Neuroscience Letters, 18 (1980) 45-51 45 © Elsevier/North-Holland Scientific Publishers Ltd. THE MESOLIMBIC DOPAMINE SYSTEM: EVIDENCE FOR A HIGH AMINE TURNOVER AND FOR A HETEROGENEITY OF THE DOPAMINE NEURON POPULATION L.F. AGNATI, K. FUXE, K. ANDERSSON, F. BENFENATI, P. CORTELLI and R. D'ALESSANDRO Department of Human Physiology, University of Bologna and Modena (Italy) and (K.F. and K.A.) Department of Histology, Karolinska Institute, S-104 O1 Stockholm 60 (Sweden) (Received February 4th, 1980) (Revised version received February 26th, 1980) (Accepted February 27th, 1980) SUMMARY By means of a new photographic method to quantitate catecholamine fluorescence in tissue sections it has been possible to demonstrate a high dopamine (DA) turnover within the DA cell bodies of the midbrain. The medially located small-sized DA cell bodies of the A10 DA cell group has a very high DA turnover which was significantly different from that found in the laterally located medium- sized DA cell bodies of the AI0 cell group. Thus, the mesolimbic DA pathways may be divided into two systems, the medial system having a very high DA turnover in their cell bodies which may reflect a high functional activity in this system. During recent years, central dopamine (DA) neurons have been extensively investigated. Areas of interest have been the study of the regulation of DA turnover in nerve terminals and the cell body-dendrite region and the differential control of DA turnover in these two parts of the nerve cell [5]. A new photographic method has been introduced to quantitate catecholamine (CA) fluorescence in tissue sections [1]. The method is based on an elaboration of microphotographs, taken from suitable Falck-Hillarp preparations [12], by means of high contrast Kodalith plates (for graphic arts, processed with homonymous bath). The method has the advantage over quantitative microfluorimetry [8] of allowing an overall evaluation of a large region. Furthermore, by means of this approach, it is possible to take into