Structure and function of the transplanted pituitary in the seawater goby, Gillichthys mirabilis. I. The rostral pars distalis

Osmoregulation in the mudskipper,Boleophthalmus boddaerti II. transepithelial potential and hormonal control

Boleophthalmus boddaerti submerged in 10%, 50% and 80% seawater (sw) for 7 days, had whole body transepithelial potentials (TEP) of 3.3, 18.3 and 22.9 mV, respectively. Hypophysectomy significantly decreased the TEP ofB. boddaerti and reversed the polarity of the TEP of the fish exposed to 10% sw.Hypophysectomy also significantly decreased the branchial Na(+)-K(+) activated adenosine triphosphatase (Na(+),K(+)-ATPase) activity but increased the activity of branchial HCO3 (-)-Cl(-) stimulated adenosine triphosphatase (HCO3 (-),Cl(-)-ATPase) inB. boddaerti exposed to 10% sw. However, survival in 10% sw was not significantly impaired by hypophysectomy and no significant change in plasma osmolality and plasma Na(+) and Cl(-) concentrations was observed.Various doses of ovine-prolactin or salmon-prolactin were unable to restore the TEP of hypophysectomizedB. boddaerti in 10% sw to that of the sham-operated fish. However, cortisol increased TEP to a positive value in hypophysectomizedB. boddaerti, though it was still lower than the sham-operated control. Cortisol treatment also affected the plasma osmolality, plasma Na(+) and Cl(-) contents and branchial Na(+),K(+)-ATPase and HCO3 (-),Cl(-)-ATPase activities. Overall, the hormonal control of osmoregulation inB. boddaerti appeared to differ from that of other teleosts.

Hypothalamic control of prolactin release in the rainbow trout,Salmo gairdneri: in vitro studies

Hypothalamic control of prolactin (PRL) release in immature rainbow troutSalmo gairdneri was investigated using anin vitro perifusion system of the rostral pars distalis. Hypothalamic extract of trout induced a dose-dependent stimulation of PRL release. A similar effect was observed when infusing the medium from a 24h static incubation of the hypothalamus. Extracts from different control tissues (muscle, liver, gut) did not changein vitro release, thus confirming the specificity of this stimulatory effect. Hypothalamic extract from adult male rat, known to contain PRL release inhibiting factors, stimulatedin vitro PRL secretion in rainbow trout. This suggests that PRL cells are predominantly influenced by PRL releasing factors. Measurement of TRH and serotonin content in trout hypothalamus indicated consistent physiological levels of these two factors. HPLC studies of hypothalamic extract showed that immunoreactive - TRH eluted at the same place as labelled TRH standard. Moreover, pizotifen, a serotonin antagonist, partially inhibited the stimulation observed with trout hypothalamic extract. These results suggest that, in immature rainbow trout, PRL release is under stimulatory hypothalamic control and that serotonin and probably TRH play a major role in this control.

Control of prolactin secretion in the teleost Oreochromis mossambicus: effects of water acidification

Prolactin secretion is stimulated markedly in fish exposed to water of pH 4. This phenomenon was used to study the control of prolactin secretion. Activation occurs irrespective of changes in plasma osmolarity or plasma sodium and calcium concentrations. After acute acidification of the water, which leads to a substantial fall in plasma osmolarity and plasma electrolyte levels, the activation of the prolactin cells is less marked than after gradual acidification of the water, when plasma osmolarity, plasma sodium, and plasma total and ionic calcium levels are not noticeably affected. When fish bearing an implanted rostral pars distalis of the pituitary gland are exposed to water of pH 4, both the in situ prolactin cells and the prolactin cells of the implant become activated only when the drop in water pH is acute and followed by a reduction in plasma osmolarity and electrolyte levels. When the rate of reduction of the pH is slow and not changing plasma osmolarity or sodium and calcium levels, the in situ prolactin cells are stimulated, but not those of the implants. We conclude that the activation of the prolactin cells in situ in fish in acid water is not mediated by reductions in plasma osmolarity, plasma sodium, total calcium, or ionic calcium, but by hypothalamic mechanisms. The drop in plasma osmolarity and electrolytes probably reflects excessive osmoregulatory stress and this may hamper instead of stimulate the response of the prolactin cells to osmoregulatory disturbance. The physiological significance of the in vitro activation of prolactin cells by reduced ambient osmolarity is unclear.

Effect of osmotic pressure on prolactin release in rainbow trout: in vitro studies

To investigate a possible effect of osmotic pressure on prolactin (PRL) release in rainbow trout, we developed a technique for in vitro perifusion of trout pituitaries. Changes in osmotic pressure similar to those observed in fish plasma during transfer experiments did not induce significant modifications of PRL release. In contrast, high-amplitude variation of osmotic pressure resulted in clear modifications of PRL secretion: hyperosmotic medium caused a reduction in PRL release, while infusion of hyposmotic medium induced a transitory increase in PRL release. By using different concentrations of mannitol, we found that the modifications of prolactin secretion could not be ascribed to alterations of the ionic composition of the medium but actually resulted from variations in the osmotic pressure of the incubation medium. In further experiments osmotic pressure was decreased from 300 to 220 mOsm/kg or from 400 to 300 mOsm/kg; a similar transitory increase in PRL release was observed. Measurement of gonadotropin (GtH) in the perifusion effluent medium showed that PRL and GtH secretion followed similar patterns. Thus, our results suggest a possible mechanical effect of wide changes in osmotic pressure on pituitary cell membranes. These data indicate that the rainbow trout differs notably from nonsalmonid teleost species thus far studied in the lack of sensitivity of its PRL cells to osmotic pressure.

The effect of prolactin on fanning behavior in the male three-spined stickleback, Gasterosteus aculeatus L

The effect of administration of homologous prolactin on fanning behavior, an important aspect of parental care in sticklebacks and many other teleost fish, was studied. Prolactin was administered by implantation of an additional pituitary prolactin lobe in the dorsal body musculature of males with a nest without eggs. The low but rather constant level of fanning behavior is significantly increased from Day 6 to Day 12 after administration of the prolactin lobe. The implantation has no noticeable effects on cell and nuclear size or on the ultrastructure of the in situ prolactin cells of the recipient fish. Recovered prolactin lobe implants show two categories of prolactin cells. The major category consists of cells that are well developed and, similar to in situ prolactin cells, show structural signs of high secretory activity. The other prolactin cell category in the implants shows signs of cellular involution. Ten days after implantation, the first category predominates, and 16 days after implantation the second category. Prolactin lobe implantation increases the number of mucocytes in the epithelium of the skin, in particular at Day 10, and to a lesser extent at Day 16. This is considered evidence for a transient rise in blood prolactin levels in the recipient fish. We conclude that prolactin stimulates fanning behavior in male sticklebacks.

Evidence for dopaminergic and serotonergic regulation of prolactin cell activity in the trout Salmo gairdneri

The control of prolactin (PRL) cell activity in Salmo gairdneri was investigated in vivo and in vitro. In some in vivo experiments treatment was followed by estimation of pituitary PRL content by gel electrophoresis or of PRL cell nuclear area by light microscopy. In the remainder, treatment was followed by incubation of the pituitary glands in drug-free medium for estimation of PRL synthesis and release. The dopamine precursor, L-dopa (20 mg/kg), reduced pituitary PRL content. Conversely, the dopamine-receptor blocker, domperidone (10 mg/kg), increased total PRL content and amount released in the subsequent incubation. The initial serotonin precursor, L-tryptophan (75 mg/kg), increased pituitary PRL content and PRL cell nuclear area. 5-HTP (20 mg/kg), the immediate serotonin precursor, increased both percentage PRL release and total PRL levels during subsequent incubation. Pargyline (25 mg/kg) treatment to inhibit serotonin catabolism elevated PRL levels in pituitary and medium during subsequent incubation. The serotonin synthesis blocker, parachlorophenylalanine (pCPA; 100 mg/kg), nonsignificantly reduced PRL cell nuclear area. When this was followed by incubation, percentage PRL release and total PRL fell significantly. During in vitro incubation, dopamine (2 micrograms/ml) reduced the release of PRL into the medium, while serotonin (10(-5) M) increased PRL release. These results suggest that both an inhibitory dopaminergic and a stimulatory serotonergic system may be involved in PRL cell regulation in S. gairdneri. The lack of any significant effect of cortisol (1 microgram/ml), somatostatin (300 ng/ml). GABA (100 mg/ml) and TRH 100 ng/ml) on PRL release in vitro suggested little or no involvement of these putative regulatory factors in PRL cell regulation.

[Ultrastructure of prolactin cells and survival of Gambusia sp. (teleost fish) in deionized water enriched in calcium or sodium]

The ultrastructure of the prolactin (PRL) cells of Gambusia was studied in animals kept in deionized water (EDes) and in EDes supplemented either with Ca2+ (10,2 and 15,3 mM) or with Na+ (10,2 and 15,3 mM). In environments supplemented with CaCl2 the maximal survival was 40 days. The stimulation of PRL cells was similar to that described in EDes. On the contrary, in environments supplemented with NaCl, 50% of the animals are still alive after 45 days. PRL cells are slightly stimulated and their ultrastructural aspect is similar to that observed in controls kept in freshwater. The significance of these results is discussed in relation with our present knowledge of the pituitary control of osmoregulation in teleost fish.

Effects of external osmolality, calcium and prolactin on growth and differentiation of the epidermal cells of the cichlid teleost Sarotherodon mossambicus

Osmolality and concentrations of divalent cations--calcium, and to a lesser extent magnesium--of the water are the main environmental factors that determine development and degree of mucification of the skin epithelium of Sarotherodon mossambicus. Epithelial thickness and number of mucocytes in fish exposed to low (freshwater level) concentrations of calcium and magnesium are directly related to the height of the osmotic gradient between water and blood plasma. No such relationship is found in fish exposed to a high (seawater level) concentration of calcium in the water, irrespective of the height of the osmotic gradient. The results strongly indicate that the effects of osmolality and divalent cations are indirect, and mediated by prolactin, since administration of ovine or fish prolactin stimulates growth and multiplication of the cells of the basal layer of the epidermis, and promotes the differentiation of the mucocytes.

Relation of mitochondria-rich chloride cells to active chloride transport in the skin of a marine teleost

Mitochondria-rich cells in the skin of the marine teleost, Gillichthys mirabilis, were found to be ultrastructurally similar to typical chloride-secreting cells of marine fish gill, but had a tall, spindlelike shape due to the thickness of the stratified epithelium. The fluorophore, dimethylaminostyrylethyl-pyridiniumiodide (DASPEI), was used to visualize and count skin chloride cells so that cell density could be regressed against the in vitro short-circuit current (ISC) measured on the same tissue. The regression (r2 = 0.76; n = 72) demonstrated that chloride cells are responsible for anion transport across Grillichthys skin.

The neurosecretory system of the adult Melanogryllus desertus Pall. (Orthoptera, Gryllidae). I. Ultrastructural study of the median neurosecretory cells in the brain

Based on the nature of their granules, eight prinicipal types of neurons, six of which are thought to be neurosecretory, are recognized in the median neurosecretory cell group of the brain of Melanogryllus desertus. Most of the neurosecretory cells contain granules with diameters of 200-300 nm. In a few the granules are smaller with diameters varying from 60-100 nm. Most of the cells have well developed Golgi areas and dense bodies of different sizes. Dense bodies are closely associated the neurosecretory granules. Accumulations of electron-dense granular material occur in expanded cisterns of endoplasmic reticulum, particularly in type-I cells.