OFF-alpha and OFF-beta ganglion cells in cat retina. I: Intracellular electrophysiology and HRP stains.

Six OFF-alpha ganglion cells and a single OFF-beta ganglion cell were penetrated with intracellular microelectrodes and marked with horseradish peroxidase (HRP) in a perfused cat eyecup. Gaussian center radii (Rc) ranging from 40 to 217 microns were measured for receptive fields mapped with slits, values in agreement with previous extracellular reports. ON and OFF response components revealed nearly identical Rc's and center locations. Although Gaussian diameters (2Rc) were about 80% of dendritic field diameters overall, in this sample dendritic and receptive fields were not well correlated. Spatial tuning of ganglion cells was evidenced in peaked amplitude-vs.-width functions, fit by difference-of-Gaussians models. Such plots yielded Rc values about 40% less than position-vs amplitude plots. Rs values for surrounds ranged from 200 to 1,700 microns. Rod and cone signals were investigated with flicker. Rod flicker signals in OFF-alpha cells were larger and of shorter latency than in either horizontal or AII amacrine cells. Cone flicker signals were also short in latency, with an ON response time constant of 9 msec, and an OFF response time constant of 3 msec. The OFF-alpha rod-cone transition involved a latency increase of 20-30 msec. The spontaneous and light-evoked impulse rates of OFF-alpha responses varied linearly with extrinsic current, but the amplitude of ON hyperpolarization was little affected. After injection of staining current, the OFF-beta cell transiently depolarized at ON, suggestive of ON inhibition with reversed chloride gradient, a result not seen in OFF-alpha responses. Events (peaked, depolarizing voltage fluctuations) of high, low, and intermediate amplitudes were studied in OFF-alpha responses. High amplitude events (impulses), were OFF-correlated with the stimulus, and exhibited mean rise times (transit time from 25 to 75% of peak amplitude) from 255 to 392 microseconds. Intermediate level events (presumed synaptic origin) were also OFF correlated and had longer rise times (325 microseconds to 1.56 microseconds). Low level events (234-685 microseconds) revealed either ON, ON/OFF, or not stimulus correlation.