Hanging In The Balance: Fluid Homeostasis In Hibernation
Hibernation is a critical strategy that allows many animals to survive without food or water over an entire winter. Despite the immense potential for hibernation biology to advance human medicine, we have an incomplete understanding of how hibernation works at the mechanistic level. In small mammals, hibernation is characterized by the alternation between two fundamentally different physiological states: “torpor” and “interbout arousal” (IBA). During torpor, animals profoundly reduce their metabolism, respiration, and body temperature (to as low as 2-4°C) for up to two weeks at a time. Torpor bouts are regularly interrupted by spontaneous IBAs, during which major physiological parameters return to normal levels for less than 24 hours. How fundamental physiological pathways have adapted to enable hibernation, and how hibernators cope with the large perturbations associated with repeated torpor-arousal cycles, remain enigmatic. My postdoctoral research examines how homeostatic pathways, specifically those controlling fluid homeostasis (water balance), function under torpor-arousal states in hibernation.
I study fluid homeostasis in the thirteen lined-ground squirrel (Ictidomys tridecemlineatus; 13LGS), which undergoes obligatory hibernation without drinking or eating for up to eight months every winter. In most terrestrial vertebrates, water deprivation leads to dehydration and increased serum osmolality, including in desert-adapted species such as the camel. Recently, I discovered that despite months-long water deprivation, 13LGS do not experience dehydration but instead lower their serum osmolality by 30mmol/kg (10%) during torpor, which is reversed in IBA (Feng et al., 2019). I also found that during IBA, animals suppress thirst and drinking behavior compared to summer active animals, but release antidiuretic hormones vasopressin and oxytocin. Thus, although thirst and antidiuretic hormone release pathways are usually synergistically activated, they are uncoupled in hibernation, enabling water retention by the kidney while suppressing the drive to leave the safety of the underground burrow in search of water. By comparing active vs. hibernating states and using a multidisciplinary approach, including developing opto- and chemo-genetic tools in 13LGS, my project will elucidate the neurohormonal mechanisms by which thirst and antidiuretic hormone release are uncoupled, and test their importance for hibernation.
Adapted from Martin 2019
People make science possible and fun, here are some of my multi-talented labmates!
Biological rhythms of vocal behavior in fish: hormonal, neuronal, and genetic mechanisms
The patterning of social acoustic signaling at multiple timescales, from day-night rhythms to acoustic temporal properties, is critical for animal communication, and enhances sender-receiver coupling and reproductive success. My PhD work in Andy Bass's lab investigated hormonal, neuronal, and genetic mechanisms underlying the timing of vocal behavior in the plainfin midshipman fish (Porichthys notatus), across behaviorally relevant timescales.
First, I demonstrated that the robust daily rhythm of the midshipman’s nocturnal courtship vocalization is under endogenous circadian and melatonin control, revealed by continuously recording vocalizations while manipulating external light cycles or internal melatonin levels (Feng and Bass, 2016).
Second, I found that melatonin also increased the excitability of the underlying vocal network, providing a neural basis for the behavioral results (Feng and Bass, 2014). These studies revealed melatonin’s remarkable versatility as a timing signal in distant vertebrate species, conveying the permissive time for vocalization in an opposing manner to permit vocalization in nocturnal fish and suppress vocalization in diurnal birds (shown by others).
Third, I demonstrated that melatonin receptor mRNA is expressed in neuroendocrine and vocal patterning centers, suggesting that melatonin’s effects on vocalization are executed by discrete neural pathways (Feng, Marchaterre, and Bass, 2019).
Finally, my transcriptome analysis of gene expression in the midshipman vocal motor nucleus (VMN) across timepoints corresponding to high and low vocal activity identified a suite of molecular candidates for shaping the precise and synchronous firing of VMN motor neurons, which produce the final motor commands for vocalization (Feng, Fergus, and Bass, 2015).
Altogether, my thesis work identified mechanisms underlying the timing of vocalization that may be applicable across vertebrates, including birds and mammals that also exhibit temporal rhythms in social communication across multiple timescales.
Throughout my PhD journey, I had the pleasure of working with the most charismatic fish and amazing people in breathtaking places.
Throughout my PhD journey, I had the pleasure of working with the most charismatic fish and amazing people in breathtaking places.
With me in these photos clockwise from upper left: Andy Bass, Joel Tripp, Dan Fergus, Joel Tripp, Midge Marchaterre
Peripheral androgen action in an acrobatic bird
As an undergraduate researcher in Barney Schlinger's lab, I investigated the androgen sensitivity of skeletal muscles involved in the acrobatic courtship dance of the Golden-collared manakins (Manacus vitellinus). I compared androgen receptor expression and testosterone metabolic enzyme activity levels in manakin wing and leg muscles across sex and to other species (Feng et al., 2010).
As part of my research, I was given the unique opportunity to study the manakins in Panama during two field seasons under the mentorship of Dr. Lainy Day. Working in the rainforest was definitely a dream come true, but I found it scary and challenging to be alone in the rainforest during the daily thunderstorms in the wet season. I also had to confront my fear of spiders (still a work-in-progress). Being in the rainforest in the dry season was a completely awe-inspiring experience!
I learned early on that the best part of research is the privilege of working with and learning from amazing animals and inspirational scientists!
As part of my research, I was given the unique opportunity to study the manakins in Panama during two field seasons under the mentorship of Dr. Lainy Day. Working in the rainforest was definitely a dream come true, but I found it scary and challenging to be alone in the rainforest during the daily thunderstorms in the wet season. I also had to confront my fear of spiders (still a work-in-progress). Being in the rainforest in the dry season was a completely awe-inspiring experience!
I learned early on that the best part of research is the privilege of working with and learning from amazing animals and inspirational scientists!