Calcium channels in rat horizontal cells regulate feedback inhibition of photoreceptors through an unconventional GABA‐ and pH‐sensitive mechanism

Abstract

Key points• Inhibitory feedback from horizontal cells to photoreceptors regulates synaptic gain and contributes to centre-surround receptive field formation via mechanisms that are not fully understood.• We show that horizontal cell calcium channels and ionotropic GABA receptors mediate the inhibitory feedback, and that the results of their actions are blocked by strong pH buffering with Hepes.• GABA appears to act not upon the photoreceptor but instead upon the horizontal cell itself.The horizontal cell GABA receptors are permeable to chloride and bicarbonate, meaning their activation can produce changes in synaptic cleft pH.• These results suggest that activation of calcium channels in a depolarized horizontal cell releases GABA, which acts in an autaptic manner to increase bicarbonate permeability. The resulting influx of bicarbonate contributes to acidification of the synaptic cleft, inhibiting photoreceptor calcium channels, the hallmark of inhibitory feedback at this synapse.Abstract Horizontal cells send inhibitory feedback to photoreceptors, helping form antagonistic receptive fields in the retina, but the neurotransmitter and the mechanisms underlying this signalling are not known. Since the proteins responsible for conventional Ca 2+ -dependent release of GABAergic synaptic vesicles are present in mammalian horizontal cells, we investigated this conventional mechanism as the means by which horizontal cells inhibit photoreceptors. Using Ca 2+ imaging in rat retinal slices, we confirm that horizontal cell depolarization with kainate inhibits and horizontal cell hyperpolarization with NBQX disinhibits the Ca 2+ signals produced by pH-sensitive activation of voltage-gated calcium channels (Ca channels) in photoreceptors. We show that while 100 μM Co 2+ reduces photoreceptor Ca 2+ signals, it disinhibits them at 10 μM, an effect reminiscent of earlier studies where low [Co 2+ ] eliminated feedback. The low [Co 2+ ] disinhibition is pH sensitive. We localized L-, N-and P/Q-type Ca channels in rat horizontal cells, and showed that both the N-type Ca channel blocker ω-conotoxin GVIA and the P/Q-type Ca channel blocker ω-agatoxin IVA increased Ca 2+ signals in photoreceptors in a pH-sensitive manner. Pronounced actions of GABAergic agents on feedback signals to photoreceptors were observed, and are pH sensitive, but are inconsistent with direct inhibition by GABA of photoreceptor [Ca 2+ ]. Patch-clamp studies revealed that GABA activates a conductance having high bicarbonate permeability in isolated horizontal cells, suggesting that the commonality of pH sensitivity throughout the results could arise from a GABA autofeedback action in horizontal * N.C. Brecha and S. Barnes are equal senior authors. cells. This could change cleft pH with concomitant inhibitory influences on photoreceptor Ca channels.

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

“…f Diagram illustrating three theories for the feedback mechanism of horizontal cell processes on the retinal terminal synapse: GABA, protons (H + is pH) and ephaptic transmission (ΔV; an electrical field effect at close range; Gardner et al 2015; Kramer and Davenport 2015). The involvement of GABA is supported by recent studies (Hirano et al 2016; also Liu et al 2013), but they suggest that GABA from horizontal cell processes activates autoreceptors directly on the horizontal cell processes, probably indirectly mediating the pH-based feedback mechanism (Color figure online)…”

“…And as noted, Hirano et al (2016) show evidence supporting a GABA mechanism. However, this latter mechanism seems to be indirect, with GABA released from horizontal cells and activating autoreceptors on the horizontal cells, possibly driving a pH-based feedback mechanism (Liu et al 2013; Hirano et al 2016); a GABA autoreceptor-based mechanism also might be present in lower vertebrates (Klooster et al 2004; Endeman et al 2012). …”

MRI of rod cell compartment-specific function in disease and treatment in vivo

“…The interpretation of these results was not clear but it has been suggested that iodate-induced damage to the photoreceptors produces a loss of glutamatergic signaling to horizontal cells (HC) which in turn impaired HC inhibitory signaling back onto the rods . Normally, HC depolarization in the dark following glutamatergic signaling results in closure of LTCCs on rods (Babai and Thoreson, 2009;Liu et al, 2013). The purpose of HC inhibitory signaling in bright light is most commonly understood in terms of i) controlling conegenerated synaptic gain and ii) lateral inhibition onto cone cells forming the basis of the center-surround organization of vision (Thoreson and Mangel, 2012;Yang and Wu, 1991).…”

Functional architecture of the retina: Development and disease

“…This pathway can also mediate feedforward inhibition by activating GABA receptors on bipolar cell dendrites (Haverkamp et al, 2000). Other feedback pathways are: (i) non-synaptic ‘ephaptic’ route involving the presence of hemichannels (Kamermans and Fahrenfort, 2004; Vroman et al, 2013), (ii) pH changes in the synaptic cleft surrounding photoreceptor terminals (Davenport et al, 2008; Vessey et al, 2005) or (iii) the autaptic action of GABA on horizontal cell GABA receptors (Liu et al, 2013). Much work remains to be carried out to identify the mechanism or mechanisms of horizontal cell action, which may vary across species (e.g.…”