Neuroscience Letters, 59 (1985) 61-66 61 Elsevier Scientific Publishers Ireland Ltd. NSL 03454 TOPOGRAPHY OF NICOTINAMIDE ADENINE DINUCLEOTIDE PHOSPHATE-DIAPHORASE STAINING NEURONS IN RAT STRIATUM NEIL W. KOWALL*, M. FLINT BEAL, ROBERT J. FERRANTE and JOSEPH B. MARTIN Department of Neurology and Neuropathology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114 (U.S.A.) (Received April 25th, 1985; Accepted May 15th, 1985) Key words. diaphorase - somatostatin - striatum - histochemistry - rat A new modification of the nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reaction was used to study the distribution of a specific subset of neurons in rat striatum. These neurons are known to also contain somatostatin-like immunoreactivity (SLI). We have previously found a heterogeneous dis- tribution of SLI in rat striatum. In the present study, we found NADPH-diaphorase neurons to be evenly distributed throughout the striatum and nucleus accumbens. There was no increase in the number of NADPH-diaphorase neurons in ventromedial striatum or nucleus accumbens where concentrations of SLI are highest. This suggests that there may be somatostatin afferents to ventromedial striatum and nucleus accumbens. In addition, the NADPH-diaphorase reaction g;as stable for up to 24 h in an animal model stimulating human autopsy conditions. The enzyme nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase is present in a specific subset of neurons which can be identified by their ability to reduce chromagens such as nitroblue tetrazolium to insoluble dark formazan reac- tion products. Vincent and Johansson [15, 16] demonstrated that these striatal neu- rons also contain somatostatin and neuropeptide Y-like immunoreactivity. Here, we report the detailed distribution of these neurons in the striatum as well as postmor- tem stability studies and our modification of the diaphorase method. Somatostatin neurons in the striatum have been shown by electron microscopy to have morphological features corresponding to medium-size aspiny neurons [7, 14]. These neurons are thought to be striatal interneurons with locally arborizing axons. Within the striatum, however, there is considerable heterogeneity in concentrations of somatostatin-like immunoreactivity. We found that concentrations are 2-3-fold greater in the ventromedial striatum and nucleus accumbens than in the dorsolateral striatum [4]. If these differences are due to the distribution of somatostatin neurons in the striatum, one would expect a larger number of neurons in the ventromedial striatum and nucleus accumbens than in the dorsolateral striatum. If, on the other hand, somatostatin neurons are evenly distributed throughout the striatum, this *Author for correspondence at: Department of Neurology, Massachusetts General Hospital, Boston, MA 02114, U.S.A. 0304-3940/85/$ 03.30 © 1985 Elsevier Scientific Publishers Ireland Ltd.