Monoamine‐accumulating ganglion cell type of the cat's retina

Abstract

A monoamine-accumulating ganglion cell type has been identified in an in vitro preparation of the cat's retina by a catecholamine-like fluorescence that appears following intravitreal injections of dopamine and the indoleaminergic transmitter analog, 5,7-dihydroxytryptamine (5,7-DHT). A subpopulation of large, weakly fluorescing neurons were identified as composing a single, morphologically distinct ganglion cell type by intracellular injections of horseradish peroxidase (HRP). In a sample of 374 HRP-filled cells soma diameter ranged from 13–21 μm (mean ± SD = 16.6 ± 1.3). Dendritic field size increased with increasing retinal eccentricity from 150–200 μm diameter at 0.5 mm from the area centralis to 600–800 μm diameter in the far retinal periphery. Dendrites are thin (∼1 μm diameter), show a characteristic branching pattern, and are narrowly stratified at the outer border of the inner plexiform layer. The monoamine-accumulating ganglion cell and the outer (OFF-center) alpha cell occupy distinct strata within sublamina a of the inner plexiform layer separated by a gap of about 5 μm. The total number of monoamine-accumulating (MA) ganglion cells was estimated at 5,400, about 3.5% of the total ganglion cell population. Spatial density of the MA ganglion cells, calculated from cell counts made in vitro, ranges from 60 cells/mm² near the area centralis to 5 cells/mm² in the far retinal periphery. A coverage factor (density x dendritic field area) of 2.2 was maintained from central to peripheral retina. The nature of the dendritic overlap was observed directly by making HRP injections into several neighboring ganglion cells. Five to seven neighboring dendritic trees extensively overlapped a given cell's dendritic field. However the dendritic processes did not intersect randomly but tended to interdigitate such that a uniform interdendritic spacing and density of dendritic processes was constructed locally within the dendritic plexus. Rotation of individual dendritic trees from their normal orientation produced a dramatic 4-5-fold increase in the number of dendritic intersections, suggesting that an active, local mechanism operates in the precise placement of individual dendrites within the plexus. The monoamine-accumulating ganglion cell appears morphologically equivalent to the delta ganglion cell (Boycott and Wässle; J. Physiol. (Lond.) 240: 397–419, '74; Kolb et al.; Vision Res. 21: 1081–1114, '81) and to the recently recognized indoleamine-accumulating ganglion cell (Wässle et al: J. Neurosci. 7: 1574–1585, '87). Comparisons with previous studies indicate that they may be physiologically equivalent to the OFF-center, sluggish sustained cells (also known as tonic W cells and Q cells) and probably project to the superior colliculus and/or the C-lamina of the dorsal lateral geniculate nucleus.