Light-evoked calcium responses of isolated melanopsin-expressing retinal ganglion cells

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Summary

This study elucidates the intracellular calcium signaling pathways in isolated ipRGCs, finding that voltage-gated calcium channels (activated by action potentials) are the primary source of light-evoked calcium influx rather than direct TRP channel flux. These foundational findings improve understanding of how melanopsin-expressing cells process light signals, which underpins circadian photoentrainment and non-visual light responses relevant to lighting design.

Key Findings

Approximately 90% of light-induced calcium responses in ipRGCs were blocked by tetrodotoxin (action potential inhibitor), indicating VGCCs — not TRP channels — are the dominant calcium source.
TRP channel blockers (2-APB, SKF-96365, flufenamic acid, lanthanum, gadolinium) inhibited calcium responses, confirming TRP channel involvement in phototransduction but with a minor contribution to calcium influx.
L-type VGCC blockers (verapamil and diltiazem) significantly reduced light-evoked calcium responses, while thapsigargin (internal store depletion) had negligible effect.
Simultaneous calcium imaging and electrophysiology demonstrated a direct correlation between action potential spike frequency and somatic calcium levels in light-stimulated ipRGCs.
ipRGCs represent less than 2% of retinal ganglion cells; immunopanning was used to achieve highly enriched cultures from neonatal rats to enable isolated-cell studies.