Abstract
Optogenetics offers a unique method to regulate the activity of select neural circuits. However, the electrophysiological consequences of targeted optogenetic manipulation upon the entire circuit remain poorly understood. Analysis of the sensory-CNS-motor circuit in Drosophila larvae expressing eHpHR and ChR2-XXL revealed unexpected patterns of excitability. Optical stimulation of motor neurons targeted to express eNpHR resulted in inhibition followed by excitation of body wall contraction with repetitive stimulation in intact larvae. In situ preparations with direct electrophysiological measures showed an increased responsiveness to excitatory synaptic activity induced by sensory stimulation within a functional neural circuit. To ensure proper function of eNpHR and ChR2-XXL they were expressed in body wall muscle and direct electrophysiological measurements were obtained. Under eNpHR induced hyperpolarization the muscle remained excitable with increased amplitude of excitatory postsynaptic synaptic potentials. Theoretical models to explain the observations are presented. This study aids in increasing the understanding of the varied possible influences with light activated proteins within intact neural circuits.
Highlights
The ability to alter selective neural circuits at various times with high temporal resolution is possible with a variety of optical techniques
The ChR2-XXL expressed in motor neurons is very sensitive to white light, even without supplementing the food with All trans-retinal (ATR)
Note that the 1st light exposure resulted in the larvae relaxing and becoming flaccid but with subsequent light exposures the larvae were contracted for the motor neurons expressing eNpHR
Summary
The ability to alter selective neural circuits at various times with high temporal resolution is possible with a variety of optical techniques. Recent approaches are advancing in the use of flexible and very thin optic fibers to control neurons in deep regions of the CNS in model animals by the use of light activated proteins (i.e., optogenetics) with good spatial and temporal resolution [4,5,6,7,8]. To measure synaptic efficacy within neural circuits, direct measures of the postsynaptic target’s membrane potential and frequency of activity are needed. Both can be relatively hard to measure with imaging techniques.
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