Glucocorticoid-dependent regulation of molecular clocks and dendritic spines in the ventromedial prefrontal cortex
Date
2022
Authors
Miller, Alex M., author
Tobet, Stuart, advisor
Vigh, Jozsef, committee member
Bamburg, Jim, committee member
Maresh, Ryan, committee member
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Abstract
Biological rhythms in the brain and periphery are governed by the suprachiasmatic nucleus of the hypothalamus (SCN) and the SCN's control of rhythmic adrenal glucocorticoid (GC) secretions via the hypothalamic pituitary adrenal (HPA) axis. The daily surge of GC secretions aid in the entrainment of molecular clocks throughout the body which physically, mentally, and metabolically prime an organism to function in accordance with the external light:dark (L:D) cycle. When key events of biological rhythms fail to match up with the external L:D cycle, various pathologies arise in the brain and periphery. To better understand the neural basis of pathologies caused by the disruption of biological rhythms, further investigation of key limbic regulatory brain regions is required. Thus, the studies described in this dissertation examine how biological rhythms in the ventromedial prefrontal cortex (vmPFC) are regulated by GC secretions. The vmPFC regulates fear acquisition, fear extinction, mood, and HPA axis function. Multiple brain regions exhibit time-of-day dependent variations in learning, long term potentiation (LTP), and dendritic morphology. GCs have been implicated in the regulation of dendritic structure in the context of stress. GCs are also known to regulate molecular clock entrainment via upregulation of Per1 transcription in a variety of tissues. In the present study, C57BL/6N mice were sacrificed at 3 distinct times of day (ZT3, ZT12, and ZT16, lights off at ZT12) and Per1 mRNA expression was measured in the infralimbic and prelimbic vmPFC subregions using droplet digital (dd)PCR after recovering from adrenalectomy or sham surgery for 10 days. Sham mice showed Per1 rhythmicity in both IL and PL, with peak expression occurring at ZT12. Adrenalectomized mice showed reductions in Per1 amplitude at ZT12 in both IL and PL, suggesting that the vmPFC molecular clock is entrained by diurnal GC oscillations. Thy1-eGFP mice were used to visualize and quantify dendritic spine density on layer V pyramidal dendrites at ZT 3, 12, and 16. Spine density in both PL and IL exhibited changes between the light (inactive) and dark (active) phases, with peak spine density observed at ZT16 and trough spine density observed at ZT3. These changes in spine density were restricted to changes in long thin and stubby type spines. To determine if changes in spine density is regulated by diurnal GC oscillations, the 11β-hydroxylase inhibitor metyrapone was administered 2 hours prior to the onset of the active phase (ZT10) daily for 7 days. Metyrapone administration blocked both the diurnal peak of plasma corticosterone and peak spine densities in the IL and PL at ZT16. These results suggest that vmPFC molecular clock gene and dendritic spine diurnal rhythms depend on intact diurnal GC oscillations. These findings establish a link between diurnal GC oscillations, the molecular clock, and synaptic plasticity. Additionally, these findings describe how the vmPFC changes across 24-hour periods, which provides a foundation for further investigation into how biological rhythms in the vmPFC may be altered in the context of circadian disruption, and how specific disease states may arise as a result.
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Subject
diurnal rhythm
dendritic spine
glucocorticoid