Fig. 5

GnRH-Expressing Terminal Nerve Is a CO2 Sensor for Avoidance Behavior (A) Dorsal view of Tg(GnRH3:GFF;UAS:G-CaMP7) zebrafish larva expressing G-CaMP7 in the terminal nerve (TN). Dotted rectangle marks the location of the terminal nerve shown in (B). Scale bar, 100 μm. (B) Left: resting-state fluorescence in the terminal nerve. Right: calcium increase in the terminal nerve after CO2 application. Scale bar, 20 μm. (C) Temporal profiles of G-CaMP7 fluorescence (ΔF/F) in the terminal nerve upon application of vehicle (black), taurocholic acid (blue), and CO2 (red) to the nose. Mean ΔF/F (solid line) and SEM (transparency) (n = 4). Gray-shaded areas indicate the period of CO2 application (4 s). (D) Dorsal fluorescence views of Tg(GnRH3:GFF;UAS:GFP) zebrafish larvae before (left) and after (right) the laser ablation of terminal nerve. Arrows indicate the location of the terminal nerve. (E) CO2-evoked avoidance response in intact (gray, n = 12) and terminal nerve-ablated (red, n = 12) zebrafish after exposure to a high concentration of CO2 (700–780 mg/L). Wilcoxon signed rank sum test for comparisons between intact fish and terminal nerve-ablated fish. ∗∗∗p < 0.001. (F) Schematic model summarizing the neural circuit mechanism underlying the CO2-evoked avoidance behavior in larval zebrafish. CO2 activates four distinct neural pathways: terminal nerve (red), trigeminal (orange), olfactory (green), and habenulo-interpeduncular (blue). The terminal nerve-trigeminal pathway (nose → terminal nerve → trigeminal ganglion → trigeminal sensory nucleus [TSN] → reticulospinal neurons [RSNs]) contributes to the emergence of the slow avoidance response.