Circadian- and Light-Dependent Regulation of Resting Membrane Potential and Spontaneous Action Potential Firing of Drosophila Circadian Pacemaker Neurons

Vasu Sheeba,Todd C Holmes,Huaiyu Gu,Vijay K Sharma,Diane K O'Dowd

doi:10.1152/jn.00930.2007

Abstract

The ventral lateral neurons (LNvs) of adult Drosophila brain express oscillating clock proteins and regulate circadian behavior. Whole cell current-clamp recordings of large LNvs in freshly dissected Drosophila whole brain preparations reveal two spontaneous activity patterns that correlate with two underlying patterns of oscillating membrane potential: tonic and burst firing of sodium-dependent action potentials. Resting membrane potential and spontaneous action potential firing are rapidly and reversibly regulated by acute changes in light intensity. The LNv electrophysiological light response is attenuated, but not abolished, in cry(b) mutant flies hypomorphic for the cell-autonomous light-sensing protein CRYPTOCHROME. The electrical activity of the large LNv is circadian regulated, as shown by significantly higher resting membrane potential and frequency of spontaneous action potential firing rate and burst firing pattern during circadian subjective day relative to subjective night. The circadian regulation of membrane potential, spontaneous action potential firing frequency, and pattern of Drosophila large LNvs closely resemble mammalian circadian neuron electrical characteristics, suggesting a general evolutionary conservation of both physiological and molecular oscillator mechanisms in pacemaker neurons.

Highlights

Circadian clocks in animals synchronize the timing of physiological and behavioral events to environmental cycles of day and night
Flies were killed during a wide 7-h “daytime” window: Zeitgeber Time (ZT) 1 to ZT8
To determine the electrophysiological properties of the circadian pacemaker neurons of adult Drosophila melanogaster, we recorded from neurons in acutely dissected adult Drosophila whole brain preparations with intact photoreceptors

Summary

Introduction

Circadian clocks in animals synchronize the timing of physiological and behavioral events to environmental cycles of day and night. The primary environmental cue that entrains the circadian clock and behavior, is transduced in Drosophila pacemaker neurons cell-autonomously by the blue light–sensing protein CRYPTOCHROME (CRY) and by light-driven synaptic inputs (Emery et al 1998; Helfrich-Forster et al 2001; Stanewsky et al 1998). Neurophysiological characterization of Drosophila pacemaker neurons has lagged behind our molecular understanding of the circadian clock. This has begun to change, first by molecular genetic analysis of transgenic flies that express modified ion channels in pacemaker neurons and ion channel mutant flies (de la Paz Fernandez et al 2007; Lear et al 2005; Nitabach et al 2002, 2005b, 2006). This work was followed more recently by direct patch-clamp analysis of Drosophila pacemaker neurons along with single-cell fills that permitted an unprecedented level of morphological detail of single large ventral lateral neurons (LNvs), this study did not depict spontaneous action potentials in the large LNvs (Park and Griffith 2006)

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Conclusion