Changes in LH, progesterone and sexual behavior associated with preovulatory luteinization in the bitch.

LH and progesterone were assayed in plasma collected at 0800, 1600 and 2400 h daily from each of 6 Beagles during proestrus and estrus. The bitches’ sexual behavior in response to males was assessed daily. Preovulatory LH surges lasting 24-40 h peaked (4.4-11.1 ng/ml) at 2400 or 0800 h between three days before and one day after the onset of estrus. Estrous behavior in each bitch was observed only after an increase in progesterone. The initial increase in progesterone occurred either just prior to, or concomitant with, the initiation of the LH surge. Mean (± SEM) progesterone at -56, -40, -24, -16, -8, 0, 24 and 48 h from the LH peak was 0.7 ± 0.1, 0.9 ± 0.1, 1.2 ± 0.1, 1.4± 0.2, 1.5 ± 0.2, 2.6 ± 0.3, 3.2 ± 0.4 and 5.4 ± 0.6 ng/ ml. LH at the same times was 1.7 ± 0.3, 1.1 ± 0.2, 1.5 ± 0.3, 1.8 ± 0.3, 4.9 ± 1.0, 7.3 ± 1.0, 2.0 ± 0.3 and 1.4 ± 0.4 ng/ml. Ovulation time in the bitch was estimated by correlating results of gross and histological examination of ovaries removed from each of 10 Beagles during proestrus or estrus with LH and progesterone measured in plasma collected two to three times a day prior to obtaining the ovaries. Ovaries were removed from 6 of these bitches at approximately 4, 8, 20, 38, 44 and 50 h after the LH peak, when plasma progesterone was 1.3, 2.8, 4.2, 3.3, 4.4 and 6.9 ng/ml, respectively. The respective ovulated:nonovulated mature follicle ratios were 0:10, 0:6, 0:7, 0:8, 4:3 and 11:0. The sexual behavior of 6 ovariectomized bitches was observed daily for 5 weeks during which 16 days of estrogen treatment was initiated either 8 days before or 8 days after initiation of a 16 day progesterone treatment. Initial progesterone (4.8 ± 0.4 ng/ml plasma) had no effect. Induction of full sexual receptivity occurred 8 days after superimposing estrogen (65 ± 5 pg/ml plasma) on progesterone. Initial estrogen caused bitches to be attractive to males but failed to induce any acceptance of the males. Full sexual receptivity was induced within 18 h of superimposing progesterone on estrogen, was not affected by removal of estrogen 8 days later but was lost entirely within 18 h of progesterone removal after another 8 days.

The results suggest that, in the bitch: 1) ovulation occurs between 1 day before and 5 days after the onset of estrus and approximately 38-44 h following the LH peak; 2) follicle luteinization and increased progesterone secretion begins just prior to or concomitant with initiation of the LH surge some 60-70 h prior to ovulation and 3) the initial preovulatory rise in progesterone is synergistic with the preceding late-proestrous estrogen peak in eliciting estrous behavior.

Sexual behavior in ovariectomized bitches in response to estrogen and progesterone treatments.

Sex behavior was quantitated in 13 groups of 2-5 ovariectomized bitches receiving various estrogen and progesterone treatments. In addition, one of the scoring systems used to quantitate sex behavior was applied to the changes observed during proestrus and estrus in 7 intact cycling bitches for which daily serum progesterone and LH levels were assayed. In 6 of the 7 bitches, the onset of full sexual receptivity characteristic of estrus occurred 0-2 (1.2 ± 0.5) days after the LH peak and only after a detectable increase in serum progesterone levels.

Ovariectomized bitches administered estradiol-17β s.c. in 4.7 mm (o.d.) Silastic capsules, at a dose of 0.6 cm/kg for 30 days, became increasingly attractive to males and in some instances showed the passive behavior characteristic of late proestrus. However, none showed the positive reflexes associated with the full sexual receptivity characteristic of estrus. When bitches were administered progesterone implants (1.8 cm/kg) after 8 days of estradiol treatment they displayed within the next 1-5 days a full estrous behavior which lasted for about 1 week. When the estradiol dose was reduced from 0.6 to 0.1 cm/kg at the time of progesterone treatment, each of the bitches showed full sexual receptivity within the next 18 h and this behavior was maintained for 5 days.

Other groups of ovariectomized bitches were injected initially with an estradiol benzoate (EB) dose of 0.4 µg/kg and every 8 h thereafter with steadily increasing doses in increments of 0.2µ g/kg for 3, 6 or 9 days. After 3 days of EB treatment, proestrous but not estrous behavior was observed regardless of whether progesterone was administered after the EB treatment. EB treatment for 6 days caused some bitches to show proestrous behavior prior to termination of treatment and to display estrous behavior following the last EB injection. When 6 days of EB were followed in 8-32 h by progesterone implants (1.6 cm/kg), estrous behavior was observed within 8 h and lasted for 8 days. Estrus occurred earlier and lasted longer in these bitches than in those receiving no additional treatment or small estradiol implants instead of progesterone. Of 9 bitches receiving EB injections for 9 days, 3 showed estrous behavior 1-2 days prior to the last injection while the other 6 displayed estrus only after the last EB injection was given. In each bitch, including those displaying estrous behavior prior to the termination of EB treatment, serum estradiol levels immediately prior to the onset of estrus were lower than those present 12 h earlier. In bitches administered progesterone implants (3.2 cm/kg) 6 h after 9 days of EB treatment, the mean duration of estrous behavior was 9 days vs the 5 days in bitches not subsequently administered progesterone. When progesterone was administered 3 days prior to the end of EB treatment, 1 of 3 bitches showed estrus 8 h following progesterone while the others did not display estrus until after the last EB injection.

These results are considered consistent with the hypothesis that behavioral estrus in the bitch is potentiated by exposure to elevated levels of estrogen and subsequently initiated by a decrease in the estrogen:progesterone ratio effected by either a fall in estrogen and/or a rise in progesterone.

Relationship of serum estrone, estradiol-17b and progesterone to LH, sexual behavior and time of ovulation in the bitch.

Serum estrone, estradiol-17β and progesterone were measured in bitches in which LH, sexual behavior and laparoscopically confirmed time of ovulation had been previously studied. Steroid values from 25 estrous periods were normalized to day of peak LH (Day 0). Estrone, estradiol-17β and progesterone concentration did not differ significantly in either dogs subjected to laparoscopy at 48 h intervals or those bitches undergoing no laparoscopy.

During the preovulatory interval, mean peripheral estrone concentrations were more variable than estradiol-17β; however, both estrogens tended to rise gradually throughout this time period. Overall maximal estradiol-17β concentration was detected on Day -2 with estrone peaking later (Day -0.5). Both estrogens declined (P<0.05) coincidentally with the LH surge. Progesterone from Days -14.0 through -1.5 was maintained at basal concentrations. An increase (P<0.05) in mean progesterone occurred on the day of the preovulatory LH peak which correlated with direct observation of apparent preovulatory follicle luteinization. Progesterone increased gradually from Day -0.5 through 8 and then varied for the remainder of the sampling period.

Analysis of individual cycles indicated that circulating serum concentrations of reproductive steroids fluctuated from day to day or within days of the proestrous-estrous interval. In dogs subjected to laparoscopy, initial evidence of the presence of visible preovulatory follicles was closely related to the first detectable elevations in both estrone and estradiol-17β. Either estrone or estradiol-17β concentrations or both surged sharply in 24 of the 25 cycles examined; however, only titers of estradiol-17β were elevated above baseline in all cycles prior to, or at the time of, the LH peak. In 16 of 25 cycles, estradiol-17β concentrations declined by 20 pg/ml or more 12-24 h prior to the onset of sexual receptivity. Bitches remained in estrus during periods of declining estrone and estradiol-17β and increasing progesterone levels.

These data integrate the hormonal, ovarian and behavioral events of the estrous cycle of the bitch and suggest that: 1) a preovulatory estradiol-17β surge exists and is likely responsible for triggering LH release; 2) estrone may play a supportive role to estradiol-17β in the endocrine control of LH secretion; 3) preovulatory changes in follicular morphology are distinct and can be correlated with a shift from estrone and estradiol-17β to progesterone secretion; 4) prolonged sexual receptivity in this species is supported in the presence of declining estrogen and continuously rising progesterone concentrations.