Effects of background illumination on the horizontal cell responses in the tiger salamander retina

Synaptic transmission between photoreceptors and horizontal cells (HCs) was studied in the flat-mounted isolated retinas of the tiger salamander. Background illumination expedited the rise time of the HC light response, and the HC response rise time (HCRRT) reached steady state about 2 sec after the onset of the background illumination. The change in HCRRT is probably responsible for the background-induced enhancement of the HC responses to short light stimuli. The amplitude of the HC responses to 100 msec light steps in the presence of background illumination was 2-5 times larger than that measured under dark-adapted conditions. Background illumination exerted little effect on the response rise time in cones and bipolar cells, and thus it caused no significant response enhancement in those cells. The background-induced change in HCRRT correlated closely with the rod voltage but not with the HC voltage. These results suggest that the background-induced change in HCRRT is probably mediated by postsynaptic events in HCs because no significant time course change is observed in photoreceptors and bipolar cells (which share the same synapses with the HCs). A suppressive rod action on the cone inputs in HCs may be responsible for modulating the HCRRT. By shortening the HCRRT, background illumination regulates the frequency response of the photoreceptor-HC synapse and alters the capacity of spatial resolution of retinal bipolar cells.