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Ji-Jie Pang, Fan Gao, Samuel M Wu; Rod-Driven OFF Pathway in the Vertebrate Retina. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):3222. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
Rod photoreceptors make chemical synapses with OFF-center hyperpolarizing bipolar cells (HBCs) in the vertebrate retina, including mammals. However, rods are known for lacking responses to light offset, therefore the significance of rod chemical synapses in the transient OFF pathway is not clear. To address this, we studied signal transmission between rod photoreceptors and HBCs in the tiger salamander retina.
Pairs of rods and their postsynaptic HBCs were recorded by dual-electrode whole-cell patch clamp technique. Under current and voltage-clamp conditions, light responses from the pairs of rod/HBC, as well as ganglion cells, were studied. Neurons were filled with fluorescent markers during recording and their morphology was observed with a confocal microscope.
1) In dark-adapted retinas, rods responded to light generally with a hyperpolarization. The light response to scotopic stimuli showed a rising and a decay phase. For light stimuli in the mesopic range, the responses displayed four phases: rising, sustained, decay/depolarization and re-hyperpolarization phase. 2) Under current clamp conditions, some rods repetitively generate a single spike with duration of ~350 ms during the decay-depolarization phase. 3) HBCs could generate light responses with a time course and dynamic range similar to rods. 4) Rod depolarization evoked transient cation currents in postsynaptic HBCs, whose amplitudes were positively correlated with both the amplitude and slope of the depolarization in rods. 5) Presynaptic hyperpolarization caused a small sustained hyperpolarization and a decrease of the spontaneous EPSC amplitude in HBCs. 6) Most GCs (~80%) showed transient ON-OFF light responses. The transient off responses in these cells could be present in either low illumination(-7~-4 log I), high illumination (-2~0 log I), or both conditions.
Transient OFF responses in ON-OFF ganglion cells can be driven by pure rod inputs. Activation of postsynaptic glutamate receptors during the rod decay/depolarization phase may mediate depolarizing OFF responses in HBCs and ON-OFF ganglion cells in low light conditions. These rod chemical synapse-mediated OFF responses are less precise compared with cone-driven OFF responses.
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