Anne Venner

Basal forebrain control of wakefulness and cortical rhythms

Wakefulness, along with fast cortical rhythms and associated cognition, depend on the basal forebrain (BF). BF cholinergic cell loss in dementia and the sedative effect of anti-cholinergic drugs have long implicated these neurons as important for cognition and wakefulness. The BF also contains intermingled inhibitory GABAergic and excitatory glutamatergic cell groups whose exact neurobiological roles are unclear. Here we show that genetically targeted chemogenetic activation of BF cholinergic or glutamatergic neurons in behaving mice produced significant effects on state consolidation and/or the electroencephalogram but had no effect on total wake. Similar activation of BF GABAergic neurons produced sustained wakefulness and high-frequency cortical rhythms, whereas chemogenetic inhibition increased sleep. Our findings reveal a major contribution of BF GABAergic neurons to wakefulness and the fast cortical rhythms associated with cognition. These findings may be clinically applicable to manipulations aimed at increasing forebrain activation in dementia and the minimally conscious state.

Dichotomous cellular properties of mouse orexin/hypocretin neurons

Non‐technical summary  Orexin/hypocretin neurons are widely projecting, ‘multi‐tasking’ brain cells that promote alertness, reward seeking and feeding. They are vital for stable consciousness in higher mammals. Loss of orexin/hypocretin cells produces narcolepsy. It was originally assumed that orexin/hypocretin neurons are one uniform population of cells, but recent studies hinted that they may be split into subsystems. To explore this, we performed unbiased statistical analysis of electrical properties of orexin/hypocretin cells in combination with 3‐D analysis of their shape. Our results pointed to an existence of two subgroups of orexin/hypocretin neurons, that have unique ‘electrical fingerprints’ and distinct ways of receiving information from other neurons.

A Novel Population of Wake-Promoting GABAergic Neurons in the Ventral Lateral Hypothalamus

The largest synaptic input to the sleep-promoting ventrolateral preoptic area (VLPO) [1] arises from the lateral hypothalamus [2], a brain area associated with arousal [3-5]. However, the neurochemical identity of the majority of these VLPO-projecting neurons within the lateral hypothalamus (LH), as well as their function in the arousal network, remains unknown. Herein we describe a population of VLPO-projecting neurons in the LH that express the vesicular GABA transporter (VGAT; a marker for GABA-releasing neurons). In addition to the VLPO, these neurons also project to several other established sleep and arousal nodes, including the tuberomammillary nucleus, ventral periaqueductal gray, and locus coeruleus. Selective and acute chemogenetic activation of LH VGAT(+) neurons was profoundly wake promoting, whereas acute inhibition increased sleep. Because of its direct and massive inputs to the VLPO, this population may play a particularly important role in sleep-wake switching.

Orexin neurons as conditional glucosensors: paradoxical regulation of sugar sensing by intracellular fuels

Non‐technical summary  Brain orexin/hypocretin neurons stimulate wakefulness, feeding, reward‐seeking and healthy glucose balance. The activity of orexin neurons is tightly regulated by several hormones, neurotransmitters and nutrients. Intriguingly, elevated glucose concentration can block or silence the activity of orexin neurons. We identified an unexpected way to control these effects of glucose on orexin neurons. We found that supplying orexin neurons with other energy‐related molecules, such as pyruvate and lactate, can stop glucose from blocking orexin neurons. We hypothesize that orexin neurons only ‘see’ glucose changes when the levels of other energy molecules are low, whereas high energy levels can stop glucose from regulating orexin cells. This may shed new light on understanding how the brain is influenced by changes in glucose levels during different metabolic situations, such as fasting, eating different diets, or in disease states such as diabetes and obesity.

Direct and indirect control of orexin/hypocretin neurons by glycine receptors

Normal wakefulness relies on brain cells called orexin/hypocretin neurons. Activity of these cells stimulates awakening while their loss produces the sleep disorder narcolepsy. By studying what makes orexin/hypocretin cells more or less active, we can thus gain insights into how the brain switches between different states of consciousness. We describe a new way to turn orexin/hypocretin cells off using a chemical called glycine. We show that glycine shuts down the electrical activity of orexin/hypocretin neurons from the adult brain, but has the opposite effect in the very young brain. Apart from these direct actions on orexin/hypocretin cells, glycine also enhances the ability of other nerve cells to communicate with orexin/hypocretin neurons. These data shed new light on the basic chemical and physical mechanisms regulating orexin/hypocretin neurons, which may also be useful in improving therapeutic strategies for disorders such as insomnia.

Supramammillary glutamate neurons are a key node of the arousal system

Basic and clinical observations suggest that the caudal hypothalamus comprises a key node of the ascending arousal system, but the cell types underlying this are not fully understood. Here we report that glutamate-releasing neurons of the supramammillary region (SuMvglut2) produce sustained behavioral and EEG arousal when chemogenetically activated. This effect is nearly abolished following selective genetic disruption of glutamate release from SuMvglut2 neurons. Inhibition of SuMvglut2 neurons decreases and fragments wake, also suppressing theta and gamma frequency EEG activity. SuMvglut2 neurons include a subpopulation containing both glutamate and GABA (SuMvgat/vglut2) and another also expressing nitric oxide synthase (SuMNos1/Vglut2). Activation of SuMvgat/vglut2 neurons produces minimal wake and optogenetic stimulation of SuMvgat/vglut2 terminals elicits monosynaptic release of both glutamate and GABA onto dentate granule cells. Activation of SuMNos1/Vglut2 neurons potently drives wakefulness, whereas inhibition reduces REM sleep theta activity. These results identify SuMvglut2 neurons as a key node of the wake−sleep regulatory system.Supramammillary nucleus (SuM) neurons have been studied in the context of REM sleep but their possible role in mediating wakefulness is not known. Here the authors elucidate the distinct functional contributions of three subpopulations in the SuM on electrographical and behavioral arousal in mice using genetically targeted approaches.

A hypothalamic circuit for the circadian control of aggression

Abstract‘Sundowning’ in dementia and Alzheimer’s disease is characterized by early-evening agitation and aggression. While such periodicity suggests a circadian origin, whether the circadian clock directly regulates aggressive behavior is unknown. We demonstrate that a daily rhythm in aggression propensity in male mice is gated by GABAergic subparaventricular zone (SPZGABA) neurons, the major postsynaptic targets of the central circadian clock, the suprachiasmatic nucleus. Optogenetic mapping revealed that SPZGABA neurons receive input from vasoactive intestinal polypeptide suprachiasmatic nucleus neurons and innervate neurons in the ventrolateral part of the ventromedial hypothalamus (VMH), which is known to regulate aggression. Additionally, VMH-projecting dorsal SPZ neurons are more active during early day than early night, and acute chemogenetic inhibition of SPZGABA transmission phase-dependently increases aggression. Finally, SPZGABA-recipient central VMH neurons directly innervate ventrolateral VMH neurons, and activation of this intra-VMH circuit drove attack behavior. Altogether, we reveal a functional polysynaptic circuit by which the suprachiasmatic nucleus clock regulates aggression.Todd et al. show a daily rhythm in aggression propensity in male mice and reveal a novel polysynaptic circuit within the hypothalamus by which the central circadian clock (the suprachiasmatic nucleus) influences neurons that regulate attack behavior.

A Genetically Defined Circuit for Arousal from Sleep during Hypercapnia

The precise neural circuitry that mediates arousal during sleep apnea is not known. We previously found that glutamatergic neurons in the external lateral parabrachial nucleus (PBel) play a critical role in arousal to elevated CO2 or hypoxia. Because many of the PBel neurons that respond to CO2 express calcitonin gene-related peptide (CGRP), we hypothesized that CGRP may provide a molecular identifier of the CO2 arousal circuit. Here, we report that selective chemogenetic and optogenetic activation of PBelCGRP neurons caused wakefulness, whereas optogenetic inhibition of PBelCGRP neurons prevented arousal to CO2, but not to an acoustic tone or shaking. Optogenetic inhibition of PBelCGRP terminals identified a network of forebrain sites under the control of a PBelCGRP switch that is necessary to arouse animals from hypercapnia. Our findings define a novel cellular target for interventions that may prevent sleep fragmentation and the attendant cardiovascular and cognitive consequences seen in obstructive sleep apnea. VIDEO ABSTRACT.

Genetic Activation, Inactivation, and Deletion Reveal a Limited And Nuanced Role for Somatostatin-Containing Basal Forebrain Neurons in Behavioral State Control

Recent studies have identified an especially important role for basal forebrain GABAergic (BFVGAT) neurons in the regulation of behavioral waking and fast cortical rhythms associated with cognition. However, BFVGAT neurons comprise several neurochemically and anatomically distinct subpopulations, including parvalbumin-containing BFVGAT neurons and somatostatin-containing BFVGAT neurons (BFSOM neurons), and it was recently reported that optogenetic activation of BFSOM neurons increases the probability of a wakefulness to non-rapid-eye movement (NREM) sleep transition when stimulated during the rest period of the animal. This finding was unexpected given that most BFSOM neurons are not NREM sleep active and that central administration of the synthetic somatostatin analog, octreotide, suppresses NREM sleep or increases REM sleep. Here we used a combination of genetically driven chemogenetic and optogenetic activation, chemogenetic inhibition, and ablation approaches to further explore the in vivo role of BFSOM neurons in arousal control. Our findings indicate that acute activation or inhibition of BFSOM neurons is neither wakefulness nor NREM sleep promoting and is without significant effect on the EEG, and that chronic loss of these neurons is without effect on total 24 h sleep amounts, although a small but significant increase in waking was observed in the lesioned mice during the early active period. Our in vitro cell recordings further reveal electrophysiological heterogeneity in BFSOM neurons, specifically suggesting at least two distinct subpopulations. Together, our data support the more nuanced view that BFSOM neurons are electrically heterogeneous and are not NREM sleep or wake promoting per se, but may exert, in particular during the early active period, a modest inhibitory influence on arousal circuitry. SIGNIFICANCE STATEMENT The cellular basal forebrain (BF) is a highly complex area of the brain that is implicated in a wide range of higher-level neurobiological processes, including regulating and maintaining normal levels of electrocortical and behavioral arousal. The respective in vivo roles of BF cell populations and their neurotransmitter systems in the regulation of electrocortical and behavioral arousal remains incompletely understood. Here we seek to define the neurobiological contribution of GABAergic somatostatin-containing BF neurons to arousal control. Understanding the respective contribution of BF cell populations to arousal control may provide critical insight into the pathogenesis of a host of neuropsychiatric and neurodegenerative disorders, including Alzheimers disease, Parkinsons disease, schizophrenia, and the cognitive impairments of normal aging.