DIRECT OBSERVATION OF THE EFFECT OF AUTORECEPTORS ON STIMULATED RELEASE OF CATECHOLAMINES FROM ADRENAL-CELLS

The direct effect of alpha(2)-autoreceptors was studied by measuring t he effects of piperoxan, an alpha(2)-autoreceptor antagonist, and clon idine, an agonist on catecholamine exocytosis, from single bovine chro maffin cells in culture. Catecholamine release was elicited by stimula tion with 100 mu M nicotine and was monitored electrochemically with a carbon-fiber microelectrode placed adjacent to the cell. These electr odes allowed the number of exocytotic release events to be monitored a nd reported as total charge for release following a specific stimulus. Repeated stimulation with 100 mu M nicotine showed that total release caused by the second exposure to nicotine was 32% of the first, and r elease caused by the third exposure to nicotine was 80% of the second. Total release of catecholamine increased significantly after applicat ion of 20 mu M piperoxan relative to a control apprication of balanced salt solution. Application of 20 mu M piperoxan alone did not cause r elease. After the cells were incubated in culture medium containing 20 mu M clonidine, a significant decrease in nicotine-stimulated catecho lamine release was observed. These results confirm that there are auto receptors on chromaffin cells and, when relatively high levels of cate cholamine are released, the catecholamine stimulates the alpha(2)-auto receptors, which inhibits subsequent release through a negative feedba ck mechanism. In addition to piperoxan, the sympathomimetic drug amphe tamine also increases quantal release after application of nicotine. A mphetamine increases the extracellular concentration of catecholamine, and these data appear to indicate that at least part of the pharmacol ogy of amphetamine might involve blocking catecholamine autoreceptors.