The tracer coupling pattern of AII amacrine cells and cone bipolar cells in the rabbit retina under changing background illumination

Purpose. To study the pattern of tracer coupling between AII amacrine cells (AIIs) and between AIIs and cone bipolar cells (CBs) under changing background light conditions. Methods. Intracellular recordings were obtained from neurons in the superfused, isolated retina-eyecup of the rabbit. Following physiological measures, cells were injected iontophoretically with Neurobiotin. Retinas were maintained under dark-adapted conditions or illuminated with a constant background light with intensity ranging from log -6.0 to -4.5 (log 0.0 -0.98 mW/cm²). Results. Injection of Neurobiotin into single AIIs invariably labeled other AIIs as well as CBs presumably by tracer movement through gap junctions. However, injection of a single CB produced, at best, labeling of 1-2 additional CBs. As reported previously (Xin and Bloomfield, 1994 ARVO abstract), the extent of coupling between AIIs could be modulated by light. Under dark-adapted conditions, AIIs were coupled in groups of only 18-69, covering an area of retina about 50-150 μm across; the size of their on-center receptive fields matched the extent of tracer coupling. Tracer-coupled CBs numbered 35-109, with an average CB:AII ratio of 1.7. With exposure to dim (log -6.0 and -5.5) background lights, the number of tracer-coupled AIIs increased dramatically to almost 700 covering an area of retina almost 650 μm across; there was an accompanying increase in the receptive field size of these cells. There was also a concomitant increase in the number of CBs labeled and so the average CB:AII ratio stayed at ∼1.7. Further increases in the background light intensity brought about a reduction in the number of tracer-coupled AIIs and CBs. However, the CB:AII ratio remained at ∼1.7. Conclusions. Changes in background light intensity modulates the extent of tracer coupling between AIIs. However, the extent of AII-CB coupling appears fixed. Thus, changes in the number of tracer-coupled CBs may be viewed as an epiphenomenon of light-induced, modulation of AII-AII coupling.