Brain Research, 322 (1984) 1-7 1 Elsevier BRE 10380 Research Reports Dopamine-Containing Amacrine Cells of Rhesus Monkey Retina Parallel Rods in Spatial Distribution ANDREW P. MARIANP.*, HELGA KOLB 2 and RALPH NELSON 3 Z Laboratory of Vision Research, National Eye Institute, National Institutes of Health, Bethesda, MD 20205, 2Department of Physiology, University of Utah, School of Medicine, Salt Lake City, UT84108 and SLaboratory of Neurochemistry, National Institute o f Neurological and Communicative Disorders and Stroke, National Institutes of Health, Bethesda, MD 20205 (U.S.A.) (Accepted March 12th, 1984) Key words: dopamine - - amacrine cells - - rhesus monkey - - retina - - rods - - spatial distribution Dopamine-containing amacrine cells of rhesus monkey were found everywhere outside of the foveola in whole, flat retinas by the formaldehyde-glutaraldehyde fluorescent method. There were about 7500 such cells in a single retina and their density, determined by cell counts and measured by a nearest neighbor method, was minimal in foveal and peripheral regions and maximal at 3 mm from the center of the fovea. Compared to density distributions of other retinal neuron types, dopamine-containing amacrine cells corre- lated only with rods, which also had a peak density at 3 mm eccentricity. Cones and ganglion cells peaked in the foveal pit, or within 1 mm of it, respectively. As the distribution of dopamine-containing cells followed that of rods, it is suggested that dopamine could be involved in the rod neuronal circuitry of primates. INTRODUCTION Histochemistry of formaldehyde-induced fluoro- phores localized dopamine to amacrine cells 9.t5 and interplexiform cells5.6 in the retinas of a variety of vertebrate species. In the macaque retina, dopamine, was found in a population of amacrine cells rather uniform in morphological characteristics, with cell bodies along the inner part of the inner nuclear layer (INL) and dendritic processes ramifying in the outer- most stratum of the inner plexiform layer (IPL)4,14. Electron microscopy of the macaque dopamine-con- taining amacrine cells following lesioning with neuro- toxic amines indicated that they were presynaptic primarily to non-dopaminergic amacrine cells, other dopamine-containing amacrines and some bipolar cells while being postsynaptic only to amacrine cells 1Ao. Yet in spite of the studies showing these cells to be primarily interamacrine, a precise functional role for the dopaminergic amacrine cells in the pri- mate retina has yet to be identified. Now, by observ- ing these cells in whole, flat preparations using an aqueous formaldehyde-glutaraldehyde method8, in- formation on the distribution and morphological or- ganization not obtainable from radial sections was found and suggests a functional role for dopaminer- gic neurons in the rod neuronal circuitry of the pri- mate retina. MATERIALS AND METHODS Six eyes of anesthetized male and female rhesus monkeys (Macaca mulatta), aged 3-7 years, were ex- cised under ordinary illumination. The anterior por- tions were cut away and the remaining eyecups put in an oxygenated tissue culture medium where the ret- inas were dissected from the pigment epithelium and choroid. Laid flat on wax sheets, the retinas were Correspondence: A. P. Mariani, Bldg. 9, Rm. 1El 16, NIH, Bethesda, MD 20205, U.S.A.