Colleen M. Novak - US grants

Numerous studies have shown that melatonin can induce sleep in humans. However, the brain mechanisms responsible for this effect are unknown. This is due to the lack of a laboratory rodent model of the soporific effects of melatonin; attempts to induce sleep with melatonin in laboratory rodents have used nocturnal rodents, meaning rodents active during the dark phase of the cycle, such as rats and Djungarian hamsters. Melatonin does not consistently induce sleep in these rodents. In fact, in most of these studies, melatonin reduced sleep and increased wakefulness. Because these nocturnal animals are most active during the night, when melatonin secretion is the highest, these results should not be surprising. A diurnal rodent model is needed to demonstrate the sleep-inducing effects of melatonin. Arvicanthis niloticus (unstriped Nile grass rat), a diurnal murid rodent, is an ideal animal in which to demonstrate the sleep-inducing effects of melatonin. Further, using this rodent as a model, the brain circuitry through which melatonin induces sleep can be investigated. Results from these studies may clarify how melatonin can be used to treat insomnia and

DESCRIPTION (provided by applicant): Brain Melanocortin Control of Activity Energy Expenditure and Obesity Resistance It is becoming widely accepted that individual differences in daily physical activity levels are inherited, and that extended inactivity can lead to weight gan and metabolic dysfunction. We have found consistently high levels of physical activity and energy expenditure in rats selectively bred for high intrinsic aerobic capacity (i.e., the ability o run long distances without prior training; HCR) compared to their low-capacity counterparts (LCR); these differences are not dependent on body mass or composition. Moreover, HCR have higher total daily energy expenditure than LCR, a result of higher non-resting (not resting) energy expenditure. Here, we hypothesize that differences in the brain melanocortin system, specifically site-specific variations in melanocortin receptor (MCR) expression, may underlie the high- and low-activity phenotypes. We will engage undergraduate research trainees in testing this hypothesis, utilizing rat models of leanness/obesity derived through artificial selection as well as through deletion of gene expression for a specific MCR. First, we will determine how differences in brain expression patterns of MCR may contribute to physical activity energy expenditure, focusing on MCR 3, 4, and 5, as well quantify how activity energy expenditure is altered in MC4R- deficient rats. Next, we will probe the molecular mechanisms underlying the ability of brain MC to increase skeletal muscle fuel utilization during physical activity, and determine if these mechanisms differ between HCR and LCR. Lastly, we will determine if caloric restriction (i.e., dieting), which causes more weight loss in high- activity HCR than in low-activiy LCR, differentially alters brain MCR and skeletal muscle gene expression patterns in HCR compared to LCR; we will also test this hypothesis using MC4R-deficient rats. Focusing on identifying unique features of central (brain) and peripheral (e.g., skeletal muscle) control of energy balance in the lean phenotype may yield novel treatments for obesity.

Mechanisms Underlying Contextual Induction of Muscle Thermogenesis While obesity is clearly a major public health concern in the US, it has proven to be stubbornly resistant to prevention and treatment. While investigating mechanisms underlying endogenous amplification of energy expenditure, we identified a pathway in the brain that increases sympathetic nervous system (SNS) activity to skeletal muscle, impacting molecular pathways that modulate muscle energy uptake and use. We hypothesize that this increased caloric use promotes leanness by enhancing energy expended during physical activity, increasing muscle thermogenesis where caloric energy is ultimately dissipated as heat. Recently, we discovered that exposure to predator odor (ferret) rapidly and robustly increases muscle thermogenesis in rats. This persists when activity is held constant, and it translates into a significant increase in energy expenditure. Here, we will investigate the neural pathway that is responsible for this effect. Frist, we will demonstrate that predator odor enhances weight loss during energy restriction, and activates muscle thermogenesis even in the absence of interscapular brown adipose tissue or beta-3 adrenergic receptor activation. Second, we hypothesize that activation of the ventromedial hypothalamus (VMH) and of steroidogenic factor-1 neurons within the VMH are critical for the ability of predator odor to activate muscle thermogenesis and enhance activity-related energy expenditure. We will use a neural inhibition tool?designer receptors exclusively activated by designer drugs (DREADDs), delivered using a viral vector?to suppress the activity of the VMH in rats, or of steroidogenic factor-1 neurons within the VMH of mice, to demonstrate the role of this cell population in the modulation of muscle metabolism and energy expenditure by predator odor. Lastly, we hypothesize that the SNS is activated by exposure to the predator odor, and that blocking the activity of beta adrenergic receptors will decrease the metabolic response to predator odor. These studies may uncover a novel mechanism for weight loss through enhanced muscle calorie use, and provide opportunities to enhance undergraduate research training in our laboratory.

Neural Mechanisms Underlying Central Induction of Skeletal Muscle Thermogenesis Though individual health and the US health-care system as a whole suffer from the deleterious consequences of obesity, weight loss and maintenance have proven difficult for the majority of people. Developing methods to increase energy expenditure would ease this process. We have found that exposing rats to the odor of their natural predator (ferret) provokes a rapid and robust rise in skeletal muscle (gastrocnemius) temperature, with a corresponding increase in energy expenditure. Here, we probe the most likely brain and muscle mediators of this response. First, we will use a chemogenetic tool we developed to target neurons expressing steroidogenic factor 1 (SF-1) in the dorsomedial/central subregions of the ventromedial hypothalamus (dmVMH), specifically predicting that inhibition of this cell population will decrease the ability of predator odor to induce muscle thermogenesis in rats. Second, we will investigate the most probable mechanism underlying the thermogenic induction at the level of the skeletal myocyte, namely sarcolipin uncoupling of sarco/endoplasmic reticulum ATPase (SERCA) Ca2+ cycling. We will determine if predator odor exposure suppresses SERCA Ca2+ transport relative to ATPase activity, while increasing sarcolipin expression. We also predict that, since unilateral sympathetic neural (lumbar sympathetic nerve) surgical denervation inhibits the ability of predator odor to induce thermogenesis in the denervated gastrocnemius muscle relative to the contralateral (intact) muscle, SERCA uncoupling and sarcolipin expression will similarly be altered in the denervated muscle compared to the intact muscle in the same rat. These studies will demonstrate the roles of brain SF-1 neurons and muscle SERCA uncoupling in central induction of skeletal muscle non-shivering thermogenesis. Altogether, we will establish multiple components of this brain-muscle thermogenic pathway as viable targets to counter weight gain, while engaging undergraduate students in research.