Preoptic Area Modulation of Arousal in Natural and Drug Induced Unconscious States

The role of the hypothalamic preoptic area (POA) in arousal state regulation has been studied since Constantin von Economo first recognized its importance in the early twentieth century. Over the intervening decades, the POA has been shown to modulate arousal in both natural (sleep and wake) as well as drug-induced (anesthetic-induced unconsciousness) states. While the POA is well known for its role in sleep promotion, populations of wake-promoting neurons within the region have also been identified. However, the complexity and molecular heterogeneity of the POA has made distinguishing these two populations difficult. Though multiple lines of evidence demonstrate that general anesthetics modulate the activity of the POA, the region’s heterogeneity has also made it challenging to determine whether the same neurons involved in sleep/wake regulation also modulate arousal in response to general anesthetics. While a number of studies show that sleep-promoting POA neurons are activated by various anesthetics, recent work suggests this is not universal to all arousal-regulating POA neurons. Technical innovations are making it increasingly possible to classify and distinguish the molecular identities of neurons involved in sleep/wake regulation as well as anesthetic-induced unconsciousness. Here, we review the current understanding of the POA’s role in arousal state regulation of both natural and drug-induced forms of unconsciousness, including its molecular organization and connectivity to other known sleep and wake promoting regions. Further insights into the molecular identities and connectivity of arousal-regulating POA neurons will be critical in fully understanding how this complex region regulates arousal states.

0160 Chemogenetic Activation of Hypothalamic Tachykinin 1-Expressing Neurons Reduces Sleep and Increases Resistance to Isoflurane

Introduction The hypothalamic preoptic area (POA) is a heterogeneous region of the brain, containing intermingled populations of excitatory and inhibitory neurons expressing a wide variety of molecular markers. Of its many important homeostatic roles, the POA modulates arousal including natural states (sleep and wakefulness) as well as drug-induced (anesthetic-induced unconsciousness) states. Although sleep and anesthetic-induced unconsciousness are undoubtedly distinct, multiple lines of evidence demonstrate that shared neuronal circuits regulate both states. Previous work identified a population of sleep-promoting, tachykinin 1-expressing (Tac1) neurons within the POA. Given evidence in the VLPO and SON, we hypothesized that Tac1 POA neurons would be another site of convergence for facilitating entry into both NREM sleep and anesthetic-induced unconsciousness. Methods Sleep and isoflurane sensitivity were assessed in Tac1-Cre mice expressing either a Cre-dependent excitatory hM3Dq DREADD or a control fluorophore in the POA. Sleep was assessed using beam break actigraphy, while isoflurane sensitivity was assessed using video tracking as well as a loss of righting reflex assay to construct dose-response curves. Results Unexpectedly, activation of Tac1 POA neurons with 3mg/kg CNO reduced cumulative sleep time by 55% compared to controls (p<0.0001). Decreased sleep time was due to a 59% reduction in sleep bout duration (p<0.01), as the number of sleep bouts remained unchanged. Activity, measured as distance traveled, also increased by 167% compared to vehicle (p<0.0001). Activation of these neurons also increased resistance to isoflurane on both induction (p<0.0001) and emergence (p<0.0001). Additionally, activation during a continuous steady-state exposure to isoflurane destabilized the unconscious state. Conclusion Our results support the concept that POA Tac1 neurons function as a point of convergence for neural circuits regulating arousal from endogenous and drug-induced states of unconsciousness, while also illustrating the complexity of sleep and wake regulation within the hypothalamus. Support R01-GM088156-06, T32-HL007953

Neural Controls of Prostaglandin 2 Pyrogenic, Tachycardic, and Anorexic Actions Are Anatomically Distributed

Fever and anorexia are induced by immune system challenges. Because these responses are adaptive when short lasting but deleterious when prolonged, an understanding of the mediating neural circuitry is important. Prostaglandins (PGE) are a critical signaling element for these immune responses. Despite the widespread distribution of PGE receptors throughout the brain, research focuses on the hypothalamic preoptic area as the mediating site of PGE action. Paraventricular nucleus of the hypothalamus (PVH), parabrachial nucleus (PBN), and nucleus tractus solitarius (NTS) neurons also express PGE receptors and are activated during systemic pathogen infection. A role for these neurons in PGE-induced fever, tachycardia, and anorexia is unexplored and is the subject of this report. A range of PGE2 doses was microinjected into third or fourth ventricles (v), or directly into the dorsal PVH, lateral PBN, and medial NTS, and core and brown adipose tissue temperature, heart rate, locomotor activity, and food intake were measured in awake, behaving rats. PGE2 delivery to multiple brain sites (third or fourth v, PVH, or PBN) induced a short- latency (<10 min) fever and tachycardia. By contrast, an anorexic effect was observed only in response to third v and PVH stimulation. NTS PGE2 stimulation was without effect; locomotor activity was not affected for any of the sites. The data are consistent with a view of PGE2-induced effects as mediated by anatomically distributed sites rather than a single center. The data also underscore a potential anatomical dissociation of the neural pathways mediating pyrogenic and anorexic effects of PGE2.