Dental pulp and gingivomucosa in rats are innervated by two morphologically and neurochemically different populations of nociceptors

Role of Neurotrophins in Orofacial Pain Modulation: A Review of the Latest Discoveries

Orofacial pain represents a multidisciplinary biomedical challenge involving basic and clinical research for which no satisfactory solution has been found. In this regard, trigeminal pain is described as one of the worst pains perceived, leaving the patient with no hope for the future. The aim of this review is to evaluate the latest discoveries on the involvement of neurotrophins in orofacial nociception, describing their role and expression in peripheral tissues, trigeminal ganglion, and trigeminal nucleus considering their double nature as "supporters" of the nervous system and as "promoters" of nociceptive transmission. In order to scan recent literature (last ten years), three independent researchers referred to databases PubMed, Embase, Google Scholar, Scopus, and Web of Science to find original research articles and clinical trials. The researchers selected 33 papers: 29 original research articles and 4 clinical trials. The results obtained by the screening of the selected articles show an interesting trend, in which the precise modulation of neurotrophin signaling could switch neurotrophins from being a "promoter" of pain to their beneficial neurotrophic role of supporting the nerves in their recovery, especially when a structural alteration is present, as in neuropathic pain. In conclusion, neurotrophins could be interesting targets for orofacial pain modulation but more studies are necessary to clarify their role for future application in clinical practice.

Acute and Chronic Pain from Facial Skin and Oral Mucosa: Unique Neurobiology and Challenging Treatment

The oral cavity is a portal into the digestive system, which exhibits unique sensory properties. Like facial skin, the oral mucosa needs to be exquisitely sensitive and selective, in order to detect harmful toxins versus edible food. Chemosensation and somatosensation by multiple receptors, including transient receptor potential channels, are well-developed to meet these needs. In contrast to facial skin, however, the oral mucosa rarely exhibits itch responses. Like the gut, the oral cavity performs mechanical and chemical digestion. Therefore, the oral mucosa needs to be insensitive, to some degree, in order to endure noxious irritation. Persistent pain from the oral mucosa is often due to ulcers, involving both tissue injury and infection. Trigeminal nerve injury and trigeminal neuralgia produce intractable pain in the orofacial skin and the oral mucosa, through mechanisms distinct from those seen in the spinal area, which is particularly difficult to predict or treat. The diagnosis and treatment of idiopathic chronic pain, such as atypical odontalgia (idiopathic painful trigeminal neuropathy or post-traumatic trigeminal neuropathy) and burning mouth syndrome, remain especially challenging. The central integration of gustatory inputs might modulate chronic oral and facial pain. A lack of pain in chronic inflammation inside the oral cavity, such as chronic periodontitis, involves the specialized functioning of oral bacteria. A more detailed understanding of the unique neurobiology of pain from the orofacial skin and the oral mucosa should help us develop novel methods for better treating persistent orofacial pain.

Nerve Growth Factor Expression and Its Receptors TrkA and p75NTR in Peri-Implantitis Lesions

BACKGROUND

Nerve growth factor (NGF) can, through its receptors TrkA and p75NTR, convey signals for cell survival or cell differentiation. These proteins are also involved in inflammation and in bone resorption. The aim of this study is to evaluate, for the first time, the expression of NGF and its receptors TrkA and p75NTR in peri-implantitis lesions.

MATERIALS AND METHODS

Fifteen biopsy specimens from patients with chronic peri-implantitis and 4 of healthy oral mucosa were immunostained with antibodies against NGF, TrkA, and p75NTR. The staining intensity and percentage of stained cells were semi-quantitatively evaluated and results were compared between the 2 groups.

RESULTS

In the peri-implant pocket epithelium and gingival epithelium, NGF and TrkA expressions were similar to the healthy oral mucosa, however, a decreased expression of p75NTR was observed. In all cases, more than 75% of the inflammatory cells stained positively for NGF and TrkA, and p75NTR was negatively expressed.

CONCLUSION

The intense expression of NGF and TrkA in the inflammatory cell infiltrate associated with decreased expression of p75NTR in both gingival and pocket epithelium suggests that these proteins may have a role in peri-implantitis lesions.

The effects of lidocaine or a lidocaine-bupivacaine mixture administered into the infraorbital canal in dogs

OBJECTIVE To determine the onset, duration, and extent of regional nerve blocks performed by administration of lidocaine or lidocaine-bupivacaine into the infraorbital canal in dogs. ANIMALS 6 healthy hound-type dogs. PROCEDURES Under general anesthesia, stimulating needles were inserted into the gingiva dorsolateral to both maxillary canine (MC) teeth and the maxillary fourth premolar (MPM4) and second molar (MM2) teeth on the treatment side. A reflex-evoked muscle potential (REMP) was recorded from the digastricus muscle after noxious electrical stimulation at each site. After baseline measurements, 1 mL of 2% lidocaine solution or a 2% lidocaine-0.5% bupivacaine mixture (0.5 mL each) was injected into the infraorbital canal (at approx two-thirds of the canal length measured rostrocaudally). The REMPs were recorded for up to 7 hours. The REMP data for the contralateral (untreated control) canine tooth were used to normalize results for all stimulation sites. RESULTS With both treatments, nerve block for MC teeth on the treated side was achieved by 5 (n = 5 dogs) or 10 (1) minutes after injection, but nerve block for ipsilateral MPM4 and MM2 teeth was successful for only 3 dogs and 1 dog, respectively. Mean duration of nerve blocks for MC teeth was 120 and 277 minutes following injection of lidocaine and lidocaine-bupivacaine, respectively. CONCLUSIONS AND CLINICAL RELEVANCE Local anesthesia, as performed in this study, successfully blocked innervation of MC teeth, but results for MPM4 and MM2 teeth were inconsistent. This specific technique should not be used during tooth extractions caudal to the MC teeth.

The paradox of painless periodontal disease

Periodontal diseases, primarily gingivitis and periodontitis, are characterised by progressive inflammation and tissue destruction. However, they are unusual in that they are not also accompanied by the pain commonly seen in other inflammatory conditions. This suggests that interactions between periodontal bacteria and host cells create a unique environment in which the pro-algesic effects of inflammatory mediators and factors released during tissue damage are directly or indirectly inhibited. In this review, we summarise the evidence that periodontal disease is characterised by an accumulation of classically pro-algesic factors from bacteria and host cells. We then discuss several mechanisms by which inflammatory sensitisation of nociceptive fibres could be prevented through inactivation or inhibition of these factors. Further studies are necessary to fully understand the molecular processes underlying the endogenous localised hypoalgesia in human periodontal disease. This knowledge might provide a rational basis to develop future therapeutic interventions, such as host modulation therapies, against a wide variety of other human pain conditions.