Pools of serotonin in the pineal gland of the mouse: the mammalian pinealocyte as a component of the diffuse neuroendocrine system.

In the pineal gland of the mouse the distribution of serotonin (5-HT) and its eventual relationships to a protein secretion were examined by means of fluorescence histochemical (Falck-Hillarp) and ultracytochemical (chromaffin and argentaffin) methods. (1) Yellow formaldehyde-induced fluorophores, characteristic of high concentrations of 5-HT, were found in pinealocytes, interstitial cells and sympathetic adrenergic nerve endings. The 5-HT content was studied according to the circadian variations and different drug treatments. (2) By use of ultracytochemical methods in untreated control mice precipitates indicative of 5-HT were found over dense-core vesicles (DCV; mean diameter: 100 nm) of the pinealocytes and the vesicular compartment of the sympathetic nerve fibers. After reserpine and p-CPA treatments, the reactivity of the DCV disappeared, but the protein secretion accumulated within the DCV was still present. After nialamide treatment the precipitates in the DCV increased and, similar to control mice, masked the protein secretion. Taking into account the specificity and sensitivity of these three complementary methods, previous biochemical data in mammals and ultracytochemical data in submammalian vertebrates, synthesis, storage, catabolism and release of 5-HT in the mouse and hamster pineals are discussed at the cellular level. Different pools of 5-HT are present: agranular and granular in pinealocytes and sympathetic nerve endings; agranular in interstitial cells. In the mouse, only a small portion of the total 5-HT content appears to bae secondarily taken up by the DCV. Within the DCV, 5-HT is possibly bound to a protein secretion of unknown significance (peptidergic neurohormone?). In the pinealocytes, which are sensitive to a large variety of inputs, processes of indole and protein secretion are found. These cells apparently are the recepto-secretory elements of the mammalian pineal gland. They can thus be classified as a member of the diffuse neuroendocrine system of the paraneuron group.