Dopamine and serotonin turnover rate in the retina of rabbit, rat, goldfish, and Eugerres plumieri: Light effects in goldfish and rat

Abstract

The concentration of dopamine, and its metabolites 3,4-dihydroxyphenylacetic and homovanillic acids, as well as serotonin and its metabolite 5-hydroxyindoleacetic acid, were determined in the retina of two teleosts, C. auratus (goldfish) and E. plumieri (mojarra), and two mammals, R. norvegicus (rat) and O. cuniculus (rabbit). The turnover rate of these monoamines were investigated in the four species by the calculation of the ratio monoamine/metabolite as an indirect index, and in goldfish and rat by the inhibition of the synthesis with alpha-methyl-p-tyrosine or p-chlorophenylalanine, by the increase in dopamine or serotonin by the corresponding precursors, 3,4-dihydroxyphenylalanine or 5-hydroxytryptophan, and by inhibition of monoaminooxidase with pargyline. The modulation by light and dark stimulation was studied in the goldfish and the rat. Differences in the concentration and turnover rate were observed among the species. Serotonin concentration was higher in the teleosts. The administration of inhibitors of dopamine and serotonin synthesis differentially decreased the levels of the monoamines in the retina of goldfish and rat. The rate of formation of dopamine and serotonin by the corresponding precursors was much higher in the goldfish than in the rat. Pargyline administration decreased 3,4-dihydroxyphenylacetic and 5-hydroxyindoleacetic acids at different rates and time dependency in the retina of goldfish and rat. Dopamine and serotonin concentration did not exhibit high modifications by the inhibitor, suggesting the function of regulatory mechanisms or additional effect of pargyline at other sites different from monoaminooxidase.(ABSTRACT TRUNCATED AT 250 WORDS)

Dopamine and Serotonin Modulate Human GABAρ1 Receptors Expressed in Xenopus laevis Oocytes

“…In conclusion, we found that DA and 5-HT directly modulate homomeric GABAρ1 receptors but not GABA A receptors. Since the concentrations of DA and 5-HT in the retina vary between 0.9 and 470 μM and 0.1 and 590 μM, ,− respectively, both of which are within the range of modulation we found for GABAρ1, it is plausible that these interactions occur in the retina. Such possibilities will be explored in future studies.…”

The serotonergic system in fish

“…Hence, we can not conclude whether 5-HT acts as a melatonin precursor in the retina and/or whether it modulates retinal function. Notably, in goldfish (Carassius auratus) and mojarra (Eugerres plumieri) 5-HT metabolism changes with photoperiod and can be blocked by p-chlorophenylalanine (PCPA), a Tph activity blocker (Lima and Urbina, 1994). Among mammals retinal 5-HT levels are low and it seems to be accumulated in some cells rather than being synthesized (Osborne and Barnett, 1990).…”

Serotonin synthesis and its light–dark variation in the rat retina

“…However, although the enzymatic pathway has now been identified in the rat retina, the major question of the origin(s) of 5-HT, as precursor of melatonin, is still unresolved. 5-HT can neither be consistently detected within the retina of adult rat (Lima and Urbina 1994), nor can its presence be demonstrated using immunohistochemical techniques (Tornqvist et al 1983), whereas N-acetyl serotonin, which is the product of AANAT, the substrate of HIOMT and precursor of melatonin, has been detected (Pang et al 1981).…”

“…However, a weak signal has been obtained for TPH mRNAs (Besançon et al 1997). TPH activity does seem to occur as the TPH inhibitor p-chlorophenylalanine decreases 5-HT concentration (Lima and Urbina 1994). In contrast, aromatic L-amino acid decarboxylase (AADC; EC 4.1.1.28), the enzyme that decarboxylates 5-hydroxytryptophan (5-HTP) to 5-HT has been localized within rat photoreceptor cells (Nguyen-Legros et al 1994).…”