Three Distinct Mechanisms of Neuro-Muscular Junction Excitation by Infrared Pulses

Abstract

Post-synaptic currents (PSCs) were recorded using whole cell voltage clamp to examine excitatory responses of the neuro-muscular junction to infrared pulses (1862nm, 200μs/pulse, 5-30 pulses/s, 20-1000 μJ/pulse over 125 nm2). In wild type Caenorhabditis elegans, optical stimuli excited the post-synaptic cell by 1) immediate opto-mechanical triggering of pulse-by-pulse miniature excitatory currents (mLEC) with 0.7ms latency to peak and 2) relatively slow (τ∼1.3s onset) thermodynamically driven reduction in a tonic outward rectified K+ current (LTC). The same optical stimuli acted on the pre-synaptic neuron to 3) rapidly increase the rate of synaptic vesicle release and the rate of miniature PSCs (mPSCs). In addition, mPSC kinetics were increased with infrared stimulation resulting in a decrease in average charge per event from 52 to 32fC. The pulse-by-pulse mLECs were enhanced in muscle degenerin gain of function mutant (unc-105) suggesting the fast response was due to opto-mechanical activation of the degenerin stretch receptor. The slow tonic current (LTC) reversed at the K+ equilibrium potential, exhibited a highly rectified outward conductance, and a thermal-dependent closure analogous to shaker related channels including Kv1.1. In the pre-synaptic neuron, the spontaneous rate of synaptic vesicle release and the laser-evoked increase was nearly eliminated in a loss of function mutation of the voltage insensitive cation leak channel (unc-77, nca-1). The increased mPSC rate (presynaptic action) and reduction in the tonic outward K+ current (post-synaptic action) contributed in nearly equal proportions at −60mV holding potential and accounted for over 90% of the total laser-evoked PSC. [Supported by NIH R01 DC006685 & R01DC011481]