Mitochondrial ATP-dependent K+ channels downregulate ionotropic glutamate receptors during hypoxia in retinal horizontal cells of goldfish

Abstract

Retinal neurons rely on continuous oxygen supply to maintain function, yet some species, like goldfish, withstand prolonged hypoxia due to adaptations that remain unclear. Horizontal cells (HCs) are inhibitory interneurons of the retina that express ionotropic glutamate receptors (iGluRs) and secrete gama-aminobutyric acid (GABA), allowing them to shape visual signalling by feeding back to photoreceptors. We investigated whether iGluRs in dissociated goldfish HCs are suppressed during hypoxia, and whether this process is mediated by oxygen-sensitive activation of mitochondrial ATP-sensitive K+ (mKATP) channels. Through perforated patch-clamp electrophysiology, we determined that 20 min of hypoxia reduced peak inward current density associated with glutamate application by ~25% (P = 0.0059). When mKATP channels were selectively blocked with 100 microM glibenclamide, hypoxic suppression of iGluRs was abolished. Inhibiting mitochondrial Ca2+ uptake via the mitochondrial Ca2+ uniporter with ruthenium red, or blocking Ca2+ release through ryanodine receptors in the endoplasmic reticulum, also abolished the hypoxia-induced suppression. Similarly, inhibition of calmodulin with calmidazolium, or of protein phosphatase 1 and 2A (PP1/PP2A) activity with calyculin A, prevented the hypoxia-induced suppression of iGluRs. Taken together, these results indicate that mKATP activation is required to mediate an intracellular Ca2+ signal that downregulates iGluR activity during hypoxia. Through the reduction of iGluR activity, HCs may lower their metabolic demand, allowing them to maintain cell function during hypoxia. Additionally, GABA release from HCs, as measured by mass spectrometry, was not affected by hypoxia, and we propose that downregulation of iGluR may account for hypoxic suppression of the retina in hypoxia-tolerant organisms.