Changes in Rat Pineal Stores of 5‐Hydroxytryptamine after Inhibition of its Synthesis or Break‐Down

The formation and break‐down of 5‐HT in the rat pineal gland during daylight conditions has been investigated with a combination of fluorescence microscopic and fluorimetric methods. 5‐HT occurs in large and equal amounts in the pinealocytes and in the intrapineal sympathetic nerve fibres. Inhibition of 5‐HT synthesis either at the hydroxylation or at the decarboxylation step causes parenchymal 5‐HT to disappear within one to two hours, while little change is found in the nervous indole level. This indicates a turn‐over rate for 5‐HT in the pinealocytes considerably higher than in the intrapineal nerves. An intact sympathetic innervation of the pinealocytes is not essential for the 5‐HT turn‐over. Although monoamine oxidase is active in the degradation of pineal 5‐HT, inhibition of the enzyme does not increase pineal 5‐HT levels overtly, nor does it interfere with the turnover rate in the pinealocytes. Various explanations for this are offered.