Characterization and distribution of gonadotrophs in the pars distalis and pars tuberalis of the equine pituitary gland during the estrous cycle and seasonal anestrus

Little is known about the neuroendocrine control of fertility in the horse. In this species, unusual features characterize the normal estrous cycle such as a prolonged preovulatory LH surge during the follicular phase and a distinctive FSH surge during the midluteal phase. This study investigated the distribution and hormonal identity of gonadotrophs in the pars distalis (PD) and pars tuberalis (PT) of the equine pituitary gland as possible morphological bases for the referred unusual endocrine characteristics. In addition, the proportion of gonadotrophs in relation to other pituitary cell types during both the estrous cycle and anestrus were investigated. Pituitary glands were collected from sexually active (n = 5) and seasonally anestrous (n = 5) mares in November, and single or double immunofluorescent staining was carried out on 6-microm sections using monoclonal antibodies to the LHbeta or FSHbeta subunits and a polyclonal antibody to ovine LHbeta. Gonadotrophs were densely distributed around the pars intermedia in the PD and in the caudal ventral region of the PT. In addition to isolated cells, clusters of gonadotrophs were found surrounding the capillaries. No significant differences were detected in the number of gonadotrophs between sexually active and anestrous mares in either the PD or PT. In the PD, gonadotrophs represented 22.7 +/- 5. 8% and 19.1 +/- 2.1% of the total cell density in sexually active and anestrous animals, respectively (P: > 0.05). However, in the PT, gonadotrophs accounted for a higher proportion of the total cell population in sexually active (6 +/- 0.1%) than in anestrous (1.2 +/- 0.05%) mares (P: < 0.02). Double immunofluorescence revealed that the majority of gonadotrophs were bihormonal (i.e., positive for LH and FSH); however, in the sexually active mare, a larger proportion of gonadotrophs (22.5 +/- 3.6%) were monohormonal for either LH or FSH, when compared to anestrous animals (9.7 +/- 1.2%; P: < 0.02). Based on these findings we conclude that: 1) although the relative distribution of gonadotrophs is similar to those reported for other species, a significantly larger proportion of gonadotroph cells is present in the equine pituitary gland; 2) gonadotroph density does not appear to differ between sexually active and anestrous mares in the PD; 3) a larger proportion of gonadotrophs is apparent in the PT of sexually active animals; and 4) although a large incidence of bihormonal gonadotrophs is present in the horse, specific LH or FSH cells differentiate predominantly during the sexually active phase.     

Gonadotropin-releasing hormone stimulates prolactin release from lactotrophs in photoperiodic species through a gonadotropin-independent mechanism

Previous studies have provided evidence for a paracrine interaction between pituitary gonadotrophs and lactotrophs. Here, we show that GnRH is able to stimulate prolactin (PRL) release in ovine primary pituitary cultures. This effect was observed during the breeding season (BS), but not during the nonbreeding season (NBS), and was abolished by the application of bromocriptine, a specific dopamine agonist. Interestingly, GnRH gained the ability to stimulate PRL release in NBS cultures following treatment with bromocriptine. In contrast, thyrotropin-releasing hormone, a potent secretagogue of PRL, stimulated PRL release during both the BS and NBS and significantly enhanced the PRL response to GnRH during the BS. These results provide evidence for a photoperiodically modulated functional interaction between the GnRH/gonadotropic and prolactin axes in the pituitary gland of a short day breeder. Moreover, the stimulation of PRL release by GnRH was shown not to be mediated by the gonadotropins, since immunocytochemical, Western blotting, and PCR studies failed to detect pituitary LH or FSH receptor protein and mRNA expressions. Similarly, no gonadotropin receptor expression was observed in the pituitary gland of the horse, a long day breeder. In contrast, S100 protein, a marker of folliculostellate cells, which are known to participate in paracrine mechanisms within this tissue, was detected throughout the pituitaries of both these seasonal breeders. Therefore, an alternative gonadotroph secretory product, a direct effect of GnRH on the lactotroph, or another cell type, such as the folliculostellate cell, may be involved in the PRL response to GnRH in these species.     

Seasonal variations of gonadotropins in the pars distalis male viscacha pituitary. Effect of chronic melatonin treatment

The gonadotropes, LH and FSH cells, were immunohistochemically identified in the pituitary pars distalis of the adult male viscacha (Lagostomus maximus maximus) using specific antibodies against hLHbeta and hFSHbeta with the streptavidin-biotin-peroxidase complex. The distribution, size and percentage immunopositive area of these cells were analyzed by image analysis in viscachas captured during the annual reproductive cycle and after the chronic administration of melatonin. The LHbeta and FSHbeta cells showed seasonal changes in the distribution, size and percentage immunopositive area. The LHbeta cells were found widely distributed throughout the pars distalis during the reproductive period, and they were found in the ventro-medial region in the pars distalis during the gonadal regression and gonadal recovery periods. The LHbeta cells reached the largest size and immunopositive area during the reproductive period and the smallest size and immunopositive area during the gonadal regression period. The FSHbeta cells were found in the ventro-medial region during reproductive and gonadal regression periods. The FSHbeta cells were found widely distributed throughout the pars distalis during the gonadal recovery period when they showed the maximum percentage immunopositive area. A decrease in the size of LHbeta and FSHbeta cells was observed after the chronic administration of melatonin. Moreover, it produces a decrease in the immunopositive area occupied by the LHbeta cells but not in the immunopositive area occupied by the FSHbeta cells. Our results show great activity of LHbeta and FSHbeta cells in different moments of the annual reproductive cycle demonstrating that these cells do not secrete in parallel. Moreover, melatonin acts differentially on the activity of the gonadotrope cells.     

A physiological model of a circannual oscillator

Recent evidence based on studies in hypothalamo-pituitary disconnected Soay sheep suggests that the generation of circannual rhythms may be local to specific tissues or physiological systems. Now, the authors present a physiological model of a circannual rhythm generator centered in the pituitary gland based on the interaction between melatonin-responsive cells in the pars tuberalis that act to decode photoperiod, and lactotroph cells of the adjacent pars distalis that secrete prolactin. The model produces a self-sustained, circannual rhythm in endocrine output that the authors explore by mathematical modeling. The circannual oscillation requires a delayed negative feedback mechanism. The authors highlight specific features of the pituitary dynamics as a guide to future research on circannual rhythms.     

Expression of the common α-subunit mRNA of glycoprotein hormones during the chick pituitary organogenesis, with special reference to the pars tuberalis

The expression of a common alpha-subunit mRNA of glycoprotein hormones was examined in the pituitary of chick embryos at various stages of development by in situ hybridization with a digoxigenin-labeled quail alpha-subunit cRNA probe. As a comparison with the expression of alpha-subunit mRNA, the onset of luteinizing hormone (LH) immunoreactivity was examined by immunohistochemical staining with a chicken LH antiserum. Both alpha-subunit mRNA and LH immunoreactivity began to appear in the basal-posterior region of the Rathke's pouch at embryonic day (E) 3.5. At E4.5 when the cephalic and caudal lobes of the pars distalis could be distinguished in the Rathke's pouch, intense signal for alpha-subunit mRNA was restricted to the cephalic lobe, consisting of a high columnar epithelium. At E6, gonadotrophs that were ovoid in shape, expressed intense signal for alpha-subunit mRNA, and revealed intense immunoreactivity for LH, were first detected in the cephalic lobe. At this stage, alpha-subunit mRNA expression became weak in the undifferentiated columnar cells of the cephalic lobe. At E8, the pars tuberalis primordium located close to the median eminence was formed at the lateral-apical end of the cephalic lobe. The primordium expressed intense signal for alpha-subunit mRNA. Gonadotrophs showing immunoreactivity for LH were densely distributed throughout the cephalic and caudal lobes in 8-day-old embryos. The pars tuberalis primordium expressing alpha-subunit mRNA progressively extended along the median eminence with embryonal age and reached the rostoral end by E14. Thus, both primordia of the pars distalis and pars tuberalis expressed intense signal for the common alpha-subunit mRNA. This subunit may play a role in the cytodifferentiation of the adenohypophysis.     

Cytological changes in hypophysis of immatur rats after administration of anti-LH antibodies (author's transl)]

In the present work, cytological changes produced in rats' pars distalis were studied by administration of anti-LH antibodies. The anti-LH sera was obtained by active immunization of adult rabbits using bovine LH hormone with Freund's complete adjuvant. Newborn albino Wistar male and female rats were daily and subcutaneously inoculated with 0.1 ml. to 0.8 ml of anti-LH sera from the first 24 hr. on during 5 weeks. For controls, a similar schedule of inoculations with normal rabbit serum was used. At weekly intervals during the treatment, 5 lots of rats were sacrificed and the rest after a recovery period of 95 days. At the end of the 2nd week of immunization, both male and female animals showed degranulated gonadotroph cells in the central part of the pars distalis; at the 3rd week these cells were hypertrophied and presented developed Golgi complex. At the 4th week, the first large vacuoles of "castration" were seen in the gonadotroph cells of male pars distalis, besides in the females the degranulation of gonadotrophs continued. During the last week of treatment, the gonadotrophs of the male animals presented a highly dilated Golgi complex, more "castration-like cells" and numerous mitosis figures. The gonadotrophs of females pars distalis presented degranulation, but not vacuoles. After the recovery period the gonadotrophs of male rats were similar to those of the control hypophisis and did not show castration vacuoles in their cytoplasm. In female rats, the gonadotrophic cells showed "castration-like vacuoles" and a raised number of mitosis cells compared to control pituitaries. The significance of these findings will be discussed.     

Effects of prolactin on the luteinizing hormone response to gonadotropin- releasing hormone in primary pituitary cell cultures during the ovine annual reproductive cycle

In the sheep pituitary, the localization of prolactin (PRL) receptors in gonadotrophs and the existence of gonadotroph-lactotroph associations have provided morphological evidence for possible direct effects of PRL on gonadotropin secretion. Here, we investigated whether PRL can readily modify the LH response to GnRH throughout the ovine annual reproductive cycle. Cell populations were obtained from sheep pituitaries during the breeding season (BS) and the nonbreeding season (NBS), plated to monolayer cultures for 7 days, and assigned to receive one of the following treatments: 1) nil (control), 2) acute (90- min) bromocriptine (ABr), 3) chronic (7-day) bromocriptine (CBr), 4) ABr and PRL, 5) CBr and PRL, 6) PRL alone, or 7) thyrotropin-releasing hormone. Cells were treated as described above, with the aim of decreasing or increasing the concentrations of PRL in the culture, and simultaneously treated with GnRH for 90 min. The LH concentrations in the medium were then determined by RIA. GnRH stimulated LH in a dose-dependent manner during both stages of the annual reproductive cycle. During the NBS, single treatments did not significantly affect the LH response to GnRH. However, when PRL was combined with bromocriptine, either acutely or chronically, GnRH failed to stimulate LH release at all doses tested (P < 0.01). In contrast, during the BS, the LH response to GnRH was not affected by any of the experimental treatments. These results reveal no apparent effects of PRL alone, but an interaction between PRL and dopamine in the regulation of LH secretion within the pituitary gland, and a seasonal modulation of this mechanism.     

Reproductive cycles of horses

Horses are long-day breeders. During the breeding season, cycle length is about 22 days with 5-7 days of oestrus. Gonadotroph cells are localized in the pars distalis as well the pars tuberalis of the pituitary and heterogeneity in the pattern of LH and FSH storage within the gonadotroph population is considered the basis for the differential regulation of gonadotrophin secretion throughout the reproductive cycle. No short and distinct periovulatory LH peak exists in the mare. The equine ovary has an extreme large size and weight. One to two major follicular waves develop per cycle. The preovulatory follicle reaches an average size of 40 mm. Only granulosa cells develop into luteal cells. Progesterone increases at the time of ovulation and reaches maximal concentrations on day 8. Functional luteolysis occurs around day 15 and is initiated by endometrial secretion of PGF(2α). In contrast to other species, no significant luteal oxytocin synthesis exists in the mare. During the oestrous cycle, uterus, vagina and endometrium undergo pronounced changes related to variations in the endocrine milieu. Seasonal reproductive activity is stimulated by photoperiod together with exogenous factors. The anovulatory season can be differentiated into an autumn transitional phase, a mid-anovulatory period and a spring transitional phase bringing the mare back into cyclic activity. During the mid-anovulatory period, follicular development is minimal. The beginning of the spring transitional period is characterized by the development of 1-3 anovulatory follicular waves before ovulation occurs and the most important factor for the re-initiation of ovulatory activity is the occurrence of repeated pronounced increases in circulating LH.     

Localization and development of chromogranin A and luteinizing hormone immunoreactivities in the secretory-specific cells of the hypophyseal pars tuberalis of the chicken

 The pars tuberalis mainly consists of the secretory cells specific to this portion of the pituitary. We examined the localization and development of luteinizing hormone (LH) and chromogranin A in the chicken pars tuberalis by immunohistochemistry. The vast majority of the chicken pars tuberalis was occupied by cells immunoreactive for both LH and chromogranin A. Furthermore, immunoblot analysis of chicken pars tuberalis extracts with LH antiserum demonstrated that two bands, the large α-subunit and small β-subunit of the LH molecule, were expressed in this tissue as well as in the pars distalis. A band for chromogranin A was also detected in pars tuberalis extracts with chromogranin A antiserum. In contrast to the cells of mammalian species that contain only a few small secretory granules, the specific cells of the chicken pars tuberalis were characterized by the presence of many secretory granules ranging from 90 to 400 nm in diameter. Postembedding immunogold labeling showed that gold particles representing immunoreactivity for LH were densely located on all secretory granules of the secretory-specific cells. Many secretory granules, especially the large ones, of the cells were also loaded with immunogold particles for chromogranin A. Double immunogold labeling confirmed that LH and chromogranin A were colocalized on the same secretory granules. During embryonic development, the primordium of the pars tuberalis was first detected at 8 days of incubation as a small group of cells containing LH- and chromogranin-immunoreactive cells. In the pars distalis, the onset of LH and chromogranin expression occurred earlier, at 6 days of incubation. At 10 days of incubation, the pars tuberalis primordium became large cell masses consisting of LH- and chromogranin-immunoreactive cells, which were located close to the median eminence. Subsequently, the primordium extended along the median eminence progressively with age. At 14 days of incubation, it reached to the rostral end and surrounded the median eminence as slender cell cords. These results indicate that specific cells of the chicken pars tuberalis synthesize a glycoprotein hormone related to the LH molecule, which is stored in the secretory granules together with chromogranin A. The pars tuberalis may be involved in the regulation of gonadal function in a different way from that of the pars distalis. Accepted: 26 August 1997     

Effect of [D-Trp6]LHRH infusion on prolactin secretion by perifused rat pituitary cells

The effect of a superactive agonistic analog of luteinizing hormone-releasing hormone (LHRH), [D-Trp6]LHRH on prolactin (PRL) secretion by perifused rat pituitary cells was investigated. Constant infusion of [D-Trp6]LHRH (0.5 ng/min) for 2-3 h elicited a significant decrease in PRL secretion by these cells. This decrease in PRL release started ca. 30 min after the beginning of the infusion with the LHRH analog and lasted up to 1.5-2 h. [D-Trp6]LHRH significantly stimulated luteinizing hormone (LH) secretion during the first 30 min of peptide infusion; thereafter, LH levels began to return to control values. In animals pretreated in vivo with 50 micrograms of [D-Trp6]LHRH (s.c.) 1 h before sacrifice, PRL secretion by the rat pituitary cell perifusion system was significantly lower than vehicle-injected controls throughout the entire [D-Trp6]LHRH infusion period. On the other hand, thyrotropin-releasing hormone (TRH)-stimulated PRL secretion was slightly, but significantly imparied by [D-Trp6]LHRH infusion, while dopamine (DA) inhibition of PRL release was unaffected by this same treatment. These results reinforce previous observations of a modulatory effect of [D-Trp6]LHRH, probably mediated by pituitary gonadotrophs, on PRL secretion by the anterior pituitary. In addition, our findings suggest that basal PRL secretion by the lactotroph may be dependent on a normal function of the gonadotroph. The collected data from this and previous reports support the existence of a functional link between gonadotrophs and lactotrophs in the rat pituitary gland.     

Seasonal changes in the pituitary gland of the feral hawaiian mongoose (Herpestes auropunctatus)

The Hawaiian mongoose (Herpestes auropunctatus) is a seasonally breeding mammal whose pituitary gland resembles that of other Viverridae. Certain features, such as a prominent pars tuberalis interna and a double-layered pars intermedia forming a cup for the neurohypophysis, are unique. With the light microscope, five different cell types can be recognized in the pars distalis after staining with periodic acid-Schiff (PAS)-orange G. Two types of acidophils are seen, a small yellow-staining cell and a large angular orange cell. Two basophilic cells are also seen, one with fine PAS-positive cytoplasmic granules and the other with coarse PAS-positive granules in the cytoplasm. The last cell type seen is the chromophobe. Differential cell counts indicated an altered distribution of chromophils in the ventral pars distalis of the female mongoose with changing season and reproductive status, but the most striking change was a decreased percentage of basophils in the pars distalis during the nonbreeding season.     

Immunocytochemical studies on the pituitary pars distalis of the Japanese long-fingered bat,Miniopterus schreibersii fuliginosus

Summary Immunocytochemical studies were performed to describe the characteristics of cell types and their distribution in the pars distalis of Japanese long-fingered bat, Miniopterus schreibersii fuliginosus, collected at various stages of the reproductive cycle. Six distinct cell types have been identified in the pars distalis by the unlabeled immunoperoxidase technique and by the ABC method. Growth hormone (GH) and prolactin (PRL) cells were immunostained with antisera against chicken GH and ovine PRL. The GH-immunoreactive cells were round or oval orangeophilic cells distributed throughout the pars distalis with prominent aggregation in the posterolateral region. The PRL cells were pleomorphic carminophilic cells that occurred in small groups within the central and dorsocaudal regions of the pars distalis. They were sparsely distributed in the central region of the pars distalis in the hibernating bats, but increased significantly in the pregnant and lactating bats. The adrenocorticotropic (ACTH) cells were large round or polygonal amphophilic cells in the rostroventral and ventrolateral regions of the pars distalis. The thyrotropic (TSH) cells were small rounded or polygonal and distributed mainly in the ventrolateral region of the pars distalis. Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) cells were identified immunocytochemically with antisera against the specific beta subunits of ovine LH and rat FSH. There were two populations of LH and FSH cells, one aggregated in the zona tuberalis and the other scattered singly throughout the rest of the pars distalis. The aggregated cells were immunoreactive with both antisera directed to LH and FSH, while scattered cells were reactive solely with antiserum to either LH or FSH and exhibited seasonal variations. In females, the proportional volume of the pars distalis occupied by LH cells was significantly reduced during pregnancy and lactation. No evidence of involution was observed in pars distalis cells except for PRL cells in males or females during hibernation.     

Cellular associations of pituitary gonadotrophs in a rodent (Lagostomus maximus maximus) with photoperiod-dependent reproduction

The morphological characteristics and percentage of the cellular associations between gonadotrophs (LH- and FSH-secreting cells) and other cellular types were studied in pituitary pars distalis of adult male viscachas (Lagostomus maximus maximus) by double immunohistochemistry using specific antibodies to LH, FSH, PRL, GH, ACTH, TSH and S-100 protein (by folliculostellate cells; FSC), during long and short photoperiods. Bihormonal gonadotrophs were observed in ventro-medial and dorsal regions, interspersed between monohormonal gonadotrophs, and their number increased in short photoperiod. LH- and FSH-gonadotrophs were found around lactotrophs, enclosed by somatotrophs in the dorsal region, and associated with irregular corticotrophs. Gonadotrophs and thyrotrophs were associated along blood vessels and follicular structures. The cytoplasmic prolongations of FSC were in contact with both gonadotrophs. The percentage of LH–FSH, LH–ACTH, LH–FSC, FSH–LH, FSH–PRL, FSH–GH, FSH–ACTH, FSH–TSH and FSH–FSC associations decreased, whereas LH–PRL increased in short as compared to long photoperiod. The most abundant associations were LH–GH and LH–TSH during long photoperiod, but LH–GH and LH–PRL during short photoperiod. FSH–GH and FSH–PRL were the most numerous associations, and LH–FSC and FSH–FSC were the less abundant ones in both photoperiods. These results provide the morphological evidence for specific cellular associations between gonadotrophs and other cellular types of viscacha pituitary.     

Leucine-enkephalin-like immunoreactivity is localized in luteinizing hormone-producing cells in the axolotl (Ambystoma mexicanum) pituitary

In this study, we used immunohistochemical techniques to determine the cell type of leucine-enkephalin (Leu-ENK)-immunoreactive cells in the axolotl (Ambystoma mexicanum) pituitary. Immunoreactive cells were scattered throughout the pars distalis except for the dorso-caudal portion. These cells were immuno-positive for luteinizing hormone (LH), but they were immuno-negative for adrenocorticotrophic, growth, and thyroid-stimulating hormones, as well as prolactin. Immunoelectron microscopy demonstrated that Leu-ENK-like substance and LH co-localized within the same secretory granules. Leu-ENK secreted from gonadotrophs may participate in LH secretion in an autocrine fashion, and/or may participate in the release of sex steroids together with LH.     

Differences in the reproductive endocrine status of ewes in the early post-partum period and during seasonal anoestrus

More (P less than 0.05) post-partum acyclic ewes (8/9) showed evidence of pulsatile LH release than did seasonally anoestrous ewes (2/8). Mean plasma prolactin concentrations were higher (P less than 0.05) in the post-partum ewes than in the seasonally anoestrous ewes. Hypothalamic and pituitary cytosol oestrogen receptor number, median eminence GnRH content and pituitary LH, FSH and prolactin contents were similar in the two groups of ewes. The number of nuclear-bound oestrogen receptors was greater (P less than 0.01) in pituitaries from seasonally anoestrous ewes than in pituitaries from post-partum ewes. These data suggest that the basis of acyclicity in seasonally anoestrous ewes and in post-partum ewes is probably due to their inability to generate LH pulse frequencies similar to that in the follicular phase of the oestrous cycle. The higher LH pulse frequency in post-partum ewes, compared to that in seasonally anoestrous ewes, suggests that pregnancy may partly negate the reduction in LH secretion that is associated with photoperiod-induced acyclicity. The lower number of nuclear-bound oestrogen receptors in post-partum ewes suggests that there may be lower plasma oestrogen levels and less ovarian follicular growth than in non-suckling anoestrous ewes.     

Localization of an endocannabinoid system in the hypophysial pars tuberalis and pars distalis of man

The hypophysial pars tuberalis (PT) acts as an important interface between neuroendocrine brain centers (hypothalamus, pineal organ) and the pars distalis (PD) of the hypophysis. Recently, we have identified an endocannabinoid system in the PT of hamsters and provided evidence that 2-arachidonoylglycerol is a messenger molecule that appears to play an essential role in seasonal reproduction and prolactin release by acting on the cannabinoid receptors in the PD. We now demonstrate the enzymes involved in endocannabinoid synthesis and degradation, namely sn-1-selective diacylglycerol lipase α, N-acylphosphatidylethanolamine-specific phospholipase D, and monoacylglycerol lipase, in the PT of man by means of immunohistochemistry. High-performance liquid chromatography coupled with tandem mass spectrometry revealed 2-arachidonoylglycerol and other endocannabinoids in the human PT. Furthermore, we detected the expression of the cannabinoid receptor 1 (CB1), a primary receptor for endocannabinoids, in the PD. Double-immunofluorescence staining for CB1 and various hypophysial hormones or S-100, a marker for folliculostellate (FS) cells, revealed that CB1 immunoreactivity was mainly localized to corticotrophs and FS-cells. A limited number of lactotrophs and somatotrophs also showed CB1 immunoreactivity, which was however absent from gonadotrophs and thyrotrophs. Our data thus indicate that the human PT comprises an endocannabinoid system, and that corticotrophs and FS-cells are the main target cells for endocannabinoids. The functional significance of this newly discovered pathway remains to be elucidated in man; it might be related to the control of stress responses and/or reflect a remnant seasonal control of hypophysial hormonal secretion.     

Immunocytochemistry of somatotrophs,gonadotrophs, prolactin and adrenocorticotropin cells in larval sea bream (Sparus auratus) pituitaries

Summary The chronological appearance of endocrine cells in the pituitary of sea-bream (Sparus auratus) larvae was studied using antisera against salmon prolactin, trout growth hormone, salmon gonadotropin and N-terminal human adrenocorticotropin. The larval pituitary (1–12 days after hatching) was oval in shape and was composed of a dense mass of cells with few neurohypophysial fibres. By 60 days after hatching it began to resemble the adult and was divisible into a distinct rostral pars distalis containing prolactin and adrenocorticotropin cells; a proximal pars distalis containing somatotrophs and gonadotrophs and a pars intermedia. Cells immunoreactive with antisera against growth hormone were observed immediately after hatching (2 days post-fertilization). Weakly staining prolactin cells were observed 2 days later in the region corresponding to the rostral pars distalis. Cells immunoreactive with anti-gonadotropin and anti-adrenocorticotropin sera were observed in the pituitary 6 and 8 days after hatching, respectively. All the cell-types studied were immunoreactive from the time they were first identified until the final samples 90 days after hatching.     

Orexin-B-like immunoreactivity localizes in both luteinizing-hormone-containing cells and melanin-concentrating hormone-containing fibers in the red-bellied piranha (Pygocentrus nattereri) pituitary

We examined orexin-like immunoreactivity in the pituitary of the red-bellied piranha (Pygocentrus nattereri). Orexin-B-immunoreactive (IR) cells corresponded to luteinizing hormone (LH)-containing cells in the pars distalis, and orexin-B-IR fibers corresponded to melanin-concentrating hormone (MCH)-containing fibers in the pars nervosa. In the pars distalis, orexin-B-IR puncta that were also immunoreactive for MCH were observed around the orexin-B-IR cells. In the ventral hypothalamus, orexin-B-IR and MCH-IR neurons were found in the nucleus lateralis tuberis. Immunoelectron-microscopic analysis revealed that the orexin-B-like substance co-localized with LH in secretory granules and with MCH in MCH-containing neurons. Some of the MCH secreted in the pituitary might participate in the modulation of LH secretion from the gonadotrophs, together with orexin-B, leading to food intake by the stimulation of growth hormone secretion from the somatotrophs.     

Prolactin receptor signaling mediates the osmotic response of embryonic zebrafish lactotrophs

The pituitary hormone prolactin (PRL) regulates salt and water homeostasis by altering ion retention and water uptake through peripheral osmoregulatory organs. To understand the role of osmotic homeostasis in the development of PRL-secreting lactotrophs, we generated germline transgenic zebrafish coexpressing red fluorescent protein directed by Prolactin regulatory elements (PRL-RFP) and green fluorescent protein by the Pro-opiomelanocortin promoter (POMC-GFP). Transparent embryos expressing fluorescent markers specifically targeted to lactotrophs and corticotrophs, the two pituitary lineages involved in teleost osmotic adaptation, allowed in vivo dynamic tracing of pituitary ontogeny during altered environmental salinity. Physiological osmotic changes selectively regulate lactotroph but not corticotroph proliferation during early ontogeny. These changes are not suppressed by pharmacological dopamine receptor blockade but are completely abrogated by morpholino knockdown of the PRL receptor. PRL receptor signaling exerts robust effects on lactotroph development and plays a permissive role in lactotroph osmo-responsiveness, reflecting the dual peripheral and central interactions required for early pituitary development and embryonic homeostasis.