Haris Alexopoulos

University of Oxford, Oxford, England, United Kingdom

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Publications (4)34.08 Total impact

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    ABSTRACT: Brain glucose sensing is critical for healthy energy balance, but how appropriate neurocircuits encode both small changes and large background values of glucose levels is unknown. Here, we report several features of hypothalamic orexin neurons, cells essential for normal wakefulness and feeding: (i) A distinct group of orexin neurons exhibits only transient inhibitory responses to sustained rises in sugar levels; (ii) this sensing strategy involves time-dependent recovery from inhibition via adaptive closure of leak-like K(+) channels; (iii) combining transient and sustained glucosensing allows orexin cell firing to maintain sensitivity to small fluctuations in glucose levels while simultaneously encoding a large range of baseline glucose concentrations. These data provide insights into how vital behavioral orchestrators sense key features of the internal environment while sustaining a basic activity tone required for the stability of consciousness.
    Proceedings of the National Academy of Sciences 09/2008; 105(33):11975-80. · 9.81 Impact Factor
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    ABSTRACT: Glucose-inhibited neurons orchestrate behavior and metabolism according to body energy levels, but how glucose inhibits these cells is unknown. We studied glucose inhibition of orexin/hypocretin neurons, which promote wakefulness (their loss causes narcolepsy) and also regulate metabolism and reward. Here we demonstrate that their inhibition by glucose is mediated by ion channels not previously implicated in central or peripheral glucose sensing: tandem-pore K(+) (K(2P)) channels. Importantly, we show that this electrical mechanism is sufficiently sensitive to encode variations in glucose levels reflecting those occurring physiologically between normal meals. Moreover, we provide evidence that glucose acts at an extracellular site on orexin neurons, and this information is transmitted to the channels by an intracellular intermediary that is not ATP, Ca(2+), or glucose itself. These results reveal an unexpected energy-sensing pathway in neurons that regulate states of consciousness and energy balance.
    Neuron 07/2006; 50(5):711-22. · 15.77 Impact Factor
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    ABSTRACT: The activity of hypothalamic neurons that release the neuropeptides orexin-A and orexin-B is essential for normal wakefulness. Orexin neurons fire spontaneously and are hyperpolarized and inhibited by physiological neuromodulators, but the intrinsic determinants of their electrical activity are poorly understood. We show that mouse orexin neurons coexpress orexin-A and orexin-B, and possess a low-voltage-activated A-type K(+) current (A-current) likely to be composed of Kv4.3 subunits. The A-current enhances the inhibitory influence of hyperpolarizing currents via two mechanisms: by delaying the resumption of spiking after hyperpolarization and by increasing the slope of the relation between the firing frequency and injected current. These results identify an important determinant of the firing dynamics of orexin neurons, and support the idea that the A-current can control neuronal gain.
    European Journal of Neuroscience 01/2005; 20(12):3281-5. · 3.75 Impact Factor
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    Denis Burdakov, Haris Alexopoulos
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    ABSTRACT: Hypothalamic neurons that produce the peptide transmitters hypocretins/orexins have attracted much recent attention. They provide direct and predominantly excitatory inputs to all major brain areas except the cerebellum, with the net effect of stimulating wakefulness and arousal. These inputs are essential for generating sustained wakefulness in mammals, and defects in hypocretin signalling result in narcolepsy. In addition, new roles for hypocretins/orexins are emerging in reward-seeking, learning, and memory. Recent studies also indicate that hypocretin/orexin neurons can alter their intrinsic electrical activity according to ambient fluctuations in the levels of nutrients and appetite-regulating hormones. These intriguing electrical responses are perhaps the strongest candidates to date for the elusive neural correlates of after-meal sleepiness and hunger-induced wakefulness. Hypocretin/orexin neurons may thus directly translate rises and falls in body energy levels into different states of consciousness.
    Journal of Cellular and Molecular Medicine 01/2005; 9(4):795-803. · 4.75 Impact Factor