Experimental trigeminal nerve injury

Alterations in the spinal cord, trigeminal nerve ganglion, and infraorbital nerve through inducing compression of the dorsal horn region at the upper cervical cord in trigeminal neuralgia

BACKGROUND

Idiopathic trigeminal neuralgia (TN) cases encountered frequently in daily practice indicate significant gaps that still need to be illuminated in the etiopathogenesis. In this study, a novel TN animal model was developed by compressing the dorsal horn (DH) of the upper cervical spinal cord.

METHODS

Eighteen rabbits were equally divided into three groups, namely control (CG), sham (SG), and spinal cord compression (SCC) groups. External pressure was applied to the left side at the C3 level in the SCC group. Dorsal hemilaminectomy was performed in the SG, and the operative side was closed without compression. No procedure was implemented in the control group. Samples from the SC, TG, and ION were taken after seven days. For the histochemical staining, damage and axons with myelin were scored using Hematoxylin and Eosin and Toluidine Blue, respectively. Immunohistochemistry, nuclei, apoptotic index, astrocyte activity, microglial labeling, and CD11b were evaluated.

RESULTS

Mechanical allodynia was observed on the ipsilateral side in the SCC group. In addition, both the TG and ION were partially damaged from SC compression, which resulted in significant histopathological changes and increased the expression of all markers in both the SG and SCC groups compared to that in the CG. There was a notable increase in tissue damage, an increase in the number of apoptotic nuclei, an increase in the apoptotic index, an indication of astrocytic gliosis, and an upsurge in microglial cells. Significant increases were noted in the SG group, whereas more pronounced significant increases were observed in the SCC group. Transmission electron microscopy revealed myelin damage, mitochondrial disruption, and increased anchoring particles. Similar changes were observed to a lesser extent in the contralateral spinal cord.

CONCLUSION

Ipsilateral trigeminal neuropathic pain was developed due to upper cervical SCC. The clinical finding is supported by immunohistochemical and ultrastructural changes. Thus, alterations in the DH due to compression of the upper cervical region should be considered as a potential cause of idiopathic TN.

Food-related oral discomfort: A cross-sectional survey assessing the sensory dimension of oral discomfort in French independently living adults

BACKGROUND

Discomfort encountered during oral processing may cause food avoidance and increase the risk of malnutrition in older adults. The aim of the present survey was to explore the relationships between oral health and the oral discomfort experienced while eating in senior people.

MATERIALS AND METHODS

In a cross-sectional study, oral discomfort was assessed in a sample of 119 independently living participants aged between 35 and 81 years. Statistical analyses were used to identify categorical variables associated with food-related oral discomfort, including age, oral health-related quality of life (OHQoL), saliva secretion, occluding support, oral diseases and denture use.

RESULTS

Food-related oral discomfort concerned 28.5% of the participants. Within the sample study, the risk of experiencing food-related oral discomfort was increased for participants perceiving average (OR = 7.968, CI 95% = 2.603-24.381, p = .000) or poor OHQoL (OR = 17.109, CI 95% = 4.398-66.552, p = .000) and presenting strictly fewer than 7 occlusal functional units (OFUs) (OR = 3.396, CI 95% = 1.206-9.561, p = .020). Textured foods including fibrous (66.6%), heterogenous (60.6%) and grainy foods (42.4%) were mostly cited as food-related oral discomfort trigger factors. Within the 66-80 years group, participants having 0-6 OFUs were four times more likely to experience oral discomfort related to fibrous foods than participants having more than 7 OFUs (OR = 4.812, CI 95% = 1.192-19.415, p = .024). Within this group, participants having their teeth replaced by denture were also four times more likely to develop oral discomfort related to foods with heterogeneous textures (OR = 4.714, CI 95% = 1.030-21.562, p = .045) and grainy foods (OR = 7.285, CI 95% = 1.308-40.568, p = .023) than non-denture wearers.

CONCLUSION

Poor oral health conditions may generate oral discomfort with foods in the elderly especially with fibrous, heterogenous and grainy textures, and thus affect mealtime experience.

Neurophysiological aspects of the trigeminal sensory system: an update

The trigeminal system is one of the most complex cranial nerve systems of the human body. Research on it has vastly grown in recent years and concentrated more and more on molecular mechanisms and pathophysiology, but thorough reviews on this topic are lacking, certainly on the normal physiology of the trigeminal sensory system. Here we review the current literature on neurophysiology of the trigeminal nerve from peripheral receptors up to its central projections toward the somatosensory cortex. We focus on the most recent scientific discoveries and describe historical relevant research to substantiate further. One chapter on new insights of the pathophysiology of pain at the level of the trigeminal system is added. A database search of Medline, Embase and Cochrane was conducted with the search terms ‘animal study’, ‘neurophysiology’, ‘trigeminal’, ‘oral’ and ‘sensory’. Articles were manually selected after reading the abstract and where needed the article. Reference lists also served to include relevant research articles. Fifty-six articles were included after critical appraisal. Physiological aspects on mechanoreceptors, trigeminal afferents, trigeminal ganglion and central projections are reviewed in light of reference works. Embryologic and anatomic insights are cited where needed. A brief description of pathophysiology of pain pathways in the trigeminal area and recent advances in dental stem cell research are also discussed. Neurophysiology at the level of the central nervous system is not reviewed. The current body of knowledge is mainly based on animal and cadaveric studies, but recent advancements in functional imaging and molecular neuroscience are elucidating the pathways and functioning of this mixed nerve system. Extrapolation of animal studies or functioning of peripheral nerves should be warranted.

Tissue engineering: Dentin - pulp complex regeneration approaches (A review)

Dental pulp is a highly specialized tissue that preserves teeth. It is important to maintain the capabilities of dental pulp before a pulpectomy by creating a local restoration of the dentin-pulp complex from residual dental pulp. The articles identified were selected by two reviewers based on entry and exit criteria. All relevant articles indexed in PubMed, Springer, Science Direct, and Scopus with no limitations from 1961 to 2016 were searched. Factors investigated in the selected articles included the following key words: Dentin-Pulp Complex, Regeneration, Tissue Engineering, Scaffold, Stem Cell, and Growth Factors. Of the 233 abstracts retrieved, the papers which were selected had evaluated the clinical aspects of the application of dentin-pulp regeneration. Generally, this study has introduced a new approach to provoke the regeneration of the dentin-pulp complex after a pulpectomy, so that exogenous growth factors and the scaffold are able to induce cells and blood vessels from the residual dental pulp in the tooth root canal. This study further presents a new strategy for local regeneration therapy of the dentin-pulp complex. This review summarizes the current knowledge of the potential beneficial effects derived from the interaction of dental materials with the dentin-pulp complex as well as potential future developments in this exciting field.

Rat Nerve Regeneration with the Use of a Polymeric Membrane Loaded with NGF

Exogenous neurotrophic factors, delivered by various systems, are used to improve nerve regeneration. This study tested the effectiveness of a polymeric membrane loaded with Nerve Growth Factor (NGF) on mental nerve regeneration after a crush injury in rats. We tested NGF application, known to play a role in afferent fiber repair in dental neurobiology, to see if it could improve the regeneration. Afferent neurogram recordings and histological analyses of the trigeminal ganglion neurons were performed. One month after the crush injury, early regeneration was observed independently of exogenous NGF. However, as compared with the activity level recorded before the injury, the afferent activity was reduced by 28.5% without NGF, and the mean number of labeled neurons decreased. With NGF, activity was increased by 30.8%, with no significant histological difference compared with animals without lesions. NGF application through a polymeric membrane can influence degenerative and/or regenerative processes after a crush injury.

Response properties of whisker-associated primary afferent neurons following infraorbital nerve transection with microsurgical repair in adult rats

The rodent whisker/trigeminal system, characterized by high spatial and temporal resolution, provides an experimental model for developing new therapies for improving sensory functions of damaged peripheral nerves. Here, we use controlled whisker stimulation and single-unit recordings of trigeminal ganglion cells to examine in detail the nature and time course of functional recovery of mechanoreceptive afferents following nerve transection with microsurgical repair of the infraorbital nerve (ION) branch of the trigeminal nerve in adult rats. Response measures include rapid vs. slow adaptation, firing rate, interspike intervals, latency, and angular (directional) tuning. Whisker-evoked responses, readily observable by 3 wk post-transection, recover progressively for at least the next 5 wk. All cells in transected animals, as in control cases, responded to deflections of single whiskers only, but topography within the ganglion was clearly disrupted. The time course and extent of recovery of quantitative response measures were receptor dependent. Cells displaying slowly adapting (SA) properties recovered more quickly than rapidly adapting (RA) populations, and for some response measures-notably evoked firing rates-closely approached or attained control levels by 8 wk post-transection. Angular tuning of RA cells was slightly better than control units, whereas SA tuning did not differ from control values. Nerve conduction times and refractory periods, examined separately using electrical stimulation of the ION, were slower than normal in all transected animals and poorly reflected recovery of whisker-evoked response latencies and interspike intervals. Results underscore the need for multiple therapeutic strategies that target different aspects of functional restitution following peripheral nerve injury.

Developmental time course of peripheral cross-modal sensory interaction of the trigeminal and gustatory systems

Few sensory modalities appear to engage in cross-modal interactions within the peripheral nervous system, making the integrated relationship between the peripheral gustatory and trigeminal systems an ideal model for investigating cross-sensory support. The present study examined taste system anatomy following unilateral transection of the trigeminal lingual nerve (LX) while leaving the gustatory chorda tympani intact. At 10, 25, or 65 days of age, rats underwent LX with outcomes assessed following various survival times. Fungiform papillae were classified by morphological feature using surface analysis. Taste bud volumes were calculated from histological sections of the anterior tongue. Differences in papillae morphology were evident by 2 days post-transection of P10 rats and by 8 days post in P25 rats. When transected at P65, animals never exhibited statistically significant morphological changes. After LX at P10, fewer taste buds were present on the transected side following 16 and 24 days survival time and remaining taste buds were smaller than on the intact side. In P25 and P65 animals, taste bud volumes were reduced on the denervated side by 8 and 16 days postsurgery, respectively. By 50 days post-transection, taste buds of P10 animals had not recovered in size; however, all observed changes in papillae morphology and taste buds subsided in P25 and P65 rats. Results indicate that LX impacts taste receptor cells and alters epithelial morphology of fungiform papillae, particularly during early development. These findings highlight dual roles for the lingual nerve in the maintenance of both gustatory and non-gustatory tissues on the anterior tongue.

Bilateral sectioning of the anterior ethmoidal nerves does not eliminate the diving response in voluntarily diving rats

The diving response is characterized by bradycardia, apnea, and increased peripheral resistance. This reflex response is initiated by immersing the nose in water. Because the anterior ethmoidal nerve (AEN) innervates the nose, our hypothesis was that intact AENs are essential for initiating the diving response in voluntarily diving rats. Heart rate (HR) and arterial blood pressure (BPa) were monitored using implanted biotransmitters. Sprague-Dawley rats were trained to voluntarily swim 5 m underwater. During diving, HR decreased from 480 ± 15 to 99 ± 5 bpm and BPa increased from 136 ± 2 to 187 ± 3 mmHg. Experimental rats (N = 9) then received bilateral AEN sectioning, while Sham rats (N = 8) did not. During diving in Experimental rats 7 days after AEN surgery, HR decreased from 478 ± 13 to 76 ± 4 bpm and BPa increased from 134 ± 3 to 186 ± 4 mmHg. Responses were similar in Sham rats. Then, during nasal stimulation with ammonia vapors in urethane-anesthetized Experimental rats, HR decreased from 368 ± 7 to 83 ± 4 bpm, and BPa increased from 126 ± 7 to 175 ± 4 mmHg. Responses were similar in Sham rats. Thus, 1 week after being sectioned the AENs are not essential for initiating a full cardiorespiratory response during both voluntary diving and nasal stimulation. We conclude that other nerve(s) innervating the nose are able to provide an afferent signal sufficient to initiate the diving response, although neuronal plasticity within the medullary dorsal horn may be necessary for this to occur.

Orofacial pain models and behavior assessment

Orofacial pain remains an understudied area in pain research given that most attention has been focused on the spinal system. In this chapter, animal models of neuropathic and inflammatory orofacial pain are presented. Four different types of pain behavior tests are then described for assessing evoked and spontaneous pain behavior in addition to conditional reward behavior. The use of a combination of different pain models and behavior assessments is needed to aid in understanding the mechanisms contributing to orofacial pain in humans for developing effective therapy.

Partial infraorbital nerve ligation as a model of trigeminal nerve injury in the mouse: behavioral, neural, and glial reactions

Trigeminal nerve damage often leads to chronic pain syndromes including trigeminal neuralgia, a severely debilitating chronic orofacial pain syndrome. Options for treatment of neuropathic pain are limited in effectiveness and new approaches based on a better understanding of the underlying pathologies are required. Partial ligation has been shown to effectively mimic many of the qualities of human neuropathic pain syndromes. We have devised a mouse model of trigeminal neuralgia using a partial infraorbital nerve ligation (pIONL) that induces persistent pain behaviors and morphological changes in the brainstem. We found that the pIONL effectively induced mechanical allodynia lasting for more than 3 weeks. Cell proliferation (bromodeoxyuridine), activation of astrocytes and microglia in the ipsilateral caudal medulla, and persistent satellite cell reaction in the ipsilateral ganglion were observed. Neurochemical markers calcitonin gene-related peptide, substance P were decreased in medullary dorsal horn ipsilateral to the injury side, whereas substance P receptor NK1 expression was increased after 8 days. Nerve injury marker ATF3 was markedly increased in ipsilateral trigeminal ganglion neurons at 8 days after pIONL. The data indicate that partial trigeminal injury in mice produces many persistent anatomical changes in neuropathic pain, as well as mechanical allodynia.

PERSPECTIVE

This study describes the development of a new mouse model of trigeminal neuropathic pain. Our goal is to devise better treatments of trigeminal pain, and this will be facilitated by characterization of the underlying cellular and molecular neuropathological mechanisms in genetically designed mice.

The application of tissue engineering to regeneration of pulp and dentin in endodontics

Caries, pulpitis, and apical periodontitis increase health care costs and attendant loss of economic productivity. They ultimately result in premature tooth loss and therefore diminishing the quality of life. Advances in vital pulp therapy with pulp stem/progenitor cells might give impetus to regenerate dentin-pulp complex without the removal of the whole pulp. Tissue engineering is the science of design and manufacture of new tissues to replace lost parts because of diseases including cancer and trauma. The three key ingredients for tissue engineering are signals for morphogenesis, stem cells for responding to morphogens and the scaffold of extracellular matrix. In preclinical studies cell therapy and gene therapy have been developed for many tissues and organs such as bone, heart, liver, and kidney as a means of delivering growth factors, cytokines, or morphogens with stem/progenitor cells in a scaffold to the sites of tissue injury to accelerate and/or induce a natural biological regeneration. The pulp tissue contains stem/progenitor cells that potentially differentiate into odontoblasts in response to bone morphogenetic proteins (BMPs). There are two strategies to regenerate dentin. First, is in vivo therapy, where BMP proteins or BMP genes are directly applied to the exposed or amputated pulp. Second is ex vivo therapy and consists of isolation of stem/progenitor cells from pulp tissue, differentiation into odontoblasts with recombinant BMPs or BMP genes and finally transplanted autogenously to regenerate dentin. This review is focused on the recent progress in this area and discusses the barriers and challenges for clinical utility in endodontics.

Contribution of free nerve endings in the laryngeal epithelium to CO2 reception in rats

The superior laryngeal nerve (SLN) contains CO2-sensitive fibers. In the laryngeal epithelium, two candidates for CO2 reception have been identified, namely the intraepithelial free nerve endings and the taste buds. To elucidate the contribution of free nerve endings to CO2 reception, electrophysiological activities were recorded during various stages of regeneration of nerve endings following SLN-crush in rats. The left SLN was crushed surgically and maintained from 4 to 40 days for regeneration of nerve endings. Laryngeal sections were processed for immunohistochemical staining of protein gene product 9.5 to observe regeneration of free nerve endings and taste buds in the epithelium. By day 4 after SLN-crush, both the free nerve endings and taste buds had disappeared. Regeneration of the free nerve endings was recognized from day 8, while that of the taste buds started at day 16. On day 40, the number of taste buds on SLN-crush side was similar to that on the untreated side. Electrophysiological recording of SLN throughout the regeneration period (excluding day 4), showed response to intralaryngeal 9% CO2 (stimulation or inhibition) whether or not taste buds were present. Our results showed intralaryngeal CO2 reception without taste bud involvement, indicating that the free nerve endings in the laryngeal epithelium are receptive to intralaryngeal CO2.

Dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration

This review covers current information about the ability of dental nerves to regenerate and the role of tooth pulp in recruitment of regenerating nerve fibers. In addition, the participation of dental nerves in pulpal injury responses and healing is discussed, especially concerning pulp regeneration and reinnervation after tooth replantation. The complex innervation of teeth is highly asymmetric and guided towards specific microenvironments along blood vessels or in the crown pulp and dentin. Pulpal products such as nerve growth factor are distributed in the same asymmetric gradients as the dentinal sensory innervation, suggesting regulation and recruitment of those nerve fibers by those specific factors. The nerve fibers have important effects on pulpal blood flow and inflammation, while their sprouting and cytochemical changes after tooth injury are in response to altered pulpal cytochemistry. Thus, their pattern and neuropeptide intensity are indicators of pulp status, while their local actions continually affect that status. When denervated teeth are injured, either by pulp exposure on the occlusal surface or by replantation, they have more pulpal necrosis than occurs for innervated teeth. However, small pulp exposures on the side of denervated crowns or larger lesions in germ-free animals can heal well, showing the value of postoperative protection from occlusal trauma or from infection. Current ideas about dental neuroplasticity, neuro-pulpal interactions, and nerve regeneration are related to the overall topics of tooth biomimetics and pulp/dentin regeneration.

Application of a pro-inflammatory agent to the orbital portion of the rat infraorbital nerve induces changes indicative of ongoing trigeminal pain

The present experiments investigated the behavioral and immunocytchemical (ICC) effects of applying complete Freund's adjuvant (CFA) to the orbital portion of the infraorbital nerve (IOn). Two control groups, the first had saline applied to the IOn and the second underwent sham operation, were included in the study. In the CFA group, significant hyper-responsiveness to von Frey (analysis of variance <0.05) and to pinprick stimulation (Kruskal Wallis <0.05) in the vibrissal pad was observed on the fourth and the fifth days post-operative (dpo). This was accompanied by a reduced bite force and altered bite patterns of similar duration. Histology of the IOn in CFA rats revealed immune cell infiltration and edema around and in the nerve trunk with only mild axonal damage confirmed by neuropeptide Y immunoreactivity in trigeminal ganglion. Histological areas of inconsistent and mild inflammation were observed in the saline group that were accompanied by similarly attenuated behavioral and ICC changes. This model of inflammation-induced neuropathic pain is highly applicable to the study of neuroinflammatory orofacial pain.

Dental injury models: experimental tools for understanding neuroinflammatory interactions and polymodal nociceptor functions

Recent research has shown that peripheral mechanisms of pain are much more complex than previously thought, and they differ for acutely injured normal tissues compared with chronic inflammation or neuropathic (nerve injury) pain. The purpose of the present review is to describe uses of dental injury models as experimental tools for understanding the normal functions of polymodal nociceptive nerves in healthy tissues, their neuroinflammatory interactions, and their roles in healing. A brief review of normal dental innervation and its interactions with healthy pulp tissue will be presented first, as a framework for understanding the changes that occur after injury. Then, the different types of dental injury that allow gradation of the extent of tissue damage will be described, along with the degree and duration of inflammation, the types of reactions in the trigeminal ganglion and brainstem, and the type of healing. The dental injury models have some unique features compared with neuroinflammation paradigms that affect other peripheral tissues such as skin, viscera, and joints. Peripheral inflammation models can all be contrasted to nerve injury studies that produce a different kind of neuroplasticity and neuropathic pain. Each of these models provides different insights about the normal and pathologic functions of peripheral nerve fibers and their effects on tissue homeostasis, inflammation, and wound healing. The physical confinement of dental pulp and its innervation within the tooth, the high incidence of polymodal A-delta and C-fibers in pulp and dentin, and the somatotopic organization of the trigeminal ganglion provide some special advantages for experimental design when dental injury models are used for the study of neuroinflammatory interactions.