Female Naked Mole-rats Research Articles

The curious case of the hypothalamic-pituitary-gonadal axis dysfunction in subordinate female naked mole-rats (Heterocephalus glaber): No apparent role of opioids and glucocorticoids.

The naked mole-rat (Heterocephalus glaber) is a unique model mammal in which to study socially induced inhibition of the hypothalamic-pituitary-gonadal (HPG) axis. Naked mole-rat groups exhibit a high degree of reproductive bias in which breeding is restricted to one female (the queen) and one male, with subordinate non-breeding colony members rarely, if ever, having the opportunity to reproduce due to a dysfunctional HPG axis. It is posited that aggression directed at subordinates by the queen suppresses reproduction in these subordinates, yet the underlying physiological mechanisms causing this dysfunction are unknown. This study aimed to investigate the possible factors contributing to the dysfunction of the HPG axis in subordinate female naked mole-rats with a specific focus on the role of ovarian feedback and stress-related factors such as circulating glucocorticoid and endogenous opioid peptides. The results showed that stress-related factors appear to not mediate the suppression of reproductive function in subordinate female naked mole rats. Indeed, in some cases, the activation of the stress axis may lead to reproductive activation instead of deactivation. At the same time, the role of ovarian sex steroid feedback in reproductive suppression is likely limited and not clearly delineated. This study highlights the need for detailed studies to elucidate the mechanism of reproductive suppression in this unique model mammalian species which may shed light on, and reveal novel mechanisms, in the social regulation of reproduction.

ANESTHETIC EFFECT OF DEXMEDETOMIDINE-KETAMINEMIDAZOLAM COMBINATION ADMINISTERED INTRAMUSCULARLY TO ZOO-HOUSED NAKED MOLE-RATS (HETEROCEPHALUS GLABER).

In this study, adult intact male and female (n = 10) naked mole-rats (Heterocephalus glaber) were anesthetized using a combination of dexmedetomidine (0.06 mg/kg intramuscularly [IM]), ketamine (20 mg/kg IM), and midazolam (1.0 mg/kg IM). Atipamezole (1.0 mg/kg IM) and flumazenil (0.1 mg/kg IM) were administered 40 min after induction. Induction and recovery times were monitored and recorded. Vital parameters, including heart rate, respiratory rate, and SpO2, and reflexes were monitored every 5 min during the anesthetic period. Anesthetic induction was smooth and rapid. All monitored reflexes were lost within a median time of 60 sec (interquartile range, 15 sec). Heart rate and respiratory rate were significantly decreased from baseline, whereas there was no difference in SpO2 over the anesthetic period. The mean time to recovery was 15 ± 7 min (mean ± SD). One animal was found dead 40 min after apparent recovery, which is suspected to be an anesthetic-related death. Based on these findings, dexmedetomidine-ketamine-midazolam anesthesia is an effective anesthetic protocol in naked mole rats that provides a consistent anesthetic plane but should be used with caution in animals with underlying conditions.

Abstract 9857: Cardiac Function in an Extraordinarily Long-lived Rodent, the Naked Mole-rat

Mechanisms underlying prolonged maintenance of cardiac function in long-lived species may differ from those involved in short-lived species. We assessed cardiac function in young (2-4 year old) male and female naked mole-rats (NMRs; Heterocephalus glaber) and compared these data to those of young (7-8 month old) male and female C57BL/6 mice to establish the use of the NMR for cardiac aging studies. NMRs have a maximum lifespan potential of 32 years (8-fold longer than mice) despite being comparably sized. Furthermore, NMRs maintain good health for at least 75% of their extremely long lifespan, equivalent to >80 year old humans. As such, NMRs provide us a unique opportunity to explore mechanisms that enable prolonged maintenance of cardiac function. Like humans NMRs have a lower than predicted heart rate for their body size. Interestingly, unlike in mice, isofluorane anesthesia had little depressive effect on NMR cardiac function, as heart rates stayed similar between anesthetized and unanesthetized NMRs. Echocardiography revealed significant differences between species (see table). While there were no species differences in average diastolic wall thickness, average systolic wall thickness was greater in mice compared to NMRs. Baseline fractional shortening (FS) was significantly lower in NMRs than in mice. Dobutamine stress echocardiography (3 μg/g body weight i.p.) elicited a greater increase in FS in NMRs than in mice, suggesting that NMRs have greater cardiac reserve. Doppler analysis of blood flow revealed that diastolic (E/A ratio) and systolic (aortic peak velocity) function were significantly different between species. The higher NMR E/A ratio is indicative of more pliable ventricle muscle and less reliance on atrial filling. The interspecies differences observed may indicate mechanisms employed by NMRs to prolong cardiac health into old age. Future studies will probe the existence of such mechanisms and investigate age-related cardiac changes in NMRs.

Distribution of oxytocin in the brain of a eusocial rodent

Naked mole-rats are highly social rodents that live in large colonies characterized by a rigid social and reproductive hierarchy. Only one female, the queen, breeds. Most colony members are non-reproductive subordinates that work cooperatively to rear the young and maintain an underground burrow system. Little is known about the neurobiological basis of the complex sociality exhibited by this species. The neuropeptide oxytocin (Oxt) modulates social bonding and other social behaviors in many vertebrates. Here we examined the distribution of Oxt immunoreactivity in the brains of male and female naked mole-rats. As in other species, the majority of Oxt-immunoreactive (Oxt-ir) cells were found in the paraventricular and supraoptic nuclei, with additional labeled cells scattered throughout the preoptic and anterior hypothalamic areas. Oxt-ir fibers were found traveling toward and through the median eminence, as well as in the tenia tecta, septum, and nucleus of the diagonal band of Broca. A moderate network of fibers covered the bed nucleus of the stria terminalis and preoptic area, and a particularly dense fiber innervation of the nucleus accumbens and substantia innominata was observed. In the brainstem, Oxt-ir fibers were found in the periaqueductal gray, locus coeruleus, parabrachial nucleus, nucleus of the solitary tract, and nucleus ambiguus. The high levels of Oxt immunoreactivity in the nucleus accumbens and preoptic area are intriguing, given the link in other rodents between Oxt signaling in these regions and maternal behavior. Although only the queen gives birth or nurses pups in a naked mole-rat colony, most individuals actively participate in pup care.

Phenotypic plasticity in female naked mole-rats after removal from reproductive suppression

Naked mole-rats are fossorial African rodents that live in large, eusocial groups. Adult subordinate female mole-rats are reproductively suppressed by the dominant breeding female in their colonies. As a result, subordinate females remain reproductively quiescent for their entire lives unless they are removed from the suppressive presence of the dominant female. This makes subordinate female mole-rats a tractable model for studying phenotypic plasticity. We measured skeletal growth of subordinate, suppressed females as they changed reproductive status. After housing subordinate female mole-rats separately from their home colonies, these animals experienced a growth surge that dramatically increased their body mass and length. After removal from reproductive suppression, females showed an 82% increase in body mass and a 37% increase in the length of their lumbar spines. The lumbar vertebrae were the only skeletal structures that exhibited this puberty-like growth. After colony separation, body mass and lumbar vertebrae growth rates peaked and remained elevated for several weeks before returning to control levels--suggestive of a puberty-like 'growth spurt'. Although previous studies have characterized pregnancy-induced lumbar spine elongation in female mole-rats, we demonstrate a significant change in the body morphology of female mole-rats after removal from reproductive suppression but before the first pregnancy.

Litter Sizes and Mammary Numbers of Naked Mole-Rats: Breaking the One-Half Rule

Among mammals generally and rodents particularly mean litter sizes usually are about one-half the number of mammae, and maximum litter sizes approximate mammary numbers. Naked mole-rats ( Heterocephalus glaber : Bathyergidae) are exceptions to both generalizations. Field-caught litters averaged 11.3 young ± 6.2 SD ( n = 82), and captive-born litters averaged 11.4 ± 5.6 young ( n = 190). Similarly, numbers of mammae on breeding females averaged 11.6 ± 1.1 ( n = 43) in the field and 11.5 ± 2.0 ( n = 29) in captivity. Maximum litter sizes were 28 in the field and 27 in captivity, whereas the maximum number of mammae was 15. More than one-half of field-caught and captive males and females had different numbers of mammae on the two sides of their body. Neither total numbers of mammae nor fluctuating asymmetries in mammary numbers differed significantly between males and females, nor between breeders and nonbreeders. There was no relationship between litter sizes and numbers of mammae or fluctuating asymmetries in mammary numbers. Breeding female naked mole-rats can bear and successfully rear litters that are far more numerous than their mammae because, on a proximate level, young take turns nursing from the same mammary and, on an ultimate level, breeding females are fed and protected by colony mates, enabling them to concentrate their reproductive efforts on gestation and lactation.

Combined Olfactory Contact with the Parent Colony and Direct Contact with Nonbreeding Animals Does Not Maintain Suppression of Ovulation in Female Naked Mole-Rats (Heterocephalus glaber)

The study investigated the role of odor cues from two naked mole-rat colonies, in conjunction with behavioral cues from nonbreeding colony members, in maintaining suppression of ovulation in subordinate female naked mole-rats isolated from the two parent colonies. Four high ranking nonbreeding female naked mole-rats were removed from their respective parent colonies and singly housed in separate burrow systems. For a 64-day period, the removed females were maintained in daily odor contact with their parent colony by daily rotating soiled bedding material between the parent colony and the burrow systems of removed females. In addition, subsets of nonbreeding animals from the respective parent colony were regularly moved into the burrow systems of removed females for 2-day periods during this 64-day period. Removed females were therefore in continual social contact with subsets of parent colony animals except for the breeding pair. All four removed females exhibited raised levels of urinary progesterone (< 2 ng/mg Cr) indicative of the onset of ovarian function within 3 days of being separated from the parent colony. Removed females exhibited a normal ovulatory cycle with levels of progesterone remaining elevated for 25–35 days (mean concentration of progesterone ± SEM; 16.2 ± 2 ng/mg Cr). Initiation of aggression and sexual behavior by removed females increased significantly when they were isolated from the parent colony. The results demonstrated that odor cues from the complete colony in conjunction with behavioral / tactile / vocal cues from the nonbreeding colony members were not the major cues maintaining reproductive suppression in nonbreeding female naked mole-rats. Instead, our results suggest that female reproductive suppression in naked mole-rats is caused by a dominance-related behavioral mechanism requiring direct contact with the breeding female.

Behavioral and Hormonal Changes in Female Naked Mole-Rats (Heterocephalus glaber) Following Removal of the Breeding Female from a Colony

Colonies of naked mole-rats (Heterocephalus glaber) contain a single dominant, breeding female, or "queen," which suppresses reproduction in subordinate females. In this study, the queen and breeding male were removed from a colony at Brookfield Zoo. We examined behavioral and endocrine changes in the remaining colony females. Behavioral observations were conducted weekly and urine samples were collected beginning 3 months prior to the planned removal of animals. Prior to the removal, only the queen displayed the high frequencies of aggressive shoving typical of breeding females. In the first 2 months following removal of the queen, three non-breeding females markedly increased their frequencies of shoving. Urinary progesterone showed that one of these three females had probably ovulated both before and after queen removal. Histological examination following fatal agonistic interactions confirmed ovulatory function in two of these three females. Similar behavioral and endocrinological results were obtained for three more females, following the combat deaths of the first three females. These findings suggest that ovarian activation facilitates intrasexual aggression in female naked mole-rats and might contribute to attainment of breeding status.

Evidence that primer pheromones do not cause social suppression of reproduction in male and female naked mole-rats (Heterocephalus glaber)

The role of chemical cues in mediating the reproductive suppression of nonbreeding males and females in colonies of naked mole-rats (Heterocephalus glaber) was examined by comparing reproductive activation in isolated nonbreeders with that of isolated nonbreeders maintained in olfactory and gustatory contact with their parent colony. Animals were either removed from their colony and paired directly with a member of the opposite sex from the same colony, or were removed, housed singly for 40 days, and then paired with a member of the opposite sex from the same parent colony that had also been housed singly for 40 days. Approximately half of these separated animals received a daily transfer of soiled bedding and litter from their parent colony. In females, there was no significant difference between the control and bedding transfer group, in the time taken from separation to the first sustained increase of urinary progesterone above 2 ng mg-1 Cr indicative of the luteal phase of an ovarian cycle (7.8 +/- 2.6 versus 9.4 +/- 2.0 days, respectively). In both control and bedding transfer groups, singly-housed females commenced ovarian cyclicity, revealing that the presence of a male was not required for reproductive activation and ovulation. In males, concentrations of both urinary testosterone and plasma LH increased after separation from their parent colonies in both control and bedding transfer groups, to reach values equivalent to those of breeding males.(ABSTRACT TRUNCATED AT 250 WORDS)

LH responses of female naked mole-rats, Heterocephalus glaber, to single and multiple doses of exogenous GnRH

To investigate possible differential pituitary secretion of LH in breeding and non-breeding female naked mole-rats, the LH responses to administration of exogenous GnRH were measured in 55 females from 20 captive colonies. Single doses of 0.1, 0.5 or 1.0 micrograms GnRH produced a significant rise in plasma LH concentrations 20 min after s.c. injection in breeding and non-breeding females at all doses (P less than 0.001). While at the highest dose of 1.0 microgram there was no difference in the LH response between breeding and non-breeding females, as the dose was lowered there was a progressive decline in the LH response in non-breeding females such that, at the 0.1 microgram dose, GnRH produced only a small, but significant, increase in plasma LH (1.3 +/- 0.2 to 2.9 +/- 0.5 mi.u./ml, N = 5) compared with breeding females (3.4 +/- 0.8 to 9.6 +/- 2.0 mi.u./ml, N = 6). The LH responses of the latter were not significantly reduced at the lower doses of GnRH. The apparent lack of sensitivity to low doses of exogenous GnRH in non-breeding females was reversed by 4 consecutive 1-h injections of 0.1 microgram, which produced a rise in LH from 1.2 +/- 0.2 to 9.0 +/- 0.2 mi.u./ml (N = 4), comparable to that of breeding females given a single injection of 0.1 microgram GnRH. These results suggest that the anterior pituitary in non-breeding female naked mole-rats is less sensitive to low doses of exogenous GnRH than in breeding females, possibly due to a lack of priming by endogenous GnRH. Therefore, the socially-induced block to ovulation in non-breeding female naked mole-rats may be due to inhibition of hypothalamic GnRH secretion.

Social suppression of ovarian cyclicity in captive and wild colonies of naked mole-rats, Heterocephalus glaber

To investigate the endocrine cause of reproductive suppression in nonbreeding female naked mole-rats, animals from 35 colonies were studied in captivity. Urinary and plasma progesterone concentrations were elevated in pregnant females (urine: 10.0-148.4 ng/mg Cr, 27 samples from 8 females; plasma: 3.6-30.0 ng/ml, 5 samples from 5 females; Days 21-40 of pregnancy) and cyclic breeding females (urine: 0.5-97.8 ng/mg Cr, 146 samples from 7 females; plasma: less than 1.0-35.4 ng/ml, 25 samples from 7 females). The latter group showed cyclic patterns of urinary progesterone, indicating a mean ovarian cycle length of 34.4 +/- 1.6 days (mean +/- s.e.m.) with a follicular phase of 6.0 +/- 0.6 days and a luteal phase of 27.5 +/- 1.3 days (19 cycles from 9 breeding females). In non-breeding females urinary and plasma progesterone values were undetectable (urine: less than 0.5 ng/mg Cr, 232 samples from 64 females; plasma: less than 1.0 ng/ml, 7 samples from 6 females). Breeding females had higher (P less than 0.001) plasma LH concentrations (3.0 +/- 0.2 mi.u./ml, 73 samples from 24 females) than did non-breeding females (1.6 +/- 0.1 mi.u./ml, 57 samples from 44 females). Urinary and plasma progesterone concentrations in non-breeding females from wild colonies situated near Mtito Andei, Kenya, were either below the assay sensitivity limit (urine: less than 0.5 ng/mg Cr, 11 females from 2 colonies; plasma: less than 1.0 ng/ml, 25 females from 4 colonies), or very low (plasma: 1.6 +/- 0.6 ng/ml, 15 females from 4 colonies). In captivity, non-breeding females removed from their colonies (i.e. the dominant breeding female) and either paired directly with a non-breeding male (N = 2), or removed and housed singly for 6 weeks before pairing with a non-breeding male (N = 5) may develop a perforate vagina for the first time in as little as 7 days. Urinary progesterone concentrations rose above 2.0 ng/mg Cr (indicative of a luteal phase) for the first time 8.0 +/- 1.9 days after being separated. These results suggest that ovulation is suppressed in subordinate non-breeding female naked mole-rats in captive and wild colonies, and show that plasma LH concentrations are significantly lower in these non-breeding females. This reproductive block in non-breeding females is readily reversible if the social factors suppressing reproduction are removed.