Pineal responses to stress.

When laboratory rats are sensitized by appropriate environmental manipulations (e.g., protracted exposure to light, fasting), significant increases in melatonin synthesis and secretion can be induced by the acute imposition of stress (e.g., physical immobilization). In the absence of such priming pre-treatment, however, a stress-induced increment in melatonin levels may not be detectable. The mechanisms responsible may involve concurrent changes in the sensitivity of the pineal to catecholamines, sympathetic neural input to the gland, and circulating levels of catecholamines. The experimental use of stress-induced changes in pineal function may enhance the utility of the laboratory rat's pineal gland as a model for studying changes in the rhythmic secretion of melatonin in humans as a consequence of endogenous processes.