SELECTIVE NEURAL REGULATION OF EPINEPHRINE AND NOREPINEPHRINE CELLS IN THE ADRENAL MEDULLA-CARDIOVASCULAR IMPLICATIONS

The innervation of the adrenal medulla regulates the release of catech olamines from the two, epinephrine (EPI) and norepinephrine (NE), popu lations of chromaffin cells. Adjustments in the neural output to the a drenal medulla are made by centers in the brain that integrate the sen sory input arising from a variety of challenges and the resulting chan ges in secretion assist in the restoration of homeostasis. Interesting ly, the adrenal medullary secretory responses do not simply reflect in crements a fixed ratio of EPI to NE as might be expected if release wa s proportional to the number EPI and NE cells. Instead, the ratio of E PI to NE changes depending on the magnitude and type of stimulus that initiates neural activation of the medulla. The variability in the EPI :NE release ratio implies that the EPI and NE cells can be differentia lly stimulated. Although the underlying mechanisms are not fully chara cterized, this review presents an emerging view that the selective con trol of EPI and NE cells is accounted for, first, by the existence of separate neural circuits between brain centers and the chromaffin cell s, and second, through neuromodulation that selectively influences EPI and NE cells. The presence of mechanisms that allow for separate cont rol of the EPI and NE cells may significantly augment the range of car diovascular and metabolic responses mediated through activation of the adrenal medulla.