Brainstem activation of the adrenal medulla in the cat

Stimulation of the locus coeruleus in 40 cats leads to generalized activation of the sympathetic nervous system characterized by an immediate pressor response which was followed in the post-stimulus period by an increase of 49 +/- 10 and 44 +/- 6% in common carotid arterial resistance ipsilateral and contralateral to stimulation, respectively. This later response was not affected by vagotomy or bilateral cervical sympathectomy but was blocked by high spinal cord section. The post-stimulus carotid vasoconstrictor response could be entirely eliminated by acute bilateral physiological adrenalectomy in the form of adrenal hilar clamping, an effect which was reversible if the clamps were removed. The carotid vasoconstrictor response was associated with a rise in the circulating level of noradrenaline (260%) and adrenaline (196%), which was prevented by clamping the adrenal hilum. This response was not mediated via the hypothalamus because it persisted in the decerebrate animal, nor was it merely excitation of fibers of passage since it was reproduced by microinjection of glutamate into the locus coeruleus. The response was blocked by phentolamine suggesting it is mediated by alpha-adrenoceptors. These data represent the first conclusive demonstration that cell bodies in the brainstem are capable of activating the adrenal medulla. This fact is central to our present concept of the organization of the sympatho-adrenal axis.