Ontogeny of central serotonergic neurons in the directly developing frog, Eleutherodactylus coqui

Abstract

Embryonic development of the central serotonergic neurons in the directly developing frog, Eleutherodactylus coqui, was determined by using immunocytochemistry. The majority of anuran amphibians (frogs) possess a larval stage (tadpole) that undergoes metamorphosis, a dramatic post-embryonic event, whereby the tadpole transforms into the adult phenotype. Directly developing frogs have evolved a derived life-history mode where the tadpole stage has been deleted and embryos develop directly into the adult bauplan. Embryonic development in E. coqui is classified into 15 stages (TS 1-15; 1 = oviposition/15 = hatching). Serotonergic immunoreactivity was initially detected at TS 6 in the raphe nuclei in the developing rhombencephalon. At TS 7, immunopositive perikarya were observed in the paraventricular organ in the hypothalamus and reticular nuclei in the hindbrain. Development of the serotonergic system was steady and gradual during mid-embryogenesis. However, starting at TS 13 there was a substantial increase in the number of serotonergic neurons in the paraventricular, raphe, and reticular nuclei, a large increase in the number of varicose fibers, and a differentiation of the reticular nuclei in the hindbrain. Consequentially, E. coqui displayed a well-developed central serotonergic system prior to hatching (TS 15). In comparison, the serotonergic system in metamorphic frogs typically starts to develop earlier but the surge of development that transpires in this system occurs post-embryonically, during metamorphosis, and not in the latter stages of embryogenesis, as it does in E. coqui. Overall, the serotonergic development in E. coqui is similar to the other vertebrates.

A novel whole-embryo culture model for pharmaceutical and developmental studies

“…Many developmental studies have utilized E. coqui, and much of their development, particularly neural, bone, and cartilage ontogeny has been described (Elinson, 2013;Hanken, Klymkowsky, Summers, Seufert, & Ingebrigtsen, 1992;Kerney, Gross, & Hanken, 2010;Kerney & Hanken, 2008;Kulkarni, Singamsetty, & Buchholz, 2010;Ten Eyck et al, 2005). Whole-embryo culture and staining could also yield studies that more easily examine developmental stages.…”

“…Serotonin (5-HT) has been found in both vertebrate and invertebrate zygotes and has been shown to regulate early embryological events (Buznikov et al, 1996;Lauder, 1993) and bone morphogenesis (Yadav et al, 2008). In particular, 5-HT, 5-HT 1A and 5-HT 2 receptors, and 5-HT T (transporter) are detectable as early as blastocoele and gastrulation events in amphibians, chicks, fish, rodents, and sea urchins (Moiseiwitsch & Lauder, 1995;Nebigil et al, 2001;Ten Eyck et al, 2005).…”

“…Investigations have shown that monoaminergic systems, particularly serotonin, are essential for normal neuronal development (Buznikov, Shmukler, & Lauder, 1996;Kruzelock & Barbato, 1991;Lauder, 1993;Lauder & Bloom, 1974;Nebigil et al, 2001;Revilla, Díez-Alarcia, Mostany, Pérez, & Fernández-López, 2001;Siuciak, Gamache, & Dubocovich, 1992;Ten Eyck, Jermakowicz, Chinn, & Summers, 2005). Serotonin (5-HT) has been found in both vertebrate and invertebrate zygotes and has been shown to regulate early embryological events (Buznikov et al, 1996;Lauder, 1993) and bone morphogenesis (Yadav et al, 2008).…”