Figure 2.

GABA appears within the modulatory input system to the STG after metamorphosis. A1, A2, Position of the main STNS ganglia depicted on schematic drawings of the embryonic (A1) and adult (A2) STNS. B, Microphotographs of GABA immunoreactivity in the CoG (top), STG (middle), and OG (bottom) observed at different developmental stages. Note the intense staining in the CoG of embryonic, larval, and postmetamorphic (LIV, juvenile, and adult) stages. In juvenile CoG, letters indicate the presence of stained neuropil (n), somata (s), and fibers (f). GABA staining can only be detected in the STG after metamorphosis. Indeed no staining is visible in the STG in premetamorphic (embryo, LIII) animals whereas in postmetamorphic animals, a faint neuropil is visible within the most anterior part of the STG. Note that the inset in the adult STG micrograph illustrates strongly stained fibers entering an other STG (see Results, Acquisition of GABA in the developing lobster nervous system). Scale bar, 100 μm. Similarly, embryonic and larval (LIII) OG usually display no GABA-immunoreactive cells whereas, typically, several soma are stained in the LIV, juvenile, and adult OG. All images are maximal projections of 20–40 optical sections. C, Distribution of the number of OG-stained soma for each developmental stage. D, Diagrams summarizing the chronology of appearance of GABA within the modulatory input system to the STG. The top panel shows the percentage of preparations in which a stained neuropil was observed within the STG for each developmental stage. The bottom panel shows the mean number of stained soma in the OG for each developmental stage (mean ± SEM). Note that the mean number of GABAergic OG soma in embryo, LII, and LIII is statistically different from the mean number observed in adult (*p < 0.005, one way ANOVA followed by Mann–Whitney test), whereas no statistically significant difference can be observed for LI OG (p = 0.05).