Antagonistic effect of D-myo-inositol-1,2,6-trisphosphate (PP56) on neuropeptide Y-induced vasoconstriction in the feline dental pulp

Dental nerves: a neglected mediator of pulpitis

As one of the most densely innervated tissues, the dental pulp contains abundant nerve fibres, including sensory, sympathetic and parasympathetic nerve fibres. Studies in animal models and human patients with pulpitis have revealed distinct alterations in protein expression and histological appearance in all types of dental nerve fibres. Various molecules secreted by neurons, such as classical neurotransmitters, neuropeptides and amino acids, not only contribute to the induction, sensitization and maintenance of tooth pain, but also regulate non-neuronal cells, including fibroblasts, odontoblasts, immune cells and vascular endothelial cells. Dental nerves are particularly important for the microcirculatory and immune responses in pulpitis via their release of a variety of functional substances. Further, nerve fibres are found to be involved in dental soft and hard tissue repair. Thus, understanding how dental nerves participate in pulpitis could have important clinical ramifications for endodontic treatment. In this review, the roles of dental nerves in regulating pulpal inflammatory processes are highlighted and their implications for future research on this topic are discussed.

Involvement of TRPA1 in the cinnamaldehyde-induced pulpal blood flow change in the feline dental pulp

Objectives

The purpose of this study was to investigate the involvement of TRPA1 in the cinnamaldehyde-induced pulpal blood flow (PBF) change in the feline dental pulp.

Materials and Methods

Mandibles of eight cats were immobilized and PBF was monitored with a laser Doppler flowmetry at the mandibular canine tooth. To evaluate the effect of cinnamaldehyde on PBF, cinnamaldehyde was injected into the pulp through the lingual artery at a constant rate for 60 seconds. As a control, a mixture of 70% ethanol and 30% dimethyl sulfoxide (DMSO, vehicle) was used. To evaluate the involvement of transient receptor potential ankyrin 1 (TRPA1) in PBF change, AP18, a specific TRPA1 antagonist, was applied into the pulp through the Class V dentinal cavity followed by cinnamaldehyde-administration 3 minutes later. The paired variables of experimental data were statistically analyzed using paired t-test. A p value of less than 0.05 was considered as statistically significant.

Results

Administration of cinnamaldehyde (0.5 mg/kg, intra-arterial [i.a.]) induced significant increases in PBF (p < 0.05). While administration of a TRPA1 antagonist, AP18 (2.5 - 3.0 mM, into the dentinal cavity [i.c.]) caused insignificant change of PBF (p > 0.05), administration of cinnamaldehyde (0.5 mg/kg, i.a.) following the application of AP18 (2.5 - 3.0 mM, i.c.) resulted in an attenuation of PBF increase from the control level (p < 0.05). As a result, a TRPA1 antagonist, AP18 effectively inhibited the vasodilative effect of cinnamaldehyde (p < 0.05).

Conclusions

The result of the present study provided a functional evidence that TRPA1 is involved in the mechanism of cinnamaldehyde-induced vasodilation in the feline dental pulp.

An overview of the dental pulp: its functions and responses to injury

The dental pulp is a unique tissue and its importance in the long-term prognosis of the tooth is often ignored by clinicians. It is unique in that it resides in a rigid chamber which provides strong mechanical support and protection from the microbial rich oral environment. If this rigid shell loses its structural integrity, the pulp is under the threat of the adverse stimuli from the mouth, such as caries, cracks, fractures and open restoration margins, all of which provide pathways for micro-organisms and their toxins to enter the pulp. The pulp initially responds to irritation by becoming inflamed and, if left untreated, this will progress to pulp necrosis and infection. The inflammation will also spread to the surrounding alveolar bone and cause periapical pathosis. The magnitude of pulp-related problems should not be underestimated since their most serious consequence is oral sepsis, which can be life threatening, and hence correct diagnosis and management are essential. Clinicians must have a thorough understanding of the physiological and pathological features of the dental pulp as well as the biological consequences of treatment interventions.

Quantification of neuropeptides (calcitonin gene-related peptide, substance P, neurokinin A, neuropeptide Y and vasoactive intestinal polypeptide) expressed in healthy and inflamed human dental pulp

AIM

To quantify the expression of calcitonin gene-related peptide (CGRP), substance P (SP), neurokinin A (NKA), neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) in healthy and inflamed human dental pulp tissue.

METHODOLOGY

Six pulp samples were obtained from teeth having a clinical diagnosis of acute irreversible pulpitis. Another 12 pulp samples were obtained from premolars where extraction was indicated for orthodontic purposes. In six of these premolar teeth inflammation was induced by mechanical pulp exposure prior to sample collection. All samples were processed and 125I-labelled; neuropeptides were quantified by competition assays. ANOVA and Mann-Whitney's (post hoc) tests were used to establish statistically significant differences between the groups.

RESULTS

Expression of five neuropeptides was found in all human pulp samples. Statistical analysis revealed a significantly higher (P < 0.05) expression of CGRP, SP, NKA and NPY in both inflammatory conditions compared with healthy pulp control values. VIP expression remained stable during the inflammatory conditions.

CONCLUSION

Expression of CGRP, SP and NKA released from C-fibres and NPY released from sympathetic fibres is significantly higher in the inflamed human pulp compared with healthy pulp. Expression of VIP released from parasympathetic fibres is not increased during the inflammatory conditions of human dental pulp.

Intrinsic regulation of CGRP release by dental pulp sympathetic fibers

Neurotransmission from sympathetic and peptidergic afferent fibers participates in the regulation of pulpal blood flow (PBF) via opposing effects. In this study, we directly tested the hypothesis that activation of pulpal sympathetic terminals inhibits exocytosis of immunoreactive calcitonin gene-related peptide (iCGRP) from peptidergic afferents innervating bovine dental pulp. The results demonstrate that norepinephrine inhibits capsaicin-evoked iCGRP release. The application of alpha-adrenergic antagonists (phentolamine or phenoxybenzamine) increased spontaneous release of iCGRP. Moreover, administration of agents that evoke the release of sympathetic neurotransmitters (guanethidine or reserpine) inhibited capsaicin-evoked iCGRP release. Collectively, these results indicate that sympathetic neurotransmission inhibits exocytosis from pulpal peptidergic afferent fibers. Analysis of these data supports the hypothesis that peripheral sympathetic vasomotor control may operate by a direct mechanism (vasoconstriction) as well as by an indirect mechanism (e.g., inhibition of exocytosis from afferent fibers). Since capsaicin-sensitive neurons are nociceptors, it is possible that certain sympathetic neurotransmission may modulate pain.

beta 2-Adrenoceptor regulation of CGRP release from capsaicin-sensitive neurons

Previous studies have suggested that neurotransmitter substances from the sympatho-adrenomedullary system regulate pulpal blood flow (PBF), in part, by the inhibition of vasoactive neuropeptide release from pulpal sensory neurons. However, no study has evaluated the role of beta-adrenoceptors. We evaluated the hypothesis that activation of beta-adrenoceptors inhibits immunoreactive calcitonin gene-related peptide (iCGRP) release from capsaicin-sensitive nociceptive neurons via in vitro superfusion of bovine dental pulp. Either norepinephrine or epinephrine inhibited capsaicin-evoked iCGRP. The norepinephrine effect was blocked by the selective beta(2)-adrenoceptor antagonist, ICI 118,551, but not by pre-treatment with the selective beta(1)-adrenoceptor antagonist, atenolol. In addition, application of albuterol, a selective beta(2)-adrenoceptor agonist, significantly blocked capsaicin-evoked release of iCGRP. Collectively, these studies demonstrate that activation of beta(2)-adrenoceptors in dental pulp significantly reduces exocytosis of neuropeptides from capsaicin-sensitive nociceptors. This effect may have physiologic significance in regulating PBF. Moreover, since capsaicin selectively activates nociceptors, beta(2)-adrenoceptor agonists may have clinical utility as peripherally acting therapeutics for dental pain and inflammation.

Age-related changes in the human pulpal blood flow measured by laser Doppler flowmetry

The purpose of this study was to examine the age-related changes in human pulpal blood flow (PBF). Recordings were taken from 22 clinically healthy upper central incisors in 22 healthy participants (age: 8-75 years). A Moor blood flow monitor (type MBF3D) was used to measure PBF; and subjects' electrocardiogram (ECG), mean blood pressure (BP) and heart rate (HR) were also recorded. The resting PBF was significantly decreased with the increased age of the participants (Pearson's correlation coefficient, P < 0.001). The examined tooth crown was briefly(1 s) cooled using a dental coolant, and significant reductions in PBF were induced (one-way repeated measures anova, P < 0.05). The magnitude of the reduction (%) was significantly decreased with the increased age of the participants (Pearson's correlation coefficient, P < 0.05). The findings indicate that the hemodynamics in the human pulp is reduced with age.

Effects of BP 2-94, a selective H(3)-receptor agonist, on blood flow and vascular permeability of the rat mandibular incisor pulp

Pulpal blood-flow changes were monitored by laser Doppler flowmetry after electrical stimulation of the mandibular incisor. Stimuli of 10 s (50 microA, 2 ms, 20 Hz) were applied to the incisors of untreated animals and longer stimulations (5 min) were applied in animals treated with the alpha-blocker phenoxybenzamine. Changes in vascular permeability in the dental pulp were measured by Evans blue extravasation following resection of the superior cervical ganglion. In these groups, a selective agonist of H(3) receptors, BP2-94 (1.5 and 15 mg/kg), and an H(3)-antagonist ciproxifan (1 mg/kg) were administered. The effects of these drugs were compared with those obtained from animals treated only with the vehicle (methylcellulose 1%). Basal pulpal blood-flow was not affected significantly by BP2-94 or ciproxifan. The vasoconstriction induced in the group of intact rats by electrical stimulation of 10 s is decreased in amplitude and duration at the higher dose of BP2-94 by 58 and 40%, respectively (P<0.05, n=5). In the sympathectomized animals, plasma extravasation was significantly increased at 15 mg/kg BP2-94 (+100%, P<0.01, n=5). These results suggest that H(3) receptors may participate in the regulation of changes in vessel contraction and permeability provoked by electrical stimulation of the dental pulp. However, the non-selective effects of the H(3) agonists reacting on adrenergic sites and H(1) receptors could explain a part of the results.

D-myo inositol 1,2,6, triphosphate (alpha-trinositol, pp56): selective antagonist at neuropeptide Y (NPY) Y-receptors or selective inhibitor of phosphatidylinositol cell signaling?

1. D-myo inositol 1,2,6 trisphosphate (alpha-trinositol, pp56), an isomer of the second messenger substance, D-myo inositol 1,4,5 trisphosphate, has an interesting pharmacological profile that includes anti-inflammatory and analgesic effects and antagonism of neuropeptide Y (NPY)-mediated cellular responses. 2. However, not all responses elicited by this neuropeptide are sensitive to antagonism by pp56. Evidence is emerging, at least in certain tissues, that other receptor populations, in addition to those for NPY, are also sensitive to inhibition by pp56. 3. A direct or allosteric interaction of pp56 at receptors for NPY is now considered unlikely and it is more probable that pp56 might interfere at some point in the phosphatidylinositol signaling pathway, possibly at the level of the plasmalemmal inositol 1,3,4,5, tetrakisphosphate receptor. 4. Full realization of the therapeutic potential of this novel compound, however, must await a thorough characterization of the cellular mechanism(s) associated with the various pharmacological effects of pp56.

Immunocytochemical and electron-microscopic features of tooth pulp innervation in hereditary sensory and autonomic neuropathy

Characteristics of the pulpal innervation in teeth obtained from a 4-year-old Asian boy with hereditary sensory and autonomic neuropathy, type II (HSAN) were investigated. Four minimally carious primary teeth were split longitudinally and prepared for either fluorescent immunocytochemistry or electron microscopy. The occurrence and distribution of specific neuropeptides were determined by the use of antisera to calcitonin gene-related peptide (CGRP), substance P (SP), neuropeptide Y (NPY), and vasoactive intestinal polypeptide (VIP). The overall innervation of the pulps was visualized using antiserum to protein gene product 9.5; an antiserum to dopamine beta-hydroxylase was used to identify postganglionic sympathetic fibres. Pulpal innervation in HSAN was notably different from that of normal teeth: in comparison with the controls, HSAN teeth had an overall marked reduction in pulpal innervation with an absence of large nerve bundles and the subodontoblastic plexus. CGRP- and SP-immunoreactivity was absent in HSAN specimens and VIP-immunoreactivity was reduced. However, NPY-immunoreactivity appeared to be increased within certain regions of the pulp/dentine complex. In addition, there was evidence of NPY-immunoreactive fibres extending into dentine, a feature not seen in the controls. Electron microscopy revealed an absence of myelinated nerve fibres and a paucity of unmyelinated fibres. CGRP and SP have a well-established role in nociceptive processing and their absence in the HSAN teeth would seem to correspond with the clinical presentation of marked peripheral sensory deficit, characteristic of this condition. An up-regulation of NPY-immunoreactivity has previously been reported in animal teeth following nerve injury and a similar mechanism may have stimulated increased NPY expression in HSAN teeth, but the functional significance of its presence within dentinal nerves is not known.