The hypothalamic-pituitary-ovarian axis and manipulations of the oestrous cycle in the brushtail possum

Emerging arguments for reproductive technologies in wildlife and their implications for assisted reproduction and conservation of threatened marsupials

Assisted reproductive technologies (ARTs) have significant potential to make a meaningful contribution to the conservation of threatened wildlife. This is true of Australia's iconic, and endangered koala (Phascolarctos cinereus). If developed, ARTs could offer a solution to manage genetic diversity and costs in breeding programs and may provide frozen repositories for either insurance or the practical production of genetically resilient koalas for release and on-ground recovery. Holding back the wider use of ARTs for koalas and other wildlife is a lack of funding to close the remaining knowledge gaps in the marsupial reproductive sciences and develop the reproductive tools needed. This lack of funding is arguably driven by a poor understanding of the potential contribution ARTs could make to threatened species management. We present a review of our cross-disciplinary and accessible strategy to draw much needed public attention and funding for the development of ARTs in wildlife, using emerging cost and genetic modelling arguments and the koala as a case study.

Human chorionic gonadotrophin does not induce ovulation in the tammar wallaby

We investigated whether human chorionic gonadotrophin (hCG), rather than porcine luteinising hormone (pLH), could induce ovulation in the tammar wallaby (Notamacropus eugenii) after priming with pregnant mare serum gonadotrophin (PMSG). pLH is effective, but is more expensive and supply is problematic. On Day 0 pouch young were removed (RPY) and 5mgkg−1 of bromocriptine mesylate was administered to all non-pregnant wallabies (n=12) to reactivate an oestrous cycle. On Day 20 RPY each female received 20IU PMSG i.m. and three days later (Day 23 RPY), the Single-Dose group (n=6) received 500IU of hCG suspended in oil at 0900 hours, and the Multi-Dose group (n=6) received 500IU of hCG at 0900, 1230 and 1630 hours. From Day 25 to Day 28 RPY copulatory plugs were observed in the Single-Dose (n=4 of 6; Day 25.3 to 28.3 RPY) and Multi-Dose (n=5 of 6; Day 25 to 28 RPY) groups. Autopsy on Day 31 RPY revealed that all females had reactivated, and despite all having large preovulatory follicles (4–6mm) in both ovaries, no ovulations or embryos were observed. We conclude that hCG is not an effective source of exogenous LH for the induction of ovulation in the tammar wallaby.

Induction of synchronous oestrus but not ovulation after pre-treatment with the GnRH agonist, Lucrin® Depot, in the tammar wallaby

We investigated the capacity for pregnant mare serum gonadotrophin (PMSG) and human chorionic gonadotrophin (hCG) to induce synchronous oestrus and ovulation in the tammar wallaby (Notamacropus eugenii) after follicular suppression with Lucrin® Depot, a one-month GnRH agonist. On Day 0 pouch young were removed (RPY) to reactivate a normal oestrous cycle and then two groups were treated with vehicle (Control; n = 5 and Superovulation (SOvn); n = 5) and two groups were treated with 7.5 mg of Lucrin Depot (Lucrin; n = 6; Lucrin+SOvn, n = 6). On Day 20 RPY the SOvn and Lucrin+SOvn Group received 20 IU of PMSG, which was followed on Day 23 RPY with 500 IU of hCG. The Lucrin+SOvn females underwent a more synchronous oestrus with 5 of 6 mating on Day 26 RPY while the SOvn (n = 5/5) and Control (n = 5/5) Groups copulated over two days, between Day 25-27 RPY and Day 27-29 RPY respectively. Mating plugs were not detected in any of the females in the Lucrin Group by Day 31 RPY. Autopsy on Day 31 RPY confirmed all females in each treatment group had undergone a reactivated cycle as evidenced by the presence of a large corpus luteum (CL) in one ovary. At autopsy the females in the Lucrin+SOvn Group had highly stimulated reproductive tracts, and their ovaries contained many follicles >3 mm; 14 ± 2.1 and 15.3 ± 2.1 follicles >3 mm in the CL-bearing ovary and contralateral ovary respectively. Similarly, females in the SOvn Group had 11.4 ± 2.4 and 17.4 ± 1.9 follicles >3 mm in each respective ovary. Uterine flushing and ovarian histology confirmed that females in Lucrin+SOvn and SOvn Groups had not ovulated, but normal oocytes were present in the follicles. By comparison, the Control Group had ovulated with a single embryo being recovered from the uterus of 4 of 5 females. In contrast to all groups, females in the Lucrin Group showed follicular suppression (all follicles <1.5 mm) and an unstimulated reproductive tract. We conclude that a suppression plus stimulation regimen using Lucrin Depot followed by PMSG and hCG has the capacity to synchronise oestrus, and that 20 IU of PMSG stimulates the development of antral follicles >3 mm in both ovaries. However, a single 500 IU treatment of hCG on Day 23 RPY was not able to induce ovulation in the tammar wallaby.

Marsupials: Progress and Prospects

The chapter provides a review of the application of reproductive science to technologies for marsupial conservation and population management and discusses prospects for the future. This includes the status of technologies such as sperm freezing, artificial insemination, and exogenous hormone treatments to stimulate ovarian activity and cycling in the female. Fertility-based population management for introduced pest species and over-abundant native marsupials remain an elusive goal. Immune-contraceptive approaches, despite demonstration of basic effectiveness, have not progressed to field deliverable agents. Emerging genetic technologies such as gene drives offer great promise, but gene modifications face major challenges to be broadly accepted both socially and politically. A main theme is the potential advantages, both genetic and economic, of integrating frozen stored genomic material, such as sperm, into the captive breeding component of threatened species strategies. However, the sperm of many marsupial species display no or very poor recovery of motility on thawing. For this reason, it is proposed that the traditional assisted breeding paradigm for conservation-cervical artificial insemination with thawed frozen sperm, based on cattle breeding-is not a viable default strategy. Rather, techniques such as sperm injection and emerging stem cell technologies that utilize stored frozen cells, and in the case of sperm, immotile cells, are better candidates for the development of a more generic approach. In addition, this change in focus encourages wide scale proactive genome storage when genetic diversity is greatest, without the need to demonstrate success in traditional sperm cryopreservation and thawing. However, the promise of the potential of reproductive science to conservation and non-lethal population management is problematic without far greater recognition of, and investment in, the needs of wildlife by society.

Breeding in the fat-tailed dunnart following ovarian suppression with the gonadotrophin-releasing hormone agonist Lucrin® Depot

Lucrin Depot (AbbVie), a 1-month microsphere gonadotrophin-releasing hormone (GnRH) agonist preparation, was investigated as a potential agent to synchronise cycling in the fat-tailed dunnart (Sminthopsis crassicaudata). Forty-eight randomly selected females were treated with 5 or 10mgkg-1 Lucrin Depot (n=24 per dose). Eighteen females per treatment had their reproductive activity scored at 4, 8, 12 and 16 weeks using two ovarian (Graafian follicle and corpus luteum status) and two reproductive tract (uterine and vaginal muscularity and vascularity) parameters that formed a reproductive activity score. Six females per treatment were paired with a male at 4 weeks. Fertility was assessed between 8 and 16 weeks by pouch check, and thereafter by dissection. The effects of the 5 and 10mgkg-1 doses were statistically equivalent. Females showed suppression at 4-8 weeks, an increase in reproductive activity at 8-12 weeks and all were cycling normally at 16 weeks. Six pouch young were born at 12 weeks to two females treated with the 5mgkg-1 dose. Nine embryos were recovered at 16 weeks from two females treated with the 10mgkg-1 dose. In conclusion, Lucrin Depot can suppress breeding, and fertile mating can occur in subsequent cycles in the dunnart. There is potential for Lucrin Depot to be used as an assisted breeding tool, but it may need to be combined with ovarian stimulation treatment to achieve practical levels of synchronisation in the fat-tailed dunnart.

Recent advances in tools and technologies for monitoring and controlling ovarian activity in marsupials

Components of assisted reproduction technologies (ART), such as sperm cryopreservation, artificial insemination, superovulation and pouch young surrogacy, have been developed for a range of Australian and American marsupials. However, methods to effectively control ovarian function, arguably the key limiting factors in applying and integrating ART as a practical tool in conservation management, remain poorly developed. This is largely due to unique characteristics of the marsupial corpus luteum and its failure to respond to agents used to synchronize ovarian function in eutherian mammals. This paper presents an overview of relevant aspects of marsupial reproductive biology across marsupial taxonomic groups including information on the long-established technique of removal of suckling young to activate ovarian cycles. Ovarian monitoring tools for marsupials are reviewed and their usefulness for ART assessed (laparotomy, hormone cycling, vaginal cytology, laparoscopy and ultrasonography). We also discuss promising recent work examining the potential of manipulating hypothalamic-pituitary function using GnRH agonists and antagonists as the basis of ovarian control (female synchronization) strategies.

Ovarian suppression in a marsupial following single treatment with a gonadotrophin-releasing hormone agonist in microspheres

The effect of treatment with Lucrin Depot (1 month), a microsphere gonadotrophin-releasing hormone agonist preparation, was investigated in the fat-tailed dunnart (Sminthopsis crassicaudata) as a potential strategy to synchronise cycling. The status of the ovaries (ovarian size, number and size of Graafian follicles and corpora lutea) and reproductive tract (weight, vascularity and muscularity) in twelve untreated females were assessed to establish the activity parameters for randomly selected cycling animals. Thirty-six females were treated with 1 mg kg–1 (n = 12), 10 mg kg–1 (n = 12) or 20 mg kg–1 (n = 12) Lucrin Depot. At 4, 6 and 8 weeks the reproductive tracts were assessed using the criteria developed in the untreated females. All of the females treated with 10 mg kg–1 showed suppression at 4 weeks and 25% showed return of reproductive activity at 8 weeks. A dose of 1 mg kg–1 did not appear to suppress reproductive activity and 20 mg kg–1 gave equivocal results, with evidence of both suppression and activity. The results indicate that Lucrin Depot appears to be a promising agent to regulate and potentially synchronise breeding activity in the fat-tailed dunnart.

Plasma prolactin concentrations during lactation, pouch young development and the return to behavioural oestrus in captive koalas (Phascolarctos cinereus)

Plasma prolactin (PRL) concentrations in captive koalas during lactation were determined by serial blood sampling. PRL concentrations were low (1.3 ± 0.1 ng mL-1; n = 5) during early lactation until pouch young (PY) began to emerge from the pouch (around Day 130) before significantly (P < 0.05) increasing between Day 161 and Day 175 (5.3 ± 1.0 ng mL-1). A significant (P < 0.001) peak in PRL (7.7 ± 0.6 ng mL-1) coincided with maturing young between Day 189 and Day 231. All females failed to exhibit any signs of oestrous behaviour until Day 268.8 ± 8.5 (n = 4), some 102 ± 19 days before PY were weaned following achieving target weights of 2.5-2.7 kg. Throughout lactation, plasma LH concentrations were relatively high (range 4.9-8.7 ng mL-1) and LH responses to exogenous gonadotrophin-releasing hormone were observed in all koalas at all times during lactation.

The use of a synthetic progesterone, levonorgestrel (LNG), to control the oestrous cycle in the koala

This study investigated the efficacy of a synthetic progestogen, levonorgestrel (LNG), to control koala ovarian activity for the purposes of oestrous synchronisation. Captive koalas were administered either saline control or a 70-mg LNG implant on Day 2 of oestrus. Urogenital cytology, oestrous behaviour and plasma oestradiol-17β and LH concentrations were monitored over a 6-week period. After LNG implant removal females were monitored to determine if the return to oestrus was synchronised. LNG-treated koalas immediately ceased displaying oestrous behaviour, showed no evidence of cornified epithelial cells in smears of urogenital cytology and exhibited low plasma oestradiol-17β concentrations throughout the implantation period. In contrast, oestradiol-17β levels in control koalas showed evidence of continued cyclic activity associated with behavioural oestrus and increased cornified epithelial cells in urogenital smears on Days 33 to 35 after saline injection. After implant removal, LNG-treated koalas exhibited oestrus at 13, 14, 17 and 30 days after implant removal. Plasma LH concentrations varied throughout the study period with no significant time (P = 0.49) or treatment (P = 0.13) effect. Overall results from this study suggest that LNG implants in koalas can inhibit oestrous behaviour and reduce circulating oestradiol-17β levels before oestrus, most likely by preventing development of the pre-ovulatory follicle. However, there was no evidence of LH suppression by the LNG implants. Removal of LNG implants resulted in the synchronous return to oestrus in three of the four treated koalas. Further studies on a larger population are required to validate these findings.

Molecular analysis of the koala reproductive hormones and their receptors: gonadotrophin-releasing hormone (GnRH), follicle-stimulating hormone β and luteinising hormone β with localisation of GnRH

During evolution, reproductive hormones and their receptors in the brain-pituitary-gonadal axis have been altered by genetic mechanisms. To understand how the neuroendocrine control of reproduction evolved in mammals, it is important to examine marsupials, the closest group to placental mammals. We hypothesised that at least some of the hormones and receptors found in placental mammals would be present in koala, a marsupial. We examined the expression of koala mRNA for the reproductive molecules. Koala cDNAs were cloned from brain for gonadotrophin-releasing hormones (GnRH1 and GnRH2) or from pituitary for GnRH receptors, types I and II, follicle-stimulating hormone (FSH)β and luteinising hormone (LH)β, and from gonads for FSH and LH receptors. Deduced proteins were compared by sequence alignment and phylogenetic analysis with those of other vertebrates. In conclusion, the koala expressed mRNA for these eight putative reproductive molecules, whereas at least one of these molecules is missing in some species in the amniote lineage, including humans. In addition, GnRH1 and 2 are shown by immunohistochemistry to be expressed as proteins in the brain.