Influence of the gonad on the degree of suppression induced by an LHRH agonist implant in the marmoset monkey

Contraceptive efficacy and dose-response effects of the gonadotrophin-releasing hormone (GnRH) agonist deslorelin in Tasmanian devils (Sarcophilus harrisii)

Contraception is increasingly used to manage breeding opportunities in conservation-dependent species. This study aimed to determine the efficacy, duration of effect, optimal dose and potential side effects of Suprelorin contraceptive implants in Tasmanian devils, for use in the conservation breeding program. In our pilot study, Suprelorin was found to effectively suppress oestrous cycles in female devils, yet caused a paradoxical increase in testosterone in males. Therefore, we focussed on females in further trials. Females received one (n=5), two (n=5) or no (n=5) Suprelorin implants, with quarterly gonadotrophin-releasing hormone (GnRH) challenges used to test pituitary responsiveness over two breeding seasons. Both Suprelorin doses suppressed pituitary responsiveness for at least one breeding season, with a reduced effect in the second. There was a dose-response effect on duration rather than magnitude of effect, with high-dose devils remaining suppressed for longer than low-dose animals. There were no apparent negative effects on general health, yet captivity and contraception together may cause weight gain. Suprelorin contraceptive implants are now routinely used in the Save the Tasmanian Devil Program insurance metapopulation to meet the aims of maintaining genetic and behavioural integrity by controlling individual reproductive contributions in group housing situations.

Gonadotrophin-Releasing Hormone Agonists and Other Contraceptive Medications in Exotic Companion Animals

The use of a gonadotrophin-releasing hormone agonist slow-release implant (GnRH A-SRI) has become increasingly popular as an alternative for surgical contraception in many species. Although these implants have proven to be very effective in some species (eg, ferrets, rats, chicken, psittacines, and iguanas), they have been found less effective in other species (eg, male guinea pigs and rabbits, veiled chameleons, slider turtles, and leopard geckos). This review provides an overview of the available literature on the effects of GnRH A-SRIs in companion exotic animals.

Pituitary and testicular endocrine responses to exogenous gonadotrophin-releasing hormone (GnRH) and luteinising hormone in male dogs treated with GnRH agonist implants

The present study tested whether exogenous gonadotrophin-releasing hormone (GnRH) and luteinising hormone (LH) can stimulate LH and testosterone secretion in dogs chronically treated with a GnRH superagonist. Twenty male adult dogs were assigned to a completely randomised design comprising five groups of four animals. Each dog in the control group received a blank implant (placebo) and each dog in the other four groups received a 6-mg implant containing a slow-release formulation of deslorelin (d-Trp6-Pro9-des-Gly10-LH-releasing hormone ethylamide). The same four control dogs were used for all hormonal challenges, whereas a different deslorelin-implanted group was used for each challenge. Native GnRH (5 microg kg(-1) bodyweight, i.v.) was injected on Days 15, 25, 40 and 100 after implantation, whereas bovine LH (0.5 microg kg(-1) bodyweight, i.v.) was injected on Days 16, 26, 41 and 101. On all occasions after Day 25-26 postimplantation, exogenous GnRH and LH elicited higher plasma concentrations of LH and testosterone in control than deslorelin-treated animals (P < 0.05). It was concluded that, in male dogs, implantation of a GnRH superagonist desensitised the pituitary gonadotrophs to GnRH and also led to a desensitisation of the Leydig cells to LH. This explains, at least in part, the profound reduction in the production of androgen and spermatozoa in deslorelin-treated male dogs.

Chronic Treatment of Male Tammar Wallabies with Deslorelin Implants During Pouch Life: Effects on Development, Puberty, and Reproduction in Adulthood1

The present study evaluated the effects of chronic GnRH agonist (deslorelin) treatment on sexual maturation in the male tammar wallaby. Slow-release deslorelin or placebo implants were administered to male pouch young (n = 10/group) when they were between 180 and 200 days old, to determine if disruption of the pituitary-testicular axis during development altered the timing of sexual maturation or had long-term effects on adult reproductive function. Deslorelin treatment caused retardation of testicular growth and reduced the serum FSH and testosterone concentrations between 12 and 24 mo of age. Maturation of the hypothalamic-pituitary-testicular axis was also delayed in treated animals at 13 and 19 mo of age. Despite these alterations in the pattern and timing of neuroendocrine development, sexual maturation was not permanently blocked in these animals and deslorelin-treated animals reached sexual maturity at the same age as treated animals, as evidenced by a fully functional pituitary-testicular axis and proven fertility at 25 mo of age. The ability of the treated animals to reach puberty at the same time as control animals, despite delayed maturation of the hypothalamic-pituitary-testicular axis, suggests that puberty in the male tammar wallaby is additionally regulated by other, gonadotropin-independent factors.

Effects of deslorelin implants on reproduction in the common brushtail possum (Trichosurus vulpecula)

The present study investigated the effects of slow-release implants containing the gonadotrophin-releasing hormone (GnRH) agonist deslorelin on reproduction in the common brushtail possum (Trichosurus vulpecula). Captive female brushtail possums were assigned to control (placebo implant), low dose (4.7 mg deslorelin) or high dose (9.4 mg deslorelin) groups; males were assigned to control or high dose (9.4 mg deslorelin) groups. The acute effects of deslorelin treatment at the level of the pituitary gland were similar between the two sexes, where a transient rise in luteinising hormone concentration was induced over the first 24 h. In females, this was associated with the disruption of the normal oestrous cycle and mating within 2–10 days in some treated individuals, but no young were subsequently detected. By 3 weeks after treatment, treated females became anoestrus and remained infertile for at least one breeding season. The effects of treatment were reversible in a subset of females that had their implants removed, although the time taken to produce offspring was variable. Paradoxically, male brushtail possums remained fertile during chronic deslorelin exposure. Despite significant declines in basal follicle-stimulating hormone and testosterone concentrations, as well as an inability to respond to a GnRH challenge, treated males sired as many offspring as control males and there was no evidence of testicular regression. In conclusion, there is potential to control reproduction in female brushtail possums by using chronic GnRH agonist treatment.

Social suppression of cortisol in female marmosets: role of luteinizing hormone/chorionic gonadotropin

Behaviorally subordinate female common marmosets (Callithrix jacchus) undergo suppression of ovulation and chronic reductions in basal plasma cortisol concentrations. Indirect evidence suggests that hypophyseal chorionic gonadotropin (CG; the major pituitary luteinizing gonadotropin in marmosets) may elevate cortisol concentrations in female marmosets, and therefore that social suppression of CG may contribute to diminution of cortisol in subordinates. To test this hypothesis, we determined whether pharmacological inhibition of pituitary CG release decreases basal and adrenocorticotropin (ACTH)-stimulated cortisol secretion. We characterized cortisol and reproductive hormone concentrations in six ovary-intact and six ovariectomized marmosets during long-term treatment with leuprolide acetate, a gonadotropin-releasing hormone (GnRH) agonist, and vehicle. Leuprolide suppressed basal plasma CG concentrations, abolished the CG response to exogenous GnRH, and, in intact animals, blocked ovarian cyclicity. During treatment with vehicle, plasma cortisol concentrations were elevated during the periovulatory phase in intact females, compared to the follicular phase, the luteal phase, and ovariectomized females. Leuprolide suppressed basal cortisol concentrations of intact females as compared to the periovulatory phase, but did not affect basal cortisol in ovariectomized animals and did not alter responses to exogenous ACTH. These findings suggest that elevations in circulating CG concentrations are associated with elevated cortisol concentrations in female marmosets, and that this relationship requires simultaneous increases in ovarian hormones that occur only during the periovulatory period. Thus, suppression of CG release in anovulatory subordinate females may not play an important role in socially induced diminution of cortisol.

Fertility control in female eastern grey kangaroos using the GnRH agonist deslorelin. 2. Effects on behaviour

In recent years fertility control has been proposed as an ethically acceptable alternative to lethal control techniques when managing overabundant kangaroo populations. A promising non-steroidal, non-immunological approach to contraception in female kangaroos involves the use of slow-release implants containing the gonadotrophin-releasing hormone (GnRH) agonist deslorelin. The practicality of using deslorelin implants as a management option is dependant on its effective inhibition of reproduction without negative physical or behavioural side-effects. This study investigated the behavioural effects of deslorelin implants in female eastern grey kangaroos. Treatment had no detectable effects on crepuscular activity. Alterations in the frequency of sexual interactions were observed in deslorelin-treated females, with a behavioural oestrus induced ~3 days after combined removal of pouch young and deslorelin administration. Copulation was observed during this early oestrous period, but conception was not achieved and pouch young were not observed in any treated females. Control females gave birth within 69.6 ± 10.4 days (mean ± s.e.m., n = 9) of placebo implant administration. The first births observed in treated animals were on Days 510, 637 and 643 after treatment. The remaining seven treated animals had not bred by the end of the study, a period of 647 days.

Fertility control in wildlife: humans as a model

In a time of accelerated extinction rates and biodiversity loss, it may seem illogical to be concerned with methods limiting the reproduction of wild populations; however, there is an urgent need to inhibit the proliferation of a wide variety of species. The range of animals for which fertility control is desired makes the development of a single method impossible. The various reproductive strategies used by individual species, the desired outcome of contraceptive programs (reversible or irreversible; male or female directed) and our ignorance of the reproductive biology of many endangered species necessitate thorough species-specific investigations. As fertility control in humans is a reality and research into methods of fertility control is more advanced, humans serve as a model for developing contraceptive approaches for wild species. Population control by traditional methods (indirect or direct intervention by culling, poisoning, translocation, etc.) is increasingly unacceptable to the public, making human studies even more valuable for finding solutions to overabundant wild populations. This review compares and contrasts the range of contraceptive methods used in both wildlife and humans.

Long-term effects of deslorelin implants on reproduction in the female tammar wallaby (Macropus eugenii)

The contraceptive and endocrine effects of long-term treatment with implants containing the GnRH agonist deslorelin were investigated in female tammar wallabies (Macropus eugenii). Fertility was successfully inhibited for 515 ± 87 days after treatment with a 5 mg deslorelin implant (n= 7), while control animals gave birth to their first young 159 ± 47 days after placebo implant administration (n= 8). The duration of contraception was highly variable, ranging from 344 to 761 days. The strict reproductive seasonality in the tammar wallaby was maintained once the implant had expired. This inhibition of reproduction was associated with a significant reduction in basal LH concentrations and a cessation of oestrous cycles, as evidenced by low progesterone concentrations. There was evidence to suggest that some aspect of either blastocyst survival, luteal reactivation, pregnancy or birth may be affected by deslorelin treatment in some animals. These results show that long-term inhibition of fertility in the female tammar wallaby is possible using slow-release deslorelin implants. The effects of deslorelin treatment were fully reversible and there was no evidence of negative side effects. Slow-release GnRH agonist implants may represent a practicable method for reproductive management of captive and semi-wild populations of marsupials.

Review on testicular development, structure, function, and regulation in common marmoset

BACKGROUND

The common marmoset (Callithrix jacchus) is a New World primate that has been used increasingly in toxicological evaluations including testing for testicular toxicity of pharmaceutical and environmental chemicals. Information on structural and functional characteristics of the testis in common marmosets ("marmoset" in this review) is critical for designing experiments, interpreting data collected, and determining relevance to humans in risk assessment.

METHODS

This study provides a comprehensive review on testicular development, structure, function, and regulation in common marmosets.

RESULTS

There is little information regarding testicular formation and development during gestation. Based on the overall pattern of embryonic development in marmosets, it is postulated that gonadal formation and testicular differentiation most likely takes place during gestational Week 6-12. After birth, the neonatal period of the first 2-3 weeks and the pubertal period from Months 6-12 are critical for establishment of spermatogenesis in the adult. In the adult, a nine-stage model has been used to describe the organization of seminiferous epithelium and multiple stages per tubular cross-section have been observed. Seminiferous epithelium is organized in a wave or partial-wave manner. There are on average two stages per cross-section of seminiferous tubules in adult marmoset testis. Sertoli cells in the marmoset have a uniform morphology. Marmoset spermatogenesis has a high efficiency. The prime determinant of germ cell production is proliferation and survival of spermatogonia. Sertoli cell proliferation during the neonatal period is regulated by follicle-stimulating hormone (FSH), but chorionic gonadotropin (CG), instead of luteinizing hormone (LH), is the only gonadotropin with luteinizing function in marmoset. The receptor gene for CG in marmoset is unique in that it does not have exon 10. Marmosets have a "generalized steroid hormone resistance," i.e., relatively high levels of steroid hormones in circulation and relatively low response to exogenous steroids. Blockage of FSH, CG, and testosterone production during the first 3 months after birth does not cause permanent damage to the male reproductive system. Initiation of spermatogenesis in the marmoset requires unique factors that are probably not present in other mammals. Normal male marmosets respond to estradiol injection positively (increased LH or CG levels), a pattern seen in normal females or castrated males, but not usually in normal males of other mammalian species.

CONCLUSIONS

It seems that the endocrine system including the testis in marmosets has some unique features that have not been observed in rodents, Old World primates, and humans, but detailed comparison in these features among these species will be presented in another review. Based on the data available, marmoset seems to be an interesting model for comparative studies. However, interpretation of experimental findings on the testicular effects in marmosets should be made with serious caution. Depending on potential mode of testicular actions of the chemical under investigation, marmoset may have very limited value in predicting potential testicular or steroid hormone-related endocrine effects of test chemicals in humans.

Effects of a Gonadotropin-Releasing Hormone Agonist Implant on Reproduction in a Male Marsupial, Macropus eugenii1

This study evaluated the potential of slow-release GnRH agonist (deslorelin) implants to inhibit reproductive function in the male tammar wallaby. The specific aim was to measure the effects of graded dosages of deslorelin on testes size and plasma LH and testosterone concentrations. Adult male tammar wallabies were assigned to four groups (n = 6 per group) and received the following treatment: control, placebo implant; low dose, 5 mg deslorelin; medium dose, 10 mg; high dose, 20 mg. All dosages of deslorelin induced acute increases (P < 0.001) in plasma LH and testosterone concentrations within 2 h, with concentrations remaining elevated during the first 24 h but returning to pretreatment levels by Day 7. Thereafter, there was no evidence of a treatment-induced decline in plasma testosterone concentrations. There was no detectable difference in basal LH concentrations between treated and control animals, nor was there a significant change in testes width or length (P > 0.05). These results suggest that the male tammar wallaby is resistant to the contraceptive effects of chronic GnRH agonist treatment. Despite the maintenance of testosterone secretion, the majority of male tammars (10 of 17) failed to respond to a GnRH challenge with a release of LH between Days 186 and 197 of treatment. The failure of animals to respond to exogenous GnRH suggests a direct effect of deslorelin on the pituitary, resulting in a level of desensitization that was sufficient to inhibit a LH surge but insufficient to inhibit basal LH secretion. The variation between animals is believed to result from earlier recovery of some individuals, in particular those that received a lower dose, or individual resistance to the desensitization process.

Effect of deslorelin implants on follicular development, parturition and post-partum oestrus in the tammar wallaby (Macropus eugenii)

The effect of treatment with slow release implants containing the GnRH agonist, deslorelin, was investigated in female tammar wallabies. Pouch young were removed from 16 wallabies presumed to be carrying quiescent blastocysts. Eight received a 5 mg deslorelin implant and eight received a placebo implant. Animals were caught daily from day 25 to day 30 and their pouches inspected for newborn young and their urogenital sinus checked for a copulatory plug. Treatment with deslorelin did not affect reactivation of a dormant blastocyst and subsequent birth in 4/8 animals, but post-partum mating was inhibited in these animals. Five control and five treated animals were killed within 0–48 h post partum and their reproductive tracts analysed. At autopsy, all five control animals had large preovulatory follicles but only one deslorelin-treated animal showed signs of follicular development. These differences were also reflected in the weights of the lateral vaginae, with treated animals showing no evidence of oestrogenic stimulation. The remaining three control and three treated animals were monitored for approximately 2 years. The long-term contraceptive effects of a single 5 mg deslorelin implant lasted for just under one year. These results indicate that slow release deslorelin implants inhibit follicular development in the female tammar wallaby for extended periods of time and may have potential application in reproductive management of captive marsupials in the kangaroo family.

Restoration of LH output and 17beta-oestradiol responsiveness in acutely ovariectomised holstein dairy cows pre-treated with a GnRH agonist (deslorelin) for 10 days

The objectives of the study were firstly to identify the role of the ovary in maintaining plasma luteinising hormone (LH) concentrations in cows treated with an implant of a potent GnRH agonist (deslorelin), and secondly to characterise the changes in LH following ovariectomy (OVX) in the same animals. Oestrus was synchronised in mature Holstein dairy cows and deslorelin implants were inserted 17 days later into two-third of the cows. A further 10 days later (day 0) all cows had bilateral OVX performed. A control group (CON; n=4) received no treatment and had blood samples collected at 15-min intervals for 8h on the day prior to OVX (day -1) and similarly on days 4 and 10. One group (DES_IN; n=4) had implants in place for the duration of the study while another group had implants removed (DES_OUT; n=4) at the time of OVX. DES_IN cows were sampled hourly at each sampling session (days -1, +4 and +10), whereas DES_OUT cows were sampled similarly to CON except on day -1 when hourly samples were collected. Predictable post-operative increases in mean LH (0.61 ng/ml versus 1.79 ng/ml; P<0.01) and LH pulse amplitude (0.66 ng/ml versus 1.56 ng/ml; day -1 versus day +10; P<0.01) occurred after CON cows were ovariectomised. Smoothed LH means showed a delayed effect of time compared to arithmetic means. Pulse frequency was unchanged following OVX in CON cows. A comparison of all cows that had been treated with deslorelin from day -1 showed a significant elevation of smoothed mean LH compared to untreated cows (0.80 ng/ml versus 0.34 ng/ml; DES_IN and DES_OUT versus CON; P<0.05). DES_IN cows had a 54% reduction in mean LH from day -1 to +4 following OVX (1.05 ng/ml versus 0.48 ng/ml; P<0.01) indicating the probable involvement of the ovary in the maintenance of elevated basal LH. No further reduction was detected by day +10. The LH response to an intramuscular (IM) injection of 500 microg 17beta-oestradiol (E2) on day +11 varied significantly between treatment groups (P<0.01). CON cows showed a typical LH surge, reaching maximum concentrations (10.3 ng/ml) at 17.3h post-injection. Even though low amplitude LH pulsatility had been restored in DES_OUT cows by day +4, there was an inconsistent response to E2 on day +12; one cow had an apparently normal surge yet, others showed only attenuated responses. Pulse amplitude in DES_OUT cows was lower at days +4 and +10 compared to CON (P<0.05). DES_IN cows did not produce any surge after E2. Mean LH prior to OVX (day -1) remained unchanged following the 500 microg oestradiol injection (0.38 ng/ml versus 0.45 ng/ml pre-E2 versus post-E2 compared to 1.05 ng/ml pre-OVX). The results of this experiment implicated ovarian involvement in maintaining elevated basal LH output in cows that were chronically treated with a GnRH agonist. Individual cows varied in their LH surge response to exogenous E2 given 12 days after implant removal, even though LH pulse amplitude and frequency had been restored.

Differential responses in anterior pituitary luteinizing hormone (LH) content and LH beta- and alpha-subunit mRNA, and plasma concentrations of LH and testosterone, in bulls treated with the LH-releasing hormone agonist deslorelin

Anterior pituitary gland contents of LH and LH beta- and alpha-subunit mRNAs, and circulating concentrations of LH and testosterone, were determined in bulls treated with the LH-releasing hormone (LHRH) agonist deslorelin. Brahman (Bos indicus) bulls (14-month-old) were allocated to two groups and received the following: Control (n = 5), no treatment; Deslorelin (n = 4), four deslorelin implants (approximately 200 micrograms total deslorelin/day) for 36 d. Plasma concentrations of LH were higher in bulls treated with deslorelin on Day 1, had returned to typical levels by Day 8, and did not differ for control bulls and bulls treated with deslorelin from Day 8 to Day 29. Pituitary content of LH on Day 36 was reduced (P < 0.001) in bulls treated with deslorelin (33 +/- 4 ng/mg) compared with control bulls (553 +/- 142 ng/mg). Relative pituitary content of LH beta-subunit mRNA was also reduced on Day 36 in bulls treated with deslorelin (Control, 0.65 +/- 0.10; Deslorelin, 0.22 +/- 0.04; P = 0.003). However, alpha-subunit mRNA relative content did not differ (Control, 0.73 +/- 0.15; Deslorelin, 1.06 +/- 0.12; P > 0.05). Plasma concentrations of testosterone were increased over the period of the experiment in the bulls treated with deslorelin compared with control bulls. This is the first demonstration of reduced pituitary content of LH beta-subunit mRNA and LH, and unaltered content of alpha-subunit mRNA, in bulls treated with LHRH agonist. This was associated with apparently typical plasma concentrations of LH and elevated plasma testosterone. The anterior pituitary in bulls treated with LHRH agonist, therefore, undergoes classical desensitization and downregulation, but plasma LH and testosterone are not suppressed.

Increased testosterone secretion in bulls treated with a luteinizing hormone releasing hormone (LHRH) agonist requires endogenous LH but not LHRH

The requirement for endogenous LHRH and LH action in the maintenance of elevated plasma concentrations of testosterone in bulls receiving the LHRH agonist deslorelin was examined. In Experiment 1, bulls were either (i) left untreated (control); (ii) implanted with deslorelin; (iii) actively immunized against LHRH; or (iv) implanted with deslorelin and immunized against LHRH. Experiment 2 was of similar design to Experiment 1, except that bulls were immunized against LH in place of LHRH. In Experiment 1, plasma LH declined in bulls immunized against LHRH, but not in the bulls immunized against LHRH and implanted with deslorelin. Also in Experiment 1, plasma testosterone declined in bulls immunized against LHRH but was elevated in bulls treated with deslorelin and bulls treated with deslorelin and immunized against LHRH. In Experiment 2, bulls immunized against LH and treated with deslorelin had plasma concentrations of testosterone similar to controls, whereas bulls treated only with deslorelin had elevated plasma testosterone. It was concluded from these experiments that endogenous LHRH action was not required for increased steroidogenic activity in bulls treated with a LHRH agonist. However, circulating LH was necessary for increased plasma testosterone in bulls implanted with deslorelin. LH is therefore involved in mediating the response of bulls to treatment with deslorelin, either by acting directly at the testes or through a permissive role that allows a direct action of deslorelin at the testes.

Effects of long-term continuous GnRH administration on the pituitary–gonadal axis in Eld's deer stags (Cervus eldi thamin)

Eld's deer stags (Cervus eldi thamin) (in groups of three) were continuously administered gonadotropin-releasing hormone (GnRH) in control, low, medium, or high doses (0, 20.1 ± 0.7, 83.3 ± 2.6, and 292.9 ± 4.9 ng∙kg−1∙d−1, respectively) via osmotic minipumps for ~80 d to investigate the potential for precociously reactivating the pituitary–testicular axis during the nonbreeding season. Secretory patterns of LH, FSH, and testosterone concentrations were qualitatively similar among treatments. However, in the low-dose group, basal LH and FSH concentrations were both increased (p < 0.05) and pituitary responsiveness to a superimposed GnRH challenge was augmented (p < 0.05) after 12 weeks of treatment compared with all other groups. Despite these endocrine changes, continuous low-dose GnRH administration was not effective for precociously inducing testicular activity in this seasonally breeding species. High-dose GnRH administration initially induced a transient increase in LH, FSH, and testosterone secretion and delayed, but did not prevent, the seasonal decline in spermatogenesis. After 6–12 weeks of high-dose GnRH administration, however, attenuated pituitary responsiveness appeared to delay the normal seasonal reactivation of the pituitary–gonadal axis. In conclusion, prolonged, continuous low-dose GnRH administration did not effectively translate into a precocious onset of testicular activity; therefore, this specific approach is unlikely to be useful for prolonging the fertile period in this seasonally breeding species.