Biology subjects

Prajapati, J. N.

Publications and source records attributed to Prajapati, J. N..

6 recordsLinked to original sources

bioRxiv2026

Regulation of itch-induced scratching by nucleus accumbens dopamine receptor-expressing neurons

Scratching provides transient relief from itch, yet the neural circuit mechanisms that transform scratching into itch relief remain poorly understood. Midbrain dopaminergic neurons and their downstream targets in the lateral shell of the nucleus accumbens (NAc LaSh) are implicated in itch-scratch processing. Previous studies show that pharmacological manipulation of dopamine D1 and D2 receptors in the NAc LaSh alters scratching behavior, but the specific contributions of D1R- and D2R-expressing neurons during acute and chronic itch remain unclear. Here, we show that NAc LaShD1R and D2R neurons bidirectionally regulate scratching behavior across itch states. NAc LaShD1R neurons activity promotes scratching bouts, whereas NAc LaShD2R neurons preferentially facilitate scratch termination. Anterograde viral tracing revealed distinct brain-wide projection patterns of NAc LaShD1R and D2R neurons, which we functionally tested using projection-specific optogenetic manipulations. We found that NAc LaShD2R neurons terminate scratching by inhibiting neurons in the lateral parabrachial nucleus (LPBN), a key hub for itch processing. Furthermore, dopamine levels in the NAc LaSh were elevated during chronic itch compared with acute itch, suggesting enhanced dopaminergic signaling contributes to persistent scratching. Together, these findings identify circuit mechanisms linking reward pathways to itch regulation.

neuroscience↗

bioRxiv2025

Scratcher: An automated machine-vision tool for dissecting the neural basis of itch

Itch or pruritus invokes a specific reflexive and repetitive directed nocifensive behavioural response, known as scratching. Recent decades have revealed neural circuits that are involved in the sensory and affective-motivational aspects of itch-induced scratching. However, most of these studies relied on manual subjective methods of quantifying scratching in laboratory mice and rats. Recent advances in deep learning have opened avenues for the development of computational tools to analyze animal behaviour in a reliable and automated manner. Further, combined with optogenetic and chemogenetic strategies, these tools can accelerate our understanding of neural circuits underlying itch and scratching. To that end, we have developed Scratcher, a GUI-based computational tool based on a real-time object detection algorithm that allows semi-supervised automated analysis of scratching behaviour in mice in a computationally inexpensive manner. We recorded chloroquine-induced acute itch as it developed and determined the consequence of nail-trimming on acute-itch induced scratching with Scratcher. To probe the neural mechanisms underlying itch, we combined Scratcher with genetic circuit dissection using the Fos-TRAP mouse line. By targeting itch-activated neurons in the lateral parabrachial nucleus (LPBN) - a key brainstem hub for pruritic signal transmission - we demonstrated that LPBN activity modulates itch-evoked scratching. Together, we present a novel, easy-to-use computational tool to dissect molecular, cellular, and circuit mechanisms of itch and scratching.

neuroscience↗

bioRxiv2025

Lateral hypothalamus directs stress-induced modulation of acute and psoriatic itch

Stress and anxiety are well-known modulators of both physiological and pathological itch. Acute stress suppresses itch, while chronic stress exacerbates it. These effects are mediated by neural circuits within the brain, though the precise mechanisms remain poorly understood. In this study, we investigate the role of neurons in the stress-sensitive lateral hypothalamic area (LHA) in modulating itch. Using neural activity-dependent genetic labeling and chemogenetic tools, we selectively engaged a population of LHA neurons (LHAstress-TRAP neurons) responsive to stress. Transient stimulation of these neurons induced anxiety-like behaviors, conditioned place aversion, and suppressed acute (chloroquine-induced) and chronic (psoriatic) itch. Conversely, the inhibition of the LHAstress-TRAP neurons enhanced acute and chronic itch. Interestingly, LHAstress-TRAP neurons did not respond to acute itch stimuli, but their activity was temporally correlated with scratching episodes in mice with psoriasis. Ex vivo whole-cell patch-clamp recordings revealed that these neurons exhibit heightened excitability in psoriatic animals. Anterograde viral tracing demonstrated that LHAstress-TRAP neurons project to brainstem regions implicated in itch modulation, including the periaqueductal gray (PAG), rostral ventromedial medulla (RVM), and lateral parabrachial nucleus (LPBN). Furthermore, chemogenetic activation and optogenetic silencing of LHAstress-TRAP axon terminals revealed that bidirectional modulation of itch is primarily mediated through projections to the PAG. Together, these findings identify a previously unrecognized central mechanism by which stress modulates itch, centered on a specific population of LHA neurons and their downstream brainstem targets.

neuroscience↗

bioRxiv2025

A brainstem-thalamic circuitry for affective-motivational responses to cold pain

The medial thalamus is crucial for the sensory and affective-motivational responses to chronic pain. However, a mechanistic understanding of how the distinct subnuclei of the medial thalamus mediate behavioral responses to pain remains lacking. Taking advantage of intersectional viral genetics, chemogenetics, optogenetics, in-vivo imaging, and ex-vivo physiology, we reveal that the neurons in the parafascicular (PF) nuclei of the medial thalamus receive monosynaptic inputs from the lateral parabrachial nuclei (LPBN) in mice. LPBN is an essential nucleus in the ascending pain pathway, receiving projections from the dorsal horn of the spinal cord. The PF neurons downstream of LPBN (PFpost-LPBN) are nociceptive, sensitized by peripheral neuropathy, acutely aversive, and, when activated, drive both sensory and affective-motivational responses to cold pain. In contrast, the LPBN target neurons in the intralaminar centromedian thalamus (CMpost-LPBN), another nociceptive nucleus of the medial thalamus, are primarily involved in the affective-motivational aspects of pain. Together, we reveal that the LPBN, through two closely related thalamic nuclei, influences behavior in mice with cold hypersensitivity due to peripheral neuropathies.

neuroscience↗

bioRxiv2024

An Intra-brainstem Circuit for Pain-induced Inhibition of Itch

Pain and itch are unpleasant and distinct sensations that give rise to behaviors such as reflexive withdrawal and scratching in humans and mice. Interestingly, it has been observed that pain modulate itch through the neural circuits housed in the brain and spinal cord. However, we are yet to fully understand the identities of and mechanisms by which specific neural circuits mediate pain-induced modulation of itch. Independent studies indicate that brainstem nuclei such as the lateral parabrachial nucleus (LPBN) and rostral ventromedial medulla (RVM) are important for the suppression of itch by painful stimuli. Here, using mouse and viral genetics, rabies tracing, chemogenetics, and calcium imaging, we show that the synaptic connections between LPBN and RVM plays an instrumental role in the interactions between pain and itch. Notably, we found that the LPBN neurons that express the gene encoding the substance P receptor, Tacr1 (LPBNTacr1), synapse onto Tacr1-expressing RVM neurons (RVMTacr1). The RVMTacr1 neurons were found to be nociceptive, sufficient for inhibiting itch, and necessary for pain-induced itch suppression. Moreover, through brain-wide anterograde and retrograde viral tracing studies, we found that the RVMTacr1 neurons are bidirectionally connected with LPBN, periaqueductal gray (PAG), and lateral hypothalamic area (LHA). Thus, together, our data indicate that the RVMTacr1 neurons integrate nociceptive information to mediate itch-induced scratching and can mediate the physiological effects of itch through their downstream targets.

neuroscience↗

bioRxiv2023

A deep-learning driven investigation of circuit basis for reflexive hypersensitivity to pain

Objectively measuring animal behavior is key to understanding the neural circuits underlying pain. Recent progress in machine vision has presented us with unprecedented scope in behavioral analysis. Here, we apply DeeplabCut (DLC) to dissect mouse behavior on the thermal-plate test -- a commonly used paradigm to ascertain supraspinal contributions to noxious thermal sensation and pain hypersensitivity. We determine the signature characteristics of the pattern of mouse movement and posture in 3D in response to a range of temperatures from innocuous to noxious on the thermal-plate test. Next, we test how acute chemical and chronic inflammatory injuries sensitize mouse behaviors. Repeated exposure to noxious temperatures on the thermal-plate can induce learning, and in this study, we design a novel assay and formulate an analytical pipeline that will facilitate the dissection of plasticity mechanisms in pain circuits in the brain. Last, we record and test how activating Tacr1 expressing PBN neurons -- a population responsive to sustained noxious stimuli-affects mouse behavior on the thermal plate test. Taken together, we demonstrate that by tracking a single body part of a mouse, we can reveal the behavioral signatures of mice exposed to noxious surface temperatures, report the alterations of the same when injured, and determine if a molecularly and anatomically defined pain responsive circuit plays a role in the reflexive hypersensitivity to thermal pain.

neuroscience↗