Neurotransmitter coupling through gap junctions in the retina

Area of Science:

• Neuroscience
• Retinal Biology
• Cellular Signaling

Background:

• Bipolar cells in the retina primarily use glutamate.
• Some cone bipolar cells also contain glycine, an inhibitory neurotransmitter.
• Rod amacrine cells contain glycine and form gap junctions with some cone bipolar cells.

Purpose of the Study:

• To investigate the source of glycine in cone bipolar cells.
• To test the hypothesis that cone bipolar cells obtain glycine from amacrine cells via gap junctions.

Main Methods:

• Examined expression of the glycine transporter GLYT1 in amacrine and bipolar cells.
• Used the gap-junction blocker carbenoxolone to assess glycine transfer.
• Depleted endogenous glycine stores and observed replenishment.
• Injected tracers into amacrine cells to identify connected bipolar cells.

Main Results:

• GLYT1 was found in amacrine cells but not bipolar cells, indicating amacrine cells are the primary site of glycine uptake.
• Carbenoxolone treatment inhibited glycine accumulation in bipolar cells, but not amacrine cells.
• Glycine depletion and replenishment experiments confirmed transfer from amacrine to bipolar cells.
• Neurobiotin tracing revealed that connected cone bipolar cells exhibit glycine immunoreactivity.

Conclusions:

• Cone bipolar cells receive glycine from glycinergic amacrine cells through heterologous gap junctions.
• Glycine in cone bipolar cells is acquired via cell coupling, not local synthesis or uptake.
• Transmitter content alone may not accurately reflect a neuron's functional transmitter role when gap junctions are involved.