Ontogenesis of proopiomelanocortin gene expression and regulation in the rat pituitary intermediate lobe

Genetic Approaches to Hypothalamic-Pituitary-Adrenal Axis Regulation

The normal function of the hypothalamic-pituitary-adrenal (HPA) axis, and resultant glucocorticoid (GC) secretion, is essential for human health. Disruption of GC regulation is associated with pathologic, psychological, and physiological disease states such as depression, post-traumatic stress disorder, hypertension, diabetes, and osteopenia, among others. As such, understanding the mechanisms by which HPA output is tightly regulated in its responses to environmental stressors and circadian cues has been an active area of investigation for decades. Over the last 20 years, however, advances in gene targeting and genome modification in rodent models have allowed the detailed dissection of roles for key molecular mediators and brain regions responsible for this control in vivo to emerge. Here, we summarize work done to elucidate the function of critical neuropeptide systems, GC-signaling targets, and inflammation-associated pathways in HPA axis regulation and behavior, and highlight areas for future investigation.

Developmental programming of energy balance and its hypothalamic regulation

Developmental programming is an important physiological process that allows different phenotypes to originate from a single genotype. Through plasticity in early life, the developing organism can adopt a phenotype (within the limits of its genetic background) that is best suited to its expected environment. In humans, together with the relative irreversibility of the phenomenon, the low predictive value of the fetal environment for later conditions in affluent countries makes it a potential contributor to the obesity epidemic of recent decades. Here, we review the current evidence for developmental programming of energy balance. For a proper understanding of the subject, knowledge about energy balance is indispensable. Therefore, we first present an overview of the major hypothalamic routes through which energy balance is regulated and their ontogeny. With this background, we then turn to the available evidence for programming of energy balance by the early nutritional environment, in both man and rodent models. A wealth of studies suggest that energy balance can indeed be permanently affected by the early-life environment. However, the direction of the effects of programming appears to vary considerably, both between and within different animal models. Because of these inconsistencies, a comprehensive picture is still elusive. More standardization between studies seems essential to reach veritable conclusions about the role of developmental programming in adult energy balance and obesity.

Postnatal ontogeny of the glucocorticoid receptor in the hippocampus

Corticosteroid hormones are important intrinsic factors that not only mediate the response to stress but also largely contribute to the main physiological processes. The biological actions of these steroids involve, first of all, the activation of specific receptors, namely mineralocorticoid (MR) and glucocorticoid (GR) receptors. These two receptor types govern a flexible and well-balanced mechanism that leads to the often opposing changes in the cell. The hippocampus is the central part of the extrahypothalamic feedback loop in the control of the hypothalamic-pituitary-adrenal (HPA) axis activity. The coexpression of both MR and GR in the hippocampus serves a coordinated response to corticosteroids in the hippocampal neurons, thereby mediating the neuronal excitability, stress response, and behavioral adaptation. Each receptor type reveals distinct ontogenetic pattern over the postnatal period. This review addresses the issues relating to postnatal development of the HPA axis and especially the hippocampal expression of the GR proteins in intact and prenatally stressed rats.

Origin of adenohypophysial lobes and cells from Rathke's pouch in Swiss albino mice. Proliferation and expression of Pitx 2 and Calbindin D28K in corticotropic and somatotropic cell differentiation

A developmental study of the adenohypophysis of the mouse was carried out in response to several as yet unanswered questions about its organogenesis and differentiation. The main objectives were to establish the origin of adenohypophysial lobes and cells from the Rathke's pouch (RP) and elucidate the mechanisms of development and functional differentiation of the gland. Using diverse techniques, the morphological development, proliferation and differentiation were studied in order to delimit different proliferative regions in the RP, and provide a satisfactory explanation for the distribution of each cell type in the adult gland. Combining the proliferation and differentiation studies, corticotropic and somatotropic cells appear to mainly originate from undifferentiated precursors located within each of these proliferative regions. The involvement of transcription factor Pitx 2 and calcium-binding protein Calbindin D 28K in the differentiation of corticotropic and somatotropic cells is further clarified.

GABAB receptor subunit mRNAs are differentially regulated in pituitary melanotropes during development and detection of functioning receptors coincides with completion of innervation

This study examines the developmental expression of GABAB receptor subunits (GABAB(1a), GABAB(1b), GABAB(2)) in the pituitary intermediate lobe using in situ hybridization, reverse transcriptase-polymerase chain reaction, immunohistochemistry, and Western blots. Receptor functionality was studied by baclofen-stimulated GTPgammaS binding. In the adult rat pituitary all three transcripts were detected in melanotropes, but not in glia, of the intermediate lobe. No transcripts of any subunit were detected in the neural lobe. Transcripts of GABAB(1a) and GABAB(1b), but not of GABAB(2), were detected in specific subpopulations of cells in the anterior lobe. All three transcripts were detected in melanotropes on gestational day 18 using in situ hybridization. Reverse transcriptase-polymerase chain reactions comparing postnatal day 2 and adult transcript levels in the neurointermediate lobe support in situ hybridization data that GABAB(1a) mRNA levels do not change, GABAB(1b) levels increase, and GABAB(2) levels decrease as the rat matures. Thus, GABAB receptor subunit transcripts are differentially regulated in melanotropes during development. In the adult rat both GABAB(1) and GABAB(2) proteins were detected in the neurointermediate lobe using Western blotting and in melanotropes by immunohistochemistry. Developmentally, GABAB(1) protein was not detected until postnatal day 7, but was clearly expressed by postnatal day 15 while GABAB(2) protein could not be detected until postnatal day 15. Functional receptors were found in the intermediate lobe at postnatal day 15 and in the adult. The demonstration of transcripts for GABAB(1a), GABAB(1b) and GABAB(2) subunits at gestational day 18 contrasted with the failure to detect any protein before postnatal day 7, suggesting that the regulation of GABAB subunit isoforms occurs differentially at both the transcriptional and translational level as development progresses. The disparity in the regulation of the receptor subunits may suggest that GABAB(1) could have other functions besides being part of the GABAB receptor heterodimer.

Deregulated E2F activity induces hyperplasia and senescence-like features in the mouse pituitary gland

The retinoblastoma gene, RB1, is one of the most frequently mutated genes in human cancer. Rb heterozygous mice develop pituitary tumors with 100% incidence, and the E2F transcription factors are required for this. To assess whether deregulated E2F activity is sufficient to induce pituitary tumors, we generated transgenic mice expressing an inducible E2F3 protein in the intermediate lobe of the pituitary gland. We found that short-term deregulation of E2F activity, similar to the earliest stages of Rb loss, is able to induce abnormal proliferation of otherwise quiescent melanotrophs. However, while long-term exposure to deregulated E2F activity results in hyperplasia of the intermediate lobe, it did not lead to tumor formation. In fact, melanotrophs become insensitive to sustained E2F stimulation and enter an irreversible senescence-like state. Thus, although deregulated E2F activity results in hyperproliferation, it is not sufficient to mimic loss of Rb, sustain proliferation of melanotrophs, and ultimately induce pituitary tumors. Similarly, we found that primary cells in tissue culture become insensitive to sustained E2F3 activation and undergo premature senescence in a pRB-, p16Ink4a-, and p19Arf-dependent manner. Thus, we conclude that deregulated E2F activity is not sufficient to fully mimic loss of Rb due to the engagement of a senescence response.

Axonal projections from the hypothalamus to the pituitary intermediate lobe in rats during ontogenesis: DiI tracing study

This study has determined ontogenetic schedule of axonal arrival from the hypothalamus in the pituitary intermediate lobe (IL) in rats using 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. The brains with attached pituitaries were dissected in rats from the 20th embryonic day (E20) to the 20th postnatal day (P20). Anterior lobe was mechanically detached from the IL, material fixed in paraformaldehyde, and DiI crystals were applied on the IL laying on the posterior lobe (PL). The labeling of IL + PL resulted in staining of hypothalamic magnocellular neurons, which send their axons to the PL, and hypothalamic parvocellular neurons contributing to the innervation of the IL. Therefore, the magnocellular neurons were not taken into account when identifying the neurons projecting axons to the IL. Rare fluorescent neurons projecting their axons to the IL were detected as early as on E20 in the ventral part of the periventricular nucleus (Pe) and in the rostral part of the arcuate nucleus. Few DiI-labeled neurons were seen in Pe from P1 to P3. At P5, the fluorescent neurons were accumulated giving rise to the prominent cluster in the Pe, which was enlarged on later stages and occupied all the Pe. In addition to the Pe, fluorescent neurons first appeared in the retrochiasmatic region and around the ventromedial nucleus in young rats. Thus, the axons of hypothalamic neurons of the Pe and mediobasal hypothalamus first arrive in the IL in rats at the end of intrauterine development, although the principal innervation of the IL is the postnatal event.

The onset of dopaminergic innervation during ontogeny decreases melanotrope proliferation in the intermediate lobe of the rat pituitary

The onset of dopaminergic innervation and its effects on melanotrope proliferation were investigated in the rat pituitary intermediate lobe. Dopamine, and its synthetic rate-limiting enzyme tyrosine hydroxylase, were first detected immunohistochemically on late post-natal day 3 or early postnatal day 4. Axon density was highest at the neural lobe/intermediate lobe border, and decreased toward the pituitary cleft. By postnatal day 10, the adult pattern of tyrosine hydroxylase immunoreactivity was established and remained through post-natal day 14. Neurointermediate lobe dopamine levels, measured by HPLC, correlated well with the increased axon density observed in the immunohistochemical studies. Dopamine could not be measured by our assay (100 fg limit) until post-natal day 3 (439.32 fg/NIL). Dopamine concentration increased to 2.09 +/- 0.425 ng at PN 4, 86.31 +/- 20.42 ng at PN 7, 168.72 +/- 18.37 ng at PN 10. Melanotrope proliferation was determined by [3H]thymidine incorporation before and after innervation. Concomitant with the onset of innervation, the proliferation index dropped from 13.4 +/- 0.01% to 6.5 +/- 0.002% at PN 4, and continued to decrease until a level of 3 +/- 0.003% was established by PN 10. To confirm the inhibitory action of dopaminergic innervation on melanotrope proliferation, rat neonates were injected intracisternally with 150 mg 6-hydroxydopamine to destroy dopaminergic axons within the intermediate lobe. Measurement of dopamine concentrations in neurointermediate lobes of injected animals showed a decrease in dopamine levels as compared to controls. From PN 4 (0.88 +/- 0.165 ng), DA levels gradually increased during development: at PN 5, [DA] = 0.689 +/- 0.104 ng; PN 6 [DA] = 11.60 +/- 2.24 ng; PN 7 [DA] = 20.93 +/- 3.80 ng; and PN 10 [DA] = 27.95 +/- 3.46 ng. Melanotrope proliferation also increased in 6-hydroxydopamine-treated animals. At PN 4, the onset of innervation reduced the pre-innervation proliferation index to 8.75 +/- 0.002%, only a 30% reduction in contrast to the greater than 50% decrease observed in control animals. A stable proliferation level of approximately 7.5% persisted in all subsequent stages with 6-OHDA administration. Our results demonstrated the time of dopamine innervation onset and a characteristic developmental pattern for axons within the rat intermediate lobe. The onset of innervation and increased dopamine concentration suggests increased dopaminergic control of the melanotropes, illustrated specifically by a decrease in their level of proliferation. This is the first presentation of evidence showing that dopaminergic innervation within the intermediate lobe of the rat pituitary regulates melanotrope proliferation.

Postnatal decrease of sodium current density in rat pituitary melanotropes following the onset of dopaminergic innervation

Peptide secretion from rat melanotropes is tonically inhibited by a dopaminergic synaptic input that develops after birth and acts through D2 dopamine receptors. In this study, whole-cell Na(+) currents were recorded from melanotropes that were isolated from rat pituitary intermediate lobes at postnatal days 1-20 (P1-P20) and maintained in culture for 5-24 h. Coincident with the development of innervation, melanotropes exhibited a progressive decrease in peak Na(+) current density from P3 to P14. The decrease involved a 50% reduction in maximal Na(+) conductance with no detectable changes in channel gating. Subcutaneous injections of the D2 antagonist sulpiride, applied from P11 to P13, restored melanotrope Na(+) channel activity to pre-innervation levels. Thus, the activation of D2 receptors by the dopaminergic input reduces the functional expression of Na(+) channels in melanotropes.

Ontogenesis of proopiomelanocortin and its processing to beta-endorphin by the fetal and neonatal rat brain

A number of reports suggest that beta-endorphin (beta-END) may play an important role in the regulation of cell proliferation and neuronal differentiation. Proopiomelanocortin (POMC), the common precursor ofadrenocorticotropic hormone and beta-END, is detected very early in embryonic life in hypothalamic neurons of the developing rat. However, very little is known about the degree to which POMC is processed to beta-END during fetal and early postnatal life. Thus, it was the objective of the present study to estimate the hypothalamic content of POMC mRNA, as well as the biosynthesis and posttranslational processing of POMC by hypothalamic neurons on fetal day 20 and on days 1, 8 and 22 of postnatal life. Hypothalamic POMC mRNA, as determined by Northern blot analysis, was higher on fetal day 20 than on postnatal days 1, 8 and 22. A higher rate of incorporation of [(3)H]phenylalanine into beta-END immunoreactive peptides was observed on fetal day 20 than on postnatal day 1. However, the rate of incorporation was significantly increased by day 8 of postnatal life and was similar to that on day 22. POMC was processed to beta-lipotropin (beta-LPH) and beta-END at all ages examined, but the relative proportions of POMC:beta-LPH:beta-END changed during development. Thus, beta-END accounted only for 34.89 +/- 6.14% of the total [(3)H]phenylalanine-labeled beta-END immunoreactive peptides on fetal day 20, while it accounted for 57. 37 +/- 5.20, 62.81 +/- 1.38 and 79.25 +/- 6.57% on days 1, 8 and 22 of postnatal life, respectively. Thus, POMC is processed to a considerable extent into beta-END-sized peptides by the fetal hypothalamus and may influence brain development. Furthermore, the rate of processing of hypothalamic POMC into beta-END increases with development, probably due to the increased activity of the enzymes specific for POMC processing.

Membranes from pituitary intermediate lobe cells enhance differentiation of fetal hypothalamic dopaminergic neurons in primary culture

Coculture of adult pituitary intermediate lobe (IL) cells, a target for hypothalamic dopaminergic neurons, with fetal rat hypothalamic cells accelerate differentiation of dopaminergic neurons. This involves long range diffusible as well as additional factors which may be membrane-bound. To determine whether IL membrane-bound factors contribute to the differentiating effect of IL cells, IL membranes were added to dispersed fetal hypothalamic neurons. This stimulated the outgrowth of dopaminergic neurites and elevated TH levels. Limited trypsin proteolysis of IL cell surface abolished the effect on TH levels. Addition of adenohypophyseal membranes was ineffective. Joint treatment with IL membranes, and medium conditioned (CM) over IL cells, produced the same effect on TH levels as did coculture with the same number of IL cells. The results demonstrate that IL cells express on their surface a membrane-bound factor promoting differentiation of fetal dopaminergic neurons in vitro; this factor acts in addition to diffusible activities.

The development of POMC gene expression in the medial basal hypothalamus of prepubertal rats

Alterations in brain opioid gene expression may underlie the dramatic change in the latency to display parental behavior in juvenile rats. Male and female juvenile rats (18-25 days of age) exhibit parental behavior either immediately or within 1-2 days after coming in contact with foster pups. By 30 days of age, however, their response latencies increase to adult levels of 5-10 days. Given the established involvement of the endogenous opioid system in adult maternal and juvenile parental behaviors, the objective of the present report was to determine possible changes in proopiomelanocortin (POMC) gene expression in the medial basal hypothalamus (MBH) during this early developmental window. We compared POMC gene expression in the MBH of male and female juvenile rats from 21 to 33 days of age by in situ hybridization histochemistry. A significant increase in the number of POMC cells in males and females was detected at 30 days of age in the central portion of the arcuate nucleus. This increase in POMC mRNA may contribute to the shift in parental behavior that occurs in male and female juvenile rats.

Differentiation of rat hypothalamic dopaminergic neurons is stimulated in vitro by target cells: the melanotrophs

We have investigated in vitro the influence of pituitary intermediate lobe melanotrophs on the differentiation of their afferent hypothalamic dopaminergic neurons. The presence of melanotrophs in primary cultures of foetal hypothalamic neurons induces an increase of the number of dopaminergic neurons (while the total neuronal population remains unchanged) and induces a stimulation of their neuritic outgrowth. These effects are mediated by diffusible factors since they are reproduced by application of conditioned medium issued from co-cultures with intermediate lobe cells from newborn rats. Moreover, by immunoneutralization of alpha-melanocyte-stimulating hormone (alphaMSH) in the co-culture or conditioned medium, or by application of the peptide itself, we demonstrate that the neuritotrophic effect on dopaminergic neurons is mediated by alphaMSH, the main secretory product of melanotrophs, whereas the inductive effect on the number of dopaminergic neurons is attributable to another diffusible neurotrophic factor(s) present in foetal, but not adult, adenohypophysis. Similar effects are observed on cultures of newborn hypothalamic neurons. However, at this stage of neuronal development, alphaMSH also increases the number of dopaminergic neurons, which could be due to a change of neuronal receptivity. We show that the neuritotrophic influence of alphaMSH is restricted to the dopaminergic neurons connected to the melanotrophs, and that in addition, these neurons systematically co-express the tyrosine hydroxylase and glutamate decarboxylase as the neurons innervating the melanotrophs in situ. These findings indicate that the differentiation of dopaminergic hypothalamic neurons is influenced by the target cells, melanotrophs, and that this trophic influence implicates alphaMSH.

G-protein expression in melanotropes changes coincident with innervation of the developing rat pituitary intermediate lobe

The two isoforms of the dopamine D2 receptor, the D2short and the D2long differ in a 29 amino acid insert in the third cytoplasmic loop with which G proteins interact. We have previously reported that in rat melanotropes, expression of D2short increases markedly at the end of the first postnatal week which is concurrent with innervation of the intermediate lobe. Using immunohistochemistry, this study examined expression of G alpha i1/2, G alpha i3, G alpha o and G alpha s proteins before and after dopaminergic innervation. G alpha i3 increased through gestational day 20, and then remained level to postnatal day 6. At this time, coinciding with the induction of D2short expression, G alpha i3 immunoreactive intensity increased markedly, possibly indicating co-regulation of these proteins. On postnatal day 6, G alpha s immunoreactive intensity increased in some, but not all, melanotropes. The resulting heterogeneity in Gs expression persisted in the adult. G alpha i1/2 immunoreactivity did not change and G alpha o was detected only subsequent to the event of innervation. Thus, dopamine released from axons and acting through D2 receptor stimulation could increase G alpha i3 immunoreactivity and decrease G alpha s immunoreactive intensity in some melanotropes.

Co-culture of hypothalamic neurons and melanotrope cells: a model to study synaptogenesis between central neurons and endocrine cells

As a first step towards elucidating mechanisms involved in neuroendocrine synaptogenesis, we developed a model of co-culture based on hypothalamic-intermediate pituitary interactions. Dissociated hypothalamic neurons from fetal rats at embryonic day 15 were cultured in a defined medium together with melanotrope cells of the pituitary intermediate lobe from neonatal rats. In these co-cultures, establishment of synaptic contacts between GABAergic or dopaminergic neurons and an endocrine target cell the melanotrope cell, was studied by morphofunctional approaches. Using double immunostaining with antibodies directed against glutamate decarboxylase or tyrosine hydroxylase and alpha-melanocyte-stimulating hormone, we demonstrated morphological contacts between GABAergic or dopaminergic neurons and melanotrope cells as early as three days in vitro. Furthermore, using an antibody directed against synapsin I, we showed a modification of synapsin I immunoreactivity from diffuse to punctate distribution correlated with the establishment of contacts and the observation of characteristic neuroendocrine synapses by electron microscopy. These results were further confirmed by electrophysiological studies. Patch-clamp recordings demonstrated that, at six days in vitro, some melanotrope cells displayed GABAergic synaptic currents, which occurred either spontaneously and/or could be evoked chemically by 50 mM KCl or 100 microM kainate. The proportion of the melanotrope cells receiving functional synaptic inputs increased until 10 days in culture, a stage at which virtually all melanotrope cells in contact with neurons possessed functional synapses. The results presented here describe the establishment of neuroendocrine synapses in vitro, studied by combining morphofunctional and electrophysiological approaches.

Early loss of the retinoblastoma gene is associated with impaired growth inhibitory innervation during melanotroph carcinogenesis in Rb+/- mice

To better understand the cell lineage-specific character of retinoblastoma (Rb) gene inactivation during tumor formation, the earliest stages of spontaneous melanotroph carcinogenesis in Rb+/- heterozygous mice have been subjected to sequential analyses. The first atypical cells are detected in the pituitary intermediate lobe during a period corresponding to the cessation of melanotroph proliferation between 35 and 60 days after birth. Atypical cells contain no wild-type copy of the Rb gene and synchronously form early atypical proliferates (EAP) in the subsequent 30-60 day period. In contrast to surrounding mature melanotrophs with the wild-type Rb gene, Rb-negative cells in EAP continue to proliferate well past postnatal day 60, and fail to be innervated by growth inhibitory dopaminergic nerve terminals. Atypical melanotrophs remain competent for dopamine D2 receptor stimulation and undergo S-phase apoptosis in close proximity to nerve terminals. These results indicate a key role for the Rb protein in the onset of neuron-neuroendocrine cell interactions. This role may explain cell-type-specific neuroendocrine carcinogenesis associated with inactivation of the ubiquitously expressed Rb gene.

Ca2+ current expression in pituitary melanotrophs of neonatal rats and its regulation by D2 dopamine receptors

1. We have examined the voltage-dependent Ca2+ channel activity of rat melanotrophs during the early postnatal period. The cells were dissociated from pituitary intermediate lobes, kept in culture for 5-24 h and then subjected to whole-cell patch-clamp experiments. 2. Like their adult counterparts, neonatal melanotrophs were able to generate Na+ currents, K+ currents and Ca2+ currents in response to membrane depolarization. Ca2+ currents were carried by both low- and high-threshold Ca2+ channels. 3. High-threshold Ca2+ current density decreased sharply between postnatal day 4 (P4) and P12. This period coincides with the onset of dopaminergic innervation within the intermediate lobe. Accordingly, the developmental decrease in Ca2+ current density was largely reversed by chronic in vivo treatment with sulpiride, a dopamine D2 receptor antagonist. 4. Prolonging the time in culture from 5 h to 8 days did not significantly alter the Ca2+ channel activity of P3 melanotrophs, whereas the high-threshold Ca2+ current in previously innervated (P14) melanotrophs stayed small for the first 24 h and then increased 3-fold during the subsequent 4-5 days. This increase required RNA and protein synthesis and was prevented by adding D2 agonists to the culture medium. 5. These results provide evidence for a postnatal suppression of high-threshold Ca2+ current expression in pituitary melanotrophs mediated by presynaptic dopamine neurons through D2 dopamine receptors.

Melanotrope dopamine D2 receptor isoform expression in the developing rat pituitary

This study measured melanotrope mRNA and protein expression for the dopamine D2 receptor, and its long isoform, in relation to the appearance of dopamine in axons of the postnatal rat pituitary intermediate lobe. At postnatal day 2, prior to the onset of dopaminergic innervation, D2 receptor (D2T) mRNA was expressed heterogeneously in a subpopulation of melanotropes which also expressed the long isoform (DL). The D2L mRNA appeared to be predominant during early postnatal development, since the D2T probe, which did not discriminate between the isoforms, and the D2L probe hybridized generally to the same cells, as demonstrated in serial sections. Immunohistochemical methods, using two different antisera for the D2T receptor, however, indicated a low level of protein in most melanotropes. Localization of D2L protein corresponded well to D2T receptor mRNA distribution. At day 10, representing a time when dopamine is present in axons throughout the lobe, both D2T receptor mRNA and protein were detected in a significantly larger population of melanotropes than those expressing D2L mRNA and protein. This suggests the appearance of detectable short isoform (D2S) mRNA in virtually all melanotropes and implicates dopamine as a possible signal for increasing D2S isoform mRNA expression.

Glial-like cells of the rat pituitary intermediate lobe change morphology and shift from vimentin to GFAP expression during development

This study demonstrated morphological changes in glial-like cells of the rat pituitary intermediate lobe during early postnatal development, and a subsequent shift in protein expression from vimentin to GFAP. Vimentin immunoreactivity was detected in the lobe at embryo day 14 and was localized in radially-oriented, bipolar cells whose processes spanned the thickness of the intermediate lobe. At electron microscopical resolution, processes contained intermediate filaments, cell nuclei were indented while secretory vesicles characteristic of the endocrine cells were not found. Vimentin immunoreactive intensity began to decrease at postnatal day 5. By postnatal day 7, vimentin-positive, stellate cells were observed, with few radial processes found by day 10. The intensity of vimentin immunoreactivity decreased through day 25. Within the lobe parenchyma, vimentin was localized in glial-like cells since double-label immunohistochemistry revealed no colocalization of beta-endorphin and vimentin, or fibronectin and vimentin. Dopamine-containing axons were in close apposition to vimentin-positive processes. GFAP immunoreactivity first appeared on postnatal day 20 and, by day 25, stellate cell bodies with three to six extended processes were evident. Cells were primarily distributed in the caudal third of the lobe. The characteristic adult pattern of cell clusters in latero-dorsal and ventral portions of the lobe was fully established by postnatal day 55. The transition from vimentin to GFAP expression and concurrent morphological changes resemble those described for radial glial during cerebral cortical development.

Transient expression of S-100 by melanotropes of the rat pituitary intermediate lobe during development

S-100, an acidic calcium-binding protein, is co-localized with vimentin in glial-like cells in the adult rat pituitary intermediate lobe. S-100 and melanotrope markers were not co-localized in the adult. During development, S-100 and vimentin were not co-localized but appeared in cells with different morphological characteristics. S-100 was co-localized with POMC mRNA and beta-endorphin during prenatal time and the first three postnatal weeks. This was demonstrated by double-label immunohistochemistry, using combinations of antisera against S-100, vimentin and beta-endorphin, and in situ hybridization histochemistry for POMC mRNA combined with immunohistochemistry for S-100. In the second and third weeks of postnatal development, S-100 was observed in fewer melanotropes and more frequently in stellate cells, which also expressed vimentin. Thus, S-100 appeared to be transiently expressed in melanotropes during prenatal and early postnatal development. S-100 serves as a neurotrophic and glial maturation factor in the CNS. Since S-100 expression in melanotropes coincides with the onset of dopaminergic innervation and morphological changes in glial-like cells of the lobe, it could have similar functions in the rat pituitary intermediate lobe.

Expression of neural cell adhesion molecules, NCAMs, and their polysialylated forms, PSA-NCAMs, in the developing rat pituitary gland

Neural cell adhesion molecules (NCAMs) can undergo post-translational modifications, such as the addition of polysialic acid chains, thus generating PSA-NCAMs, which are expressed mainly during development. Since polysialylation considerably modifies NCAM adhesivity, expression of NCAMs and PSA-NCAMs has been investigated in the developing hypophysis by immunohistochemistry. At embryonic day 13 (E13), an antibody against NCAM outlined all cellular profiles in the entire Rathke's pouch; this labelling persisted until adulthood. NCAM expression increased in all lobes during development and concerned all pituitary cell types. In contrast, at E13, PSA-NCAMs were only detected in the neural lobe, solely constituted of pituicytes at this stage, and the tuberal lobe, the only lobe expressing hormonal mRNA at the same stage. PSA-NCAMs expression increased in the neural lobe at E17 with the arrival of the neurosecretory fibres and persisted into adulthood. In the anterior lobe, PSA-NCAMs appeared at E15 where their distribution was similar to that of the differentiating corticotrophic cells; at subsequent stages, their expression extended to the whole anterior lobe. Only two cell types, corticotrophic and somatotrophic cells, remained labelled in the adult gland. In the intermediate lobe, melanotrophic cells never expressed PSA-NCAMs but these were expressed on folliculo-stellate cells at birth, preceding the onset of innervation. These results suggest that NCAMs and PSA-NCAMs play a role in pituitary histogenesis, cell differentiation and neurointermediate lobe innervation.

Ontogeny of glucocorticoid and D2 receptors in the rat pituitary: an in situ hybridization study

The expression of glucocorticoid and D2 dopamine receptors (GR and D2R) during rat pituitary ontogenesis was studied by in situ hybridization (ISH). On early stages, E13-E14, a weak specific signal for GR mRNA was obvious in the whole Rathke's pouch (RP) whereas subsequently, from E17-E18, strong labelling was restricted to the anterior lobe (AL) and the neural lobe (NL). At the same time, D2R mRNAs appeared in the intermediate lobe (IL) and the long isoform of the D2R (D2R 444) was detectable with specific probes. On the postnatal stages, until adult, GR mRNA, if present, was always undetectable in the IL using the conventional ISH technique. These data indicate a possible early regulation of POMC gene expression by glucocorticoid in corticotrophic cells of the AL and by dopamine in the melanotrophic cells of the IL. The possibility of a negative regulation of GR mRNA by dopamine (DA) in the IL as soon as E17 is discussed.

Proopiomelanocortin gene expression during pig pituitary and brain development

The expression of proopiomelanocortin (POMC) mRNA, which plays an important role in neural, endocrine, neuroendocrine and immune systems, was studied by in situ hybridization during the development of pituitary in the domestic pig. The POMC gene was activated as early as fetal day 30 (E30). The signal for POMC mRNA in the anterior lobe progressively increased from E30 to E80 and then remained at relatively constant level. In contrast, POMC transcripts in the intermediate lobe first appeared at E40 and steadily increased during development. POMC transcripts in the brain were first found at E40 and were scattered in the arcuate nucleus (AN) and nucleus medialis thalami (NMT). At E50 the extra-pituitary POMC mRNA was located not only in the AN and NMT but also in the fasciculus tegmenti and entorhinal cortex. In the posterior lobe, no signal was detected. The specific pattern of expression of the pig POMC gene in the pituitary and in specific regions of the central nervous system suggests important roles for POMC in fetal development. These results also suggest that POMC is excellent for studying the expression and regulation of pituitary hormone genes because of its tissue-specific regulation and developmental pattern.

Ontogeny of prodynorphin gene expression in the rat hypothalamus

Opioid peptides, deriving from prodynorphin, proenkephalin and proopiomelanocortin genes, have been shown to modulate brain development. Prodynorphin gene expression was studied here by in situ hybridization in the developing rat hypothalamus using oligodeoxynucleotide probes. Prodynorphin mRNA-synthetizing cells were observed in the ventromedial hypothalamic nucleus, the supraoptic and the paraventricular nuclei from embryonic days 16, 18 and 21, respectively. We detected no transient expression of prodynorphin gene in the rat hypothalamus. Prodynorphin mRNA-containing cells were also observed prenatally in the striatum, the cortex, the hippocampus and the amygdala. When compared with data from the literature, our results suggest that translation may immediately follow transcription of prodynorphin gene in the supraoptic nucleus. The presence of prodynorphin mRNA in the developing rat hypothalamus also raises the possibility of an involvement of prodynorphin-derived peptides in developmental processes and/or in the maturation of adult neural regulations.

Ontogeny of the prohormone convertases PC1 and PC2 in the mouse hypophysis and their colocalization with corticotropin and alpha-melanotropin

In the adult pituitary, anterior lobe corticotrophs and intermediate lobe melanotrophs differentially process proopiomelanocortin (POMC). Within the corticotrophs, POMC is processed mainly to corticotropin (ACTH) and beta-lipotropin, while alpha-melanotropin (alpha MSH) and beta-endorphin are the major end products in the melanotrophs. The observed transient presence of alpha MSH-like immunoreactivity during ontogeny suggested an age-dependent variation in POMC processing in the adenohypophysis. In this tissue, cell-specific POMC products are likely the result of differential expression of the two known prohormone convertases PC1 and PC2. In the present ontogeny study done in the mouse intermediate and anterior pituitary, we examined how the expression pattern of PC1 and PC2 mRNA transcripts correlates with that of ACTH and alpha MSH-like immunoreactivities. Our data demonstrated that both PC1 and PC2 transcripts can be detected in the presumptive adenohypophysis starting on embryonic day 15 (E15). In the intermediate lobe, PC1 and PC2 mRNAs appear on E18 and E16, respectively, and their levels increased during ontogeny, reaching maximal expression in the adult. Similarly, PC1 expression in the anterior pituitary increased from E15 to adulthood. However, PC2 mRNA expression peaked between postnatal days 1 (P1) and 14 (P14) and then decreased to adult levels. The distribution of PC1 and PC2 immunoreactivity is nicely correlated with the in situ hybridization data. In the anterior lobe, during the P1-P14 postnatal period, PC2 immunoreactivity was detected within cells synthesizing an alpha MSH-like peptide(s). This observation substantiates our earlier biochemical data suggesting that PC2 is the important convertase in the processing of POMC into alpha MSH. Furthermore, the demonstrated variation in the relative ratio of PC1/PC2 expression during ontogeny rationalizes the observed plasticity of POMC processing in the adenohypophysis. It is expected that beta-endorphin processing will follow that of alpha MSH.

Differentiation of the melanotrophic cells of rat pituitary primordium in organotypic culture in defined medium

Organotypic cultures, in defined medium, of pituitary primordia obtained from 15-day-old rat fetuses were performed in order to study the in vitro differentiation of melanotrophic cells. The morphological and ultrastructural features of the transplants resembled those of the gland developing in vivo. In situ hybridization on semi-thin sections, using a 35S-labelled oligonucleotide probe, revealed pro-opiomelanocortin-mRNA-containing cells on the first day of culture in the anterior lobe and after 2-3 days in the intermediate lobe. Immunoperoxidase labelling of adjacent sections showed that the same cells reacted with antibodies against alpha-melanocyte-stimulating hormone (alpha MSH), gamma 3MSH and adrenocorticotropic hormone in both lobes. The pro-opiomelanocortin-mRNA-containing cells formed progressively conspicuous areas in the intermediate lobe, which was almost uniformly labelled after 6 days. In the anterior lobe, these cells remained scattered in small cell groups, and colloidal gold immunolabelling showed the progressive disappearance of alpha MSH labelling from the secretory vesicles in cells exhibiting morphological features of adult corticotrophic cells. Both the alpha MSH content of the explants and alpha MSH release into the culture medium increased with time. Treatment with the dopamine agonist bromocriptine induced a strong dose-dependent decrease in alpha MSH secretion, which was significant after 3 days in culture, indicating that dopamine D2 receptors are able to regulate hormonal release of melanotrophic cells at early stages. This system constitutes a suitable model for further studies of factors controlling cell differentiation and cellular interactions involved in histogenesis.

Ontogeny of proenkephalin gene expression in the rat hypothalamus

In the rat hypothalamus, proenkephalin (PE) mRNA synthetizing cells were detected by in situ hybridization, using synthetic oligodeoxy-nucleotides, from embryonic day 14 (E14) in the presumptive anterior hypothalamic area (AHA) and preoptic part of the bed nucleus of the stria terminalis (BST), and from E18 in the developing median preoptic area, perifornical area, suprachiasmatic nucleus, dorsomedial and ventromedial hypothalamic nuclei. In the paraventricular nucleus, cells expressed PE gene in the late prenatal stages; both parvo- and magnocellular neurons synthetized PEmRNA in the early postnatal stages. Cells expressing PE gene were observed after birth in the lateral preoptic area, lateral hypothalamus, medial and lateral parts of the BST. PEmRNA was also found from E14 in the striatum, from E18 in the central and medial amygdaloid nuclei, the medial group of the thalamic nuclei, and postnatally in a second more anterior structure of the thalamus. In the hypothalamus, a clear similarity was observed between adult and developmental distributions of PE gene expressing cells. The early onset of PE gene expression in the developing rat diencephalon suggests an involvement of PE in developmental processes, such as cell proliferation and differentiation; the presence of PE during the perinatal period may also indicate the appearance of adult neural regulations.

Activation of protein kinase C differentially regulates corticotropin-releasing factor-stimulated peptide secretion and cyclic AMP formation of intermediate and anterior pituitary cells in culture

The present study was aimed at investigating the effect of protein kinase C (PKC) activation on CRF receptor function of proopiomelanocortin (POMC) cells in culture. Incubation of tissues with the phorbol ester PMA selectively potentiated corticotropin-releasing factor (CRF)-stimulated ACTH secretion and cyclic AMP formation of anterior pituitary (AP) cells, while, in sharp contrast, it failed to similarly affect intermediate pituitary (IP) cells and AtT-20 corticotrophs exposed to CRF. Unexpectedly, however, long-term treatment of cultures with PMA, which depletes cell stores of PKC, resulted in a similar dramatic attenuation of stimulated peptide release from both corticotrophs and melanotrophs, while being without significant effect on cyclic AMP production. Exposure of cells to PMA did not change either basal or CRF-enhanced levels of POMC mRNA. We conclude that activation of PKC fails to synergize with CRF-mediated signalling in IP and AtT-20 cells, although optimal CRF receptor expression requires the presence of a functional kinase C pathway, thus suggesting cross-talks between both messenger systems.