The Role of Dendritic T-Type Calcium Channels in the Functional Organization of Retinal Ganglion Cells

Abstract

Abstract: : Purpose: Determine the presence and function of T-type calcium channels in retinal ganglion cells Methods: Immunoblotting and immunohistochemical studies were combined with physiological and computer simulation techniques to determine the presence of T-type calcium channels in retinal ganglion cells and evaluate their contribution to cell function. Antibodies to the three major classes of T-type calcium channel subunits (G,H,I) were developed and applied to the rat and tiger salamander retinas. Results: Immunoblotting results and immunohistochemistry revealed the presence of T-type calcium channels in the retinas of the rat and tiger salamander. The rat retina revealed sparse staining for alpha 1G, whereas the salamander retina revealed selective staining for Muller cells with this antibody. Immunoreactivity was minimal in both rat and salamander for the alpha 1H subunit. The most pronounced immunoreactivity, observed in both rat and salamander retina was for the alpha 1I T-type calcium channel subunit. Alpha 1I immunoreactivity was found in both the outer and innerplexiform layers and in single, dissociated ganglion cells. Physiological and biophysical studies of dissociated retinal ganglion cells were carried out to determine the proprties of T-type calcium channels. Comparing the isolated T-type calcium currents in cells with and without processes revealed that cells with dendrites had much larger T-type currents than soma-only cells and we concluded that the dendrites had 4 to 12 times the channel density as that observed in the soma. Voltage-clamp data were used to construct a model of the T-type currents and this model was used to study the roles which T-type calcium channels can play in contributing to ganglion cell function. These included enhancement of synaptic currents, acceleration of sodium spiking and compensation for dendritic loading which determines the maxium rate of impulse activity, as established in previous modeling studies. Conclusions: T-type calcium channels are present in retinal ganglion cells with enhanced distribution in the dendrites. These voltage-gated ion channels can contribute to several functional reqirements of ganglion cells.

Keywords: ganglion cells • ion channels • calcium