Characteristics of Calcium Signals in Developing Starburst Amacrine Cells

Abstract

Purpose: : Starburst amacrine cells (SACs) play a critical role in generating spontaneous waves in the developing retina. The dynamics of calcium signals in SACs shapes the spatiotemporal properties of retinal waves. This study investigated the characteristics of calcium signals in the developing rabbit retina.

Methods: : Displaced SACs were studied in flat-mount retinas of New Zealand White rabbits aged between embryonic day 29 and postnatal day 2. Whole-cell patch-clamp recordings were made from SACs using pipettes containing the calcium indicator, Oregon Green BAPTA-1. Ca²⁺ imaging was performed simultaneously with patch-clamp recording using a fast CCD camera (Cascade 128, Roper Scientific) at acquisition rates between 20 and 200 frames/s.

Results: : Upon depolarization by current injection under current clamp, developing SACs generated robust action potentials, mediated by voltage-gated Na⁺ and, to a larger extent, Ca²⁺ currents. The spikes were associated with prominent increases in intracellular Ca²⁺, detectable from both the soma and proximal and distal dendrites. The Ca²⁺ responses appeared as fast step increases followed by a prolonged decay phase. Each step increase corresponded to a somatic spike. Little Ca²⁺ increase was detected during the period of sub-threshold depolarization immediately before a spike. Bursts of spikes were most effective in generating large Ca²⁺ responses, which rose to peak in a staircase-like waveform, followed by a slow decay.Depolarization under voltage clamp evoked a Na⁺ current and large Ca²⁺ currents, as well as Ca²⁺-activated currents. It also induced Ca²⁺ increases in both the somata and dendrites of SACs. The rising phase of the Ca²⁺ transients under voltage clamp suggested potential contributions to Ca²⁺ signals from sources in addition to voltage-gated Ca²⁺ currents.

Conclusions: : The results suggest that action potential generation, particularly Ca²⁺ action potential generation, is essential for Ca²⁺ signaling in developing SACs. The intracellular Ca²⁺ transients in SACs changed from a more-or-less whole-cell to a more distally localized distribution pattern during development.

Keywords: amacrine cells • calcium • electrophysiology: non-clinical