Modification of the β- and α2-adrenergic sensitivity of rat submandibular glands by environmental stimuli and stress

In man, the rate of resting salivary secretion can be influenced by environmental stimuli related to light–dark cycles or by noxious stimuli (stressors) of psychological origin. The sympathetic branch of the autonomic nervous system and the adrenal medulla play an important part in homeostatic responses. Previous observations have shown that chronic exposure of rats to constant light promotes degranulation of parotid acini and desensitization of submandibular β-adrenergic receptors. Now the submandibular secretory response elicited by β- and α2-adrenergic agonists was studied in rats chronically exposed to environmental conditions that modified the activities of sympathetic efferents to the pineal, salivary and adrenal glands. Adult male rats were exposed to constant light (LL) or constant darkness (DD) for 20 days, or to stress (2 h daily immobilization) for 14 days. Control animals were kept under the usual lighting conditions and without immobilization. Dose–response curves to isoproterenol (i.v), before and after administration (i.v.) of a dose (20 μg/kg) of clonidine were obtained. β-adrenergic desensitization was observed in all the experimental groups, while α2-adrenergic desensitization was only observed in the stress and LL groups. The results suggest that circulating catecholamines could mediate light and stress effects on submandibular β-adrenergic secretory responses. Extrasynaptic α2-adrenoceptors might modulate the submandibular secretory response when predictable environmental stimuli (daily light phase) or unpredictable stressors raise the concentrations of circulating catecholamines.