Influence of distant femtosecond laser pulses on growth cone fillopodia

Spatial temperature gradients guide axonal outgrowth

Formation of neural networks during development and regeneration after injury depends on accuracy of axonal pathfinding, which is primarily believed to be influenced by chemical cues. Recently, there is growing evidence that physical cues can play crucial role in axonal guidance. However, detailed mechanism involved in such guidance cues is lacking. By using weakly-focused near-infrared continuous wave (CW) laser microbeam in the path of an advancing axon, we discovered that the beam acts as a repulsive guidance cue. Here, we report that this highly-effective at-a-distance guidance is the result of a temperature field produced by the near-infrared laser light absorption. Since light absorption by extracellular medium increases when the laser wavelength was red shifted, the threshold laser power for reliable guidance was significantly lower in the near-infrared as compared to the visible spectrum. The spatial temperature gradient caused by the near-infrared laser beam at-a-distance was found to activate temperature-sensitive membrane receptors, resulting in an influx of calcium. The repulsive guidance effect was significantly reduced when extracellular calcium was depleted or in the presence of TRPV1-antagonist. Further, direct heating using micro-heater confirmed that the axonal guidance is caused by shallow temperature-gradient, eliminating the role of any non-photothermal effects.

Signalling effect of NIR pulsed lasers on axonal growth

In this work we show that a pulsed laser light placed at a distance is able to modulate the growth of axons of primary neuronal cell cultures. In our experiments continuous wave (CW), chopped CW and modelocked fs (FS) laser light was focused through a microscope objective to a point placed at a distance of about 15 microm from the growth cone. We found that CW light does not produce any significant influence on the axon growth. In contrast, when using pulsed light (chopped CW light or FS pulses), the beam was able to modify the trajectory of the axons, attracting approximately 45% of the observed cases to the beam spot. Such effect could possibly indicate the capacity of neurons to interpret the pulsating NIR light as the source of other nearby cells, resulting in extension of processes in the direction of the source.

Automated laser guidance of neuronal growth cones using a spatial light modulator

The growth cone of a developing neuron can be guided using a focused infra-red (IR) laser beam [1]. In previous setups this process has required a significant amount of user intervention to adjust continuously the laser beam to guide the growing neuron. Previously, a system using an acousto-optical deflector (AOD) has been developed to steer the beam [2]. However, to enhance the controllability of this system, here we demonstrate the use of a computer controlled spatial light modulator (SLM) to steer and manipulate the shape of a laser beam for use in guided neuronal growth. This new experimental setup paves the way to enable a comprehensive investigation into beam shaping effects on neuronal growth and we show neuronal growth initiated by a Bessel light mode. This is a robust platform to explore the biochemistry of this novel phenomenon.