Christian R. Burgess

Mania-like behavior induced by genetic dysfunction of the neuron-specific Na+,K+-ATPase α3 sodium pump.

Bipolar disorder is a debilitating psychopathology with unknown etiology. Accumulating evidence suggests the possible involvement of Na+,K+-ATPase dysfunction in the pathophysiology of bipolar disorder. Here we show that Myshkin mice carrying an inactivating mutation in the neuron-specific Na+,K+-ATPase α3 subunit display a behavioral profile remarkably similar to bipolar patients in the manic state. Myshkin mice show increased Ca2+ signaling in cultured cortical neurons and phospho-activation of extracellular signal regulated kinase (ERK) and Akt in the hippocampus. The mood-stabilizing drugs lithium and valproic acid, specific ERK inhibitor SL327, rostafuroxin, and transgenic expression of a functional Na+,K+-ATPase α3 protein rescue the mania-like phenotype of Myshkin mice. These findings establish Myshkin mice as a unique model of mania, reveal an important role for Na+,K+-ATPase α3 in the control of mania-like behavior, and identify Na+,K+-ATPase α3, its physiological regulators and downstream signal transduction pathways as putative targets for the design of new antimanic therapies.

An Endogenous Glutamatergic Drive onto Somatic Motoneurons Contributes to the Stereotypical Pattern of Muscle Tone across the Sleep–Wake Cycle

Skeletal muscle tone is modulated in a stereotypical pattern across the sleep–wake cycle. Abnormalities in this modulation contribute to most of the major sleep disorders; therefore, characterizing the neurochemical substrate responsible for transmitting a sleep–wake drive to somatic motoneurons needs to be determined. Glutamate is an excitatory neurotransmitter that modulates motoneuron excitability; however, its role in regulating motoneuron excitability and muscle tone during natural sleep–wake behaviors is unknown. Therefore, we used reverse-microdialysis, electrophysiology, pharmacological, and histological methods to determine how changes in glutamatergic neurotransmission within the trigeminal motor pool contribute to the sleep–wake pattern of masseter muscle tone in behaving rats. We found that blockade of non-NMDA and NMDA glutamate receptors (via CNQX and d-AP-5) on trigeminal motoneurons reduced waking masseter tone to sleeping levels, indicating that masseter tone is maximal during alert waking because motoneurons are activated by an endogenous glutamatergic drive. This wake-related drive is switched off in non-rapid eye movement (NREM) sleep, and this contributes to the suppression of muscle tone during this state. We also show that a functional glutamatergic drive generates the muscle twitches that characterize phasic rapid-eye movement (REM) sleep. However, loss of a waking glutamatergic drive is not sufficient for triggering the motor atonia that characterizes REM sleep because potent activation of either AMPA or NMDA receptors on trigeminal motoneurons was unable to reverse REM atonia. We conclude that an endogenous glutamatergic drive onto somatic motoneurons contributes to the stereotypical pattern of muscle tone during wakefulness, NREM sleep, and phasic REM sleep but not during tonic REM sleep.

Narcolepsy: neural mechanisms of sleepiness and cataplexy.

### Introduction Narcolepsy is a common cause of chronic sleepiness and is often accompanied by symptoms that include odd mixtures of sleep and wakefulness. A patient of ours is an intelligent and highly motivated young woman who developed unrelenting sleepiness during law school. No matter how

Arcuate hypothalamic AgRP and putative POMC neurons show opposite changes in spiking across multiple timescales

Agouti-related-peptide (AgRP) neurons—interoceptive neurons in the arcuate nucleus of the hypothalamus (ARC)—are both necessary and sufficient for driving feeding behavior. To better understand the functional roles of AgRP neurons, we performed optetrode electrophysiological recordings from AgRP neurons in awake, behaving AgRP-IRES-Cre mice. In free-feeding mice, we observed a fivefold increase in AgRP neuron firing with mounting caloric deficit in afternoon vs morning recordings. In food-restricted mice, as food became available, AgRP neuron firing dropped, yet remained elevated as compared to firing in sated mice. The rapid drop in spiking activity of AgRP neurons at meal onset may reflect a termination of the drive to find food, while residual, persistent spiking may reflect a sustained drive to consume food. Moreover, nearby neurons inhibited by AgRP neuron photostimulation, likely including satiety-promoting pro-opiomelanocortin (POMC) neurons, demonstrated opposite changes in spiking. Finally, firing of ARC neurons was also rapidly modulated within seconds of individual licks for liquid food. These findings suggest novel roles for antagonistic AgRP and POMC neurons in the regulation of feeding behaviors across multiple timescales. DOI: http://dx.doi.org/10.7554/eLife.07122.001

Amygdala Lesions Reduce Cataplexy in Orexin Knock-Out Mice

Narcolepsy is characterized by excessive sleepiness and cataplexy, sudden episodes of muscle weakness during waking that are thought to be an intrusion of rapid eye movement sleep muscle atonia into wakefulness. One of the most striking aspects of cataplexy is that it is often triggered by strong, generally positive emotions, but little is known about the neural pathways through which positive emotions trigger muscle atonia. We hypothesized that the amygdala is functionally important for cataplexy because the amygdala has a role in processing emotional stimuli and it contains neurons that are active during cataplexy. Using anterograde and retrograde tracing in mice, we found that GABAergic neurons in the central nucleus of the amygdala heavily innervate neurons that maintain waking muscle tone such as those in the ventrolateral periaqueductal gray, lateral pontine tegmentum, locus ceruleus, and dorsal raphe. We then found that bilateral, excitotoxic lesions of the amygdala markedly reduced cataplexy in orexin knock-out mice, a model of narcolepsy. These lesions did not alter basic sleep–wake behavior but substantially reduced the triggering of cataplexy. Lesions also reduced the cataplexy events triggered by conditions associated with high arousal and positive emotions (i.e., wheel running and chocolate). These observations demonstrate that the amygdala is a functionally important part of the circuitry underlying cataplexy and suggest that increased amygdala activity in response to emotional stimuli could directly trigger cataplexy by inhibiting brainstem regions that suppress muscle atonia.

Dynamic GABAergic afferent modulation of AgRP neurons

Agouti-related peptide (AgRP) neurons of the arcuate nucleus of the hypothalamus (ARC) promote homeostatic feeding at times of caloric insufficiency, yet they are rapidly suppressed by food-related sensory cues before ingestion. Here we identify a highly selective inhibitory afferent to AgRP neurons that serves as a neural determinant of this rapid modulation. Specifically, GABAergic projections arising from the ventral compartment of the dorsomedial nucleus of the hypothalamus (vDMH) contribute to the preconsummatory modulation of ARCAgRP neurons. In a manner reciprocal to ARCAgRP neurons, ARC-projecting leptin receptor-expressing GABAergic vDMH neurons exhibit rapid activation upon availability of food that additionally reflects the relative value of the food. Thus, leptin receptor-expressing GABAergic vDMH neurons form part of the sensory network that relays real-time information about the nature and availability of food to dynamically modulate ARCAgRP neuron activity and feeding behavior.

A noradrenergic mechanism functions to couple motor behavior with arousal state.

BACKGROUND Appropriate levels of skeletal muscle tone are needed to support routine motor behaviors. But, the brain mechanisms that function to couple muscle tone with waking behaviors are unknown. We addressed this question by studying mice with cataplexy--a condition caused by a decoupling of motor and arousal behaviors. Cataplexy is characterized by involuntary loss of muscle tone during wakefulness, which results in postural collapse during otherwise normal consciousness. Cataplexy is caused by loss of hypocretin (orexin) cells, but it is unknown how this loss triggers motor inactivity during cataplexy. Here, we used hypocretin knockout mice to identify the neurochemical cause of cataplexy and to determine the biochemical mechanisms that normally function to couple arousal and motor systems. RESULTS Using genetic, behavioral, electrophysiological, and pharmacological approaches, we show that the noradrenergic system acts to synchronize motor and arousal systems. Specifically, we show that an excitatory noradrenergic drive maintains postural muscle tone during wakefulness by activating α1 receptors on skeletal motoneurons. Loss of this normal excitatory drive triggers motor inactivity during cataplexy by reducing motoneuron excitation. However, loss of this drive does not affect arousal since mice remain awake during cataplexy, suggesting the noradrenergic system is not required for maintaining wakefulness. Artificial restoration of noradrenergic drive to motoneurons prevents motor inactivity and rescues cataplexy. CONCLUSIONS We conclude that hypocretin deficiency causes cataplexy by short-circuiting the noradrenergic drive to skeletal motoneurons. We suggest that the noradrenergic system functions to couple the brain systems that control postural muscle tone and behavioral arousal state.

Homeostatic circuits selectively gate food cue responses in insular cortex

Physiological needs bias perception and attention to relevant sensory cues. This process is ‘hijacked’ by drug addiction, causing cue-induced cravings and relapse. Similarly, its dysregulation contributes to failed diets, obesity, and eating disorders. Neuroimaging studies in humans have implicated insular cortex in these phenomena. However, it remains unclear how ‘cognitive’ cortical representations of motivationally relevant cues are biased by subcortical circuits that drive specific motivational states. Here we develop a microprism-based cellular imaging approach to monitor visual cue responses in the insular cortex of behaving mice across hunger states. Insular cortex neurons demonstrate food-cue-biased responses that are abolished during satiety. Unexpectedly, while multiple satiety-related visceral signals converge in insular cortex, chemogenetic activation of hypothalamic ‘hunger neurons’ (expressing agouti-related peptide (AgRP)) bypasses these signals to restore hunger-like response patterns in insular cortex. Circuit mapping and pathway-specific manipulations uncover a pathway from AgRP neurons to insular cortex via the paraventricular thalamus and basolateral amygdala. These results reveal a neural basis for state-specific biased processing of motivationally relevant cues.

Bidirectional Anticipation of Future Osmotic Challenges by Vasopressin Neurons

Ingestion of water and food are major hypo- and hyperosmotic challenges. To protect the body from osmotic stress, posterior pituitary-projecting, vasopressin-secreting neurons (VPpp neurons) counter osmotic perturbations by altering their release of vasopressin, which controls renal water excretion. Vasopressin levels begin to fall within minutes of water consumption, even prior to changes in blood osmolality. To ascertain the precise temporal dynamics by which water or food ingestion affect VPpp neuron activity, we directly recorded the spiking and calcium activity of genetically defined VPpp neurons. In states of elevated osmolality, water availability rapidly decreased VPpp neuron activity within seconds, beginning prior to water ingestion, upon presentation of water-predicting cues. In contrast, food availability following food restriction rapidly increased VPpp neuron activity within seconds, but only following feeding onset. These rapid and distinct changes in activity during drinking and feeding suggest diverse neural mechanisms underlying anticipatory regulation of VPpp neurons.

Attenuation of mania-like behavior in Na+,K+-ATPase α3 mutant mice by prospective therapies for bipolar disorder: Melatonin and exercise

Bipolar disorder is a neuropsychiatric disease characterized by states of mania with or without depression. Pharmacological treatments can be inadequate at regulating mood for many individuals. Melatonin therapy and aerobic exercise are independent prospective therapies for bipolar disorder that have shown potential as mood stabilizers in humans. Myshkin mice (Myk/+) carry a heterozygous missense mutation in the neuronal Na(+),K(+)-ATPase α3 and model mania-related symptoms of bipolar disorder including increased activity, risk-taking behavior and reductions in sleep. One cohort of Myk/+ and wild-type littermates (+/+) was treated with melatonin and a separate cohort was treated with voluntary exercise. Mania-related behavior was assessed in both cohorts. The effect of melatonin on sleep and the effect of exercise on brain-derived neurotrophic factor (BDNF) expression in the hippocampus were assayed. Melatonin and voluntary wheel running were both effective at reducing mania-related behavior in Myk/+ but did not affect behavior in +/+. Melatonin increased sleep in Myk/+ and did not change sleep in +/+. Myk/+ showed higher baseline levels of BDNF protein in the hippocampus than +/+. Exercise increased BDNF protein in +/+ hippocampus, while it did not significantly affect BDNF levels in Myk/+ hippocampus. These findings support initial studies in humans indicating that melatonin and exercise are useful independent adjunct therapies for bipolar disorder. Their effects on mood regulation should be further examined in randomized clinical trials. Our results also suggest that hippocampal BDNF may not mediate the effects of exercise on mania-related behavior in the Myk/+ model of mania.