Looking at neurotransmitters in the microscope

This review article covers the early period of my career. I first summarize research initiated by the late Nils-Åke Hillarp, after his appointment in 1962 as professor in the Department of Histology at Karolinska Institutet. He only lived for three more years, but during this short period he started up a group of ten students who explored various aspects of the three monoamine transmitters, dopamine, noradrenaline and 5-hydroxytryptamine, using the new formaldehyde fluorescence method developed by Bengt Falck and Hillarp in Lund. This method allowed visualization of the cellular localization in the microscope of these monoamines, which introduced a new discipline in neurobiology—chemical neuroanatomy. I then deal with work aiming at localizing the monoamines at the ultrastructural level, as well as attempts to use radioactively labeled aminoacids, especially γ-aminobutyric acid (GABA), and autoradiography, to identify, in the microscope, neurons using such transmitters. Finally, our immunohistochemical work together with Kjell Fuxe and the late Menek Goldstein, using antibodies to four monoamine-synthesizing enzymes is summarized, including some aspects on the adrenaline neurons, which had escaped detection with the Falck–Hillarp technique.