The Effect of Capsaicin Sensory Primary Afferents on the Eruption Rate Of Rats’ Mandibular Incisors

Abstract

Background and Aims: Tooth eruption is defined as the movement of a tooth from its site of development within the alveolar process to its functional position in the oral cavity. Several studies aimed to investigate the neural effect on the eruption mechanism. Most of reported results were descriptive based on clinical observations and were not supported by experimental evidence. The present study aims to investigate (1) the possible involvement of the sensory nervous system in the eruption rate of mandibular incisors in rats, and 2) the possible involvement of substance P in this process. Materials and methods: Rats (adult female Sprague-Dawley) were divided into 5 groups; group 1, (n=7) had no chemical or surgical intervention (control group); group 2, (n=7) was subjected to the exposure and sectioning of the inferior alveolar nerve (IAN); group 3, (n=6) was subjected to selective ablation of the sensory afferent fibers to the incisor by topical application of capsaicin on the left IAN; group 4, (n=5), was the sham capsaicin group, where the capsaicin solvent was applied to the left IAN; group 5, (n=7) consisted of rats subjected to systemic ablation of their capsaicin sensitive primary afferents (CSPA); group 6, (n=8) was subjected to daily treatment with substance P antagonist, Spantide II. The method adopted to measure tooth eruption was based on two fixed reference points; the first reference was localized, with a groove, at the junction of the buccal and distal surface of the tooth and the second was a tattoo mark on the gingiva at the tooth base. A digital caliper (accuracy 0.01mm) was used for measurements, which were performed every 48hr over a period of 144 hours. Each measurement was repeated three times on each rat. Rats were sacrificed under deep anesthesia and the brainstem of rats of three groups (Control, Spantide II, and systemic capsaicin ablation) were collected and processed for the determination of substance P by Immunohistochemistry (IHC). The result obtained for each time point was presented as the mean and standard error of the mean (SEM) of measurements made on all rats in the same experimental group. Statistics were made using GraphPad Instat 3, and the significance of variations were calculated. Results: A general trend of reduction in the rate of eruption was observed in all groups. Significant reduction was observed in group 2 during the second (48-96hrs) and the third (96-144hrs) time segments, (0.44 ± 0.13mm) and (0.47 ± 0.11mm), respectively compared to control (0.79 ± 0.15mm) and (1.02 ± 0.14mm). Same pattern of attenuation was noticed in group 3 (0.41 ± 0.13mm, p < 0.01) as compared to the control (1.08 ± 0.09mm), as well as in group 6 (0.73±0.08mm, p < 0.05) compared to control (1.08±0.09 mm) at time point 48-96 hrs group 5 showed a significant attenuation of the eruption rate of the mandibular incisors at the initial time segment (0.64 ± 0.04 mm, p < 0.001) as compared to the control group (1.18±0.15 mm). IHC results showed a marked attenuation of substance P immunoreactivity in group 6 and a complete absence of immunoresctivity in G6 as compared to control. Conclusion: Capsaicin sensitive primary afferents play a major role in the control of the eruption process; however, their absence does not produce a permanent impairment.