Dopamine Signaling Inversely Affects Motor Neuron and V2 Interneuron Numbers Lateral trunk views at 33 hpf are shown (drug exposure: 24–33 hpf). (A) Treatment with the dopamine agonist pergolide leads to an increase of HB9 immunoreactive motor neurons and a reduction of transgenically labeled presumptive V2 interneurons (vsx1:GFP) in the same embryos. A small subpopulation of double-labeled cells is also strongly reduced (Mann-Whitney U test; p < 0.0001, p < 0.001). (B) NPA, another dopamine agonist, increases the number of HB9:GFP+ motor neurons and decreases the number of vsx1:GFP+ cells (left column). The D4 antagonist L-745870 has the inverse effect on numbers of these cell types (right column). Error bars represent SEM. Scale bars, 50 μm. See also Figure S2.