Horizontal axons exhibit pause behavior in response to high-frequency photostimulation directed at the soma of labeled neurons.

<p><b>A,</b> A horizontal axon from a cell cotransfected with DsRed and ChR2-EYFP was observed. Photostimulation (4 Hz, 1 min) with 475-nm excitation was applied to the cell body after a 60-min observation of axon growth at 560-nm excitation. After the photostimulation at 475-nm excitation, the axon was further observed at 560-nm excitation. Scale bar, 5 µm. <b>B,</b> Growth of the horizontal axon shown in A. The arrow indicates the time of photostimulation. <b>C,</b> Growth of individual axons before and after photostimulation. Despite differences in growth rates, most axons display pause behavior after high-frequency photostimulation (arrow), n = 11. <b>D,</b> Axonal growth rates before and after high-frequency stimulation. After high-frequency photostimulation, axonal growth rate decreased significantly and this decline persisted for 20 min (Student’s <i>t</i>-test, *p<0.005). <b>E,</b> Axonal tip morphology before and after photostimulation. Scale bar, 2 µm.