Chapter 3 The GnRH System and the Neuroendocrine Regulation of Reproduction

The Expression Pattern of Insulin-Like Growth Factor Subtype 3 (igf3) in the Orange-Spotted Grouper Epinephelus coioides and Its Function on Ovary Maturation

A new insulin-like growth factor (Igf) subtype 3 (igf3) has recently been found in the bony fish orange-spotted grouper (Epinephelus coioides). However, the role of igf3 in the maturation of the ovary and sex differentiation in E. coioides is currently unknown. We examined the ovarian localization and receptor binding of the novel ortholog Igf3 using qRT-PCR, and Western blotting, combined with in situ hybridization and immunohistochemistry methods. Results demonstrated the presence of igf3 mRNA and protein in mature oocytes. Furthermore, Igf3 protein expression was not detected in testis, brain, kidney and liver homogenates. The calculated molecular weight of Igf3 was 22 kDa, which was consistent with the deduced amino acid sequence from the full-length open reading frame. The immunoreactivity showed that Igf3 was strongly present in the follicle staining fully-grown stage. The igf3 mRNA expression level was significantly positively correlated with ovarian follicular maturation. Meanwhile, Igf3 increased germinal-vesicle breakdown in a time- and dose-dependent manner. In vitro, treatment of primary ovarian cells with Igf3 up-regulated significantly the mRNA expression level of genes related to sex determination and reproduction such as forkhead boxl2 (foxl2), dosage-sensitive sex reversal adrenal hypoplasia critical region on chromosome x gene 1 (dax1), cytochrome P450 family 19 subfamily member 1 a (cyp19a1a), cytochrome P450 family 11 subfamily a member 1 a (cyp11a1a) and luteinizing hormone receptor 1 (lhr1). Overall, our results demonstrated that igf3 promotes the maturation of the ovary and plays an important role in sex differentiation in E. coioides.

A chronic exposure to bisphenol A reduces sperm quality in goldfish associated with increases in kiss2, gpr54, and gnrh3 mRNA and circulatory LH levels at environmentally relevant concentrations

The bisphenol A (BPA)-disrupted reproductive functions have been demonstrated in male animals. In fish, it has been shown that environmentally relevant concentrations of BPA decrease sperm quality associated with inhibition of androgen biosynthesis. However, BPA effects on neuroendocrine regulation of reproduction to affect testicular functions are largely unknown. In the present study, reproductive functions of hypothalamus and pituitary were studied in mature male goldfish exposed to nominal 0.2, 2.0 and 20.0 μg/L BPA. At 90 d of exposure, sperm volume, velocity, and density and motility were decreased in goldfish exposed to 0.2, 2.0, and 20.0 μg/L BPA, respectively (p < 0.05). At 30 d of exposure, there were no significant changes in circulatory LH levels and mRNA transcripts of kiss1, Kiss2, gpr54, and gnrh3. At 90 d of exposure, circulatory LH levels showed trends toward increases in BPA exposed goldfish, which was significant in those exposed to 2.0 μg/L (P < 0.05). At this time, Kiss2, gpr54, and gnrh3 mRNA levels were increased in goldfish exposed to any concentrations of BPA (p < 0.05). This study shows that BPA-diminished sperm quality was accompanied by an increase in circulatory LH levels associated with increases in mRNA transcripts of upstream neuroendocrine regulators of reproduction in goldfish. Further, this is the first study to report circulatory levels of LH in fish exposed to BPA.

Coordinate regulation of feeding, metabolism, and growth: Perspectives from studies in fish

This paper develops a model for coordinate regulation of feeding, metabolism, and growth based on studies in fish. Many factors involved with the control of feeding [e.g., cholecystokinin (CCK) and ghrelin (GRLN)], energy metabolism [e.g., insulin (INS), glucagon (GLU), glucagon-like peptide (GLP), and somatostatins (SS), produced in the endocrine pancreas; and leptin (LEP) produced broadly], and growth [e.g., GRLN, growth hormone (GH), insulin-like growth factors (IGFs), GH receptors (GHR), IGF receptors (IGFR)] interact at various levels. Many such interactions serve to coordinate these systems to favor anabolic processes (i.e., lipid and protein synthesis, glycogenesis) and growth, including GH promotion of feeding and stimulation of INS production/secretion and the upregulation of GHR and IGFR by GRLN. As nutrient and stored energy status change, various feedbacks serve to curtail feeding and transition the animal from an anabolic/growth state to a catabolic state. Many factors, including LEP and IGF, promote satiety, whereas SS downregulates INS signaling as well as IGF production and GHR and IGFR abundance. As INS and IGF levels fall, GH becomes disconnected from growth as a result of altered linkage of GHR to cell signaling pathways. As a result, the catabolic actions of GH, GLU, GLP, LEP, and SS prevail, mobilizing stored energy reserves. Coordinate regulation involves relative abundances of blood-borne hormones as well as the ability to adjust responsiveness to hormones (via receptor and post-receptor events) in a cell-/tissue-specific manner that results from genetic and epigenetic programming and modulation by the local milieu of hormones, nutrients, and autocrine/paracrine interactions. The proposed model of coordinate regulation demonstrates how feeding, metabolism, and growth are integrated with each other and with other processes, such as reproduction, and how adaptive adjustments can be made to energy allocation during an animal's life history and/or in response to changes in environmental conditions.

Molecular Characterization and Expression of Cytochrome P450 Aromatase in Atlantic Croaker Brain: Regulation by Antioxidant Status and Nitric Oxide Synthase During Hypoxia Stress

We have previously shown that nitric oxide synthase (NOS, an enzyme) is significantly increased during hypoxic stress in Atlantic croaker brains and modulated by an antioxidant (AOX). However, the influence of NOS and AOX on cytochrome P450 aromatase (AROM, CYP19a1, an enzyme) activity on vertebrate brains during hypoxic stress is largely unknown. In this study, we characterized brain AROM (bAROM, CYP19a1b) cDNA in croaker and examined the interactive effects of hypoxia and a NOS-inhibitor or AOX on AROM activity. The amino acid sequence of croaker bAROM cDNA is highly homologous (76–80%) to other marine teleost bAROM cDNAs. Both real-time PCR and Northern blot analyses showed that bAROM transcript (size: ∼2.8 kb) is highly expressed in the preoptic-anterior hypothalamus (POAH). Hypoxia exposure (dissolved oxygen, DO: 1.7 mg/L for 4 weeks) caused significant decreases in hypothalamic AROM activity, bAROM mRNA and protein expressions. Hypothalamic AROM activity and mRNA levels were also decreased by pharmacological treatment with N-ethylmaleimide (NEM, an alkylating drug that modifies sulfhydryl groups) of fish exposed to normoxic (DO: ∼6.5 mg/L) conditions. On the other hand, treatments with Nω-nitro-L-arginine methyl ester (NAME, a competitive NOS-inhibitor) or vitamin-E (Vit-E, a powerful AOX) prevented the downregulation of hypothalamic AROM activity and mRNA levels in hypoxic fish. Moreover, NAME and Vit-E treatments also restored gonadal growth in hypoxic fish. Double-labeled immunohistochemistry results showed that AROM and NOS proteins are co-expressed with NADPH oxidase (generates superoxide anion) in the POAH. Collectively, these results suggest that the hypoxia-induced downregulation of AROM activity in teleost brains is influenced by neuronal NOS activity and AOX status. The present study provides, to the best of our knowledge, the first evidence of restoration of AROM levels in vertebrate brains by a competitive NOS-inhibitor and potent AOX during hypoxic stress.

Effect of Fishmeal Content in the Diet on the Growth and Sexual Maturation of Olive Flounder (Paralichthysolivaceus) at a Typical Fish Farm

Simple Summary

Increasing demand for an efficient and economic fishmeal feed for sustainable aquaculture has urged the aquafeed sector to seek an optimum fish-feed formulation. This study investigated the physiological response in olive flounder fed various fishmeal diets in a typical fish farm. The fish were farmed for 20 weeks, using the following experimental feeds: a control feed (CON), a replacement by 20% (F20), and 30% (F30) of the fish meal content of the CON. All groups showed no significant difference in growth and survival rates. However, due to investigating hormone expression associated with maturation, high expression of PSS-I and low expression of FSH-β, ER-α, and ER-β in FM30 compared to other experimental groups were observed. Therefore, up to 30% fishmeal replacement does not affect growth, but it appears to have a slight effect on the sexual development of olive flounder.

Abstract

Olive flounder (Paralichthys olivaceus) is a commercially important and valuable species for aquaculture in Korea. Due to the unstable supply of fishmeal for farmed fish, an optimum fish-feed formulation should be researched to ensure the sustainability of P. olivaceus aquaculture. This study investigated the effect of three experimental diets: Con (basal diet); FM₂₀ (20% fishmeal replacement of CON); and FM₃₀ (30% fishmeal replacement of CON) on P. olivaceus over 20 weeks at a typical farm by monitoring the growth and factors relating to sexual maturation. The results showed that no differences in growth were observed between the CON and diet-replacement groups. Gonadal oocyte development was similar between the CON and diet-replacement groups. Moreover, sbGnRH and GH expression did not differ between the CON and diet-replacement groups. The levels of Erβ and Vtg expression were significantly higher in the FM₂₀ group than in the CON and FM₃₀ groups after the experimental period. The expression of PSS-I was significantly higher in the FM₃₀ group than in the CON and FM₂₀ groups. Therefore, although growth occurred when 30% of the fishmeal was replaced, such high dietary protein replacement may be ill-advised during the maturation of olive flounder at the commercial fish farm.

Structure and function analysis of various brain subregions and pituitary in grass carp (Ctenopharyngodon idellus)

It has been generally acknowledged that environment could alter the morphology and functional differentiation of vertebrate brain. However, as the largest group of all vertebrates, studies about the structures and functions of various brain subregions in teleost are still scarce. In this study, using grass carp as a model, histology method and RNA-sequencing were recruited to examine the microstructure and transcript levels among different brain subregions and pituitary. Histological results showed that the grass carp brain was composed of six parts, including olfactory bulb, telencephalon, hypothalamus, optic tectum, cerebellum, and medulla oblongata. In addition, compared to elasmobranchs and non-teleost bony ray-finned fishes, grass carp lost the hypothalamo-hypophyseal portal system, instead the hypophysiotropic neurons were directly terminated in the pituitary cells. At the transcriptomic level, our results suggested that the olfactory bulb might be related to reproduction and immune function. The telencephalon was deemed to be involved in the regulation of appetite and reproduction. The optic tectum might play important roles in the vision system and feeding. The hypothalamus could regulate feeding, and reproduction process. The medulla oblongata was related with the auditory system. The pituitary seemed to play pivotal roles in energy metabolism, organ development and reproduction. Finally, the correlation analysis suggested that the hypothalamus and the telencephalon were highly related, and close anatomical connection and overlapping functions suggested that the telencephalon and hypothalamus might be the regulation center of feeding and reproduction among teleost brain. This study provided a global view of the microstructures and specific functions of various brain subregions and pituitary in teleost. These results will be very helpful for further study in the neuroendocrinology regulation of growth and reproduction in teleost brain-pituitary axis.

Gemfibrozil and carbamazepine decrease steroid production in zebrafish testes (Danio rerio)

Gemfibrozil (GEM) and carbamazepine (CBZ) are two environmentally relevant pharmaceuticals and chronic exposure of fish to these compounds has decreased androgen levels and fish reproduction in laboratory studies. The main focus of this study was to examine the effects of GEM and CBZ on testicular steroid production, using zebrafish as a model species. Chronic water borne exposures of adult zebrafish to 10 μg/L of GEM and CBZ were conducted and the dosing was confirmed by chemical analysis of water as 17.5 ± 1.78 and 11.2 ± 1.08 μg/L respectively. A 67 day exposure led to reduced reproductive output and lowered whole body, plasma, and testicular 11-ketotestosterone (11-KT). Testicular production of 11-KT was examined post exposure (42 days) using ex vivo cultures to determine basal and stimulated steroid production. The goal was to ascertain the step impaired in the steroidogenic pathway by each compound. Ex vivo 11-KT production in testes from males chronically exposed to GEM and CBZ was lower than that from unexposed males. Although hCG, 25-OH cholesterol, and pregnenolone stimulation increased 11-KT production in all treatment groups over basal levels, hCG stimulated 11-KT production remained significantly less in testes from exposed males compared to controls. 25-OH cholesterol and pregnenolone stimulated 11-KT production was similar between GEM and control groups but the CBZ group had lower 11-KT production than controls with both stimulants. We therefore propose that chronic GEM and CBZ exposure can reduce production of 11-KT in testes through direct effects independent of mediation through HPG axis. The biochemical processes for steroid production appear un-impacted by GEM exposure; while CBZ exposure may influence steroidogenic enzyme expression or function.

A game of two? Gene expression analysis of brain (cyp19a1b) and gonadal (cyp19a1a) aromatase in females of a Neotropical cichlid fish through the parental care period and removal of the offspring

For many species parental behavior is essential for the survival of the offspring. While the ultimate causes of teleost parental behavior have been widely studied, comparatively little is known about its proximate causes. The aim of this study was to analyze the yet unexplored, potential dual role of brain and gonadal aromatases, the enzymes responsible for the conversion of androgens to estrogens in the brains and gonads of teleosts, respectively, on the different stages of the maternal care period of the biparental cichlid Cichlasoma dimerus, locally known as chanchita. By immunohistochemistry we analyzed the neural distribution of brain aromatase and observed it exclusively within the forebrain, including areas involved in the regulation of parental behavior. We next analyzed the gene expression of brain aromatase in the brain, and gonadal aromatase in the ovary, of female chanchitas through the parental care period. To further characterize the physiological environment associated to maternal care, we also evaluated sex steroid levels (17β-estradiol, testosterone and 11-ketotestoterone) and ovarian follicle percentage. The onset of parental behavior specifically downregulated sex steroids synthesis and the rate of ovarian maturation, as denoted by a more than 10-fold decrease in steroid levels and delayed detection of mature follicles in females with offspring, compared to females which eggs were removed. Gene expression levels of both aromatases were independent of maternal care at the evaluated time points, even though they varied during the parental care period.

Dynamics of ovarian maturation throughout the reproductive cycle of the Neotropical cichlid fishCichlasomadimerus(Teleostei, Cichliformes)

In this study, we analyzed gene expression profiles, plasma steroids concentrations, and gonadal morphology throughout the reproductive cycle of female Cichlasoma dimerus (Heckel, 1840), a monogamous cichlid fish exhibiting social hierarchies. Fish were analyzed at six phases encompassing their annual cycle, namely resting (during the nonreproductive period), prespawning, 30 h post spawning, 4 days post spawning, 10 days post spawning, and subordinate (during the reproductive period). The histological and histomorphometric analysis showed that C. dimerus exhibits asynchronous ovarian development. Similar to resting females, subordinate females showed low gonadosomatic index, reduced expression levels of vitellogenin (vtgAb), zona pellucida (zpB), gonadal aromatase (cyp19a1A), and low concentrations of plasma sex steroids, thus indicating that social intimidation by dominant conspecifics elicited reproductive arrest. In reproductively active females, a direct positive correlation between plasma estradiol, vtgAb expression, percentage of late vitellogenic oocytes, and gonadosomatic index was observed. These parameters were maximal at the prespawning phase, decreased at 30 h post spawning and 4 days post spawning, and then reached a peak at 10 days post spawning. Our results indicate that female C. dimerus become spawning capable after 10 days post spawning, coincidently with the shortest time interval between successive spawns recorded in captivity.

Transcripts of genes encoding reproductive neuroendocrine hormones and androgen receptor in the brain and testis of goldfish exposed to vinclozolin, flutamide, testosterone, and their combinations

Vinclozolin (VZ) is a pesticide that acts as an anti-androgen to impair reproduction in mammals. However, VZ-induced disruption of reproduction is largely unknown in fish. In the present study, we have established a combination exposure in which adult goldfish were exposed to VZ (30 and 100 μg/L), anti-androgen flutamide (Flu, 300 μg/L), and androgen testosterone (T, 1 μg/L) to better understand effects of VZ on reproductive endocrine system. mRNA levels of kisspeptin (kiss-1 and kiss-2) and its receptor (gpr54), salmon gonadotropin-releasing hormone (gnrh3) and androgen receptor (ar) in the mid-brain, and luteinizing hormone receptor (lhr) in the testis were analyzed and compared with those of control following 10 days of exposure. kiss-1 mRNA level was increased in goldfish exposed to 100 µg/L VZ and to Flu, while kiss-2 mRNA level was increased following exposure to Flu and to combinations of 30 µg/L VZ with Flu, 100 µg/L VZ with T, and Flu with T. gpr54 mRNA level was increased in goldfish exposed to Flu and to combination of 30 µg/L VZ with Flu and 100 µg/L VZ with T. gnrh3 mRNA level was increased in goldfish exposed to 100 µg/L VZ, to Flu, and to combinations of 30 µg/L VZ with Flu, 100 µg/L VZ with T, and Flu with T. The mid-brain ar mRNA level was increased in goldfish exposed to Flu and to combinations of 30 µg/L VZ with Flu, 100 µg/L VZ with T, and Flu with T. Testicular lhr mRNA level was increased in goldfish exposed to Flu and to combination of 30 µg/L VZ with Flu. These results suggest that VZ and Flu are capable of interfering with kisspeptin and GnRH systems to alter pituitary and testicular horonal functions in adult goldfish and the brain ar mediates VZ-induced disruption of androgen production.

Progestin increases the expression of gonadotropins in pituitaries of male zebrafish

Our previous study showed that the in vivo positive effects of 17α,20β-dihydroxy-4-pregnen-3-one (DHP), the major progestin in zebrafish, on early spermatogenesis was much stronger than the ex vivo ones, which may suggest an effect of DHP on the expression of gonadotropins. In our present study, we first observed that fshb and lhb mRNA levels in the pituitary of male adult zebrafish were greatly inhibited by 3 weeks exposure to 10nM estradiol (E2). However, an additional 24h 100nM DHP exposure not only reversed the E2-induced inhibition, but also significantly increased the expression of fshb and lhb mRNA. These stimulatory effects were also observed in male adult fish without E2 pretreatment, and a time course experiment showed that it took 24h for fshb and 12h for lhb to respond significantly. Because these stimulatory activities were partially antagonized by a nuclear progesterone receptor (Pgr) antagonist mifepristone, we generated a Pgr-knockout (pgr(-/-)) model using the TALEN technique. With and without DHP in vivo treatment, fshb and lhb mRNA levels of pgr(-/-) were significantly lower than those of pgr(+/+) Furthermore, ex vivo treatment of pituitary fragments of pgr(-/-) with DHP stimulated lhb, but not fshb mRNA expression. Results from double-colored fluorescent in situ hybridization showed that pgr mRNA was expressed only in fshb-expressing cells. Taken together, our results indicated that DHP participated in the regulation of neuroendocrine control of reproduction in male zebrafish, and exerted a Pgr-mediated direct stimulatory effect on fshb mRNA at pituitary level.

Physiological impacts of elevated carbon dioxide and ocean acidification on fish

Most fish studied to date efficiently compensate for a hypercapnic acid-base disturbance; however, many recent studies examining the effects of ocean acidification on fish have documented impacts at CO2 levels predicted to occur before the end of this century. Notable impacts on neurosensory and behavioral endpoints, otolith growth, mitochondrial function, and metabolic rate demonstrate an unexpected sensitivity to current-day and near-future CO2 levels. Most explanations for these effects seem to center on increases in Pco2 and HCO3− that occur in the body during pH compensation for acid-base balance; however, few studies have measured these parameters at environmentally relevant CO2 levels or directly related them to reported negative endpoints. This compensatory response is well documented, but noted variation in dynamic regulation of acid-base transport pathways across species, exposure levels, and exposure duration suggests that multiple strategies may be utilized to cope with hypercapnia. Understanding this regulation and changes in ion gradients in extracellular and intracellular compartments during CO2 exposure could provide a basis for predicting sensitivity and explaining interspecies variation. Based on analysis of the existing literature, the present review presents a clear message that ocean acidification may cause significant effects on fish across multiple physiological systems, suggesting that pH compensation does not necessarily confer tolerance as downstream consequences and tradeoffs occur. It remains difficult to assess if acclimation responses during abrupt CO2 exposures will translate to fitness impacts over longer timescales. Nonetheless, identifying mechanisms and processes that may be subject to selective pressure could be one of many important components of assessing adaptive capacity.

An integrative overview of the role of gonadotropin-inhibitory hormone in behavior: applying Tinbergen's four questions

The integration of various fields of investigation is of key importance to fully comprehending endocrine function. Here, I enact the theoretical framework of Nikolaas Tinbergen's four questions for understanding behavior to help bridge the wide gap that exists between our relatively reductionist molecular knowledge of a particular neurohormone, gonadotropin-inhibitory hormone (GnIH), and its place in animal behavior. Hypothalamic GnIH, upon its discovery in 2000, was so named because of its inhibitory effect on the release of the gonadotropins, luteinizing hormone (LH) and follicle stimulating hormone (FSH), from the pituitary. Because gonadotropins are necessary for reproduction, this finding stimulated questions about the functional significance of GnIH in reproduction and sexual behavior. After over a decade of research, invaluable knowledge has been gained regarding the mechanistic attributes of GnIH (mammalian homolog, RFamide-related peptide (RFRP)) in a variety of vertebrate species. However, many questions remain regarding the effect of the environment on GnIH and the subsequent effects of GnIH on behavior. I review the role of GnIH in shaping behavior using the framework of Tinbergen's four questions of mechanism, ontogeny, function and phylogeny. The studies I review were conducted in various species of mammals, birds, and in one species of fish. Because GnIH can play a role in mediating behaviors such as those important for reproduction, sociality, feeding, and the stress response in a variety of species, an integrative approach to the study of GnIH will help provide a multipronged schema for answering questions of GnIH function. By using the framework highlighted by Tinbergen's four questions, we will deepen and enhance our knowledge of the role of hormones in behavior from the point of view of the mechanisms involved.

Fundulus heteroclitus: ovarian reproductive physiology and the impact of environmental contaminants

Fundulus heteroclitus, the mummichog or Atlantic killifish, is the dominant small-bodied fish species of the east coast estuaries and salt marshes of Canada and the USA, where it is present as two subspecies, the northern F. h. macrolepidotus and the southern F. h. heteroclitus. Recently identified as the premier teleost model in environmental biology, the species has long been of value in understanding evolved tolerance to toxicants and more lately in adding to our knowledge about reproductive effects of environmental endocrine disruptors. The body of literature on F. heteroclitus ovarian physiology and reproduction, from both field and laboratory studies, provides the foundation for present work focused on understanding the reproductive effects and modes of action of environmental toxicants. In this paper, we review the environmental and endocrine factors controlling ovarian and reproductive cycling in F. heteroclitus, noting specifics related to field and laboratory studies on the two subspecies as well as key research gaps compared to other fish species. We also summarize recent development of methodologies to study the effects of environmental contaminants on endocrine signalling and egg production in F. heteroclitus. Continued efforts to progress both our fundamental understanding of reproductive physiology in mummichog, coupled with studies focused on the modes of action of environmental contaminants, have high potential to further develop this teleost model. While the model may presently lag behind those based on other species of fish, the unique biochemical and physiological adaptations which allow F. heteroclitus to adapt to changing environmental and toxic conditions provide a valuable experimental system for comparative physiologists, ecotoxicologists and evolutionary biologists.

Tryptophan hydroxylase: a target for neuroendocrine disruption

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, Atlantic croaker, indicated that hypothalamic TPH is a major site of interference of hypothalamic-pituitary-gonadal function by environmental stressors. The effects of exposure to two different types of environmental stressors, low dissolved oxygen (hypoxia) and a polychlorinated biphenyl mixture (Aroclor 1254), on the stimulatory brain serotonergic system controlling reproductive neuroendocrine function in Atlantic croaker are reviewed. Exposure to both stressors produced decreases in TPH activity, which were accompanied by a fall in hypothalamic 5-HT and gonadotropin-releasing hormone (GnRH I) content in the preoptic-anterior hypothalamic area and were associated with reduction in luteinizing hormone (LH) secretion and gonadal development. Pharmacological restoration of hypothalamic 5-HT levels after exposure to both stressors also restored neuroendocrine and reproductive functions, indicating that the serotonergic system is an important site for hypoxia- and Aroclor 1254-induced inhibition of reproductive neuroendocrine functions. The mechanisms underlying downregulation of TPH activity by these stressors remain unclear but may involve alterations in hypothalamic antioxidant status. In support of this hypothesis, treatment with an antioxidant, vitamin E, was found to reverse the inhibitory effects of Aroclor 1254 on TPH activity. The results suggest that TPH is a major target for neuroendocrine disruption by diverse environmental stressors.