Ultrastructure of the eta cells in the pituitary gland of adult migratory sockeye salmon (Oncorhynchus nerka)

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.

Prolactin and growth hormone secretion during long-term incubation of the pituitary pars distalis of mature chum salmon, Oncorhynchus keta

The effects of hypotonicity of the medium on prolactin (PRL) and growth hormone (GH) release from the pars distalis (PD) of mature chum salmon pituitary were examined during culture for 4 days. Large amounts of PRL and GH were released during the first 6 hr and secretion decreased rapidly thereafter. The amounts of GH released as well as the residual content in PD were 10 times greater than those of PRL. There was no difference in PRL or GH release between the PD incubated in isotonic medium (325 mOsm) and those in hypotonic medium (250 mOsm) even during the first 6 hr. Female PD secreted more PRL than male PD during the first 6 hr, whereas a sex difference was not observed in GH release. Considerable amounts of both PRL and GH remained in the PD at the end of the culture period. The amount of PRL released during 4 days of culture from female PD (37-39%) was greater than that from male PD (23-26%), and the amount of GH released was less in females (28-32%) than that in males (53-54%). Release of PRL and GH appears to be mainly under stimulatory hypothalamic control in the mature chum salmon.

Plasma and pituitary prolactin levels in rainbow trout during adaptation to different salinities

The development of a highly specific radioimmunoassay for salmonid prolactin (PRL) using chinook salmon PRL allowed us to study plasma and pituitary PRL profiles in large sedentary rainbow trout (Salmo gairdneri) transferred from fresh water to seawater and vice versa. Plasma osmotic pressure and chloride levels were also measured for 3 weeks following change of salinity. Within 1 day after transfer to full seawater we observed a plasma PRL decrease, which stayed significantly lower (3-5 ng/ml) than the fresh water control group (10-15 ng/ml) during the entire experiment. Pituitary PRL content showed an initial abrupt increase, but after 3 weeks in seawater pituitary PRL content had decreased to the same level as in the fresh water control group. On the contrary, transfer from seawater to fresh water was followed within 1 day by a rise in plasma PRL levels, which stayed high (10-15 ng/ml) after 3 weeks in fresh water. Simultaneously, pituitary PRL content decreased significantly. These results may indicate an important role of PRL in fresh water adaptation of sedentary rainbow trout.

Immunocytochemistry of gonadotropic cells and identification of cell types in ultrathin cryosections of the pituitary of the rainbow trout, Salmo gairdneri

The most frequently occurring cell types in the pars distalis of the pituitary gland of the rainbow trout, (i) the lactotropic, (ii) the gonadotropic, and (iii) the somatotropic cells, were identified in cryosections. Their morphological characteristics were compared with those of Epon-embedded material. Cell location, cell form, position of the nucleus, arrangement of rough endoplasmic reticulum and sizes of secretory granules proved to be useful parameters for identification. The size distribution of secretory granules of corresponding cells in cryosections and Epon sections proved to be similar. Additionally, both the immunoferritin and the unlabeled antibody enzyme method were applied for the immunocytochemical labeling of gonadotropic hormone-producing cells in cryosections. Anti-salmon-GTH as well as anti-carp-GTH serum showed the presence of GTH in both the smaller and the larger granules of the classical GTH cells, but also produced a reaction in TSH cells. Labelling of TSH cells was absent when using anti-beta-carp-GTH. Specificity of the reaction depended upon the degree of dilution of the anti-GTH serum. Results with dilutions of 1:4,000 and 1:8,000 in the unlabeled antibody enzyme method, and of 1:8,000 up to 1:32,000 in the immunoferritin technique were optimal. Acid phosphatase activity in the smaller granules was demonstrated by enzyme cytochemistry in Epon sections. The relationship of the presence of hormone in these granules is discussed. The high sensitivity of the immunocytochemical labeling procedure is discussed with respect to cryo-ultramicrotomy.

Cellular composition of the rostral pars distalis of the anterior pituitary gland of the alewife, Alosa pseudoharengus, during the spawning run

The rostral pars distalis of the anterior pituitary gland of the marine alewife, Alosa pseudoharengus, during its annual spawning run to fresh water was examined histologically. The rostral pars distalis is composed of many interconnecting follicles of various sizes. Contrary to earlier reports, the follicular epithelium contains not only prolactin (PRL) cells but corticotropic (ACTH) cell and thyrotropic (TSH) cells (in addition to two nonendocrine cell types). Basally all three endocrine cell types make direct contact with the basement membrane which separates the follicles from the neurohypophysial processes. Apically, however, only the prolactin cells, the largest of the three, protrude into the follicular lumen by means of the small ciliated apical protruberance. All other cellular elements are sealed from the follicular lumen by a layer of covering cells which have properties of transitional epithelial cells. In the follicular epithelium, the slender TSH cells are intercalated between the large conspicuous prolactin cells. The ACTH cells, the smallest of the three endocrine cells, lie in deep invaginations in the basal regions of the individual PRL cells in such a way that on cursory examination they can be mistaken for the nuclei of the latter. Only a small portion of the cellular surface of the ACTH cell escapes the enveloping prolactin cell to make contact with the basement membrane of the follicle. In teleosts, prolactin, ACTH, and TSH have all been implicated in the regulation of hydromineral metabolism and reproductive development. The intimate spatial relation between the three endocrine cells in the alewife rostral pars distalis thus raises the possibility of some functional interactions at the adenohypophysial level, perhaps as an adaptation of this anadromous teleost whose reproductive development and behavior is associated with large changes in ambient salinity. The functional significance of the follicular lumen is discussed together with possible sensory functions of the PRL cells.

The influence of social rank on adenohypophysial cell activity in Salmo irideus. With special reference to basophil ACTH-cells of the proximal pars distalis

In nine cell types of the adenohypophysis in untreated adult rainbow trout, histologically different activity phases, seasonal changes in activity, and the relation between certain cell types and the interrenal gland, thyroid or gonads were investigated by light and, occasionally, by electron microscopy. Special attention was given to the effect of social rank on the synthetic activity in adenohypophysial cells of trout kept in small groups in which a social hierarchy with one (light) dominant and several (dark) submissives is established. Cell types in the rostral pars distalis were azocarminophil (I) or amphiphil (II). Proximal pars distalis cell types were slightly basophil (IV), orangeophil (V), strongly basophil (VI) or chromophobe (VII). In the pars intermedia, cell types were amphiphil (VIII) or very slightly basophil (IX). Type III was a non-secretory supporting (?) cell. Histologically different activity phases abounded in type IV cells, which mainly occurred in the proximal pars distalis but were also found dispersed in the rostral pars distalis, the pars intermedia and the neurohypophysis. Influences of social rank were pronounced in type IV cells. Phases with a high synthetic activity were exclusively found in submissive animals, phases with a low synthetic activity occurred in dominants. As a positive relation existed between type IV cell activity and the social rank dependent activity of the interrenal gland, it was suggested that type IV cells produce ACTH. In (dominant) male trout treated with DOCG or ACTH, colloid-containing type IV cell phases, reflecting accumulation of the secretory product, were found. This supported the earlier suggestion that ACTH in the trout is produced in the basophil type IV cells and not, as reported in the literature, in cells comparable to type II.