Removal of reproductive suppression reveals latent sex differences in brain steroid hormone receptors in naked mole-rats, Heterocephalus glaber

Abstract

BackgroundNaked mole-rats are eusocial mammals, living in large colonies with a single breeding female and 1–3 breeding males. Breeders are socially dominant, and only the breeders exhibit traditional sex differences in circulating gonadal steroid hormones and reproductive behaviors. Non-reproductive subordinates also fail to show sex differences in overall body size, external genital morphology, and non-reproductive behaviors. However, subordinates can transition to breeding status if removed from their colony and housed with an opposite-sex conspecific, suggesting the presence of latent sex differences. Here, we assessed the expression of steroid hormone receptor and aromatase messenger RNA (mRNA) in the brains of males and females as they transitioned in social and reproductive status.MethodsWe compared in-colony subordinates to opposite-sex subordinate pairs that were removed from their colony for either 1 day, 1 week, 1 month, or until they became breeders (i.e., produced a litter). Diencephalic tissue was collected and mRNA of androgen receptor (Ar), estrogen receptor alpha (Esr1), progesterone receptor (Pgr), and aromatase (Cyp19a1) was measured using qPCR. Testosterone, 17β-estradiol, and progesterone from serum were also measured.ResultsAs early as 1 week post-removal, males exhibited increased diencephalic Ar mRNA and circulating testosterone, whereas females had increased Cyp19a1 mRNA in the diencephalon. At 1 month post-removal, females exhibited increased 17β-estradiol and progesterone. The largest changes in steroid hormone receptors were observed in breeders. Breeding females had a threefold increase in Cyp19a1 and fivefold increases in Esr1 and Pgr, whereas breeding males had reduced Pgr and increased Ar.ConclusionsThese data demonstrate that sex differences in circulating gonadal steroids and hypothalamic gene expression emerge weeks to months after subordinate animals are removed from reproductive suppression in their home colony.

Assessment of Stress on Serum Estradiol and Cortisol Levels in Female Subordinate Naked Mole Rats Following Isolation from Natal Colony

“…Similar studies showed that repeated acute stress caused an increase in cortisol, as well as reproductive hormones in Wistar rats [5]. The lower mean serum estradiol levels in female subordinate naked mole rats between day 1 to week 6 compared to the levels measured during isolation period agree with the previous studies [6] where female naked mole rats exhibited increased estradiol and progesterone levels immediately after one month of removal from their colony. There was accompanying increased steroid hormone receptors in the female breeders compared to non-breeding female subordinate naked mole rats.…”

RFamide-related peptide-3 (RFRP-3) suppresses sexual maturation in a eusocial mammal

“…RFRP-3-treated animals received 200 ng/μL RFRP-3 released at 0.25 μL/h, resulting in 1,200 ng/d, for a total of 4 wk. Treatment duration corresponds with previous reports on the time course of gonadal activation following colony removal (46). Infusion volume and flow rate did not differ among SAL and RFRP-3 animals.…”

“…Circulating T and P were detected by ELISA at an absorbance of 405 nm, as per manufacturer's instructions. All samples were run in duplicate with a Synergy-HT-Bio-Tek 96-well microplate reader and measured as described (46,47).…”

“…Indeed, although colony-housed subordinates have low progesterone (P), testosterone (T), and luteinizing hormone (LH) concentrations compared with same-sex breeders (37,38), they show elevated urinary P and T levels within 1 wk of separation from the colony (37,40), with females developing a perforated vagina during that time (37). At longer intervals, sexually maturing individuals begin to exhibit mating behavior, as well as alterations in sociosexual neural circuits (41)(42)(43)(44)46). Contrary to expectations, NMRs do not show substantial variation in GnRH or kisspeptin expression according to sex or reproductive state, with differences in kisspeptin immunoreactivity confined to breeder and subordinate females (44).…”

“…In colonies of up to 300 individuals, breeding is restricted to one dominant female (the queen) and one to three males, with other colony members being socially subordinate and reproductively suppressed. Though pronounced alterations in neural and endocrine function accompany the change (37)(38)(39)(40)(41)(42)(43)(44)(45)(46)(47), male and female subordinates are capable of transitioning to breeding status following the death/removal of breeders or separation from the colony. Indeed, although colony-housed subordinates have low progesterone (P), testosterone (T), and luteinizing hormone (LH) concentrations compared with same-sex breeders (37,38), they show elevated urinary P and T levels within 1 wk of separation from the colony (37,40), with females developing a perforated vagina during that time (37).…”