotropin secretion. Reflexive ovulation is considered as a special instance of external environmental influence. Data suggests that both lower mammals and human primates may show reflexive as well as spontaneous ovulation.

Coitus between 1700 and 1730 caused more ova to be ovulated than in unmated rats. It is clear that cardiac puncture (CP) 0.5 hr after mating inhibited the increases in the number of ova ovulated as a result of mating. Cardiac puncture also lowered terminal LH levels in the no mate group in comparison to the levels in the no mate—no CP group. Serum LH in the early evening of proestrus and terminal LH and FSH serum levels were not increased by mating. In earlier studies coitus between 1800 and 1830 increased the number of ova ovulated, while in the present study coitus was effective between 1700 and 1730. Cardiac puncture in combination with mating affects ovulation and LH levels differently than does CP subsequent to pentobarbital blockade of ovulation.

The influence of mating during the afternoon and early evening of proestrus on serum LH levels was evaluated in 4–day cycling rats of the Charles River CD strain. Plasma for RIA was obtained by sequential or single cardiac punctures in mated and unmated females. Values of serum LH between 1430 and 1530 were low and resemble those obtained on the morning of estrus in this strain. Serum LH in both mated and unmated animals was high between 1530 and 2030 hr on the day of proestrus. Serum FSH was assayed in animals which were mated or placed in an empty mating arena between the time of 1750 and 1830 on the day of proestrus. There was no significant elevation, over unmated females, in serum FSH between 1900 and 2130 on the day of proestrus or estrus in mated females. The data showed no significant relationship between the levels of serum LH present in proestrous samples and the number of ova ovulated on the following morning. It is clear that mating during the afternoon of proestrus failed to elevate serum LH or FSH. (Endocrinology92: 1475, 1973)

Ovariectomy in the rat increases brain norepinephrine synthesis and turnover. The present study investigated the effect of ovariectomy and subsequent replacement of ovarian steroids on tyrosine hydroxylase activity (TH), which is purported to be the rate—limiting step in catecholamine (CA) synthesis. Animals were ovariectomized and tyrosine and tyrosine hydroxylase activity assayed 2, 4, 26 and 60 days postoperatively. Estrogen (1.5 μg/rat) or vehicle was injected for one week beginning on day 21. Progesterone (0.5 mg or 2 mg) or vehicle was administered at 1000 hours on day 26 and the animals sacrificed 3 or 6 hours later. Ovariectomy caused a 2–3 fold increase in TH having an onset of 4 days and a duration of at least 60 days. Daily injection of estrogen for 1 week caused a 30% increase in TH. TH was significantly reduced 6 hours after injection of 0.5 mg progesterone in estrogen—treated ovariectomized controls. After injection of 2 mg progesterone, TH of ovariectomized and ovariectomized estrogen—treated rats was unchanged after 3 hours, but significantly decreased at 6 hours. Thus, CA synthesis in the rat hypothalamus may be altered by ovarian steroids, with progesterone having an inhibitory effect and estrogen a stimulatory effect. The inhibitory effect of progesterone on TH was facilitated by the presence of small amounts of estrogen. (Endocrinology91: 276, 1972)

Female rats which displayed a regular pattern of four-day estrous cycles were used to study the effects of surgical cuts placed at the posterior border of the optic chiasm on mating, ovulation, and plasma LH concentration. Two additional groups of four-day cycling females served as mated and unmated controls, autopsied on a morning of estrus. It was found that interruption of fibers in the anterior region of the basomedial hypothalamus results in constant behavioral receptivity maintained over several weeks of testing. Mating did not induce ovulation; however, females with the cut at the chiasmatic border were capable of ovulating in response to exogenous LH treatment 38-40 days after surgical treatment. Ovarian histology suggested that the largest (atretic) follicles did not ovulate, but that a developing cohort of follicles ovulated to exogenous LH. Plasma LH was significantly elevated at the time of autopsy in the operated animals in contrast to the controls. Uterine weights suggest that endogenous estrogen secretion is fairly continuous and high in the operated animals. The neural fibers which mediate reflexive and spontaneous ovulation in the normal cycling animal are interrupted by cuts placed at the chiasmatic border. However, sexual receptivity could be elicited daily, and a steady state of high estrogen and LH secretion rates appears to occur. (Endocrinology90: 461, 1972)

Female rats with a record of regular 4-day estrous cycles received different quantities of coital stimulation at specified times on the afternoon of proe3trus. The effects of copulation on numbers of ova ovulated were evaluated on the following morning of estrus between 0930 and 1055. The results show that the amount of copulation received between the hours of 1230 and 1730 failed to produce increases in the normal number of ova ovulated. If, however, intromissions were received between 1750 and 1830 on the afternoon of proestrus, there were increased numbers of ova released. It was suggested that the increases result from an additional release of LH acting on mature follicles which would not ordinarily ovulate. Those subjects responding early in the afternoon had significantly higher uterine weights than those responsive later, suggesting a relationship between high preovulatory estrogen titers and the timing of sexual behavior. The present experiment provides evidence that, if the mating stimulus is properly timed, LH may be reflex ively affected in a normal spontaneous ovulator. (Endocrinology88: 433, 1971)

Adrenalectomy was performed 4 weeks prior to evaluating the effects on mating behavior, ovulation, and the first 10 days of gestation in 4-day cycling female rats. Mating behavior was essentially normal and occurred at times similar to sham-adrenalectomized animals on the afternoon of predicted proestrus. The number of ova ovulated was significantly reduced in mated adrenalectomized subjects in contrast to data reported by Rodgers (1971) for intact females mated at similar times in proestrus. Embryonic swellings were significantly fewer at day 10 of gestation than in the sham-adrenalectomized group providing additional evidence of reduced numbers of ova ovulated. Embryonic weights at day 10 of gestation were not affected by adrenalectomy. Removal of adrenocortical hormones significantly increased uterine weight on the morning of vaginal estrus. The data fail to show a significant delay in sexual receptivity as a result of removing adrenal progesterone, but suggest that adrenalectomy interferes with the neuroendocrine reflex mediating ovulation.

Virgin female rats were tested to determine the beginning of sustained sexual behavior with reference to vaginal cycling. Records of vaginal smear sequences established that the subjects cycled every 4 days. Coitus was first observed in 29% of the females in the early afternoon of proestrus. Eighty-eight per cent were fully receptive by 1930 on the day of proestrus. It is suggested that receptivity may occur before systemic progesterone reaches peak concentrations. The data demonstrate that coitus occurs in close proximity to the critical period for ovulatory release of LH on the day of proestrus. (Endocrinology86: 1181, 1970)