Dissertations, Theses, and Capstone Projects

Date of Degree

2-2020

Document Type

Dissertation

Degree Name

Ph.D.

Program

Biology

Advisor

Daniel McCloskey

Committee Members

Abdeslem El Idrissi

Jeffrey Goodman

Jonathan Levitt

Kai Kaila

Subject Categories

Animal Studies | Biology | Nervous System Diseases | Neuroscience and Neurobiology | Research Methods in Life Sciences

Keywords

seizure, glaber, GABA, febrile, RFID, hypercapnia

Abstract

A Neural Mechanism for Capnotaxis in the Naked Mole-Rat

The Naked Mole-Rat (NM-R) is a small hairless rodent that has thrived underground in the Horn of Africa for the past 40 million years. It was first discovered in 1847, but has only been maintained in captivity and studied as a laboratory animal over the past 40 years. To date, more than 400 scientific papers have been published on this species. This is relatively few compared to established lab animals like rats and mice, but those papers contain a trove of information that has changed our understanding of the constraints of mammalian physiology.

Naked mole-rats maintain a cooperative breeding system of eusociality by suppressing the reproductive system and can tolerate prolonged periods of low oxygen.

Using a combination of behavioral analysis, in vitro and in vivo electrophysiology, and molecular biology, this thesis demonstrates a capnotaxic drive toward aggregation in adult NM-R. By extending the developmental tolerance for hypercapnia which is present at birth, NM-R have come to depend upon the enhanced neuronal inhibition that CO2 provides, at the expense of adequate inhibitory tone in “normal” atmosphere.

RFID tracking of whole colonies shows a majority of activity concentrated around a single nest chamber. Individuals of the colony remain in the nest with forays to the food chamber and toilet chambers. Infusing CO2 into an empty chamber showed an increase in forays through the chamber compared to no infusion and to air infusion (5.9±0.92 trips per hour, compared to 1.2±0.29 trips and 1.4±0.31 trips per hour respectively).

While NM-Rs are well-equipped for a gain of CO2, I demonstrate here that they are inadequately equipped for what could be considered hypocapnia, relative to their normal environment. When adult NM-Rs were exposed to room air at 42 °C, respiration rate more than doubled (room temperature: 77±7 breaths per minute; heated: 168±18 breaths per minute, t(4)=4.7, p= 0.01). In 8 of 9 animals exposed to this simulated surface environment, signs of seizure activity (head bobbing, mouth automatisms, generalized convulsions) appeared 14.4±1.2 minutes after the chamber temperature reached 42 °C. These seizures are likely due to the loss of CO2 during hyperventilation, and subsequent brain hyperactivity due to respiratory alkalosis. As support for this, seizures were prevented with simulated nest air at 42 °C (2.5% CO2 / 21% O2 / 76.5% N2). If administered after seizure began, high amplitude cortical seizure events subsided in the EEG but seizure-related motor activity continued.

This vulnerability to heated normal air, which is nearly identical to typical conditions a naked mole-rat would experience on the ground just above its burrow, are hallmarks of an “overspecialization” in this species. NM-Rs are often touted as “superheroes”, but every comic book superhero has a vulnerability. I contend that NM-R depend on the hypercapnic environment they have created in their burrows for typical brain function, and are vulnerable outside it.

The extended sensitivity to hyperthermia-induced seizures into adulthood does make the NM-R a superheroic animal model of febrile seizure, however, overcoming the limitations of other animal models, which are restricted to the early postnatal period. I have established a collaboration with experts in the mechanisms of febrile seizures from the University of Helsinki to inquire whether the same mechanisms of hyperexcitability they have identified in neonatal rats and mice are implicated in the seizure susceptibility of NM-R neonates and adults, and in fact they are. We also observe neonatal levels of KCC2 (chloride potassium) neuronal symporter and an impairment of GABAergic tone. Remarkably, injection of diazepam, a positive allosteric modulator of GABA, causes seizure in normal air in nearly all adult NM-R. This phenomenon is suppressed in nest air.

This work will add a neuronal requirement for hypercapnia to the growing list of remarkable NM-R traits. This understanding of how certain physiological states in NM-R render them more vulnerable to environmental conditions may yield important new insight to other species who have the same challenges at times, including our own.

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