Figure 1.

Dendritic Ca²⁺ signals evoked by depolarization and rebound excitation. A, Morphology of ON (top) and OFF-T (bottom) RGCs in isolated intact mouse retina. Two-photon z-projections (top) and side-projections (bottom) following patch-clamp recordings with 140 μm OGB-1 included in the patch pipette. Side-projections show overlay of simultaneously acquired green fluorescence from filled cells (black) and red fluorescence from bath-applied sulforhodamine (magenta), respectively. IPL borders indicated as 0% and 100%. Circles show linescan positions in B and C; arrows indicate axons. Scale bars, 25 μm. B, Ca²⁺ signals during spiking responses to depolarizing current injection in ON (top) and OFF-T (bottom) RGCs (+300 pA for ON cell; +200 pA for OFF-T cell). Each plot shows three individual responses. Red traces highlight an individual response and are shown in insets on faster time scale. Scale bars (main panel) for ON cell are 20 mV, 50% ΔF/F, 0.5 s, OFF cell 30 mV, 45% ΔF/F, 0.5 s. Scale bars in insets are 40 mV, 40% ΔF/F, 0.5 s for both ON and OFF cell. C, Ca²⁺ signals in response to hyperpolarizing current injection in the same cells (−260 pA for ON cell; −200 pA for OFF-T cell). Note rebound burst fired by OFF-T cell at termination of negative current step. Ca²⁺ signals measured at the same dendritic locations as in B. Colors and insets as in B. Maximum spike frequencies for OFF-T cell in B and C were similar (∼270 Hz). D, Delay time between Ca signal onset (see Materials and Methods) and occurrence of first AP was 38.5 ± 12.7 ms and 44.3 ± 24.7 ms for depolarizing stimuli in ON and OFF cell respectively. Delay time for rebound excitation in OFF cell was 8.4 ± 2.5 ms. Bar graph (right hand panel) showing rising slopes of Ca²⁺ signals evoked by depolarizing steps and rebound excitation in the two cell types. Symbols are mean ± SEM (n = 5 ON, 4 OFF-T cells). Recordings made in the presence of synaptic blockers mixture (in μm: 50 l-APB, 20 CNQX, 50 APV, 1 strychnine, 50 picrotoxin).