Date of Award
Doctor of Philosophy (PhD)
Circadian rhythms have evolved to enable organisms to adapt to daily (24 hour) changes in the environment. In mammals, including humans, the suprachiasmatic nucleus (SCN), located in the hypothalamus, is the master circadian pacemaker that governs daily rhythms in physiology and behavior. It is known that neurons in the SCN generate circadian changes in spontaneous repetitive firing rates (higher during the day than at night) that regulate and synchronize daily physiological and behavioral rhythms, although the critical ionic conductances driving these rhythms remain elusive. Considerable evidence suggests that the daily rhythms in the repetitive firing rates of SCN neurons are linked to changes in cellular input resistances, driven by changes in subthreshold potassium (K+) conductance(s). An alternative model, “bicycle” model, for circadian regulation of membrane excitability in Drosophila clock neurons, however, has also been proposed. In this model, an increase in the NALCN-encoded sodium (Na+) leak conductance underlies the daytime increase in repetitive firing rates.In this dissertation, I combined cellular electrophysiology, pharmacology, in vivo molecular genetics, and computational approaches to explore the role of NALCN-encoded Na+ leak currents and the “bicycle” model mechanism in regulating daily rhythms in the repetitive firing rates of adult SCN neurons. I focused on three subtypes of neurons in the mouse SCN: vasoactive intestinal peptide (VIP)-expressing, neuromedin S (NMS)-expressing, and gastrin-releasing peptide (GRP)-expressing. I discovered that VIP-, NMS-, and GRP-producing SCN neurons have distinct daily rhythms in firing rates and membrane properties. I also found that Na+ leak current amplitudes in all three SCN cell types are similar during the day and at night, but have a larger impact on membrane potentials during the day than at night, suggesting that Na+ leak currents may play a role in regulating daily rhythms in the excitability of mature SCN neurons. Using an in vivo conditional knockout approach, I also demonstrated that the NALCN-encoded Na+ leak currents specifically regulate daytime, but not nighttime, repetitive firing rates of SCN neurons. Lastly, I developed computational models of Na+ leak and subthreshold K+ channels and employed the dynamic clamp technique to demonstrate that the effects of NALCN-encoded Na+ leak currents on the repetitive firing rates of adult SCN neurons depend on K+ current-driven changes in the input resistances. In all, this work defines a mechanism for Na+ leak currents to regulate daily rhythms in the excitability of adult SCN neurons in which the impact of Na+ leak currents is coupled to the K+ current-mediated rhythmic changes in intrinsic membrane properties. The results presented in this dissertation also provide new insights into how specific subthreshold ionic conductances independently, and collectively, regulate daily rhythms in the spontaneous repetitive firing rates of SCN neurons. The framework and tools developed in this dissertation will also drive future studies to define the roles of specific ionic conductances in distinct SCN subtypes and, in turn, to different aspects of physiology and behavior.
Jeanne M. Nerbonne
Sarah K. England, Erik D. Herzog, Daniel W. Moran, Jonathan R. Silva,
Available for download on Wednesday, May 15, 2024