Izhar A. Khan

Texas A&M University-Commerce · Department of Biological and Environmental Science

The arginine vasotocin (AVT)‐V1a receptor mediates critical reproductive behaviors of the nonapeptide vasotocin in the teleost brain. In this study, we report the molecular characterization of the AVT‐V1a2 receptor and its messenger RNA (mRNA) and protein expressions in the Atlantic croaker brain after exposure to the planar polychlorinated bipheny...

Oocyte maturation can be a target of endocrine disruption by environmental chemicals capable of acting as hormone mimics, receptor blockers, and/or enzyme inhibitors. Six environmental chemicals (genistein, endosulfan, malathion, iprodione, carbaryl, and glyphosate) were selected to determine their ability to interfere with oocyte maturation in zeb...

Aims: Cerebral atrophy resulting from neurodegeneration is highly prevalent in individuals with diabetes; however, the underlying mechanisms for diabetic neurodegeneration are not fully understood. Here we hypothesized that hyperglycemia induces molecular signaling that favors induction of proliferation in post mitotic, fully differentiated hippoc...

The arginine vasotocin/vasopressin (AVT/AVP) and gonadotropin releasing hormone (GnRH) systems are known to control sexual behaviors and reproduction, respectively, in different vertebrate groups. However, a direct functional connection between these two neuroendocrine systems has not been demonstrated for any vertebrate species. Therefore, the obj...

Nonapeptides and their receptors have important functions in mediating social behavior across vertebrates. Where these nonapeptides are synthesized in the brain has been studied extensively in most vertebrate lineages, yet we know relatively little about the neural distribution of nonapeptide receptors outside of mammals. As nonapeptides play influ...

Tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-HT) synthesis, performs an essential role in the maintenance of serotonergic functions in the central nervous system (CNS), including regulation of the neuroendocrine system controlling reproduction. The results of recent studies in a teleost model of neuroendocrine disruption,...

Rock hind male behavior. Typical rock hind Epinephelus adscensionis male patrolling behavior, “head shake” and tuxedo pattern display. (MP4)

Hermaphroditism, associated with territoriality and dominance behavior, is common in the marine environment. While male sex-specific coloration patterns have been documented in groupers, particularly during the spawning season, few data regarding social structure and the context for these color displays are available. In the present study, we defin...

The current study sought to clarify the role of cholecystokinin (CCK) in the digestion of larval red drum (Sciaenops ocellatus) in order to better characterize the processes limiting the utilization of microparticulate diets at first feeding. The red drum CCK cDNA, isolated from adult anterior intestine and pyloric caeca, contains a 414 base pair (...

Recent studies have demonstrated that the G protein-coupled receptor 54 (GPR54) and its natural ligand kisspeptin (KiSS) play important roles in the control of puberty and reproductive maturation in mammals. In addition, GPR54 mRNA has been localized on the neurons expressing three GnRH mRNAs in tilapia (Parhar et al., 2004). However, localization...

This study was designed to characterize the amount of androgen implant and the time period necessary to cause sexual inversion in sevenband grouper Epinephelus septemfasciatus and to determine whether administration of follicle-stimulating hormone (FSH) with or without androgens had any effect on the sexual state of this species. Seventy-two fish w...

The cDNAs of FSH and LH beta subunits were isolated from Atlantic croaker pituitary. The isolated genes in croaker showed 35-75% and 60-75% identities with FSH and LH beta of other teleosts, and 41% and 45% with human FSH and LH beta, respectively. The homology models of croaker FSH and LH beta were constructed using the currently known X-ray cryst...

The long-term effects on marine fish populations of the recent increase worldwide in the incidence of coastal hypoxia are unknown. Here we show that chronic environmental exposure of Atlantic croaker (Micropogonias undulatus) to hypoxia in a Florida estuary caused marked suppression of ovarian and testicular growth which was accompanied by endocrin...

Reproduction in fishes as in other vertebrates is dependent on the coordinated actions of various hormones associated with the hypothalamic- pituitary-gonadal (HPG) axis. Although gonadotropin-releasing hormone (GnRH) is considered the master molecule of reproduction that regulates the HPG axis in all vertebrate groups, GnRH itself is regulated by...

The G protein-coupled receptor 54 (GPR54) and its natural ligand Kisspeptin have been shown to exert a strong stimulatory influence on gonadotropin-releasing hormone (GnRH) release in several mammalian species. In addition, GPR54 mRNA has been localized on the neurons expressing three forms of GnRH in tilapia. However, a natural ligand for GPR54 ha...

The cDNAs of the G protein-coupled receptor 54 (GPR54) and three prepro-gonadotropin-releasing hormones, GnRH-I (seabream GnRH), GnRH-II (chicken GnRH-II), and GnRH-III (salmon GnRH) were isolated and cloned from the brain of the teleost fish cobia, Rachycentron canadum. The cobia GPR54 cDNA was 95 and 51-56% identical to those of tilapia and mamma...

Atlantic croaker (Micropogonias undulatus) were exposed to the polychlorinated biphenyl (PCB) mixture (Aroclor 1254) or one of three individual congeners (planar PCB 77 or ortho-substituted PCB 47 and PCB 153) in the diet for 30 days to investigate the effects of PCBs on circulating thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Aroclo...

Steroid profiles of cultured and captive red drum (Sciaenops ocellatus L.) were investigated to evaluate the potential use of circulating sex steroid levels as a tool for gender identification in this species. Cultured 18-month-old fish were maintained on a 120-day shortened photothermal cycle to induce precocious maturation. Additionally, wild-cau...

Exposure of Atlantic croaker to Aroclor 1254 has been shown to impair reproductive neuroendocrine function in this species. In addition, we have identified hypothalamic tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin synthesis, as a target of PCB neuroendocrine toxicity. A previous study in rats has implicated di-ortho-substitut...

Three gonadotropin releasing hormones, seabream GnRH (GnRH-I), chicken GnRH-II (GnRH-II) and salmon GnRH (GnRH-III) cDNAs were isolated from the brain of the red drum, Sciaenops ocellatus. The GnRH cDNA sequences of red drum showed more similarity to those of Atlantic croaker, sea bass and sea bream. The real-time quantitative RT-PCR study revealed...

Three prepro-gonadotropin-releasing hormones, seabream GnRH (sbGnRH), chicken GnRH-II (cGnRH-II), and salmon GnRH (sGnRH) were isolated by cDNA cloning from the brain of the Atlantic croaker, Micropogonias undulatus. The amino acid sequences of croaker GnRH precursors show greatest similarities to those of the gilthead and red sea breams and Europe...

Over the last decade there has been a heightened awareness and concern among the scientific community, policy makers, and general public over the reproductive hazards of endocrine-disrupting environmental chemicals, particularly xenobiotic estrogens (xenoestrogens), to fish, wildlife, and humans [1, 2]. Feminization of male birds, alligators, and f...

We have shown previously that exposure of Atlantic croaker to a PCB mixture (Aroclor 1254) results in impaired reproductive neuroendocrine function. In addition, we have identified hypothalamic tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-hydroxytryptamine) synthesis, as a target of PCB neuroendocrine toxicity. In order to...

Previous studies have shown that oral exposure of rats to polychlorinated biphenyls (PCBs) results in reduced 5-hydroxytryptamine (5-HT) concentrations in certain brain regions. In the present study, we investigated whether the PCB mixture Aroclor 1254 (0.33 mg/g body weight as a single oral dose) can inhibit the activity of tryptophan hydroxylase...

On Oct 16, 2003 this sequence version replaced gi:33356449.

On Oct 16, 2003 this sequence version replaced gi:33356451.

On Oct 16, 2003 this sequence version replaced gi:33356453.

The effects of treatment with the maturation-inducing steroid 17,20beta,21-trihydroxy-4-pregnen-3-one (20beta-S) on luteinizing hormone-releasing hormone analog (LHRHa)-induced LH secretion were examined during several phases of the gonadal cycle in Atlantic croaker, Micropogonias undulatus. 20beta-S (1 and 5 microg/g of body wt) was administered b...

Exposure of Atlantic croaker (Micropogonias undulatus) to the polychlorinated biphenyl mixture (Aroclor 1254, PCB; 1 mg/kg body wt/day for 30 days) during the early-recrudescence phase of the gonadal cycle results in the impairment of LH secretion and gonadal growth. In order to determine whether impairment was due to disruption of the stimulatory...

Mechanisms governing the effect of polychlorinated biphenyl (PCB) toxicity on hypothalamic serotonergic function and the neuroendocrine system controlling LH secretion were investigated in Atlantic croaker (Micropogonias unulatus) exposed to the PCB mixture Aroclor 1254 (1 microg x g body weight(-1) x day(-1)) in the diet for 30 days. PCB treatment...

We have previously shown that lead (lead chloride) and a polychlorinated biphenyl (PCB) mixture (Aroclor 1254) can alter hypothalamic serotonin (5-hydroxytryptamine, 5-HT) content, and pituitary gonadotropin II (GTH II) release in vitro, in the Atlantic croaker (Micropogonias undulatus). In the present study we investigated whether impairment of th...

Involvement of gonadal steroids in the control of gonadotropin II (GTH II) (homologous to LH) secretion was investigated in the Atlantic croaker (Micropogonias undulatus) using gonadectomy (Gx) and steroid replacement paradigms. Gonadectomy in males and females during the late gonadal recrudescence phase elicited significant increases in the gonado...

Possible involvement of γ-aminobutyric acid (GABA) in the control of gonadotropin II (GTH II) secretion was investigated during different phases of the seasonal gonadal cycle of the Atlantic croaker (Micropogonias undulatus). Intraperitoneal administration of GABA (100 μg/g) elicited a significant increase in plasma GTH II levels in croaker with re...

Xenobiotic estrogens have the potential to act at a variety of estrogen-responsive target tissues on the hypothalamic-pituitary-gonadal axis. However, to date most studies in fish have focused on stimulation of vitellogenin synthesis by the liver. In the present study the effects of the xenoestrogen o,p′-DDT and estradiol-17β on the neuroendocrine...

Male Atlantic croaker (Micropogonias undulatus) were exposed to Aroclor 1254 (a PCB mixture) in the diet (0.1 mg/100 g body wt./day) for 30 days during gonadal recrudescence to investigate the effects of the PCB mixture on reproductive neuroendocrine function. The concentrations of biogenic amines (epinephrine [E], norepinephrine [NE], dopamine [DA...

The role of melatonin in the control of gonadotropin II (GtH II) secretion was investigated in Atlantic croaker (Micropogonias undulatus) during different phases of the day-night cycle and seasonal reproductive cycle. Intraperitoneal injection of melatonin during the late-light phase of the day-night cycle elicited a significant elevation in plasma...

To investigate the effects of a polychlorinated biphenyl (PCB) mixture (Aroclor 1254) on reproductive neuroendocrine function, male Atlantic croaker (Micropogonias undulatus) were exposed to the PCB mixture in the diet (0.1 mg/100 g body wt/day) for 30 days during gonadal recrudescence. Concentrations of the monoamines (5-hydroxytryptamine (5-HT),...

The effects of serotonin (5-hydroxytryptamine; 5-HT) alone, and in combination with luteinizing hormone-releasing hormone analog (LHRHa), on plasma levels of maturational gonadotropin (GtH II) in the post-vitellogenic female and spermiating male Atlantic croaker were investigated during mid-light, late-light, mid-dark, and late-dark phases of the d...

The distribution of serotonin-immunoreactive (5-HT-ir) and gonadotropin-releasing hormone-immunoreactive (GnRH-ir) elements in the brain and pituitary gland of the Atlantic croaker (Micropogonias undulatus) was studied using immunoperoxidase and immunofluorescence techniques. 5-HT- and GnRH-ir perikarya were observed in the ventrolateral region of...

The effects of serotonin (5-HT) injection alone, and in combination with des Gly10 [D-Trp6]-luteinizing hormone-releasing hormone ethylamide (LHRHa), on plasma maturational gonadotropin (GtH) levels in the Atlantic croaker, Micropogonias undulatus, were examined. An injection of 5-HT (20 micrograms/g body wt) alone did not significantly elevate GtH...

In Channa punctatus, pinealectomy results in an accelerated growth (stimulatory effect) of the ovary in preparatory phase (March), but had no significant effect in prespawning (May-June) or postspawning (September) phases. Administration of melatonin (25 (μg/fish or 0.4 mg/kg BW at 2-day intervals for 30 days) inhibits the stimulatory effect of a l...

Hypothalamic monoamine oxidase (MAO) activity displayed biphasic responses to administration of estradiol-17 beta (E2) depending on season. Lower doses of 0.05 and 0.10 microgram/g BW of E2 had enhanced enzyme activity and higher doses (0.5 and 1.0 microgram/g BW) depressed it significantly in preparatory season. In prespawning phase, the lowest do...

In Channa punctatus maintained under ambient photothermal conditions (10.5L:13.5D; 13 ± 2°C) in the postspawning season, hypothalamic dopamine (DA) and norepinephrine (NE) content varied diurnally, but serotonin (5-HT) and monoamine oxidase (MAO) did not. Exposure of the fish to high temperature (25 ± 2°C), with or without altering the photopheriod...

In Channa punctatus, day-night variations in hypothalamic 5-HT (serotonin) and monoamine oxidase (MAO) activity were noticed in preparatory but not other phases (prespawning and postspawning) of the annual reproductive cycle. Hypothalamic MAO, 5-HT, and norepinephrine (NE) activity was found to be high in the prespawning phase and low in the postsp...

Hypothalamic dopamine (DA), noradrenaline (NA) and 5-hydroxytryptamine (5-HT) levels exhibited marked day-night variations under ambient photoperiod and temperature (12L12D; 171C) with peak values at mid-light phase. The 16L8D; 221C treatment reversed the diurnal rhythm of 5-HT, but not that of DA and NA. However, there was an overall increase in t...

In Channa punctatus, a significant daily variation in hypothalamic 5-HT level and monoamine oxidase (MAO) activity was noticed in preparatory phase (February), but not in prespawning (May) or postspawning (November) phases. Hypothalamic dopamine (DA) and noradrenaline (NA), on the other hand, showed marked daily variation in their levels during all...