Abstract
Abstract: :
Purpose: Starburst amacrine cells exhibit directionally–selective light responses and play an essential role in the generation of direction selectivity in the retina. We therefore examined their light response properties and the effects of light and dark adaptation on these properties. Methods: DAPI–stained displaced starburst amacrine cells in the adult rabbit eyecup were studied using the whole cell patch–clamp recording technique. The superfusion solution consisted of a modified Ames medium. The composition of the pipette solution was (in mM) 100 K–gluconate, 14 KCl, 5 EGTA, 5 HEPES, 3 MgATP, 0.5 Na3GTP, 20 Na2phospho–creatine, 0.5 CaCl2, and 4.1 NaHCO3. The identity of the starburst cells was confirmed in some cases by injection of biocytin tracer. Light stimuli were generated with a custom–made computer program and focused onto the retinal surface through the microscope objective. Results: When whole–cell recording commenced, the average resting potential of the starburst amacrine cells was –51.0 ± 1.2 (SEM) mV. Little change from this value was observed under dark–adapted conditions throughout the course of the experiments. The receptive fields of the cells were mapped using spots, annuli and slits of light. Under dark–adapted conditions, a clear center–surround organization was observed. Spot stimuli (200 µm diam) depolarized the cells by 20 mV and annular stimulation hyperpolarized the cells by 25 mV. Mapping the receptive field profile by flashing stationary slits of light (0.5 x 0.03 mm) at various locations from the cell body indicated the presence of a hyperpolarizing surround 100 µm from the center. As reported previously (Gavrikov et al., 2003), the starburst amacrine cells depolarized (8.9 ± 0.6 mV) to stimuli that moved centrifugally from their somata to the periphery, but hyperpolarized (11.3 ± 1.0 mV) to stimuli that moved centripetally from the periphery to their somata. Light adaptation hyperpolarized the starburst amacrine cells, increased the size of the center response, and reduced the magnitude of surround inhibition and the hyperpolarizing response to centripetal motion. As previously reported (Cohen, 2001), we did not observe light–evoked spiking when recording starburst amacrine cells in current clamp mode, but depolarizing current injections consistently evoked spikes that were approximately 15–20 mV in amplitude. Conclusions: Starburst amacrine cell light responses exhibit a clear center–surround organization under dark–adapted conditions. Light adaptation decreases the magnitude of surround inhibition and the hyperpolarizing response to centripetal motion.
Keywords: amacrine cells • retina: proximal (bipolar, amacrine, and ganglion cells) • receptive fields