Seasonal & Circadian Rhythms
Research interests: Biological rhythms, reproductive strategies, reproductive physiology, neuroendocrinology, and behavior.
Current research focuses on characterizing neural networks that mediate responses to melatonin. Animals must adapt to temporal, as well as spatial, niches. For example, confining reproduction so that offspring are born during the spring and summer months may contribute to an individual’s reproductive fitness in temperate regions. To accurately synchronize breeding with the environment many animals rely on changes in day length or photoperiod. In many rodent species, including the Siberian hamster, short day lengths experienced during the fall and winter inhibit reproduction whereas long day lengths in spring and summer stimulate reproduction. It has been well-established that day length is represented endogenously by the duration of the nightly secretion of the pineal-derived hormone melatonin. Our present research is designed to characterize the neural substrates and circuits that mediate melatonin’s effects.
Melatonin-containing cannula location near the SCN
Neuroendocrine intermediates in the response to melatonin: It is well-established that melatonin regulates many aspects of the reproductive axis in both male and female mammals. The final common pathway to access the reproductive axis is through gonadotropin releasing hormone (GnRH)-containing neurons in the hypothalamus. These neurons are sensitive to steroid feedback, both positive and negative, as well as to many other stimuli capable of impacting reproduction. Exposure to short day lengths is thought to reduce the release of GnRH from hypothalamic neurons. The mechanism by which day length and melatonin alter GnRH activity remains unknown; largely because GnRH neurons do not express melatonin receptors. Thus, some intermediate target tissue must exist whereby melatonin exerts its effects on GnRH. Recently, several neuropeptide/neuroendocrine candidates have been identified that may subserve this role. These include, kisspeptin, RFamide-related peptide (RFRP), and thyroid hormones.
Kisspeptin and RFRP: We aim to identify the melatonin target tissues that impact RFamide expression. Kisspeptin and RFRP are thought to act as positive and negative regulators of GnRH, respectively. Hamsters exposed to short photoperiods, or winter-like melatonin profiles, exhibit significant alterations in kisspeptin expression in the arcuate nucleus (ARC) and the anteroventral periventricular nucleus (AVPV). In Syrian hamsters, exogenous administration of kisspeptin stimulates the reproductive axis in short-day housed males. Kisspeptin neurons may be involved in steroid-feedback to the reproductive system as they express steroid receptors, including estrogen receptor alpha (ERa) and androgen receptors (AR). Removal of gonadal steroids following castration alters the expression of kisspeptin. The neural pathways and mechanisms by which melatonin alters these neuropeptides remains uncharacterized and is one of the current areas of inquiry in the lab.
Steroid receptors and RF-amides
ER alpha-ir AVPV Kisspeptin-ir AVPV
Thyroid hormone: Results from our laboratory and others have revealed a role for thyroid hormone in seasonal control of testis size. Photoperiod appears to regulate the expression of one or more enzymes (deiodinase type 2 and 3) involved in the conversion of the thyroid hormone thyroxine to a more potent metabolite triiodothyronine (T3). The pattern of enzyme expression favors increased T3 availability during the long days of summer and decreased T3 availability during the short days of winter. This pattern of enzyme expression is melatonin-dependent. We demonstrated that daily injections of T3 administered to short-day housed hamsters resulted in stimulation of testis growth and a delay in the timing of recrudescence. It remains to be determined whether thyroid hormones and the RFamides interact in the regulation of reproduction, or if they represent parallel, independent pathways.
Photoperiod-dependent alterations in aggression via changes in steroid receptor expression: Exposure to short day lengths elicits increases in aggressive behavior in Siberian hamsters. This appears puzzling because hamsters housed in short day lengths exhibit very low circulating concentrations of testosterone. Aggression is typically positively correlated with testosterone; this observation led to the hypothesis that seasonal aggression in these hamsters is independent of gonadal steroids. Alternatively, we hypothesized that increased aggression in winter may be mediated by increased sensitivity to steroids through a photoperiod-dependent increase in ERa expression in brain sites that mediate aggressive behavior. Interestingly, we found significant increases in ERa in several brain nuclei known to be involved in aggression; the bed nucleus of the stria terminalis (BST), the medial amygdale (MeA), and the central amygdale (CeA). We plan to directly manipulate ERa expression in these brain regions via viral vector transfection to characterize their roles in seasonal alterations in aggression.
ERa-ir in the AVPV after unilateral transfection with the human ERa gene using an adeno-associated viral vector (AAV)
Current projects in the lab include:
- The interaction of melatonin, RF-amide neuropeptides (RF-amide-related peptide [RFRP], kisspeptin), and steroid receptors in seasonality
- Neural substrates mediating responses to both long and short day lengths
- Neural mechanisms by which day length affects aggressive behavior, stress responses, and immunity
- Day length and melatonin regulation of steroid receptor expression related to physiology and behavior
- The role of the intergeniculate leaflet of the thalamus in seasonal and circadian rhythms
Eusocial Damaraland mole-rat adults and juvenile
Research interests: Cooperative breeding, reproductive strategies & physiology, endocrine regulation of sex behaviors, conspecific recognition & xenophobia, incest avoidance, neuroendocrinology, aging, and behavior.
Damaraland mole-rats (Fukomys damarensis) and the related naked mole rat (Heterocephalus glaber), are the only known eusocial mammals. Thus, these subterranean rodents live in colonies of up to 40 individuals that usually consist of a single breeding pair (a queen and her male breeding partner) and several generations of reproductively suppressed offspring. The non-breeding individuals help the breeding pair to raise and care for offspring.
Castes: There are two types of non-breeders; one subset of non-breeders do the majority of the colony work (e.g., excavating tunnels with their large front teeth, foraging for food, and defending the colony from predators), whereas a second subset of males do very little work. These so-called “lazy males” may conserve energy until environmental conditions are favorable for leaving the colony to find mates and establish a new colony. In support of this hypothesis, the lazy males exhibit increased burrowing activity during the rainy season when the soil is easier to excavate.
Incest avoidance and reproductive suppression: Within a colony, only the breeding queen and her male partner mate and have offspring. The remaining individuals are non-reproductive workers. The mechanisms by which reproduction is suppressed in the non-breeders remains unknown. Currently, it is hypothesized that several factors may maintain reproductive suppression in the non-breeders.
- There is strong incest avoidance within colonies, and because all of the non-breeders are siblings, this precludes mating among them or with the parents. Non-breeding mole-rats may require the presence of an unfamiliar opposite-sex conspecific before exhibiting reproductive maturation and/or the full repertoire of sex behaviors.
- Additionally, the dominant breeding individuals may actively suppress reproduction in non-breeders by behavioral and/or pheromonal mechanisms.
Individual Recognition: The mechanism by which DMRs avoid incest remains speculative. It is possible that DMRs are capable of determining genetic similarities in other individuals and refuse to mate with close relatives. Alternatively, it is possible that mole-rats become “familiar” with other mole-rats in their burrow system and avoid mating with familiar individuals.
Sex differences in reproductive suppression: Female non-breeders have immature ovaries and uteri, and fail to ovulate. These observations suggest inhibition of the hypothalamic-pituitary-gonadal (HPG) axis in female non-breeders; most will fail to ever undergo puberty, even though they may live for many years. Interestingly, female non-breeders will rapidly exhibit sexual behavior when introduced to an unfamiliar male; often within minutes. The purpose of this behavior remains unknown, since it is unlikely that these females are capable of ovulating in such a short time; perhaps mating stimulates reproductive maturation, or pair-bonding.
Male breeders and non-breeders exhibit similar gonadal function; that is, their testes are similar in size and make similar amounts of both sperm and testosterone. Non-breeder males do not exhibit sex behavior within their colony, suggesting that reproductive behaviors may be suppressed in non-breeding males. Male non-breeders will mate within minutes of being introduced to an unfamiliar female.
One area of research is to determine the neuroendocrine and behavioral mechanisms by which reproduction is suppressed in DMRs, and to determine the cues and mechanisms that trigger sex behavior between unfamiliar individuals.
Pair bonding, partner preference, and monogamy: DMR colonies are made up of a single breeding pair and their offspring from one or more litters. The same individuals remain paired, often resulting in multiple generations of offspring. This pattern of reproduction conforms to the criteria defining monogamy, although it is unclear whether the breeders form a “pair bond” or have a “partner preference”, wherein the breeders “prefer” to associate with each other rather than with “unfamiliar” opposite-sex conspecifics. The apparent monogamy may be by “default” since the breeders do not encounter opposite-sex unfamiliars within their burrow systems. Data indicate that if an unfamiliar DMR infiltrates a colony, it will be attacked by a same-sex breeder, e.g., if a female infiltrates a colony, she is most often attacked by the breeding female, whereas an unfamiliar male is most often repelled by the breeding male. In this way, monogamy may be due to the mole-rats’ xenophobic behavior and colony defense, rather than through the formation of pair bonds. It will be exciting to determine whether, and to what extent, DMRs form pair bonds or partner preferences.
Steroid-independent expression of sex behavior? Pilot data indicate that both male and female mole-rats will exhibit mating behavior many months after gonadectomy. This finding suggests that, in contrast to most mammals, the expression of sex behavior is not intimately linked to circulating gonadally-derived sex steroids (i.e., testosterone and estrogen). This rare system may have evolved as a consequence of the DMR social system. Thus, it is advantageous for subordinate individual’s to forgo attempts at reproduction within their natal colony. As a consequence, undergoing puberty while in the colony may be strongly selected against as there are negative consequences associated with steroid actions. In contrast, upon encountering an unfamiliar opposite-sex conspecific, it may be beneficial to express sex behaviors very rapidly, since such encounters are rare. Thus, natural selection may favor individuals that are capable of exhibiting sex behavior in the absence of mature gonads and steroid hormones. We will attempt to determine the mechanism underlying this pattern of behavior.
Xenophobia: DMRs vigorously defend their burrows against intrusion by unfamiliar conspecifics. Interestingly, when removed from their “home cages” many individuals will rapidly attempt to mate with unfamiliar opposite-sex individuals. Thus, the expression of aggression appears to be highly dependent on environmental factors. The mechanisms by which aggressive behavior is altered by environmental context remains unknown. The photograph above illustrates the typical defensive posture.
Aging: DMRs can live for 14-22 years, this is an extraordinarily long life-span for a mammal of this size (~70-200 grams body mass). Aging and senescence in many species is thought to be related to oxidative stress. Oxidative stress is the result of exposure to reactive oxygen species (ROS), which can damage cellular macromolecules, including DNA, lipids, and proteins. ROS are the product of aerobic metabolism; this suggests that animals with higher metabolic rates will generate ROS at a higher rate than organisms with lower metabolic rates. This led to the “oxidative stress theory” of aging, which posits that accumulated oxidative damage is related to aging; and that organisms with higher metabolic rates age more rapidly and have shorter life spans. Oxidative stress in a physiological setting can be defined as the excessive exposure to ROS; which is the net result of an imbalance between production and destruction of ROS. ROS are inactivated by antioxidants.
Several vertebrates fail to conform to the oxidative stress theory of aging (i.e., their life spans exceed what is predicted based on their metabolic rate), including many avian species, the great apes (including humans), naked mole-rats, and DMRs. In the species that have been examined (some avian and humans), a common factor may be very efficient antioxidant capabilities. This may counteract the relatively high metabolic rates of these species by effectively inactivating the associated high rate of ROS production, leading to longer life spans.
The oxidative stress theory of aging leads to several exciting experiments in DMRs:
- It will be interesting to determine whether DMR generate ROS at a lower rate than similarly sized mammals, or if they have very robust antioxidant capabilities.
- We will also test whether the presence of helpers in their cooperative breeding life-style leads to a reduction in the generation of ROS by decreasing the amount of work required of each individual. In this way, eusociality may increase life span. Perhaps the number of helpers in each colony decreases the rate of oxidative damage in a dose-dependent manner (i.e., the more helpers, the less oxidative stress).
Sensory Physiology: DMR exhibit several interesting sensory adaptations related to living underground. First, their eyes are used to sense changes in air pressure, rather than for vision. When they reach the end of a tunnel, they open their eyes, possibly to sense air movements generated by colony mates or predators.
Second, inside of their tunnels, DMRs often exhibit a behavior that resembles doing a push-up, or busting a breakdance move called “the worm”. This may be to generate air currents as a means of communication throughout the tunnel system. See the behavior here: DMR movie