Biomedical subjects

B Senthilkumaran

Publications and source records attributed to B Senthilkumaran.

At least 19 recordsLinked to original sources

PubMed2006

Co-administration of C-Phycocyanin ameliorates thioacetamide-induced hepatic encephalopathy in Wistar rats.

Fulminant hepatic failure (FHF) is a condition with a sudden onset of necrosis followed by degeneration of hepatocytes, without any previously established liver disease, generally occurring within hours or days. FHF is associated with a wide spectrum of neuropsychiatric alterations ranging from stupor to coma, culminating in death. In the present study FHF was induced in rats by the administration of thioacetamide (TAA). Oxidative stress is thought to play a prominent role in the pathophysiology of cerebral changes during FHF leading to the assumption that antioxidants might offer protection. Hence, in the present study the protective effect of C-Phycocyanin (C-PC), a natural antioxidant, was evaluated on TAA-induced tissue damage. C-Phycocyanin was administered intraperitoneally twice at 24 h interval (50 mg/kg body weight) along with the hepatotoxin TAA (300 mg/kg body weight). The animals were sacrificed 18 h after the second injection of TAA treatment and various biochemical parameters were analysed in liver, serum and brain tissues. These studies revealed significant prevention of TAA-induced liver damage by C-PC, as evidenced by a) increase in survival rate; b) the prevention of leakage of liver enzymes (AAT and AST) and ammonia into serum; c) increase in prothrombin time and d) liver histopathology. Ultrastructural studies of astrocytes of different regions of brain clearly showed a decrease in edema after C-PC treatment. TAA-induced histopathological lesions in different regions of the brain namely cerebral cortex, cerebellum and pons medulla were significantly reduced by the co-administration of C-PC with TAA. Further C-PC treatment resulted in a) decrease in the levels of tryptophan and markers of lipid peroxidation and b) elevation in the activity levels of catalase, glutathione peroxidase in different regions of brain. These studies reveal the potential of C-PC in ameliorating TAA-induced hepatic encephalopathy by improving antioxidant defenses.

Albumins↗

PubMed2006

Thiourea-induced thyroid hormone depletion impairs testicular recrudescence in the air-breathing catfish, Clarias gariepinus.

We used thiourea-induced thyroid hormone depletion as a strategy to understand the influence of thyroid hormones on testicular recrudescence of the air-breathing catfish, Clarias gariepinus. Treatment with 0.03% thiourea via immersion for 21 days induced hypothyroidism (thyroid hormone depletion) as evidenced by significantly reduced serum T(3) levels. Thiourea-treated males had narrowed seminiferous lobules with fewer spermatozoa in testis, very little or no secretory fluid, reduced protein and sialic acid levels in seminal vesicles when compared to controls. The histological changes were accompanied by reduction in serum and tissue levels of testosterone (T) and 11-ketotestosterone (11-KT), a potent male specific androgen in fish. Qualitative changes in the localization of catfish gonadotropin-releasing hormone (cfGnRH) and luteinizing hormone (LH, heterologous system) revealed a reduction in the distribution of immunoreactive neuronal cells and fibers in thyroid depleted fish. Interestingly, thiourea-withdrawal group showed physiological and histological signs of recovery after 21 days such as reappearance of spermatozoa and partial restoration of 11-KT and T levels. These data demonstrate that thyroid hormones play a significant role in testicular function of catfish. The mechanism of action includes modulating sex steroids either directly or through the hypothalamo (GnRH)-hypophyseal (LH) axis.

Animals↗

PubMed2006

Phospholipid and cholesterol alterations accompany structural disarray in myelin membrane of rats with hepatic encephalopathy induced by thioacetamide.

Fulminant hepatic failure is often associated with a wide range of neurological symptoms which are collectively referred to as hepatic encephalopathy. Fulminant hepatic failure with associated hepatic encephalopathy has a poor prognosis with the currently available sure treatment being only liver transplantation. This is largely owing to the lack of understanding of critical factors involved in the etiology of the condition. Lipid changes have been implicated in cerebral derangements characteristic of hepatic encephalopathy. About 79% of the brain lipid is concentrated in the myelin fraction where they play an important role in ion balance and conduction of nerve impulses. Hence, in the present study we aimed to investigate changes in myelin lipid composition and structure. Myelin was isolated by sucrose density gradient centrifugation from cerebral cortex of male Wistar rats (250-300 g body weight) treated with 300 mg/kg body weight thioacetamide administered twice at 24h interval to induce hepatic encephalopathy. Significant decrease was observed in the cholesterol and phospholipids content of myelin from treated rats. Sphingomyelin, phosphatidylserine and phosphatidylethanolamine content also decreased significantly following 18 h of thioacetamide administration. However, phosphatidylcholine levels remained unaltered. Transmission electron microscopic observation of myelin membrane from cerebral cortex sections showed considerable disorganization in myelin structure. Increase in malondialdehyde levels precede lipid changes leading to the speculation that oxidative damage may be the critical factor leading to decrease in the anionic phospholipids. Changes in myelin were evident only in later stages of hepatic encephalopathy indicating that myelin alteration may not play a role in early stages of hepatic encephalopathy. Nevertheless, myelin alteration may have a crucial role to play in various psycho-motor alterations during later stages of hepatic encephalopathy.

Animals↗

PubMed2006

Membrane alterations and fluidity changes in cerebral cortex during acute ammonia intoxication.

Acute ammonia intoxication is known to cause alterations in activities of several membrane bound enzymes like Na+ K+ ATPase, acetylcholine esterase and glutamate uptake in brain. The alteration in these membrane associated activities could be a consequence of altered membrane architecture. To probe this, the effect of pathophysiological concentrations of ammonia on lipid composition and fluidity of membranes isolated from cerebral cortex of rats, were investigated in the present study. Administration of acute doses of ammonium acetate caused depletion of membrane sphingomyelin and cholesterol levels thereby reducing cholesterol: phospholipid (C: PL) ratio. Levels of phosphatidylserine increased while those of phosphatidylcholine and phosphatidylethanolamine remain unaltered. Membrane fluidity estimations using 1,6-diphenyl-1,3,5-hexatriene (DPH), 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene (TMA-DPH) indicated no changes in core and surface membrane fluidity following ammonium acetate administration. Acute ammonia toxicity induced no alteration in bulk fluidity but a decrease in annular fluidity of membranes, as determined using pyrene fluorescence. Elevated levels of malondialdehyde and declined level of total thiols in cerebral cortex membranes of rats under acute ammonia intoxication indicated the existence of oxidative stress.

Ammonia↗

PubMed2006

Changes in cerebral membrane lipid composition and fluidity during thioacetamide-induced hepatic encephalopathy.

Lipids are an essential structural and functional component of cellular membranes. Changes in membrane lipid composition are known to affect the activities of many membrane-associated enzymes, endocytosis, exocytosis, membrane fusion and neurotransmitter uptake, and have been implicated in the pathophysiology of many neurodegenerative disorders. In the present study, we investigated changes in the lipid composition of membranes isolated from the cerebral cortex of rats treated with thioacetamide (TAA), a hepatotoxin that induces fulminant hepatic failure (FHF) and thereon hepatic encephalopathy (HE). HE refers to acute neuropsychiatric changes accompanying FHF. The estimation of membrane phospholipids, cholesterol and fatty acid content in cerebral cortex membranes from TAA-treated rats revealed a decrease in cholesterol, phosphatidylserine, sphingomyelin, a monounsaturated fatty acid, namely oleic acid, and the polyunsaturated fatty acids gamma-linolenic acid, decosa hexanoic acid and arachidonic acid compared with controls. Assessment of membrane fluidity with pyrene, 1,6-diphenyl-1,3,5-hexatriene and 1-[4-(trimethylammonio)phenyl]-6-phenyl-1,3,5-hexatriene revealed a decrease in the annular membrane fluidity, whereas the global fluidity was unaffected. The level of the thiobarbituric acid reactive species marker for lipid peroxidation also increased in membranes from TAA-treated rats, thereby indicating the prevalence of oxidative stress. Results from the present study demonstrate gross alterations in cerebral cortical membrane lipid composition and fluidity during TAA-induced HE, and their possible implications in the pathogenesis of this condition are also discussed.

Animals↗

PubMed2005

Cloning, expression and characterization of three types of 17beta-hydroxysteroid dehydrogenases from the Nile tilapia, Oreochromis niloticus.

In order to elucidate the roles of 17beta-HSDs in fish gonadal steroidogenesis, three types of 17beta-HSDs (17beta-HSD1, 17beta-HSD8 and putative 17beta-HSD12) were cloned and characterized from the Nile tilapia, Oreochromis niloticus. The cloned cDNAs of 17beta-HSD type 1, 8 and 12 were 1504, 1006 and 1930 bp long, with open reading frames encoding proteins of 289, 256 and 314 aminoacids, respectively. Tissue distribution pattern analyzed by RT-PCR and Northern blot showed that 17beta-HSD1 was dominantly expressed in the ovary, while the putative 17beta-HSD12, one of the two duplicates found in fish, is a male specific enzyme and expressed exclusively in testis (detected by RT-PCR only). On the other hand, 17beta-HSD8 was expressed in the brain, gill, heart, liver, intestine, gonad, kidney and muscle of both male and female. Enzymatic assays of the three types of 17beta-HSDs were performed using recombinant proteins expressed in E. coli or HEK 293 cells. Tilapia 17beta-HSD1 expressed in E. coli had the preference for NADP(H) as cofactor and could catalyze the inter-conversion between estrone and estradiol efficiently as well as the inter-conversion between androstenedione and testosterone, but less efficiently. Tilapia 17beta-HSD8 recombinant protein expressed in HEK 293 cells could catalyze the conversion of testosterone to androstenedione, as well as the inter-conversion between estrone and estradiol. However, the putative 17beta-HSD12 expressed in E. coli or in HEK 293 cells showed no conversion to any of the four substrates tested in this study. Based on enzyme characterization and tissue distribution, it is plausible to attribute crucial roles to 17beta-HSDs in the gonadal steroidogenesis of teleosts.

17-Hydroxysteroid Dehydrogenases↗

PubMed2003

Isolation, characterization and expression of 11beta-hydroxysteroid dehydrogenase type 2 cDNAs from the testes of Japanese eel (Anguilla japonica) and Nile tilapia (Oreochromis niloticus).

The Japanese eel (Anguilla japonica) and Nile tilapia (Oreochromis niloticus) 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD2) cDNAs were isolated from their respective testes cDNA libraries. The cDNAs predict two peptides of 436 and 406 amino acid residues that share about 42% homology with mammalian 11beta-HSD type 2 proteins. Analysis of the tissue distribution pattern by RT-PCR reveals that 11beta-HSD2 is expressed in a wide variety of tissues in tilapia, with higher expression in kidney and gill of both sexes, and with the highest expression in testis. 11beta-Dehydrogenase activity of the eel 11beta-HSD2 was confirmed by demonstrating the conversion of cortisol to cortisone by the recombinant protein after transient expression of this cDNA clone in COS-1 cells. Bands of approximately 2.7 and approximately 3.8 Kb were detected in Northern blot of eel and tilapia testes respectively, which is consistent with the cloned cDNA sizes of the two species. Northern blot analysis also revealed that the expression of the eel testis 11beta-HSD2 gene could be induced by human chorionic gonadotropin (hCG) injection, implying a role of 11beta-HSD2 in hCG-induced 11-ketotestosterone production and spermatogenesis in the Japanese eel.

11-beta-Hydroxysteroid Dehydrogenases↗

PubMed2001

Seasonal changes in adrenal and gonadal activity in the quail, Perdicula asiatica: involvement of the pineal gland.

The present study assessed annual adrenal gland activity in the Indian tropical Jungle bush quail, Perdicula asiatica. We also elucidated the role of the annual variations in gonadal steroids and melatonin in the regulation of its activity. Increasing day length (photoperiod), ambient temperature and rainfall are positively correlated with adrenal and gonadal functions, and inversely related to pineal gland activity. Pineal, adrenal and gonadal weights showed cyclical patterns relative to environmental factors, which were also correlated with plasma melatonin, corticosterone and gonadal steroids, respectively. In both sexes of P. asiatica, pineal gland weight and/or plasma melatonin levels were inversely related to adrenal lipids, (e.g. phospholipids, free and esterified cholesterol) and plasma corticosterone levels. Melatonin levels also showed an inverse relationship with plasma testosterone and estradiol levels. These studies indicate that changes in environmental factors promote annual variations in adrenal and gonadal activity probably by modulating the pineal gland. Melatonin receptors have been localized in the pars tuberalis, adrenal gland and gonads of birds, the pineal gland may, therefore, mediate environmental stimuli indirectly and directly to down regulate adrenal and gonadal activity, which run in parallel in this species.

Adrenal Glands↗

PubMed2001

Effects of serotonin, GABA and neuropeptide Y on seabream gonadotropin releasing hormone release in vitro from preoptic-anterior hypothalamus and pituitary of red seabream, Pagrus major.

The effects of serotonin (5-HT), GABA and neuropeptide Y (NPY) on in vitro release of seabream (sb) gonadotropin releasing hormone (GnRH) from slices of the preoptic-anterior hypothalamus (P-AH) and pituitary of red seabream were studied. 5-HT, GABA and NPY all stimulated the release of sbGnRH from the P-AH but not from the pituitary of immature red seabream. They also stimulated sbGnRH release from the P-AH with a similar potency during the course of gonadal development. Specific agonists and/or antagonists of 5-HT, GABA and NPY showed that 5-HT and GABA utilize 5-HT(2) and GABAA receptor subtypes, respectively, to mediate their action, and that NPY employs at least NPY(Y1) and NPY(Y2) receptor subtypes to stimulate sbGnRH release. Combinations of different antagonists for 5-HT, GABA and noradrenaline/adrenaline did not block the stimulatory influence of NPY on release of sbGnRH, indicating that the action of NPY on the sbGnRH neuronal system is probably direct.

Animals↗

PubMed2001

Periovulatory changes in catfish ovarian oestradiol-17beta, oestrogen-2-hydroxylase and catechol-O-methyltransferase during GnRH analogue-induced ovulation and in vitro induction of oocyte maturation by catecholoestrogens.

In the catfish Heteropneustes fossilis and Clarias batrachus, ovarian oestrogen-2-hydroxylase (OE-2-H) activity increased significantly at 8 h after the injection of an ovulatory dose (0.15 microg/g body weight) of a mammalian GnRH analogue ([d -Ala(6)-Pro(9)]-LHRH ethylamide) and was restored to the 0 h (control) level after egg-stripping at 16 h. On the other hand, ovarian oestradiol-17beta (OE2) level and catechol-O-methyltransferase (COMT) activity decreased significantly at 8 h. While the OE2 level was restored to the 0 h level, COMT activity increased significantly at 16 h. Changes in ovarian OE2 level and enzymes indicate higher synthesis of 2-hydroxylated catecholoestrogens and their degradation during the periovulatory period. Under in vitro conditions, the synthetic catecholoestrogens (CEs, 2- and 4-hydroxylated oestradiol17beta and oestrone (OE1)) induced germinal vesicle break down (GVBD) in a dose- (0.01-10 microg/ml) and duration- (1-36 h) dependent manner, the mean values of the responses being in the order 2-OH OE2>4-OH OE2> 2-OH OE1>4-OH OE1. The CE-induced GVBD response (8 h induction) was not blocked by prior and subsequent incubations with steroid synthesis inhibitors (cyanoketone, epostane and aminoglutethimide) up to 36 h, suggesting that de novo steroidogenesis is not essential for the response. The percentage of GVBD response to 2-h induction by CEs was significantly inhibited by actinomycin D (a transcriptional inhibitor) and cycloheximide (a translational inhibitor), indicating the involvement of both RNA and protein synthesis. The CE-induced 8-h stimulation of GVBD was mildly blocked by propranolol, the beta-adrenergic inhibitor, suggesting the response was partly mediated through a beta-adrenergic receptor mechanism. Incubations with phentolamine, an alpha-adrenergic inhibitor, did not interfere with the CE-induced GVBD response. The results demonstrate CE-related enzymatic changes in teleost (catfish) ovaries and maturation-inducing substance activity of CEs.

Animals↗

PubMed2000

Thyroidectomy modulates rat prostatic monosaccharides.

Earlier studies have shown that thyroid hormones play an important role in glycoprotein metabolism in the prostate. In this paper, hypothyroidism induced changes in prostatic glycoprotein-associated monosaccharides and fructose concentrations were studied in 30-day-old peripubertal rats. Hypothyroidism was induced by total surgical thyroidectomy and confirmed by radioimmunoassay (RIA) of thyroid hormones and thyroid stimulating hormone (TSH). Animals were killed after 30 and 60 days post-thyroidectomy. To test the direct effect of thyroid hormones on prostatic monosaccharides, prostatic tissue was incubated for 24 h at 37 degrees C with 25, 50 or 100 ng/mL T3. Ventral prostatic hexosamines and sialic acid concentrations were significantly decreased in hypothyroid rats aged 30 and 60 days, whereas fucose concentration was increased at 30 days but decreased at 60 days in hypothyroid rats. In the case of the dorsolateral prostate, hypothyroidism enhanced the concentration of hexosamines but caused a decrease in levels of fucose, sialic acid and fructose, irrespective of the duration of hypothyroidism. Concentrations of fucose, fructose and hexosamines in the anterior prostate were decreased and sialic acid concentration was increased in 60-day hypothyroid rats. In in vitro studies, the concentrations of fucose, sialic acid and hexosamines were significantly increased in all three prostatic lobes irrespective of the T3 dose with which they had been incubated. A consistent increase in fructose concentration was observed in anterior and dorsolateral prostates when incubated with different doses of T3 stimulation. It is concluded that the influence of hypothyroidism on prostatic monosaccharides seems to be specific for each sugar in different lobes of the prostate. These results clearly show that thyroid hormones differentially regulate prostatic glycoprotein metabolism.

Animals↗

PubMed2000

Unique expression of gonadotropin-I and -II subunit genes in male and female red seabream (Pagrus major) during sexual maturation.

Two distinct gonadotropins (GTHs) have been demonstrated in a number of teleost fishes. Although the physiological roles of GTHs have been extensively studied in salmonids, little is known about their biological functions in nonsalmonid fishes. In this study, to elucidate the role of GTH-I and GTH-II in reproduction, we cloned the alpha-glycoprotein subunit (alphaGSU) and gonadotropin beta subunits (Ibeta and IIbeta) of red seabream using the 5'- and 3'-RACE methods and used these cDNA probes to reveal changes in mRNA levels of each subunit during sexual maturation of both male and female red seabream. The nucleotide sequences of alphaGSU, Ibeta, and IIbeta are 629, 531, and 557 base pairs long, encoding peptides of 117, 120, and 146 amino acids, respectively. The deduced amino acid sequence of each mature subunit showed high homology with those of other teleosts. Northern blot analysis showed that Ibeta mRNA levels of males increase in association with gonadal development, whereas those of females remain low throughout sexual maturation, indicating sexual dimorphism in the expression pattern of Ibeta. In contrast, IIbeta mRNA levels of both sexes are maintained at high levels from the beginning of gametogenesis to spawning season. These results are different than those of salmonids and suggest that GTH-I may have important roles in male, but not female, gametogenesis. GTH-II may be involved in regulation of early and late gametogenesis in both male and female red seabream.

Amino Acid Sequence↗

PubMed1999

Distribution and seasonal variations in levels of three native GnRHs in the brain and pituitary of perciform fish.

Specific and sensitive radioimmunoassays (RIAs) were newly developed for two types of gonadotropin-releasing hormone (GnRH), namely, seabream (sb) GnRH and chicken (c) GnRH-II. We employed these two RIAs together with a previously reported RIA for salmon (s) GnRH to study the presence and regional distribution of these three GnRHs in the brains and pituitaries of four perciform fishes (red seabream, Pagrus major; black seabream, Acanthopagrus schlegeli; striped knifejaw, Oplegnathus fasciatus; and Nile tilapia, Oreochromis niloticus), as well as clarify seasonal changes in levels of these GnRHs in the brain and pituitary of red seabream. All three GnRHs were found in brains of all fishes examined, with regional distributions in the brains of the three GnRHs being rather similar. sbGnRH was abundant in telencephalon and hypothalamus. cGnRH-II was concentrated from the middle to posterior part of the brain and distributed throughout the brain. sGnRH was concentrated in the olfactory bulb and distributed all over the brain, as was cGnRH-II. The dominant form of GnRH in the pituitary was sbGnRH, with levels 500- to 2400-fold higher than those of sGnRH, while cGnRH-II was undetectable in all four species. In the brain and pituitary of female red seabream, levels of both brain and pituitary sbGnRH increased from October (immature phase) and reached a peak in April (spawning phase), reflecting the increase in gonadosomatic index and vitellogenesis. However, levels of sbGnRH remained high only in the pituitary of completely regressed fish in June. Levels of both sGnRH and cGnRH-II in the brain were higher in the regressed phase and remained lower during the spawning phase. From these and previous results, it appears that sbGnRH is physiologically the most important form of GnRH in reproduction in red seabream and, probably, in other perciforms also.

Animals↗

PubMed1998

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.

Animals↗

PubMed1998

Periovulatory changes in hypothalamic and pituitary monoamines following GnRH analogue treatment in the catfish Heteropneustes fossilis: a study correlating changes in plasma hormone profiles.

In Heteropneustes fossilis, administration of a single dose (0.15 micrograms/g body weight, i.p.) of [D-Ala6,Gly10]-gonadotrophin-releasing hormone analogue (GnRHa) induced ovulation (in 35 of 35 fish) when mild-stripped at 16 h. Plasma gonadotrophin II (GTH II) levels showed a highly significant increase at 2, 4, 8, 12 and 16 h with the peak at 8 h. Plasma cortisol, progesterone and testosterone showed significant elevations at 2, 4, 8 and 12 h with peaks at 8 h (cortisol and testosterone) and 4 h (progesterone). The levels declined to control values at 16 and 48 h except that of testosterone which decreased even further. In contrast, plasma levels of oestradiol-17 beta decreased significantly at 2, 4, 8 and 12 h, with the lowest value at 8 h, but increased at 16 and 18 h. The contents of hypothalamic and pituitary serotonin and noradrenaline increased at 8 h, coinciding with the peak GTH II rise, and decreased at 16 h. In contrast, dopamine content declined at 8 h in both the hypothalamus and pituitary, but increased at 16 h only in the hypothalamus. The hypothalamic adrenaline level decreased at 8 h but increased significantly at 16 h. Hypothalamic levels of monoamine oxidase, catechol O-methyltransferase and dopamine beta-hydroxylase were elevated significantly at 8 h; the dopamine beta-hydroxylase activity decreased at 16 h. Phenylethanolamine N-methyltransferase activity was elevated only at 16 h, coinciding with the rise in adrenaline content. It is inferred that the preovulatory decrease in dopamine content concomitant with rises in serotonin and noradrenaline levels, triggered by the low titre of oestradiol, might have potentiated the GnRHa/GnRH (endogenous)-induced release of GTH II for a prolonged period.

Animals↗

PubMed1996

Effects of administration of some monoamine-synthesis blockers and precursors on ovariectomy-induced rise in plasma gonadotropin II in the catfish Heteropneustes fossilis.

In the present study, effects of three daily i.p. injections of some monoamine (MA)-synthesis blockers and pre-cursors on plasma gonadotropin (GTH-II) levels were investigated in 3-week ovariectomized catfish, and the effects were correlated with changes in hypothalamic MA contents. Administration of alpha-methylparatyrosine (alpha-MPT; 250 micrograms/g BW, a tyrosine hydroxylase inhibitor) significantly decreased the ovariectomy-induced rise in GTH-II compared to that of the sham control group. Injection of L-dihydroxyphenylalanine (10 micrograms/g BW) in the alpha-MPT-treated fish elevated the GTH-II level significantly over that of the sham control group but not to the level of the ovariectomized fish. Administration of noradrenaline (NA; 5 micrograms/g BW) in combination with alpha-MPT counteracted the effect of the latter and maintained the plasma GTH-II level at that of the ovariectomized fish. Plasma GTH-II level was decreased significantly in the diethyldithiocarbamate (10 micrograms/g BW, a dopamine-beta-hydroxylase inhibitor)-injected fish compared to that of the sham and ovariectomized control groups. Administration of para-chlorophenylalanine (p-CPA; 100 micrograms/g BW, a tryptophan hydroxylase inhibitor) decreased the GTH-II level significantly compared to that of the ovariectomized group. Supplementation of 5-hydroxytryptophan (20 micrograms/g BW) with p-CPA nullified the latter's effect and maintained the GTH-II level at that of the ovariectomized fish. The administration of both p-CPA and alpha-MPT significantly reduced plasma GTH-II to the lowest mark compared to that of the sham, ovariectomized, and all other treatment groups. These results clearly show that the ovariectomy-induced rise in GTH-II was mediated through simultaneous activation of hypothalamic serotonergic and NA-ergic and suppression of dopaminergic mechanisms.

Animals↗

PubMed1995

A turnover study of hypothalamic monoamine oxidase (MAO) and effects of MAO inhibition on gonadotropin secretion in the female catfish, Heteropneustes fossilis.

In the present study, the rate constants of degradation (k) and synthesis (S) and half-life of hypothalamic monoamine oxidase were determined to explain annual variations and biphasic effects of the enzyme to low and high doses of estradiol-17 beta (E2) in 3-week ovariectomized Heteropneustes fossilis. In the preparatory phase, the half-life (t1/2) of the enzyme was the longest (21.16 days) with low k (0.03275 days-1) and S (0.000845 Units/day) values, suggesting a low turnover of the enzyme. In the prespawning phase the t1/2 was the shortest (11.65 days) with high k (0.0595 days-1) and S (0.011 Units/day) values. The low and high turnovers of the enzyme, respectively, in these two seasons could be correlated to low and high profiles of plasma E2 levels. In the resting phase, the values were in between (t1/2 = 18.83 days, k = 0.0368 days-1, S = 0.00211 Units/day) but the plasma E2 level was undetectable. Three weeks of ovariectomy increased the t1/2 (19.04 days) compared to that of the control (11.61 days) with decreases in both k and S values. The administration of a low dose of E2 (0.1 micrograms/g BW) further increased the t1/2 (19.63 days) over that of the ovariectomized fish with a significant rise in the S value. However, a high dose of E2 (1.0 micrograms/g BW) decreased it (13.33 days) by reducing the S and elevating the k values. These results suggest that the stimulatory effect of low doses of E2 on the enzyme activity is produced by elevating its synthesis rate and the inhibitory effect of high doses of E2 by simultaneously decreasing the synthesis and increasing the degradation rates of the enzyme. The administration of a single dose (75 mg/kg BW) of pargyline has elevated plasma gonadotropin (GTH) level after 3 and 6 hr and 7 days in the sham-ovariectomized (control), ovariectomized, and ovariectomized low-E2-dose groups; the peak increase was found at 6 hr. On the contrary, in the ovariectomized high-E2 group the GTH level was inhibited at 3 and 6 hr postinjection. These changes in the GTH level could be correlated with changes in the profiles of hypothalamic monoamines. The data show that E2 exerts its feedback regulation of GTH by modifying monoaminergic activity at the level of oxidative deamination.

Animals↗

PubMed1995

Changes in hypothalamic catecholamines, dopamine-beta-hydroxylase, and phenylethanolamine-N-methyltransferase in the catfish Heteropneustes fossilis in relation to season, raised photoperiod and temperature, ovariectomy, and estradiol-17 beta replacement.

In Heteropneustes fossilis, contents and turnovers of hypothalamic catecholamines (CA) and activities of dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyltransferase (PNMT) showed significant seasonal variations with significantly high day values. The seasonal pattern of dopamine (DA) on one hand and that of noradrenaline (NA) and adrenaline (A) on the other hand showed an inverse relationship, the former decreasing and the latter increasing during the progress of gonadal recrudescence. The DBH and PNMT levels were low in the resting phase and increased to the peak in the prespawning (DBH) and spawning (PNMT) phases. Maintenance of the fish under long photoperiods (16L:8D) and high temperature (28 +/- 2 degrees) stimulated the NA and A, and DBH and PNMT activities, and suppressed the DA mechanism, the changes being maximal in the raised temperature groups. In the resting phase (December), ovariectomy (OVX) or estradiol-17 beta (E2) replacement in 4-week ovariectomized fish did not produce any significant effects on the CA and enzyme activities. On the contrary, in the prespawning phase (May), OVX produced differential and biphasic responses on CA and the enzymes. The contents and turnovers of both NA and A increased significantly at 2-5 weeks and decreased in the sixth week. However, the reverse was true for DA. The DBH and PNMT activities (assayed only 3, 4, and 6 weeks after OVX) were elevated significantly in the third and fourth weeks but decreased in the sixth week. Plasma levels of gonadotropin (GTH) increased significantly at all durations of OVX in a bimodal pattern while the E2 levels decreased consistently. Supplementation with a low dose (0.1 microgram/g BW) of E2 restored the NA and A and enzyme activities while the higher doses (0.5, 1.0, and 5.0 micrograms/g BW) depleted them. The reverse was true for DA. The low dose of E2 restored the GTH level while the higher ones inhibited it significantly. These results indicate that both environmental photoperiod and temperature and E2-negative feedback act on the CA to modulate GTH secretion.

Animals↗