Saliva plays an important role in most biologic events occiuring in the oral cavity and contributes to the maintenance of systemic health. Secretion occurs in response to a salivary reflex. During the fasting period, mouth dryness controls basal secretion. Chewing is the main stimulus for secretion. In human, salivary secretion can be influenced by light/darkness and other environmental stimuli. Postganglionic fibers originated in the superior cervical ganglia provide sympathetic innervation to salivary and pineal glands. Postsynaptic α- and β-adrenoceptors mediate salivary responses, including synthesis, storage and release of proteins. In mammals, the suprachiasmatic nuclei generate and synchronize endogenous circadian rhythms and the sympathetic nervous system plays an important role in the propagation of circadian signals. In immature rats, parotid α-amylase and total soluble protein circadian rhythms are controlled by periodic factors whose timing is independent of that imposed by nocturnal chewing of solid food. In rats exposed to various environmental stimuli, such as photoperiod, constant darkness or light, the submandibular secretory response and fine structure of parotid gland, suggest that both presence and absence of light can induce adrenergic responses. Homeostatic mechanisms would be required for a differential control of adrenergic responses during the two photoperiod phases. Circulating catecholamines would control light induced response, while sympathetic nerve efferents to the salivary glands would mediate dark induced responses. Postsynaptic α2-adrenoceptors with extrasynaptic localization might act preferentially in the control of adrenergic responses induced by light and other environmental stimuli which stimulate the adrenal medulla to release catecholamines into the bloodstream.
Biomedical Reviews 1998; 9: 61-68.
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