A new method for induction and synchronization of oestrus and fertile ovulations in mice by using exogenous hormones

Management of the oestrous cycle in the mouse, despite being a species widely used for research purposes worldwide, ^1,2 is scarce. Usual methods for the induction of behavioural oestrus and ovulation are ‘male effect’, by the introduction of a male, fertile or vasectomized ³ and ‘Whitten effect’, by the exposition to male pheromones; ⁴ however, the degree of oestrus synchronization was early reported to be very limited. ⁵ Thus, the application of a reliable method for oestrus synchronization, with fertile ovulation and mating, in a group of animals, in a short time-range, would be of interest for improving animal management (controlling and planning the reproduction of different individuals and groups in the animal units, in the presence or absence of superovulatory effects), biotechnology (artificial insemination, combination with gonadotrophin superovulatory protocols for embryo recovery and induction of pseudopregnant fosters for embryo transfer), research (scheduling standardized and consistent experimental groups with synchronized mates for generation of synchronized pregnancies either for stage-specific studies on embryonic development or for obtaining synchronized deliveries in studies on neonates) and wellbeing (reducing the number of experimental animals and refining the studies).

Induction and/or synchronization of oestrous cycle is routinely achieved, both in humans and other animal species, by supplying different combinations of hormones; luteolytic (prostaglandin F_2α, PGF_2α and its analogues) and progestative (progesterone and its analogues) factors are commonly used. Secretion of PGF_2α is the event that provokes the lysis of the corpus luteum in mammals, ⁶ giving way to a follicular phase with oestrus behaviour and therefore, ovulation. Administration of exogenous PGF_2α, or its analogues, induces a rapid and controlled luteolysis. If given as a single injection, some of the females will be in non-luteal phase or in the very early or very late luteal phase and will fail to respond. The use of two injections several days apart ensures that, at the second injection, all the animals will be at the correct stage of the cycle to respond to oestrus behaviour and ovulation. On the other hand, progestative hormones are applied during several days (either by daily supply or using systems for a slow release) for mimicking the secretory activity of a corpus luteum during the luteal phase; withdrawal of the hormones would induce a follicular phase and, thus, oestrus and ovulation. To be effective, the duration of the progestative treatment may surpass the active life of a possible corpus luteum in the ovary or may be combined with a luteolytic agent.

In view of these considerations, the current paper describes the results obtained in several consecutive experiments developed to evaluate the suitability of different hormonal protocols based either in the administration, alone and combined, of exogenous progesterone and prostaglandin analogues for induction and synchronization of oestrus in mice. The experiments were consecutive, arising from the results obtained in the precedents, and evaluated the reproductive features (appearance of oestrus behaviour and ovulation, fertility and prolificacy rates) obtained in response to the application of the different treatments. For determining the influence of genotype on breeding performance and evaluating the possibilities for the application of the protocol in different animal facilities using different strains, we assessed the response of three breeds widely used and which are the basis of other strains. The final aim was to design a simple and effective protocol able to be used in practice, SINCEL, and the patent for this is currently pending (application number P-200702376).

Materials and methods

Animals and husbandry

A total of 72 adult mice, between 6 and 8 weeks old, were used. Different inbred (37 BALB/cAnNCrl and 10 C57BL/6J) and outbred strains (25 Crl:CD1(ICR)) were combined in the different experiments for testing breed effects. BALB/c and CD1 were obtained from Charles River Laboratories Inc (Wilmington, MA, USA) while C57BL/6 mice were obtained from The Jackson Laboratory (Bar Harbor, ME, USA). All the animals were maintained for at least one month before experimental procedures at the specific pathogen-free facilities of the CNIC Animal Laboratory Unit in Madrid, Spain, so they were adapted to husbandry, under microbiological control and free of infections and parasites. The lighting cycle was 14 h:10 h light:dark, respectively (07:30 h on; 21:30 h off). Humidity was maintained at 50 ± 5% and the temperature was 20 ± 2ºC. Females were grouped with a maximum of eight animals in plastic cages of 800 cm² of floor area (1291H, Eurostandard Type III; Tecniplast, Biosis, Barcelona, Spain); males were individualized in plastic cages of 370 cm² of floor area (1264C, Eurostandard Type II; Tecniplast) for at least 15 days prior to mating. CNIC Animal Unit meets the requirements of the European Union for Scientific Procedure Establishments. The experiment was carried out under Project Licence PA176/07 from the CNIC Scientific Ethic Committee. Animal manipulations were performed accordingly with the Spanish Policy for Animal Protection RD1201/05, which meets the European Union Directive 86/609 on the protection of animals used in experimentation.

Experimental procedure

Experiment 1: Comparison of the effect obtained by the administration of either progestative or luteolytic hormones

The objective of this experiment was to test the effectiveness of treatments either with progestative or luteolytic hormones for synchronizing fertile oestrus in mice. A total of 15 CD1 females, divided into three groups of five each, was considered (Figure 1). All the treatments, as well as introduction of males, in this experiment and the following were performed between 13:00 h and 14:00 h of the day. The first group (P4CR) was treated by the subcutaneous insertion, on Day −6, of a device for controlled release (Alzet Osmotic Pumps, Durect Co, Cupertino, CA, USA) containing 9 μg of progesterone (4-pregnen-3,20-diona, Siemsgluss Iberica, Barcelona, Spain); the device was changed by a new one on Day −3, which was maintained until Day 0. The second group (2PGF) was treated by the administration of two intraperitoneal doses of 0.5 μg of cloprostenol (Estrumate, Mallinckrodt Vet GmbH, Friesoythe, Germany) three days apart (Days −3 and 0). A third group was used as control (group C), being treated by the introduction of fertile males at the rate of 1:1 on Day 0. Fertile males were also introduced at a rate of 1:1 to groups P4 and PGF on Day 0. Thereafter, appearance of oestrus was determined every morning at 07:30 h of the day, for four days, by evaluating the presence of a vaginal plug as a result of overnight mating; the male was taken out in the morning when a vaginal plug was detected. In case there was no vaginal plug, the male was removed and introduced again at 13:00 h to the female. Fertility rate (percentage of pregnant animals) and prolificacy rate (number of newborns) were established at delivery.

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Figure 1

Schematic representation of the treatment cycle in Experiment 1; days are referred to introduction of male. Group P4CR was treated with the insertion of a device for controlled progesterone release; group 2PGF was treated by the administration of two doses of cloprostenol; group C was the untreated control

Experiment 2: Effect of the administration of protocols combining prostaglandins and a short-acting dose of progesterone

The objective of this experiment arises from the previous one and was to test if effectiveness of treatments with prostaglandins may be increased by a supply of progesterone with the first prostaglandin dose, mimicking a luteal phase in all the animals, or even a single dose of prostaglandin and progesterone would be successful. A total of 25 animals (15 BALB/c and 10 C57BL/6) were used. Two groups of BALB/c (2PGF, n = 5 and C, n = 5) were treated as described in Experiment 1 (Figure 2); breed effects can be determined by comparing both trials. The remaining animals were treated by combining a single short-acting dose of progesterone and PGF_2α, either in a single (P4/PGF, n = 5 C57BL/6) or double dose (P4/2PGF, n = 10, being 5 C57BL/6 and 5 BALB/c for testing strain effects). Treatment of the group P4/2PGF was similar to group 2PGF but including the subcutaneous administration of a single dose of 3 μg of progesterone in 0.2 mL of saline, coincidentally with the first injection of cloprostenol on Day −3. Treatment of the group P4/PGF consisted of a single dose of progesterone plus a single dose of cloprostenol on Day 0. Fertile males were introduced at a rate of 1:1 to all the groups on Day 0. Appearance of oestrus behaviour and rates of fertility and prolificacy were determined as previously described.

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Figure 2

Schematic representation of the treatment cycle in Experiment 2; days are referred to introduction of male. Group P42PGF was treated with a single dose of progesterone and two doses of cloprostenol; group P4PGF was treated by the administration of a single dose of progesterone and a single cloprostenol dose; group C was the untreated control

Experiment 3. Effect of the progesterone dose in protocols combining single doses of prostaglandins and progesterone

The objective of this experiment was to determine if effectiveness of treatments with a single dose of prostaglandin and progesterone would be improved by increasing the dose of progesterone applied. A total of 15 BALB/c, divided in three groups, were used. All the groups were treated, on Day 0, with a single dose of cloprostenol plus a single dose of 3, 6 and 9 μg progesterone. Fertile males were introduced at a rate of 1:1 to all the groups on Day 0. Appearance of oestrus behaviour and rates of fertility and prolificacy were determined as previously described.

Experiment 4. Repeatability of the response to the administration of a single progesterone dose and a double prostaglandin injection (protocol SINCEL)

The objective of this experiment arises from the previous to compare repeatability of the protocol SINCEL in different trials and strains. Two replicates, having seven BALB/c and 10 CD1 females, were treated with two intraperitoneal doses of 0.5 μg of cloprostenol on Days −3 and 0 and a subcutaneous single dose of 3 μg of progesterone in saline on Day −3. Fertile males were introduced at a rate of 1:1 to all the groups on Day 0. Appearance of oestrus behaviour and rates of fertility and prolificacy were determined as previously described.

Statistical analysis

Statistical treatments of data regarding the effect of treatments on the day of onset of oestrus behaviour and mating and on fertility and prolificacy rates were examined by analysis of variance (ANOVA) for the effects of treatment and genetic background within treatments. Analysis of data expressed as percentages were done after transformation to the arcsine. Results were expressed as mean ± SEM and the statistical significance was accepted from P < 0.05.

Results

Experiment 1: Comparison of the effect obtained by the administration of either progestative or luteolytic hormones

Three of five (60%) of the control females, exposed to the male without prior hormonal treatment, showed vaginal plugs during the first 96 h after male introduction; one of them showed the plug at 24 h, the remaining were found at 72 h (Figure 3a). Four of five (80%) females showed signs of mating in both groups treated with exogenous hormones. In the group treated with progesterone (P4CR), most of the animals (3/4, 75%) showed oestrus at 24 h after device removal, while the remaining showed vaginal plugs at 72 h. Conversely, the appearance of plugs was delayed until 72 and 96 h (3/4 and 1/4, respectively) in the group treated with prostaglandins (2PGF). Fertility rates were 100% in control and P4CR, but decreased to 50% in 2PGF (P < 0.05); on the other hand, significant differences in prolificacy rate were not found (12.6 ± 0.4, 11.3 ± 0.6 and 10.0 ± 0.4, respectively).

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Figure 3

Percentage of mice showing vaginal plugs at different times after treatment for induction and synchronization of oestrus and ovulation based on the use, alone and combined, of exogenous progesterone and prostaglandin analogues. (a) Response in control untreated group (white bar) and in mice treated either with prostaglandins or progesterone (light and dark grey bars, respectively). (b) Response in control untreated group (white bar) and in mice treated either with prostaglandin (light grey bar) or with a single short-acting dose of progesterone and a single or double dose of prostaglandin (dotted grey and dotted black bars, respectively). (c) Response in females treated with a single dose of prostaglandin and a single progesterone dose of 3 (dotted grey bar), 6 and 9 μg (no response). (d) Response in two replicates treated with a single progesterone dose and a double dose of prostaglandin (dotted black bars)

Experiment 2: Effect of the administration of protocols combining prostaglandins and a short-acting dose of progesterone

All the control females showed appearance of vaginal plugs, but expanded at 48 h (2/5, 40%), 72 h (2/5, 40%) and 96 h (1/5, 20%); pregnancy and prolificacy rates were 60% and 6.6 ± 0.4. In the group treated with prostaglandins alone (2PGF) only three of five animals responded to the treatment (P < 0.05); the timing of appearance of plugs was similar to that found in the previous experiment, with most of the mice (2/3, 66.7%) showing plugs at 72 h (Figure 3b). All the females became pregnant, with a mean number of offspring of 5.6 ± 0.6. In the groups treated with a single short-acting dose of progesterone and PGF_2α, either in a single (P4/PGF) or double dose (P4/2PGF), the animals showing vaginal plugs were 4 of 5 and 8 of 10, respectively. In the first group, 60% of the animals showed vaginal plugs at 24 h; the remaining mice showed that they had been mated at 96 h. Appearance of oestrus signs was more concentrated in P4/2PGF, with 75% of animals showing oestrus at 24 h (P < 0.05), without significant differences between strains. Pregnancy rates were 100% in both treatments and the litter size was 5.3 ± 0.4 for P4/PGF, and 5.3 ± 0.4 for BALB/c and 6.2 ± 0.6 for C57BL/6 in P4/2PGF.

Experiment 3: Effect of the progesterone dose in protocols combining single doses of prostaglandins and progesterone

Four of five mice showed vaginal plugs in the group treated with 3 μg of progesterone; half of them showed mating signs at 24 h and half of them at 72 h (Figure 3c). On the other hand, there was no mating behaviour in the groups treated with 6 and 9 μg of progesterone.

Experiment 4: Repeatability of the response to the administration of a single progesterone dose and a double prostaglandin injection (protocol SINCEL)

Overall, 13 of 16 mice (4/6 in the first and 9/10 of the mice in the second replicate) showed oestrus behaviour at 24 h, as depicted in Figure 3d; the remaining showed vaginal plugs at 48 h. Fertility rate was 100% again and prolificacy was 8.3 ± 0.6 for BALB/c and 11.7 ± 0.6 for CD1. When combining with the results of the group P4/2PGF in Experiment 2, the mean percentage of animals mated at 24 h was 78.4%. There were no significant differences found between replicates and strains, except the expected effect of the strain on prolificacy.

Discussion

Current results clearly demonstrate the usefulness of protocols based on the combination of progesterone and prostaglandins, mainly of the proposed protocol SINCEL, for the induction and synchronization of fertile oestrus and ovulation in mice.

A detailed analysis of the results found in the different experiments performed indicates, in the first trial, that the use of two luteolytic doses of prostaglandin analogues induces appearance of oestrus and ovulation. However, the results are quite similar to that found in the control group and in the literature described for male effect; ⁵ most of the animals showed vaginal plugs around 72 h after the second injection. By comparing the results in the first and second replicates, we can hypothesize that, as with the male effect, the response and timing of response, and even fertility and prolificacy, by using two doses of prostaglandins alone are very dependent on the moment of the oestrous cycle when applied. These findings resemble those previously found in large animals, pointing to a lower fertility ⁷ and a higher variability in the timing of ovulation after PGF_2α-induced luteolysis; ⁸ features related to a disruption of ovulatory follicle dynamics in different phases of the oestrous cycle. ⁹

On the other hand, results from the first experiment confirm the suitability of a long-term sustained release of progesterone for synchronizing reproductive features in mice in a short period, 24 h after progesterone release. These results would be very useful for the design of a simple and effective protocol for reproductive management of mice in practice. However, the use of subcutaneous devices has the inconveniences derived from the necessity of a surgical approach for applying/removing the tool under the skin.

Thus, a possible alternative is the application of progesterone by injecting subcutaneous doses; however, this makes necessary the use of several consecutive injections to surpass the active life of a possible corpus luteum in the ovary or may be combined with a luteolytic agent to avoid the permanence of such corpus luteum. Furthermore, the use of a single dose of progesterone coincidentally with a prostaglandin treatment would mimic the presence of a secretory corpus luteum and would unify the ovarian cycle and the response in the entire group of animals. In fact, the administration of progesterone in a single intramuscular dose is used in large animals from early studies on oestrus synchronization. ¹⁰ In ruminants, the appearance and quality of ovulations are known to be dependent on the stage of follicle development at the time of male introduction. A single injection of progesterone stimulates the emergence of a new follicular wave, able to grow and give adequate ovulations. ¹¹

Results of the second experiment confirmed that the combined use of prostaglandin and a single dose of progesterone induced the appearance of synchronized and fertile oestrus and ovulation. It can be established a progesterone dosage of around 3 μg, from the results of the third experiment, since higher doses generate anoestrus; possible causes, mechanisms and applications of such findings deserve further studies but a long-acting effect of the gestagen due to tissue storage and slow release may be hypothesized. The comparison of the results found by applying single doses of progesterone and prostaglandin in the second and third experiments showed that there are two peaks of appearance of vaginal plugs, at 24 h and 96 h, which we theorize to be determined by the stage of oestrous and ovarian cycle at treatment. On the other hand, the response was more uniform in the group treated with progesterone and two prostaglandin doses (protocol SINCEL); from the results of a second and fourth experiment, almost 100% of the animals were mated before 48 h of the treatment; most of them (around 78.4%) at 24 h. The response to SINCEL was highly repeatable between replicates, which may be related to a higher synchronization of the ovarian stage at induction of luteolysis and introduction of the male.

In conclusion, the use of the protocol SINCEL synchronizes the appearance of fertile ovulation at 24 h after the second dose of cloprostenol. This method arises as an adequate alternative to the classical methods based on the use of male pheromones due to: (a) the short time elapsed for the appearance of oestrus and ovulations, (b) the high degree of synchronization of such features and (c) the high fertility rate after a programmed mating. The suggested protocol for oestrus synchronization of mice will also be helpful for all those dealing with problems associated with reproduction and timed mating.