Ablation of V2a interneurons in the larval zebrafish. (A) V2a interneurons were ablated over 10 segments in the midbody region of larval zebrafish. (B) Reconstruction of 1.5 segments of the spinal cord in zebrafish using confocal microscopy before the ablation of V2a interneurons. (C) The spinal cord was scanned and reconstructed after photoablation of eight V2a interneurons (asterisks). (Right) Expanded image of the area indicated by the dashed boxes. (Left scale bar, 10 μm and Right scale bar, 5 μm.) (D) Motoneurons were prelabeled with rhodamine dextran in larval zebrafish expressing GFP in V2a interneurons. Ablation of V2a interneurons did not produce any secondary damage in adjacent motoneurons. (Scale bar, 10 μm.)

Ablation of glycinergic (GlyT2) interneurons does not alter swimming activity. (A, Upper) Drawing of a zebrafish showing the region where GlyT2 interneurons are ablated. (Lower) Confocal reconstruction of 1.5 segment of the spinal cord before and after photoablation of GlyT2 interneurons. Asterisks indicate the position of the ablated interneurons. (Right) High magnification of the regions indicated by the dashed boxes. (Left scale bar, 10 μm and Right scale bar, 5 μm.) (B) Electrical stimulation at threshold intensity (1 × T; arrow) induces a bout of swimming activity in animals with GlyT2 interneurons ablated. (C) Application of a lower NMDA concentration (75 μM) induces swimming activity both in the region of the ablation and in more caudal segments. (D) Burst frequency and duration of the swimming bout recorded in animals with GlyT2 interneurons ablated is similar to control animals. (E) Cumulative distribution of the burst frequency in control and in animals with GlyT2 interneurons ablated.