Seasonal changes in membrane structure and excitability in retinal neurons of goldfish (Carassius auratus) under constant environmental conditions

Adaptations to hypoxia in the vertebrate retina

As part of the CNS, the retina maintains high metabolic activity yet is more tolerant to the adverse effects of low oxygen (hypoxia) than the brain. Tissue hypoxia is a component of ischaemia and has devastating effects on the CNS, including neuronal death. Differences in blood supply, oxygen consumption, and the expression of excitatory receptors and oxygen-binding proteins contribute to regional differences in the sensitivity of retinal neurons to hypoxia. Although highly variable and species-dependent, the mammalian retina has the capacity to withstand the effects of oxygen deprivation through ischaemic preconditioning, where a brief, sublethal stimulus imparts protection against subsequent bouts of hypoxia or ischaemia. Decades of research have demonstrated that hypoxia- and anoxia-tolerant ectotherms are adapted to survive weeks or months in the absence of oxygen without damage to the CNS, nor without the need for a preconditioning stimulus. Recent studies provide evidence that, in the retinas of tolerant animals, such as crucian carp, freshwater turtle and goldfish, prolonged oxygen deprivation elicits a reversible reduction in electroretinogram activity. Moreover, inhibitory interneurons of the outer retina involved in visual processing, called horizontal cells, sense hypoxia via mitochondrial ATP-dependent K+ channels and regulate intracellular Ca2+ homeostasis. An understanding of how these mechanisms may protect the retina from hypoxia has only recently begun to emerge but may involve downregulation of excitatory receptor activity or release of the inhibitory neurotransmitter, GABA, to reduce neuronal activity and reduce the metabolic demands of the retina during oxygen deprivation.

Seasonal regulation and PIP2 dependence of inwardly rectifying potassium channels (Kir) in retinal horizontal cells of goldfish (Carassius auratus)

Many species face extreme changes in their environment throughout the year, during which time they must modify their neuronal activity to survive. Horizontal cells (HCs) are interneurons in the goldfish (Carassius auratus) retina that demonstrate seasonal changes in membrane structure and excitability. Excitability in neurons is strongly influenced by inwardly rectifying potassium channels (Kir), suggesting that Kir in HCs may be a candidate for modulating seasonal changes. Kir channel function depends on the plasma membrane phospholipid, PIP2, to which it is bound. However, it remains unclear whether the PIP2-Kir channel interaction contributes to regulation of Kir in goldfish HCs. Using perforated patch voltage-clamp recording, we identified Kir current by inhibition with cesium (Cs+) and compared peak inward Kir current density in goldfish HCs isolated during the summer and winter months. Significantly more current was inhibited by Cs+ in winter HCs, suggesting more open Kir channels. To examine the role of PIP2 in regulating Kir, tamoxifen or activation of phospholipase C by m-3M3FBS were used to disrupt the PIP2-Kir channel interaction and subsequently reduced Kir current. Spermine, through strengthening the PIP2-Kir interaction, reduced the inhibitory effects of tamoxifen. Additionally, in current-clamp recordings, HCs treated with m-3M3FBS displayed a depolarized membrane potential (Vm) and a decrease in spontaneous Ca2+-based action potentials. We demonstrate that Kir activity is seasonally regulated in goldfish HCs and that Kir is dependent upon PIP2. Our results suggest PIP2 as a potential target for coordinating seasonal changes in the activity of neurons in the goldfish retina.

Three steps down: Metabolic depression in winter-acclimatized crucian carp (Carassius carassius L.)

Acclimatization of certain ectothermic vertebrates to winter conditions is associated with reduced energy consumption (winter dormancy). Principally, this may be achieved by reducing movement activity, depression of basal cellular functions, or by switching from aerobic to anaerobic energy production to sustain low energy consumption during anoxia. Therefore, we determined standard (SMR), routine (RMR) and anoxic (AMR) metabolic rates in summer- (SumA; 20 °C) and winter-acclimatized (WinA; 2 °C) crucian carp (Carassius carassius), an anoxia-tolerant teleost fish. At 20 °C, RMR was 39% lower in WinA than SumA fish (p < 0.05), indicating reduced movements in winter. SMR, measured for the first time in crucian carp, was 45% lower in WinA than SumA fish at 10 °C (p < 0.05), indicating significant reduction of energy consumption in vital cellular processes. At 2 °C, AMR - measured from ethanol production- was 78% and 97% of the SMR and RMR levels at the same temperature, respectively. The current study revealed that the winter dormancy in anoxia-tolerant crucian carp is achieved in 3 different steps: (1) by active reduction of SMR, possibly in anticipation of seasonal anoxia, (2) acute Arrhenius Q10 effect that slows SMR as well as RMR, and (3) direct response to the absence of oxygen (AMR). Furthermore, the anoxic energy production was strongly dependent on body mass with scaling exponents of -0.335 and - 0.421 for WinA and SumA fish, respectively.