Monika Schmitz

Endocrine Evidence that Silvering, a Secondary Metamorphosis in the Eel, Is a Pubertal Rather than a Metamorphic Event

Silvering (transition from yellow to silver eel) has been traditionally considered as a metamorphosis in view of the numerous morphological, physiological and behavioral changes preparing the eel for the oceanic migration. However, some changes, such as increases in gonad weight and steroidogenesis, suggest that silvering could also be considered as a pubertal event. In order to assess which endocrine axis may be involved in the induction of silvering, we compared the profiles of pituitary and peripheral hormones during the transition from yellow to silver female eels. A strong activation of the gonadotropic axis was shown during silvering. Follicle-stimulating hormone (FSH) mRNA levels increased during the early stages of silvering, followed by a later increase in luteinizing hormone (protein and mRNA) levels. In addition, plasma levels of sexual steroids (estradiol, E2; testosterone, T, and 11-ketotestosterone) and of vitellogenin significantly increased. In contrast, thyrotropin mRNA levels did not change and no or weak variations in plasma thyroid hormones were observed, indicating no or moderate change of the thyrotropic axis during silvering. Similarly, the somatotropic axis was not activated, as shown by pituitary growth hormone expression (protein and mRNA) and plasma levels. In addition, we studied the effects of chronic treatments of female yellow eels with thyroid hormone (thyroxine, T4) and sex steroids (T and E2) on biometrical parameters characteristics of silvering. T induced an increase in eye size and a reduction of digestive tract, whereas T4 and E2 had no effect. These hormonal profiles and experimental data lead to the conclusion that eel silvering should be considered as an onset of puberty rather than a ‘genuine’ metamorphosis.

Use of the Passive Integrated Transponder (PIT) in a Fish Identification and Monitoring System for Fish Behavioral Studies

Abstract Use of the passive integrated transponder (PIT) as a fish identification and monitoring system for behavioral study of Arctic char Salvelinus alpinus was evaluated. The system was developed in order to track individual differences, mainly in rheotactic behavior. In a preliminary experimental setup, Arctic char (N = 20, 9–140 g each) were PIT-tagged and their movements were recorded at regular intervals in a circular stream channel. Two PIT tag loop detectors, placed on a narrow part of the channel, automatically recorded identity and swimming direction of tagged fish. We demonstrate some of the applications in behavioral research, such as individual, general, and diel locomotor activity patterns, rheotactic behavior, and sociograms showing activity relationships among individuals within a group.

Differential Regulation of Luteinizing Hormone and Follicle-Stimulating Hormone Expression during Ovarian Development and under Sexual Steroid Feedback in the European Eel

Pituitary gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are, in teleosts as in mammals, under the control of hypothalamic factors and steroid feedbacks. In teleosts, feedback regulations largely vary depending on species and physiological stage. In the present study the regulation of FSH and LH expression was investigated in the European eel, a fish of biological and phylogenetical interest as a representative of an early group of teleosts. The eel FSHβ subunit was cloned, sequenced and together with earlier isolated eel LHβ and glycoprotein hormone α (GPα) subunits used to study the differential regulation of LH and FSH. In situ hybridization indicated that FSHβ and LHβ are expressed by separate cells of the proximal pars distalis of the adenohypophysis, differently from the situation in mammals. The profiles of LHβ and FSHβ subunit expression were compared during experimental ovarian maturation, using dot-blot assays. Expression levels for LHβ and GPα increased throughout ovarian development with a positive correlation between these two subunits. Conversely, FSHβ mRNA levels decreased. To understand the role of sex steroids in these opposite variations, immature eels were treated with estradiol (E2)and testosterone (T), both steroids being produced in eel ovaries during gonadal development. E2 treatment induced increases in both LHβ and GPα mRNA levels, without any significant effect on FSHβ. In contrast, T treatment induced a decrease in FSHβ mRNA levels, without any significant effect on the other subunits. These data demonstrate that steroids exert a differential feedback on eel gonadotropin expression, with an E2-specific positive feedback on LH and a T-specific negative feedback on FSH, leading to an opposite regulation of LH and FSH during ovarian development.

Seasonal changes in hypoosmoregulatory ability in landlocked and anadromous populations of Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar)

SynopsisSeasonal changes in hypoosmoregulatory ability were compared in landlocked and anadromous strains of Arctic charr and Atlantic salmon. Seawater adaptability was assessed using periodic 48 h seawater challenge tests with 25‰. seawater. The landlocked strains of Arctic charr, two from northern Sweden and one from Southern Norway, displayed similar seasonal changes in seawater adaptability as the anadromous strain. Seawater tolerance increased during spring and remained high until the end of July — early August after which it declined. The two strains of Atlantic salmon displayed different seasonal patterns in hypoosmoregulatory ability. The anadromous strain showed a pronounced seasonal pattern with maximal seawater adaptability in early June. In contrast, seawater tolerance in the landlocked strain improved steadily during spring and remained high until late autumn. During the period of enhanced seawater tolerance, hypoosmoregulatory ability increased significantly with body size in both Arctic charr and anadromous Atlantic salmon. The minimum size at which fish were able to regulate plasma sodium following seawater transfer at a level comparable to freshwater levels (<170 mmol I−1) differed significantly between anadromous Atlantic salmon (ca. 14 cm) and Arctic charr (ca. 22 cm). The results show that seasonal changes in hypoosmoregulatory ability are present in both Atlantic salmon and Arctic charr, and that these physiological traits are retained in the corresponding landlocked strains. However, the seasonal pattern of seawater adaptability as well as the minimum size at which seawater tolerance occurs differs between the two species.

Growth hormone response to seawater challenge in Atlantic salmon, Salmo salar, during parr-smolt transformation

Abstract The relation between plasma growth hormone (GH) levels and hypo-osmoregulatory ability during parr-smolt transformation was studied in 2-year-old immature parr and previously mature male parr of two different Baltic stocks of Atlantic salmon ( Salmo salar ). At several times between early May and the end of June fish were transferred to seawater (25‰ salinity) and sampled after 24 h. Among freshwater controls plasma GH levels increased only slightly in one stock during the smoltification period, whereas GH levels were significantly elevated in mid-June in the other stock. Seawater adaptability reached an optimum by the end of May to early June in one stock and 2 weeks later in the other one. Plasma GH levels in fish exposed to seawater were similar to those of freshwater fish until mid-May, but increased to significantly higher levels at the time of peak seawater adaptability. Changes in plasma sodium levels during seawater exposure were negatively correlated to plasma GH levels. At the time of peak seawater adaptability two groups were detectable among previously mature males: one group had plasma sodium levels comparable to those of immature fish, while the other had significantly higher sodium levels. In one stock plasma GH showed a similar bimodal distribution, where the group with high GH levels corresponded to low plasma sodium levels and the group with low GH levels had significantly higher sodium levels. The results indicate that some Baltic stocks of Atlantic salmon develop an ability to respond to seawater exposure with a rapid increase in GH levels during the parr-smolt transformation. It is suggested that increasing environmental salinity triggers the final development of hypo-osmoregulatory ability.

Effect of androgens on seawater adaptation in Arctic char, Salvelinus alpinus.

Sexually immature two-year old Arctic char (Salvelinus alpinus) were implanted with Silastic capsules containing testosterone or 11-ketoandrostenedione in early spring. Seawater adaptability of the hormone-treated and sham-operated fish was tested periodically from May to August using a 48h seawater challenge test with 25‰ seawater. The sham-operated control fish displayed a seasonal pattern in seawater adaptation, showing a good hypoosmoregulatory ability until mid June followed by a marked increase in plasma sodium and magnesium levels in July and August. Gill Na+xadK+-ATPase activity decreased concurrently with the observed decrease in seawater adaptability. Over the same period the androgen-treated fish displayed a similar pattern in seawater adaptability, however, in May and June the plasma sodium levels were significantly higher in both androgen-treated groups compared to the control group. Plasma magnesium regulation was impaired in both androgen-treated groups in August. Gill Na+xadK+-ATPase activity in the testosterone-treated fish was lower in June compared to the control fish, whereas the activity was not affected by 11-ketoandrostenedione treatment. The results show that while androgens impair the hypoosmoregulatory capacity in Arctic char, the seasonal pattern of seawater adaptability is not affected.

LH-β and FSH-β mRNA expression in nesting and post-breeding three-spined stickleback, Gasterosteus aculeatus, and effects of castration on expression of LH-β, FSH-β and spiggin mRNA

The mRNA expression of the LH-β and FSH-β subunits were measured in nesting and post-breeding male three-spined sticklebacks, Gasterosteus aculetaus, as well as in castrated and sham-operated nesting males. Furthermore, expression of an androgen induced kidney protein, spiggin, and 11-ketotestosterone (11KT) levels, were measured in the castrated and sham-operated males. Nesting males had significantly higher levels of both LH-β and FSH-β mRNA expression compared to post-breeding males. Furthermore, sham-operated males had significantly higher levels of LH-β mRNA and spiggin mRNA expression than the castrated fish. Expression of FSH-β, on the other hand, did not differ between castrated and sham-operated males. There were strong positive individual correlations between circulating levels of 11KT on the one hand and expressions of LH-β and spiggin mRNA, whereas the correlation between 11KT levels and FSH-β mRNA was weak. The negative effect of castration on β-LH mRNA indicates that gonadal hormones stimulate this expression, whereas this was not the case for β-FSH. The observed decline in β-LH expression after the end of the breeding season may be the result of cessation of the gonadal stimulation of the pituitary. On the other hand, it is not likely that this can explain the decline in FSH-β expression.

Differential effect of insulin-like growth factor I on in vitro gonadotropin subunits expression in Atlantic salmon

In the present study a serum-free primary culture of pituitary cells was used to study the direct effect of insulin-like growth factors (IGFs) on gonadotropin gene expression in immature Atlantic salmon. Treatment with IGFxa0I increased the expression of the luteinizing hormone (LH) β subunit in a dose-dependent manner. IGFxa0I and IGFxa0II had similar potencies, but insulin was less effective in stimulating LH-β subunit expression. On the other hand, IGFs or insulin had no effect on the expression of the follicle-stimulating hormone (FSH) β-subunit. The results indicate that the stimulatory effect of IGFxa0I on LH synthesis is exerted through a specific increase of the transcription level of the LH β subunit.

Cloning and sequencing of FSH-β and LH-β subunits and changes in their expression during the reproductive cycle in the three-spined stickleback, Gasterosteus aculeatus

To achieve a better understanding of the role of gonadotropins (GTHs) in the stickleback we have cloned the full-length cDNAs of the β-subunits of follicle stimulating hormone (FSH) and luteinizing hormone (LH), and analysed the expression during the seasonal cycle. In females, LH-β levels were low during winter and early spring, increased to a peak in late May and declined to low levels again in July. FSH-β expression peaked earlier, in January and declined spring. In males, LH-β expression peaked in May. During June–September, when spermatogenesis occurs, LH-β levels were very low. FSH-β expression peaked earlier, in January, and reached the lowest levels in July. Thus, when spermatogenesis starts, the expression of both GTH-β mRNAs display their lowest levels.