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

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 cognitive impact of light: illuminating ipRGC circuit mechanisms

Ever-present in our environments, light entrains circadian rhythms over long timescales, influencing daily activity patterns, health and performance. Increasing evidence indicates that light also acts independently of the circadian system to directly impact physiology and behaviour, including cognition. Exposure to light stimulates brain areas involved in cognition and appears to improve a broad range of cognitive functions. However, the extent of these effects and their mechanisms are unknown. Intrinsically photosensitive retinal ganglion cells (ipRGCs) have emerged as the primary conduit through which light impacts non-image-forming behaviours and are a prime candidate for mediating the direct effects of light on cognition. Here, we review the current state of understanding of these effects in humans and mice, and the tools available to uncover circuit-level and photoreceptor-specific mechanisms. We also address current barriers to progress in this area. Current and future efforts to unravel the circuits through which light influences cognitive functions may inform the tailoring of lighting landscapes to optimize health and cognitive function.

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.

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.

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.