Identification of feedback mechanisms from horizontal cells to cone photoreceptors in the mouse retina using two‐photon calcium imaging and pharmacology

Abstract

In neurons, transmitter release from axon terminals is directly linked to the calcium level (Thoreson, 2007; Jackman et al., 2009). Thus, one key mechanism to control transmitter release is to modulate presynaptic calcium by synaptic feedback (reviewed in Kamermans and Fahrenfort, 2004). “Traditional” GABAergic feedback but also more unconventional mechanisms like ephaptic and pH‐mediated feedback are found in many parts of the central nervous system (reviewed in Voronin, 2000; Chesler, 2003). However, little is known if these mechanisms operate in parallel to control transmitter release – that is, form a complex feedback system –, and if so, to what extent they fulfil distinct functions. An excellent system to study such feedback mechanisms is the photoreceptor synapse in the retina. This study investigated how the glutamatergic output of cone photoreceptors (cones) in the mouse retina is shaped by different feedback mechanisms from postsynaptic GABAergic horizontal cells using a combination of two‐photon calcium imaging and pharmacology at the level of individual cone axon terminals. I provide evidence that ephaptic feedback sets the cone output gain by defining the basal calcium level, a mechanism that may be crucial for adapting cones to the ambient light level. In contrast, pH‐mediated feedback did not modulate the cone basal calcium level, but affected the size and shape of light‐evoked cone calcium signals in a contrast‐dependent way: low contrast light responses were amplified, whereas high contrast light responses were reduced. Finally, I provide functional evidence that GABA shapes light‐evoked calcium signals in cones. Because we could not localize ionotropic GABA receptors on cone axon terminals using electron microscopy, this suggests that GABA may act through GABA auto‐receptors on horizontal cells, thereby possibly modulating ephaptic and/or pH‐mediated feedback. Taken together, the results of my thesis suggest that at the cone synapse, ephaptic and pH‐mediated feedback may fulfil distinct functions to adjust the output of cones to changing ambient light levels and stimulus contrasts, and the efficacy of these feedback mechanisms is likely modulated by GABA release in the outer retina. Such an intricate feedback system at the first synapse of our visual system could be important for reliable information transfer from one neuron to the next. It is possible that similarly complex synapses with different feedback mechanisms also play a role in other parts of the nervous system.