Serotonin synthesis and accumulation by neurons of the anuran retina

Abstract

Serotonin-synthesizing and serotonin-accumulating neurons were studied in the retinas of Xenopus laevis and Bufo marinus. All previously identified cell types exhibiting serotonin-like immunoreactivity (SLI) were labeled by intravitreal injection of 5,7-dihydroxytryptamine (5,7-DHT). They included two amacrine cell types (large and small) in both species, and one bipolar cell type in Xenopus. Incubation of retinas in culture medium in the ambient light reduced SLI in amacrine cells and enhanced the labeling in bipolar cells. After incubation, some photoreceptor cell bodies and large numbers of outer segments also displayed SLI in both species. Incubation with the serotonin-uptake inhibitor, fluoxetine, reduced immunolabeling in bipolar cells and outer segments to the level in the untreated retinas. Both large SLI and 5,7-DHT-accumulating amacrine cells in Xenopus and Bufo were labeled with an antibody raised against phenylalanine hydroxylase (PH), which binds to tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin. Small SLI and 5,7-DHT-accumulating amacrine cells in both species represented two populations, one with and the other without PH-like immunoreactivity (PH-LI). The anti-PH antibody failed to label any SLI or 5,7-DHT-accumulating bipolar cells in Xenopus. These observations indicate that all large and some small SLI amacrine cells in the retinas of Xenopus and Bufo synthesize serotonin, while other small SLI amacrine, bipolar and photoreceptor cell bodies, and outer segments only accumulate serotonin.

Colocalization of GABA-immunoreactivity in neuropeptide- and monoamine-containing amacrine cells in the retina of Bufo marinus.

“…In the present study we showed that only a portion of 5HT-immunoreactive cells contained GABA. We assume that both 5HT-synthesizing and -accumulating neurons (ZHU et al, 1992) colocalize with GABA in the Bufo retina. It has been demonstrated recently that the large-sized 5HT-synthesizing amacrine cell type is immunoreactive for microtubule-associated protein 2 (MAP2) in Bufo retina (GABRIEL et al, 1992b); however, no overlap between MAP2-and GABAimmunoreactive cells has been found.…”

“…The latter antibody, apart from phenylalanine hydroxylase which is not present in nervous tissue, recognizes tryptophan 5-hydroxylase and tyrosine hydroxylase (HAAN et al, 1987). Thus with some precaution, this antibody can be used as a marker for 5HT-synthesizing cells in the anuran retina (ZHu et al, 1992). The primary antibodies were applied overnight at room temperature, the sections then being washed in PBS.…”

“…Preabsorption controls for the peptide antigens (HISCOCK and STRAZNICKY, 1989a, b) and control labelling experiments in the brain stem with the antibodies raised against TH and PH (WILHELM et al, 1993) have already been described. Cross-reactivities between non-corresponding primary and secondary antibodies as well as leakage of nonspecific wavelength light through the microscope filters were tested according to previous descriptions (ZHu et al, 1992). The labelling pattern achieved by the mouse and rabbit anti-GABA antibodies were directly compared, revealing that they labelled the same structures in the retina of Bufo marinus.…”

Interaction between the serotoninergic and GABAergic systems in frog retina as revealed by electroretinogram

“…Serotonin (5-hydroxytryptamine; 5-HT) is one of the monoamine neurotransmitters in the retina, which is better represented in lower than higher vertebrate retinas (Osborne et al 1982). It is released by serotoninergic amacrine cells obtained in the retinas of many species (Ehinger 1983, Zhu et al 1992, Hurd and Eldred 1993, Gábriel 2000, Vigh et al 2000. Some serotonin-accumulating neurons are found among horizontal and bipolar cells, but it is thought that they are only able to transport and metabolize, but not synthesize serotonin (Marc et al 1988, Zhu et al 1992, Wilhelm et al 1993, Schutte 1994, Ghai et al 2009).…”

“…It is released by serotoninergic amacrine cells obtained in the retinas of many species (Ehinger 1983, Zhu et al 1992, Hurd and Eldred 1993, Gábriel 2000, Vigh et al 2000. Some serotonin-accumulating neurons are found among horizontal and bipolar cells, but it is thought that they are only able to transport and metabolize, but not synthesize serotonin (Marc et al 1988, Zhu et al 1992, Wilhelm et al 1993, Schutte 1994, Ghai et al 2009). Serotonin is also released by retinopetal axons that originate in dorsal raphe in fishes, rodents and primates (Villar et al 1987, Shen and Semba 1994, Lima and Urbina 1998, Gastinger et al 2005.…”