Barnes Lab Group

We recently introduced a radical new model to explain the synaptic processing that tunes photoreceptor output to produce center-surround antagonistic receptive fields, a key visual processing mechanism that sharpens spatial acuity and color discrimination. The model unites earlier theories, showing that synaptic cleft pH changes, directly affecting photoreceptor synaptic output, occur due to actions of the neurotransmitter GABA. Our current goal is to test whether the GABA receptor-mediated pH changes outlined in the model account for the retinal receptive field processing by recording responses of cones and horizontal cells to spatial and chromatic light stimuli, fully characterizing this synaptic interaction.

The key features of visual information processing in the outer retina are spatial and color contrast enhancement. In vertebrates, center-surround antagonism of retinal neurons begins with inhibition of cone photoreceptors by laterally extending horizontal cells. Cones are always hyperpolarized by direct illumination, but illumination of laterally displaced stimuli can depolarize them, or more commonly, reduced the hyperpolarization caused by direct light stimulation. All proposed models must explain how horizontal cells disinhibit the calcium channels in cones that are responsible for synaptic output. Our new model unites theories, specifying that GABA is released by and onto the horizontal cells themselves, activating bicarbonate-permeable channels, with resulting voltage-dependent efflux of bicarbonate from the cell alkalinizing the pH in the synaptic cleft, regulating the cone calcium channels that mediate neurotransmitter release.

The research aims to reveal how endogenous reactive oxygen species (ROS) produced by cellular metabolism contribute to and change the light-elicited spike firing properties of identified mouse retinal ganglion cells (RGCs). The relationship between the effects of ROS production by and on neuronal excitability is a research area receiving increasing attention. It has significance to both physiological and pathophysiological states since local metabolic activity alters the oxidant status of neurons in health and disease, impacting their neurophysiological responses. Spike metabolic efficiency is represented in part by the degree of temporal overlap of the opposing Na+ and K+ current flow during action potentials, and is a highly relevant constraint underlying neuronal efficiency. An additional dimension has been added recently to the theory of spike metabolic efficiency. Metabolic by-products produced by mitochondria, in particular, ROS including H2O2, are potent intrinsic neuromodulators of neurotransmitter- and voltage-gated ion channels. This emphasizes that rather than being hardwired, neuronal excitability and metabolic efficiency may be flexibly controlled by the balance of cellular antioxidants and mitochondrion-produced ROS.

We are investigating how RGC responses to stimuli that have large intensity fluctuations around a mean light level (e.g. high contrast) are reduced both on short (<1 s) and longer (>1 m) time scales. By producing fewer spikes in response to high contrast stimuli, this adaptation reduces metabolic cost. Contrast adaptation can involve both the presynaptic network as well as components intrinsic to RGCs themselves, e.g. altered Na+ channel activation and inactivation and voltage-gated K+ channel modulation.

Dopaminergic actions mediated by D1Rs in horizontal cells are activated by light exposure and poised to influence multiple aspects of feedback to photoreceptors. In these cells, D1Rs signal via G-protein α subunits to regulate cAMP-dependent phosphorylation of gap junctions and AMPA receptors, altering horizontal cell response properties, and via G-protein βγ subunits that directly inhibit voltage-gated calcium channels, which mediate horizontal cell GABA release. Dopamine and light also differentially regulate the balance of GABAR activity in horizontal cells. This action changes the signaling pathways developed in the GABA-pH hybrid model of feedback, in which GABA acts autaptically on horizontal cell GABARs, contributing to increased acidity of the synaptic cleft through depolarization, or to increased alkalinity through bicarbonate efflux during hyperpolarization. As a consequence of these interventions to horizontal cell autaptic GABA reception, the model aids identification of the basis by which important changes in feedback signaling occur.