New data on serotoninergic mechanisms in ovulation in the cyclic female rat]

Injection of parachlorophenylalanine on dioestrus II at 18:00 was shown to decrease ovulation frequency in 4-day cyclic female rats. This effect was overcome by either HTP, or oestradiol benzoate, when administered on dioestrus II, at 18:00 and 10:00 - 11:00 respectively. No antiovulatory action of PCPA was observed on prooestrus at 13:00. These results provide support to the specificity of action of serotonin in the control of ovulation in the cyclic rat. They also suggest an interaction of serotonin and oestrogens in this control.     

Role played by the adrenal cortex on the luteotrophic action of estrogens during the rat estrus cycle]

Estrogen-induced changes in peripheral blood progesterone concentration have been studied in dexamethasone (DEX) and metopyrone (MET) treated 4-day cyclic female rats. Estradiol benzoate (EB) was injected at 10--11 h on diestrus I and peripheral blood was collected at 16--17 h on diestrus II for progesterone radioimmunoassay. The EB induced-increase in blood progesterone concentration was more pronounced, compared to non-injected females in intact DEX-treated females and in adrenalectomized females treated or not with DEX than in their intact counterparts. The adrenal cortex was then supposed to inhibit the luteotrophic action of EB. When injected for 10--12 days, MET caused an increase in blood progesterone concentration compared to uninjected control animals. No cumulative effects of EB and MET were observed. These results are discussed in the light of knowledge, on the feed-back mechanisms which are involved in the action of estrogen on the pituitary-ovarian-adrenocortical system.     

Episodic LH secretion patterns in the mare during the oestrous cycle.

Jugular blood samples were obtained from 8 mares at 5- and/or 20-min intervals for 2 to 5 days during various phases of the oestrous cycle for plasma LH determination. An episodic release pattern was observed in 1 of 3 mares sampled during the ovulatory period. One mare had one secretory burst and the other mare had several periods of fluctuating plasma LH concentration. During dioestrus, episodic secretions were observed in 2 mares sampled 11 to 13 days before and, in 1 mare, 9 days after ovulation. During the 2 to 5-day period before ovulation, episodic secretion was not observed (3 mares) but plasma LH concentrations fluctuated as much as 6 ng/ml during a period of 3--4 h. Daily plasma samples were obtained form 10 mares (1--8 oestrous cycles/mare) during which 22 single, 18 double and 2 luteal-phase ovulations occurred. Dioestrous ovulations were accompanied by small increases in plasma LH (1--4 ng/ml), but many similar increases in LH were not accompanied by ovulation. No significant differences in secretory patterns were observed between single and multiple ovulations. In one mare, 4 ovulations occurred in the presence of a prolonged luteal phase; 3 were accompanied by increasing LH concentrations and the other occurred when LH was at a low concentration.     

Effect of exogenous progesterone on 17 beta estradiol secretion in 4-day cyclic rats]

The rate of ovarian 17 beta oestradiol was showed to be decreased on the decreased on the afternoon of dioestrus 2 in 4-day cyclic female rats with cycles prolonged to 5 days following progesterone treatment as compared to untreated 4-day cyclic females. OEstradiol values also appeared lower on dioestrus 3 and prooestrus in experimental 5-day cycles than, on prooestrus, in natural 4-day cycles.     

The adrenal cortex and the luteotrophic action of estrogens during the estrous cycle in the rat

The aim of this study was to evaluate the effects of estradiol benzoate (EB) on ovarian progesterone secretion in the presence or in the absence of the adrenals. 4-day cyclic female rats were injected with 10 microgram EB on the morning of diestrus I. An increase in the rate of ovarian progesterone secretion in diestrus II at either 10--11 a.m. or at 2 : 30--3 : 30 p.m. was only observed in one of two experimental series. A very significant increase in the peripheral blood progesterone concentration was noted in adrenalectomized EB-treated females as compared to EB-injected intact females, thus suggesting that the adrenals might inhibit the luteotrophic action exerted by EB on the ovary. Experiments in dexamethasone (DEX)-EB-treated females confirmed this view. Peripheral blood progesterone concentration was significantly greater in DEX-EB-treated females than in EB-treated females. The possible mechanisms were discussed in the light of experiments involving the administration of metyrapone (MET) prior to EB injection. While blood progesterone concentration increased following MET-treatment only, no cumulative effects resulted from combined MET and EB-treatment. Progesterone of adrenal origin was then supposed to be implicated in the inhibitory action of the adrenal cortex on the luteotrophic action of EB in cyclic female rats.     

A dopamine agonist stimulates progesterone secretion from adrenal gland

Administration of either progesterone (P) or a dopamine agonist, Legrotrile mesylate (LM), have been shown to induce the ovulatory release of LH in rats. In order to elucidate the mode of action of dopamine agonists we studied the effects of LM on P secretion by the adrenals. A subcutaneous injection of LM, in doses which induce ovulation, stimulated adrenal P secretion in ovariectomized, estrogen-primed rats and in castrate male rats. Peak plasma P concentrations were found at 2 h with a gradual return to pre-injection levels at 6 hrs following LM injection. These results raise the possibility that P increments following LM administration may be responsible for inducing ovulation in young and old rats.There is considerable evidence to show that modification of hypothalamic monoamine metabolism by pharmacologic agents results in markedly altered pituitary gonadotropin secretion [1]. Lergotrile mesylate (LM), a dopamine agonist, has been shown to advance the ovulatory release of LH on proestrus and induce ovulation and cyclicity in anovulatory aged rats [2,3]. Everett [4] observed that progesterone (P) administration induced ovulation in rats rendered anovulatory under constant light and also advanced ovulation by one day if administered to 5-day cycling rats. A similar effect of LM on ovulation advancement has been noted (Clemens, personal communication). These similarities in action of P and LM on ovulation in young and aged rats, led us to speculate that LM may stimulate P secretion which in turn may elicit the discharge of ovulatory hormones. To test this hypothesis we examined the effects of LM on progesterone secretion in gonadectomized rats.     

Compensatory responses after unilateral ovariectomy in rabbits

Compensatory ovarian and gonadotropic responses to unilateral ovariectomy (ULO) were examined in the rabbit doe, an induced ovulator. On Days 2, 4, 5, 6, 8, 10, 15 and 20 after ULO, ovaries from 3 hemiovariectomized does and 1 sham-hemiovariectomized doe were examined macro- and microscopically for number, size and signs of atresia of follicles. The number of surface follicles increased initially to 7 or 8 follicles 2 days after ULO, followed by an increase to 10 or more follicles by Day 15 (control ovaries had 5.7 +/- 0.4 follicles). Total numbers of antral follicles and the proportion of follicles which were atretic did not vary relative to day after ULO. However, distributions of antral follicles in classes of 0.2-mm increments were significantly different between sham-ovariectomized and hemiovariectomized does after Day 2 due to shifts of follicles into larger size classes. Peripheral serum concentrations of follicle-stimulating hormone (FSH), but not luteinizing hormone (LH), increased temporarily during the 48 h after ULO. Follicular compensation after ULO in the doe entailed nonlinear increases in numbers of preovulatory follicles, due to increased growth within the antral population of follicles, probably the result of an acute surge of FSH. A period of more than 10 days was necessary to restore the number of preovulatory follicles after ULO. Exogenous human chorionic gonadotropin (hCG) induced ovulation of recruited follicles.     

Demonstration of a critical period of ovarian activity in the control of early receptivity induced by estrogen in the female rat with 4-day cycles]

A significant decrease in early receptivity during the night from dioestrus II to prooestrus was observed in estrogen-treated 4-day cyclic rats following bilateral ovariectomy on dioestrus II at 4-5 p.m. Early receptivity appeared then to be dependent on the presence of ovary at this stage of the cycle. The mechanism whereby the adrenals may compensate for the ovaries when removed on dioestrus I, at 10-11 a. m. (Roos et al. 1973), was not observed in the present experimental conditions.     

The effect of frozen ovarian autografts on compensatory ovulation and steroid production in unilaterally ovariectomized rats.

In the present study rats were unilaterally ovariectomized (ULO) and the surgically removed ovary was frozen for 13 days. After allowing the remaining ovary to compensate with respect to number of ova shed, the frozen graft was thawed and transplanted subcutaneously to determine the effect on ovulation number, cycle length, uterine weight, ovarian weight and plasma levels of estradiol-17beta (E2) and progesterone. Rats ULO at 45 days of age, which received an autograft 13 days later, had a decrease in the number of eggs shed as compared to control ULO rats (6.4 +/- 0.8 vs. 11.1 +/- 0.9 eggs, respectively) and a decrease in plasma E2 (14.5 +/- 1.7 VS. 21.0 +/- 1.5 PG/ML, respectively). No differences were observed in progesterone concentration, uterine weight, ovarian weight or cycle length. In contrast, rats ULO at 31 days of age, which received an autograft 13 days later, showed no differences in comparison to control ULO rats. Castrates which received ovarian autografts developed cycling vaginal smears and had increased E2 (31.9 +/- 4.3 pg/ml) and decreased progesterone (18.3 +/- 1.9 ng/ml) levels. Since ULO animals with autografts shed fewer ova, the present study demonstrates that the amount of ovarian tissue influences ovulation number either by utilization of gonadotropins or by an, as yet, undefined mechanism.     

Prostaglandin biosynthesis by the rat uterus during the oestrus cycle, temporal correlation with plasma oestradiol and progesterone concentrations, identification of 6 keto PGF1α as the major metabolite

Prostaglandin biosynthesis was studied in the rat uterus during the oestrous cycle. Uterine homogenates were incubated for 20 minutes in the presence of exogenous substrate (2.10⁻⁵M). PGF_2α and PGE₂ were measured by R.I.A.. A sharp peak PGF_2α and a smaller peak of PGE₂ were observed at prooestrus, 20 h. Another small PGE₂ peak occurred at dioestrus II, 15 h. The lowest values of both PGs were found on dioestrus, 15 h. Plasma oestradiol concentration were highest at proestrus, 15 h and 20 h. A sharp progesterone peak occurred at prooestrus, 20 h. The PGF_2α peak is next to the oestradiol peak and is superimposable or lags slightly beyond the progesterone peak.Incubation with ¹⁴C arachidonic acid and subsequent analysis of extracts by TLC and scanning showed that the major metabolite is PGI₂, identified as 6 keto PGF_1α. The conversion rate of arachidonic acid into 6 keto PGF_1α is 5 times higher than into PGF_2α. 6 keto PGF_1α was further identified by GC/MS. No significant difference was observed between 6 keto PGF_1α production during oestrus and dioestrus.     

Influence of ovaries and photoperiod on reproductive function in the mare.

A 16 h daily photoperiod hastened the onset of the ovulatory season (first ovulation); gonadotrophin and follicular changes prior to the onset were similar in intact light-treated and control mares. A preovulatory decline in FSH concentrations before the onset of the ovulatory season preceded the decrease in number of follicles (15--25 mm) and the rise in LH concentrations which was temporally associated with the growth of an ovulatory follicle. Seasonal changes of FSH and LH concentrations were found in ovariectomized mares and were influenced by photoperiod. During the anovulatory season, there was no ovarian influence on gonadotrophin concentrations. However, during the ovulatory season the ovaries exerted a positive influence on seasonally elevated LH concentrations during oestrus and a negative influence during dioestrus. The ovaries exerted a negative influence on seasonally elevated FSH concentrations throughout the oestrous cycle. The onset of the ovulatory season occurred at the time of the first sustained increase in LH concentrations resulting from positive seasonal (increasing photoperiod) and ovarian influences.     

Short- and long-term effects of unilateral ovariectomy in sheep: causative mechanisms

The mechanisms of ovulatory compensation following unilateral ovariectomy (ULO) are still not understood. In the present study, we investigated the short- and long-term effects of ULO in sheep using transrectal ovarian ultrasonography and hormone estimations made during the estrous cycle in which surgery was done, the estrous cycle 2 mo after surgery, and the 17-day period during the subsequent anestrus. The ULOs were done when a follicle in the first follicular wave of the cycle reached a diameter > or =5 mm, leaving at least one corpus luteum and one ovulatory-sized follicle in the remaining ovary. Ovulation rate per ewe was 50% higher in the ULO ewes compared with the control ewes at the end of the cycle during which surgery was performed, but it did not differ between groups at the end of the cycle, 2 mo later. This compensation of ovulation rate in ULO ewes was due to ovulation of follicles from the penultimate follicular wave in addition to those from the final wave of the cycle. Ovulation from multiple follicular waves appeared to be due to a prolongation of the static phase of the largest follicle of the penultimate wave of the cycle. Interestingly, the length of the static phase of waves was prolonged in ULO ewes compared with control ewes in every instance where the length of the static phase could be determined. Changes in follicular dynamics due to ULO were not associated with alterations in FSH and LH secretion. In conclusion, ovulatory compensation in ULO sheep involves ovulation from multiple follicular waves due to the lengthened static phase of ovulatory-sized follicles. These altered antral follicular dynamics do not appear to be FSH or LH dependent. Further studies are required to examine the potential role of the nervous system in the enhancement of the life span of the ovulatory-sized follicles leading to ovulatory compensation by the unpaired ovary in ULO sheep.     

Ovarian and endocrine responses in the cat after coitus

LH release leading to ovulation was induced in 17 of 29 oestrous periods. The time of ovulation after coitus was determined by histological examination or by observation at laparotomy of ovaries in situ. Histological methods revealed that ovulation was complete in most follicles (9 of 13) at 32 h post coitum and in all follicles that were involved in the ovulatory process by 36 h. When laparotomy was used, no signs of preovulatory change were noted at the first observation time, 22 h post coitum, but in 4 cycles in which the entire process of ovulation was observed, the ovulatory process occurred between 23 and 28 h (3 follicles), 23 and 27 h (2 follicles), 25 and 28 h (3 follicles), and 25 and 29 h (3 follicles) post coitum. The first ovulatory process noted was complete at 25 h post coitum. In cats, LH release continued over a 16-h period before returning to baseline (long surge), values being 616 +/- 180 ng/ml at 1/2 h and 941 +/- 154 ng/ml at 2 h post coitum. In 6 cats the LH release pattern was limited to a 4-h period (short surge), values being 537 +/- 218 ng/ml at 1/2 h and 353 +/- 245 ng/ml plasma at 2 h and basal (49 +/- 18 ng/ml) by 4 h post coitum. Decreased secretion of oestrogen by follicles in animals undergoing ovulation was first observed at 16 h post coitum. It is concluded that coitus induces LH release within minutes in the cat and that ovulation begins about 24 h later and finishes by about 32 h post coitum. Only one coital input can cause LH release for as long as 16-20 h although shorter periods of LH release (4 h or less) can result in ovulation.     

Corticosteroid regulation of gonadotropin secretion and induction of ovulation in the rat

In the human polycystic ovarian syndrome, glucocorticoids have been demonstrated to have beneficial effects in inducing ovulation in a number of cases. These beneficial effects were assumed to be due to suppression of adrenal overproduction of androgens. However, the possibility exists that glucocorticoids may directly regulate gonadotropin secretion and thereby improve menstrual rhythm and ovulatory activity. Herein, we report that the corticoid, deoxycorticosterone, and the synthetic glucocorticoid, triamcinolone acetonide, like progesterone (P4), are able to induce luteinizing hormone and follicle-stimulating hormone surges and facilitate ovulation in the pregnant mare serum gonadotropin-primed rat. This effect is not shared by cortisol. Prolactin release was also stimulated by deoxycorticosterone, cortisol, and progesterone, but not by triamcinolone acetonide. Similar to progesterone, triamcinolone acetonide and deoxycorticosterone administration caused a loss of fluid retention in the uterus. This effect of triamcinolone acetonide and deoxycorticosterone may be related to progesterone action as opposed to anti-inflammatory action since cortisol had no effect on uterine fluid retention. These findings raise the possibility that the beneficial effects seen with glucocorticoids in inducing ovulation in polycystic ovarian syndrome may be due in part to their direct effects upon the release of gonadotropins.     

New data on luteotropic mechanism of action of estrogens during the rat estrus cycle]

An increase in peripheral blood progesterone concentration was observed in diestrus II, at 17:30 in 4-day cyclic female rats subcutaneously injected with 10 microgram estradiol benzoate (EB) at 10:00-11:00 on diestrus I. Pentobarbital injection (30 mg/kg) at 13:30 on diestrus II did not prevent this effect on EB. By contrast PB injected at 13:30 on diestrus II as above completely suppressed the luteinizing or ovulating effects of EB. The action of estrogen on blood progesterone level was therefore concluded to be unrelated to the mechanisms underlying estrogen-induced ovulation luteinization in the cyclic female rat.     

An early single dose of progesterone agonist attenuates endogenous progesterone surge and reduces ovulation rate in immature rat model of induced ovulation

Inhibition of preovulatory synthesis and action of progesterone impairs ovulation in rodents. We evaluated effects of supplementation of exogenous progesterone on human chorionic gonadotropin (hCG)-induced ovulatory response in immature rats. Equine CG-primed mature follicles responded to hCG with induction of immunoreactive steroidogenic acute regulatory protein (StAR) mainly in thecal layers and a transient enhancement in progesterone synthesis peaking at 6h after hCG (hCG6h). A single dose of natural progesterone or a synthetic agonist (MP) at hCG0h both decreased ovulation rates in dose-dependent manners. MP was still effective when treated at hCG4h. Treatment with these agents at hCG0h reduced circulating progesterone and thecal expression of StAR at hCG6h. The treatments further attenuated induction of cyclooxygenase (COX)-2 in mural granulosa cells and ovarian prostaglandin (PG) E(2) level at hCG8h. We also found a significant reduction in bromo-deoxyuridine incorporation by mural granulosa cells. Obtained results show that the early treatment with exogenous progesterone agonist caused attenuated amplitude of endogenous progesterone surge, reduced COX-2/PGE(2) system, dysregulated mitosis of granulosa cells, and decreased oocytes release. We suggest that optimal progesterone synthesis and action are an early critical component of hCG-initiated ovulatory cascade that regulates biochemical function of granulosa cells.     

Reproductive performance of heifers induced to oestrous asynchrony by suprabasal plasma progesterone levels

Suprabasal progesterone concentrations around oestrus have induced disturbances in oestrous behaviour and ovulation. To determine whether fertility in such an altered oestrus can be maintained at normal levels with additional inseminations (AI) until ovulation, fertility was compared in heifers (n = 11) inseminated in normal oestrous cycles and thereafter in cycles in which the animals were treated with progesterone in order to create suprabasal concentrations after luteolysis. The treatment consisted of silicone implants containing 10.6 mg kg⁻¹ of progesterone inserted subcutaneously on Day 8 of the oestrous cycle (day of ovulation designated Day 0) and removed on Day 25. Both in control oestrous cycles and oestrous cycles under progesterone treatment, growth of the ovulatory follicle and ovulation were determined by frequent ultrasound scanning. Blood was collected frequently for further analysis of progesterone, oestradiol-17β and luteinising hormone (LH). Insemination was performed 12 h after onset of standing oestrus. if ovulation did not occur 24 h after AI, heifers were inseminated again until ovulation. Pregnancy was diagnosed by ultrasound 25 days after ovulation.In control oestrous cycles, plasma progesterone decreased to 0.3 ± 0.3 nmol 1⁻¹. Duration of oestrus was 22.9 ± 2.0 h, the interval from onset of oestrus to ovulation was 32.4 ± 2.3 h and the interval from LH peak to ovulation was 28.6 ± 1.4 h. The interovulatory interval was 20.7 ± 0.6 days. In oestrous cycles in treated heifers, progesterone decreased to 1.0 ± 0.3 nmol l⁻¹ (P > 0.10) and the interovulatory interval was prolonged to 23.5 ± 1.0 days (P < 0.05). Standing oestrus lasted 47.2 ± 12.0 h (P = 0.09, n = 7). The interval from the onset of oestrus to ovulation was 59.4 ± 13.0 h (P = 0.08) and the interval from LH peak to ovulation 25.8 ± 1.3 h (P > 0.10). The prolonged oestrus was associated with increased (P < 0.05) growth of the ovulatory follicle and higher (P < 0.05) release of oestradiol-17β. Conception rates were 90% and 46% (P < 0.05), and the numbers of AI per heifer were 1.1 ± 0.1 and 3.4 ± 0.6 (P < 0.01) for control oestrous cycles and after treatment, respectively.The induction of suprabasal concentrations of progesterone caused asynchronies similar to those observed in cases of repeat breeding. The repeated AI did not maintain fertility at normal levels. It is suggested that the extended growth of the ovulatory follicle may cause impaired oocyte maturation or it may alter the maternal milieu owing to the prolonged release of oestradiol.     

Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles

The aim was to define precisely the FSH secretion pattern in mares during the two ovulatory cycles before, and for 24 days after, the last ovulation of the season and to compare this with the profiles of other reproductive hormones and follicular growth to identify changes which may lead to the termination of follicular cycles. Jugular blood was collected every 6 h from ten light horse mares for 6 weeks in autumn. Samples were assayed for FSH, LH, prolactin, inhibin, oestrone conjugates and progesterone. Luteolysis occurred earlier and periovulatory oestrone, but not inhibin, concentrations were significantly lower in the last than in the second to last cycles. In ovulatory and anovulatory cycles, daily mean FSH concentrations were low at the expected time of ovulation and high between days 9 and 11 (day 0 = ovulation), which were usually after luteolysis. However, the periovulatory FSH nadir was prolonged in the last compared with the second to last cycles, and the difference between peak and trough values was not significant in anovulatory cycles. Between day 5 and day 8, the FSH interpulse interval was approximately 2 days, and did not vary in successive cycles. The LH profile also showed progressive changes as mares entered acyclicity; the surge terminated sooner in the last than in the second to last cycles, and failed to occur when expected in acyclicity. Sporadic prolactin pulses occurred at luteolysis in a similar proportion of ovulatory and anovulatory cycles. These results indicate that inadequate gonadotrophin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, and eventually acyclicity. Moreover, the time relationships amongst changes in pituitary and ovarian hormones and follicular growth become increasingly disrupted during the autumn transition, which may contribute to the cessation of cyclicity.     

Roles of pulsatile release of LH in the development and maintenance of corpus luteum function in the goat

The roles of the pulsatile release of LH in the functional development and maintenance of the corpus luteum (CL) during the estrus cycle in the goat were examined using a potent GnRH antagonist. In Experiment 1, to assess the inhibitory effects of the GnRH antagonist on the release of LH during the estrus cycle, 9 goats were divided into 3 groups. Goats in Group I received only saline on Days 0 (day of ovulation), 5, 10 and 15. Goats in Group II received the GnRH antagonist (50 microg/kg, s.c.) on the days mentioned for Group I to inhibit endogenous LH during the periods of luteal development and maintenance. Goats in Group III received saline on Days 0 and 5 and then the GnRH antagonist on Days 10 and 15 to inhibit LH during the period of luteal maintenance. Serial blood sampling took place on Days 1, 3, 5, 8, 13 and 18 to characterize the LH pulses. The LH pulses were observed throughout the estrus cycle in Group I but were completely abolished in Group II. In Group III, the pulsatile release of LH was observed from Day 1 to 8, but the LH pulses were completely abolished on Days 13 and 18. In Experiment 2, 16 goats were divided into the same 3 groups as in Experiment 1 to examine the effects of the GnRH antagonist on the luteal function. The concentration of progesterone in the plasma in Group I increased after ovulation, reached a maximum level around Day 12, and subsequently returned to the basal level on Day 17. The concentrations of progesterone in Group II rose after ovulation, but reached a plateau around Day 6 and maintained the level up to Day 9, then rapidly decreased from Day 9 to 10 to the basal level. The concentrations of progesterone in Group II were lower on Days 7 to 15 than those in Group I (P<0.01). The concentrations of progesterone in Group III increased after ovulation, reached a maximum level around Day 8, then dropped from Day 10 to 13 to the basal level. The concentrations of progesterone in Group III on Days 11 to 15 were lower than those in Group I (P<0.05 on Day 11, P<0.01 on Days 12 to 15). These results demonstrate that endogenous LH is essential for normal development and maintenance of the CL function during the estrus cycle in the goat. Further, this study suggests that while the functional maintenance of the caprine CL depends entirely on LH support, such functional dependence during early CL development is only partial.     

Changes in the concentration of LH, FSH and estrogen in the immature female rat during precocious sexual maturation induced by electrochemical stimulation of the brain.

Electrochemical stimulation of the hypothalamus of 23-day-old female rats induced precocious puberty as judged by occurrence of vaginal opening, the degree of uterine hypertrophy, changes in ovarian steroid content and incidence of first ovulation. Three types of responses were observed: (I) pubertal ovulation within 96 h; (II) pubertal ovulation within 120 h, and (III) vaginal opening at 120 h not followed by ovulation. All treated animals showed a sustained increase in the LH/FSH ratio in both pituitary and plasma. Plasma estrogen was also increased 1 h after stimulation. A preovulatory rise in plasma estrogen and gonadotropins was noted in type I and type II animals. These data lend further support to the suggestion that brain stimulation causes a release of gonadotrophins which induced ovarian steroidogenesis leading to an ovulatory gonadotropin surge via a positive feedback effect.