Dendritic calcium signaling in ON and OFF mouse retinal ganglion cells.

Retinal ganglion cells (RGCs) are the output cells of the retina; they convert synaptic input into spike output that carries visual information to the brain. Synaptic inputs are received, integrated and communicated to the spike initiation zone of the axon by dendrites whose properties are poorly understood. Here simultaneous patch-clamp recording and 2-photon Ca(2+) imaging are used to study voltage- and light-evoked Ca(2+) signals in the dendrites of identified types of mouse RGCs from parallel ON and OFF pathways, which encode the onset and offset of light, respectively. The results show pathway-specific differences in voltage-dependent Ca(2+) signaling. While both ON and OFF cells express high-voltage-activated (HVA) Ca(2+) channels, only OFF RGCs also express low-voltage-activated (LVA) Ca(2+) channels. LVA Ca(2+) channels in OFF cells are deinactivated by hyperpolarization from the resting potential and give rise to rebound excited Ca(2+) spikes at the termination of a step of either hyperpolarizing current or light. This suggests that the differential expression of voltage-gated Ca(2+) channels in ON and OFF RGC dendrites contributes to differences in the way the two cell types process visual stimuli.