Centrally evoked increase in adrenal sympathetic outflow elicits immediate secretion of adrenaline in anaesthetized rats.

To examine whether feedforward control by central command activates preganglionic adrenal sympathetic nerve activity (AdSNA) and releases catecholamines from the adrenal medulla, we investigated the effects of electrical stimulation of the hypothalamic locomotor region on preganglionic AdSNA and secretion rate of adrenal catecholamines in anaesthetized rats. Pre- or postganglionic AdSNA was verified by temporary sympathetic ganglionic blockade with trimethaphan. Adrenal venous blood was collected every 30 s to determine adrenal catecholamine output and blood flow. Hypothalamic stimulation for 30 s (50 Hz, 100-200 microA) induced rapid activation of preganglionic AdSNA by 83-181% depending on current intensity, which was followed by an immediate increase of 123-233% in adrenal adrenaline output. Hypothalamic stimulation also increased postganglionic AdSNA by 42-113% and renal sympathetic nerve activity by 94-171%. Hypothalamic stimulation induced preferential secretion of adrenal adrenaline compared with noradrenaline, because the ratio of adrenaline to noradrenaline increased greatly during hypothalamic stimulation. As soon as the hypothalamic stimulation was terminated, preganglionic AdSNA returned to the prestimulation level in a few seconds, and the elevated catecholamine output decayed within 30-60 s. Adrenal blood flow and vascular resistance were not affected or slightly decreased by hypothalamic stimulation. Thus, it is likely that feedforward control of catecholamine secretion from the adrenal medulla plays a role in conducting rapid hormonal control of the cardiovascular system at the beginning of exercise.