Abstract
Abstract: :
Purpose: To study how photoreceptor degeneration and associated synaptic changes in retina in dystrophic RCS rats affect response properties of retinal neurons. Methods: We compared response properties of neurons in the ganglion cell layer, recorded with extracellular microelectrodes in vivo in the retina of normal Long Evans rats (2 – 6 month old) and those recorded in dystrophic pigmented RCS rats of the same age range, under Ketamine–Xylazine anesthesia. The stimuli were spots of light, annuli and gratings of different sizes, orientations, spatial and temporal frequencies and contrasts, presented on a display (mean luminance 30 cd/m2), with computer–controlled stimulus presentation and data collection. Results: In normal rats, majority of recorded neurons, which presumably were ganglion cells, showed typical center–surround organization of receptive fields (RF), with diameter of central part in around 2.5 – 5º. Many of them responded better to stripes than to uniform flashing spots, with spatial frequency cut–offs at 1–1.2 c/º, and were able to follow high temporal frequencies (at least up to ∼15 Hz). Some cells were orientation–selective and showed orientation–specific center–surround interactions, sometimes indistinguishable from those recorded in the rat primary visual cortex. In dystrophic rats, spatial frequency cut–offs started to drop significantly after age of 2 months reaching the value ∼0.05 c/º at the age over 4 months. The size of RF centers showed progressive increase with age reaching ∼20º by 3.5 – 4 months. Neurons responded to slower moving gratings as compared to normal animals. Orientation selectivity of neurons was less affected by retinal degeneration, so that even at age over 3 months some neurons were sharply orientation–tuned. With age the variability of responses increased, and after age of 5 months the neurons rarely showed reliable responses. Conclusions: Cells of the ganglion cell layer of RCS rats show deterioration of acuity cutoff that parallels deteriorating acuity measures recorded in behavioral studies. In addition there are complex changes in detailed response properties suggesting more elaborate modifications of retinal circuitry. Support contributed by: NEI (EY 14038), FFB and Wynn Foundation.
Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • pathology: experimental • electrophysiology: non–clinical