Effects of irradiation on neuropeptide expression in rat salivary gland and spinal cord

On approaches to the functional restoration of salivary glands damaged by radiation therapy for head and neck cancer, with a review of related aspects of salivary gland morphology and development

Radiation therapy for cancer of the head and neck can devastate the salivary glands and partially devitalize the mandible and maxilla. As a result, saliva production is drastically reduced and its quality adversely altered. Without diligent home and professional care, the teeth are subject to rapid destruction by caries, necessitating extractions with attendant high risk of necrosis of the supporting bone. Innovative techniques in delivery of radiation therapy and administration of drugs that selectively protect normal tissues can reduce significantly the radiation effects on salivary glands. Nonetheless, many patients still suffer severe oral dryness. I review here the functional morphology and development of salivary glands as these relate to approaches to preventing and restoring radiation-induced loss of salivary function. The acinar cells are responsible for most of the fluid and organic material in saliva, while the larger ducts influence the inorganic content. A central theme of this review is the extent to which the several types of epithelial cells in salivary glands may be pluripotential and the circumstances that may influence their ability to replace cells that have been lost or functionally inactivated due to the effects of radiation. The evidence suggests that the highly differentiated cells of the acini and large ducts of mature glands can replace themselves except when the respective pools of available cells are greatly diminished via apoptosis or necrosis owing to severely stressful events. Under the latter circumstances, relatively undifferentiated cells in the intercalated ducts proliferate and redifferentiate as may be required to replenish the depleted pools. It is likely that some, if not many, acinar cells may de-differentiate into intercalated duct-like cells and thus add to the pool of progenitor cells in such situations. If the stress is heavy doses of radiation, however, the result is not only the death of acinar cells, but also a marked decline in functional differentiation and proliferative capacity of all of the surviving cells, including those with progenitor capability. Restoration of gland function, therefore, seems to require increasing the secretory capacity of the surviving cells, or replacing the acinar cells and their progenitors either in the existing gland remnants or with artificial glands.

Early radiation effects on muscarinic receptor-induced secretory responsiveness of the parotid gland in the freely moving rat

Although the salivary glands have a low rate of cell turnover, they are relatively radiosensitive. To study the possible mechanism behind this inherent radiosensitivity, a rat model was developed in which saliva can be collected after local irradiation of the parotid gland without the use of anesthetics or stressful handling. Saliva secretion was induced by the partial muscarinic receptor agonist pilocarpine (0.03-3 mg/kg) with or without pretreatment with the beta-adrenoceptor antagonist propranolol (2.5 mg/kg), or the full muscarinic receptor agonist methacholine (0.16-16 mg/min), and measured during 5 min per drug dose before and 1, 3, 6 and 10 days after irradiation. The maximal secretory response induced by pilocarpine plus propranolol was increased compared to that with pilocarpine alone but did not reach the level of methacholine-induced secretion, which was about five times higher. One day after irradiation a decrease in maximal pilocarpine-induced secretion was observed (-22%) using the same dose of pilocarpine that induces 50% of the maximal response (ED(50)), in both the absence and presence of propranolol, indicating that the receptor-drug interaction was not affected by the radiation at this time. The secretory response to methacholine 1 day after irradiation, however, was normal. At day 3 after irradiation, the maximal methacholine-induced secretion was also affected, whereas pilocarpine (+/-propranolol)-induced maximal secretion decreased further. At day 6 after irradiation, maximal secretory responses had declined to approximately 50% regardless of the agonist used, whereas ED(50) values were still unaffected. No net acinar cell loss was observed within the first 10 days after irradiation, and this therefore could not account for the loss in function. The results indicate that radiation does not affect cell number or receptor-drug interaction, but rather signal transduction, which eventually leads to the impaired response. We hypothesize that the early radiation effect, within 3 days, may be membrane damage affecting the receptor-G-protein signaltransfer. Later critical damage, however, is probably of a different nature and may be located in the second-messenger signal transduction pathway downstream from the G protein, not necessarily involving cellular membranes.

Short- and long-term effects of irradiation on laryngeal mucosa of the rat

Although radiotherapy is often used to treat laryngeal carcinoma, there is little information on the effects of this treatment on laryngeal structures. Rats were irradiated to the head and neck region and the larynges were studied by light- and electron-microscopy and immunohistochemistry. Ten days after irradiation, a change in the ultrastructural appearance of the granules of the subglottic glands was observed. Substance P-, bombesin- and enkephalin-like immunoreactivity was increased in local ganglionic cells and glandular nerve fibres. The mast cells were reduced in number. At examination 4 6 months after irradiation, there were no obvious differences compared with controls concerning mast-cell numbers and neuropeptide expression. The ultrastructural changes seen in the subglottic glands remained to some extent. The results show that structural changes in the subglottic glands occur concomitantly with an increased expression of certain neuropeptides in the innervation of these glands, which implies a relationship between these two parameters. The mast cells respond drastically to irradiation, but in the long run, regeneration of these cells occurs.

Does bombesin-like peptide mediate radiation-induced anorexia and satiety?

Bombesin (BN) and its mammalian counterpart gastrin-releasing peptide (GRP) act as neuroregulatory hormones and peripheral and central satiety-inducing agents. Previously, we demonstrated that irradiation induces an increase in the expression of BN/GRP in the innervation of the salivary glands in rats. We therefore carried out a study using radioimmunoassay (RIA) analysis and immunohistochemistry to examine whether saliva contains BN and whether irradiation affects the BN release to saliva in rats. Immunoreactivity for BN was detected not only in the innervation of the parenchyma but also in the duct cells and in the lumina of the ducts, suggesting entrance of BN into saliva. The RIA analysis confirmed that rat saliva contains a BN-like peptide. The observation shows that saliva contains this peptide but that there is no significant increase following the radiation schedule used. Nevertheless, the occurrence of an enhanced expression of BN in different peripheral tissues such as the salivary and laryngeal glands should be taken into consideration when discussing the clinically important problem of reduced food intake and anorexia in cancer patients.

Parallel increase in substance P and VIP in rat duodenum in response to irradiation

Irradiation was administered to the upper abdomen of rats, whereupon the duodenum was examined. Numerous vasoactive intestinal peptide (VIP)- and substance P (SP)-like immunoreactive nerve fibers were seen in the damaged mucosa, often in close association to each other. The intensity of the SP- and VIP-like immunoreaction was increased in several of the tissue compartments and, as measured with radioimmunoassay, the contents of SP- and VIP-like materials were increased after 30 Gray. The results show that SP and VIP levels increase after irradiation and suggest that SP and VIP are involved in interactive reactions in the reorganization and inflammatory processes in the gut after abdominal irradiation.

Main sensory neuropeptides, but not VIP and NPY, are involved in bone remodeling during orthodontic tooth movement in the rat

During orthodontic tooth movement (OTM) a remodeling of the periodontal ligament (PDL) and the alveolar bone occurs. We have recently observed that the expression of CGRP and substance P (SP) increases in the PDL and dental pulp in response to buccally directed OTM of the upper first molar in the rat. We have now examined whether there is also an involvement of VIP and NPY in this type of OTM. A sectional arch wire cemented to the upper incisors exerted an orthodontic force, mediated by a coil on the lingual side of the tooth, for 24 hours. It was observed that the blood vessels in the periodontal ligament were supplied with VIP- and NPY-immunoreactive (IR) nerve fibers, whereas VIP-IR nerve fibers in contrast to NPY-IR fibers were only occasionally observed in the dental pulp. No significant changes were observed in response to OTM. The observations suggest that VIP and NPY, in contrast to the main sensory neuropeptides CGRP and SP, are not involved in the tissue processes that occur in the remodeling of PDL and alveolar bone during orthodontic tooth movement.

Enhanced expression of neuropeptides in human breast cancer cell lines following irradiation

Previously, we have observed that the expression of the neuropeptides bombesin (BN-), the mammalian counterpart being gastrin-releasing peptide (GRP), and substance P (SP) in intact normal tissues, such as salivary and laryngeal glands, increases in response to irradiation. In the present study, the aim was to evaluate whether irradiation can have effects on individual cells that normally synthesize neuropeptides. In addition, since these neuropeptides are potentially mitogenic, we studied tumor cells. Therefore, the estrogen receptor-negative human breast cancer cell line MDA-MB-231 and its subline, with acquired doxorubicin resistance, MDA-MB-231 Dox were examined before irradiation and 4, 10, and 15 days after irradiation with 4 Gy (195 kV, 2 Gy fractions with 4 hours interval). Potential dose related changes were studied by delivering single doses of 2 or 9 Gy with the same technique. Immunohistochemical and radioimmunoassay (RIA) methods were used for detection of the SP and BN/GRP. Before, and at all time points following irradiation, a subpopulation in both cell lines displayed an intense immunostaining of SP and BN/GRP. A partial reorganization of the immunoreactive material was observed 10 days after irradiation. The RIA-analyses displayed signs of a dose-related increase, and a time-dependent transient and significant increase in the content of both peptides. The pattern of changes differed between the two peptides, and was especially pronounced in the doxorubicin resistant cells with regard to SP. Another neuropeptide, calcitonin gene related peptide (CGRP), was not detected in the cells used. The results suggest that irradiation has effects on a population of cultured neuropeptide-synthesizing cells. The occurrence and the specific changes obtained in the levels of neuropeptides, in response to irradiation, might imply an importance in the growth of breast cancer cells and in explaining repair processes following irradiation.

Bombesin-like peptide is present in duct cells in salivary glands: studies on normal and irradiated animals

Bombesin (BN) and its mammalian counterpart gastrin-releasing peptide act as neuroregulatory hormones and tissue-specific growth factors, and have been implicated as peripheral and central satiety-inducing agents. In the present study, the immunohistochemical expression of BN in submandibular, sublingual and parotid glands of rats was examined 10 days after 5 consecutive days with daily doses of 6-8 Gy irradiation. Radioimmunoassay (RIA) methods were also used. Immunoreactive granular structures were observed within duct cells of both controls and irradiated animals. In the parenchyma of irradiated animals, very few nerve fibres showing BN-like immunoreactivity were observed. The RIA analysis showed that the content of BN-like material significantly increased in submandibular and parotid glands in response to irradiation. The results suggest that mainly a non-neural form of BN is detected in the salivary glands in the immunohistochemical analysis. Thus, the immunohistochemical observations suggest that BN-like peptides may be present in the duct system, where they may be constituents of the saliva. The observations of an increase in BN content in response to irradiation are of interest as BN has mitogenic effects, may stimulate secretion and contributes to satiety.

VIP in the rat parotid gland in response to irradiation

In the present study, the immunohistochemical expression of vasoactive intestinal polypeptide (VIP) in the parotid gland of rats exposed to fractionated irradiation was examined. VIP concentration was analyzed by radioimmunoassay (RIA). Irradiation lead to a marked increase in the immunohistochemical expression of VIP in the innervation of the gland parenchyme. VIP-like immunoreactivity (LI) frequently coexisted with SP (substance P)-LI in these nerve fibers. The pattern of VIP-innervation in association with large ducts and blood vessel walls was unchanged. RIA analysis revealed a more than three-fold elevation in VIP content in the gland in response to irradiation. The increase in VIP immunoreaction and VIP content was seen at examination ten days after cessation of a five-day treatment with a total dose of 30-40 Gray. The upregulation of VIP may be related to changes in the production of neurotrophic factors and to an increased demand for a potentiation of secretagogue effects of SP.

Time- and dose-related changes in the expression of substance P in salivary glands in response to fractionated irradiation

PURPOSE

The expression of different neuropeptides in the innervation of submandibular and parotid glands of the rats was examined 2 and 5 days after initiation of radiation treatment as well as 10 and 180 days following the termination of irradiation.

METHODS AND MATERIALS

The irradiation was given on 2 or 5 consecutive days with daily doses of 4-8 Gy up to a total dose of 20-40 Gy. Immunohistochemical methods were used for the demonstration of substance P (SP), calcitonin gene-related peptide (CGRP), neuropeptide Y (NPY) and, the rate-limiting enzyme in the catecholamine synthetic pathway, tyrosine hydroxylase (TH). The content of SP was also analyzed by the use of radioimmunoassay (RIA).

RESULTS

In the parenchyma of both the submandibular and the parotid glands of control animals as well as after 2 days of irradiation treatment, a few nerve fibers showing SP-like immunoreactivity (LI) were observed. A marked increase in the expression of SP in the innervation of the parenchyma in both glands was observed 10 days after cessation of radiation treatment. The number of stained nerve fibers and the intensity of fluorescence in the fibers seemed to be dose dependent because the group subjected to a total dose of 40 Gy displayed a more pronounced staining intensity than that treated with 30 Gy. These results were supported by the RIA analysis. One hundred eighty days after treatment no obvious differences in SP-expression were seen between control and irradiated animals. No acute and long-term alterations were seen with regard to the other peptides and TH.

CONCLUSIONS

These results suggest that specific dose- and time-dependent changes in the expression of SP in the parenchyma of both submandibular and parotid glands occur in response to fractionated irradiation. The observations add further aspects to the tissue differences in physiological response and sensitivity to irradiation.

Sucralfate: prophylaxis of mucosal damage during cancer therapy

BACKGROUND

Chemotherapy and radiotherapy of different malignancies may be complicated by a variety of side effects, some of which may be related mucosal damage.

RESULTS

There is increasing evidence that sucralfate reduces the severity of radiation-induced mucositis in the head and neck, esophagus, and the lower gastrointestinal tract. Sucralfate also seems to protect the skin during radiotherapy and to reduce chemotherapy-induced mucositis.

CONCLUSION

Further studies could be of interest to define the clinical significance of sucralfate in reducing the mucosal damage and increasing quality of life during an following cancer therapy.