Effect of ionizing radiation on sympathetic nerve function in rat parotid glands

Radiation produces irreversible chronic dysfunction in the submandibular glands of the rat

The exposure to high doses of ionizing radiation during radiotherapy results in severe morphological and functional alterations of the salivary glands, such as xerostomia. In the present study we investigated the chronic effect of a single radiation dose of 15 Gray (Gy) limited to head and neck on rat salivary gland function (salivary secretion and gland mass) and histology. Results indicate that norepinephrine (NE)-induced salivary secretion was reduced significantly at 30, 90, 180 and 365 days after the administration of a single dose of 15 Gy of ionizing radiation compared to non-irradiated animals. The maximal secretory response was reduced by 33% at 30 and 90 days post irradiation. Interestingly, a new fall in the salivary response to NE was observed at 180 days and was maintained at 365 days post irradiation, showing a 75% reduction in the maximal response. The functional fall of the salivary secretion observed at 180 days post irradiation was not only associated with a reduction of gland mass but also to an alteration of the epithelial architecture exhibiting a changed proportion of ducts and acini, loss of eosinophilic secretor granular material, and glandular vacuolization and fibrosis. On the basis of the presented results, we conclude that ionizing radiation produces irreversible and progressive alterations of submandibular gland (SMG) function and morphology that leads to a severe salivary hypo-function.

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.

Suppression of radiation-induced salivary gland dysfunction by IGF-1

Background

Radiation is a primary or secondary therapeutic modality for treatment of head and neck cancer. A common side effect of irradiation to the neck and neck region is xerostomia caused by salivary gland dysfunction. Approximately 40,000 new cases of xerostomia result from radiation treatment in the United States each year. The ensuing salivary gland hypofunction results in significant morbidity and diminishes the effectiveness of anti-cancer therapies as well as the quality of life for these patients. Previous studies in a rat model have shown no correlation between induction of apoptosis in the salivary gland and either the immediate or chronic decrease in salivary function following γ-radiation treatment.

Methodology/Principal Finding

A significant level of apoptosis can be detected in the salivary glands of FVB mice following γ-radiation treatment of the head and neck and this apoptosis is suppressed in transgenic mice expressing an activated mutant of Akt (myr-Akt1). Importantly, this suppression of apoptosis in myr-Akt1 mice preserves salivary function, as measured by saliva output, three and thirty days after γ-radiation treatment. In order to translate these studies into a preclinal model we found that intravenous injection of IGF1 stimulated activation of endogenous Akt in the salivary glands in vivo. A single injection of IGF1 prior to exposure to γ-radiation diminishes salivary acinar cell apoptosis and completely preserves salivary gland function three and thirty days following irradiation.

Conclusions/Significance

These studies suggest that apoptosis of salivary acinar cells underlies salivary gland hypofunction occurring secondary to radiation of the head and neck region. Targeted delivery of IGF1 to the salivary gland of patients receiving head and neck irradiation may be useful in reducing or eliminating xerostomia and restoring quality of life to these patients.

Effect of ionizing radiation on rat parotid gland

A common side effect of radiotherapy used in the treatment of oral cancer is the occurrence of structural and physiological alterations of the salivary glands due to exposure to ionizing radiation, as demonstrated by conditions such as decreased salivary flow. The present study evaluated ultrastructural alterations in the parotid glands of rats receiving a fractionated dose (1,500-cGy) of radiation emitted by a Cesium-137 source and rats that were not subjected to ionizing radiation. After sacrifice, the parotid glands were removed and examined by transmission electron microscopy. Damage such as cytoplasmic vacuolization, dilatation of the endoplasmic reticulum and destruction of mitochondria, as well as damage to the cellular membrane of acinar cells, were observed. These findings lead to the conclusion that ionizing radiation promotes alterations in the glandular parenchyma, and that these alterations are directly related to the dose level of absorbed radiation. Certain phenomena that appear in the cytoplasm and nuclear material indicate that ionizing radiation causes acinar cell death (apoptosis).

Natural history and prevention of radiation injury

Radiation therapy for cancers of the head and neck can irreversibly damage the salivary glands. Xerostomia (subjective oral dryness) develops within the first week of therapy and is progressive, with devastating effects on the quality of life of the individual. The xerostomia does not correlate with the degree of salivary gland hypofunction. The mechanism of tissue injury in humans is still unclear, but much progress has been made with animal models. This paper reviews the natural history of radiation damage to human salivary glands and highlights the inter-individual variations in the responses to and recovery from therapeutic radiation. The degree of salivary gland damage is correlated to the dose of radiation delivered and the volume of gland included in the field of radiation. The molecular mechanism of acute radiation damage is not fully understood; however, long-term salivary gland dysfunction is associated with both loss of gland weight and loss of acinar cells. Various strategies have been used to prevent or alleviate the problem of salivary gland hypofunction following therapeutic radiation. This paper reviews the progress made to date and the possibilities for future interventions to prevent radiation damage.

Acquired salivary dysfunction. Drugs and radiation

When considering the effects of drugs on salivary glands, a distinction should be drawn between the complaint of oral dryness (xerostomia), a symptom, and measurable secretory hypofunction, a sign. In general, the symptom of xerostomia is often not accompanied by objective reductions in salivary output, and xerostomia is not a reliable indicator of secretory hypofunction. Whereas therapeutic pharmaceutical side effects represent the most prominent cause of xerostomia, with over 500 drugs associated with this symptom, only a small number of drugs have been demonstrated to reduce salivary output substantially. There are examples in which drugs with a high prevalence of xerostomia complaints do not affect secretory function. The mechanisms responsible for this discrepancy between subjective and objective findings have not been fully identified. It is hypothesized that alterations in systemic or mucosal hydration may play a role. Of the drugs with true salivary hypofunctional actions, most have direct anticholinergic properties. In almost all cases, the salivary effects of pharmaceuticals are not permanent, and function returns to pretreatment levels when the medication is stopped. By contrast, the effects of irradiation on the salivary glands are permanent when exposures exceed 50 Gy. About 40,000 individuals per year receive irradiation that involves the salivary glands (by external beam or internal sources--radon implants and 1311) for treatment of cancers of the head and neck region. Although these radiation effects have been recognized as a significant clinical problem for more than 80 years, the specific mechanisms responsible for radiation-induced salivary gland dysfunction are still not understood. With the exception of studies documenting the secretory functional deficits following head and neck irradiation, limited studies have been done in humans. The majority of experimental work has been done in rodents. A variety of mechanisms, including mitotic and interphase cell death, direct DNA damage or effects of secondary metabolites, damage to progenitor cells, or altered gene expression, have all been proposed to explain the salivary epithelial cell death observed. Recent experimental studies with models of radiation-induced salivary damage in rats and a human salivary cell line suggest that the small percentage of surviving epithelial cells are capable of performing functions such as signal transduction and ion transport normally. Apoptotic cell death following irradiation has not been a prominent feature in these model systems. The effects of head and neck radiation on the salivary glands and oral cavity continue to present multiple significant clinical problems both during and after radiotherapy. In recent years, there has been some progress in minimizing these effects through more careful shielding and pretreatment planning. Additionally, there are preliminary results from a clinical trial suggesting that the use of a secretagogue, pilocarpine HCl, given during the course of radiotherapy, may reduce the secretory hypofunctional effects. A multicenter trial is now underway to test this hypothesis. There is still a real need to develop more effective treatments for this condition.

Protection against irradiation-induced damage to salivary glands by adrenergic agonist administration

PURPOSE

Irradiation [IR]-induced damage to major salivary glands is an entity first described at the beginning of our century, yet its underlying mechanism is still enigmatic. Exposure of the salivary glands to IR is often inevitable when delivering radiotherapy for malignancies of the head and neck region. Frequently, this results in rapidly developing, life-long severe xerostomia for which no adequate prevention or treatment is available. The purpose of this study was to examine the role of secretion granules in serous cells of the parotid (P) and submandibular (SM) glands as mediators in the IR-induced salivary damage. Functional parameters (flow rate and gland weight), and total body weight were examined at both early term (4 days) and extended term (2 months) post-IR in male Wistar rats exposed to 15 Gy of head and neck irradiation following stimulation for granule secretion (degranulation).

METHODS AND MATERIALS

At 4 days, it was demonstrated that IR reduced P flow rate, P gland weight, total body weight, and submandibular/sublingual gland weight by 89, 33, 30, and 32% (p < 0.01), respectively, while SM flow rate was not altered significantly. At 2 months, these parameters were reduced by 59, 37, 31, and 37%, respectively, and the SM flow rate was reduced by 39% (p < 0.01).

RESULTS

Pilocarpine, a muscarinsic agonist which, albeit its efficacy as a salivary watery secretion stimulator, causes only limited degranulation, did not protect significantly any of the reduced parameters at either term. In contrast, cyclocytidine, an adrenergic agonist that is a very potent salivary degranulating agent, protected the P against the weight loss at 4 days and 2 months, and against the flow rate reduction at 2 months. The P weight and flow rate were protected to the extent that their values were not significantly different than those of the nonirradiated controls. Cyclocytidine also partially protected against the body weight reduction at 2 months. Our results emphasize the importance of secretion granules as mediatory agents in IR-induced P damage, and more so at the extended term. The demonstrated protective role of adrenergic agonists against IR damage to the P may be of importance in the clinical setting.

A 2 week pair-fed study of early X-irradiation effects on rat major salivary gland function

Salivary gland function is affected shortly after irradiation of the head and neck. An intense oral mucositis develops after exposure and may interfere with ingestion. The effects of restricted food and water intake on the secretory output of rat major salivary glands were examined. Parotid and submandibular salivary output and body weight were measured in rats at 4, 8, 11 and 14 days after 15 Gy X-irradiation of the head and neck. Comparisons were made with two groups: a non-irradiated group with food and water intake restricted to that of the irradiated group (pair-fed), and a non-irradiated, ad libitum-fed control group. Parotid saliva output was significantly decreased in the irradiated group at 4, 8, and 11 days compared with the control group. The pair-fed rats also had significantly decreased parotid output at these time points and their parotid function did not differ from that of the irradiated animals. At 14 days, all three groups demonstrated similar parotid function. Submandibular salivary output was not affected to the same extent. Only at a single time point (11 days) was flow significantly decreased in the irradiated group. Total body weight was less than that of control rats for both the irradiated and pair-fed animals at all time points. These results suggest that the early effects of radiation on salivary glands are due, in part, to limited intake of food and water during the immediate post-irradiation period.

Radiation response of murine eccrine sweat glands

Following irradiation of the left-hind feet of mice, we measured the ability of the eccrine glands to secrete sweat following stimulation by pilocarpine. Silicone elastomer impression moulds of the foot pads gave repeatable, detailed localization of sweat ducts by retaining the impression of each emerging sweat droplet. Loss of gland function occurred rapidly following irradiation (within 2 weeks) and the rate of loss was dose-dependent, being over three times greater following a dose of 13.0 Gy than after 6.8 Gy. There was a dose-dependent nadir of function at around 8 weeks, followed by a gradual recovery that was complete by about 30 weeks after irradiation, leaving a dose-dependent residual functional deficit. Eccrine sweat glands are very radiosensitive organs compared with the epidermis. A single dose of 13 Gy resulted in complete loss of eccrine gland function at 8 weeks whilst about 23 Gy would be required to elicit transient moist desquamation, in oxygen-breathing mice. Substantial sparing was seen when two doses were split by intervals of up to 24 h.

Rat salivary gland blood flow and blood-to-tissue partition coefficients following X-irradiation

Effects of a single high dose of X-irradiation (15 Gy) to the head and neck region in adult male rats were examined. Three days after radiation, rats were infused with [14C]-iodoantipyrine (IAP) via the femoral vein. Animals were killed at designated times thereafter and the three major salivary glands removed. The uptake of IAP by these tissues was analysed using a pharmacokinetic model. Radiation had relatively modest effects on all measured variables when compared with results from the non-irradiated control and pair-fed groups.

New approaches for determining the site of salivary fluid secretory disorders

Considerable progress has been made in understanding the events which underlie salivary fluid secretion. We have utilized novel approaches to evaluate the functional status of three distinct sites in the secretory cascade. First, salivary sympathetic nerve function studies used isotopic dopamine to measure catecholamine uptake and metabolism in vivo. Second, salivary muscarinic-cholinergic receptors were characterized in vivo by use of stereo-specific ligands and pharmacokinetic analyses. Finally, microfluorometric methods were used to study intracellular Ca2+ signaling in dispersed cells prepared from biopsied tissue. We conclude that it is possible to determine the functional status of key steps in salivary fluid generation with isotopic and fluorometric imaging techniques.