Abstract
Purpose:
The functional properties of bipolar cells are shaped by the inventory and sub-cellular distribution of voltage-gated ion channels they express. Here, we examine the localization and functional expression of the T-type calcium channel subunit, CaV3.1, in bipolar cells of the macaque monkey retina.
Methods:
Macaque eyes were obtained under the Tissue Distribution Program of the Oregon National Primate Research Center. For electrophysiology, whole-cell voltage-clamp and current-clamp recordings were made from bipolar cells in a light-adapted retinal slice preparation. For immunohistochemistry, paraformaldehyde fixed retinal sections were immunolabelled for CaV3.1 (Neuromab) in combination with subtype-specific bipolar cell markers.
Results:
Depolarizing voltage steps from a holding potential of -90mV, revealed transient, mibefradil-sensitive, inward currents in cells that had morphological features consistent with DB2, DB4 and DB5 bipolar cells. In contrast, DB1, DB6 and midget bipolar cells lacked T-type currents. DB3 bipolar cells comprised two functionally distinct groups; one type, which we denote as DB3a, expressed large voltage-gated sodium currents but lacked T-type calcium currents, while a second type, which we denote as DB3b, exhibited large T-type calcium currents but lacked sodium currents. T-type currents remained in cells in which the axon terminal system had been removed, suggesting localization of channels in the somatic and/or dendritic compartments. Current-clamp recordings showed that T-type channels produce calcium spikes in response to depolarizing current injection. Immunohistochemistry for the T-type calcium channel subunit, CaV3.1, revealed expression in the somatodendritic compartment of the same bipolar cell types that exhibited T-type currents by electrophysiology.
Conclusions:
These data demonstrate the presence of CaV3.1-mediated currents in sub-types of primate bipolar cells. We propose that these channels serve to boost signaling within transient visual pathways.
Keywords: 435 bipolar cells •
508 electrophysiology: non-clinical •
569 ion channels