Neuropeptide y gates a stress-induced, long-lasting plasticity in the sympathetic nervous system

Acute stress evokes the fight-or-flight reflex, which via release of the catecholamine hormones affects the function of every major organ. Although the reflex is transient, it has lasting consequences that produce an exaggerated response when stress is reexperienced. How this change is encoded is not known. We investigated whether the reflex affects the adrenal component of the sympathetic nervous system, a major branch of the stress response. Mice were briefly exposed to the cold-water forced swim test (FST) which evoked an increase in circulating catecholamines. Although this hormonal response was transient, the FST led to a long-lasting increase in the catecholamine secretory capacity measured amperometrically from chromaffin cells and in the expression of tyrosine hydroxylase. A variety of approaches indicate that these changes are regulated postsynaptically by neuropeptide Y (NPY), an adrenal cotransmitter. Using immunohistochemistry, RT-PCR, and NPY(GFP) BAC mice, we find that NPY is synthesized by all chromaffin cells. Stress failed to increase secretory capacity in NPY knock-out mice. Genetic or pharmacological interference with NPY and Y1 (but not Y2 or Y5) receptor signaling attenuated the stress-induced change in tyrosine hydroxylase expression. These results indicate that, under basal conditions, adrenal signaling is tonically inhibited by NPY, but stress overrides this autocrine negative feedback loop. Because acute stress leads to a lasting increase in secretory capacity in vivo but does not alter sympathetic tone, these postsynaptic changes appear to be an adaptive response. We conclude that the sympathetic limb of the stress response exhibits an activity-dependent form of long-lasting plasticity.