In-vitro uptake and metabolism of [3H]-5-hydroxytryptamine in the pineal glands of the rabbit, rat and hamster. A comparative study with the use of autoradiography, chromatography and liquid-scintillation counting.

Pineal glands of rat, rabbit and hamster were incubated during day or night in Merlis' fluid containing [3H]-5-hydroxytryptamine (=[3H]-HT) by the use of a 20-min pulse with or without postincubation in "cold" medium for 15, 30, 45 or 60 min. (1) Selective autoradiographic labeling was observed in sympathetic nerve terminals; this reaction was missing after bilateral surgical removal of the superior cervical ganglia. In contrast, a scarce and diffuse labeling was found in pinealocytes (Pi) and interstitial cells (IC) of both untreated and ganglionectomized animals. (2) With the use of thin-layer chromatography, it could be shown in the rat that the well-known indoles of the pineal gland are formed from [3H]-HT. (3) During preparation for electron microscopy (EM), the total loss of indoles from pineal glands was studied by means of liquid-scintillation counting; approximately 57% of the radioactivity of the pineal glands was released into EM-processing solutions, mainly into the glutaraldehyde fixative. In summary, our results show that in this type of experiment with pineal glands of mammals, the routinely used EM-procedures are inadequate to visualize the uptake and metabolism of exogenous indoles in Pi and IC. Furthermore, the data differ considerably from those obtained with the pineal organs of several reptilian and avian species when a similar cytological procedure is used. It appears that protein(s) located in the densely packed vesicles of the pineal cells of sauropsids, homologous to mammalian pinealocytes, may play a crucial role in indole binding (specific indole-binding proteins); this may help to interpret the diverging results obtained in different amniotes.