Immunohistochemical localization of P2X3 receptors in the zebrafish gill at 16 days post-fertilization.

Co-labelling with antibodies against P2X3 (red) and serotonin (5-HT, green) are shown at low magnification in a whole gill complex in ventral view (AC), and at higher magnification in an isolated gill arch (DF). (A) Many P2X3-positive cells were observed on all four gill arches, as indicated by numerals IIV on the left side of the gill complex. (B) Labelling by anti-5-HT identified primarily Merkel-like cells (arrows) of the gill arches with few neuroepithelial cells (NECs, arrowheads) of the gill filaments visible. (C) Merge of panels A and B. (D) In isolated gills, the gill arch (GA) displayed P2X3-positive neurons (arrow), and a nerve bundle (arrowheads); gill filament primordia (F) retain several P2X3-positive cells (numbered 14; note that cell 3 is P2X3-negative). In addition, the filament at the right displayed a pattern of P2X3 activity that was reminiscent of nerve fibres (arrowhead). (E) 5-HT immunoreactivity revealed that P2X3-positive neurons were also 5-HT positive. From D, cell 1 was 5-HT-negative; cells 2 and 4 were P2X3- and 5-HT-positive; cell 3 was 5-HT-positive and P2X3-negative. (F) Merge of panels D and E. Scale bar in A = 50 um; in B = 10 µm.