Physiological and morphological correlations of horizontal cells in the mudpuppy retina

1. Horizontal cells (HCs) of the mudpuppy (Necturus maculosus) retina were physiologically characterized with the use of intracellular recordings in a superfused, dark-adapted, retina-eyecup preparation. 2. Physiological analysis included an evaluation of rod versus cone input and a determination of the receptive field size with the use of a displaced slit of light. 3. The morphology of HCs was established through intracellular staining with horseradish peroxidase (HRP) and Lucifer yellow mixed in a single electrode. 4. Three types of horizontal cells were identified, each associated with a distinct morphology. Physiological subtypes included luminosity (L) and chromaticity (C) cells. Morphological diversities included single axon-bearing, multiple axon-bearing and, nonaxon-bearing cells. All C-type HCs lacked axons. 5. Approximately 90% of HCs encountered in this study were L-type cells, which received sign-conserving inputs from both rods and cones. These cell types contained one or more long axons that often stretched greater than 500 microns. This group was morphologically diverse, particularly with respect to variations in the number of axons, but we were unable to correlate this diversity with any unique set of physiological properties. 6. Several C-type HCs were identified (n = 8). These cells depolarized to a low-intensity, short-wavelength (SW) stimulus, whereas they hyperpolarized to high-intensity, long-wavelength stimuli. Morphologically, these cells were axonless (n = 4), with relatively small dendritic fields. 7. A third group of HCs were classified as "short wavelength preferring" HCs (n = 7). These cells responded better to a SW stimulus at all intensity levels. They were thus dissimilar to the common L-type HCs, which showed an apparent rod to cone transition as the stimulus intensity increased, suggestive of a shift from rod to cone preference. Morphologically, these cells were axonless (n = 2), but had broader dendritic fields than the C-type HCs. 8. Our observations indicate that the horizontal cell population of the mudpuppy retina is considerably more complex than previously supposed. The existence of both axon-bearing and axonless HCs, which could be correlated with L- and C-type physiology, implies that HCs may support more than one function in outer retina processing.