Studies on 5‐hydroxytryptamine Stores in Pineal Gland of Rat

Summary— The pineal gland of the rat has been studied using a highly specific and sensitive fluorescence microscopical method for the cellular localization of certain biogenic monoamines, in combination with fluorimetric determinations of 5‐hydroxytryptamine (5‐HT) levels under normal and experimental conditions. Exceptionally large amounts of 5‐HT have been found in the gland. This amine is located partly in the parenchymal cells and partly in the pineal sympathetic nerves. There is strong evidence to support the view that the sympathetic nerves are noradrenergic, but take up 5‐HT from the parenchymal cells into their terminal portions, which run within the gland. A unique feature thus exists in the sympathetic nerves of the rat's pineal in that they contain two different monoamines. The experiments have shown that half of the pineal 5‐HT is located in the sympathetic nerves, and that the rest is stored in the parenchyma. A small portion of this parenchymal pool is dependent on an intact innervation and is lost on denervation. After the injection of noradrenaline, the fluorescence of the pineal nerves changes to show the characteristics of a catecholamine. Half of the pineal 5‐HT is then depleted, showing that the neuronal 5‐HT is replaced by the administered noradrenaline. No change can be registered in pineal 5‐HT levels 22 hours after the administration of α‐methyl‐m‐tyrosine. However, metaraminol causes a pronounced depletion of pineal 5‐HT. A reaccumulation of the 5‐HT fluorescence in the pineal nerves can be brought about by the injection of nialamide to reserpinized animals, as it occurs in the cerebral nerves containing 5‐HT.