Patterns of appearance of serotonin and proctolin immunoreactivities in the developing nervous system of the American lobster

Serotonin (5‐HT) and proctolin, neurohormones widely distributed in the lobster nervous system, have been implicated in a variety of behaviors and also are known to coexist in large pairs of identified neurons in the fifth thoracic (T5) and first abdominal ganglia (A1) of adults (Siwicki, Beltz, and Kravitz, 1987). Earlier studies also have shown that these paired neurons already contain 5‐HT in embryos approximately halfway through development, whereas protolin immunoreactivity does not appear in these cells until near the time of hatching (Beltz and Kravitz, 1987a). In the current studies, the brain and ventral nerve cord have been screened for the appearance of serotonin and proctolin immunoreactivities using immunocytochemical and biochemical methods, in order to determine whether the late appearance of proctolin in the paired T5 and A1 cells is a general feature of development in other neurons as well. In embryos approximately halfway through development, the adult complement of 5‐HT‐staining cells is already present. In several cases, embryonic serotonin cells are proportionally very large and prominent, suggesting possible developmental roles. In contrast to serotonin, fewer than 10% of the proctolin‐staining neurons of juvenile animals are seen in embryos halfway through development. The number of immunoreactive cells gradually increases, but even by the sixth larval stage only half the number of cells that will eventually stain for proctolin are observed. Therefore, the developmental appearance of proctolin in lobster neurons, assayed using immunocytochemical methods, is relatively late and protracted compared to the appearance of serotonin. Quantitative measurements for 5‐HT in lobster larvae were performed using high pressure liquid chromatography (HPLC) with dual electrochemical detection, and for proctolin using radioimmunoassay. A gradual, probably growth‐related increase in the amounts of serotonin and proctolin were seen during larval development. The implications of the biochemical data, in light of the immunocytochemical studies, are discussed.