title: Proton-mediated feedback inhibition of presynaptic calcium channels at the cone photoreceptor synapse.
abstract: Generation of center-surround antagonistic receptive fields in the outer retina occurs via inhibitory feedback modulation of presynaptic voltage-gated calcium channels in cone photoreceptor synaptic terminals. Both conventional and unconventional neurotransmitters, as well as an ephaptic effect, have been proposed, but the intercellular messaging that mediates the inhibitory feedback signal from postsynaptic horizontal cells (HCs) to cones remains unknown. We examined the possibility that proton concentration in the synaptic cleft is regulated by HCs and that it carries the feedback signal to cones. In isolated, dark-adapted goldfish retina, we assessed feedback in the responses of HCs to light and found that strengthened pH buffering reduced both rollback and the depolarization to red light. In zebrafish retinal slices loaded with Fluo-4, depolarization with elevated K(+) increased Ca signals in the synaptic terminals of cone photoreceptors. Kainic acid, which depolarizes HCs but has no direct effect on cones, depressed the K(+)-induced Ca signal, whereas CNQX, which hyperpolarizes HCs, increased the Ca signals, suggesting that polarization of HCs alters inhibitory feedback to cones. We found that these feedback signals were blocked by elevated extracellular pH buffering, as well as amiloride and divalent cations. Voltage clamp of isolated HCs revealed an amiloride-sensitive conductance that could mediate modulation of cleft pH dependent on the membrane potential of these postsynaptic cells.
authors.affiliation: Department of Physiology and Biophysics, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4H7.
authors.forename: John P
authors.initials: JP
authors.lastname: Vessey
authors.forename: Anna K
authors.initials: AK
authors.lastname: Stratis
authors.forename: Bryan A
authors.initials: BA
authors.lastname: Daniels
authors.forename: Noel
authors.initials: N
authors.lastname: Da Silva
authors.forename: Michael G
authors.initials: MG
authors.lastname: Jonz
authors.forename: Melanie R
authors.initials: MR
authors.lastname: Lalonde
authors.forename: William H
authors.initials: WH
authors.lastname: Baldridge
authors.forename: Steven
authors.initials: S
authors.lastname: Barnes
mesh_terms: D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D003035:Cobalt / Q000494:pharmacology; D004307:Dose-Response Relationship, Radiation; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005246:Feedback / Q000502:physiology*; D018698:Glutamic Acid / Q000494:pharmacology; D006054:Goldfish; D006531:HEPES / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007608:Kainic Acid / Q000494:pharmacology; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology / Q000528:radiation effects; D008704:Methazolamide / Q000494:pharmacology; D018613:Microscopy, Confocal / Q000379:methods; D018408:Patch-Clamp Techniques / Q000379:methods; D011188:Potassium / Q000494:pharmacology; D011522:Protons; D012160:Retina / Q000166:cytology*; D017949:Retinal Cone Photoreceptor Cells / Q000166:cytology / Q000502:physiology*; D051248:Retinal Horizontal Cells / Q000502:physiology; D026941:Sodium Channel Blockers / Q000494:pharmacology; D013569:Synapses / Q000502:physiology*; D015027:Zebrafish
chemical_list: D001639:Bicarbonates; D015220:Calcium Channels; D002257:Carbonic Anhydrase Inhibitors; D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D011522:Protons; D026941:Sodium Channel Blockers; D003035:Cobalt; D018698:Glutamic Acid; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D000584:Amiloride; D011188:Potassium; D006531:HEPES; D007608:Kainic Acid; D002118:Calcium; D008704:Methazolamide