Autoradiographic and chemical localization of 5‐hydroxytryptamine in identified neurons in the leech

The large leech neurons known as Retzius cells contain substantial amounts of 5‐hydroxytryptamine (5‐HT) stored in granules. Previous studies show that these cells are able to make 5‐HT from precursors. The present study, which is a combined chemical and autoradiographic analysis of these cells after the administration of radioactive 5‐HT or 5‐HTP, confirms that the cells make 5‐HT from precursors. In addition, this study shows that the cells can acquire and store 5‐HT when it is present in the environment. Thus, if 5‐HT is available, these cells need not manufacture this compound from precursors. Furthermore, the newly acquired 5 HT seems to be stored in the granules that characterize these cells. Finally, this study shows that the Retzius acquire or make the 5‐HT in vivo.