The avian pineal organ. Distribution of exogenous indoleamines: a qualitative study of the rudimentary photoreceptor cells by electron microscopic radioautography.

The uptake of 5-hydroxytryptophan (3H-5-HTP) and serotonin (3H-5-HT) and their distribution have been demonstrated, at the ultrastructural level, in the secretory rudimentary photoreceptor cells (SRP cells), in the pineal organ of the parakeet and the duck. In vivo experiments. When the parakeets were injected with 3H-5-HTP alone or pretreated with drugs interfering with monoamine metabolism (e.g. R04-4602, nialamide) prior to injection of the label, the SRP cells always exhibited specific radioautographic reactions, in the regions of the dense secretory granules of the cell body and asynaptic pedicles. The labeling concentration avried quantitatively with time (interval between injection of the label and sacrifice) and drugs used. In vitro experiments. The uptake of 3H-5-HTP and 3H-5-HT in the pineal organ of the duck has also been studied, using an incubation procedure. The results with the two precursors confirmed those obtained "in vivo" with the parakeet. From these results it is suggested that the uptake of precursors in some compartments of SRP cells is qualitatively independent of: a) the experimental period (experiments performed in winter, spring, and summer; uptake of precursors between 11.00 a.m. and 12.00) b) the precursor (3H-5-HTP or 3H-5-HT) c) the observation times. It is concluded that the SRP cell, an important step in the cytological and functional evolution of the pineal organ (Collin, 1969, 1971), is responsible for the synthesis of 5-HT. The metabolism of 5-HT seems also possible in such cells.