Activation of CamKIIα expressing neurons on ventrolateral periaqueductal gray improves behavioral hypersensitivity and thalamic discharge in a trigeminal neuralgia rat model

Optogenetic inhibition of ventrolateral orbitofrontal cortex astrocytes facilitates ventrolateral periaqueductal gray glutamatergic activity to reduce hypersensitivity in infraorbital nerve injury rat model

BACKGROUND

Trigeminal neuropathic pain (TNP) is a chronic condition characterized by heightened nociceptive responses and neuroinflammatory changes. While astrocytes are recognized as critical players in pain modulation, their specific role in influencing descending trigeminal pain pathways via ventrolateral orbitofrontal cortex (vlOFC) activity modulation remains underexplored. Therefore, we investigated the impact of optogenetic modulation of astrocytes in the vlOFC on pain hypersensitivity in a rat model of chronic constriction injury of the infraorbital nerve (CCI-ION).

METHOD

Adult female Sprague Dawley rats underwent ION constriction to mimic TNP symptoms, with naive and sham animals serving as controls. AAV8-GFAP-hChR2-mCherry, AAV8-GFAP-eNpHR3.0-mCherry, or AAV8-GFAP-mCherry were delivered to the vlOFC for in vivo optogenetic manipulation. Pain behaviors were assessed using acetone, von Frey, and elevated plus maze tests, while electrophysiological recordings from the ventrolateral periaqueductal gray (vlPAG) and ventral posteromedial (VPM) thalamus were obtained.

RESULTS

Orofacial hyperalgesia, reduced vlPAG activity, and thalamic hyperexcitability were associated with vlOFC astrocytic hyperactivity in the TNP animals. In contrast, optogenetic inhibition of vlOFC astrocytes restored vlOFC glutamatergic signaling, increased vlPAG glutamatergic neuronal activity, and reduced hyperactivity in the VPM thalamus. Behavioral assessments also revealed alleviation of hyperalgesia, allodynia, and anxiety-like behaviors during the stimulation-ON phase, alongside reduced neuroinflammatory markers, including P2 × 3 and Iba-1. However, astrocytic excitation and null virus controls did not alter TNP responses, underscoring the specificity of astrocytic inhibition.

CONCLUSION

These findings suggest that the astrocytic subpopulation in the vlOFC and its robust influence on vlPAG glutamatergic neurons play a crucial role in restoring descending pain processing pathways, potentially contributing to the development of novel therapeutic approaches for TNP management.

Potential hypothalamic mechanisms in trigeminal neuropathic pain: a comparative analysis with migraine and cluster headache

Trigeminal neuropathic pain (TNP), migraine, and cluster headache (CH) profoundly impact the quality of life and present significant clinical challenges due to their complex neurobiological underpinnings. This review delves into the pivotal role of the hypothalamus in the pathophysiology of these facial pain syndromes, highlighting its distinctive functions and potential as a primary target for research, diagnosis, and therapy. While the involvement of the hypothalamus in migraine and CH has been increasingly supported by imaging and clinical studies, the precise mechanisms of its role remain under active investigation. The role of the hypothalamus in TNP, in contrast, is less explored and represents a critical gap in our understanding. The hypothalamus's involvement varies significantly across these conditions, orchestrating a unique interplay of neural circuits and neurotransmitter systems that underlie the distinct characteristics of each pain type. We have explored advanced neuromodulation techniques, such as deep brain stimulation (DBS) and optogenetics, which show promise in targeting hypothalamic dysfunction to alleviate pain symptoms. Furthermore, we discuss the neuroplastic changes within the hypothalamus that contribute to the chronicity of these pains and the implications of these findings for developing targeted therapies. By offering a comprehensive examination of the hypothalamus's roles, this paper aims to bridge existing knowledge gaps and propel forward the understanding and management of facial neuralgias, underscoring the hypothalamus's critical position in future neurological research.

A neural circuit from thalamic paraventricular nucleus via zona incerta to periaqueductal gray for the facilitation of neuropathic pain

Top-down projections transmit a series of signals encoding pain sensation to the ventrolateral periaqueductal gray (vlPAG), where they converge with various incoming projections to regulate pain. Clarifying the upstream regulatory hierarchy of vlPAG can enhance our understanding of the neural circuitry involved in pain modulation. Here, we show that a in a mouse model of spared nerve injury (SNI), activation of a circuit arising from posterior paraventricular thalamic nucleus CaMKIIα-positive neurons (PVPCaMKIIα) projects to gamma-aminobutyric acid neurons in the rostral zona incerta (ZIrGABA) to facilitate the development of pain hypersensitivity behaviors. In turn, these ZIrGABA neurons project to CaMKIIα-positive neurons in the vlPAG (vlPAGCaMKIIα), a well-known neuronal population involved in pain descending modulation. In vivo calcium signal recording and whole-cell electrophysiological recordings reveal that the PVPCaMKIIα→ZIrGABA→vlPAGCaMKIIα circuit is activated in SNI models of persistent pain. Inhibition of this circuit using chemogenetics or optogenetics can alleviate the mechanical pain behaviors. Our study indicates that the PVPCaMKIIα→ZIrGABA→vlPAGCaMKIIα circuit is involved in the facilitation of neuropathic pain. This previously unrecognized circuit could be explored as a potential target for neuropathic pain treatment.

Ventral posteromedial nucleus of the thalamus gates the spread of trigeminal neuropathic pain

BACKGROUND

Widespread neuropathic pain usually affects a wide range of body areas and inflicts huge suffering on patients. However, little is known about how it happens and effective therapeutic interventions are lacking.

METHODS

Widespread neuropathic pain was induced by partial infraorbital nerve transection (p-IONX) and evaluated by measuring nociceptive thresholds. In vivo/vitro electrophysiology were used to evaluate neuronal activity. Virus tracing strategies, combined with optogenetics and chemogenetics, were used to clarify the role of remodeling circuit in widespread neuropathic pain.

RESULTS

We found that in mice receiving p-IONX, along with pain sensitization spreading from the orofacial area to distal body parts, glutamatergic neurons in the ventral posteromedial nucleus of the thalamus (VPMGlu) were hyperactive and more responsive to stimulations applied to the hind paw or tail. Tracing experiments revealed that a remodeling was induced by p-IONX in the afferent circuitry of VPMGlu, notably evidenced by more projections from glutamatergic neurons in the dorsal column nuclei (DCNGlu). Moreover, VPMGlu receiving afferents from the DCN extended projections further to glutamatergic neurons in the posterior insular cortex (pIC). Selective inhibition of the terminals of DCNGlu in the VPM, the soma of VPMGlu or the terminals of VPMGlu in the pIC all alleviated trigeminal and widespread neuropathic pain.

CONCLUSION

These results demonstrate that hyperactive VPMGlu recruit new afferents from the DCN and relay the extra-cephalic input to the pIC after p-IONX, thus hold a key position in trigeminal neuropathic pain and its spreading. This study provides novel insights into the circuit mechanism and preclinical evidence for potential therapeutic targets of widespread neuropathic pain.

Optogenetics in oral and craniofacial research

Optogenetics combines optics and genetic engineering to control specific gene expression and biological functions and has the advantages of precise spatiotemporal control, noninvasiveness, and high efficiency. Genetically modified photosensory sensors are engineered into proteins to modulate conformational changes with light stimulation. Therefore, optogenetic techniques can provide new insights into oral biological processes at different levels, ranging from the subcellular and cellular levels to neural circuits and behavioral models. Here, we introduce the origins of optogenetics and highlight the recent progress of optogenetic approaches in oral and craniofacial research, focusing on the ability to apply optogenetics to the study of basic scientific neural mechanisms and to establish different oral behavioral test models in vivo (orofacial movement, licking, eating, and drinking), such as channelrhodopsin (ChR), archaerhodopsin (Arch), and halorhodopsin from Natronomonas pharaonis (NpHR). We also review the synergic and antagonistic effects of optogenetics in preclinical studies of trigeminal neuralgia and maxillofacial cellulitis. In addition, optogenetic tools have been used to control the neurogenic differentiation of dental pulp stem cells in translational studies. Although the scope of optogenetic tools is increasing, there are limited large animal experiments and clinical studies in dental research. Potential future directions include exploring therapeutic strategies for addressing loss of taste in patients with coronavirus disease 2019 (COVID-19), studying oral bacterial biofilms, enhancing craniomaxillofacial and periodontal tissue regeneration, and elucidating the possible pathogenesis of dry sockets, xerostomia, and burning mouth syndrome.

Optogenetic Approach in Trigeminal Neuralgia and Potential Concerns: Preclinical Insights

The integration of optogenetics in the trigeminal pain circuitry broadens and reinforces existing pain investigations. Similar to research on spinal neuropathic pain, the exploration of the underlying determinants of orofacial pain is expanding. Optogenetics facilitates more direct, specific, and subtle investigations of the neuronal circuits involved in orofacial pain. One of the most significant concerns of both dentistry and medicine is trigeminal neuralgia (TN) management due to its substantial impact on a patient's quality of life. Our objective is to gather insights from preclinical studies conducted in TN employing an optogenetic paradigm, thereby extending the prospects for in-depth neurobiological research. This review highlights optogenetic research in trigeminal pain circuitry involving TN. We outline the central and peripheral regions associated with pain-that have been investigated using optogenetics in the trigeminal pain circuitry. The study further reports its scope and limitations as well as its potential for future applications from bench to bedside.

Progress in animal models of trigeminal neuralgia

OBJECTIVE

This review aims to systematically summarize the methods of establishing various models of trigeminal neuralgia (TN), the scope of application, and current animals used in TN research and the corresponding pain measurements, hoping to provide valuable reference for researchers to select appropriate TN animal models and make contributions to the research of pathophysiology and management of the disease.

DESIGN

The related literatures of TN were searched through PubMed database using different combinations of the following terms and keywords including but not limited: animal models, trigeminal neuralgia, orofacial neuropathic pain. To find the maximum number of eligible articles, no filters were used in the search. The references of eligible studies were analyzed and reviewed comprehensively.

RESULTS

This study summarized the current animal models of TN, categorized them into the following groups: chemical induction, photochemical induction, surgery and genetic engineering, and introduced various measurement methods to evaluate animal pain behaviors.

CONCLUSIONS

Although a variety of methods are used to establish disease models, there is no ideal TN model that can reflect all the characteristics of the disease. Therefore, there is still a need to develop more novel animal models in order to further study the etiology, pathological mechanism and potential treatment of TN.

Role of 5-HT2A receptor in modulating glutamatergic activity in the ventrolateral orbital cortex: implication in trigeminal neuralgia

5-HT_2A receptors (5-HT_2ARs) are widely expressed in the central nervous system, including in the ventrolateral orbital cortex (VLO). The VLO is an important cortical component for pain processing. Brain 5-HT_2ARs are implicated in both pro- and anti- nociceptive functions. However, the roles of 5-HT_2ARs in the VLO in trigeminal neuralgia and neuronal synaptic function remain to be understood. We used chronic constriction injury of infraorbital nerve (IoN-CCI) model and shRNA mediated gene knockdown in mice to investigate the role of 5-HT_2ARs in the VLO in trigeminal neuralgia. We found that knockdown of 5-HT_2ARs in the VLO aggravated spontaneous pain and mechanical allodynia in mice after IoN-CCI. At the synaptic level, decreasing 5-HT_2AR expression by shRNA or inhibition of 5-HT_2AR activity by its antagonist ketanserin decreased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs) of the neurons in the VLO, whereas 5-HT_2AR partial agonist 2,5-Dimethoxy-4-iodoamphetamine (DOI) enhanced sEPSCs of the neurons in the VLO. In summary, 5-HT_2ARs in the VLO modulate the trigeminal pain by regulating neuronal glutamatergic activity.

Pain Relief in a Trigeminal Neuralgia Model via Optogenetic Inhibition on Trigeminal Ganglion Itself With Flexible Optic Fiber Cannula

The trigeminal ganglion (TG) is the primary site of aberration in trigeminal neuralgia (TN), and hence a crucial site where afferent input can be modulated. Here, we postulated that inhibiting TG via optogenetics using flexible optic cannula would diminish brainstem trigeminal nucleus caudalis (TNC) neuronal activity and pain behavior in TN rat model. Infraorbital nerve constriction was employed to induce TN in female Sprague-Dawley rats, while naive and sham rats served as controls. TG-directed microinjections of AAV virus containing either the optogenetic or null vector were delivered to rats in each group. In vivo electrophysiological responses were obtained from the ventral posteromedial nucleus (VPm) of the thalamus with simultaneous TG optogenetic stimulation using flexible optic cannula as well the effects on behavioral responses were investigated. Recordings in TN rats revealed a decrease in burst firing activity during yellow laser driven inhibition on TG, as well as considerably improved behavioral responses. In contrast, we noticed persistent hypersensitivity and increased tonic firing with blue laser stimulation which indicates that TG inhibition can synchronize trigeminal pain signal transmission in a TN animal model. The potential of an optogenetic approach in TG itself with flexible optic fiber to directly disrupt the trigeminal pain circuitry delivers fundamental underpinnings toward its prospective as a trigeminal neuralgia management.