Anxiogenic drug administration and elevated plus-maze exposure in rats activate populations of relaxin-3 neurons in the nucleus incertus and serotonergic neurons in the dorsal raphe nucleus

Genetically defined nucleus incertus neurons differ in connectivity and function

The nucleus incertus (NI), a conserved hindbrain structure implicated in the stress response, arousal, and memory, is a major site for production of the neuropeptide relaxin-3. On the basis of goosecoid homeobox 2 (gsc2) expression, we identified a neuronal cluster that lies adjacent to relaxin 3a (rln3a) neurons in the zebrafish analogue of the NI. To delineate the characteristics of the gsc2 and rln3a NI neurons, we used CRISPR/Cas9 targeted integration to drive gene expression specifically in each neuronal group, and found that they differ in their efferent and afferent connectivity, spontaneous activity, and functional properties. gsc2 and rln3a NI neurons have widely divergent projection patterns and innervate distinct subregions of the midbrain interpeduncular nucleus (IPN). Whereas gsc2 neurons are activated more robustly by electric shock, rln3a neurons exhibit spontaneous fluctuations in calcium signaling and regulate locomotor activity. Our findings define heterogeneous neurons in the NI and provide new tools to probe its diverse functions.

The Role of Dorsal Raphe Nucleus Serotonergic Systems in Emotional Learning and Memory in Male BALB/c Mice

Selective serotonin reuptake inhibitors (SSRIs) are the first-line pharmacological treatment for a variety of anxiety-, trauma- and stressor-related disorders. Although they are efficacious, therapeutic improvements require several weeks of treatment and are often associated with an initial exacerbation of symptoms. The dorsal raphe nucleus (DR) has been proposed as an important target for the modulation of emotional responses and the therapeutic effects of SSRIs. Using a fear-conditioning paradigm we aimed to understand how SSRIs affect emotional learning and memory, and their effects on serotonergic circuitry. Adult male BALB/c mice were treated with vehicle (n = 16) or the SSRI fluoxetine (18 mg/kg/d) acutely (n = 16), or chronically (21d, n = 16), prior to fear conditioning. Treatment was stopped, and half of the mice (n = 8/treatment group) were exposed to cued fear memory recall 72 h later. Activation of DR serotonergic neurons during fear conditioning (Experiment 1) or fear memory recall (Experiment 2), was measured using dual-label immunohistochemistry for Tph2 and c-Fos. Acute and chronic fluoxetine treatment reduced associative fear learning without affecting memory recall and had opposite effects on anxiety-like behaviour. Acute fluoxetine decreased serotonergic activity in the DR, while chronic treatment led to serotonergic activity that was indistinguishable from that of control levels in DRD and DRV subpopulations. Chronic fluoxetine facilitated fear extinction, which was associated with rostral DRD inhibition. These findings provide further evidence that SSRIs can alter aspects of learning and memory processes and are consistent with a role for discrete populations of DR serotonergic neurons in regulating fear- and anxiety-related behaviours.

Sex differences in amygdalohippocampal oscillations and neuronal activation in a rodent anxiety model and in response to infralimbic deep brain stimulation

Introduction

Depression and anxiety are highly comorbid mental disorders with marked sex differences. Both disorders show altered activity in the amygdala, hippocampus, and prefrontal cortex. Infralimbic deep brain stimulation (DBS-IL) has anxiolytic and antidepressant effects, but the underlying mechanisms remain unclear. We aimed to contribute to understanding sex differences in the neurobiology of these disorders.

Methods

In male and female rats, we recorded neural oscillations along the dorsoventral axis of the hippocampus and the amygdala in response to an anxiogenic drug, FG-7142. Following this, we applied DBS-IL.

Results

Surprisingly, in females, the anxiogenic drug failed to induce most of the changes observed in males. We found sex differences in slow, delta, theta, and beta oscillations, and the amygdalo-hippocampal communication in response to FG-7142, with modest changes in females. Females had a more prominent basal gamma, and the drug altered this band only in males. We also analyzed c-Fos expression in both sexes in stress-related structures in response to FG-7142, DBS-IL, and combined interventions. With the anxiogenic drug, females showed reduced expression in the nucleus incertus, amygdala, septohippocampal network, and neocortical levels. In both experiments, the DBS-IL reversed FG-7142-induced effects, with a more substantial effect in males than females.

Discussion

Here, we show a reduced response in female rats which contrasts with the higher prevalence of anxiety in women but is consistent with other studies in rodents. Our results open compelling questions about sex differences in the neurobiology of anxiety and depression and their study in animal models.

Under or Absent Reporting of Light Stimuli in Testing of Anxiety-Like Behaviors in Rodents: The Need for Standardization

Behavioral neuroscience tests such as the Light/Dark Test, the Open Field Test, the Elevated Plus Maze Test, and the Three Chamber Social Interaction Test have become both essential and widely used behavioral tests for transgenic and pre-clinical models for drug screening and testing. However, as fast as the field has evolved and the contemporaneous involvement of technology, little assessment of the literature has been done to ensure that these behavioral neuroscience tests that are crucial to pre-clinical testing have well-controlled ethological motivation by the use of lighting (i.e., Lux). In the present review paper, N = 420 manuscripts were examined from 2015 to 2019 as a sample set (i.e., n = ~20–22 publications per year) and it was found that only a meager n = 50 publications (i.e., 11.9% of the publications sampled) met the criteria for proper anxiogenic and anxiolytic Lux reported. These findings illustrate a serious concern that behavioral neuroscience papers are not being vetted properly at the journal review level and are being released into the literature and public domain making it difficult to assess the quality of the science being reported. This creates a real need for standardizing the use of Lux in all publications on behavioral neuroscience techniques within the field to ensure that contributions are meaningful, avoid unnecessary duplication, and ultimately would serve to create a more efficient process within the pre-clinical screening/testing for drugs that serve as anxiolytic compounds that would prove more useful than what prior decades of work have produced. It is suggested that improving the standardization of the use and reporting of Lux in behavioral neuroscience tests and the standardization of peer-review processes overseeing the proper documentation of these methodological approaches in manuscripts could serve to advance pre-clinical testing for effective anxiolytic drugs. This report serves to highlight this concern and proposes strategies to proactively remedy them as the field moves forward for decades to come.

Acute treatment with 5-hydroxytryptophan increases social approach behaviour but does not activate serotonergic neurons in the dorsal raphe nucleus in juvenile male BALB/c mice: A model of human disorders with deficits of sociability

BACKGROUND

The BALB/c mouse has been proposed as a model of human psychiatric disorders characterised by elevated anxiety and altered sociability. Juvenile BALB/c mice show decreased social exploratory behaviour, increased anxiety, and reduced brain serotonin synthesis compared to other strains including C57BL/6J mice.

AIM

To determine whether supplementation of brain serotonin synthesis alters social behaviour and activation of serotonergic neurons across subregions of the dorsal raphe nucleus (DR) in BALB/c mice.

METHODS

Juvenile male BALB/c mice were assigned to one of four treatment conditions: vehicle/vehicle, carbidopa (25 mg/kg)/vehicle, vehicle/5-HTP (10 mg/kg), carbidopa (25 mg/kg)/5-HTP (10 mg/kg). Social behaviour was measured using the three-chamber social approach test, followed by immunohistochemical staining for TPH2 and c-Fos to measure activation of serotonergic neurons across subregions of the DR.

RESULTS

Mice treated with carbidopa/5-HTP spent more time in the social cage zone and covered more distance in the social approach test compared to other treatment groups. There was no difference between treatment groups in the activation of serotonergic neurons across subregions of the DR. However, the DRD was associated with increased social approach behaviour in carbidopa/5-HTP treated animals.

CONCLUSIONS

Supplementation of serotonin synthesis can increase social approach behaviour in juvenile BALB/c mice. An increase in locomotor behaviour was also observed suggesting that increasing central serotonin synthesis may have led to a reduction in state anxiety, manifesting in increased exploratory behaviour. As no effect on serotonergic activation within the DR was found, alternative mechanisms are likely important for the effects of 5-HTP on social behaviour.

The Oscillatory Profile Induced by the Anxiogenic Drug FG-7142 in the Amygdala-Hippocampal Network Is Reversed by Infralimbic Deep Brain Stimulation: Relevance for Mood Disorders

Anxiety and depression exhibit high comorbidity and share the alteration of the amygdala–hippocampal–prefrontal network, playing different roles in the ventral and dorsal hippocampi. Deep brain stimulation of the infralimbic cortex in rodents or the human equivalent—the subgenual cingulate cortex—constitutes a fast antidepressant treatment. The aim of this work was: (1) to describe the oscillatory profile in a rodent model of anxiety, and (2) to deepen the therapeutic basis of infralimbic deep brain stimulation in mood disorders. First, the anxiogenic drug FG-7142 was administered to anaesthetized rats to characterize neural oscillations within the amygdala and the dorsoventral axis of the hippocampus. Next, deep brain stimulation was applied. FG-7142 administration drastically reduced the slow waves, increasing delta, low theta, and beta oscillations in the network. Moreover, FG-7142 altered communication in these bands in selective subnetworks. Deep brain stimulation of the infralimbic cortex reversed most of these FG-7142 effects. Cross-frequency coupling was also inversely modified by FG-7142 and by deep brain stimulation. Our study demonstrates that the hyperactivated amygdala–hippocampal network associated with the anxiogenic drug exhibits an oscillatory fingerprint. The study contributes to comprehending the neurobiological basis of anxiety and the effects of infralimbic deep brain stimulation.

Immunomodulation Eliminates Inflammation in the Hippocampus in Experimental Autoimmune Encephalomyelitis, but Does Not Ameliorate Anxiety-Like Behavior

Multiple sclerosis (MS) is an autoimmune disease targeting the central nervous system, characterized by an unpredictable disease course and a wide range of symptoms. Emotional and cognitive deficits are now recognized as primary disease manifestations and not simply the consequence of living with a chronic condition, raising questions regarding the efficacy of current therapeutics for these specific symptoms. Mechanisms underlying psychiatric sequelae in MS are believed to be similar to those underlying pathogenesis, that is mediated by cytokines and other inflammatory mediators. To gain insight into the pathogenesis of MS depression, we performed behavioral assays in the murine experimental autoimmune encephalomyelitis (EAE) MS model, in the presence or absence of immunomodulation using the drug FTY720, an analogue of the lipid signaling molecule sphingosine-1-phosphate (S1P). Specifically, mice were challenged with the elevated plus maze (EPM) test, a validated experimental paradigm for rodent-specific anxiety-like behavior. FTY720 treatment failed to ameliorate anxiety-like symptoms, irrespective of dosage. On the other hand, it was effective in reducing inflammatory infiltration, microglial reactivity and levels of pro-inflammatory molecules in the hippocampus, confirming the anti-inflammatory capacity of treatment. To explore the absence of FTY720 effect on behavior, we confirmed expression of S1P receptors (S1PR) S1PR1, S1PR3 and S1PR5 in the hippocampus and mapped the dynamics of these receptors in response to drug treatment alone, or in combination with EAE induction. We identified a complex pattern of responses, differing between (1) receptors, (2) dosage and (3) hippocampal sub-field. FTY720 treatment in the absence of EAE resulted in overall downregulation of S1PR1 and S1PR3, while S1PR5 exhibited a dose-dependent upregulation. EAE induction alone resulted in overall downregulation of all three receptors. On the other hand, combined FTY720 and EAE showed generally no effect on S1PR1 and S1PR3 expression except for the fimbrium region, but strong upregulation of S1PR5 over the range of doses examined. These data illustrate a hitherto undescribed complexity of S1PR response to FTY720 in the hippocampus, independent of drug effect on effector immune cells, but simultaneously emphasize the need to explore novel treatment strategies to specifically address mood disorders in MS.

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.

Effects of plus-maze experience and chlordiazepoxide on anxiety-like behavior and serotonin neural activity in the dorsal raphe nucleus in rats

The extent to which rats express anxiety-like behavior on the elevated plus-maze (EPM) depends on their previous maze experience. Open-arm avoidance develops in maze-experienced rats, and is often accompanied by a diminished anxiolytic response to benzodiazepines. Regions of the dorsal raphe nucleus (DRN) were examined in male Sprague-Dawley rats using c-Fos and serotonin immunohistochemistry following a single exposure, a second exposure or no exposure to the EPM. We then examined the effect of the benzodiazepine anxiolytic chlordiazepoxide (CDP, 5 mg/kg) on EPM behavior and DRN neural activity. Enhanced open-arm avoidance was evident on the second EPM trial in both experiments. The observed pattern of c-Fos expression suggests that the first exposure to the maze activates serotonin cells in the rostral and dorsal regions of the DRN and that only the dorsal subregion is activated by a second exposure. CDP increased open-arm exploration during the first trial, which corresponded to decreased 5-hydroxytryptamine (5-HT) activity in the rostral and ventral subregions of the DRN. However, 5-HT activity in the DRN was reduced in rats on the second maze trial compared with the first trial, when CDP had no effect on open-arm exploration. These results suggest that open-arm avoidance in maze-experienced rats can be characterized as a coping response that is mediated by specific populations of 5-HT neurons in the DRN.

Platelets Drive Inflammation and Target Gray Matter and the Retina in Autoimmune-Mediated Encephalomyelitis

Despite growing evidence for platelets as active players in infection and immunity, it remains unresolved whether platelets contribute to, or are key elements in the development of neuroinflammation. Using the experimental autoimmune encephalomyelitis (EAE) model of multiple sclerosis, we identified platelet accumulation in the circulation by 7-day postinduction (dpi), ahead of clinical onset which occurs at 13-14 dpi. By inducing platelet depletion between 7 and 16 dpi, we demonstrate an association between platelet accumulation in the spinal cord and disease development. Additionally, we provide evidence for platelet infiltration in the white and gray matter parenchyma, but with different outcomes. Thus, while in white matter platelets are clearly associated with lesions, in gray matter large-scale platelet infiltration and expression of the platelet-specific molecule PF4 are detectable prior to T cell entry. In the retina, platelet accumulation also precedes clinical onset and is associated with significant increase in retinal thickness in experimental relative to control animals. Platelet accumulation increases over the disease course in this tissue, but without subsequent T cell infiltration. These findings provide definitive confirmation that platelet accumulation is key to EAE pathophysiology. Furthermore, they suggest an undescribed and, most importantly, therapeutically targetable mechanism of neuronal damage.

Platelet Depletion is Effective in Ameliorating Anxiety-Like Behavior and Reducing the Pro-Inflammatory Environment in the Hippocampus in Murine Experimental Autoimmune Encephalomyelitis

The neuropsychiatric symptoms of multiple sclerosis (MS), such as anxiety and depression, can result from disease activity itself as well as psychological reaction to an unfavorable diagnosis. Accordingly, the literature reports evidence of increased anxiety-like behavior in experimental autoimmune encephalomyelitis (EAE), an accepted MS model. Due to the recently described critical role of platelets in inflammation and autoimmune disease, we examined the relationship between platelets, inflammation, and anxiety-like behavior in EAE. In the elevated plus maze, EAE-induced C57BL/6J mice showed decreased time spent in the open arms relative to vehicle-only controls, demonstrating an increase in anxiety-like behavior. This effect occurred in the presence of platelet–neuron association, but absence of lymphocytic infiltration, in the hippocampal parenchyma. Platelet depletion at the pre-clinical disease stage, using antibody-mediated lysis prevented the EAE-induced increase in anxiety-like behavior, while no significant difference in distance moved was recorded. Furthermore, platelet depletion was also associated with reduction of the pro-inflammatory environment to control levels in the hippocampus and prevention of EAE disease symptomology. These studies demonstrate the high efficacy of a platelet-targeting approach in preventing anxiety-like symptoms and clinical manifestations of EAE and have implications for the treatment of neuropsychiatric symptoms in MS.

Multiple pathological mechanisms contribute to hippocampal damage in the experimental autoimmune encephalomyelitis model of multiple sclerosis

Emotional and cognitive deficits and associated hippocampal damage observed in multiple sclerosis (MS) are now recognized as primary disease manifestations. However, the pathological substrate of these dysfunctions is unclear. In the experimental autoimmune encephalomyelitis (EAE) MS model, impaired hippocampal-dependent functions are concomitant with severe microglial reactivity and neurodegeneration, but reports vary with respect to evidence of lymphocytic infiltration, raising questions as to the nature of the underlying neurodegenerative mechanisms. Our investigations of EAE-induced inflammation across the hippocampal formation showed CD3 infiltration only in regions adjacent to inflamed meningeal membranes interposed between the ventral aspect of the hippocampus and the dorsal aspect of the mid-brain, but widespread microglial reactivity across the structure. Regions that contact the lateral ventricles do not show inflammation, but CD3 cells are observed in the adjacent ventricular space and choroid plexus, suggesting that microglial reactivity in these regions results from exposure to proinflammatory mediators released into the ventricles. These data indicate that multiple pathophysiological mechanisms underlie hippocampal damage during EAE. Treatment with the immunomodulator FTY720 eliminates microglial reactivity across the whole structure, suggesting potential benefit for neuropsychological symptoms in MS.

Dual-transmitter systems regulating arousal, attention, learning and memory

An array of neuromodulators, including monoamines and neuropeptides, regulate most behavioural and physiological traits. In the past decade, dramatic progress has been made in mapping neuromodulatory circuits, in analysing circuit dynamics, and interrogating circuit function using pharmacogenetic, optogenetic and imaging methods This review will focus on several distinct neural networks (acetylcholine/GABA/glutamate; histamine/GABA; orexin/glutamate; and relaxin-3/GABA) that originate from neural hubs that regulate wakefulness and related attentional and cognitive processes, and highlight approaches that have identified dual transmitter roles in these behavioural functions. Modulation of these different neural networks might be effective treatments of diseases related to arousal/sleep dysfunction and of cognitive dysfunction in psychiatric and neurodegenerative disorders.

Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Relaxin' the brain: a case for targeting the nucleus incertus network and relaxin-3/RXFP3 system in neuropsychiatric disorders

Relaxin‐3 has been proposed to modulate emotional–behavioural functions such as arousal and behavioural activation, appetite regulation, stress responses, anxiety, memory, sleep and circadian rhythm. The nucleus incertus (NI), in the midline tegmentum close to the fourth ventricle, projects widely throughout the brain and is the primary site of relaxin‐3 neurons. Over recent years, a number of preclinical studies have explored the function of the NI and relaxin‐3 signalling, including reports of mRNA or peptide expression changes in the NI in response to behavioural or pharmacological manipulations, effects of lesions or electrical or pharmacological manipulations of the NI, effects of central microinfusions of relaxin‐3 or related agonist or antagonist ligands on physiology and behaviour, and the impact of relaxin‐3 gene deletion or knockdown. Although these individual studies reveal facets of the likely functional relevance of the NI and relaxin‐3 systems for human physiology and behaviour, the differences observed in responses between species (e.g. rat vs. mouse), the clearly identified heterogeneity of NI neurons and procedural differences between laboratories are some of the factors that have prevented a precise understanding of their function. This review aims to draw attention to the current preclinical evidence available that suggests the relevance of the NI/relaxin‐3 system to the pathology and/or symptoms of certain neuropsychiatric disorders and to provide cognizant directions for future research to effectively and efficiently uncover its therapeutic potential.

Linked Articles

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc

Nucleus incertus contributes to an anxiogenic effect of buspirone in rats: Involvement of 5-HT1A receptors

The nucleus incertus (NI), a brainstem structure with diverse anatomical connections, is implicated in anxiety, arousal, hippocampal theta modulation, and stress responses. It expresses a variety of neurotransmitters, neuropeptides and receptors such as 5-HT1A, D2 and CRF1 receptors. We hypothesized that the NI may play a role in the neuropharmacology of buspirone, a clinical anxiolytic which is a 5-HT1A receptor partial agonist and a D2 receptor antagonist. Several preclinical studies have reported a biphasic anxiety-modulating effect of buspirone but the precise mechanism and structures underlying this effect are not well-understood. The present study implicates the NI in the anxiogenic effects of a high dose of buspirone. Systemic buspirone (3 mg/kg) induced anxiogenic effects in elevated plus maze, light-dark box and open field exploration paradigms in rats and strongly activated the NI, as reflected by c-Fos expression. This anxiogenic effect was reproduced by direct infusion of buspirone (5 μg) into the NI, but was abolished in NI-CRF-saporin-lesioned rats, indicating that the NI is present in neural circuits driving anxiogenic behaviour. Pharmacological studies with NAD 299, a selective 5-HT1A antagonist, or quinpirole, a D2/D3 agonist, were conducted to examine the receptor system in the NI involved in this anxiogenic effect. Opposing the 5-HT1A agonism but not the D2 antagonism of buspirone in the NI attenuated the anxiogenic effects of systemic buspirone. In conclusion, 5-HT1A receptors in the NI contribute to the anxiogenic effect of an acute high dose of buspirone in rats and may be functionally relevant to physiological anxiety.

Central monoaminergic systems are a site of convergence of signals conveying the experience of exercise to brain circuits involved in cognition and emotional behavior

Physical activity can enhance cognitive function and increase resistance against deleterious effects of stress on mental health. Enhanced cognitive function and stress resistance produced by exercise are conserved among vertebrates, suggesting that ubiquitous mechanisms may underlie beneficial effects of exercise. In the current review, we summarize the beneficial effects of exercise on cognitive function and stress resistance and discuss central and peripheral signaling factors that may be critical for conferring the effects of physical activity to brain circuits involved in cognitive function and stress. Additionally, it is suggested that norepinephrine and serotonin, highly conserved monoamines that are sensitive to exercise and able to modulate behavior in multiple species, could represent a convergence between peripheral and central exercise signals that mediate the beneficial effects of exercise. Finally, we offer the novel hypothesis that thermoregulation during exercise could contribute to the emotional effects of exercise by activating a subset of temperature-sensitive serotonergic neurons in the dorsal raphe nucleus that convey anxiolytic and stress-protective signals to forebrain regions. Throughout the review, we discuss limitations to current approaches and offer strategies for future research in exercise neuroscience.

Nucleus incertus promotes cortical desynchronization and behavioral arousal

Arousal and vigilance are essential for survival and relevant regulatory neural circuits lie within the brainstem, hypothalamus and forebrain. The nucleus incertus (NI) is a distinct site within the pontine periventricular gray, containing a substantial population of GABAergic neurons with long-range, ascending projections. Existing neuroanatomical data and functional studies in anesthetized rats, suggest the NI is a central component of a midline behavioral control network well positioned to modulate arousal, vigilance and exploratory navigation, yet none of these roles have been established experimentally. We used a chemogenetic approach-clozapine-N-oxide (CNO) activation of virally delivered excitatory hM3Dq-DREADDs-to activate the NI in rats and examined the behavioral and physiological effects, relative to effects in naïve rats and appropriate viral-treated controls. hM3Dq activation by CNO resulted in long-lasting depolarization of NI neurons with action potentials, in vitro. Peripheral injection of CNO significantly increased c-Fos immunoreactivity in the NI and promoted cortical electroencephalograph (EEG) desynchronization. These brain changes were associated with heightened arousal, and increased locomotor activity in the homecage and in a novel environment. Furthermore, NI activation altered responses in a fear conditioning paradigm, reflected by increased head-scanning, vigilant behaviors during conditioned fear recall. These findings provide direct evidence that the NI promotes general arousal via a broad behavioral activation circuit and support early hypotheses, based on its connectivity, that the NI is a modulator of cognition and attention, and emotional and motivated behaviors.

Increased alcohol consumption in relaxin-3 deficient male mice

Relaxin-3 is a neuropeptide expressed in the brainstem, and predominantly localized in the gray matter of the midline dorsal pons termed the nucleus incertus. Relaxin-3-expressing neurons densely project axons rostrally to various forebrain regions including the septum, hippocampus, and lateral hypothalamus. Several relaxin-3 functions have been reported including food intake, stress responses, neuroendocrine function, emotion, and spatial memory. In addition, recently relaxin-3 and its receptor, RXFP3, were shown to regulate alcohol intake using an RXFP3 antagonist and RXFP3 gene knockout mice. In the present study, we investigated alcohol consumption in relaxin-3 knockout mice, and found that male but not female mice significantly drank more alcohol than wild-type mice in the two-bottle choice test. However, after chronic alcohol vapor exposure, wild-type and mutant mice did not show this difference in alcohol intake, although both genotypes exhibited increased alcohol consumption compared with non-alcohol-exposed control mice. There was no genotype difference in sucrose or quinine preference. These results suggest that the relaxin-3 neuronal system modestly affects alcohol preference and consumption.