Gain control by sparse, ultra-slow glycinergic synapses.

Area of Science:

• Neuroscience
• Retinal circuitry
• Visual processing

Background:

• ON starburst amacrine cells (SACs) are key to direction-selective retinal circuits.
• The sources of inhibition shaping SAC responses were previously unclear.
• Most SAC inhibitory synapses were thought to be GABAergic.

Purpose of the Study:

• To identify the sources and nature of inhibitory currents in SACs.
• To understand how inhibition shapes SAC function in direction selectivity.
• To elucidate the role of specific amacrine cells in retinal computation.

Main Methods:

• Electrophysiological recordings to measure light-evoked currents in SACs.
• Pharmacological identification of neurotransmitter receptor involvement.
• Serial electron microscopy for ultrastructural analysis and cell reconstruction.
• Analysis of synaptic connections between amacrine cells and SACs.

Main Results:

• Light-evoked inhibitory currents in SACs are predominantly glycinergic, contrary to prior assumptions.
• Glycinergic inhibition is slow, mediated by non-canonical glycine receptors (α4 subunits).
• Glycine inputs originate from both ON and OFF retinal pathways.
• Three types of narrow-field amacrine cells are identified as the likely sources of glycinergic inhibition.
• Glycine receptor properties allow for summation, controlling SAC output gain and expanding directional computation range.

Conclusions:

• Glycinergic inhibition, rather than GABAergic, plays a critical role in shaping SAC responses.
• Specific glycinergic amacrine cells are essential for retinal direction computation.
• The slow kinetics and summation properties of glycine inputs are crucial for SAC function in direction selectivity.