Molecular and neural basis of contagious itch behavior in mice

The neural basis of affective empathy: What is known from rodents

Empathy is the cornerstone of social interactions between conspecies for human beings and other social animals. Human beings with empathy defects might either suffer unpleasant or failed social interactions as ASD patients, or even display antisocial behaviors. To find efficient cure for empathy defects, first of all, the neural mechanisms underpinning various empathy behaviors should be well studied and understood. And the research in the field of affective empathy thrives fast in recent years. It is necessary to review the important contributions in this field, especially for understanding the delicate neural mechanisms of diverse forms of affective empathy. Here, we have summarized the characteristics of various types of affective empathy. We also discuss the distinctions between empathy for pain and fear, as well as instinctive and experienced empathy. Our analysis further highlights the findings in the complex neural mechanisms and potential brain regions underlying different affective empathy behaviors. Above all, this work is expected to help enhance our comprehension of behavioral dynamics and neural basis of affective empathy along with its role in emotional regulation and social behavior.

Distinct oxytocin signaling pathways synergistically mediate rescue-like behavior in mice

Spontaneous rescue behavior enhances the well-being and survival of social animals, yet the neural mechanisms underlying the recognition and response to conspecifics in need remain unclear. Here, we report that observer mice experience distress when encountering anesthetized conspecifics, prompting spontaneous rescue-like behavior toward the unconscious mice. This behavior facilitates the earlier awakening of anesthetized mice while simultaneously alleviating stress in the helper mice. Our findings reveal that endogenous oxytocin (OXT) release from the hypothalamic paraventricular nucleus (PVN) to the oxytocin receptor (OXTR) in the central nucleus of the amygdala (CeA) regulates the emotional component of rescue-like behavior. In contrast, OXT release from the PVN to OXTR in the dorsal bed nucleus of the stria terminalis (dBNST) mediates the motor component of the behavior. Furthermore, we demonstrate that these two pathways exhibited distinct temporal dynamics and functional roles. The OXTPVN-OXTRCeA pathway is activated in a transient and intense manner, acting as a trigger for rescue-like behavior, whereas the OXTPVN-OXTRdBNST pathway responds in a sustained manner, ensuring the continuation of the behavior. These findings highlight the remarkable ability of rodents to engage in targeted helping behavior and suggest that distinct subcortical oxytocinergic pathways selectively and synergistically regulate the motor and emotional aspects of rescue-like behavior.

Monkeys increase scratching when encountering unexpected good fortune

Elucidating how ancestral behavioural traits have been repurposed for psychological and social functions is critical to advancing our understanding of human behavioural evolution. Self-scratching, originally a hygienic response and known to exhibit social contagion, serves as a model for this process. Although human scratching behaviour is traditionally linked to negative emotions, evidence from non-human animals has produced inconsistent results, casting doubt on its association with negative emotions. Here, we examined scratching in Japanese macaques (Macaca fuscata) during cognitive tasks. In Experiment 1, we manipulated reward outcomes by introducing rare, unexpected increases ("fortunate" condition) or omissions ("unfortunate" condition) of rewards; scratching increased significantly under the fortunate condition, but not the unfortunate one. In Experiment 2, we presented unexpected visual stimuli without altering reward outcomes and observed increased scratching. Our results challenge the traditional view that scratching is primarily linked to negative emotions. Our findings suggest that scratching, which originally evolved as a response to unexpected skin stimulation, may later have been repurposed as a coping mechanism to manage heightened uncertainty.

Neuroimaging Characteristics of Pruritus Induced by Eczema: An fMRI Study

OBJECTIVE

To explore the neuroimaging characteristics of eczema-induced pruritus with resting-state functional magnetic resonance imaging (rs-fMRI).

METHODS

A total of 42 patients with eczema were recruited in the PE group, and 42 healthy participants were included in the HC group. The Visual Analogue Score (VAS), 12-Item Pruritus Severity Scale (12-PSS), Pittsburgh Sleep Quality Index (PSQI), and Self-Rating Anxiety Scale (SAS) were recorded in the PE group. The different values of fraction amplitude of low-frequency fluctuation (fALFF) and functional connectivity (FC) were compared after rs-fMRI scanning.

RESULTS

Compared with the HC group, the fALFF values of the left precentral gyrus, left postcentral gyrus, left supplementary motor area (SMA), and left midcingulate cortex in the PE group were increased. The FC values between the left precentral gyrus, bilateral superior temporal gyrus, bilateral hippocampus, and left inferior occipital gyrus in the PE group were decreased. The FC values between left SMA and bilateral superior temporal gyrus in the PE group were decreased. The 12-PSS score was positively correlated with fALFF value of the left precentral gyrus and left postcentral gyrus.

CONCLUSION

Pruritus caused increased spontaneous activity in given cerebral regions, involving the perception of itch, control of scratching movements, and expression of itch-related emotions. Meanwhile, there is a correlation between fALFF values of given cerebral regions and clinical scales, which provided potential neurobiological markers for the future study of pruritus.

Affective bodily responses in monkeys predict subsequent pessimism, but not vice versa

Affect is a complex interplay of behaviour, physiology, cognition and subjective feelings. The James-Lange theory, which posits that affective bodily responses occur first and subsequently shape our affective experiences, has been extensively studied in humans but remains underexplored in non-human animals. This study employs a judgement bias test to explore the relationship between peripheral bodily responses, specifically self-scratching associated with negative affect, and shifts in cognitive information-processing modes (i.e. a 'pessimistic' judgement bias characterized by heightened anticipation of undesirable outcomes) in Japanese macaques (Macaca fuscata). Our findings support the hypothesis that bodily responses precede and influence changes in cognitive modes, demonstrating that self-scratching predicts subsequent pessimistic judgements, but not vice versa. This study provides comparative insights into the biological underpinnings of affect, highlighting that peripheral behaviours can shape cognitive processes in non-human primates. These results have broader implications for understanding the evolutionary continuity and adaptive significance of affective response systems across species.

Light-eye-body axis: exploring the network from retinal illumination to systemic regulation

The human body is an intricate system, where diverse and complex signaling among different organs sustains physiological activities. The eye, as a primary organ for information acquisition, not only plays a crucial role in visual perception but also, as increasing evidence suggests, exerts a broad influence on the entire body through complex circuits upon receiving light signals which is called non-image-forming vision. However, the extent and mechanisms of light's impact on the body through the eyes remain insufficiently explored. There is also a dearth of comprehensive reviews elucidating the intricate interplay between light, the eye, and the systemic connections to the entire body. Herein, we propose the concept of the light-eye-body axis to systematically encapsulate the extensive non-image-forming effects of light signals received by the retina on the entire body. We reviewed the visual-neural structure basis of the light-eye-body axis, summarized the mechanism by which the eyes regulate the whole body and the current research status and challenges within the physiological and pathological processes involved in the light-eye-body axis. Future research should aim to expand the influence of the light-eye-body axis and explore its deeper mechanisms. Understanding and investigating the light-eye-body axis will contribute to improving lighting conditions to optimize health and guide the establishment of phototherapy standards in clinical practice.

Visual and olfactory signals of conspecifics induce emotional contagion in mice

Emotional contagion occurs in many animals, including rodents. To determine the social signals of emotional state-matching between individuals in mice, we conducted an empirical laboratory experiment using visual, olfactory and auditory stimuli. The Japanese wild-derived mouse strain MSM/Ms (MSM) was tested as observers, since our initial experiments indicated that MSM mice showed higher sensitivity to others' pain compared with the laboratory strain C57BL/6J (B6). MSM observers were shown footage of an unfamiliar B6 mouse receiving painful foot shocks via a screen. For olfactory stimuli, one of the following was presented during observation: (i) urine collected from a shocked B6 mouse, (ii) urine collected from an unshocked B6 mouse, or (iii) reverse osmosis water. Consequently, MSM mice observing the footage with urine from shocked mice demonstrated significantly higher fear-induced freezing behaviour than in the other two conditions. Regarding visual and auditory stimuli, observing the pixelated video clip was significantly associated with reduced freeze responses, whereas blocking auditory cues did not affect the duration of freezing. These results provide clear-cut evidence that multiple cues, including olfactory and visual information, are sufficient social signals for emotional contagion in mice.

Contagious yawning and scratching in captive lemurs

Behavioral contagion is thought to play a significant role in social synchronization and coordination across animal taxa. While there is extensive evidence of behavioral contagion in Haplorrhines (i.e. monkeys and apes), limited research exists in Strepsirrhines (i.e. lemurs). Here, we aimed to investigate the presence of contagious yawning and scratching in two captive groups of black-and-white ruffed lemurs (Varecia variegata) (N = 4) and red ruffed lemurs (Varecia rubra) (N = 4), and further test whether behavioral contagion is modulated by the model's social integration in the group. We conducted all occurrence sampling to examine whether individuals observing a yawning or scratching event (i.e. trigger event) were more likely to yawn or scratch in the following 2 min, as compared to individuals who did not observe it. We ran generalized linear mixed models and found that the likelihood of yawning and scratching was higher for subjects observing the trigger event than for subjects who did not observe the event, although the model's social integration had no modulating effect on the probability of showing behavioral contagion. Our findings represent the first evidence of behavioral contagion in this genus and contribute to shed light on the distribution and the possible adaptive function of this phenomenon in primates.

Proper Frequency of Perinatal Retinal Waves Is Essential for the Precise Wiring of Visual Axons in Nonimage-Forming Nuclei

The development of the visual system is a complex and multistep process characterized by the precise wiring of retinal ganglion cell (RGC) axon terminals with their corresponding neurons in the visual nuclei of the brain. Upon reaching primary image-forming nuclei (IFN), such as the superior colliculus and the lateral geniculate nucleus, RGC axons undergo extensive arborization that refines over the first few postnatal weeks. The molecular mechanisms driving this activity-dependent remodeling process, which is influenced by waves of spontaneous activity in the developing retina, are still not well understood. In this study, by manipulating the activity of RGCs in mice from either sex and analyzing their transcriptomic profiles before eye-opening, we identified the Type I membrane protein synaptotagmin 13 (Syt13) as involved in spontaneous activity-dependent remodeling. Using these mice, we also explored the impact of spontaneous retinal activity on the development of other RGC recipient targets such as nonimage-forming (NIF) nuclei and demonstrated that proper frequency and duration of retinal waves occurring prior to visual experience are essential for shaping the connectivity of the NIF circuit. Together, these findings contribute to a deeper understanding of the molecular and physiological mechanisms governing activity-dependent axon refinement during the assembly of the visual circuit.

Play contagion in African elephants: The closest, the better

Behavioural contagion is an automatic process through which a behaviour performed by an individual (trigger) is reproduced by an observer (responder) without necessarily replicating the exact motor sequence. It has been linked to inter-individual synchronisation and possibly emotional contagion. Play can convey emotions and enhance social bonding, although its contagiousness is understudied. To verify social play contagion presence and modulating factors, we gathered audio-video data on social play, distance and affiliation on a group of savannah African elephants (15 individuals) at Parque de la Naturaleza de Cabarceno (Cantabria, Spain). Social play was contagious as it was more likely started by uninvolved elephants (within 3-min) in Post-Play Condition (PP) - after that other elephants had started playing - than in Matched-control Condition (MC; no previous play). Social play contagion mostly occurred within 30 m - probably due to elephants' limited visual acuity - and it was highest between individuals that affiliated the most, with the distance-affiliation interaction having no effect. The most prominent individuals in the social play network were also the most influential in the play contagion network (Eigenvector-centrality measure). Play contagion was socially modulated, thus suggesting it may extend from motor replication to the replication of the underlying affective state.

The COL6A5-p.Glu2272* mutation induces chronic itch in mice

Pruritus is a common irritating sensation that provokes the desire to scratch. Environmental and genetic factors contribute to the onset of pruritus. Moreover, itch can become a major burden when it becomes chronic. Interestingly, the rare Collagen VI alpha 5 (COL6A5) gene variant p.Glu2272* has been identified in two families and an independent patient with chronic neuropathic itch. These patients showed reduced COL6A5 expression in skin and normal skin morphology. However, little progress has been made until now toward understanding the relationships between this mutation and chronic itch. Therefore, we developed the first mouse model that recapitulates COL6A5-p.Glu2272* mutation using the CRISPR-Cas technology and characterized this new mouse model. The mutant mRNA, measured by RT-ddPCR, was expressed at normal levels in dorsal root ganglia and was decreased in skin. The functional exploration showed effects of the mutation with some sex dysmorphology. Mutant mice had increased skin permeability. Elevated spontaneous scratching and grooming was detected in male and female mutants, with increased anxiety-like behavior in female mutants. These results suggest that the COL6A5-p.Glu2272* mutation found in patients contributes to chronic itch and induces in mice additional behavioral changes. The COL6A5-p.Glu2272* mouse model could elucidate the pathophysiological mechanisms underlying COL6A5 role in itch and help identify potential new therapeutic targets.

Social interactions and olfactory cues are required for contagious itch in mice

The phenomenon of contagious itch, observed in both humans and rodents, remains a topic of ongoing debate concerning its modulators and underlying pathways. This study delves into the relationship between contagious itch and familiar olfactory cues, a non-visual factor contributing to this intriguing behavior. Our findings showed that contagious itch in observer mice occurs during physical interaction with the cagemate itch-demonstrator but not with a stranger demonstrator or in a non-physical encounter condition. Notably, itch-experienced observer mice displayed an increased contagious itch behavior, highlighting the relevance of itch-associated memory in this phenomenon. Furthermore, anosmic observer mice, whether itch-naïve or itch-experienced, displayed no contagious itch behavior. These results demonstrate that the familiar olfactory cues, specifically cagemate body odors, are required for contagious itch behaviors in mice. In line with these behavioral findings, our study reveals increased activity in brain regions associated with olfaction, emotion, and memory during contagious itch, including the olfactory bulb, the amygdala, the hypothalamus, and the hippocampus, with this activity diminished in anosmic mice. In conclusion, our study unveils the critical role of familiar olfactory cues in driving contagious itch in mice, shedding light on the interplay between social factors, sensory perception, and memory in this phenomenon.

An insular cortical circuit required for itch sensation and aversion

Itch encompasses both sensory and emotional dimensions, with the two dimensions reciprocally exacerbating each other. However, whether a shared neural circuit mechanism governs both dimensions remains elusive. Here, we report that the anterior insular cortex (AIC) is activated by both histamine-dependent and -independent itch stimuli. The activation of AIC elicits aversive emotion and exacerbates pruritogen-induced itch sensation and aversion. Mechanistically, AIC excitatory neurons project to the GABAergic neurons in the dorsal bed nucleus of the stria terminalis (dBNST). Manipulating the activity of the AIC → dBNST pathway affects both itch sensation and itch-induced aversion. Our study discovers the shared neural circuit (AIC → dBNST pathway) underlying the itch sensation and aversion, highlights the critical role of the AIC as a central hub for the itch processing, and provides a framework to understand the neural mechanisms underlying the sensation and emotion interaction.

The functional role of the visual and olfactory modalities in the development of socially transferred mechanical hypersensitivity in male C57BL/6J mice

An increasing body of evidence suggests that the state of hyperalgesia could be socially transferred from one individual to another through a brief empathetic social contact. However, how the social transfer of pain develops during social contact is not well-known. Utilizing a well-established mouse model, the present study aims to study the functional role of visual and olfactory cues in the development of socially-transferred mechanical hypersensitivity. Behavioral tests demonstrated that one hour of brief social contact with a conspecific mouse injected with complete Freund's adjuvant (CFA) was both sufficient and necessary for developing socially-transferred mechanical hypersensitivity. One hour of social contact with visual deprivation could not prevent the development of socially-transferred mechanical hypersensitivity, and screen observation of a CFA cagemate was not sufficient to develop socially-transferred mechanical hypersensitivity in bystanders. Methimazole-induced olfactory deprivation, a compound with reversible toxicity on the nasal olfactory epithelium, was sufficient to prevent the development of socially-transferred mechanical hypersensitivity. Intriguingly, repeated but not acute olfactory exposure to the CFA mouse bedding induced a robust decrease in 50 % paw withdrawal thresholds (50 %PWTs) to mechanical stimuli, an effect returned to the baseline level after two days of washout with clean bedding. The findings strongly indicate that the normal olfactory function is crucial for the induction of mechanical hypersensitivity through brief empathetic contact, offering valuable insights for animal housing in future pain research.

IL-6 Enhances the Activation of PI3K-AKT/mTOR-GSK-3β by Upregulating GRPR in Hippocampal Neurons of Autistic Mice

Autism spectrum disorder (ASD) is a neurological disorder associated with brain inflammation. The underlying mechanisms could be attributed to the activation of PI3K signaling in the inflamed brain of ASD. Multiple studies highlight the role of GRPR in regulating ASD like abnormal behavior and enhancing the PI3K signaling. However, the molecular mechanism by which GRPR regulates PI3K signaling in neurons of individuals with ASD is still unclear. In this study, we utilized a maternal immune activation model to investigate the effects of GRPR on PI3K signaling in the inflamed brain of ASD mice. We used HT22 cells with and without GRPR to examine the impact of GRP-GRPR on the PI3K-AKT pathway with IL-6 treatment. We analyzed a dataset of hippocampus samples from ASD mice to identify hub genes. Our results demonstrated increased expression of IL-6, GRPR, and PI3K-AKT signaling in the hippocampus of ASD mice. Additionally, we observed increased GRPR expression and PI3K-AKT/mTOR activation in HT22 cells after IL-6 treatment, but decreased expression in HT22 cells with GRPR knockdown. NetworkAnalyst identified GSK-3β as the most crucial gene in the PI3K-AKT/mTOR pathway in the hippocampus of ASD. Furthermore, we found that IL-6 upregulated the expression of GSK-3β in HT22 cells by upregulating GRP-GRPR. Our findings suggest that IL-6 can enhance the activation of PI3K-AKT/mTOR-GSK-3β in hippocampal neurons of ASD mice by upregulating GRPR.

Role of stress in skin diseases: A neuroendocrine-immune interaction view

Psychological stress is a crucial factor in the development of many skin diseases, and the stigma caused by skin disorders may further increase the psychological burden, forming a vicious cycle of psychological stress leading to skin diseases. Therefore, understanding the relationship between stress and skin diseases is necessary. The skin, as the vital interface with the external environment, possesses its own complex immune system, and the neuroendocrine system plays a central role in the stress response of the body. Stress-induced alterations in the immune system can also disrupt the delicate balance of immune cells and inflammatory mediators in the skin, leading to immune dysregulation and increased susceptibility to various skin diseases. Stress can also affect the skin barrier function, impair wound healing, and promote the release of pro-inflammatory cytokines, thereby exacerbating existing skin diseases such as psoriasis, atopic dermatitis, acne, and urticaria. In the present review, we explored the intricate relationship between stress and skin diseases from a neuroendocrine-immune interaction perspective. We explored the occurrence and development of skin diseases in the context of stress, the stress models for skin diseases, the impact of stress on skin function and diseases, and relevant epidemiological studies and clinical trials. Understanding the relationship between stress and skin diseases from a neuroendocrine-immune interaction perspective provides a comprehensive framework for targeted interventions and new insights into the diagnosis and treatment of skin diseases.

Visuotactile integration facilitates mirror-induced self-directed behavior through activation of hippocampal neuronal ensembles in mice

Remembering the visual features of oneself is critical for self-recognition. However, the neural mechanisms of how the visual self-image is developed remain unknown because of the limited availability of behavioral paradigms in experimental animals. Here, we demonstrate a mirror-induced self-directed behavior (MSB) in mice, resembling visual self-recognition. Mice displayed increased mark-directed grooming to remove ink placed on their heads when an ink-induced visual-tactile stimulus contingency occurred. MSB required mirror habituation and social experience. The chemogenetic inhibition of dorsal or ventral hippocampal CA1 (vCA1) neurons attenuated MSB. Especially, a subset of vCA1 neurons activated during the mirror exposure was significantly reactivated during re-exposure to the mirror and was necessary for MSB. The self-responding vCA1 neurons were also reactivated when mice were exposed to a conspecific of the same strain. These results suggest that visual self-image may be developed through social experience and mirror habituation and stored in a subset of vCA1 neurons.

Xanthotoxol relieves itch in mice via suppressing spinal GRP/GRPR signaling

Although pruritus, commonly known as itch, is a common and debilitating symptom associated with various skin conditions, there is a lack of effective therapies available. Xanthotoxol (XAN), a biologically active linear furocoumarin, shows potential in the treatment of various neurological disorders. In this study, we discovered that administering XAN either through intraperitoneal or intrathecal injections effectively reduced scratching behavior induced by compound 48/80 or chloroquine. Importantly, XAN also substantially alleviates chronic itch in dry skin and allergic contact dermatitis mice. Substantial progress has highlighted the crucial role of gastrin-releasing peptide (GRP)-gastrin-releasing peptide receptor (GRPR) signaling in the dorsal spinal cord in transmitting various types of itch. Our behavior tests revealed that XAN significantly alleviated scratching behaviors induced by intrathecal administration of GRP or GRPR agonist bombesin. Furthermore, XAN reduced the activation of neurons in the spinal cord caused by intrathecal administration of GRP in mice. Moreover, XAN attenuates the activation of spinal GRPR-positive neurons in itchy mice. These findings suggest that XAN mitigates itch in mice by suppressing spinal GRP/GRPR signaling, thereby establishing XAN as a promising therapeutic option for treating pruritus.

Chemokine receptor CXCR2 in primary sensory neurons of trigeminal ganglion mediates orofacial itch

The CXCR2 chemokine receptor is known to have a significant impact on the initiation and control of inflammatory processes. However, its specific involvement in the sensation of itch is not yet fully understood. In this study, we aimed to elucidate the function of CXCR2 in the trigeminal ganglion (TG) by utilizing orofacial itch models induced by incision, chloroquine (CQ), and histamine. Our results revealed a significant up-regulation of CXCR2 mRNA and protein expressions in the primary sensory neurons of TG in response to itch stimuli. The CXCR2 inhibitor SB225002 resulted in notable decrease in CXCR2 protein expression and reduction in scratch behaviors. Distal infraorbital nerve (DION) microinjection of a specific shRNA virus inhibited CXCR2 expression in TG neurons and reversed itch behaviors. Additionally, the administration of the PI3K inhibitor LY294002 resulted in a decrease in the expressions of p-Akt, Akt, and CXCR2 in TG neurons, thereby mitigating pruritic behaviors. Collectively, we report that CXCR2 in the primary sensory neurons of trigeminal ganglion contributes to orofacial itch through the PI3K/Akt signaling pathway. These observations highlight the potential of molecules involved in the regulation of CXCR2 as viable therapeutic targets for the treatment of itch.

Role of the GRP/GRPR System in Regulating Brain Functions

Re-examining the relationship between neuropeptide systems and neural circuits will help us to understand more intensively the critical role of neuropeptides in brain function as the neural circuits responsible for specific brain functions are gradually revealed. Gastrin-releasing peptide receptors (GRPRs) are Gαq-coupling neuropeptide receptors and widely distributed in the brain, including hippocampus, amygdala, hypothalamus, nucleus tractus solitarius (NTS), suprachiasmatic nucleus (SCN), paraventricular nucleus of the hypothalamus (PVN), preoptic area of the hypothalamus (POA), preBötzinger complex (preBötC), etc., implying the GRP/GRPR system is involved in modulating multiple brain functions. In this review, we focus on the functionality of GRPR neurons and the regulatory role of the GRP/GRPR system in memory and cognition, fear, depression and anxiety, circadian rhythms, contagious itch, gastric acid secretion, food intake, body temperature, and sighing behavior. It can be found that GRPR is usually centered on a certain brain nucleus or anatomical structure and modulates richer or more specific behaviors by connecting with additional different nuclei. In order to explain the regulatory mechanism of the GRP/GRPR system, more precise intervention methods are needed.

Pushing the Frontiers: Optogenetics for Illuminating the Neural Pathophysiology of Bipolar Disorder

Bipolar disorder (BD), a disabling mental disorder, is featured by the oscillation between episodes of depression and mania, along with disturbance in the biological rhythms. It is on an urgent demand to identify the intricate mechanisms of BD pathophysiology. Based on the continuous progression of neural science techniques, the dysfunction of circuits in the central nervous system was currently thought to be tightly associated with BD development. Yet, challenge exists since it depends on techniques that can manipulate spatiotemporal dynamics of neuron activity. Notably, the emergence of optogenetics has empowered researchers with precise timing and local manipulation, providing a possible approach for deciphering the pathological underpinnings of mental disorders. Although the application of optogenetics in BD research remains preliminary due to the scarcity of valid animal models, this technique will advance the psychiatric research at neural circuit level. In this review, we summarized the crucial aberrant brain activity and function pertaining to emotion and rhythm abnormities, thereby elucidating the underlying neural substrates of BD, and highlighted the importance of optogenetics in the pursuit of BD research.

Effects of NALCN-Encoded Na+ Leak Currents on the Repetitive Firing Properties of SCN Neurons Depend on K+-Driven Rhythmic Changes in Input Resistance

Neurons in the suprachiasmatic nucleus (SCN) generate circadian changes in the rates of spontaneous action potential firing that regulate and synchronize daily rhythms in physiology and behavior. Considerable evidence suggests that daily rhythms in the repetitive firing rates (higher during the day than at night) of SCN neurons are mediated by changes in subthreshold potassium (K+) conductance(s). An alternative "bicycle" model for circadian regulation of membrane excitability in clock neurons, however, suggests that an increase in NALCN-encoded sodium (Na+) leak conductance underlies daytime increases in firing rates. The experiments reported here explored the role of Na+ leak currents in regulating daytime and nighttime repetitive firing rates in identified adult male and female mouse SCN neurons: vasoactive intestinal peptide-expressing (VIP+), neuromedin S-expressing (NMS+) and gastrin-releasing peptide-expressing (GRP+) cells. Whole-cell recordings from VIP+, NMS+, and GRP+ neurons in acute SCN slices revealed that Na+ leak current amplitudes/densities are similar during the day and at night, but have a larger impact on membrane potentials in daytime neurons. Additional experiments, using an in vivo conditional knockout approach, demonstrated that NALCN-encoded Na+ currents selectively regulate daytime repetitive firing rates of adult SCN neurons. Dynamic clamp-mediated manipulation revealed that the effects of NALCN-encoded Na+ currents on the repetitive firing rates of SCN neurons depend on K+ current-driven changes in input resistances. Together, these findings demonstrate that NALCN-encoded Na+ leak channels contribute to regulating daily rhythms in the excitability of SCN neurons by a mechanism that depends on K+ current-mediated rhythmic changes in intrinsic membrane properties.SIGNIFICANCE STATEMENT Elucidating the ionic mechanisms responsible for generating daily rhythms in the rates of spontaneous action potential firing of neurons in the suprachiasmatic nucleus (SCN), the master circadian pacemaker in mammals, is an important step toward understanding how the molecular clock controls circadian rhythms in physiology and behavior. While numerous studies have focused on identifying subthreshold K+ channel(s) that mediate day-night changes in the firing rates of SCN neurons, a role for Na+ leak currents has also been suggested. The results of the experiments presented here demonstrate that NALCN-encoded Na+ leak currents differentially modulate daily rhythms in the daytime/nighttime repetitive firing rates of SCN neurons as a consequence of rhythmic changes in subthreshold K+ currents.

How video calls affect mimicry and trust during interactions

Many social species, humans included, mimic emotional expressions, with important consequences for social bonding. Although humans increasingly interact via video calls, little is known about the effect of these online interactions on the mimicry of scratching and yawning, and their linkage with trust. The current study investigated whether mimicry and trust are affected by these new communication media. Using participant-confederate dyads (n = 27), we tested the mimicry of four behaviours across three different conditions: watching a pre-recorded video, online video call, and face-to-face. We measured mimicry of target behaviours frequently observed in emotional situations, yawn and scratch and control behaviours, lip-bite and face-touch. In addition, trust in the confederate was assessed via a trust game. Our study revealed that (i) mimicry and trust did not differ between face-to-face and video calls, but were significantly lower in the pre-recorded condition; and (ii) target behaviours were significantly more mimicked than the control behaviours. This negative relationship can possibly be explained by the negative connotation usually associated with the behaviours included in this study. Overall, this study showed that video calls might provide enough interaction cues for mimicry to occur in our student population and during interactions between strangers. This article is part of a discussion meeting issue 'Face2face: advancing the science of social interaction'.

A carotid body-brainstem neural circuit mediates sighing in hypoxia

Increased ventilation is a critical process that occurs when the body responds to a hypoxic environment. Sighs are long, deep breaths that prevent alveolar collapse, and their frequency is significantly increased by hypoxia. In this study, we first show that sighing is induced by hypoxia as a function of increased hypoxic severity and that hypoxia-induced sighing is capable of increasing the oxygen saturation in a mouse model. We next found that the gastrin-releasing peptide (Grp) expressing neurons in the nucleus of the solitary tract (NTS) are important in mediating hypoxia-induced sighing. Retrograde tracing from these Grp neurons reveals their direct afferent input from the petrosal ganglion neurons that innervate the carotid body, the major peripheral chemoreceptor that senses blood oxygen. Acute hypoxia preferentially activates these Grp neurons in the NTS. Photoactivation of these neurons through their projections in the inspiratory rhythm generator in the ventral medulla induces sighing, whereas genetic ablation or chemogenetic silencing of these neurons specifically diminishes the sighs, but not other respiratory responses, induced by hypoxia. Finally, the mice with reduced sighing in hypoxia exhibit an elevated heart-rate increase, which may compensate for maintaining the blood oxygen level. Therefore, we identified a neural circuit that connects the carotid body to the breathing control center in the ventral medulla with a specific function for hypoxia-induced sighing, which restores the oxygen level.

Structures of human gastrin-releasing peptide receptors bound to antagonist and agonist for cancer and itch therapy

Gastrin releasing peptide receptor (GRPR), a member of the bombesin (BBN) G protein-coupled receptors, is aberrantly overexpressed in several malignant tumors, including those of the breast, prostate, pancreas, lung, and central nervous system. Additionally, it also mediates non-histaminergic itch and pathological itch conditions in mice. Thus, GRPR could be an attractive target for cancer and itch therapy. Here, we report the inactive state crystal structure of human GRPR in complex with the non-peptide antagonist PD176252, as well as two active state cryo-electron microscopy (cryo-EM) structures of GRPR bound to the endogenous peptide agonist gastrin-releasing peptide and the synthetic BBN analog [D-Phe⁶, β-Ala¹¹, Phe¹³, Nle¹⁴] Bn (6-14), in complex with G_q heterotrimers. These structures revealed the molecular mechanisms for the ligand binding, receptor activation, and G_q proteins signaling of GRPR, which are expected to accelerate the structure-based design of GRPR antagonists and agonists for the treatments of cancer and pruritus.

The prelimbic cortex regulates itch processing by controlling attentional bias

Itch is a complex and unpleasant sensory experience. Recent studies have begun to investigate the neural mechanisms underlying the modulation of sensory and emotional components of itch in the brain. However, the key brain regions and neural mechanism involved in modulating the attentional processing of itch remain elusive. Here, we showed that the prelimbic cortex (PrL) is associated with itch processing and that the manipulation of itch-responsive neurons in the PrL significantly disrupted itch-induced scratching. Interestingly, we found that increasing attentional bias toward a distracting stimulus could disturb itch processing. We also demonstrated the existence of a population of attention-related neurons in the PrL that drive attentional bias to regulate itch processing. Importantly, itch-responsive neurons and attention-related neurons significantly overlapped in the PrL and were mutually interchangeable in the regulation of itch processing at the cellular activity level. Our results revealed that the PrL regulates itch processing by controlling attentional bias.

Tickle contagion in the rat somatosensory cortex

The cellular mechanisms of emotional contagion are unknown. We investigated tickle contagion and the underlying neuronal representations in playful rats. We recorded trunk somatosensory cortex activity of observer rats while they received tickling and audiovisual playback of tickling footage and while they witnessed tickling of demonstrator rats. Observers vocalized and showed "Freudensprünge" ("joy jumps") during witnessing live tickling, while they showed little behavioral responses to playbacks. Deep layers in the trunk somatosensory neurons showed a larger correlation between direct and witnessed tickling responses compared to superficial layers. Trunk somatosensory neurons discharged upon emission of own and demonstrator's vocalizations and might drive contagious "laughter". We conclude that trunk somatosensory cortex might represent ticklishness contagion.

Imitative and contagious behaviors in animals and their potential roles in the study of neurodevelopmental disorders

Social learning in the forms of imitative and contagious behaviors are essential for learning abilities and social interaction. However, children with neurodevelopmental disorders and intellectual disabilities show impairments in these behaviors, which profoundly affect their communication skills and cognitive functions. Although these deficits are well studied in humans, pre-clinical animal model assessments of imitative and contagious behavioral deficits are limited. Here, we first define various forms of social learning as well as their developmental and evolutionary significance in humans. We also explore the impact of imitative and contagious behavioral deficits in several neurodevelopmental disorders associated with autistic-like symptoms. Second, we highlight imitative and contagious behaviors observed in nonhuman primates and other social animals commonly used as models for neurodevelopmental disorders. Lastly, we conceptualize these behaviors in the contexts of mirror neuron activity, learning, and empathy, which are highly debated topics. Taken together, this review furthers the understanding of imitative and contagious behaviors. We hope to prompt and guide future behavioral studies in animal models of neurodevelopmental disorders.

Protocols for the social transfer of pain and analgesia in mice

We provide protocols for the social transfer of pain and analgesia in mice. We describe the steps to induce pain or analgesia (pain relief) in bystander mice with a 1-h social interaction with a partner injected with CFA (complete Freund's adjuvant) or CFA and morphine, respectively. We detail behavioral tests to assess pain or analgesia in the untreated bystander mice. This protocol has been validated in mice and rats and can be used for investigating mechanisms of empathy. For complete details on the use and execution of this protocol, please refer to Smith et al. (2021).

A non-canonical retina-ipRGCs-SCN-PVT visual pathway for mediating contagious itch behavior

Contagious itch behavior informs conspecifics of adverse environment and is crucial for the survival of social animals. Gastrin-releasing peptide (GRP) and its receptor (GRPR) in the suprachiasmatic nucleus (SCN) of the hypothalamus mediates contagious itch behavior in mice. Here, we show that intrinsically photosensitive retina ganglion cells (ipRGCs) convey visual itch information, independently of melanopsin, from the retina to GRP neurons via PACAP-PAC1R signaling. Moreover, GRPR neurons relay itch information to the paraventricular nucleus of the thalamus (PVT). Surprisingly, neither the visual cortex nor superior colliculus is involved in contagious itch. In vivo calcium imaging and extracellular recordings reveal contagious itch-specific neural dynamics of GRPR neurons. Thus, we propose that the retina-ipRGC-SCN-PVT pathway constitutes a previously unknown visual pathway that probably evolved for motion vision that encodes salient environmental cues and enables animals to imitate behaviors of conspecifics as an anticipatory mechanism to cope with adverse conditions.

It has been shown that GRP-GRPR neuropeptide signaling in the SCN is important for contagious itch behavior in mice. Gao et al. find that SCN-projecting ipRGCs are sufficient to relay itch information from the retina to the SCN by releasing neuropeptide PACAP to activate the GRP-GRPR pathway.

Inhibitions and Down-Regulation of Motor Protein Eg5 Expression in Primary Sensory Neurons Reveal a Novel Therapeutic Target for Pathological Pain

The motor protein Eg5, known as kif11 or kinesin-5, interacts with adjacent microtubules in the mitotic spindle and plays essential roles in cell division, yet the function of Eg5 in mature postmitotic neurons remains largely unknown. In this study, we investigated the contribution and molecular mechanism of Eg5 in pathological pain. Pharmacological inhibition of Eg5 and a specific shRNA-expressing viral vector reversed complete Freund's adjuvant (CFA)-induced pain and abrogated vanilloid receptor subtype 1 (VR1) expression in dorsal root ganglion (DRG) neurons. In the dorsal root, Eg5 inhibition promoted VR1 axonal transport and decreased VR1 expression. In the spinal cord, Eg5 inhibition suppressed VR1 expression in axon terminals and impaired synapse formation in superficial laminae I/II. Finally, we showed that Eg5 is necessary for PI3K/Akt signalling-mediated VR1 membrane trafficking and pathological pain. The present study provides compelling evidence of a noncanonical function of Eg5 in primary sensory neurons. These results suggest that Eg5 may be a potential therapeutic target for intractable pain.

No Evidence for Contagious Yawning in Juvenile Ravens (Corvus corax): An Observational Study

The overt and reflexive matching of behaviors among conspecifics has been observed in a growing number of social vertebrates, including avian species. In general, behavioral contagion-such as the spread of yawning-may serve important functions in group synchronization and vigilance behavior. Here, we performed an exploratory study to investigate yawn contagion among 10 captive juvenile ravens (Corvus corax), across two groups. Using observational methods, we also examined the contagiousness of three other distinct behaviors: stretching, scratching, and shaking. A total of 44 20 min observations were made across both groups, including 28 in the morning and 16 in the afternoon. The time and occurrence of all the behaviors from each bird were coded, and the temporal pattern of each behavior across both groups was then analyzed to assess the degree of social contagion. Overall, we found no evidence for contagious yawning, stretching, scratching, or shaking. However, yawns were relatively infrequent per observation (0.052 ± 0.076 yawns/bird) and thus experimental methods should be used to support this finding.

Glutamine Maintains Satellite Glial Cells Growth and Survival in Culture

Satellite glial cells (SGCs) tightly surround neurons and modulate sensory transmission in dorsal root ganglion (DRG). At present, the biological property of primary SGCs in culture deserves further investigation. To reveal the key factor for SGCs growth and survival, we examined the effects of different culture supplementations containing Dulbecco's Modified Eagle Medium (DMEM)/F12, DMEM high glucose (HG) or Neurobasal-A (NB). CCK-8 proliferation assay showed an increased proliferation of SGCs in DMEM/F12 and DMEM/HG, but not in NB medium. Bax, AnnexinV, and propidium iodide (PI) staining results showed that NB medium caused cell death and apoptosis. We showed that glutamine was over 2.5 mM in DMEM/F12 and DMEM/HG, whereas it was absence in NB medium. Interestingly, exogenous glutamine application significantly reversed the poor proliferation and cell death of SGCs in NB medium. These findings demonstrated that DMEM/F12 medium was optimal to get high-purity SGCs. Glutamine was the key molecule to maintain SGCs growth and survival in culture. Here, we provided a novel approach to get high-purity SGCs by changing the key component of culture medium. Our study shed a new light on understanding the biological property and modulation of glial cells of primary sensory ganglia.

Enhancement of behavioral nociceptive responses but not itching responses by viewing mirror images in adult mice

Can mice recognize themselves in a mirror? The answer is unclear. Previous studies have reported that adult mice - when shown itch-like videos - demonstrated itch empathy. However, this was proven to be unreproducible in other studies. In the present study, we wanted to examine whether adult mice were able to recognize their mirror image. In our testing, we found that mice spent more time in the central area in an open field with mirrors surrounding the chamber than those in a normal open field. In a similar open field test with four mice placed in four directions, mice showed similar behavioral responses to those with mirrors. These results indicate that mice are able to recognize images in the mirror, however, they cannot distinguish their own mirror images from the mirror images of other mice. To repeat the experiments of itch empathy, we compared the itch responses of mice in the mirrored environment, to those without. No significant difference in itching responses was detected. Differently, in the case of chemical pain (formalin injection), animals' nociceptive responses to formalin during Phase II were significantly enhanced in the mirrored open field. A new format of heat map was developed to help the analysis of the trace of mice in the open field. Our results suggest that mice do recognize the presence of mice in the mirror, and their nociceptive - but not itch - responses are enhanced.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

Behavioral mimicry of eating in mice

Nonconscious behavioral mimicry is prevalent in humans and highly social animals who imitate the behaviors of others without awareness. Previous studies indicated people tended to match their amount of eating to that of their eating companions due to behavioral mimicry, however whether such phenomena were existed in rodents is still unknown. Developing a behavioral paradigm would be helpful to better understand the molecular and circuit mechanisms underlying behavioral mimicry. In this study, we examined behavioral mimicry of eating in C57BL/6J mice which had strong sociality and were widely used as animal model in researches. We set up an eating monitoring platform, and this platform could record the data of mice eating behavior in real time. We further examined the behavioral mimicry of eating in mice, and found that the mice observing eating behavior of food-restricted mice would consume more food than control mice, and visual cue with eating behavior was sufficient to increased food consumption in mice. Our study indicated behavioral mimicry of eating was existed in mice and provided a mouse model for future studies on the mechanism of behavioral mimicry of eating.

Selective glycinergic input from vGluT3 amacrine cells confers a suppressed-by-contrast trigger feature in a subtype of M1 ipRGCs in the mouse retina

KEY POINTS

M1 intrinsically photosensitive retinal ganglion cells (ipRGCs) are known to encode absolute light intensity (irradiance) for non-image-forming visual functions (subconscious vision), such as circadian photoentrainment and the pupillary light reflex. It remains unclear how M1 cells respond to relative light intensity (contrast) and patterned visual signals. The present study identified a special form of contrast sensitivity (suppressed-by-contrast) in M1 cells, suggesting a role of patterned visual signals in regulating non-image-forming vision and a potential role of M1 ipRGCs in encoding image-forming visual cues. The study also uncovered a synaptic mechanism and a retinal circuit mediated by vesicular glutamate transporter 3 (vGluT3) amacrine cells that underlie the suppressed-by-contrast response of M1 cells. M1 ipRGC subtypes (M1a and M1b) were revealed that are distinguishable based on synaptic connectivity with vGluT3 amacrine cells, receptive field properties, intrinsic photo sensitivity and membrane excitability, and morphological features, suggesting a division of visual tasks among discrete M1 subpopulations.

ABSTRACT

The M1 type ipRGC (intrinsically photosensitive retinal ganglion cell) is known to encode ambient light signals for non-image-forming visual functions such as circadian photo-entrainment and the pupillary light reflex. Here, we report that a subpopulation of M1 cells (M1a) in the mouse retina possess the suppressed-by-contrast (sbc) trigger feature that is a receptive field property previously found only in ganglion cells mediating image-forming vision. Using optogenetics and the dual patch clamp technique, we found that vesicular glutamate transporter 3 (vGluT3) (vGluT3) amacrine cells make glycinergic, but not glutamatergic, synapses specifically onto M1a cells. The spatiotemporal and pharmacological properties of visually evoked responses of M1a cells closely matched the receptive field characteristics of vGluT3 cells, suggesting a major role of the vGluT3 amacrine cell input in shaping the sbc trigger feature of M1a cells. We found that the other subpopulation of M1 cells (M1b), which did not receive a direct vGluT3 cell input, lacked the sbc trigger feature, being distinctively different from M1a cells in intrinsic photo responses, membrane excitability, receptive-field characteristics and morphological features. Together, the results reveal a retinal circuit that uses the sbc trigger feature to regulate irradiance coding and potentially send image-forming cues to non-image-forming visual centres in the brain.

Bombesin-like peptide recruits disinhibitory cortical circuits and enhances fear memories

Disinhibitory neurons throughout the mammalian cortex are powerful enhancers of circuit excitability and plasticity. The differential expression of neuropeptide receptors in disinhibitory, inhibitory, and excitatory neurons suggests that each circuit motif may be controlled by distinct neuropeptidergic systems. Here, we reveal that a bombesin-like neuropeptide, gastrin-releasing peptide (GRP), recruits disinhibitory cortical microcircuits through selective targeting and activation of vasoactive intestinal peptide (VIP)-expressing cells. Using a genetically encoded GRP sensor, optogenetic anterograde stimulation, and trans-synaptic tracing, we reveal that GRP regulates VIP cells most likely via extrasynaptic diffusion from several local and long-range sources. In vivo photometry and CRISPR-Cas9-mediated knockout of the GRP receptor (GRPR) in auditory cortex indicate that VIP cells are strongly recruited by novel sounds and aversive shocks, and GRP-GRPR signaling enhances auditory fear memories. Our data establish peptidergic recruitment of selective disinhibitory cortical microcircuits as a mechanism to regulate fear memories.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Circuit Mechanisms of Itch in the Brain

Itch is an unpleasant somatic sensation with the desire to scratch, and it consists of sensory, affective, and motivational components. Acute itch serves as a critical protective mechanism because an itch-evoked scratching response will help to remove harmful substances invading the skin. Recently, exciting progress has been made in deciphering the mechanisms of itch at both the peripheral nervous system and the CNS levels. Key neuronal subtypes and circuits have been revealed for ascending transmission and the descending modulation of itch. In this review, we mainly summarize the current understanding of the central circuit mechanisms of itch in the brain.

New Frontiers in Psoriatic Disease Research, Part II: Comorbidities and Targeted Therapies

Although psoriasis and psoriatic arthritis (PsA) have been classically considered to be diseases of the skin and joints, respectively, emerging evidence suggests that a combination of innate and environmental factors creates widespread immune dysfunction, affecting multiple organ systems. A greater understanding of the pathogenesis of psoriasis and the systemic effects of psoriatic inflammation has allowed for the development of new, more effective treatments. The second portion of this two-part review series examines the comorbidities associated with psoriasis and PsA as well as the most recent advances in targeted systemic therapies for these conditions.

Early evidence for emotional play contagion in juvenile ravens

Perceiving, evaluating and reacting towards conspecifics' emotional states are important challenges of social group living. Emotional contagion describes an alignment of emotional states between individuals and is widely believed to be based on behavioral synchronization, i.e., behavioral contagion. As basic empathy-like processes, the occurrence of both forms of contagion seems to underlie early ontogenetic trajectories in humans and non-human species. In the present study, we assessed play as a context for studying the development of emotional contagion and its interlink with behavioral contagion in ten juvenile common ravens. Ravens are exceptional players that engage in all three forms of play: object, locomotion and social play. To assess potential ontogenetic patterns of both behavioral and emotional contagion, we tested juvenile ravens at two different periods of early development, at three- and six-month post-hatching. We elicited object play in one or several ravens (demonstrators) in a standardized experimental environment, using a playground setup. At both test ages, we found evidence for emotional contagion as observer ravens showed an increase of locomotion and social play after we provided the demonstrator(s) with the playground setup, but no significant changes in the amount of object play. Hence, observers did not copy motor patterns from demonstrator(s) but engaged in other forms of play. Our findings speak for a transfer of a general mood state in the context of play in ravens as young as 3 months and against behavioral mimicry as a precondition for emotional contagion.

Presynaptic inputs to vasopressin neurons in the hypothalamic supraoptic nucleus and paraventricular nucleus in mice

Arginine vasopressin (AVP) neurons in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus (PVN) are involved in important physiological behaviors, such as controling osmotic stability and thermoregulation. However, the presynaptic input patterns governing AVP neurons have remained poorly understood due to their heterogeneity, as well as intermingling of AVP neurons with other neurons both in the SON and PVN. In the present study, we employed a retrograde modified rabies-virus system to reveal the brain areas that provide specific inputs to AVP neurons in the SON and PVN. We found that AVP neurons of the SON and PVN received similar input patterns from multiple areas of the brain, particularly massive afferent inputs from the diencephalon and other brain regions of the limbic system; however, PVN^AVP neurons received relatively broader and denser inputs compared to SON^AVP neurons. Additionally, SON^AVP neurons received more projections from the median preoptic nucleus and organum vasculosum of the lamina terminalis (a circumventricular organ), compared to PVN^AVP neurons, while PVN^AVP neurons received more afferent inputs from the bed nucleus of stria terminalis and dorsomedial nucleus of the hypothalamus, both of which are thermoregulatory nuclei, compared to those of SON^AVP neurons. In addition, both SON^AVP and PVN^AVP neurons received direct afferent projections from the bilateral suprachiasmatic nucleus, which is the master regulator of circadian rhythms and is concomitantly responsible for fluctuations in AVP levels. Taken together, our present results provide a comprehensive understanding of the specific afferent framework of AVP neurons both in the SON and PVN, and lay the foundation for further dissecting the diverse roles of SON^AVP and PVN^AVP neurons.

Lack of Spinal Neuropeptide Y Is Involved in Mechanical Itch in Aged Mice

Neuropeptide Y (NPY) signaling plays an essential role in gating the pruritic afferent information in the spinal cord. Recent studies revealed that the aging process down-regulated the expression of NPY in the central nervous system. We propose that the lack of spinal NPY may be involved in certain types of pruritus in the elderly population. This study was designed to investigate the role of NPY in aging-induced itch using the senile mouse model. The expression of NPY in the spinal dorsal horn was compared between young (2 months old) and aged (24 months old) mice. Western blotting and immunohistochemistry showed that the expression of NPY was significantly reduced in the spinal dorsal horn in aged mice. In addition, a neuronal maker of apoptosis, TUNEL, was detected in the NPY positive neurons only in the aged spinal cord. Behavioral assay indicated that light mechanical stimulus evoked significantly more scratching in the aged than in the young mice, whereas chemical-evoked itch and pain-related behaviors were not altered. Intrathecal injection of either NPY or LP-NPY, a NPY receptor 1 (NPY1R) agonist, significantly alleviated the mechanically evoked itch in aged mice without altering the responses to chemical pruritogens. Our study suggested that downregulation of spinal NPY in the aged mice might play a role in the higher incidence of the mechanically evoked itch than that in the young mice. Therapies targeting the NPY system might serve as a potential strategy for alleviating the pruritic symptoms among the elderly population.

Chemogenetic activation of central gastrin-releasing peptide-expressing neurons elicits itch-related scratching behavior in male and female mice

Several lines of evidence have clarified that the key transmission pathways of itching sensation travel from the periphery to the central nervous system (CNS). Despite the functional significance of gastrin-releasing peptide (GRP) and its cognate receptor in the itch processing mechanism in the spinal dorsal horn (SDH), the roles of GRP-expressing (GRP+ ) neurons in different regions remain unclear. This study aimed to determine whether GRP+ neurons in the CNS directly modulated itch processing. To specifically activate spinal and supraspinal GRP neurons by the designer receptors exclusively activated by designer drugs (DREADDs) system, CAG-LSL-Gq-DREADD mice were crossed with GRP-Cre mice, resulting in the development of GRP-hM3Dq mice. Immunohistochemistry showed that hM3Dq was highly expressed in the SDH and brainstem closely related to sensory processing. The intraperitoneal, intrathecal, or intracerebroventricular administration of clozapine-N-oxide, an agonist of hM3Dq, strongly elicited dermatome-dependent itch-related scratching behavior, but did not change pain sensitivity. Importantly, GRP-Gq-DREADD-mediated scratching behavior in GRP-hM3Dq mice was not affected by the ablation of transient receptor potential vanilloid 1+ sensory C-fibers, and it was also observed to a similar degree under chronic itch conditions. Furthermore, there were no significant sex differences in the scratching behavior elicited by GRP-Gq-DREADD, suggesting that itch-dominant roles of central GRP+ neurons might be common in both sexes, at least under normal physiological conditions. These novel findings not only contribute to understanding the functional roles of central GRP+ neurons further, but also propose the development of future effective therapeutics for intractable itching.

dmPFC-vlPAG projection neurons contribute to pain threshold maintenance and antianxiety behaviors

The dorsal medial prefrontal cortex (dmPFC) has been recognized as a key cortical area for nociceptive modulation. However, the underlying neural pathway and the function of specific cell types remain largely unclear. Here, we show that lesions in the dmPFC induced an algesic and anxious state. Using multiple tracing methods including a rabies-based transsynaptic tracing method, we outlined an excitatory descending neural pathway from the dmPFC to the ventrolateral periaqueductal gray (vlPAG). Specific activation of the dmPFC/vlPAG neural pathway by optogenetic manipulation produced analgesic and antianxiety effects in a mouse model of chronic pain. Inhibitory neurons in the dmPFC were specifically activated using a chemogenetic approach, which logically produced an algesic and anxious state under both normal and chronic pain conditions. Antagonists of the GABAA receptor (GABAAR) or mGluR1 were applied to the dmPFC, which produced analgesic and antianxiety effects. In summary, the results of our study suggest that the dmPFC/vlPAG neural pathway might participate in the maintenance of pain thresholds and antianxiety behaviors under normal conditions, while silencing or suppressing the dmPFC/vlPAG pathway might be involved in the initial stages and maintenance of chronic pain and the emergence of anxiety-like behaviors.

Paraventricular thalamic nucleus plays a critical role in consolation and anxious behaviors of familiar observers exposed to surgery mice

Background: Consolation behaviors toward the sick are common in humans. Anxiety in the relatives of the sick is also common. Anxiety can cause detrimental effects on multiple systems. However, our understanding on the neural mechanisms of these behaviors is limited because of the lack of small animal models.

Methods: Five of 6- to 8-week-old CD-1 male mice were housed in a cage. Among them, 2 mice had right common artery exposure (surgery) and the rest were without surgery. Allo-grooming and performance in light and dark box and elevated plus maze tests of the mice were determined.

Results: Mice without surgery had increased allo-grooming toward mice with surgery but decreased allo-grooming toward non-surgery intruders. This increased allo-grooming toward surgery mice was higher in familiar observers of surgery mice than that of mice that were not cage-mates of surgery mice before the surgery. Familiar observers developed anxious behavior after being with surgery mice. Surgery mice with familiar observers had less anxious behavior than surgery mice without interacting with familiar observers. Multiple brain regions including paraventricular thalamic nucleus (PVT) were activated in familiar observers. The activated cells in PVT contained orexin receptors. Injuring the neurons with ibotenic acid, antagonizing orexin signaling with an anti-orexin antibody or inhibiting neurons by chemogenetic approach in PVT abolished the consolation and anxious behaviors of familiar observers.

Conclusions: Mice show consolation behavior toward the sick. This behavior attenuates the anxious behavior of surgery mice. The orexin signaling in the PVT neurons play a critical role in the consolation of familiar observers toward surgery mice and their anxious behavior. Considering that about 50 million patients have surgery annually in the United States, our study represents the initial attempt to understand neural mechanisms for consolation and anxiety of a large number of people.