Skyler J. Mooney

Peripheral administration of oxytocin increases social affiliation in the naked mole-rat (Heterocephalus glaber)

The neuropeptide oxytocin regulates a wide variety of social behaviors across diverse species. However, the types of behaviors that are influenced by this hormone are constrained by the species in question and the social organization that a particular species exhibits. Therefore, the present experiments investigated behaviors regulated by oxytocin in a eusocial mammalian species by using the naked mole-rat (Heterocephalus glaber). In Experiment 1, adult non-breeding mole-rats were given intraperitoneal injections of either oxytocin (1mg/kg or 10mg/kg) or saline on alternate days. Animals were then returned to their colony and behavior was recorded for minutes 15-30 post-injection. Both doses of oxytocin increased huddling behavior during this time period. In Experiment 2, animals received intraperitoneal injections of either oxytocin (1mg/kg), an oxytocin-receptor antagonist (0.1mg/kg), a cocktail of oxytocin and the antagonist, or saline across 4 testing days in a counterbalanced design. Animals were placed in either a 2-chamber arena with a familiar conspecific or in a small chamber with 1week old pups from their home colony and behaviors were recorded for minutes 15-30 post-injection. Oxytocin increased investigation of, and time spent in close proximity to, a familiar conspecific; these effects were blocked by the oxytocin antagonist. No effects were seen on pup-directed behavior. These data suggest that oxytocin is capable of modulating affiliative-like behavior in this eusocial species.

Social regulation of adult neurogenesis in a eusocial mammal

The present study examined the effects of social status on adult neurogenesis in an extreme cooperative breeder: the naked mole rat. These animals live in large colonies of up to 300 individuals, with a strict reproductive dominance hierarchy; one female and one to three males breed, and all other members are socially subordinate and reproductively suppressed. We examined the effects of social and gonadal cues on doublecortin (DCX; a marker for immature neurons) immunoreactivity in the dentate gyrus (DG), piriform cortex (PCx) and basolateral amygdala (BLA) by comparing dominant breeding animals to non-breeding subordinates from intact colonies. We also examined DCX expression in subordinate animals that had been removed from their colony and paired with an opposite- or same-sex conspecific for 6months. Compared to subordinates, dominant breeders had significantly reduced DCX immunoreactivity in all brain areas, with BLA effects confined to females. By contrast, the effects of same- versus opposite-sex housing were region-specific. In the DG and PCx, more DCX immunoreactivity was observed for opposite- than same-sex-paired subordinates. Conversely, same-sex-paired females had more DCX immunoreactivity than opposite-sex-paired females in the BLA. Gonadectomy did not affect DCX expression in opposite-sex-paired animals, and no significant relationships between gonadal steroids and DCX immunoreactivity were detected, suggesting that group differences in neurogenesis are independent of gonadal hormones. The apparent lower neurogenic capacity displayed by breeders contrasts previous reports on neurogenesis and social rank, challenging the conventional view that subordination is stressful and impairs neurogenesis. Future work will clarify whether the present findings can be attributed to status-dependent differences in stress, behavioral plasticity, or life stage.

Social condition and oxytocin neuron number in the hypothalamus of naked mole-rats (Heterocephalus glaber).

The naked mole-rat is a subterranean colonial rodent. In each colony, which can grow to as many as 300 individuals, there is only one female and 1-3 males that are reproductive and socially dominant. The remaining animals are reproductively suppressed subordinates that contribute to colony survival through their cooperative behaviors. Oxytocin is a peptide hormone that has shown relatively widespread effects on prosocial behaviors in other species. We examined whether social status affects the number of oxytocin-immunoreactive neurons in the paraventricular nucleus and the supraoptic nucleus by comparing dominant breeding animals to subordinate non-breeding workers from intact colonies. We also examined these regions in subordinate animals that had been removed from their colony and paired with an opposite- or same-sex conspecific for 6 months. Stereological analyses indicated that subordinates had significantly more oxytocin neurons in the paraventricular nucleus than breeders. Animals in both opposite- and same-sex pairs showed a decreased oxytocin neuron number compared to subordinates suggesting that status differences may be due to social condition rather than the reproductive activity of the animal per se. The effects of social status appear to be region specific as no group differences were found for oxytocin neuron number in the supraoptic nucleus. Given that subordinate naked mole-rats are kept reproductively suppressed through antagonism by the queen, we speculate that status differences are due either to oxytocins anxiolytic properties to combat the stress of this antagonism or to its ability to promote the prosocial behaviors of subordinates.

Region-specific associations between sex, social status, and oxytocin receptor density in the brains of eusocial rodents

Naturally occurring variations in neuropeptide receptor distributions in the brain contribute to numerous mammalian social behaviors. In naked mole-rats, which live in large social groups and exhibit remarkable reproductive skew, colony-related social behaviors vary with reproductive status. Here we examined whether variation in social status is associated with variations in the location and/or density of oxytocin binding in this species. Autoradiography was performed to assess forebrain oxytocin receptor (OTR) densities in breeding and non-breeding naked mole-rats of both sexes. Overall, males exhibited higher OTR binding in the medial amygdala in comparison to females. While there were no main effects of reproductive status in any region, a sex difference in OTR binding in the nucleus accumbens was mediated by status. Specifically, breeding males tended to have more OTR binding than breeding females in the nucleus accumbens, while no sex difference was observed in subordinates. These effects suggest that oxytocin may act in a sex- and region-specific way that corresponds to reproductive status and associated social behaviors.

Task specialization and task switching in eusocial mammals

Eusocial insects often display a certain degree of task specialization, which may help maximize the efficiency of a colony. Here we tested for the presence of task specialization in a eusocial mammal. Naked mole-rats, Heterocephalus glaber, were videorecorded across multiple days in their home colony and in a neutral arena with an unfamiliar conspecific for determination of short-term behavioural profiles. They were also recorded in these settings across the birth of multiple litters to assess the stability of behaviour patterns over months. Pup care behaviour, working behaviour and colony defence were unevenly distributed among subordinate mole-rats. Furthermore, these behaviours were stable across days and months. Across days, age was positively related to colony defence and negatively related to pup carrying. We also tested whether behaviours were stable across contexts by observing pup care behaviour outside of the colony in a neutral arena. We further attempted to determine whether mole-rats’ behaviours were contingent on the demands of the colony by removing the most frequent performers of pup care, colony defence and work behaviour from each colony. Results from these experiments suggest that when task specialists were no longer present, remaining animals adjusted their behaviour to fill the needs of the colony. Under these circumstances, younger animals engaged in the majority of working and pup-carrying behaviour while older animals engaged in the majority of colony defence behaviours. Thus, subordinate naked mole-rats show both task specialization and task switching.

Sex, social status, and CRF receptor densities in naked mole‐rats

Naked mole‐rats (Heterocephalus glaber) live in groups that are notable for their large size and caste structure, with breeding monopolized by a single female and a small number of males. Recent studies have demonstrated substantial differences between the brains of breeders and subordinates induced by changes in social standing. Corticotropin‐releasing factor (CRF) receptors—which bind the hormone CRF as well as related peptides—are important regulators of stress and anxiety, and are emerging as factors affecting social behavior. We conducted autoradiographic analyses of CRF1 and CRF2 receptor binding densities in female and male naked mole‐rats varying in breeding status. Both globally and in specific brain regions, CRF1 receptor densities varied with breeding status. CRF1 receptor densities were higher in subordinates across brain regions, and particularly in the piriform cortex and cortical amygdala. Sex differences were present in CRF2 receptor binding densities, as is the case in multiple vole species. CRF2 receptor densities were higher in females, both globally and in the cortical amygdala and lateral amygdalar nucleus. These results provide novel insights into the neurobiology of social hierarchy in naked mole‐rats, and add to a growing body of work that links changes in the CRF system with social behavior. J. Comp. Neurol. 524:228–243, 2016.

Subcaste differences in neural activation suggest a prosocial role for oxytocin in eusocial naked mole-rats

The neuropeptide oxytocin (OT) influences prosocial behavior(s), aggression, and stress responsiveness, and these diverse effects are regulated in a species- and context-specific manner. The naked mole-rat (Heterocephalus glaber) is a unique species with which to study context-dependent effects of OT, exhibiting a strict social hierarchy with behavioral specialization within the subordinate caste: soldiers are aggressive and defend colonies against unfamiliar conspecifics while workers are prosocial and contribute to in-colony behaviors such as pup care. To determine if OT is involved in subcaste-specific behaviors, we compared behavioral responses between workers and soldiers of both sexes during a modified resident/intruder paradigm, and quantified activation of OT neurons in the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON) using the immediate-early-gene marker c-fos co-localized with OT neurons. Resident workers and soldiers were age-matched with unfamiliar worker stimulus animals as intruders, and encounters were videorecorded and scored for aggressive behaviors. Colony-matched controls were left in their home colony for the duration of the encounters. Brains were extracted and cell counts were conducted for OT immunoreactive (ir), c-fos-ir, and percentage of OT-c-fos double-labeled cells. Results indicate that resident workers were less aggressive but showed greater OT neural activity than soldiers. Furthermore, a linear model including social treatment, cortisol, and subcaste revealed that subcaste was the only significant predictor of OT-c-fos double-labeled cells in the PVN. These data suggest that in naked mole-rats OT promotes prosocial behaviors rather than aggression and that even within subordinates status exerts robust effects on brain and behavior.

A game of thrones: Neural plasticity in mammalian social hierarchies

Social status is a key regulator of health and reproduction in mammals, including humans. Despite this, relatively little is known about how social status influences the mammalian brain. Furthermore, the extent to which status is an independent construct, i.e., not simply acting as a psychosocial stressor, is yet to be determined. Research to date reveals several promising mechanisms and/or systems associated with social status, including monoamine systems, hypothalamic neuroendocrine axes, and the hippocampus, though whether these differences are the cause or effect of status is often unclear. We review these candidates and propose how best to approach this research question in the future.

Retinal ganglion cell survival and axon regeneration after optic nerve injury in naked mole-rats

In the adult mammalian central nervous system (CNS), axonal damage often triggers neuronal cell death and glial activation, with very limited spontaneous axon regeneration. In this study, we performed optic nerve injury in adult naked mole‐rats, the longest living rodent, with a maximum life span exceeding 30 years, and found that injury responses in this species are quite distinct from those in other mammalian species. In contrast to what is seen in other mammals, the majority of injured retinal ganglion cells (RGCs) survive with relatively high spontaneous axon regeneration. Furthermore, injured RGCs display activated signal transducer and activator of transcription‐3 (STAT3), whereas astrocytes in the optic nerve robustly occupy and fill the lesion area days after injury. These neuron‐intrinsic and ‐extrinsic injury responses are reminiscent of those in “cold‐blooded” animals, such as fish and amphibians, suggesting that the naked mole‐rat is a powerful model for exploring the mechanisms of neuronal injury responses and axon regeneration in mammals. J. Comp. Neurol. 525:380–388, 2017.

Successful intracerebroventricular cannulation of a eusocial mammal.

BACKGROUND Manipulating neural activity in live animals within a colony would allow researchers to more fully explore the neurobiology of complex social behaviors. However, some colony-living animals like the naked mole-rat (Heterocephalus glaber) cannot be reintroduced to a colony after the extended recovery time required following cranial surgery. Furthermore, the colony setting creates increased risk of infection and interruption of cranial surgical sites. NEW METHOD A protocol for intracerebroventricular cannulations was developed for securing and minimizing exposure of the intracranial apparatus. We tested whether animals could be reintroduced to the colony immediately following surgery and whether they showed full recovery and expression of normal behavior a week later, after intracerebroventricular infusion of saline. RESULTS Animals were successfully reincorporated into their home colony and showed normal behavior. No animals lost guide cannulae within their colony and loss of dummy cannulae was minimized. Any loss of animals was due to surgical complications or multiple intracerebroventricular infusions of saline rather than recovery in the colony, per se. COMPARISON WITH EXISTING METHODS Standard cranial cannulation methods for small rodents were used with the addition of implanting a shortened guide cannula under the skin for limited exposure of cannulae to the external environment. Furthermore, dummy cannulae were sealed to guides to avoid loss in-colony. CONCLUSION The use of intracranial cannulations is a viable option for colony-living rodents when the proper care is taken to minimize cannula exposure and when animals are carefully and promptly reintroduced to the colony setting after surgery.