Trigeminal neuropathic pain causes changes in affective processing of pain in rats

Strategies for measuring non-evoked pain in preclinical models of neuropathic pain: Systematic review

The development of new analgesics for neuropathic pain treatment is crucial. The failure of promising drugs in clinical trials may be related to the over-reliance on reflex-based responses (evoked pain) in preclinical drug testing, which may not fully represent clinical neuropathic pain, characterized by spontaneous non-evoked pain (NEP). Hence, strategies for assessing NEP in preclinical studies emerged. This systematic review identified 443 articles evaluating NEP in neuropathic pain models (mainly traumatic nerve injuries in male rodents). An exponential growth in NEP evaluation was observed, which was assessed using 48 different tests classified in 12 NEP-related outcomes: anxiety, exploration/locomotion, paw lifting, depression, conditioned place preference, gait, autotomy, wellbeing, facial grooming, cognitive impairment, facial pain expressions and vocalizations. Although most of these outcomes showed clear limitations, our analysis suggests that conditioning-associated outcomes, pain-related comorbidities, and gait evaluation may be the most effective strategies. Moreover, a minimal part of the studies evaluated standard analgesics. The greater emphasis on evaluating NEP aligning with clinical pain symptoms may enhance analgesic drug development, improving clinical translation.

Deciphering the functional role of insular cortex stratification in trigeminal neuropathic pain

Trigeminal neuropathic pain (TNP) is a major concern in both dentistry and medicine. The progression from normal to chronic TNP through activation of the insular cortex (IC) is thought to involve several neuroplastic changes in multiple brain regions, resulting in distorted pain perception and associated comorbidities. While the functional changes in the insula are recognized contributors to TNP, the intricate mechanisms underlying the involvement of the insula in TNP processing remain subjects of ongoing investigation. Here, we have overviewed the most recent advancements regarding the functional role of IC in regulating TNP alongside insights into the IC's connectivity with other brain regions implicated in trigeminal pain pathways. In addition, the review examines diverse modulation strategies that target the different parts of the IC, thereby suggesting novel diagnostic and therapeutic management of chronic TNP in the future.

Sex differences in carbamazepine effects in a rat model of trigeminal neuropathic pain

Carbamazepine (CBZ) represents the first-line treatment for trigeminal neuralgia, a condition of facial pain that affects mainly women. The chronic constriction of the infraorbital nerve (CCI-ION) is a widely used model to study this condition, but most studies do not include females. Thus, this study aimed to characterize sensory and affective changes in female rats after CCI-ION and compare the effect of CBZ in both sexes. Mechanical allodynia was assessed 15 days after CCI-ION surgery in rats treated with CBZ (10 and 30 mg/kg, i.p.) or vehicle, together with the open-field test. Independent groups were tested on the Conditioned Place Preference (CPP) paradigm and ultrasonic vocalization (USV) analysis. Blood samples were collected for dosage of the main CBZ metabolite. CBZ at 30 mg/kg impaired locomotion of CCI-ION male and sham and CCI-ION female rats and resulted in significantly higher plasma concentrations of 10-11-EPX-CBZ in the latter. Only male CCI-ION rats showed increased facial grooming which was significantly reduced by CBZ at 10 mg/kg. CBZ at 10 mg/kg significantly reduced mechanical allodynia and induced CPP only in female CCI-ION rats. Also, female CCI-ION showed reduced emission of appetitive USV but did not show anxiety-like behavior. In conclusion, male and female CCI-ION rats presented differences in the expression of the affective-motivational pain component and CBZ was more effective in females than males. Further studies using both sexes in trigeminal neuropathic pain models are warranted for a better understanding of potential differences in the pathophysiological mechanisms and efficacy of pharmacological treatments.

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.

Optogenetic Inhibition of Glutamatergic Neurons in the Dysgranular Posterior Insular Cortex Modulates Trigeminal Neuropathic Pain in CCI-ION Rat

In individuals with chronic neuropathic pain, the posterior insular cortex (PIC) has been found to exhibit increased glutamatergic activity, and the dysgranular portion of PIC (DPIC) has been investigated as a novel cortical target for pain modulation. However, the role of DPIC glutamatergic neurons (DPICg) in trigeminal neuropathic pain (TNP) remains unclear. Here, we examined the outcomes of DPICg inhibition in a rat model of chronic constriction injury of the infraorbital nerve (CCI-ION). Animals were randomly divided into TNP, sham, and control groups. TNP animals underwent CCI-ION surgery. Either optogenetic or null viruses were delivered to the contralateral DPICg of TNP and sham animals. In vivo single-unit extracellular recordings from the ipsilateral spinal trigeminal nucleus caudalis (TNC) and contralateral ventral posteromedial (VPM) thalamus were obtained under both "ON" and "OFF" stimulation states. Behavioral responses during the stimulation-OFF and stimulation-ON phases were examined. Expression of c-Fos, pERK, and CREB immunopositive neurons were also observed. Optogenetic inhibition of contralateral DPICg decreased the neural firing rate in both TNC and VPM thalamus, the expression of sensory-responsive cell bodies, and transcriptional factors in the DPIC of TNP group. Improvements in hyperalgesia, allodynia, and anxiety-like responses in TNP animals were also observed during stimulation-ON condition. In fine, descending pain processing is influenced by neuroanatomical projections from the DPIC to the pain matrix areas, and DPICg could play a necessary role in this neural circuitry. Therefore, the antinociceptive effect of DPICg inhibition in this study may provide evidence for the therapeutic potential of DPICg in TNP.

Cognitive Functions following Trigeminal Neuromodulation

Vast scientific effort in recent years have been focused on the search for effective and safe treatments for cognitive decline. In this regard, non-invasive neuromodulation has gained increasing attention for its reported effectiveness in promoting the recovery of multiple cognitive domains after central nervous system damage. In this short review, we discuss the available evidence supporting a possible cognitive effect of trigeminal nerve stimulation (TNS). In particular, we ask that, while TNS has been widely and successfully used in the treatment of various neuropsychiatric conditions, as far as research in the cognitive field is concerned, where does TNS stand? The trigeminal nerve is the largest cranial nerve, conveying the sensory information from the face to the trigeminal sensory nuclei, and from there to the thalamus and up to the somatosensory cortex. On these bases, a bottom-up mechanism has been proposed, positing that TNS-induced modulation of the brainstem noradrenergic system may affect the function of the brain networks involved in cognition. Nevertheless, despite the promising theories, to date, the use of TNS for cognitive empowering and/or cognitive decline treatment has several challenges ahead of it, mainly due to little uniformity of the stimulation protocols. However, as the field continues to grow, standardization of practice will allow for data comparisons across studies, leading to optimized protocols targeting specific brain circuitries, which may, in turn, influence cognition in a designed manner.

Introduction of gloved hand to cage induces 22-kHz ultrasonic vocalizations in male albino rats

Rodents emit ultrasonic vocalizations (USVs) above the human hearing threshold of ~ 20 kHz to communicate emotional states and to coordinate their social interactive behavior. Twenty-two kHz USVs emitted by adult rats have been reported in a variety of aversive social and behavioral situations. They occur not only under painful or restraining conditions but can also be evoked by gentle cutaneous touch or airflow. This study aimed to test if placement of a human hand in a cage can evoke 22-kHz USVs. It was found that 36% of the adult male Sprague-Dawley and 13% of the adult male Wistar Han rats emitted 22-kHz USVs when a gloved hand was introduced into the cages. Average vocalization onset latencies were 5.0 ± 4.4 s (Sprague-Dawley) and 7.4 ± 4.0 s (Wistar Han) and the USVs had a stable frequency (22 kHz) across the calls, ranging from 0.1 to 2.3 seconds in duration. Surprisingly, no 22-kHz USVs were found in any female Wistar Han rats tested. To further explore the mechanisms underlying this observation, we compared retinal function, basal serum corticosterone, and testosterone levels between the 22-kHz USV responders and non-responders. None of these parameters or endpoints showed any significant differences between the two cohorts. The results suggest that the introduction of a gloved-hand inside the cage can trigger adult male albino rats to emit 22-kHz ultrasonic vocalizations. This response should be considered in USV studies and animal welfare.