Female Gray Short-tailed Opossums Research Articles

Perinatal exposure to progesterone, estradiol, or mifepristone affects sexual differentiation of behavior in opossums ( Monodelphis domestica)

The effects of perinatal exposure to progesterone (P) and estradiol (E) on sexual differentiation of behavior and morphology were examined by treating male and female gray short-tailed opossums on postnatal day 8 with progesterone alone (P), P plus estradiol (E) (PE), the P receptor antagonist mifepristone/RU486 (MIF), or corn oil control (C) and gonadectomizing them before puberty. When given female hormone replacement therapy in adulthood and tested with intact stimulus males, MIF animals showed less female-typical aggressive threat behavior than animals in other treatment groups. Stimulus males scent marked in more tests involving females than males and in more tests involving MIF animals than animals in other treatment groups. Body weight was lower in females than in males and was lower in MIF animals than in animals in other treatment groups, and P females failed to show female-typical genital locks after copulation. Sexual receptivity was similar in males and females and, while not decreased by any perinatal hormone treatment, was higher in PE males than in animals of either sex in any treatment group. These findings suggest that perinatal exposure to P is associated with the organization of feminine threat behavior and the defeminization of attractivity, body weight and genital anatomy in this marsupial. Reasons for these findings and for why female sexual receptivity is enhanced by perinatal exposure to exogenous E only in an endogenous masculine environment are discussed.

Progesterone receptor in the forebrain of female gray short-tailed opossums: Effects of exposure to male stimuli

Progesterone receptor immunoreactivity (PRir) in brain areas involved in reproductive behavior in eutherian species was examined for the first time in a female marsupial, the gray short-tailed opossum ( Monodelphis domestica, hereinafter, opossum ). PRir in nuclei of neurons, measured as area covered by stained nuclei, was seen in the arcuate nucleus (Arc); anteroventral periventricular nucleus (AVPv); bed nucleus of the stria terminalis (BST); medial preoptic area (MPOA), and ventromedial hypothalamus (VMH), but not in control areas adjacent to the hypothalamus or cortex. Female opossums are induced into cytological, urogenital sinus (UGS), estrus by male pheromones and into behavioral estrus, i.e., receptivity, by pairing with a male, and both estradiol (E) and progesterone (P) are involved in induction of receptivity in intact and ovariectomized females. PRir in the AVPv, MPOA, and VMH was very low in females that had never been exposed to males or their scent marks, i.e., naïve anestrous (NVA) females, and either previous or current exposure to males or their scent marks was associated with elevated PRir. PRir was significantly higher in the AVPv and MPOA of anestrous females with previous but no current exposure to males and their scent marks, i.e., experienced anestrous (EXPA) females, than in NVA females, but PRir was significantly lower in the MPOA and VMH of EXPA females than in females that were behaviorally receptive and had recently copulated, i.e., behavioral receptive estrous (BRE) females. PRir was higher in the VMH of both UGS estrous (UGSE) and BRE females compared to that in EXPA animals, but PRir did not differ between UGSE and BRE females in any of the 3 brain areas examined, including the MPOA These results provide evidence that pheromonal induction of estrus and sexual receptivity in opossums is associated with elevation of PRir in the VMH and MPOA and that prior exposure to males or their pheromones, even in the absence of current male stimuli, is associated with persistent elevation of PRir in the AVPv and MPOA.

Behavioral and reproductive responses of female opossums to volatile and nonvolatile components of male suprasternal gland secretion

Female gray short-tailed opossums ( Monodelphis domestica) lack an estrous cycle and are induced into estrus by exposure to a pheromone in male scent marks. Behavioral and physiological responses of females to the volatile and nonvolatile components of scent marks were examined in two experiments. Young females ( n = 9) were tested prior to and during their first estrus for behavioral responses to scent marks, collected on a 7-ml glass vial rubbed over the suprasternal gland of a mature male. The response to volatile components of the scent mark, recorded when marked and unmarked vials were covered with a perforated shield, was compared to the response to these vials when unshielded. Estrous females nuzzled the shields over marked vials (55.8 ± 8.5 nuzzles/10 min) more than the shielded clean vial (10.9 ± 2.4) ( P < 0.05); a similar response was observed in anestrous females. Nuzzling of unshielded, scent-marked vials was higher ( P < 0.05) during anestrus than in the same females when in estrus. The role of nonvolatile pheromones in reproductive activation was tested in adult females ( n = 11) exposed for up to 14 days to a shielded, marked vial or to an unshielded, marked vial in a crossover design. All females exposed to unshielded vials expressed estrus, and 10 copulated. Only 2 females expressed estrus (significantly fewer, P < 0.05), when exposed to shielded marked vials, and neither copulated. These results demonstrate that females detect and respond behaviorally to both volatile and nonvolatile components of male suprasternal gland secretion, but the estrus-inducing pheromone in these secretions is nonvolatile.

Evidence for Extended Action of Gonadal Hormones on the Organization of Sexually Dimorphic Behavior and Morphology in Gray Short-Tailed Opossums (Monodelphis domestica)

Male and female gray short-tailed opossums were gonadectomized (GDX), or treated with the estrogen receptor antagonist tamoxifen citrate (TX), or corn oil (OIL) (control) during the 5th postnatal week, a time period equivalent to the 3rd postnatal week in rats and associated with high levels of circulating gonadal hormones and neural aromatase activity in this marsupial species. In adulthood following gonadectomy (for animals not previously gonadectomized) and replacement therapy with estradiol or testosterone, GDX males showed less male-typical scent marking and had shorter phalluses than OIL and TX males. Following replacement therapy with estradiol, GDX females were more likely to fight with and less likely to mate with stimulus males than TX females; OIL females were intermediate in these measures. Along with previous findings, these results suggest that gonadal hormones act over an extended postnatal period to organize sexually dimorphic behavior and morphology in male gray opossums and may have some effect on the organization of aggressive behavior in females of this species.

Partner Preference of Intact and Ovariectomized Female Gray Short-Tailed Opossums (Monodelphis domestica)

Intact, ovariectomized and ovariectomized estradiol (E)-treated female gray short-tailed opossums were placed in a test situation in which they could choose between an intact and a castrated male. Intact females chose to visit intact males first and visited them more frequently and spent more time with intact than with castrated males. Ovariectomized (OVX) females did not show this preference for visiting intact males over castrates. When compared to OVX females with blank implants, OVX females with E implants spent less time with castrated males. Like intact females, OVX and OVX-E-treated females preferred to stay in close proximity to but not actually in the cage of intact rather than castrated males. To our knowledge, this is the first experimental study of partner preference and its relationship to hormonal condition in a female marsupial.

The Hormonal Induction of Mating Behavior in Female Gray Short-Tailed Opossums (Monodelphis domestica)

The activating effects of estradiol alone and of estradiol plus progesterone on copulation and related behavior were examined in ovariectomized female gray short-tailed opossums. Following receipt of subcutaneous estradiol (E) or cholesterol (C) implants, E-females showed less screeching threat behavior toward intact males than C-females. Intact males showed more precopulatory behavior toward E- than toward C-females, and 20% of E-females and no C-females copulated with males. In a second experiment, females that had previously received subcutaneous E implants received injections of either corn oil alone (E + OIL) or 500 μg progesterone in corn oil (E + P) 4 hr before behavioral testing. Of E + P-females, 88% copulated with males, compared to 38% of E + OIL-females. All females that copulated in both experiments showed anogenital dragging just prior to copulation. Hormonal induction of estrous behavior and copulation has not been reported previously in a female marsupial.

Vomeronasal organ removal blocks pheromonal induction of estrus in gray short-tailed opossums (Monodelphis domestica).

Estrus is induced in female gray short-tailed opossums (Monodelphis domestica) by exposure to male chemical signals (pheromones). Isolated females remain anestrous, but direct exposure to a male or his scent-marked cage induces estrus within 4-6 days. The objective of this study was to investigate the importance of the vomeronasal organ in detection of and response to estrus-inducing pheromones. The vomeronasal organ was surgically removed through the palate from 8 females (VNX); 5 females (SHAM) underwent sham surgeries in which the vomeronasal organ was exposed but not removed. After a 10-day recovery period, females were placed into male scent-marked cages. Body weight and urogenital sinus cytology were monitored throughout the experiment. All females were anesthetized and perfused with 4% paraformaldehyde 12-13 days after initial pheromone exposure. Vomeronasal organ ablation was evaluated histologically in decalcified snouts. In addition, deafferentation of the accessory olfactory bulb was confirmed by use of a lectin stain specific for the vomeronasal nerve and the glomerular layer of the bulb. All females classified as completely VNX (n = 5) remained anestrous throughout the pheromonal exposure. Incompletely VNX females (n = 2) and all SHAM animals exhibited estrus within 7 days of pheromone stimulation. At perfusion, the mean uterine weight (280.71 +/- 95.6 mg/85 g BW) of SHAM females was greater (p < 0.05) than that of unresponsive, VNX females (133.33 +/- 31.14 mg/85 g). This study demonstrates that the vomeronasal organ is an essential component for transduction of male pheromones required for induction of estrus in a marsupial species.

The Effects of Neonatal Treatment with Tamoxifen on Sexually Dimorphic Behavior and Morphology in Gray Short-Tailed Opossums (Monodelphis domestica)

The effects of postnatal exposure to the antiestrogen tamoxifen on sexually dimorphic behavior and morphology in adult male and female gray short-tailed opossums were examined. Following gonadecomy and treatment with estradiol in adulthood, neonatally tamoxifen-treated (TX) females showed less female-typical screeching threat behavior and TX males showed less male-typical precopulatory behavior than same-sex controls. While testes failed to descend and phalluses were shorter in juvenile TX males than in juvenile control males, behavior did not differ between TX and control animals following treatment with testosterone in adulthood. The findings that activity level following treatment with estradiol in adulthood and body weight in juveniles and adults were significantly higher in TX than in control animals of both sexes suggest that perinatal estrogens are involved also in the organization of these characteristics. The significance of these findings is discussed with respect to the development of sex differences in behavior and morphology in eutherian mammals.

Effects of Postnatal Exposure to Alcohol on Reproductive Physiology and Sexually Dimorphic Behavior in a Marsupial, The Gray Short‐Tailed Opossum (Monodelphis domestica)

The effects of postnatal exposure to alcohol on reproductive physiology and sexually dimorphic behavior and anatomy in adult male and female gray short-tailed opossums were examined. Female responsiveness to male pheromones and fertility in both sexes were essentially normal in postnatally alcohol-treated animals. However, aspects of sexually dimorphic behavior were masculinized and defeminized in females and demasculinized in males following gonadectomy in adulthood and treatment with male (testosterone) or female (estradiol) hormones. The possible role of alterations in neural aromatase activity by perinatal alcohol exposure in mediating these behavioral effects and the potential use of this marsupial species in perinatal alcohol studies are discussed.

Sexual maturation in female gray short-tailed opossums, Monodelphis domestica, is dependent upon male stimuli.

The development of physiological responsiveness to male stimuli in peripubertal female gray short-tailed opossums was examined in this study. Prepubertal females were exposed directly or indirectly to male chemicals (odors) or to no male stimuli. All females exposed directly to deposited male stimuli (22/22) showed estrus at a mean (+/- SEM) age of 127 +/- 2 days. Only 59% (13/22) of females exposed indirectly showed estrus, and at an older mean age of 162 +/- 5 days (p less than 0.01). Vaginal estrus was not observed in any of the females isolated from male stimuli (0/11). Direct exposure of prepubertal females to male odors was associated with higher uterine weights compared to those of indirectly exposed and isolated females. Reproductive success, measured as litter production, of peripubertal females greater than 140 days old was comparable to adult reproductive success. This study has shown that reproductive maturation in pubertal female opossums requires male stimuli, that this stimulus appears to be pheromonal, and that direct exposure to deposited male odors is the most effective stimulus for female reproductive maturation.

The effects of gonadal hormones on scent marking and related behavior and morphology in female gray short-tailed opossums ( Monodelphis domestica)

The hormonal control of scent marking and related behavior and morphology was examined in female gray short-tailed opossums. Females rarely scent marked when intact or following ovariectomy. Testosterone (T) but not estradiol (E) treatment stimulated chest marking while either hormone stimulated head marking in ovariectomized females tested alone. When the same females were tested with males, T-treated females showed little scent marking of any type; E-treated females showed hip marking in significantly more tests than females in the other treatment groups. Suprasternal scent glands (absent in intact females) and phalluses of females that received T were significantly larger than those of animals that received E or control animals. These findings are discussed with respect to similarities and differences between marsupial and eutherian females and between male and female gray opossums in the hormonal control of sexually dimorphic behavior and morphology.

The effects of pheromonal stimuli on estrus and peripheral plasma estradiol in female gray short-tailed opossums (Monodelphis domestica).

Exposure to male pheromones is associated with the activation of vaginal estrus in gray short-tailed opossums. The effects of such exposure on peripheral plasma estradiol-17 beta (E) levels in this marsupial species were examined in this study. Mean E levels of 27.8 pg/ml +/- 4.4 in diestrous females living in a room containing only females were similar to those seen in other marsupials. Direct naso/oral exposure to pheromonal cues provided by males resulted in vaginal estrus in 75% of these females within 4 11 days. None of the females exposed to clean cages came into vaginal estrus. Animals that were in estrus at the time of blood sampling or came into estrus over the experimental period had significantly higher E levels (58.1 +/- 12.6 pg/ml) than females in the pheromone-exposed and control groups that did not come into estrus (23.3 +/- 8.2 pg/ml). These findings are discussed with respect to other marsupial and eutherian species.

Sex differences in aggressive behavior in a marsupial, the gray short-tailed opossum (Monodelphis domestica)

In male and female gray short-tailed opossums (Monodelphis domestica), same-sex interactions were characterized by high levels of threat behavior (open-mouth postures and screeching vocalizations). Actual fighting behavior (biting, locking, and tumbling) was seen almost exclusively in male-male interactions. In male-female interactions, females showed high levels of threat behavior and males showed little aggressive behavior of any kind. Clicking vocalizations, submissive behavior in marsupials, were displayed almost exclusively by males. These findings are discussed with respect to the uses of auditory and olfactory communication in marsupials.

Estrogen binding macromolecules in hypothalamus-preoptic area of male and female gray short-tailed opossums ( Monodelphis domestica)

Estrogen binding sites in the hypothalamus -preoptic area (HPOA) of adult male and female gray opossums ( Monodelphis domestica) were characterized by incubating cytosols from gonadectomized animals with [ 3H]estradiol (E 2) in the presence or absence of excess unlabeled E 2. Scatchard analyses revealed that HPOA cytosols from both males and females contained high concentrations (25–30 fmol/mg protein) of binding sites with a very high affinity ( K d = 0.08 – 0.12 n M) for E 2. There was no sex difference in either the number or the affinity of estrogen binding sites. Competition studies indicated that estrogen agonists and antagonists displaced [ 3H]E 2 from binding sites in HPOA cytosols more effectively than did progestins, androgens, or adrenal corticoids. These data suggest that the HPOA of both male and female gray opossums contains macromolecules with several of the steroid binding properties characteristic of estrogen receptors.