Pituitary autotransplants in Fundulus heteroclitus: effect onf thyroid function

Regulation of thyroid hormones and branchial iodothyronine deiodinases during freshwater acclimation in tilapia

Euryhaline fishes are capable of maintaining osmotic homeostasis in a wide range of environmental salinities. Several pleiotropic hormones, including prolactin, growth hormone, and thyroid hormones (THs) are mediators of salinity acclimation. It is unclear, however, the extent to which THs and the pituitary-thyroid axis promote the adaptive responses of key osmoregulatory organs to freshwater (FW) environments. In the current study, we characterized circulating thyroxine (T4) and 3-3'-5-triiodothyronine (T3) levels in parallel with the outer ring deiodination (ORD) activities of deiodinases (dios) and mRNA expression of dio1, dio2, and dio3 in gill during the acclimation of Mozambique tilapia (Oreochromis mossambicus) to FW. Tilapia transferred from seawater (SW) to FW exhibited reduced plasma T4 and T3 levels at 6 h. These reductions coincided with an increase in branchial dio2-like activity and decreased branchial dio1 gene expression. To assess whether dios respond to osmotic conditions and/or systemic signals, gill filaments were exposed to osmolalities ranging from 280 to 450 mOsm/kg in an in vitro incubation system. Gene expression of branchial dio1, dio2, and dio3 was not directly affected by extracellular osmotic conditions. Lastly, we observed that dio1 and dio2 expression was stimulated by thyroid-stimulating hormone in hypophysectomized tilapia, suggesting that branchial TH metabolism is regulated by systemic signals. Our collective findings suggest that THs are involved in the FW acclimation of Mozambique tilapia through their interactions with branchial deiodinases that modulate their activities in a key osmoregulatory organ.

Osmoregulatory effects of hypophysectomy and homologous prolactin replacement in hybrid striped bass

The effects of ovine prolactin (oPRL) and striped bass prolactin (sbPRL; Morone saxatilis) on plasma osmolality, electrolyte balance, and gill Na(+),K(+)-ATPase activity were investigated in hypophysectomized (Hx), freshwater (FW)-acclimated, hybrid striped bass (M. saxatilisxMorone chrysops). They were kept in dilute (isoosmotic) seawater for about 10 days after surgery. Seven days after transfer to FW, Hx fish had lower plasma osmolality and lower levels of Na(+), Cl(-), and Ca(2+) than sham-operated and intact fish. Fish were injected four times with oPRL (1, 5, or 20 microg/g body mass), sbPRL (10 or 100 ng/g), or hormone vehicle (0.9% NaCl) at 48-h intervals (days 0, 2, 4, and 6) in FW and then sampled for blood plasma 24 h after the fourth injection (day 7). In Hx fish, oPRL (5 and 20 microg/g) and sbPRL (10 and 100 ng/g) were effective in maintaining plasma osmolality and levels of Na(+), Cl(-), and Ca(2+) above values seen in saline-injected controls. Hypophysectomy did not affect branchial Na(+),K(+)-ATPase activity, but enzyme activity was significantly reduced in Hx fish receiving oPRL (20 mug/g) or sbPRL (10 or 100 ng/g). These results indicate that PRL acts to maintain plasma osmotic and ionic balance in FW-adapted hybrid striped bass, and that this may involve downregulation of branchial Na(+),K(+)-ATPase activity.

Effects of iodothyronines on the hepatic outer-ring deiodinating pathway in killifish

Substrate availability has been thought to be a major regulator of the outer-ring deiodinating pathway (ORD) in fish. However, current information strongly suggests that while fish iodothyronine deiodinase type 2 (D2) responds to iodothyronines in the same manner as its mammalian counterpart, fish deiodinase type 1 (D1) exhibits a distinct response. Furthermore, 3,5-T2, generally considered to be an inactive product of iodothyronine metabolism, has recently been described as bioactive, but its effects upon D1 and D2 are not yet known. We examined the effect that short-term immersion in T4, T3, and 3,5-T2 (0.1 microM; 12 or 24 h) exerts on both D1 and D2 activities and on the levels of expression of D1 and D2 mRNAs in killifish liver. In agreement with previous reports in teleosts, no iodothyronine exerted a significant effect on D1 enzymatic activity. However, all three iodothyronines significantly decreased D2 activity. Furthermore, at 24 h post-immersion T4, T3, and 3,5-T2 inhibited both D1 and D2 transcription. Together, the present results confirm the differential effect of iodothyronines upon the hepatic ORD pathway in fish and show that this effect can occur at a transcriptional level. Furthermore, we provide the first evidence that 3,5-T2 can affect both activity and transcription of hepatic deiodinases in teleosts.

Isolation and characterization of Japanese eel prolactins

A highly purified prolactin (PRL) was isolated from the pituitary of the Japanese eel (Anguilla japonica) by extraction with acid-acetone, gel filtration on Sephadex G-75, and reversed-phase HPLC on TSK-gel ODS 120T and on TSK-gel TMS 250. Eel PRL is comprised of two variants (ePRL I and II), which were separated by HPLC on an ODS column. The two PRLs were also secreted by organ-cultured pituitaries in a defined medium. After being dialyzed against distilled water and lyophilized, the medium was dissolved in 0.01 M ammonium acetate (pH 9.0), and then the insoluble fraction was subjected to HPLC on an ODS column to isolate the secreted PRLs. The ePRL I and II were equipotent in retaining plasma Na in the hypophysectomized killifish, Fundulus heteroclitus, transferred from sea-water to fresh water. The putative PRL-producing cells in the rostral pars distalis of the eel pituitary were specifically stained with the antiserum against the mixture of ePRL I and II. Both PRLs had a molecular weight of 22 kDa in SDS-PAGE, an isoelectric point of 10.1 by gel electrofocusing, and an N-terminal residue of valine. Amino acid compositions and the partial amino acid sequences of ePRL I and II show that they are highly homologous with a limited number of substitutions, and that they are more closely related to those of teleostean PRLs than to those of mammalian PRLs.

Isolation and characterization of carp prolactin

Prolactin (PRL) was extracted with acid-acetone from common carp (Cyprinus carpio) pituitary glands and purified by gel filtration on Sephadex G-75, ion-exchange chromatography on DEAE-cellulose, and reversed-phase high-performance liquid chromatography (HPLC) on TSK-gel TMS 250 with a yield of 0.7 mg/g wet tissue. At each stage of purification, fractions were monitored by HPLC on TSK-gel ODS 120T and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Carp PRL was almost equipotent with ovine PRL in retaining plasma Na concentrations in the hypophysectomized killifish, Fundulus heteroclitus. Immunocytochemistry at both the light and electron microscopic levels revealed that carp PRL antiserum specifically stained cells in the goldfish rostral pars distalis. No cross reaction with putative growth hormone (GH) cells in the proximal pars distalis was observed. The specificity of the carp PRL antiserum was confirmed by immunoblot studies. Although immunostaining of both carp and salmon PRL was observed, there was no cross reaction to GHs from these species. Carp PRL had a sole N-terminal residue of valine, a molecular weight of 23 kDa in SDS-PAGE, and an isoelectric point of 7.3 by gel electrofocusing. Based on these results, together with the knowledge of physicochemical properties of salmon and tilapia PRLs, we propose a standard procedure for isolation of fish PRLs.

Increased gonadotropin levels in goldfish do not result in alterations in circulating thyroid hormone levels

To determine whether increases in gonadotropin levels are capable of altering thyroid function in goldfish, plasma thyroid hormone levels were measured following induced changes in endogenous gonadotropin secretion and injection of carp gonadotropin. Radio-frequency lesions placed in the nucleus preopticus periventricularis or monosodium-L-glutamate-induced lesions of the posterior nucleus lateralis tuberis (NLT) of the hypothalamus were capable of stimulating significant increases in plasma gonadotropin levels, but were without effect on plasma triiodothyronine (T3) or thyroxine (T4) at time intervals ranging from 5 hr to 10 days. Likewise, injections of a superactive analog of gonadotropin-releasing hormone resulted in profound increases in gonadotropin levels without associated changes in thyroid hormones. No changes in the circulating levels of T4 or T3 were observed in response to injection of purified carp gonadotropins whereas injection of bovine thyrotropin or carp pituitary extracts stimulated significant increases in T4. Radiofrequency lesions of the pituitary stalk or of the anterior NLT also resulted in significant increases in circulating levels of T4, but not of T3, at 10 and 30 hr postlesion. These results demonstrate that direct acute stimulation of circulating thyroid hormone levels is not an intrinsic action of endogenous goldfish gonadotropin and that activation of the reproductive system, leading to ovulation in some cases, is without effect on blood total thyroid hormone levels. Additionally, these results confirm that hypothalamic inhibition of the pituitary-thyroid axis exists in this teleost fish and demonstrate that interruption of this inhibition results in a time-dependent, high-magnitude increase in circulating thyroxine levels.

Sodium-retaining activity of chum salmon prolactin in some euryhaline teleosts

Sodium-retaining activity of chum salmon prolactin (PRL) was examined in several euryhaline teleosts. Chum PRL was 100 times more potent than ovine PRL in maintaining plasma sodium levels in the hypophysectomized killifish, Fundulus heteroclitus, transferred from 50% seawater to fresh water. The effects of PRLs were parabolic, high doses of the hormones being less effective than low doses. When injected into seawater-adapted fry of the ayu, Plecoglossus altivelis, or into juvenile rainbow trout, Salmo gairdneri, adapted to 50% seawater, a dose-dependent increase in plasma sodium was observed. Chum PRL was 2-10 times more active than ovine PRL, and the effects in the ayu were also parabolic. An increase in plasma sodium also occurred when the PRLs were injected into the seawater-adapted eel, Anguilla japonica; the chum and ovine PRLs were equipotent, and hypercalcemia was also observed. In contrast, both chum and ovine PRLs were without effect on plasma sodium levels of chum salmon fry, either when injected into seawater-adapted fish kept in seawater or into fish subsequently transferred to fresh water. The absence of an effect of PRLs in chum salmon fry seems to be due, at least in part, to their good osmoregulatory ability during the period of seaward migration; effects of the exogenously administered PRLs may be compensated for by other hormones responsible for their hydromineral balance.

Ultrastructural changes in the parrotfish thyroid afterin vitro stimulation with bovine thyrotropin

Six groups of thyroid glands ofScarus dubius were examined and compared by electron microscopy after anin vitro culture for 4h with graded doses of bovine thyrotropin (bTSH). Five doses of bTSH were used encompassing the full range of the dose-response curve developed for this tissue. Upon electron microscopic examination, micrographs were taken randomly and at the same magnification, and three intracellular inclusions were quantified. The relative surface density of rough endoplasmic reticulum (rER) and the relative surface area of lysosomes and engulfed colloid droplets were recorded for each group. Three treatment groups, (1) control, no bTSH, (2) tissues exposed to 1 mIU/ml bTSH, and (3) tissues exposed to 2 mIU/ml bTSH, did not differ from each other in the quantified organelles nor in general appearance. Overall, these three groups were similar in appearance to the ultrastructure described in other teleosts except for a lack of flagellated cells. Compared to the first three groups, treatment with 5 mIU/ml bTSH, increased the density of rER, and the proportion of cell area occupied by lysosomes and engulfed colloid. This group also possessed either more microvilli or pseudopods at the lumenal surface of the follicular epithelium. After exposure to 10 mIU/ml bTSH there was an even greater increase in surface density of rER, and in surface area occupied by lysosomes and engulfed colloid droplets. The apical portion of this group was highly irregular, commonly displaying pseudopods. Group (6), (20 mIU/ml), showed a decline in cytoplasm in comparison to group (5) with many epithelial cells breaking apart. A few cells in this group were still intact but contained huge engulfed colloid droplets which extended from the basal to apical borders. This first detailed description suggests that the teleost thyroid gland undergoes ultrastructural changes with exogenous TSH stimulation in a manner similar to that seen in higher vertebrates.

The thyroid gland of the Hawaiian parrotfish and its use as an in vitro model system

Thyroid tissue in the Hawaiian parrotfish, Scarus dubius, is organized into two discrete lobes, one anterior to the first pair of afferent branchial arteries and the other between the first and second pairs. Lobes or pieces of thyroid lobes from S. dubius were incubated in static or perifusion culture using a simple defined medium. In static culture, this thyroid tissue releases thyroxine (T4) responding to bovine thyrotropin (bTSH) in a dose-related manner during 4- and 24-hr incubations. At 24 hr, however, substantially lower concentrations of bTSH are required to evoke a significant elevation of T4 than at 4 hr. Exposure to bTSH also elicits morphological changes within 4 hr, which include colloid resorption and an increase in the height of the follicular epithelium. During perifusion culture, T4 release rises rapidly within 20 to 30 min following the initiation of exposure to bTSH and remains elevated for between 4 and 8 hr thereafter. In spite of high plasma triiodothyronine (T3) concentrations, the parrotfish thyroid releases no detectable T3 during in vitro culture. This is the first direct evidence in support of the notion that plasma T3 in a teleost fish may be derived principally, perhaps exclusively, from the peripheral monodeiodination of T4.

Photoperiodic regulation of thyroid responsiveness to TSH in Fundulus heteroclitus

Treatment of the killifish, Fundulus heteroclitus, with ovine thyrotropin (oTSH) produced elevations in serum thyroxin (T4) that varied seasonally with a strong inverse correlation to environmental photoperiod (P less than 0.001). At 20 degrees C, oTSH (0.2 IU) increased serum T4 to levels ranging from 1.5 micrograms/100 ml in midsummer to 7.5 micrograms/100 ml in midwinter despite relatively stable resting levels throughout the two years of study. Similar rates of clearance of T4 in summer and winter suggest that an alteration in thyroid response to oTSH accounts for this change. The serum T4 response to oTSH is both photoperiod- and temperature-dependent. In laboratory studies, fish exposed to a short artificial photoperiod (LD 8:16) for one month responded to oTSH with T4 levels about twice as high as those in fish exposed to long days (LD 14:10). On the other hand the T4 response to oTSH was sharply reduced at 5 degrees C from that seen at 20 degrees C. This suggests that, in nature, seasonal changes in photoperiod and temperature have opposite effects on thyroidal responsiveness to TSH. Overall, the annual variation in the T4 response to oTSH appears to be driven by photoperiod, inasmuch as the alterations preceded major temperature changes in the wild and can be shown to occur at constant temperature in captivity. Photoperiodic induction of changes in thyroid sensitivity may aid in the maintenance of basal T4 levels under changing thermal conditions.

Functional specificity of gonadotropin and thyrotropin in Fundulus heteroclitus

Gonadotropic hormones (GTHs) and thyrotropic hormones (TSHs) reportedly bear close evolutionary and structural relationships, and the thyroid appears to be active in reproduction in some fish species. We tested the sensitivity of the thyroid of Fundulus heteroclitus to glycoprotein hormones from mammalian and piscine sources. Six mammalian glycoprotein hormones, including four gonadotropins and two thyrotropins, produced dose-dependent elevations in serum thyroxin. A release of endogenous gonadotropins was elicited by injecting GnRH. This resulted in gonadal stimulation, with no alteration in circulating thyroxin levels and the rate of radioiodine uptake. We also treated fish with partially purified salmon gonadotropin (SG-G100). The gonadotropic actions of this extract were confirmed by steroid elevations, and again T4 and 125I uptake remained at resting levels. The lack of response of the thyroid gland to fish gonadotropins suggests that TSH receptors in Fundulus heteroclitus can differentiate between endogenous thyrotropin and gonadotropin(s), even though most heterologous glycoprotein hormones are thyrotropic.

Photoperiod-dependent negative feedback effects of thyroid hormones in Fundulus heteroclitus

In Fundulus heteroclitus, an annual cycle in the response of the thyroid to ovine thyroid-stimulating hormone (oTSH) is characterized by maximal thyroxin (T4) secretion in mid-winter and minimal T4 secretion in summer. Four daily injections of oTSH, given in winter caused serum T4 to plateau at elevated levels for several days, while in summer fish similar treatment resulted in far more fluctuating titers of serum T4; maximum levels were similar in both groups. The difference in sustenance rather than magnitude of Peak T4 led to an examination of the negative feedback effects of thyroid hormones as they might relate to these seasonal changes. Radioiodine uptake by thyroid follicles served as a simple, but effective bioassay for endogenous TSH. Fish collected in summer were more sensitive to negative feedback of T3 than those collected in winter; feedback effects of T4 in the two groups were not significantly different. The effects of specific photoperiods on negative feedback sensitivity to T3 and T4 were also tested. Exposure of winter fish for one month to long days (LD 14:10) enhanced the degree of reduction of iodine uptake caused by T4 in the aquarium water (10 micrograms/100 ml). Negative feedback in short-day (LD 8:16) winter fish was not demonstrated. It is concluded that long days increase and short days diminish the negative feedback sensitivity of the hypothalamus-pituitary axis to thyroid hormones in F. heteroclitus. Such photoperiodically induced changes may act to aid in the year-round maintenance of T4 levels necessary for seasonal adaptation and survival.

Bioassay for salmon prolactin using hypophysectomized Fundulus heteroclitus

A bioassay for salmon prolactin (PRL) is described. This assay which is based on the sodium-retaining action of PRL in the hypophysectomized killifish, Fundulus heteroclitus, has proved to be rapid, sensitive (250 pg PRL per gram of fish), and specific. The procedure has been used to characterize the biological activity of a highly purified PRL from the pituitaries of the chinook salmon, Oncorhynchus tschawytscha, and a similar PRL isolated (by acid buffer polyacrylamide disc gel electrophoresis) from pituitaries of coho salmon (O. kisutch) (MW ca. 22,000; isoelectric point greater than 9).

Prolactin--thyroid interaction in Fundulus heteroclitus

Injections of adult Fundulus heteroclitus with 0.2 IU of oTSH over 4 days caused an increase in serum T4, with no effect on serum T3. Administration of 12 IU of oPRL in conjunction with the same dosage of TSH prevented the TSH-induced rise in T4, without affecting serum T3 levels. The failure of TSH to cause T4 levels to rise in the presence of PRL may result from either inhibition of release or acceleration of metabolic clearance of T4. We therefore conducted three experiments to examine potential effects of PRL on the kinetics of peripheral clearance of thyroid hormones. 125I T4 was cleared from serum in a biphasic pattern that was unaltered by PRL. Clearance of labeled T3 followed a similar pattern that was also not influenced by PRL treatment. Generation of labeled T3 by deiodination of a dose of 125I T4 was quantified over a 24-h period. Again, PRL-treated fish showed no significant differences. Since PRL is without effects on thyroid hormone clearance patterns or deiodination rate, we conclude that PRL prevents TSH-induced increases in serum T4 in this species by directly affecting thyroid function.