Regulation of steroid production and key genes in catfish Heteropneustes fossilis using recombinant gonadotropins

The two gonadotropins, FSH and LH, stimulate growth and development of the gonads through gonadal biosynthesis of steroid hormones and growth factors. To date, cDNA sequences encoding gonadotropin subunits have been isolated and characterized from a large number of fish species. Recently, we successfully cloned and characterized gonadotropins (LHβ, FSHβ, and GPα) from the pituitary glands of the catfish, Heteropneustes fossilis. In the present study, we describe herein the production of recombinant stinging catfish, H. fossilis (hf) FSH (rhfFSH) and LH (rhfLH) using the methylotrophic yeast P. pastoris expression system. We further explored the hypothesis that the recombinant gonadotropins can modulate the hypothalamus-pituitary-ovarian (HPO) axis genes (avt, it, gnrh2, kiss2, and cyp19a1a) and regulate their transcriptional profile and steroid levels in relation to their annual developmental stage during preparatory and pre-spawning phases under in-vitro conditions. We found that the different concentrations of recombinant rhfFSH and rhfLH significantly stimulated E2 levels in the preparatory and prespawning season, and also upregulated gonadal aromatase gene expression in a dose dependent manner. Our results demonstrate that the yeast expression system produced biologically active recombinant catfish gonadotropins, enabling the study of their function in the catfish.

Reproductive roles of the vasopressin/oxytocin neuropeptide family in teleost fishes

The vertebrate nonapeptide families arginine vasopressin (AVP) and oxytocin (OXT) are considered to have evolved from a single vasopressin-like peptide present in invertebrates and termed arginine vasotocin in early vertebrate evolution. Unprecedented genome sequence availability has more recently allowed new insight into the evolution of nonapeptides and especially their receptor families in the context of whole genome duplications. In bony fish, nonapeptide homologues of AVP termed arginine vasotocin (Avp) and an OXT family peptide (Oxt) originally termed isotocin have been characterized. While reproductive roles of both nonapeptide families have historically been studied in several vertebrates, their roles in teleost reproduction remain much less understood. Taking advantage of novel genome resources and associated technological advances such as genetic modifications in fish models, we here critically review the current state of knowledge regarding the roles of nonapeptide systems in teleost reproduction. We further discuss sources of plasticity of the conserved nonapeptide systems in the context of diverse reproductive phenotypes observed in teleost fishes. Given the dual roles of preoptic area (POA) synthesized Avp and Oxt as neuromodulators and endocrine/paracrine factors, we focus on known roles of both peptides on reproductive behaviour and the regulation of the hypothalamic-pituitary-gonadal axis. Emphasis is placed on the identification of a gonadal nonapeptide system that plays critical roles in both steroidogenesis and gamete maturation. We conclude by highlighting key research gaps including a call for translational studies linking new mechanistic understanding of nonapeptide regulated physiology in the context of aquaculture, conservation biology and ecotoxicology.

Air sac and gill vasotocin receptor gene expression in the air-breathing catfish Heteropneustes fossilis exposed to water and air deprivation conditions

Heteropneustes fossilis is a facultative air-breathing freshwater catfish and inhabits ponds, ditches, swamps, marshes and rivers that dry up in summers. It possesses a pair of unique tubular accessory respiratory organ (air sac), which is a modification of the gill chamber and enables it to live in water-air transition zones. In the catfish, three vasotocin (Vt) receptor gene paralogs viz., v1a1, v1a2 and v2a were identified for Vt actions. In the present study, the receptor gene transcripts were localized in the gill and air sac by in situ hybridization, and their expression levels in relation to water and air deprivation conditions were investigated by quantitative RT-PCR. The catfish were exposed to 1 h and 2 h in gonad inactive (resting) and gonad active (prespawning) phases. The gene paralogs showed overlapping distribution in the respiratory epithelium of primary and secondary lamellae of gills and reduced lamellae of the air sacs. In water deprivation (forced aerial mode of respiration) experiment, v2a expression showed a high fold increase in the air sac, which was unchanged or inhibited in the gill. Both v1a1 and v1a2 expression was significantly upregulated in the air sac but showed varied responses in the gill. The gill v1a1 expression was unchanged in the resting phase and modestly upregulated in the prespawning phase. The gill v1a2 expression was modestly upregulated at 1 h in both phases but unchanged at 2 h. In the air deprivation experiment (forced aquatic respiration), the v2a expression in the air sac was inhibited except for a mild stimulation at 1 h in the prespawning phase. In the gill, the v2a expression was stimulated with a steep upregulation at 2 h in the prespawning phase. Both v1a1 and v1a2 expression was significantly high in the gill but only modestly increased or unchanged in the air sac. The expression patterns point to a functional distinction; the V2 type receptor expression was higher in the air sac during forced aerial respiration, and the V1 type receptor expression was highly prominent in the gill during forced aquatic respiration. Water and air deprivation treatments caused a significant increase in plasma cortisol level, and the stimulation was higher in the water deprivation fish in the resting phase but equally prominent in the water and air deprivation groups in the prespawning phase. The results indicate that the changes in the expression patterns of Vt receptor genes may be a sequel to stress (hypoxic, metabolic and osmotic), and both Vt and cortisol may interact to counter the stress responses. This study shows that Vt has a new role in the control of air sac functions.

Vasotocin stimulates maturation-inducing hormone, oocyte maturation and ovulation in the catfish Heteropneustes fossilis: Evidence for a preferential calcium involvement

Arginine vasotocin (VT) is the basic neurohypophysial nonapeptide hormone in teleosts. VT is also distributed in the ovary of the catfish Heteropneustes fossilis and induces final oocyte maturation (FOM) and ovulation by stimulating the maturation-inducing hormone (MIH). The present study reports the effects of cAMP (0.5 mM), phosphodiesterase inhibitors (IBMX -0.5 mM and theophylline- 0.5 mM), the inositol triphosphate (IP3) receptor inhibitor heparin (10 μg/mL) and the Ca²⁺ chelator BAPTA-AM (25 μM) on VT (100 nM) - induced progestin stimulation, FOM and ovulation. Incubation of post-vitellogenic follicles with cAMP, IBMX and theophylline for 0, 8, 16 and 24 h stimulated basal secretion of progesterone (P₄), 17-hydroxyprogesterone (17-P) and 17, 20β-dihydroxy-4-pregnen-3-one (MIH) in a duration-dependent manner. The incubation of the follicles with heparin stimulated P₄ modestly, and 17-P and MIH levels in a duration-dependent manner. The incubation of the follicles with BAPTA-AM stimulated P₄ and MIH levels marginally and 17-P robustly. The stimulation was in the order cAMP > IBMX > theophylline > heparin > BAPTA-AM. The incubation of the follicles with VT stimulated P₄, 17-P, MIH, GVBD and ovulation in a duration-dependent manner. The co-incubations with VT and the test compounds inhibited the VT-induced stimulation of P₄, 17-P and MIH levels in a time-dependent manner in the order heparin > BAPTA-AM > cAMP > IBMX > theophylline. Concurrently, the VT-induced stimulation of GVBD and ovulation were also inhibited by the test compounds in the same order. The results show that VT induces FOM and ovulation preferentially acting through Ca²⁺ pathway and a crosstalk between Ca²⁺ and cAMP signaling pathways seems to integrate the processes.

Cloning and characterization of estrogen hydroxylase (cyp1a1 and cyp1b1) genes in the stinging catfish Heteropneustes fossilis and induction of mRNA expression during final oocyte maturation

Estrogen hydroxylases (EHs) are cytochrome P450 Family 1 (Cyp1, Clan 2) proteins involved in estrogen hydroxylations at 2-, 4- or 16- carbon positions to form catecholestrogens. EHs are encoded by CYP1A1, CYP1A2 and CYP1B1 in mammals. In the catfish Heteropneustes fossilis, cyp1a1 and cyp1b1 cDNAs were cloned and characterized from liver and ovary. The cyp1a1 cDNA is 2071 bp long and codes for a 518 amino acids (aa) long protein. The cloned cyp1b1 cDNA is 1927 bp long and codes for a 509 residue protein. The deduced proteins clustered distinctly into teleost Cyp1a1 and Cyp1b1 clades, distinct from the tetrapod clusters and featured common function domains and homology with other teleost proteins. In the qPCR assay, the transcripts were the most abundant in the liver, followed by brain and ovary, and moderate in gill, kidney and muscle. Evidence was presented to show the involvement of the genes in reproduction. Expression of brain and ovarian transcripts showed significant seasonal variations with the highest abundance in the spawning phase. In situ hybridization showed the transcripts in the follicular layer (theca and granulosa) of the ovarian follicles. Periovulatory changes in the expression cyp1a1 and cyp1b1 were obtained during final oocyte maturation (FOM) and ovulation induced by human chorionic gonadotropin (hCG), both in vivo and in vitro, and by 2-hydroxyestradiol-17β (catecholestrogen) in vitro. In the brain, the transcript levels increased with time but in the ovary, the increase was maximal at 16 h and decreased at 24 h. The periovulatory activation of the cyp1 genes was reported in this study and discussed on the basis of complex regulation of FOM and ovulation.

Expression profile of kisspeptin2 and gonadotropin-releasing hormone2 mRNA during photo-thermal and melatonin treatments in the female air-breathing catfish Heteropneustes fossilis

In seasonally breeding vertebrates, extrinsic factors like photoperiod and temperature are major determinants, controlling the annual reproductive cycle. In teleosts, kisspeptin, which occurs in two molecular forms: kisspeptin1 (Kiss1) and kisspetin2 (Kiss2), has been reported to alter gonadotropin (Lh and Fsh) secretion but its effect on gonadotropin-releasing hormone (Gnrh) secretion is not unequivocally proved. In the catfish Heteropneustes fossilis, we isolated and characterized kiss2 and gnrh2 cDNAs and the present work reports effects of altered photo-thermal conditions and melatonin (MT, a pineal hormone) on their expressions in the brain. The exposure of the catfish to long photoperiod (LP, 16 h light) at normal temperature (NT) or high temperature (HT, 28 °C) at normal photoperiod (NP) for 14 or 28 days stimulated both kiss2 and gnrh2 expression in both gonad resting and preparatory phases with the combination of LP + HT eliciting maximal effects. Short photoperiod (SP, 8 h light) under NT or HT altered the gene expression according to the reproductive phase and temperature. MT that mediates photo-thermal signals to the brain inhibited brain kiss2 and gnrh2 gene expression in the NP + HT, LP + NT, and SP + NT groups. The altered photo-thermal conditions elicited changes in steroidogenic pathway as evident from changes in plasma E₂, progesterone, and testosterone levels. The results show that brain kiss2-gnrh2 signaling is involved in photo-thermal-mediated mechanisms controlling reproduction.

Identification of kisspeptin2 cDNA in the catfish Heteropneustes fossilis: Expression profile, in situ localization and steroid modulation

Kisspeptin (Kiss) is considered an upstream regulator of gonadotropin-releasing hormone in mammals but its role in non-mammalian vertebrates is not unequivocally established. In the catfish Heteropneustes fossilis, a 605 bp long cDNA was identified from the brain by cloning as well as by retrieving from the catfish transcriptome database. The open reading frame (ORF, 93-405 bp) codes for a 113 amino acids long precursor protein. Homology and phylogenetic analyses showed that the predicted protein belongs to the vertebrate Kiss2 type with a high degree of conservation in the Kiss2-10 region (FNFNPFGLRF). The kiss2 transcripts were expressed highly in the brain and gonads in a dimorphic manner with a female bias. In the brain, kiss2 transcripts showed regional differences with higher expression in the medulla oblongata and forebrain regions. The kiss2 transcripts showed significant seasonal variations with the highest expression in the brain in spawning phase and in the gonads in prespawning phase. The kiss2 transcripts were localized in the brain (nucleus preopticus, habenular nucleus, nucleus recessus posterioris, nucleus recessus lateralis) and stratum periventriculare (radial glial cells) of optic tectum, pituitary and ovary (follicular layer and germinal vesicle). Ovariectomy (1, 2, 3 and 4 weeks) decreased brain kiss2 mRNA levels and a single injection of estradiol-17β (E₂; 0.5 μg/g body weight) in 3- week ovariectomized (OVX) and sham operated fish resulted in an increase in the transcript levels after 24 h. The E₂ receptor antagonist Tamoxifen (TMX) produced biphasic effects on the kiss2 expression in the dose- response study. TMX inhibited the expression in the OVX fish, but elicited a stimulatory effect in the OVX + E₂-treated fish. Testosterone (T) decreased, and progesterone (P₄) inhibited (resting phase) or stimulated (prespawning phase) the transcript level in 3-week OVX fish. In the 3-week sham groups, E₂ increased, and TMX, T and P₄ inhibited the kiss2 transcript levels. The results suggest that Kiss2 is an important regulator of the brain- pituitary- gonadal- endocrine axis, and in habenular and optic tectum functions.

Molecular cloning and characterization of a gonadotropin-releasing hormone 2 precursor cDNA in the catfish Heteropneustes fossilis: Expression profile and regulation by ovarian steroids

Gonadotropin-releasing hormone 2 (Gnrh2) is one of the three classes of Gnrh distributed in vertebrates and is highly conserved. In the present study, the cDNA encoding Gnrh2 was isolated and characterized in the ostariophysan catfish Heteropneustes fossilis (hf). The cDNA is 611 bp long with an open reading frame (ORF) of 261 bp that encodes a highly conserved protein of 86 amino acids. The deduced Gnrh2 precursor protein clustered with the vertebrate Gnrh2 type. The sequence identity of hfgnrh2 is 94% with African catfish (Clarias gariepinus) gnrh2 mRNA (accession no. X78047). The hfgnrh2 transcripts were expressed only in the brain and gonads with a higher expression in the female brain and ovary in both resting and prespawning phases. The expression was higher in the prespawning phase than the resting phase. The gnrh2 expression in the brain and ovary showed significant seasonal variations but with opposite patterns. In the brain, the expression was the highest in the preparatory phase, decreased progressively to low levels in the postspawning and resting phases. In the ovary, the transcript level was low in the resting and preparatory phases, increased sharply in the prespawning phase reaching the peak level in the spawning phase and declined sharply in the postspawning phase. The gnrh2 mRNA showed the highest expression in the hind brain-medulla oblongata and moderate to low expression in forebrain regions and pituitary. Ovariectomy resulted in a duration-dependent inhibition of hfgnrh2 mRNA levels in the resting and prespawning phases. Steroid (E₂, testosterone and progesterone) replacement treatments (0.5 μg/g body weight) in the 3- week ovariectomized fish restored the inhibition due to ovariectomy, elevated the expression over and above the sham level in the resting phase (E₂ group), and raised the levels almost to that of the sham group (testosterone and progesterone groups) in the prespawning phase. In the sham control groups, the steroid replacement resulted in a significant reduction in the mRNA levels. The expression of the gnrh2 mRNA in the brain-pituitary-gonadal axis and its regulation by gonadal steroids suggest that Gnrh2 may have a reproductive role in the catfish.

Reproductive hormones modulate differentially brain and ovarian vasotocin receptor gene expression in early and late recrudescent catfish, Heteropneustes fossilis

Investigations on the role of the reproductive hormones on VT receptor gene expression are lacking in teleosts. Previously we reported that gonadotropin and steroid hormones modulate the secretion and gene expression of brain and ovarian vasotocin (VT) in the catfish Heteropneustes fossilis. In continuation, in the present study we investigated the role of estradiol-17β (E₂), the maturation-inducing steroid (MIS) 17α, 20β-dihydroxy-4-pregnen-3-one (17, 20β-DP), and human chorionic gonadotropin (hCG) on the expression of VT receptor genes (v1a1, v1a2 and v2a) in the brain and ovary of the catfish in early (previtellogenic, preparatory) and late (post vitellogenic, prespawning) phases of the ovarian cycle. The steroid treatments (in vivo and in vitro) modulated only the v1a1 and v1a2 expression in both tissues, but not the v2a expression. The E₂-induced modulation of the v1a1 and v1a2 gene expression varied with the reproductive phase. In the preparatory phase, E₂ up regulated the expression of brain and ovarian v1a1 and v1a2 gene expression, the response varied with the dose and duration. In the prespawning phase, E₂ inhibited the expression in a dose- and duration-dependent manner. On the other hand, 17, 20β-DP up regulated the expression of brain and ovarian v1a1 and v1a2 in both phases, and the response was higher in the prespawning phase and varied with dose and duration. In contrast to the steroid effects, the hCG treatment modulated the expression of all the VT receptor genes only in the prespawning phase and the response varied with dose and duration. The results indicate differential modulatory roles of steroid hormones and hCG on the VT receptor gene expression, to mediate VT's reproductive or osmoregulatory functions. While the hCG effect on v1a type receptor expression may be steroid- dependent, that of v2a expression seems to be steroid-independent.

Stress hormones modulate lipopolysaccharide stimulation of head kidney interleukin-6 production in the catfish Heteropneustes fossilis: In vivo and in vitro studies

Interleukin-6 (IL-6) is a pleiotropic cytokine secreted by immune tissues such as monocytes/macrophages and have pro-inflammatory/anti-inflammatory and neuroendocrine actions. In this study, we report the modulatory effects of stress hormones, the cortisol agonist dexamethasone and catecholamines on lipopolysaccharide (LPS) - induced stimulation of head kidney IL-6 in the catfish Heteropneustes fossilis. In the in vivo study, the intraperitoneal administration of LPS stimulated, and dexamethasone time-dependently inhibited IL-6 level. In the in vitro study, the incubation of macrophage cultures with LPS stimulated IL-6 level significantly in all incubation times. Dexamethasone did not alter the basal IL-6 level but inhibited time-dependently the LPS-induced stimulation. Likewise, catecholamines did not alter the basal level of IL-6. Both epinephrine and norepinephrine inhibited the LPS-induced stimulation of IL-6. Dopamine, on the other hand, was ineffective. The results indicate that IL-6 is a useful marker of head kidney macrophage activity for studying endocrine-immune interactions in the catfish.

An in vitro study on noradrenergic modulation of final oocyte maturation in the catfish Heteropneustes fossilis

This study was aimed to elucidate in vitro the effects of endogenous catecholamines: dopamine (DA), noradrenaline (NA) and adrenaline (A), and the β-adrenergic blocker propranolol on induction of final oocyte maturation (FOM) in the catfish Heteropneustes fossilis. With this aim, post vitellogenic follicles from sexually mature gravid female catfish were incubated with each of DA, NA and A in a concentration range of 5-250 μM, and propranolol in a concentration range of 1-200 μg/mL at time points varying from 0 to 30 h). Translucent follicles without germinal vesicle (GV) and opaque follicles with GV were scored separately for the calculation of percentage germinal vesicle breakdown (GVBD), an index of FOM. Data were analyzed by one-way ANOVA and were considered statistically significant when P values were less than 0.05. The analysis of the data showed that the incubation with NA only stimulated GVBD in a concentration - and time-dependent manner. Though the incubation with propranolol decreased total follicular cAMP level significantly at and above 10 μg/mL concentrations, a significant effect of the GVBD increase was noticed at 50 μg/mL or higher. However, the 10 μg/mL concentration of propranolol was effective to inhibit the NA-induced GVBD significantly albeit at a low level (39%). The present study suggests that final oocyte maturation is modulated by NA through a β-adrenergic mechanism, implicating a neural control of oocyte maturation and ovulation in teleosts.

Reproductive stage- and sex-dependant effects of neurohypophyseal nonapeptides on gonadotropin subunit mRNA expression in the catfish Heteropneustes fossilis: An in vitro study

In the present study, in vitro effects of synthetic vasotocin (VT), isotocin (4Ser, 8Ile- oxytocin; ITb) and the recently cloned IT gene paralog product (8Val-Isotocin, ITa) were studied on the expression of pituitary gonadotropin (GtH) subunit mRNA levels. In male pituitaries of early (preparatory phase) and late (prespawning phase) recrudescing catfish, Heteropneustes fossilis, VT (10 nM, 100 nM and 1000 nM) stimulated fshβ expression dose-dependently. But in females, the dose-dependent effect was found only in the preparatory phase. In males, VT stimulated lhβ expression only at higher doses. In females, VT produced a significant dose-dependent increase of the lhβ expression only in the prespawning phase. VT stimulated the expression of gpα, dose-dependently in the preparatory phase in males and in the prespawning phase in females. The incubation of the pituitaries with ITb did not alter the fshβ expression in either sex in both preparatory and prespawning phases. In males, ITb stimulated the expression of lhβ and gpα only at the highest concentration (1000 nM) in both phases. In females, ITb stimulated both lhβ and gpα expression only at 1000 nM in the preparatory phase and dose-dependently in the prespawning phase. The incubation of the pituitaries with ITa produced effects similar to ITb on the expression of fshβ, lhβ, and gpα. The results show that the basic peptide VT modulates both fshβ and lhβ expressions, which are influenced by the sex and reproductive stage. The neutral peptide ITA/ITb exerts an insignificant effect on the fshβ expression regardless of sex or season. Both VT and ITa/ITb elicit a significant effect on the lhβ expression in late recrudescent phase especially in females.

Ovaprim, a commercial spawning inducer, stimulates gonadotropin subunit gene transcriptional activity: A study correlated with plasma steroid profile, ovulation and fertilization in the catfish Heteropneustes fossilis

The commercial fish spawning inducer Ovaprim (OVP) containing a salmon gonadotropin-releasing hormone analogue and domperidone (a dopamine receptor-2 antagonist) has been widely used as an effective spawning inducer in artificial breeding of fishes. It induces a preovulatory LH surge resulting in final oocyte maturation (FOM) and ovulation through a mechanism involving a steroidogenic shift to secrete a maturation-inducing steroid (MIS). In the present study, a 0.5μL/g body weight dose of OVP each injected at 0h and 24h intraperitoneally into gravid female catfish, Heteropneustes fossilis resulted in periovulatory changes in gonadotropin (GtH) subunit gene expression and steroid hormone levels. The OVP injections induced ovulation time-dependently from 6h onwards with 100% ovulation recorded from 24h to 48h. The fertilization rate was high from 6h to 18h and declined from 24h onwards. The OVP treatment up regulated the expression of GtH subunit genes differentially. The expression of glycoprotein-α (GPα) and luteinizing hormone (LHβ) peaked at 6h and 12h, and declined at 18h and 24h after the first injection. The second OVP injection at 24h elicited only a transient increase in the GPα expression at 6h and a sustained increase in the LHβ expression from 6h to 18h after the second injection, but both transcripts decreased subsequently. The follicle-stimulating hormone (FSHβ) expression responded to the OVP treatment from 12h onwards and maintained a constant level from 18h to 36h after the first injection; the second dose had little effect. Plasma steroids were differentially altered: the levels of estradiol-17β decreased while that of the MIS 17,20β-dihydroxy-4-pregnen-3-one; 17,20β-DP increased, causing the steroidogenic shift preceding FOM and ovulation. The present results indicate that LHβ expression coincides with the ovulation response and the late induction and maintenance of the FSH expression may be related to post-ovulatory events in the ovary.

Effects of altered photoperiod and temperature on expression levels of gonadotrophin subunit mRNAs in the female stinging catfish Heteropneustes fossilis

Differential effects of photoperiod and temperature on the temporal modulation of gonadotrophin subunit genes (glycoprotein α, gpα), follicle-stimulating hormone β (fshβ) and luteinizing hormone β (lhβ) expression were investigated in the stinging catfish Heteropneustes fossilis. Female H. fossilis were exposed to varying photoperiod and temperature conditions for 14 and 28 days in the early preparatory phase of the annual reproductive cycle. Gonadotrophin subunit gene expression, gonado-somatic index (I_G ), ovarian histology and plasma steroid hormone levels were evaluated. The exposure of H. fossilis to long photoperiod (LP) of 16 h light or high temperature (HT) at 28 ± 2° C (mean ± s.e.), alone or in combination, resulted in significant increases in gpα, fshβ and lhβ messenger (m)RNA levels, I_G , plasma oestradiol-17β (E₂ ), testosterone (T) and progesterone (P₄ ) levels. The ovaries were filled with advanced yolky oocytes. On the other hand, the short photoperiod (SP) of 8 h light exposure decreased the transcript levels with higher inhibition in the normal temperature (NT) group at 18 ± 2° C (mean ± s.e.) than the HT group at 28 ± 2° C. Furthermore, the inhibition reached the highest level in total darkness (TD) of 24 h light deprivation under NT conditions at 18 ± 2° C. Consequently, the SP and TD treatments inhibited the I_G , plasma E₂ and T levels and ovarian development. The exposure to high temperature at 28 ± 2° C also modified the short photoperiod effect by elevating plasma E₂ level. The plasma T level changed only mildly while the plasma P₄ level showed the greatest fluctuations; the level reached the nadir in the SP + HT group but increased in the SP + NT group on day 28. A two-way ANOVA of the data showed differential effects of photoperiod and temperature; photoperiod produced a highly significant effect on fshβ expression while temperature had a highly significant effect both on lhβ and gpα levels. Thus, the differential expression of the gpα by the environmental variables ensures temporal synchronization of ovarian development and spawning.

Identification and characterization of a catechol-o-methyltransferase cDNA in the catfish Heteropneustes fossilis: Tissue, sex and seasonal variations, and effects of gonadotropin and 2-hydroxyestradiol-17β on mRNA expression

Catechol-O-methyltransferase (COMT) is involved in the methylation and inactivation of endogenous and xenobiotic catechol compounds, and serves as a common biochemical link in the catecholamine and catecholestrogen metabolism. Studies on cloning, sequencing and function characterization comt gene in lower vertebrates like fish are fewer. In the present study, a full-length comt cDNA of 1442bp with an open-reading frame (ORF) of 792bp, and start codon (ATG) at nucleotide 162 and stop codon (TAG) at nucleotide 953 was isolated and characterized in the stinging catfish Heteropneustes fossilis (accession No. KT597925). The ORF codes for a protein of 263 amino acid residues, which is also validated by the catfish transcriptome data analysis. The catfish Comt shared conserved putative structural regions important for S-adenosyl methionine (AdoMet)- and catechol-binding, transmembrane regions, two glycosylation sites (N-65 and N-91) at the N-terminus and two phosphorylation sites (Ser-235 and Thr-240) at the C-terminus. The gene was expressed in all tissues examined and the expression showed significant sex dimorphic distribution with high levels in females. The transcript was abundant in the liver, brain and gonads and low in muscles. The transcripts showed significant seasonal variations in the brain and ovary, increased progressively to the peak levels in spawning phase and then declined. The brain and ovarian comt mRNA levels showed periovulatory changes after in vivo and in vitro human chorionic gonadotropin (hCG) treatments with high fold increases at 16 and 24h in the brain and at 16h in the ovary. The catecholestrogen 2-hydroxyE₂ up regulated ovarian comt expression in vitro with the highest fold increase at 16h. The mRNA and protein was localized in the follicular layer of the vitellogenic follicles and in the cytoplasm of primary follicles. The data were discussed in relation to catecholamine and catecholestrogen-mediated functions in the brain and ovary of the stinging catfish.

Effects of the fish spawning inducer ovaprim on vasotocin receptor gene expression in brain and ovary of the catfish Heteropneustes fossilis with a note on differential transcript expression in ovarian follicles

Ovaprim (OVP), a commercial formulation of a salmon GnRH analogue and the dopamine receptor-2 blocker domperidone, is a successful spawning inducer for fish breeding. It induces a preovulatory surge in LH, which stimulates the synthesis of a maturation-inducing steroid (MIS, 17,20β-dihydroxy-4-pregnen-3-one) that initiates germinal vesicle breakdown (GVBD) and ovulation. Coincidently, the OVP treatment also stimulates vasotocin (VT) secretion in the brain and ovary of the catfish Heteropneustes fossilis that also stimulates the synthesis of the MIS. VT mediates its effect through V1- and V2-type receptors. In the present study in the catfish, we report that OVP stimulates the expression of VT receptor genes v1a1, v1a2 and v2a in the brain and ovary. A single intraperitoneal administration of OVP (0.5μL/g body weight) or incubation of post-vitellogenic ovarian follicles with 5μL/mL OVP, for 0, 4, 8, 12, 16, and 24h stimulated ovulation and GVBD, respectively, in a time-dependent manner. The OVP treatment in vivo stimulated brain VT receptor transcript levels 4h onwards. The peak expression was noticed at 12h (v1a1), 8 and 12h (v1a2), and 8, 12 and 16h (v2a), coinciding with FOM and ovulation. The VT receptor genes are expressed in the ovarian follicles compartmentally; both v1a1 and v1a2 are expressed in the isolated follicular layer (theca and granulosa) but absent in denuded oocytes. V2a is expressed in the denuded oocytes and not in the follicular layer. The OVP injection stimulated the v1a1 and v1a2 expression from 4h onwards in both intact follicle and isolated follicular layer, the peak expression was observed at 16h. The v2a expression was up-regulated in both intact follicles and denuded oocytes at 4h (denuded oocytes) or 8h (intact follicle) onwards with the peak expression at 12h and 16h (denuded oocytes) or at 16h (intact follicles). Under in vitro conditions, the OVP incubations elicited similar pattern of changes with the peak stimulation at 16h for all the genes. In conclusion, the VT receptor genes are differentially expressed in the ovarian follicles and OVP induced periovulatory stimulation of the VT receptor genes, coinciding with FOM and ovulation.

Structural and functional diversity of nonapeptide hormones from an evolutionary perspective: A review

The article presents an overview of the comparative distribution, structure and functions of the nonapeptide hormones in chordates and non chordates. The review begins with a historical preview of the advent of the concept of neurosecretion and birth of neuroendocrine science, pioneered by the works of E. Scharrer and W. Bargmann. The sections which follow discuss different vertebrate nonapeptides, their distribution, comparison, precursor gene structures and processing, highlighting the major differences in these aspects amidst the conserved features across vertebrates. The vast literature on the anatomical characteristics of the nonapeptide secreting nuclei in the brain and their projections was briefly reviewed in a comparative framework. Recent knowledge on the nonapeptide hormone receptors and their intracellular signaling pathways is discussed and few grey areas which require deeper studies are identified. The sections on the functions and regulation of nonapeptides summarize the huge and ever increasing literature that is available in these areas. The nonapeptides emerge as key homeostatic molecules with complex regulation and several synergistic partners. Lastly, an update of the nonapeptides in non chordates with respect to distribution, site of synthesis, functions and receptors, dealt separately for each phylum, is presented. The non chordate nonapeptides share many similarities with their counterparts in vertebrates, pointing the system to have an ancient origin and to be an important substrate for changes during adaptive evolution. The article concludes projecting the nonapeptides as one of the very first common molecules of the primitive nervous and endocrine systems, which have been retained to maintain homeostatic functions in metazoans; some of which are conserved across the animal kingdom and some are specialized in a group/lineage-specific manner.

Morpho-histology of head kidney of female catfish Heteropneustes fossilis: seasonal variations in melano-macrophage centers, melanin contents and effects of lipopolysaccharide and dexamethasone on melanins

In the catfish Heteropneustes fossilis, the anterior kidney is a hemopoietic tissue which surrounds the adrenal homologues, interrenal (IR) and chromaffin tissues corresponding to the adrenal cortical and adrenal medulla of higher mammals. The IR tissue is arranged in cell cords around the posterior cardinal vein (PCV) and its tributaries and secretes corticosteroids. The chromaffin tissue is scattered singly or in nests of one or more cells around the epithelial lining of the PCV or blood capillaries within the IR tissue. They are ferric ferricyanide-positive. Leukemia-inhibitory factor (LIF)-like reactivity was noticed in the lining of the epithelium of the IR cell cords and around the wall of the PCV and blood capillaries. No staining was observed in the hemopoietic cells. IL-1β- and TNF-α-like immunoreactivity was seen in certain cells in the hemopoietic tissue but not in the IR region. Macrophages were identified with mammalian macrophage-specific MAC387 antibodies and are present in the hemopoietic mass but not in the IR tissue. Pigments accumulate in the hemopoietic mass as melano-macrophage centers (MMCs) and are PAS-, Schmorl's- and Perls'-positive. The pigments contain melanin (black), hemosiderin (blue) and lipofuscin/ceroid (oxidized lipid, yellowish tan), as evident from the Perls' reaction. The MMCs were TUNEL-positive as evident from FITC fluorescence, indicating their apoptotic nature. The MMCs showed significant seasonal variation with their density increasing to the peak in the postspawning phase. Melanins were characterized spectrophotometrically for the first time in fish anterior kidney. The predominant form is pheomelanin (PM), followed by eumelanin (EM) and alkali-soluble melanin (ASM). Melanins showed significant seasonal variations with the level low in the resting phase and increasing to the peak in the postspawning phase. Under in vitro conditions, lipopolysaccharide (10 µg/mL) treatment increased significantly the levels of PM and EM levels both at 16 and at 32 h and the ASM level at 32 h. On the other hand, the synthetic glucocorticoid dexamethasone (100 nM) decreased significantly the levels of EM, PM and ASM time-dependently. The results indicate that the anterior kidney is an important site of immune-endocrine interaction.

Vasotocin--A new player in the control of oocyte maturation and ovulation in fish

In this article, the physiological role of ovarian vasotocin (VT) on fish final oocyte maturation (FOM) and ovulation is reviewed based on the studies mainly available in the catfish Heteropneustes fossilis. The VT system is characterized in the follicular layer of the oocytes by both immunocytochemical and in situ hybridization techniques. The distribution was confirmed in isolated follicular layer preparations by HPLC characterization and quantification. Three VT receptor subtype genes are identified: V1a1 and V1a2 subtypes are distributed in the follicular layer and V2 subtype is present along the granulosa-oocyte membrane junction. The expression of peptide, VT precursor gene and VT receptor genes shows seasonal and periovulatory changes in the ovary. VT secretion is modulated by E2 differentially in a season-specific manner, and by progestin steroids positively. VT modulates E2 in a biphasic manner in early recrudescent phase and induces a steroidogenic shift inhibiting E2 and stimulating progestin steroid (P4, 17P4 and 17,20β-DP) pathways in the late recrudescent phase. VT stimulates prostaglandin secretion, germinal vesicle breakdown (GVBD), oocyte hydration and ovulation. VT acts through different receptors to stimulate these processes. It uses the V1 type receptor to stimulate GVBD and ovulation, and the V2 type to stimulate oocyte hydration. VT acts as an important link in the cascade of gonadotropin control of FOM and ovulation. More research is required in other species.

Catfish gonadotrophins: cellular origin, structural properties and physiology

Gonadotrophins (GTHs) play a central role in the regulation of gametogenesis and spawning. The structural duality of the GTHs [luteinising hormone (LH) and follicle-stimulating hormone (FSH)] is established in fishes with the exception of ancestral vertebrates. Most studies indicate that, in teleosts, the GTHs are secreted in separate cells. Phylogenetic analysis shows that the common α-subunit of the GTHs (and also of thyroid-stimulating hormone) and LHβ are highly conserved in fishes, as in tetrapods. However, FSHβ shows considerable divergence in teleosts. There may be 12 or 13 cysteine residues, with an additional one near the N-terminus. There may be one or two N-linked glycolsyation sites. In catfishes, there are 13 cysteine residues and one N-linked glycosylation site. In an extreme situation, a potential glycosylation site is lacking in some fishes. Both FSH and LH receptors are characterised in teleosts. The FSH receptor is promiscuous and can be cross-activated by LH. By contrast, the LH receptor is highly selective, being activated by its natural ligand or by heterologous ligands (e.g. human chorionic gonadotrophin). Consequently, teleosts show different patterns of LH and FSH secretion. In catfishes, in the absence of native FSH protein, LH controls all aspects of reproduction, from early gametogenesis to spawning.

Melanins as biomarkers of ovarian follicular atresia in the catfish Heteropneustes fossilis: biochemical and histochemical characterization, seasonal variation and hormone effects

Follicular atresia is a common feature of the vertebrate ovary that occurs at different stages of folliculogenesis and ovarian regression. It has physiological significance to maintain homeostasis and control fecundity, and ensure removal of post-ovulatory follicular remnants for preparing the ovary for the next cycle. Pigments appear late in the atretic process as indigestible waste formed out of the degradation of the oocytes, follicle wall and granulocytes. In the present study, pigment accumulation was demonstrated by Schmorl's and Perls' staining methods in the atretic ovarian follicles of Heteropneustes fossilis during follicular development and regression. Melanins were characterized spectrophotometrically for the first time in fish ovary. The predominant form is eumelanin, followed by pheomelanin and alkali-soluble melanin. Melanins showed significant seasonal variations with levels low in gonad resting phase, increasing to the peak in the post-spawning phase. The concentration of melanins increased time-dependently in post-ovulated ovary after human chorionic gonadotropin treatment. In the spawning phase, in vitro incubation of ovary slices with estradiol-17β or dexamethasone for 8 or 16 h decreased both eumelanin and pheomelanin levels time-dependently. The alkali-soluble melanin showed a significant decrease only in the dexamethasone group at 16 h. The results show that melanin assay can be used as a biomarker of follicular atresia in fish ovary, natural or induced by environmental toxicants.

Molecular cloning, sequencing and tissue expression of vasotocin and isotocin precursor genes from Ostariophysian catfishes: phylogeny and evolutionary considerations in teleosts

Basic and neutral neurohypophyseal (NH) nonapeptides have evolved from vasotocin (VT) by a gene duplication at the base of the gnathostome lineage. In teleosts, VT and IT are the basic and neutral peptides, respectively. In the present study, VT and IT precursor genes of Heteropneustes fossilis and Clarias batrachus (Siluriformes, Ostariophysi) were cloned and sequenced. The channel catfish Icatalurus punctatus NH precursor sequences were obtained from EST database. The catfish NH sequences were used along with the available Acanthopterygii and other vertebrate NH precursor sequences to draw phylogenetic inference on the evolutionary history of the teleost NH peptides. Synteny analysis of the NH gene loci in various teleost species was done to complement the phylogenetic analysis. In H. fossilis, the NH transcripts were also sequenced from the ovary. The cloned genes and the deduced precursor proteins showed conserved characteristics of the NH nonapeptide precursors. The genes are expressed in brain and ovary (follicular envelope) of H. fossilis with higher transcript abundance in the brain. The addition of the catfish sequences in the phylogenetic analysis revealed that the VT and IT precursors of the species-rich superorders of teleosts have a distinct phylogenetic history with the Acanthopterygii VT and IT precursors sharing a less evolutionary distance and the Ostariophysi VT and IT having a greater evolutionary distance. The genomic location of VT and IT precursors, and synteny analysis of the NH loci lend support to the phylogenetic inference and suggest a footprint of fish- specific whole genome duplication (3R) and subsequent diploidization in the NH loci. The VT and IT precursor genes are most likely lineage-specific paralogs resulting from differential losses of the 3R NH paralogs in the two superorders. The independent yet consistent retention of VT and IT in the two superorders might be directed by a stringent ligand-receptor selectivity.

Molecular cloning, sequencing and phylogeny of vasotocin receptor genes in the air-breathing catfish Heteropneustes fossilis with sex dimorphic and seasonal variations in tissue expression

Vasotocin (VT) is the ortholog of vasopressin (VP) in non-mammalian vertebrates and is known for multiple functions. Teleost fishes have a complete repertoire of known VP/VT receptor subtypes (vasopressin type, VR): two V1A subtypes (V1Aa and V1Ab or V1a1 and V1a2) and five V2 subtypes (V2A1, V1A2, V2B1, V2B2 and V2C). Full-length cDNAs of v1a1, v1a2 and v2 (v2a1) with ORFs of 1,308, 1,137 and 1,527 bp, respectively, were cloned and characterized in the catfish Heteropneustes fossilis (Siluriformes, Ostariophysi). BLAST analysis revealed that the genes encoded three VT receptors, V1a1, V1a2 and V2 of 436, 379 and 509 amino acid residues, respectively. The predicted proteins showed typical features of the seven-transmembrane domain receptor core structure with hallmark triplets Asp-Arg-Tyr/Asp-Arg-His (DRY/DRH) and the variable intracellular loop III of vertebrate neurohypophysial hormone receptors. Phylogenetic analysis of the deduced protein sequences revealed that they clustered with the V1Aa, V1Ab and V2A1, respectively, of other teleosts. The V2R has a sequence identity of 70-76% with V2A1 than with the V2B type (sequence identity 43-49%). Semiquantitative PCR analysis showed that the receptor gene transcripts were expressed ubiquitously in the tissues examined (brain, pituitary, gonads, liver, muscle, kidney and gills) and displayed sex and seasonal fluctuations in a tissue-specific manner. The results form a basis for functional studies on the VT receptors in the catfish.

An in vitro study on catecholamine modulation of ovarian steroidogenic activity in the catfish Heteropneustes fossilis

In the present study, α-methylparatyrosine (α-MPT), a tyrosine hydroxylase inhibitor was used to impair ovarian catecholaminergic activity in vitro. The consequent effects on catecholamine (CA) levels were correlated with follicular steroid production. l-dihdroxyphenylalanine (l-DOPA, the precursor of CA) and human gonadotropin (hCG) were supplemented to reverse the effect of α-MPT. The experiments were conducted in two reproductive phases, namely preparatory and pre-spawning phases in female catfish Heteropneustes fossilis. The incubation with α-MPT inhibited ovarian l-DOPA, dopamine (DA), norepinephrine (NE) and epinephrine (EP) levels and the l-DOPA supplementation compensated the inhibitory effect. The level of tyramine (TR) was increased by the α-MPT treatment but inhibited by the l-DOPA supplementation. α-MPT produced stage-specific (seasonal) effects on ovarian estradiol-17β (E2); in the preparatory phase, E2 was decreased significantly at both 12 and 24h and in the pre-spawning phase, the level was stimulated over the respective control groups. The changes were higher at 24h in both phases. l-DOPA and hCG increased the E2 level significantly in the preparatory phase and reversed the inhibitory effect of α-MPT in the co-incubation groups. In the pre-spawning phase, α-MPT-stimulated the E2 level compared to the control groups, which was reversed by l-DOPA, hCG, or by both, in co-incubations. In contrast, the α-MPT treatment decreased progesterone (P4), 17-hydroxyprogesterone and 17,20β-dihydroxy-4-prenen-3-one (17,20β-DP) in a duration-dependent manner while the co-incubations with l-DOPA, hCG, or by both, significantly reversed the inhibitory effect. These results suggest that ovarian CAs (DA, NE and EP) may exert differential and stage-specific effects on E2, inhibition in the preparative phase and stimulation in the pre-spawning phase. The progestin steroids appear to be stimulated by CAs. In conclusion, this study highlights a possible direct/causal functional interaction between CA activity and gonadotropin on steroidogenic activity, and that CAs may be involved in regulating temporal secretion of the hormones through causing the shift in steroidogenic pattern.

Effects of ovaprim, a commercial spawning inducer, on vasotocin and steroid hormone profiles in the catfish Heteropneustes fossilis: in vivo and in vitro studies

Ovaprim (OVP) is used as an effective spawning inducer for artificial breeding of fishes and contains a salmon gonadotropin-releasing hormone analogue and a dopamine receptor-2 antagonist, domperidone. Previously, we have shown that vasotocin (VT) stimulates ovarian final oocyte maturation, hydration, and ovulation through a mechanism involving induction of a steroidogenic shift, favouring the production of a maturation-inducing hormone (MIH). In the present study, we demonstrated that OVP stimulated brain, plasma and ovarian VT levels, suggesting multiple sites of action, apart from its well established role in the induction of a preovulatory LH surge. An intraperitoneal injection of 0.5μL/g body weight of OVP for different time intervals (0, 4, 8, 12, 16 and 24h) induced ovulation as well as increased significantly brain and plasma VT levels in a time-dependent manner. Plasma steroids were differentially altered; the levels of estradiol-17β (E2) and testosterone (T) decreased, and the MIH (17, 20β-dihydroxy-4-pregnen-3-one; 17, 20β-DP) level increased time-dependently. In order to demonstrate whether OVP acts at the level of the ovary directly, in vitro experiments were conducted. The incubation of ovarian slices/follicles with OVP (1, 5 and 10μL/mL) for different time points (0, 4, 8, 12, 16 and 24h) induced germinal vesicle breakdown (GVBD) in a concentration- and time-dependent manner. Ovarian VT increased significantly in a concentration- and time-dependent manner with a maximal increment at 16h. Ovarian T and E2 levels decreased concurrently with the rise in the MIH level, dose- and duration-dependently. The results show that OVP stimulates VT at the brain and ovarian level. The direct OVP-VT cascade has the potential to stimulate FOM and ovulation, sidelining the pituitary glycoprotein hormone (LH) surge.

Functional interactions between vasotocin and prostaglandins during final oocyte maturation and ovulation in the catfish Heteropneustes fossilis

Functional interactions between vasotocin (VT) and prostaglandins (PGs) in the regulation of final oocyte maturation (FOM) and ovulation were investigated in the catfish Heteropneustes fossilis. Incubation of post-vitellogenic follicles with VT resulted in significant increases of both PGF2α and PGE2 at 8 and 16h intervals. The rise was higher at 16h except in the 1000nM VT group, in which the PG levels decreased compared to the 100nM group (biphasic effect). VT was more effective to increase the PG levels in comparison to hCG or IT. The co-incubation of the follicles with both hCG (20IU/ml) and VT (100nM) increased significantly PGF2α level at 8h, higher than that elicited by each when incubated alone. Pre-incubation of the follicles with V1 receptor antagonist, alone or in co-incubation with VT, significantly inhibited the VT-stimulated PGF2α and PGE2 levels. Under similar conditions, V2 receptor antagonist did not affect the PGE2 levels. Both VT (100nM) and PGs stimulated FOM (germinal vesicle breakdown) and ovulation in a dose- and duration dependent manner, PGF2α was more effective. Incubation of postvitellogenic follicles with indomethacin (a non selective cyclooxygenase inhibitor) per se did not affect FOM and ovulation but significantly decreased VT and PG effects upon pre-incubation. The results suggest that the VT stimulation of PGs may be mediated mainly through the V1 receptor though the involvement of V2 receptor cannot be excluded. The article also discussed the positive interplay of gonadotropin, maturation-inducing steroid, VT and PG during FOM and ovulation.

Role of catecholestrogens on ovarian prostaglandin secretion in vitro in the catfish Heteropneustes fossilis and possible mechanism of regulation

Seasonal, periovulatory and 2-hydroxyestradiol-17β (2-OHE(2))-induced changes on ovarian prostaglandin (PG) E(2) and F(2α) were investigated under in vivo or in vitro in the female catfish Heteropneustes fossilis. Both PGE(2) and PGF(2α) increased significantly during ovarian recrudescence with the peak levels in spawning phase. The PGs showed periovulatory changes with the peak levels at 16 h after the hCG treatment. Incubation of postvitellogenic ovary fragments with estradiol-17β (E(2)), 2-OHE(2) or 2-methoxyE(2) produced concentration-dependent increases in PG levels; 2-OHE(2) was more effective. In order to identify the receptor mechanism involved in the 2-OHE(2)-induced PG stimulation, the ovarian pieces were incubated with phentolamine (an α-adrenergic antagonist), propranolol (a β-adrenergic antagonist) or tamoxifen (an estrogen receptor blocker) alone or in combination with 2-OHE(2). The incubation of the tissues with the receptor blockers alone did not produce any significant effect on basal PG levels. However, co- and pre-incubation of the tissues with the blockers resulted in inhibition of the stimulatory effect of 2-OHE(2) on the PGs. Phentolamine was more effective than propranolol. The signal transduction pathway(s) involved in the 2-OHE(2)-induced PG secretion was investigated. The incubation of the ovarian pieces with 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), chelerythrine (a protein kinase C inhibitor) and PD098059 (a mitogen-activated protein kinase inhibitor) significantly lowered the basal secretion of PGF(2α) and PGE(2). In contrast, H89 (a protein kinase A inhibitor) increased the basal secretion of PGs at 1 and 5 μM concentration and decreased it at 10 μM concentration. The co- or pre-incubation with IBMX, H89, chelerythrine and PD098059 significantly inhibited the stimulatory effect of 2-OHE(2) on PGF(2α) and PGE(2) levels. The inhibition was higher in the pre-incubation groups. Chelerythrine was the most effective followed by PD098059, IBMX and H89. The results suggest that 2-OHE(2) may employ both adrenergic and estrogen receptors, or a novel receptor mechanism having properties of both adrenergic and estrogen receptors.

In vitro effects of 2-hydroxyestradiol-17β on ovarian follicular steroid secretion in the catfish Heteropneustes fossilis and identification of the receptor and signaling mechanisms

Ovarian pieces containing postvitellogenic follicles were incubated in vitro with different concentrations of the catecholestrogen 2-hydroxyestradiol-17β (2-OHE(2)) to evaluate its effects on steroid production and germinal vesicle breakdown (GVBD) in the catfish Heteropneustes fossilis. The incubation with 2-OHE(2) induced a shift in steroidogenic pattern: the C(19) and C(18) steroids testosterone (T) and estradiol-17β (E(2)), respectively were significantly decreased with a concomitant significant increase in the C(21) steroids progesterone (P(4)), 17-hydroxyprogesterone (17-OHP), 17,20β-dihydroxy-4-pregnen-3-one (17,20β-DP), 17,20α-dihydroxy-4-pregnen-3-one (17,20α-DP) and cortisol (F). Concomitantly, the catecholestrogen induced dose-dependently GVBD response, the first sign of meiosis resumption. The co- and pre-incubations of the ovarian pieces with 2-OHE(2), and adrenergic (phentolamine, α-blocker and propranolol, β-blocker) or estrogen (tamoxifen) receptor blockers resulted in inhibition of the stimulatory effect of the catecholestrogen on C(21) steroids and reversed the inhibition of testosterone and E(2). The α-blocker was more effective than the β-blocker. Our results suggest that 2-OHE(2) appears to employ both adrenergic (α-type) and estrogen receptor mechanisms in mediating the effects. The co- or pre-incubation of ovarian pieces with IBMX (a cAMP elevating drug), H89 (a protein kinase A inhibitor), and PD098059 (a MAP kinase kinase inhibitor) significantly inhibited the stimulatory effect of 2-OHE(2) on the C(21) steroids. The effect of chelerythrine (a protein kinase C inhibitor), on the other hand, varied with the incubation condition. In the co-incubation, the steroids showed varied effects: 17,20β-DP, testosterone and E(2) were elevated, and P(4) and 17-OHP were decreased. In the pre-incubation set up, all the steroids were inhibited except E(2). The inhibition by the blockers was higher in the pre-incubation groups. Taken together, the data suggest the involvement cAMP-protein kinase A, protein kinase C and MAP kinase pathways in the modulation of the steroidogenic activity.

Estrogen regulation of brain vasotocin secretion in the catfish Heteropneustes fossilis: an interaction with catecholaminergic system

Vasotocin (VT) is a basic neurohypophysial nonapeptide in non-mammalian vertebrates and is involved in diverse functions like osmoregulation, reproduction, metabolism and behavior. In this study, we report that estradiol-17β (E(2)) regulates brain and plasma VT secretion through the involvement of the catecholaminergic (CA) system. To demonstrate this, E(2) level was altered through ovariectomy (OVX, 3 weeks) and replacement study with low and high E(2) doses (0.1 and 0.5 μg/g body weight). CA activity was inhibited by treatment with α-methylparatyrosine (α-MPT; 250 μg/g body weight), a competitive inhibitor of tyrosine hydroxylase. VT was assayed by an enzyme immunoassay method. In the sham group, the low E(2) dose produced 82% and 104% increase, respectively, in brain and plasma VT levels. The high E(2) dose decreased the VT levels significantly. The low E(2) dose decreased brain E(2) but elevated plasma E(2). In the high E(2) group, the E(2) level increased further in both brain and plasma. OVX resulted in a significant inhibition (69% and 25%, respectively) of both brain and plasma VT, which was correlated with low E(2) levels. The low E(2) dose not only reversed the inhibition, but increased the VT level in both brain and plasma in comparison to the sham groups. The high E(2) replacement inhibited VT levels further low in both brain and plasma. The α-MPT treatment inhibited VT levels significantly in both sham and OVX groups. The drug treatment abolished partially the restorative effect of the low E(2) dose in the ovariectomized fish. In the high E(2) dose group, α-MPT decreased brain and plasma VT levels further low compared to the sham + 0. 5 μg E(2) group or OVX + 0.5 μg E(2) group except the brain VT level, which increased in the OVX+0.5 μg E(2) group. It is inferred that E(2) may exert biphasic effects on VT through the mediation of the CA system.

Estradiol-17β modulates dose-dependently hypothalamic tyrosine hydroxylase activity inhibited by α-methylparatyrosine in the catfish Heteropneustes fossilis

The brain is a target for organizational and activational effects of oestrogens synthesized de novo or transported from the peripheral organs. A neuroprotective role of oestrogens has been documented in a variety of vertebrates. In the present study in the catfish Heteropneustes fossilis, we have demonstrated that estradiol-17β (E(2)), the major circulating oestrogen at low dosages (0.05 and 0.1 μg/g body weight of fish for 3 days) stimulated hypothalamic tyrosine hydroxylase (TH) activity, and countered the negative effects of ovariectomy (3-week) or α-methylparatyrosine (α-MPT: 250 μg/g body weight, a competitive inhibitor of TH). In contrast, high dosages of E(2) (1 and 2 μg/g body weight of fish for 3 days) were inhibitory and further amplified the inhibitory effects of ovariectomy and α-MPT. The inhibiting role of E(2) was higher in gonad-active (prespawning) phase than gonad-inactive (resting phase) phase. The dual roles of E(2) may ensure a tight regulation of catecholaminergic activity, activating and inhibiting the system against wide fluctuations that are characteristic of seasonally breeding animals.

Vasotocin induces final oocyte maturation and ovulation through the production of a maturation-inducing steroid in the catfish Heteropneustes fossilis

The study reports for the first time vasotocin (VT) induction of final oocyte maturation and ovulation through the production of the maturation-inducing steroid 17, 20β-dihydroxy-4-pregnen-3-one (MIS, 17, 20β-DP). Post-vitellogenic follicles of the catfish Heteropneustes fossilis were incubated with different concentrations of VT (1, 10, 100 and 1000 nM) for different time periods. Germinal vesicle breakdown [GVBD, as a marker of final oocyte maturation (FOM)] and ovulation were scored. In another series of experiments, the follicles were incubated with VT alone or in combination with VT receptor (V(1) and V(2)) antagonists, and GVBD and ovulation were increased with progesterone, 17-hydroxy-4-pregnene-3, 20-dione (17-P) and 17, 20β-DP levels. VT stimulated both GVBD and ovulation in a concentration and time-dependent manner, and the responses were inhibited to varying degrees in groups incubated with the VT receptor antagonists. The V(1) antagonist inhibited the responses by 2- to 3-fold and more than the V(2) antagonist, and the combination was more potent than the separate incubation. Progestins increased time-dependently in the VT groups and the fold increase was greater for the MIS. The VT-induced steroid stimulation was significantly inhibited to near the control levels in co-incubations with both V(1) and V(2) receptor antagonists, in the order 17, 20β-DP > 17-P > P(4). The inhibition by the V(1) receptor antagonist was greater than that with the V(2) blocker, and followed the same order of inhibition described above. The results suggest that VT induces FOM and ovulation mainly through the V(1) receptors.

Development, characterization, conservation and storage of fish cell lines: a review

Cell lines provide an important biological tool for carrying out investigations into physiology, virology, toxicology, carcinogenesis and transgenics. Teleost fish cell lines have been developed from a broad range of tissues such as ovary, fin, swim bladder, heart, spleen, liver, eye muscle, vertebrae, brain, skin. One hundred and twenty-four new fish cell lines from different fish species ranging from grouper to eel have been reported since the last review by Fryer and Lannan (J Tissue Culture Methods 16: 87-94, 1994). Among the cell lines listed, more than 60% were established from species from Asia, which contributes more than 80% of total fish production. This includes 59 cell lines from 19 freshwater, 54 from 22 marine and 11 from 3 brackish water fishes. Presently, about 283 cell lines have been established from finfish around the world. In addition to the listing and a scientific update on new cell lines, the importance of authentication, applications, cross-contamination and implications of overpassaged cell lines has also been discussed in this comprehensive review. The authors feel that the review will serve an updated database for beginners and established researchers in the field of fish cell line research and development.

Molecular and functional characterization of catfish (Heteropneustes fossilis) aquaporin-1b: changes in expression during ovarian development and hormone-induced follicular maturation

The oocytes of the freshwater catfish Heteropneustes fossilis hydrate during hormone-induced meiotic maturation. To investigate if this process may be mediated by aquaporins (AQPs), as it occurs in marine fish producing highly hydrated eggs, the cloning of ovarian AQPs in catfish was carried out. Using degenerate primers for conserved domains of the major intrinsic protein (MIP) family, and 5' and 3'end amplification procedures, a full-length cDNA encoding for an AQP1-like protein was isolated. The predicted protein showed the typical six transmembrane domains and two Asn-Pro-Ala (NPA) motifs conserved among the members of the AQP superfamily. Phylogenetic analysis indicated that the catfish AQP clustered with the teleost-specific aquaporin-1b subfamily, and accordingly it was termed HfAqp1b. Heterologous expression in Xenopus laevis oocytes indicated that HfAqp1b encoded for a functional AQP, water permeability being enhanced by cAMP. Site-directed mutagenesis revealed that cAMP induced the translocation of HfAqp1b into the oocyte plasma membrane most likely through the phosphorylation of HfAqp1b Ser(227). In adult catfish, hfaqp1b transcripts were detected exclusively in ovary and brain and showed significant seasonal variations; in the ovary, hfaqp1b was maximally expressed during the pre-spawning period, whereas in the brain the highest expression was detected during spawning. In vitro stimulation of isolated catfish ovarian follicles with vasotocin (VT) or human chorionic gonadotropin (hCG), which induce oocyte maturation and hydration, elevated the hfaqp1b transcript levels after 6 or 16 h of incubation, respectively. These results suggest that HfAqp1b may play a role during VT- and hCG-induced oocyte hydration in catfish, and that VT may regulate HfAqp1b at the transcriptional and post-translational level in a manner similar to the vasopressin-dependent mammalian AQP2.

Seasonal variation in tissue estrogen-2/4-hydroxylases (EH) and in vitro effects of steroids on ovarian EH activity in the catfish Heteropneustes fossilis

A radiometric assay was used to measure microsomal EH activity from tritiated H(2)O formed during the conversion of [2,4 (3)H] estradiol-17β into catecholestrogens in the microsomal fractions of liver, brain and ovary of the catfish Heteropneustes fossilis. The validation data show that enzyme activity increased with incubation time, and substrate and cofactor (NADPH) concentrations, elicited temperature optima of 30-37°C and pH optima of 6.8-7.8. EH activity was strongly NADPH-dependent and in its absence only 13.48% activity was recorded. Liver recorded the highest enzyme activity, followed by brain and ovary. EH activity showed a significant seasonal variation with the peak activity in spawning phase and the lowest activity in resting phase. In the ovary, the follicular layer (theca and granulosa) elicited the highest activity over that of the denuded oocytes. Modulatory effects of steroids on ovarian enzyme activity were further demonstrated. The incubation of postvitellogenic follicles with 1, 10 or 100 nM concentrations of various steroids for 24 h produced varied effects on EH activity. Progesterone and 2-hydroxyestradiol-17β elicited strong suppressive effects on enzyme activity. Estrogens (E(1), E(2) and E(3)) suppressed the activity in a concentration-dependent manner. Among the progestins tested, 17,20α-dihydroxy-4-pregnen-3-one, the isomer of 17,20β-dihydroxy-4-pregnen-3-one (a teleost maturation-inducing steroid) showed the lowest depressing effect. Among androgens, the testosterone metabolite 11-ketotestosterone (functional teleost androgen) showed a high suppressing effect. Corticosteroids elicited low activity with cortisol suppressed the activity at higher concentrations. The study will form a basis to understand the physiological role of catecholestrogens in ovarian functions.

Temporal and periovulatory changes in ovarian catecholamines in the catfish Heteropneustes fossilis

A high performance liquid chromatography-fluorescence detection method was employed to demonstrate temporal and periovulatory changes in ovarian catecholamines in the catfish Heteropneustes fossilis. Tyrosine, L-DOPA, dopamine (DA), norepinephrine (NE) and epinephrine showed significant seasonal and diurnal changes during the reproductive cycle. A high concentration of tyrosine was detected, the values increased in day and decreased in night during recrudescence from preparatory to spawning phases. Similarly, L-DOPA increased during the breeding phase giving the day value concentration peak in the spawning phase and the night value peak in the postspawning phase. DA activity or turnover index (calculated as a ratio of DA to 3,4-dihydroxyphenylacetic acid, DOPAC) showed a bimodal pattern with the major activity peak in the postspawning phase and the minor one in the prespawning phase, the spawning phase registered the lowest activity. NE activity or turnover index (ratio of NE to normetanephrine, NME) increased during the recrudescent phase to give the peak in the spawning phase and decreased in the quiescent phase. Epinephrine elicited an inverse relationship in the day-night pattern, the day values increased to the peak in the spawning phase. All the study correlates showed significant periovulatory changes after hCG treatment. DA activity dropped to the nadir at 8h but recovered at 16 and 24h. NE activity showed only a marginal decrease up to 16h but decreased drastically at 24h. Epinephrine levels remained unchanged but only to increase at 24h. The seasonal patterns and periovulatory changes strongly point to important functions for catecholamines in the ovary. The occurrence of tyramine (an invertebrate neurotransmitter) suggests the presence of alternate catecholamine pathway in fish ovary.

Inhibition of hCG-induced spawning by alpha-methylparatyrosine, a tyrosine hydroxylase inhibitor, in the catfish Heteropneustes fossilis (Bloch)

In this study, the effect of pharmacological inhibition of catecholaminergic activity on hCG-induced spawning was evaluated and correlated with tyrosine hydroxylase (TH) activity, the rate-limiting enzyme in catecholamine biosynthesis. Gravid female H. fossilis collected in both prespawning and spawning phases were given alpha-methylparatyrosine (alpha-MPT: 250 microg/g body weight, ip, an irreversible inhibitor of TH) and human chorionic gonadotropin (hCG: 100 IU/fish, ip) alone or in combination. The fish were sampled at different intervals for measuring hypothalamic and ovarian TH activity and checking spawning response. The administration of hCG resulted in ovulation and spawning in both phases with a higher response in the spawning phase. The administration of alpha-MPT did not induce any response, like the control fish. In the hCG + alpha-MPT groups, the spawning response of hCG was significantly inhibited and delayed by the inhibitor. The spawning response of hCG was accompanied by a significant increase in both hypothalamic and ovarian TH activity at 6 and 12 h of the injection. However, at 24hr the activity decreased except in the spawning phase. The alpha-MPT treatment inhibited TH activity significantly in a duration-dependent manner. In the hCG + alpha-MPT groups, enzyme activity was inhibited at all duration. The results indicate the involvement of catecholamines during the hCG-induced spawning and the specific functional nature of the involvement needs further investigation.

An involvement of vasotocin in oocyte hydration in the catfish Heteropneustes fossilis: A comparison with effects of isotocin and hCG

In the present investigation, in vitro effects of vasotocin (VT) on oocyte (follicular) hydration during germinal vesicle breakdown (GVBD) and ovulation were demonstrated in hCG-primed and non-primed catfish. The data were compared with that of groups incubated with isotocin, and hCG alone or in combination with VT. The priming with hCG resulted in significant increases on percentage GVBD and ovulation, and stimulated follicular hydration, as judged by the increase in diameter, volume, water content, osmolality and Ca(2+) concentration. However, Na(+), K(+) ATPase activity, and concentrations of Na(+), K(+) and Mg(2+) did not alter significantly. The incubations with hCG or VT stimulated all the above parameters. In the non-primed fish, the response of hCG was significantly higher on follicular diameter, volume and osmolality, and that of VT on ovulation. In the primed fish, the VT response was significantly higher on GVBD, ovulation, Na(+), K(+) ATPase activity and divalent cation concentrations. The co-incubation with both hCG and VT produced maximal increases in all the parameters with significantly higher effects in the primed fish. The effects of IT on various parameters were relatively low compared to hCG or VT effects. The results indicate that VT may play an important role in oocyte (follicular) hydration, which is consistent with its role in osmoregulation of fish.

Annual and periovulatory changes in tyrosine hydroxylase activity in the ovary of the catfish Heteropneustes fossilis

In the present study, tyrosine hydroxylase (TH, the rate-limiting enzyme in catecholamine synthesis) activity was demonstrated in the ovary of the catfish to elucidate the possible physiological role of catecholamines in the gonad. The ovary is innervated by seven pairs of nerves, originating from the paired sympathetic chain lying dorsal to the posterior kidney. Ovarian TH activity showed a significant annual variation (P<0.001, one-way ANOVA), correlating with gonado-somatic index. Activity was low from December to February (resting phase), increased from March to July (recrudescent phase) and then decreased in post-spawning phase (August-November). The annual pattern was similar to that of the brain. An intraperitoneal injection of 100 IU hCG/fish induced significant periovulatory changes in TH activity with the peak rise at 16 h, and the activity decreased after egg-stripping (P<0.001, one-way ANOVA). Brain TH activity showed similar periovulatory changes. The results suggest that catecholamine synthesis is increased during both ovarian recrudescence and spawning of the annual reproductive cycle, implying a functional role in ovarian growth, maturation and ovulation.

Relative in vitro seasonal effects of vasotocin and isotocin on ovarian steroid hormone levels in the catfish Heteropneustes fossilis

In the present investigation, catfish (Heteropneustes fossilis) ovarian tissues were incubated in vitro with vasotocin (VT) or isotocin (IT) to demonstrate their effects on estradiol-17beta (E(2)), progesterone (P(4)), 17alpha-hydroxy-4-pregnene-3, 20-dione (17-P) and 17alpha, 20beta-hydroxy-4-pregnen-3-one (17, 20beta-DP). Parallel incubations with human chorionic gonadotropin (hCG) alone or in combination with VT were used for a comparison. In pre-vitellogenic phase (preparatory phase, GSI-0.48+/-0.03%), both VT and hCG stimulated E(2) significantly, VT in a biphasic manner and hCG in a dose-dependent manner. In pre-spawning (post-vitellogenic, GSI-9.05+/-0.11%) and spawning (post-vitellogenic, GSI-8.01+/-0.12%) phases, both hormones decreased E(2) levels in a dose- and duration-dependent manner; the VT effect being biphasic in the spawning phase. The co-incubation with VT+hCG stimulated E(2) in the preparatory phase but inhibited it in the pre-spawning and spawning phases. The incubations with VT or hCG increased P(4) levels in a dose- and duration-dependent manner, the magnitude of the effect was higher in the pre-spawning and spawning phases. The co-incubation with VT+hCG stimulated P(4) without any additive effect. The P(4) derivatives (17-P and 17, 20beta-DP) showed similar changes except 17-P in the spawning phase which decreased at 16h of the incubation. The incubations with IT produced similar but low responses. In conclusion, like hCG, VT has differential effects on ovarian steroidogenesis and may be involved directly or indirectly in ovarian functions, as a paracrine/autocrine factor or a neurohormone.

Effects of hCG and ovarian steroid hormones on vasotocin levels in the female catfish Heteropneustes fossilis

Effects of hCG, ovariectomy and estradiol replacement on brain, plasma and/or ovarian vasotocin in vivo, and estradiol, progesterone, 17alpha, 20beta-hydroxy-4-pregnen-3-one and hCG on ovarian vasotocin in vitro were investigated in the catfish. A 100IU/fish of hCG induced ovulation and elicited both periovulatory and post-ovulatory changes in vasotocin concentrations with a significant increase up to 8h in the brain and up to 16h in both plasma and ovary. After stripping the fish at 16h, the peptide concentration decreased significantly with time, up to 4 days. Ovariectomy in early pre-spawning phase resulted in a duration-dependent significant reduction of both brain and plasma vasotocin. Estradiol replacement in 3-week ovariectomized fish produced dosage-dependent biphasic effects: the lower dosage (0.1microg/g) restored the vasotocin level while the higher dosage (0.5microg/g) decreased it significantly below the control level. In vitro incubation of ovarian tissues with estradiol produced season-dependent effects on vasotocin. The incubation of pre-vitellogenic ovarian pieces with estradiol (1, 10, and 100ng/ml) elevated vasotocin level in a dose- and duration-dependent manner while that of post-vitellogenic follicles resulted in a significant decrease. The incubation of intact post-vitellogenic follicles or follicular envelope with progesterone and 17alpha, 20beta-hydroxy-4-pregnen-3-one (1microg/ml) or hCG (20IU/ml) for 8 and 16h significantly increased vasotocin in a duration-dependent manner. The results show that both gonadotropin and ovarian steroids modulate vasotocin titer, which may influence follicular growth, ovulation and spawning in the catfish.

Immunocytochemical localization, HPLC characterization, and seasonal dynamics of vasotocin in the brain, blood plasma and gonads of the catfish Heteropneustes fossilis

Immunocytochemical distribution and dynamics of vasotocin (VT) were studied in the air-breathing catfish Heteropneustes fossilis in relation to the reproductive cycle. Vasotocin was localized in the brain and ovary by streptavidin-biotin immunocytochemistry. The immunoreactivity was found throughout the hypothalamo-hypophysial neurosecretory system consisting of the magnocellular and parvocellular neurons of the nucleus preopticus, neurosecretory axonal tract and neurohypophysis (NH). The VT neurons showed seasonal changes; they were numerically less in resting phase but increased during the recrudescent phase. The neurons were hypertrophied and degranulated in pre-spawning phase and heavily degranulated and vacuolated in spawning phase. In the NH, the density of VT fibers increased up to the pre-spawning phase and decreased thereafter. In the ovary, VT immunoreactivity was noticed in the follicular layer and varied with the growth of the follicles. Vasotocin was characterized and quantified by a high performance liquid chromatography with UV detection method in the brain, plasma and ovary. Brain and plasma VT concentrations were also assayed with an EIA method, which was more sensitive than the HPLC method with values about 2-fold higher. Vasotocin levels showed significant seasonal and sexual differences with higher concentrations in females in the recrudescent (preparative, pre-spawning and spawning) phase. Brain VT recorded the highest concentration in the preparative phase (both sexes) while plasma (both sexes) and ovarian VT in the spawning phase. The ovarian concentration of VT was 15- and 25-fold higher in the pre-spawning and spawning phases (when expressed per mg protein), respectively, than plasma but lower than brain levels. In testis, VT concentration was relatively low and apparently did not show any significant seasonal variation. The seasonal activity patterns and gonadal distribution of VT indicate a reproductive function of the peptide.

Ovarian P450 aromatase activity in the catfish Heteropneustes fossilis: seasonal changes and effects of catecholestrogens

Ovarian microsomal aromatase (P450arom) activity was studied in relation to season and incubation of follicles with catecholestrogens [(2-hydroxyestradiol-17beta (2-OHE2) and 2-methoxyestradiol-17 beta (2-methoxyE2)] using a product (estradiol-17 beta) assay. Peak P450arom activity was noticed in late preparatory phase (April) and it decreased significantly in pre-spawning, spawning and post-spawning phases to give the lowest value in resting phase. Apparent Km and Vmax of the enzyme varied significantly and the values were high in the preparatory (vitellogenic) phase (Km 74.62+/-1.73 nM, Vmax 0.81+/-0.01 pmol/mg protein/min) and low in the spawning (post-vitellogenic) phase (Km 62.01+/-1.68 nM, Vmax 0.69+/-0.002 pmol/mg protein/min). The incubation of the ovarian microsomes with 2-OHE2 elicited significant biphasic effects on enzyme activity. In the vitellogenic phase, concentrations of the steroid up to 1 microM inhibited enzyme activity significantly with the highest inhibition at 10nM. However, in the post-vitellogenic ovary, the highest inhibition was registered at 100 nM. The higher concentrations (10 microM or 100 microM) did not elicit any significant change compared to the control groups. A comparison of the aromatase inhibition index (AI50, indicates 50% inhibition of aromatase activity) of fadrozole, a known aromatase inhibitor and 2-OHE2 shows that the AI50 was 4.4 nM for fadrozole and 0.864 nM (vitellogenic phase) and 1.31 nM (post-vitellogenic phase) for 2-OHE2 indicating higher potency of the latter. The incubation of the ovarian microsomes with 2-methoxyE2 increased enzyme activity only at the higher concentrations (1-100 microM). The results show seasonality in the potential of the ovary to synthesize E2 and the potent enzyme inhibiting activity of 2-OHE2, which is reported for the first time.

Estrogen-2/4-hydroxylase activity is stimulated during germinal vesicle breakdown induced by hCG, IGF-1, GH and insulin in the catfish Heteropneustes fossilis

Estrogen-2/4-hydroxylase (EH) activity was measured radiometrically in ovaries of catfish injected with hCG intraperitoneally and in postvitellogenic follicles incubated with different concentrations of hCG, catfish (Clarias batrachus) growth hormone (GH), bovine insulin or recombinant human insulin-like growth factor-I (rhIGF-I). The change in enzyme activity was correlated with germinal vesicle breakdown (GVBD), an index of oocyte maturation. A single intraperitoneal injection of hCG (100 IU/fish) stimulated EH activity both at 8 and 16 h prior to stripping of eggs. The activity decreased significantly at 24 h, following ovulation. The follicles incubated with hCG, rhIGF-I, insulin or GH elicited biphasic effects on EH activity. rhIGF-I, insulin and GH increased enzyme activity at the lower or median concentrations. hCG and rhIGF-I stimulated EH activity higher than GH or insulin. All the hormones elicited a dose-dependent increase in GVBD, the effect was greater with rhIGF-I (100 nM) and hCG (5.0 IU/ml). The significance of changes in EH activity (E2 hydroxylation) and GVBD were discussed.

2-Hydroxyestradiol-17beta-induced oocyte maturation: involvement of cAMP-protein kinase A and okadaic acid-sensitive protein phosphatases, and their interplay in oocyte maturation in the catfish Heteropneustes fossilis

In Heteropneustes fossilis, in vitro incubation of postvitellogenic follicles with 2-hydroxyestradiol-17beta (2-OHE2, 5 micromol l(-1)) decreased significantly the total cAMP level, concomitant with germinal vesicle breakdown (GVBD). The incubation of the follicles with cAMP or cAMP-elevating drugs [phosphodiesterase (PDE) inhibitors], such as IBMX (3-isobutyl-1-methyl-xanthine), theophylline and caffeine, inhibited the 2-OHE2-induced GVBD in a concentration-dependent manner. The magnitude of the response varied: both cAMP and IBMX were effective at all concentrations (0.1-2.0 mmol l(-1)), followed by theophylline (0.5-2.0 mmol l(-1)) and caffeine (1-2.0 mmol l(-1)). The protein kinase A (PKA) inhibitor H89 stimulated oocyte maturation in a concentration-dependent manner. However, when co-incubated with 2-OHE2 for 24 h it produced a biphasic effect: low concentrations (0.1 and 1.0 micromol l(-1)) did not alter the 2-OHE2-induced GVBD, but high concentrations (5 and 10 micromol l(-1)) inhibited it. The incubation of the follicles with H89 lowered the inhibitory effect of IBMX on the 2-OHE2-induced GVBD. The incubation of the follicles with okadaic acid (OA), a protein phosphatase 1 and 2A inhibitor did not affect GVBD but when co-incubated with 2-OHE2, it enhanced the GVBD response. OA reversed the inhibitory effect of IBMX. The results suggest that OA may overcome the inhibition of 2-OHE2-induced GVBD by IBMX at a step distal to the cAMP-PKA pathway.

Involvement of mitogen-activated protein kinase in 2-hydroxyestradiol-17beta-induced oocyte maturation in the catfish Heteropneustes fossilis and a note on possible interaction with protein phosphatases

Mitogen-activated protein kinase (MAPK) was demonstrated in the postvitellogenic follicles (theca-granulosa and oocyte) of catfish by Western blotting using a polyclonal anti-rabbit serum, which recognized both ERK1 and ERK2. Two distinct protein bands resolved in the 46-48 kDa range of 12% SDS-PAGE were immunoblotted. Incubation of the follicles with 5 microM 2-OHE2 elicited GVBD significantly in a duration-dependent manner with a concomitant increase in the expression of MAPK (ERK1 and ERK2). Densitometric analysis of the immunoblots showed significant variations in the intensity of staining. The ERK1 expression increased significantly from 6 h onwards but the changes were less pronounced. On the other hand, ERK2 registered a sharp significant increase after 3h, which paralleled the GVBD response. The MEK inhibitor PD098059 alone did not induce GVBD. Co-incubation of the follicles with 2-OHE2 and PD098059 significantly inhibited the steroid-induced GVBD at all concentrations. Immunoblot analysis showed that PD098059 inhibited MAPK activity significantly compared to the 2-OHE2 group. The addition of okadaic acid (OA) in the incubation medium containing both 2-OHE2 and PD098059 reversed the inhibitory effect of the latter and GVBD was elevated significantly over that of the 2-OHE2 group but significantly lower than that of the 2-OHE2 + OA group. The results suggest an involvement of MAPK in meiotic maturation but the site(s) of action: oocyte, follicular envelope or both needs further investigation.

Relative effects of estradiol-17beta (E2), catecholestrogens and clomiphene citrate on in vitro oocyte maturation in the catfish Heteropneustes fossilis (Bloch) and E2 inhibition of 2-hydroxyestradiol-induced maturation

In vitro effects of estradiol-17beta (E(2)), the catecholestrogens 2-hydroxyE(2) (2-OHE(2)) and 2-methoxyE(2), and the nonsteroidal antiestrogen clomiphene citrate (clomid) on oocyte maturation were investigated in the catfish Heteropneustes fossilis. Incubation of postvitellogenic follicles with 2-OHE(2) induced germinal vesicle breakdown (GVBD; 86% at 5 microM for 30 h) and progression of meiosis up to metaphase II, as evident from the presence of Hoechst stained metaphase chromosomes and anti-alpha-tubulin-positive bipolar spindles. The response was both concentration (1, 2.5, 5, 10, and 20 microM)- and duration (0, 3, 6, 12, 24, and 30 h)-dependent. The diameter of the follicles increased and about 20% follicles elicited ovulation. Incubation of the follicles with clomid (20 microM) induced only about 29-35% GVBD at 30 h. This might be due to the dual properties of clomid with estrogenic (cis-isomer) and antiestrogenic (trans-isomer) actions or due to estrogen receptor binding dynamics. Incubations of the follicles with E(2) or 2-methoxyE(2) did not induce oocyte maturation. The higher concentrations of 2-methoxyE(2) caused degenerative changes in the follicles. In competition studies, E(2) inhibited the GVBD response of 2-OHE(2) (5 microM) significantly in a concentration (1, 5, 10, and 20 microM) or duration (2, 4, and 6 h)-dependent manner after pre-incubation with 20 microM E(2) (P<0.001, one-way ANOVA, P<0.05, Newman-Keuls' test). The results show that 2-OHE(2) induces maturational activity while the parent estrogen is a strong inhibitor, alone or in combination with 2-OHE(2).

Effects of gonadotrophin in vivo and 2-hydroxyoestradiol-17beta in vitro on follicular steroid hormone profile associated with oocyte maturation in the catfish Heteropneustes fossilis

An HPLC method was used to tentatively identify progesterone (P4) and its metabolites (17-hydroxyprogesterone (17-P4) and 17,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P)), corticosteroids (cortisol and corticosterone) and testosterone in ovary/follicular preparations of the catfish Heteropneustes fossilis associated with in vivo or in vitro oocyte maturation/ovulation. A single i.p. injection of human chorionic gonadotrophin (100 IU/fish, sampled at 0, 8 and 16 h) induced oocyte maturation and ovulation, which coincided with significant and progressive increases in 17,20beta-P, and P4 and 17-P4, the precursors of the former. Both cortisol and corticosterone also increased significantly. Conversely, testosterone decreased significantly and progressively over time. Under in vitro conditions, incubation of post-vitellogenic (intact) follicles or follicular envelope (layer) with 2-hydroxyoestradiol (2-OHE2, 5 microM for 0, 6 and 24 h) elicited a sharp significant increase in 17,20beta-P, the increase being higher in the follicular envelope incubate. P4 and 17-P4 also registered significant increases over the time with the peak values at 24 h. Cortisol and corticosterone increased significantly in the intact follicle, but not in the follicular envelope incubate. Testosterone decreased significantly in the intact follicle, but increased significantly (24 h) in the follicular envelope incubate. Coincident with these changes, the percentage of germinal vesicle breakdown (GVBD) increased over the time in the intact follicle incubate (48.9% at 6 h and 79.8% at 24 h). Denuded oocytes on incubation with 2-OHE2 (5 microM) did not produce any significant change in the percentage of GVBD or in the steroid profile. While corticosterone and 17,20beta-P were undetected, P4, 17-P4, cortisol and testosterone were detected in low amounts. The results show that the 2-OHE2-induced GVBD response seems to be mediated through the production of 17,20beta-P and corticosteroids. It is suggested that hydroxyoestrogens seem to be a component in the gonadotrophin cascade of regulation of oocyte maturation/ovulation in the catfish.

HPLC-electrochemical detection of ovarian estradiol-17beta and catecholestrogens in the catfish Heteropneustes fossilis: seasonal and periovulatory changes

A high performance liquid chromatography-electrochemical (HPLC-EC) detection method was used to characterize estradiol-17beta (E2) and its metabolites (2-hydroxyE2, 4-hydroxyE2, and 2-methoxyE2) and investigate their seasonal and periovulatory changes in the ovary of the catfish Heteropneustes fossilis. The retention times in minutes of standards determined by individual and mixture applications are: 2-OHE2-6.6, 4-OHE2-7.0, 4-OHE1-11.2, E2-12.0, and 2-methoxyE2-15.2. Since the retention times of 2-OHE2 and 4-OHE2 merged at higher concentrations, the elution peaks of the sample were taken as due to both (2/4-OHE2) for analysis. The steroids were not detectable in the resting and postspawning phases and 2-methoxyE2 was not detectable in the recrudescent (preparatory, prespawning, and spawning) phases as well. E2 and 2/4-OHE2 have maintained an inverse relationship in the recrudescent phase. The E2 concentration was the highest in the preparatory phase (April) with active vitellogenic activity and declined significantly across prespawning and spawning phases (P<0.001, one way ANOVA; P<0.05, Newman-Keuls' test). On the other hand, the concentration of 2/4-OHE2, which was the lowest in the preparatory phase, increased significantly to the peak level in the spawning phase. A single intraperitoneal injection of hCG (100 IU/fish) stimulated significantly the formation of 2/4-OHE2 at 8 h with a simultaneous reduction in E2. 2-MethoxyE2 was detected only after 16 h of the hCG injection. The functional significance of catecholestrogens in the seasonal reproductive cycle and during the hCG-induced ovulation of the catfish was discussed.