Dynamic characteristics of serotonin and dopamine metabolism in the rainbow trout brain: a regional study using liquid chromatography with electrochemical detection

Distribution, properties, and inhibitor sensitivity of zebrafish catechol-O-methyl transferases (COMT)

Catechol-O-methyltransferase (COMT; EC 2.1.1.6) is an enzyme with multiple functions in vertebrates. COMT methylates and thus inactivates catecholamine neurotransmitters and metabolizes xenobiotic catechols. Gene polymorphism rs4680 that influences the enzymatic activity of COMT affects cognition and behavior in humans. The zebrafish is widely used as an experimental animal in many areas of biomedical research, but most aspects of COMT function in this species have remained uncharacterized. We hypothesized that both comt genes play essential roles in zebrafish. Both comt-a and comt-b were widely expressed in zebrafish tissues, but their relative abundance varied considerably. Homogenates of zebrafish organs, including the brain, showed enzymatic COMT activity that was the highest in the liver and kidney. Treatment of larval zebrafish with the COMT inhibitor Ro41-0960 shifted the balance of catecholamine metabolic pathways towards increased oxidative metabolism. Whole-body concentrations of dioxyphenylacetic acid (DOPAC), a product of dopamine oxidation, were increased in the inhibitor-treated larvae, although the dopamine levels were unchanged. Thus, COMT is likely to participate in the processing of catecholamine neurotransmitters in the zebrafish, but the inhibition of COMT in larval fish is compensated efficiently and does not have pronounced effects on dopamine levels.

Daily rhythmic expression patterns of clock1a, bmal1, and per1 genes in retina and hypothalamus of the rainbow trout, Oncorhynchus mykiss

Living organisms show daily rhythms in physiology, behavior, and gene expression, which are due to the presence of endogenous clocks that synchronize biological processes to the 24-h light/dark cycle. In metazoans, generation of circadian rhythmicity is a consequence of specialized tissues known as "master clocks," having different locations among species. A few studies have described clock-gene expression in fish neural tissues, but none of them assessed clock-gene expression in different discrete regions. The present study was designed to explore the presence/absence of circadian clock-gene expression in the rainbow trout (Oncorhynchus mykiss) retina and hypothalamus. Juvenile fish were acclimated to a 12:12 light (L)-dark (D) cycle. Then, retina and hypothalamus were collected from animals kept under LD conditions or constant darkness (DD) for 24 h. Real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) assays were performed to quantify expression of the core circadian genes Clock1a, Bmal1, and Per1 as representative members of the circadian oscillator. All clock genes analyzed in the retina and hypothalamus showed circadian fluctuations. However, gene expression peaked in the rainbow trout hypothalamus with a 3-h (Clock1a and Bmal1) or 6-h (Per1) delay relative to that observed in the retina, the latter showing highest expression levels at zeitgeber times 9 (ZT9) for Clock1a and Bmal1, and at ZT21 for Per1. When exposed to DD, the rhythmic gene expression pattern was maintained for all genes in the rainbow trout retina, but only for Clock1a and Per1 in the hypothalamus. Bmal1 failed to cycle under DD, suggesting that hypothalamic clock function might depend on either several clock-gene isoforms or regulation from external inputs. Overall, these data indicate that representative molecular members of the core circadian clock are present in both the retina and hypothalamus of rainbow trout.

Modulation of monoamine neurotransmitters in fighting fish Betta splendens exposed to waterborne phytoestrogens

Endogenous estrogens are known to affect the activity of monoamine neurotransmitters in vertebrate animals, but the effects of exogenous estrogens on neurotransmitters are relatively poorly understood. We exposed sexually mature male fighting fish Betta splendens to environmentally relevant and pharmacological doses of three phytoestrogens that are potential endocrine disruptors in wild fish populations: genistein, equol, and β-sitosterol. We also exposed fish to two doses of the endogenous estrogen 17β-estradiol, which we selected as a positive control because phytoestrogens are putative estrogen mimics. Our results were variable, but the effects were generally modest. Genistein increased dopamine levels in the forebrains of B. splendens at both environmentally relevant and pharmacological doses. The environmentally relevant dose of equol increased dopamine levels in B. splendens forebrains, and the pharmacological dose decreased norepinephrine (forebrain), dopamine (hindbrain), and serotonin (forebrain) levels. The environmentally relevant dose of β-sitosterol decreased norepinephrine and dopamine in the forebrain and hindbrain, respectively. Our results suggest that sources of environmental phytoestrogens, such as runoff or effluent from agricultural fields, wood pulp mills, and sewage treatment plants, have the potential to modulate neurotransmitter activity in free-living fishes in a way that could interfere with normal behavioral processes.

Alterations in the brain monoaminergic neurotransmitters of rainbow trout related to naphthalene exposure at the beginning of vitellogenesis

The contents of dopamine (DA), noradrenaline (NA), serotonin (5HT), and some related metabolites were studied in different brain regions of rainbow trout at two different stages of sexual maturation (at the beginning of vitellogenesis), after naphthalene (NAP) administration. The effects of NAP varied according to duration of exposure, brain region and vitellogenesis stage of the trout, and were more significant during previtellogenesis. The changes observed in DA metabolism were generally stimulatory after exposure for 3 h, and either stimulatory or inhibitory (depending on the brain regions) after exposure for 3 days to NAP. NA levels were altered by NAP in various brain regions, but only during previtellogenesis. With respect to 5HT, treatment with NAP reduced levels of the amine and/or its main metabolite in most of the brain regions studied, particularly 3 h after treatment. The results suggest that NAP might interfere with the processes regulating brain monoamine metabolism, either locally or indirectly by altering steroid feedback to brain centres, and thus disrupt endocrine control of reproductive development through the brain-pituitary axis.

Effects of acute and prolonged naphthalene exposure on brain monoaminergic neurotransmitters in rainbow trout (Oncorhynchus mykiss)

We have shown previously that acute (1 to 6 h) and prolonged (1 to 5 days) exposure of rainbow trout to naphthalene resulted in decreased plasmatic cortisol and 17-beta-estradiol levels. In order to elucidate the mechanisms through which naphthalene might disrupt endocrine regulation, the present study investigated whether brain monoaminergic neurotransmitters are altered by the action of this polycyclic aromatic hydrocarbon. In a first experiment, immature rainbow trout were injected with vegetable oil alone or containing naphthalene (10 and 50 mg/kg, i.p.), and sacrificed 1, 3 and 6 h after treatment. In a second experiment, slow-coconut oil implants alone or containing naphthalene (doses of 10 and 50 mg/kg) were i.p. located and fish sacrificed 1, 3 and 5 days after treatment. Levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and noradrenaline (NA) were measured in several brain regions by HPLC. The results show that short-term naphthalene increases DA and 5-HT contents in hypothalamus and telencephalon, but differentially alter contents of the acid metabolites. Implants with naphthalene reduced DA content in hypothalamus and preoptic region but increased in telencephalon. 5-HT metabolism was decreased in hypothalamus, preoptic region, pituitary and brain stem after 3 to 6 days of treatment. In addition, the levels of NA were increased in hypothalamus and telencephalon after acute treatment and in hypothalamus and preoptic area after several days of exposure to naphthalene. These data suggest that brain neurotransmitter systems are sensitive to polycyclic aromatic hydrocarbons and could represent a target of the naphthalene-induced neuroendocrine disruption.

Localization of tyrosine hydroxylase and its messenger RNA in the brain of rainbow trout by immunocytochemistry and in situ hybridization

This report describes the distribution of tyrosine hydroxylase (TH)-expressing structures in the brain of rainbow trout (Oncorhynchus mykiss). TH neurons have been localized by the use of two complementary techniques, immunocytochemistry and in situ hybridization of TH messenger RNA. Results obtained from in situ hybridization and immunocytochemistry were in agreement. TH cells were observed in many areas of the brain, with a higher density at the level of the olfactory bulbs where TH-positive neurons are abundant in the internal cell layer. In the telencephalon, two populations of TH neurons can be distinguished: one group is located in the area ventralis telencephali pars dorsalis, and the other group is located in the area ventralis telencephali pars ventralis and extends laterally in the area ventralis telencephali pars lateralis. Many labeled neurons are also seen in the preoptic area as well as in the hypothalamus, where several clusters of TH-positive cells are observed. Some of these neurons located in the paraventricular organ grow a short cytoplasmic extension directed to the ventricular wall and are known to be cerebrospinal fluid-contacting cells. The most caudal TH neurons are observed at the level of the locus caeruleus. At the level of the pituitary, TH-positive fibers are observed in the neurohypophysis. The TH-immunoreactive innervation at the level of the pituitary provides a neuroanatomic basis for the effects of dopamine and/or norepinephrine on the release of pituitary hormones in fish.

Levels of specifically bound [3H]ketanserin compared with levels of serotonin (5HT) in the brain regions of juvenile and sexually recrudescing female rainbow trout, Oncorhynchus mykiss

This study compared the distribution of specifically bound [3H]ketanserin (Bsp) with serotonin (5HT) in brain regions of juvenile and sexually recrudescing female trout. Amounts of Bsp varied widely among brain regions and consistently differed between juvenile and sexually recrudescing females. Levels of Bsp were significantly greater in the hypothalamus than the olfactory lobe, which were at least threefold greater than in all other tissues examined (Kruskal-Wallis test, p < 0.05). Bsp densities in the hypothalamus, preoptic area, and optic lobe were significantly greater in juveniles compared with corresponding tissues from sexually recrudescing females (Mann-Whitney U test, p < 0.05); in contrast, Bsp in olfactory lobe and spinal cord did not differ significantly between the two classes of fish. 5HT concentration was determined by high performance liquid chromatography - electrochemical detection (HPLC-EC) analysis. Biogenic amine standards eluted in a stereotypic pattern, with peaks consistently separable in time. 5HT concentration was significantly greater in hypothalamus than in olfactory lobe and undetectable in the pituitary (Kruskal-Wallis test, p < 0.05). Trends in distribution of Bsp and 5HT were comparable in the hypothalamus and preoptic area in juvenile and sexually recrudescing females. In general, density of specific [3H]ketanserin binding sites was directly related to 5HT content of brain regions in juvenile and sexually recrudescing females. 5HT concentrations (pmol/g tissue) were approximately 900-fold greater than Bsp (fmol/g tissue) in all brain regions, and approximately 300-fold greater than Bsp in the olfactory lobe. These results suggest important regulatory role(s) for 5HT in the trout preoptic-hypothalamo-hypophysial axis, which may differ from 5HT role(s) in trout olfactory lobe.Key words: high performance liquid chromatography - electrochemical detection, [3H]ketanserin, sexually recrudescing female trout.

Changes in the pituitary metabolism of monoamines (dopamine, norepinephrine, and serotonin) in female and male rainbow trout (Oncorhynchus mykiss) during gonadal recrudescence

The dynamics of the levels and metabolism of dopamine, norepinephrine, and serotonin were studied in pituitaries of male and female rainbow trout at different stages of gonadal development. In female rainbow trout, the turnover of dopamine (calculated using the inhibitor of tyrosine hydroxylase alpha-methyl-p-tyrosine methyl-ester HCl), serotonin metabolism, and norepinephrine levels decreased in the advanced stage of exogenous vitellogenesis with respect to the initial stage. However, data obtained in males did not show changes in either serotonergic or noradrenergic metabolism during the last stages of gonadal development. However, an increase of dopaminergic turnover was noticed in the male fish at the end of spermiation. Finally, pituitary dopaminergic activity was significantly higher in immature (prepubescent stage) than in adult fish.

Annual and diurnal variations in, and effects of altered photoperiod and temperature, ovariectomy, and estradiol-17 beta replacement on catechol-O-methyltransferase level in brain regions of the catfish, Heteropneustes fossilis

The annual data show that catechol-O-methyltransferase (COMT) content increases with the progress of ovarian recrudescence in all the brain regions (telencephalon, hypothalamus, thalamus + tegmentum, and medulla oblongata) and declines after spawning to low values in quiescent phase. Diurnal variation in enzyme concentration with peak values at 24 hr was noticed in the hypothalamus throughout the reproductive cycle; in other brain regions, it was observed only in February and March. The stimulatory response of the enzyme to photoperiod and temperature alterations was differential and region-specific; an effect of photoperiod change was noticed only in the hypothalamus and was less in magnitude compared to the temperature effect. The response of the enzyme to ovariectomy (OVX) and E2 supplementation was region-specific (hypothalamus only), season-specific (prespawning phase), and varied:inhibitory at 3 and 4 weeks, stimulatory at 6-week of OVX and after 0.05, 0.1, 5, and 10 micrograms/g doses of E2, and none at week 2 and 5 of OVX and after 0.5 and 1 microgram/g doses of E2. The season-specific changes in hypothalamic COMT may be indicative of its involvement in catecholamine (and possibly catecholestrogen)-mediated neuroendocrine control of gonadotropin.

Effects of estradiol on the serotonin secretion and turnover in the hypothalamus of male tilapia, Oreochromis mossambicus, in vitro

The effects of estradiol on the secretion and turnover of serotonin in the hypothalamic fragments of male tilapia, Oreochromis mossambicus, were studied using a static incubation system. The quantitative analysis of serotonin and its related metabolite, 5-hydroxyindoleacetic acid, were performed by high-performance liquid chromatography with electrochemical detection. The hypothalamic fragments were incubated with 17 beta-estradiol at a concentration of 2 x 10(-8), 8 x 10(-8), 2 x 10(-7), 4 x 10(-7), or 4 x 10(-6) g/ml. The low dose of estradiol, 2 x 10(-8) g/ml, had no effect on the concentration of serotonin and 5-hydroxyindoleacetic acid or serotonin turnover in the hypothalamic incubation media. The moderate doses of estradiol 8 x 10(-8) and 2 x 10(-7) g/ml, increased the concentrations of serotonin and 5-hydroxyindoleacetic acid in the hypothalamic incubation media, but had no effect on the serotonin turnover. The high doses of estradiol, 4 x 10(-7) and 4 x 10(-6) g/ml, did not alter the serotonin concentration in the hypothalamic incubation media, but increased the 5-hydroxyindoleacetic acid concentration and serotonin turnover. These results demonstrate that the moderate dose of estradiol increases the serotonin activity by increasing the serotonin concentration, whereas the high dose of estradiol increases the serotonin activity by increasing the ratio of 5-hydroxyindoleacetic acid and serotonin. However, the serotonin concentration is homeostatically maintained in the extracellular fluid of hypothalamus under the high dose of E2 treatment.

Tyrosine hydroxylase activity and dopamine turnover of rainbow trout (Oncorhynchus mykiss) brain: the special status of the hypothalamus

The dynamics of catecholamine (CA)-synthesis enzymes have been poorly studied in fish. Tyrosine hydroxylase (TH), the rate-limiting enzyme of CA synthesis has been only studied inin vitro conditions. In the present report thein vivo CA synthesis and the CA metabolism were studied in different regions of the forebrain of the rainbow trout. Levels of norepinephrine (NE), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and the rate of accumulation of 3,4-dihydroxyphenylalanine (DOPA) were determined by HPLC following a treatment with hydroxybenzylhydrazine (NSD), a potential inhibitor of DOPA decarboxylase. Kinetics of the accumulation of DOPA and of the decline of DOPAC were in agreement with those found in rat, evidencing that the accumulation of DOPA following NSD can be used in trout to quantify thein vivo enzymatic activity of tyrosine hydroxylase. Experiments using treatment with NSD or with methyl-p-tyrosine reached a same conclusion: the DA neuronal activity in trout is much higher than NE neuronal activity. However, the hypothalamus had high DA levelsvs. lowin vitro andin vivo TH activities and exhibited a low CA turnover.

Effects of mercury on serotonin concentration in the brain of tilapia, Oreochromis mossambicus

In order to know the effect of mercury pollution on the serotonergic system of fish, serotonin concentrations in a discrete brain region of tilapia, Oreochromis mossambicus, were examined. Serotonin concentration was measured using a high performance liquid chromatography system with electrochemical detector. In male fish, the concentrations of serotonin were 1.468 +/- 0.350, 0.811 +/- 0.190 and 0.330 +/- 0.061 micrograms/g wet tissue in hypothalamus, telencephalon and optic lobe, respectively. The serotonin content was significantly different between each region; the hypothalamus had a higher content than that of the telencephalon and optic lobe. The serotonin concentration in female hypothalamus was 1.102 +/- 0.112 micrograms/g wet tissue which was significantly lower than that in males. However, serotonin concentration in the telencephalon and optic lobe showed no difference between male and female. After exposure to 0.015 and 0.03 ppm HgCl2 for 6 months beginning 7 days posthatching, male sample fish showed a significantly dose-dependent decrease in serotonin concentration in the hypothalamus. But a similar phenomenon was not found in other regions of the brain. These results suggest that exposure to HgCl2 results in an attenuated development of the serotonergic system in the hypothalamus of fish.

Catecholamine synthesis in the rainbow trout (Oncorhynchus mykiss) brain: modulation of tyrosine hydroxylase activity

Levels of catecholamines and tyrosine hydroxylase (TH) activity were measured in brain homogenates from female rainbow trout. In triploid fish or in diploid fish in ovarian recrudescence, the patterns of catecholamine content expressed as a function of in vitro TH activity vary in different areas of the brain. Km for the pterin cofactor is lower in the telencephalon than in the hypothalamus. Dopamine (DA) and 2-hydroxyestradiol (20HE2) inhibit TH activity (by competitive and non-competitive interaction respectively).The K1 for DA were different in the telencephalon and the hypothalamus and this could explain the different patterns of catecholamine levels and TH activity for these two structures. 20HE2 inhibits TH activity in vitro; a catechol moiety is required since estradiol (E2) is notinhibitory. However, the exact mechanism of inhibition remains unclear. The rapid effect of 20HE2 cannot explain the previously reported activation of catecholamine synthesis by E2 in vivo.

Effects of estradiol on brain aminergic turnover of the female rainbow trout (Oncorhynchus mykiss) at the beginning of vitellogenesis

Brain serotonin and dopamine (DA) turnovers in the female rainbow trout were studied at the beginning of the vitellogenesis and related to blood estradiol (E2) levels; pituitary and plasma gonadotropin (GtH) were also assayed. Ovariectomy did not modify brain aminergic turnover. E2 replacement on ovariectomized fish increased hypothalamic DA turnover (increased DA and increased DA metabolites). E2 stimulated GtH synthesis (positive feedback) but did not enhance GtH release; hypothalamic E2-mediated aminergic inhibition upon release was suspected. Individual relations between blood E2 levels and catecholaminergic neurotransmitters were determined. A linear positive correlation (r = 0.82) was found for the hypothalamus, but not for the pituitary, the preoptic area, or the telencephalon. These data suggest that an activation of hypothalamic tyrosine hydroxylase (the limiting step of catecholamines synthesis) by E2 could develop as vitellogenesis proceeds.

A reduction in pituitary dopamine turnover is associated with sex pheromone-induced gonadotropin secretion in male goldfish

In goldfish, the gonadal steroid, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17,20 beta-P), functions as a potent preovulatory female sex pheromone which stimulates rapid elevations in serum gonadotropin (GtH) levels and subsequent increases in milt production in males. GtH secretion in goldfish is known to be regulated by the stimulatory actions of gonadotropin-releasing hormone (GnRH) and the inhibitory actions of dopamine (DA). This study specifically examined whether the 17,20 beta-P-induced elevation in male GtH is caused by pheromone-mediated changes in DA inhibition at the level of the pituitary. First, we have demonstrated that dihydroxyphenylacetic acid (DOPAC) is the primary metabolite of DA catabolism in the brain and pituitary gland of goldfish. Second, we measured changes in circulating levels of GtH and changes in pituitary content of DA and its metabolite, DOPAC, as well as possible alterations in DA turnover rate (DOPAC/DA ratio) following short-term exposure of male goldfish to water-borne 17,20 beta-P. Water-borne 17,20 beta-P consistently increased serum GtH levels in males within 20 min of exposure and maintained elevated levels for up to 120 min. Although changes in pituitary DA content were not observed during periods of high GtH release, coincident reductions in pituitary levels of DOPAC were measured within 45 min of exposure to the pheromone. More importantly, there was a significant decrease in the rate of DA turnover in the pituitary, as assessed by comparing the ratio of DOPAC to DA present, at 20, 45, and 120 min of exposure. Since the reduction of DA turnover in the pituitary is inversely correlated with periods of increased GtH release, the present results suggest that water-borne 17,20 beta-P causes an abatement of DA release to the pituitary. Based on the latency of the GtH response to water-borne 17,20 beta-P, a rapid reduction of DA turnover in the pituitary appears to be at least part of the neuroendocrine trigger for 17,20 beta-P-induced GtH release in male goldfish.

Serotonin and dopamine turnover in the female rainbow trout (Oncorhynchus mykiss) brain and pituitary: changes during the annual reproductive cycle

Brain serotonin (5HT) and dopamine (DA) turnover were studied at various stages of the reproductive cycle of the female rainbow trout by simultaneous determination by HPLC of neurotransmitters and major related metabolites. An increase of 5HT turnover in telencephalon and hypothalamus and a decrease of DA turnover in pituitary and hypothalamus were observed during the periovulatory period. Some changes also occurred during vitellogenesis: decreased 5HT metabolite in telencephalon and preoptic area and increased DA content in preoptic area. These data suggest that physiological fluctuations of biogenic amines could be involved in both ovarian recrudescence and ovulation, with major effects on the hypothalamo-hypophysial complex during the periovulatory period.