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 »  Home  »  Endodontic Articles 1  »  Quantitative analysis of substance P, neurokinin A and calcitonin gene-related peptide in pulp tissue from painful and healthy human teeth
Quantitative analysis of substance P, neurokinin A and calcitonin gene-related peptide in pulp tissue from painful and healthy human teeth
Discussion - References



Discussion.
This study is the first to compare the concentrations of SP, NKA and CGRP in pulp tissue from painful and non-painful human teeth. All three neuropeptides were identified in virtually all of the pulp tissue samples, whether from painful or non-painful teeth. The results of the present study showed that the levels of all three neuropeptides were significantly higher in pulp samples from painful teeth compared with healthy control teeth. The levels, in descending order, were CGRP > NKA > SP. The increased levels of these neuropeptides strongly suggest that afferent peptidergic nerve fibres in the dental pulp do not simply conduct impulses centrally, but also play an active and important part in the overall response of the pulp to injury.
The presence of the neuropeptides in non-painful teeth suggests nonsensory functions. In support of this, SP, NKA and CGRP have been reported to be involved in the regulation of pulpal blood flow in cats and ferrets (Olgart et al . 1993, Berggreen & Heyeraas 1999). In healthy tissues, neurovascular mediators are present in small amounts to maintain vascular tone, ensure smooth flow of blood and the consistent supply of nutrients to the tissue, and to regulate the interstitial pulpal pressure within a noncompliant tissue (Goodis & Saeki 1997, Fristad 1997).
Neuropeptides are involved in neurogenic inflammation (Alstergren et al . 1995) and their increased levels in pulpitis may be an attempt to regulate pulpal blood supply and thus control the fluid exudate associated with the inflammatory process. The release of peptides such as SP, NKA and CGRP has long been implicated in neurogenic inflammation and many studies support the hypothesis that neuropeptides are involved in the pathophysiology of inflammatory diseases (Maggi 1995, Alstergren et al . 1995, Olgart 1996). Most previous studies on neuropeptide responses to pulpal injury have been carried out in animals and have demonstrated that neuropeptides are altered in inflamed pulps (Byers 1994, Buck et al . 1999). The relevance of neuropeptides to humans is less clear and caution must be exercised when extending the results of animal studies to humans. Nevertheless, the fact that SP, NKA and CGRP, which are found in many species, are highly conserved molecules in evolution suggests that they may play very similar roles in a wide variety of species (Silverman & Kruger 1987).
Although animal models of pulpal injury are fundamental to our current understanding of neuropeptides in pulpal inflammation (Byers 1994, Buck et al . 1999), it is important to extend such research to examine the role of neuropeptides in human pulpitis. Since ethical considerations are of paramount importance in research involving human subjects, it is not always possible to achieve closely matched patient groups. In the current study the control group was recruited from young patients requiring extractions for orthodontic reasons, whereas the pain group patients, who attended a casualty clinic for extraction or the first stage of root canal treatment, had a much greater age range. Although third molars were considered as an alternative source of healthy pulp tissue from age-matched individuals, this source was rejected because of difficulties associated with the extraction of these teeth coupled with the fact that teeth which are not in occlusion are known to have lower neuropeptide levels than normal (Byers & Närhi 1999). Furthermore, since logistic regression analysis of the data from the current study indicated that age was not significantly related to neuropeptide concentration, the age difference in the subject groups was considered not to have introduced a discernible biological effect. It is also acknowledged that by virtue of patient treatment, the method of harvesting pulp tissue was different for painful teeth compared with non-painful ones. Most of the painful pulp tissue samples were extirpated during root canal treatment, with the exception of six samples that were obtained from extracted painful teeth. In non-painful teeth the pulp was removed after the tooth had been extracted and split. This difference is unlikely to have affected the neuropeptides studied, because both procedures took the same amount of time (~3 min). Furthermore, any neuropeptides discharged from nerves during either tooth extraction or pulpectomy would have accumulated in the pulp tissue which was collected and boiled as a whole.
There are no published reports of quantitative studies on the levels of SP, NKA and CGRP in pulp tissue from painful human teeth to compare with the present investigation. Only four studies have reported the levels of SP, NKA and CGRP in non-painful human teeth (Brodin et al . 1981, Parris et al . 1989, Goodis & Saeki 1997, Pertl et al . 1997). In the present study, the levels of SP and NKA in healthy pulps were comparable but CGRP was substantially more abundant. This agrees with previous studies that reported similar levels of SP and NKA (Goodis & Saeki 1997) and a 10-fold increase in the pulpal concentration of CGRP over SP (Pertl et al . 1997). The higher levels of CGRP compared with SP and NKA in healthy pulps can probably be explained by a greater number of nerve fibres containing CGRP and it has been shown that trigeminal CGRP neurones are more numerous than SP neurones in animals (Dalsgaard 1988) and in humans (Matsuyama et al . 1986).
There is evidence that SP (and possibly NKA) are responsible for the initiation of vasodilation, whilst CGRP mediates the late and more dominant phase of vasodilation in pulpal inflammation (Kerezoudis et al . 1994, Olgart 1996, Berggreen & Heyeraas 1999). In addition, CGRP has been shown to potentiate the actions of SP (Greves et al . 1985) and is believed to have an important role in pulpal repair following injury (Byers & Taylor 1993). For example, it is known that CGRP stimulates human endothelial cell proliferation (Hægerstrand et al . 1990). The inhibition of T-lymphocyte proliferation by CGRP suppresses the inflammatory response, whereas SP has an opposite stimulatory effect (Umeda et al . 1988). It is possible therefore that the high levels of CGRP in painful teeth may modify the inflammatory and immune processes and thus promote healing. The abundance of CGRP in both health and disease suggests that CGRP has a role in inflammation, healing and regeneration within the dental pulp.
The proportional increase in NKA levels in painful pulps compared with healthy pulps was greater than that of either SP or CGRP. Both NKA and SP evoke the release of pro-inflammatory cytokines from human monocytes but NKA is more potent in this regard ( Lotz et al . 1988). Because NKA coexists with SP it has been tentatively concluded that they have a similar role (Wakisaka 1990). The greater proportional increase in NKA suggests an important role for this neuropeptide in promoting pulpal inflammation. However, further investigations are required to elucidate the precise function of NKA in the dental pulp.
The concentration of CGRP (but not SP or NKA) in painful teeth was significantly higher in smokers compared with non-smokers. Nicotine triggers the release of SP, NKA and CGRP from C-fibres in the lungs (Hong et al . 1995) and a significant increase in NKA and CGRP has been reported in the pulmonary effluent in guinea pigs in response to smoke inhalation (Lee et al . 1995). Therefore, it seems probable that nicotine causes release of CGRP from afferent nerves in human pulp tissue.

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