William D. Todd, Ph.D. - US grants

DESCRIPTION (provided by applicant): The subparaventricular zone (SPZ) of the hypothalamus is an important component of the circuitry that synchronizes and drives circadian rhythms of behavior and physiology. The SPZ receives most of the axonal output from the suprachiasmatic nucleus (SCN), the mammalian circadian pacemaker, and further projects to most of the SCN's downstream targets. Physiological and anatomical studies in rats suggest the SPZ is heterogeneous and comprised of subregions differentially modulating particular rhythms. Specifically, dorsal SPZ lesions, defined by the region extending ventrally from the paraventricular nucleus (PVH), selectively disrupt body temperature (Tb) rhythms with minimal impact on locomotor activity (LMA) or sleep/wake rhythms. In contrast, ventral SPZ lesions, defined by the region extending dorsally and caudally from the SCN, profoundly disrupt LMA and sleep/wake rhythms with less effect on Tb. Anterograde tracing from the SPZ in rats further suggests this area is comprised of four anatomically, and perhaps functionally, distinct subregions: the ventromedial and ventrolateral SPZ (vmSPZ, vlSPZ) and the dorsomedial and dorsolateral SPZ (dmSPZ, dlSPZ). Recent developments of certain conditional knockout mice allow for the direct testing of the neurotransmitter-specific role of SPZ subregions in vivo. We seek to gain a better understanding of SPZ organization and outflow and its role in the circadian timing system in mice. Elucidating such pathways may reveal mechanisms by which chronic circadian disruption alters behavior and adversely affects overall health. Aim 1. To assess the role of GABAergic subpopulations of the SPZ in the modulation of circadian rhythms of Tb, LMA, and sleep/wake: The SPZ is primarily comprised of GABAergic neurons, which package GABA into vesicles using VGAT. Using Cre-Lox technology, we will inject a viral vector to delete VGAT expression in SPZ subregions, preventing these cells from releasing GABA. We will record freely moving mice in constant dark, while measuring Tb and LMA using biotelemetry transmitters implanted in the intraperitoneal cavity, and sleep/wake using EEG and EMG recordings. We expect these experiments to identify which populations of GABAergic neurons in the SPZ regulate circadian rhythms of LMA, sleep/wake, and Tb. Aim 2. To delineate efferent projections from GABAergic SPZ neurons and correlate the physiological patterns in Aim 1 with deletion of GABAergic transmission in specific terminal fields: There are no published reports of a detailed analysis of SPZ efferents in mice. The vector used in Aim 1 also traces the efferent projections from transduced neurons and will reveal the projection targets that mediate these rhythms, however, it also traces from non-GABAergic neurons. To clarify the projections likely underlying our effects, we will use mice expressing Cre under the VGAT promoter and a vector expressing the gene for a Cre-dependent tracer to selectively trace SPZ GABAergic projections. We will then compare VGAT deletion in specific targets with physiological responses in Aim 1 to determine SPZ targets regulating specific circadian rhythms.

Project Summary/Abstract Alzheimer's disease and related dementias are associated with progressive disruption of circadian rhythms. One particular feature of such circadian dysfunction in patients with AD and related dementias is ?sundowning syndrome?, a poorly understood clinical phenomenon characterized by agitation, aggression, and delirium during the early evening hours. Such symptoms have a major impact on the quality of life for both the patient and their caregivers and often lead to the decision to seek institutionalization. The neurobiology of sundowning remains unknown, however the temporal periodicity of sundowning symptoms suggests a possible disturbance in the master circadian clock, the suprachiasmatic nucleus (SCN) of the hypothalamus, or in the pathways by which the SCN modulates particular rhythms. Rhythms of sleep-wake and LMA are known to be regulated by the SCN via a pathway through its major postsynaptic target, the subparaventricular zone (SPZ), to the dorsomedial hypothalamus (DMH). Additionally, I recently demonstrated that the propensity for behavioral aggression also follows a daily rhythm that is regulated by the SCN, via an additional pathway through the SPZ, to the ventromedial hypothalamus (VMH). Importantly, disrupting this SCN?SPZ?VMH pathway led to increased aggression during the early resting phase (the light period for nocturnal mice), which is temporally analogous to when AD and dementia patients who experience sundowning display increased agitation and aggression. This suggests that the function of certain structures within this circuit may be compromised in AD and dementia, and that this pathway may be a promising therapeutic target for treating circadian dysfunction and aggression in patients who display sundowning. To test this novel hypothesis, I began examining circadian rhythms in the TAPP mouse model, which develops amyloid-beta (a-beta) plaques and tau neurofibrillary tangles (both hallmarks of AD neuropathology), and my preliminary results suggest that these mice exhibit increased early resting period aggression and blunted active period LMA at ages shortly after they first develop AD-related neuropathology. In this proposal, I will examine tissue from these mice for AD-related neuropathological markers in the SCN, the SPZ and its output targets the VMH and the DMH. It has been hypothesized that circadian dysfunction associated with sundowning results instead from AD-related disturbances to areas that provide input to the circadian system, such as serotoninergic and cholinergic pathways, and I will also examine neuropathology in such areas. Additionally, I will also examine activated astrocytes in all of these circadian pathways, as such glial responses have been show to be associated with neuroinflammation and neurodegeneration in AD, and normal astrocyte functioning is known to be critical to the circadian system's ability to maintain proper time-keeping. Finally, I seek to determine the effects of manipulating SPZ activity (using chemogenetic activation) on the increased daytime aggression and blunted circadian sleep-wake rhythms in TAPP mice, and on the patterns of neuropathology and astrocyte responses.