Hyperosmotic tolerance of adult fish and early embryos are determined by discrete, single loci in the genus Oryzias.

The acquisition of environmental osmolality tolerance traits in individuals and gametes is an important event in the evolution and diversification of organisms. Although teleost fish exhibit considerable intra- and interspecific variation in salinity tolerance, the genetic mechanisms underlying this trait remain unclear. Oryzias celebensis survives in sea and fresh water during both the embryonic and adult stages, whereas its close relative Oryzias woworae cannot survive in sea water at either stage. A linkage analysis using backcross progeny identified a single locus responsible for adult hyperosmotic tolerance on a fused chromosome that corresponds to O. latipes linkage groups (LGs) 6 and 23. Conversely, O. woworae eggs fertilised with O. celebensis sperm died in sea water at the cleavage stages, whereas O. celebensis eggs fertilised with O. woworae sperm developed normally, demonstrating that maternal factor(s) from O. celebensis are responsible for hyperosmotic tolerance during early development. A further linkage analysis using backcrossed females revealed a discrete single locus relating to the maternal hyperosmotic tolerance factor in a fused chromosomal region homologous to O. latipes LGs 17 and 19. These results indicate that a maternal factor governs embryonic hyperosmotic tolerance and maps to a locus distinct from that associated with adult hyperosmotic tolerance.

Principal function of mineralocorticoid signaling suggested by constitutive knockout of the mineralocorticoid receptor in medaka fish.

As in osmoregulation, mineralocorticoid signaling is implicated in the control of brain-behavior actions. Nevertheless, the understanding of this role is limited, partly due to the mortality of mineralocorticoid receptor (MR)-knockout (KO) mice due to impaired Na+ reabsorption. In teleost fish, a distinct mineralocorticoid system has only been identified recently. Here, we generated a constitutive MR-KO medaka as the first adult-viable MR-KO animal, since MR expression is modest in osmoregulatory organs but high in the brain of adult medaka as for most teleosts. Hyper- and hypo-osmoregulation were normal in MR-KO medaka. When we studied the behavioral phenotypes based on the central MR localization, however, MR-KO medaka failed to track moving dots despite having an increase in acceleration of swimming. These findings reinforce previous results showing a minor role for mineralocorticoid signaling in fish osmoregulation, and provide the first convincing evidence that MR is required for normal locomotor activity in response to visual motion stimuli, but not for the recognition of these stimuli per se. We suggest that MR potentially integrates brain-behavioral and visual responses, which might be a conserved function of mineralocorticoid signaling through vertebrates. Importantly, this fish model allows for the possible identification of novel aspects of mineralocorticoid signaling.

High levels of plasma cortisol and impaired hypoosmoregulation in a mutant medaka deficient in P450c17I.

scl is a spontaneous medaka mutant deficient in P450c17I, which is required for production of sex steroids, but not of cortisol, the major role of which is osmoregulation in teleost fish. The scl mutant provides a new model to study the functions of these hormones. We first found that fish homozygous for this mutation have plasma cortisol constitutively at a high physiological level (1000 nM). Since we previously showed that this level reversed the seawater-type differentiation of the medaka gastrointestinal tract, hypoosmoregulation of the scl mutant was analyzed. Muscle water contents in freshwater were normal in scl homozygotes, but the contents were lower than those of the wild type (WT) after seawater transfer. There were no differences in gill mRNA levels of corticosteroid receptors or ion transporters between scl homozygotes and WT. In the intestine, expression of glucocorticoid receptors and Na(+)/K(+)/2Cl(-) cotransporter were induced in WT during seawater acclimation, but not in scl homozygotes. The high plasma cortisol may prevent hypoosmoregulation by inhibition of increased intestinal water absorption, essentially by the Na(+)/K(+)/2Cl(-) cotransporter, in seawater.

Osmotic/ionic status of body fluids in the euryhaline cephalopod suggest possible parallel evolution of osmoregulation.

Acclimation from marine to dilute environments constitutes among the dramatic evolutionary transitions in the history of life. Such adaptations have evolved in multiple lineages, but studies of the blood/hemolymph homeostasis mechanisms are limited to those using evolutionarily advanced Deuterostome (chordates) and Ecdysozoa (crustaceans). Here, we examined hemolymph homeostasis in the advanced Lophotrochozoa/mollusc, the other unexplored taxa, and its possible regulation by the vasopressin/oxytocin superfamily peptides known to be implicated in fluid homeostasis in Chordata and Arthropoda. The hemolymph osmotic and ionic status in the euryhaline cephalopod (Octopus ocellatus) following transfer from 30-ppt normal seawater to 20 ppt salinity indicate hyperosmo- and hyperionoregulatory abilities for more than 1 week, as in crustaceans and teleost fish. While ventilation frequency decreased by 1 day, Na(+)/K(+)-ATPase activity, which has been generally implicated in ion transport, was induced in two of the eight posterior gills after 1 week. In addition, the octopuses were intravenously injected with 1 or 100 ng/g octopressin or cephalotocin, which are Octopus vasopressin/oxytocin orthologs. After 1 day, octopressin, but not cephalotocin, decreased the hemolymph osmolality and Ca concentrations, as well as urinary Na concentrations. These data provide evidence for possible parallel evolution in hyperionoregulatory mechanisms and coordination by conserved peptides.

Neurohypophysial Hormones Regulate Amphibious Behaviour in the Mudskipper Goby.

The neurohypophysial hormones, arginine vasotocin and isotocin, regulate both hydromineral balance and social behaviors in fish. In the amphibious mudskipper, Periophthalmus modestus, we previously found arginine-vasotocin-specific regulation of aggressive behavior, including migration of the submissive subordinate into water. This migration also implies the need for adaptation to dehydration. Here, we examined the effects of arginine vasotocin and isotocin administration on the amphibious behavior of individual mudskippers in vivo. The mudskippers remained in the water for an increased period of time after 1-8 h of intracerebroventricular (ICV) injection with 500 pg/g arginine vasotocin or isotocin. The 'frequency of migration' was decreased after ICV injection of arginine vasotocin or isotocin, reflecting a tendency to remain in the water. ICV injections of isotocin receptor antagonist with arginine vasotocin or isotocin inhibited all of these hormonal effects. In animals kept out of water, mRNA expression of brain arginine vasotocin and isotocin precursors increased 3- and 1.5-fold, respectively. Given the relatively wide distribution of arginine vasotocin fibres throughout the mudskipper brain, induction of arginine vasotocin and isotocin under terrestrial conditions may be involved also in the preference for an aquatic habitat as ligands for brain isotocin receptors.

Diverse mechanisms for body fluid regulation in teleost fishes.

Teleost fishes are the major group of ray-finned fishes and represent more than one-half of the total number of vertebrate species. They have experienced in their evolution an additional third-round whole genome duplication just after the divergence of their lineage, which endowed them with an extra adaptability to invade various aquatic habitats. Thus their physiology is also extremely diverse compared with other vertebrate groups as exemplified by the many patterns of body fluid regulation or osmoregulation. The key osmoregulatory organ for teleosts, whose body fluid composition is similar to mammals, is the gill, where ions are absorbed from or excreted into surrounding waters of various salinities against concentration gradients. It has been shown that the underlying molecular physiology of gill ionocytes responsible for ion regulation is highly variable among species. This variability is also seen in the endocrine control of osmoregulation where some hormones have distinct effects on body fluid regulation in different teleost species. A typical example is atrial natriuretic peptide (ANP); ANP is secreted in response to increased blood volume and acts on various osmoregulatory organs to restore volume in rainbow trout as it does in mammals, but it is secreted in response to increased plasma osmolality, and specifically decreases NaCl, and not water, in the body of eels. The distinct actions of other osmoregulatory hormones such as growth hormone, prolactin, angiotensin II, and vasotocin among teleost species are also evident. We hypothesized that such diversity of ionocytes and hormone actions among species stems from their intrinsic differences in body fluid regulation that originated from their native habitats, either fresh water or seawater. In this review, we summarized remarkable differences in body fluid regulation and its endocrine control among teleost species, although the number of species is still limited to substantiate the hypothesis.

Gastric Contraction Imaging System Using a 3-D Endoscope.

This paper presents a gastric contraction imaging system for assessment of gastric motility using a 3-D endoscope. Gastrointestinal diseases are mainly based on morphological abnormalities. However, gastrointestinal symptoms are sometimes apparent without visible abnormalities. One of the major factors for these diseases is abnormal gastrointestinal motility. For assessment of gastric motility, a gastric motility imaging system is needed. To assess the dynamic motility of the stomach, the proposed system measures 3-D gastric contractions derived from a 3-D profile of the stomach wall obtained with a developed 3-D endoscope. After obtaining contraction waves, their frequency, amplitude, and speed of propagation can be calculated using a Gaussian function. The proposed system was evaluated for 3-D measurements of several objects with known geometries. The results showed that the surface profiles could be obtained with an error of [Formula: see text] of the distance between two different points on images. Subsequently, we evaluated the validity of a prototype system using a wave simulated model. In the experiment, the amplitude and position of waves could be measured with 1-mm accuracy. The present results suggest that the proposed system can measure the speed and amplitude of contractions. This system has low invasiveness and can assess the motility of the stomach wall directly in a 3-D manner. Our method can be used for examination of gastric morphological and functional abnormalities.

Potential roles of arginine-vasotocin in the regulation of aggressive behavior in the mudskipper (Periophthalmus modestus).

The hypothalamic hormones, arginine-vasotocin (VT) and isotocin (IT), play central roles in osmoregulation and in the regulation of social behaviors including aggressive behavior in many vertebrates including fish. Here, we examined whether these hormones are associated with aggressive behavior in the mudskipper (Periophthalmus modestus). The mudskipper is an amphibious fish, which lives in the brackish water of river mouths and displays unique aggressive behavior. Upon introduction to each other in an experimental tank with aquatic and terrestrial areas, a pair of males can be classified as aggressive dominant or submissive subordinate based on the frequency of their aggressive acts, which is significantly higher in dominant male. Additionally, the length of stay in terrestrial area of dominant was longer than that of the subordinate. The latter remained in aquatic area almost throughout the period of behavioral observation. The expression of brain VT mRNA was significantly higher in subordinate than in dominant, whereas neither IT mRNA expression nor plasma cortisol level differed between subordinate and dominant male. On the other hand, an intracerebroventricular injection of VT increased aggressive behaviors in mudskippers. In addition to known roles of VT in mediation of aggressive behavior, these results may shed light on the role of endogenous VT toward water migration in submissive mudskippers. The amphibious fish is a valuable experimental model to observe the relationship between effects of central VT on the osmoregulation and social behavioral regulation in vertebrates.

Tele-auscultation support system with mixed reality navigation.

The aim of this research is to develop an information support system for tele-auscultation. In auscultation, a doctor requires to understand condition of applying a stethoscope, in addition to auscultatory sounds. The proposed system includes intuitive navigation system of stethoscope operation, in addition to conventional audio streaming system of auscultatory sounds and conventional video conferencing system for telecommunication. Mixed reality technology is applied for intuitive navigation of the stethoscope. Information, such as position, contact condition and breath, is overlaid on a view of the patient's chest. The contact condition of the stethoscope is measured by e-textile contact sensors. The breath is measured by a band type breath sensor. In a simulated tele-auscultation experiment, the stethoscope with the contact sensors and the breath sensor were evaluated. The results show that the presentation of the contact condition was not understandable enough for navigating the stethoscope handling. The time series of the breath phases was usable for the remote doctor to understand the breath condition of the patient.

The role of 'mineralocorticoids' in teleost fish: relative importance of glucocorticoid signaling in the osmoregulation and 'central' actions of mineralocorticoid receptor.

It has long been held that cortisol, a glucocorticoid in many vertebrates, performs glucocorticoid and mineralocorticoid actions in the teleost fish since it lacks aldosterone. However, in addition to the counterparts of tetrapod mineralocorticoid receptors (MRs), 11-deoxycorticosterone (DOC) has been recently identified as a specific endogenous ligand for the MRs in teleosts. Here, we point out the minor role of mineralocorticoid signaling (i.e., DOC-MR) in the osmoregulation compared with those of glucocorticoid signaling (i.e., cortisol-glucocorticoid receptor [GR]), and review the current findings on the physiological roles of the DOC-MR in teleosts. Cortisol promotes both freshwater and seawater adaptation via the GRs in the osmoregulatory organs such as gills and gastrointestinal tracts, but the expressions of MR mRNA are abundant in the brains especially in the key components of the stress axis and cerebellums. Together with the behavioral effects of intracerebroventricular injection with DOC, the MR is suggested to play an important role in the brain dependent behaviors. Since the abundant expression of central MRs has been reported also in higher vertebrates and the MR is thought to be ancestral to the GR, the role of MR in fish might reflect the principal and original function of corticosteroid signaling. Functional evolution of corticosteroid systems is summarized and areas in need of research like our on-going experiments with MR-knockout medaka are outlined.

Growth, energetics and the cortisol-hepatic glucocorticoid receptor axis of medaka (Oryzias latipes) in various salinities.

We examined growth of euryhaline Japanese medaka (Oryzias latipes) after transfer to freshwater or seawater from isotonic saline. Growth was unaffected by the different salinities for 1 week, but the body weight increase and BMI of fish kept in freshwater for 2-3 weeks were significantly higher than those in the isotonic controls. These results may reflect the usual habitat of this species. To assess the basis for the difference in growth, energetics and the hepatic stress axis were evaluated 1 week after the transfer. Unexpectedly, despite the higher growth rate, the rate of routine oxygen consumption was significantly higher in freshwater. Plasma cortisol levels in freshwater were significantly higher than those in seawater, and the mRNA levels of the glucocorticoid receptor (GR1) in the liver were significantly lower in freshwater and seawater, compared to that in isotonic saline. Branchial Na(+)/K(+)-ATPase activities were also reduced significantly in freshwater and seawater, compared to that in isotonic saline. The higher levels of hepatic GR1 expression and branchial Na(+)/K(+)-ATPase activity in isotonic salinity than those in freshwater and seawater for 1 week may account for the lower growth rate under the isotonic condition. After 3 weeks, however, the Na(+)/K(+)-ATPase activity in seawater was significantly higher than that in freshwater. No significant difference in growth rate between freshwater and seawater groups indicates that medaka is a good model for studies of hypo- and hyperosmotic adaptations, since osmoregulation is not strongly associated with size and growth.

Cloning of two members of the calcitonin-family receptors from stingray, Dasyatis akajei: possible physiological roles of the calcitonin family in osmoregulation.

In cartilaginous fish, two cDNAs encoding calcitonin-family receptors were isolated for the first time from the stingray brain. The open reading frame of one receptor cDNA coded a 525-amino acid protein. The amino acid identity of this receptor to human calcitonin-receptor-like receptor (CRLR) is 64.5%, frog CRLR is 64.7%, and flounder CRLR is 61.2% and this was higher than to human calcitonin receptor (CTR) (46.1%), frog CTR (54.7%), and flounder CTR (48.9%). We strongly suggested that this receptor is a ray CRLR based on phylogenetic analysis. In case of the second receptor, amino acid identity among CRLRs (human 50.5%, frog 50.7%, flounder 48.0%) and CTRs (human 43.2%, frog 49.1%, flounder 41.8%) was similar. From phylogenetic analysis of both CRLRs and CTRs, we believe that this receptor is ray CTR. The expression of ray CRLR mRNA was predominantly detected in the nervous system (brain) and vascular system (atrium, ventricle, and gill), which reflects the similar localization of CGRP in the nervous and vascular systems as mammals. It was observed that the second receptor was expressed in several tissues, namely cartilage, brain, pituitary gland, gill, atrium, ventricle, pancreas, spleen, liver, gall bladder, intestine, rectal gland, kidney, testis and ovary. This localization pattern was very similar to flounder CTR. Both receptor mRNAs were strongly expressed in the gill. This suggests that the calcitonin-family members are involved in the osmoregulation of stingray as this fish is known to be euryhaline. When a stingray was transferred to diluted seawater (20% seawater), the expression of both receptors significantly decreased in the gill. Similar results were obtained in the kidney of the stingray. Thus, our cloning and isolation of both receptors in the stingray will be helpful for elucidation of their physiological role(s) such as osmoregulation including calcium metabolism of cartilaginous fish.

Rapid signaling of steroid hormones in the vertebrate nervous system.

Steroid hormones easily cross the blood-brain barrier because of their physicochemical lipid solubility. The hormones act through nuclear receptor-mediated mechanisms and modulate gene transcription. In contrast to their genomic actions, the non-genomic rapid action of steroid hormones, acting via various types of membrane-associated receptors, reveals pharmacological properties that are distinct from the actions of the intracellular nuclear receptors. As a result, non-genomic rapid actions have gained increased scientific interest. However, insight into the phylogenic and/or comparative actions of steroids in the brain is still poorly understood. In this review, we summarize recent findings concerning the rapid, non-genomic signaling of steroid hormones in the vertebrate central nervous system, and we discuss (using a comparative view from fish to mammals) recently published data regarding the mechanism underlying physiology and behavior.

Corticosteroids stimulate the amphibious behavior in mudskipper: potential role of mineralocorticoid receptors in teleost fish.

It has long been held that cortisol, a glucocorticoid in many vertebrates, carries out both glucocorticoid and mineralocorticoid actions in teleost fish. However, 11-deoxycorticosterone (DOC) has been identified as a specific endogenous ligand for the teleostean mineralocorticoid receptor (MR). Furthermore, the expressions of MR mRNA are modest in the osmoregulatory organs, but considerably higher in the brain of most teleosts. These recent findings suggest that the mineralocorticoid system (DOC/MR) may carry out some behavioral functions in fish. To test this possibility, we examined the effects of cortisol and DOC administration in the amphibious behavior in mudskipper (Periophthalmus modestus) in vivo. It was found that mudskippers remained in the water for an increased period of time when they were immersed into 5 µM DOC or cortisol for 8h. Additionally, an exposure to 25 µM DOC for 4 to 8 h caused a decreased migratory frequency of mudskippers to the water, reflected a tendency to remain in the water. It was further observed that after 8 h of intracerebroventricular (ICV) injection with 0.3 pmol DOC or cortisol the staying period in the water increased in fish. The migratory frequency was decreased after ICV DOC injection which indicated that fishes stayed in the water. Concurrent ICV injections of cortisol with RU486 [a specific glucocorticoid-receptor (GR) antagonist] inhibited only the partial effects of cortisol. Together with no changes in the plasma DOC concentrations under terrestrial conditions, these results indicate the involvement of brain MRs as cortisol receptors in the preference for an aquatic habitat of mudskippers. Although the role of GR signaling cannot be excluded in the aquatic preference, our data further suggest that the MR may play an important role in the brain dependent behaviors of teleost fish.

Dual in vitro effects of cortisol on cell turnover in the medaka esophagus via the glucocorticoid receptor.

AIMS: Cortisol is a glucocorticoid in mammals, but has both gluco- and mineralocorticoid activities in teleost fish. Our previous in vivo studies on osmoregulatory esophagi of euryhaline fish showed that epithelial apoptosis for the simple epithelium in seawater and cell proliferation for the stratified epithelium in fresh water are both induced by cortisol. The aim of the present study was to examine the mechanism of these dual cortisol effects on esophageal cell turnover. MAIN METHODS: We developed a tissue culture method for the esophagus from euryhaline medaka (Oryzias latipes) and assessed cell proliferation and apoptosis in vitro in response to cortisol and 11-deoxycorticosterone (DOC), a recently identified agonist of the teleostean mineralocorticoid receptor. KEY FINDINGS: Epithelial apoptosis, a well-established glucocorticoid function, was stimulated by treatment of the esophagus culture with 10nM cortisol for 8 days, but no effects were seen at higher doses (100 and 1000 nM). In contrast, cell proliferation was induced by 1000 nM cortisol treatment for 8 days and this response was dose-dependent. Both effects were blocked by RU-486, a glucocorticoid receptor antagonist. DOC showed no significant effects at 10-1000 nM. SIGNIFICANCE: In the esophageal epithelium in euryhaline fish, cortisol induces either apoptosis or cell proliferation via the glucocorticoid receptor, depending on the cortisol concentration. The glucocorticoid signaling may play a more important role than mineralocorticoid signaling in differentiation of the osmoregulatory esophagus in euryhaline fishes.

Prolactin inhibits osteoclastic activity in the goldfish scale: a novel direct action of prolactin in teleosts.

In teleosts, prolactin is involved in calcium regulation, but its role in scale/bone metabolism is unclear. Using the in-vitro system with goldfish scales developed recently, we explored the effects of teleost prolactin, growth hormone, and somatolactin on osteoclasts and osteoblasts. Addition of prolactin at concentrations of 0.01-100 ng/ml reduced osteoclastic activity, partly via osteoclast apoptosis, after 6-18 h incubation. Conversely, growth hormone and somatolactin at a concentration of 100 ng/ml increased osteoclastic activity after 18 h incubation, indicating the specificity of the inhibitory effect of prolactin on osteoclastic activity. On the other hand, these three hormones promoted osteoblastic activity at concentrations of 10-100 ng/ml. The results from this study are the first demonstration of direct effects of prolactin on scale/bone metabolism and osteoclastic activity in a teleost.

Cartilage ossiculoplasty by lever method.

CONCLUSION: We invented a new ossicular chain reconstruction by lever methods (OCRLM) in tympanoplasty type III with the canal wall down technique and investigated the postoperative hearing results of the 24 patients who had undergone this surgery. Postoperative air-bone gaps (ABGs) followed up for 2.1-3.2 years gave satisfactory improvement audiometrically compared with former reports. OBJECTIVE: To present newly devised methods applied from the viewpoint of mechanical dynamics for ossicular chain reconstruction in tympanoplasty. MATERIALS AND METHODS: A total of 24 cholesteatoma patients who underwent tympanoplasty with mastoidectomy using the canal wall down technique and ossicular chain reconstruction with OCRLM between May 2003 and April 2004 were investigated. The postoperative ABG of a four-frequency (0.5, 1, 2, and 4 kHz) average of OCRLM followed up for 2 years or more was assessed. RESULTS: Patients who underwent OCRLM showed good postoperative hearing results. Average postoperative ABGs of less than 10, 15, 20, 25, and 30 dB were 4.2, 37.5, 70.8, 91.6, and 100%, respectively. A gap of >30 dB was absent and the mean postoperative ABG was 17.2dB. The OCRLM method was satisfactory to improve hearing and effectively reduce the ABG postoperatively over the period of observation.

Prolactin receptor and proliferating/apoptotic cells in esophagus of the Mozambique tilapia (Oreochromis mossambicus) in fresh water and in seawater.

We have previously shown that esophageal epithelium of a euryhaline goby displays elevated cell proliferation in freshwater (FW) fish, but undergo apoptosis during seawater (SW) acclimation. Prolactin (PRL) injection into the goby induced the cell proliferation, whereas cortisol treatment stimulated the cell proliferation and apoptosis [Takahashi, H., Sakamoto, T., Narita, K., 2006a. Cell proliferation and apoptosis in the anterior intestine of an amphibious, euryhaline mudskipper (Periophthalmus modestus). J. Comp. Physiol. B 176, 463-468, 2006). In the euryhaline tilapia (Oreochromis mossambicus), the dynamics of changes in cortisol-glucocorticoid receptors (GR) during acclimation to different salinities also suggests a role for glucocorticoid signaling in the esophageal cell turnover, but the mode of PRL action remains largely unclear. In the present study, we report on effects in the tilapia esophagus that result from changes in environmental salinity. Specifically, we assessed: (1) mRNA expression of PRL receptor (PRLR) using quantitative real-time RT-PCR; (2) esophageal cell proliferation and apoptosis, using immunohistochemistry of proliferating cells nuclear antigen (PCNA) and in situ nick end-labeling of genomic DNA (TUNEL); (3) the possible localization of immunoreactive PRLR on proliferating/apoptotic cells. Plasma PRL increased after FW acclimation; PRLR mRNA levels in the esophagus of FW fish were significantly higher than those in SW-acclimated fish. Cell proliferation was induced randomly throughout the esophageal epithelium after FW acclimation, while cell division and increased apoptosis were concentrated at the tips of esophageal epithelial folds in SW-acclimated fish. Immunoreactive PRLR appeared to be localized at proliferating cells and at certain apoptotic cells, whereas immunoreactive GR was observed over the whole epithelium including the apoptotic/proliferating cells. Thus, PRL appears to affect cell turnover directly in the esophageal epithelium of the euryhaline tilapia.

In vivo effects of thyroid hormone, corticosteroids and prolactin on cell proliferation and apoptosis in the anterior intestine of the euryhaline mudskipper (Periophthalmus modestus).

We have previously shown that anterior intestinal epithelium of the euryhaline mudskipper (Periophthalmus modestus) undergoes apoptosis during seawater (SW) acclimation, whereas elevated cell proliferation was observed in freshwater (FW)-acclimated fish. To understand the possible endocrine regulation of the gastrointestinal cell turnover during salinity acclimation, we examined the ratios of apoptotic and proliferating cells in the anterior intestine of one-third SW-acclimated mudskipper treated with triiodothyronine (T3), cortisol, 11-deoxycorticosterone (DOC, the putative teleostean mineralocorticoid), or prolactin (PRL). In situ nick end labeling of genomic DNA (TUNEL) and immunohistochemistry of proliferating cells nuclear antigen (PCNA) were used as indicators of apoptosis and cell proliferations, respectively. Cortisol significantly elevated apoptosis (P<0.05) in the epithelia and connective tissues and also stimulated the epithelial cell proliferation (P<0.05). PRL induced epithelial cell proliferation (P<0.05), but did not affect apoptotic status of the intestinal epithelium. Neither T3 nor DOC had any impact on cell proliferation or apoptosis. Together, our results suggest a role for cortisol and PRL in the regulation of anterior intestinal epithelial turnover during salinity acclimation in this species.

Expression of glucocorticoid receptor in the intestine of a euryhaline teleost, the Mozambique tilapia (Oreochromis mossambicus): effect of seawater exposure and cortisol treatment.

Cortisol plays an important role in controlling intestinal water and ion transport in teleosts possibly through glucocorticoid receptor (GR) and/or mineralocorticoid receptor. To better understand the role of GR in the teleost intestine, in a euryhaline tilapia, Oreochromis mossambicus, we examined (1) the intestinal localizations of GR; (2) the effects of environmental salinity challenge and cortisol treatment on GR mRNA expression. The mRNA abundance of GR in the posterior intestinal region of tilapia was found to be higher than that in the anterior and middle intestine. In the posterior intestine, GR appears to be localized in the mucosal layer. GR mRNA levels in the posterior intestine were elevated after exposure of freshwater fish to seawater for 7 days following an increase in plasma cortisol. Similarly, cortisol implantation in freshwater tilapia for 7 days elevated the intestinal GR mRNA. These results indicate that seawater acclimation is accompanied by upregulation of GR mRNA abundance in intestinal tissue, possibly as a consequence of the elevation of cortisol levels. In contrast, a single intraperitoneal injection of cortisol into freshwater tilapia decreased intestinal GR mRNA. This downregulation of the GR mRNA by cortisol suggests a dual mode of autoregulation of GR expression by cortisol.