65
4.2 Model of a cone
4.2.1 Feedback from horizontal cell to cone
The mechanism of action mediating the center-surround receptive field antagonism (CS-
FRA) between horizontal cells and cones has eluded discovery since antagonistic feedback
was discovered in cone photoreceptors. Early work suggested that the synapse may be me-
diated by gamma-Aminobutyric acid (GABA) receptors in the cone that gate a chloride cur-
rent. This theory predicts that when horizontal cells hyperpolarize, they release decreasing
amounts of GABA to the cone, causing a decrease in GABA receptor opening. If the equilib-
rium potential for chloride (Eci) were very hyperpolarized, as is typical for many neurons,
this would cause a depolarization in the cone membrane potential, as observed from cone
voltage responses to surround light. It also implies that the cone membrane would see a net
conductance decrease during a surround response. However, recent work has found Eci to
be depolarized relative to the membrane potential, and direct application of GABA to the
cone was found to have an inconsistent or no response [100,102]. Furthermore, microelec-
trode recordings of salamander cones have demonstrated a net conductance increase dur-
ing a surround response (Zhang, AJ, unpublished data). While other evidence for a role for
GABA in the cone surround response exists, it appears that it cannot fully explain surround
inhibition in cones.
Another theory of the feedback synapse originally put forth by Byzov and Shura-Bura
[25] involves an ephaptic connection via gap-junction like hemichannels that causes direct
changes in the cone membrane potential around voltage gated calcium channels in the cone
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