Nerve growth factor-like activity in the rat pituitary intermediate lobe

The NGF saga: from animal models of psychosocial stress to stress-related psychopathology

The role of the neurotrophins Nerve Growth Factor (NGF) and Brain-Derived Neurotrophic Factor (BDNF) has been expanding over the last years from trophic factors involved in brain growth and differentiation, to much more complex messengers, involved in psycho-neuro-endocrine adaptations. Much of this research stems from a series of studies inspired by the life-long work of the Nobel laureate Rita Levi-Montalcini. A new field of research started when NGF was found to be released in the bloodstream as a result of psychosocial stressors in male mice. Subsequent studies have shown that, in humans, highly arousing situations also result in increased blood levels of NGF, underlying the unique role of this neurotrophin, compared to other neuroendocrine effectors, and its sensitivity to environmental variables endowed by a social nature. Data are reviewed to support the hypothesis that this neurotrophic factor, together with BDNF, could be involved in the neurobiological changes underlying physiological and pathological reactions to stress that can result in increased vulnerability to disease in humans, including risk for anxiety disorders, or in the complex pathophysiology associated with mood disorders. Indeed, numerous data indicate that neurotrophins are present in brain hypothalamic areas involved in the regulation of hypothalamic-pituitary-adrenal axis, circadian rhythms and metabolism. In addition, there is now evidence that, in addition to the nervous system, neurotrophins exert their effects in various tissue compartments as they are produced by a variety of non-neuronal cell types such as endocrine and immune cells, adipocytes, endothelial cells, keratinocytes, thus being in a position to coordinate brain and body reactions to external challenges. Aim of this review is to discuss the evidence suggesting a role for neurotrophins as multifunctional signaling molecules activated during allostatic responses to stressful events and their involvement in the complex pathophysiology underlying stress-related psychopathology.

The mammalian pituitary intermediate lobe: an update on innervation and regulation

The pituitary intermediate lobe (IL) in mammals is an area of uniform endocrine cells which synthesize and release specific peptide products of the proopiomelanocortin gene. The lobe receives direct synaptic connections onto the endocrine cells from hypothalamic dopaminergic neurons. This review updates information on the dopaminergic as well as the gamma-aminobutyric acid inhibitory neuroregulation for the IL. It also provides a discussion of stimulatory molecules which are likely to affect peptide release, particularly the neurotransmitter serotonin, which may be present via uptake into the dopaminergic nerve terminals. Other stimulatory molecules discussed which are likely to significantly affect peptide secretion are norepinephrine, corticotropin-releasing factor, and several opiate peptides. A new direction of study involves the potential interaction of neurotrophic factors, which are present in all areas of the pituitary, and may be suggested to have a supportive role for the neural elements of the IL. The endocrine cells of the IL and their direct hypothalamic innervation are considered to be an easily accessed peripheral model for study of both neural-endocrine and neurotrophic-target cell interactions.

Glial cell line-derived neurotrophic factor in the rat pituitary gland

The presence of glial cell line-derived neurotrophic factor (GDNF) is described within specific regions of the adult rat pituitary gland. Immune staining methods revealed a small number of GDNF-immunopositive cells in the anterior lobe, and in areas of the neural lobe, while no immunoreactive endocrine cells were observed in the intermediate lobe. In the neural lobe, immunofluorescence methods were also used to demonstrate that GDNF and glial fibrillary acidic protein (GFAP) are co-localized in the glial cells (pituicytes) of the neural lobe. GDNF was not co-localized with neurofilament (NF) in nerve fibers of the neural lobe, suggesting that it is not present in axonal fibers. Measurements of GDNF content in separated anterior and neurointermediate lobes were also performed, using an enzyme-linked immunoassay (ELISA). Values for GDNF were slightly higher in the neurointermediate lobe than those obtained for the anterior lobe. The presence of GDNF in areas of the pituitary is discussed in the context of its possible function to support and maintain hypothalamic innervation, as well as a potential autocrine factor within endocrine cells.

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.

Mitogenic effects of nerve growth factor on different cell types in reaggregate cell cultures of immature rat pituitary

Treatment of reaggregate pituitary cell cultures of 14-day-old female rats with nerve growth factor (NGF) augmented the number of [3H]thymidine ([3H]T)-incorporating lactotrophs in a dose-dependent manner (0.03-3 nM). At least during short-term treatment NGF increased the total number of cells expressing prolactin (PRL) mRNA and enlarged the cytoplasmic area occupied by PRL mRNA but did not affect the number of cells and the cytoplasmic area containing PRL, suggesting that NGF recruits lactotrophs expressing PRL mRNA but not yet PRL. NGF also stimulated [3H]T incorporation in ACTH cells but not in somatotrophs, thyrotrophs and gonadotrophs. In addition, NGF augmented the total number of [3H]T-incorporating cells to a much higher extent than was expected from its effect on lactotrophs and ACTH cells, suggesting NGF also stimulates [3H]T-incorporation in non-hormone producing cells (progenitors or stem cells?). Around 40% of these [3H]T-incorporating cells in both control and NGF treated cultures showed immunoreactivity for the transcription factor Pit-1 in the nuclei, which is twice the percentage expected (18%) if these [3H]T-incorporating cells were the only known Pit-1 expressing cells in the pituitary i.e. lactotrophs, somatotrophs and thyrotrophs. The present data suggest that NGF has a mitogenic effect on several cell lineages in the pituitary: lactotrophs, corticotrophs and non-hormone-containing cells. The high proportion of mitotic non-hormone containing cells that express Pit-1 is consistent with the proposed role of Pit-1 in cell proliferation in the developing lactosomatotroph lineage.

NGF content and expression in the rat pituitary gland and regulation by thyroid hormone

The involvement of nerve growth factor (NGF) in neuroendocrine regulation is supported by several lines of evidence. In this paper, we investigated the NGF content and expression in the pituitary gland and other endocrine organs during dysendocrine states (thyroidectomized, adrenalectomized and gonadectomized male rats). We found an increase of NGF-IR in the pituitary gland and testis of hypothyroid rats whereas no differences were found in the adrenal gland and blood. Also, NGF mRNA expression had increased in the anterior pituitary of hypothyroid rats whereas it had not changed after adrenalectomy and gonadectomy. Moreover, other neurotrophins and neurotrophin high-affinity receptors were unchanged in the anterior pituitary of hypothyroid rats. These data indicate that pituitary NGF is selectively modulated by thyroid status of the animal, further supporting a close link between NGF and thyroid hormone.

Polysialylated neural cell adhesion is involved in target-induced morphological differentiation of arcuate dopaminergic neurons

We have previously shown that the morphological and biochemical maturation of developing rat hypothalamic dopaminergic neurons is accelerated when they are cocultivated with pituitary intermediate lobe cells, one of their targets. Only two subsets of hypothalamic dopaminergic neurons (arcuate, A12, and periventricular, A14, nuclei) may project to the pars intermedia. In order to determine whether the two populations are equally responsive to coculture conditions, we microdissected the hypothalamus of 17-day-old rat fetuses in two fragments containing cell bodies from the A12 and from the A14 regions, prepared neuronal cultures from both portions and incubated them separately with intermediate lobe cells. The presence of intermediate lobe cells increased tyrosine hydroxylase levels in both dopaminergic neuron subsets, but morphological differentiation was accelerated in dopaminergic neurons originating in the arcuate nucleus only. We then investigated whether physical contact between developing arcuate neurons and their target cells was a prerequisite of the morphological effect by interposing a semipermeable membrane between cultivated neurons and intermediate lobe cells in transwell culture dishes. The morphological effect was no longer observed under transwell coculture conditions, pointing to the involvement of membrane-bound molecules. Accordingly, the stimulating effect of coculture on arcuate dopaminergic neurons was completely abolished by the removal of polysialic acid on neural cell adhesion molecules by endoneuraminidase N treatment. Thus, maturation of A12 and A14 dopaminergic neurons exhibits differential susceptibility to intermediate lobe target cells, and polysialylated-NCAM is required for the contact-dependent effect.

Pituitary cytokine and growth factor expression and action

The complex range of pituitary regulatory mechanisms reviewed here underlies the critical function of the pituitary in sustaining all higher life forms. Thus, the ultimate net secretion of pituitary hormones is determined by signal integration from all three tiers of pituitary control. It is clear from our current knowledge that the trophic hormone cells of the anterior pituitary are uniquely specialized to respond to these signals. Unravelling their diversity and complexity will shed light upon the normal function of the master gland. Understanding these control mechanisms will lead to novel diagnosis and therapy of disordered pituitary function (357).

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.

Expression of platelet-derived growth factor B-chain in the mature rat brain and pituitary gland

For better understanding of the role of platelet-derived growth factor (PDGF) B-chain in the brain, the expression of PDGF B-chain was studied in the mature rat brain at both protein and mRNA levels, by assay of PDGF B-chain-related mitogenic activity, Northern blot, in situ hybridization and immunohistochemistry. It was shown that (1) mature rat brain contained substantial PDGF B-chain-related mitogenic activity, (2) significant amounts of two sizes of transcripts (3.5 kb, widely, and 2.6 kb, weakly and in narrower areas) were expressed in the brain, and (3) the transcripts were localized in ubiquitous neurons by in situ hybridization, with the strongest signal in hippocampal pyramidal cells, which distribution almost corresponded with that of the immunoreactive products. The abundant neuronal localization of the transcript and protein, as well as the neuronal expression of the receptor reported elsewhere, suggests the role of the growth factor in neuronal cells as a neuronal regulatory and/or trophic agent acting by autocrine loop or by neuron to neuron interaction. However, there was an apparent discrepancy in part, in the distribution between transcripts and immunoreactivity; that is, transcripts were expressed intensely in the intermediate pituitary lobe with only a scattered immunoreactivity, and the opposite situation was observed in the accessory olfactory nerve and posterior pituitary lobe. This might suggest that PDGF B-chain is transported or secreted in these foci.

Update of the NGF saga

Nerve growth factor (NGF), initially characterized for its survival and differentiating actions on embryonic sensory and sympathetic neurons, is now known to display a greatly extended spectrum of biological functions. NGF exerts a profound modulatory role on sensory nociceptive nerve physiology during adulthood which appears to correlate with hyperalgesic phenomena occurring in tissue inflammation. Other newly detected NGF-responsive cells belong to the hematopoietic-immune and neuroendocrine systems. In particular, mast cells and NGF both appear to be involved in neuroimmune interactions and tissue inflammation, with NGF acting as a general "alert" molecule capable of recruiting and priming both local tissue and systemic defense processes following stressful events. NGF can thus be viewed as a multifactorial mediator modulating neuroimmune-endocrine functions of vital importance to the regulation of homeostatic interactions, with potential involvement in pathological processes deriving from dysregulation of either local or systemic homeostatic balances.

Distribution of neurofilament proteins and peripherin in the rat pituitary gland

The distribution of neurofilament proteins and peripherin in the pituitary gland of the rat was studied with a panel of monoclonal and polyclonal antibodies recognizing different neurofilament subunits. In the posterior lobe, a dense plexus of neurofilament- and peripherin-immunoreactive fibers was seen. In the intermediate lobe neurofilament- and peripherin-immunoreactivity was seen only in nerve fibers in the connective tissue septa, while no immunoreactivity was seen in parenchymal nerve fibers. Bilateral sympathetic ganglionectomy did not affect peripherin-immunoreactivity, indicating that the peripherin-immunoreactive fibers are of central origin. In the anterior lobe, a few solitary neurofilament- and peripherin-immunoreactive fibers were observed. Western blotting confirmed the presence of 150 kD and 200 kD neurofilament proteins in the posterior lobe. No neurofilament protein was detected in the intermediate and anterior lobes. Abundant intermediate filaments were seen with electron microscopy in the nerve fibers of the connective tissue septa in the intermediate lobe. In the parenchymal nerve fibers only microtubules were seen, indicating that the lack of neurofilament immunoreactivity is due to absence of neurofilaments.

Colocalization of NGF and TSH-like immunoreactivity in cultures of adult rat anterior pituitary cells

Nerve growth factor (NGF) has been well-characterized with respect to its role as a trophic agent for various peripheral nervous system (PNS) and central nervous system (CNS) neuronal populations. Recent evidence indicates that NGF may also play a functional role in endocrine systems, although investigations in this field are only beginning to define sites of action and molecular mechanisms involved in NGF-endocrine interactions. A potential site for such an interaction to occur is within the pituitary. Previous investigations have demonstrated the presence of NGF and NGF receptors in the pituitary and our group has recently reported the presence of NGF-like immunoreactivity exclusively within the thyrotrophic cells of the anterior pituitary of the adult rat. Since many questions regarding how NGF interacts in the anterior pituitary will be more efficiently addressed using an in vitro system, it was necessary to first determine if cultured adult anterior pituitary cells retain the NGF-like staining and unique association of NGF with thyroid-stimulating hormone-producing cells seen in vivo. Results of the present investigation confirm that cultured anterior pituitary cells retain the characteristics previously observed in vivo and further demonstrate the stability of these cells and their specific NGF and pituitary hormone contents in culture for as long as 6 days.

Neurotrophins and their receptors in the rat pituitary gland: regulation of BDNF and trkB mRNA levels by adrenal hormones

We studied the expression of messenger ribonucleic acids (mRNAs) for neurotrophins and neurotrophin receptors in the rat pituitary gland and examined the influence of adrenal hormones on their mRNA levels, using in situ hybridization and Northern blot analysis. The only neurotrophin present at detectable levels in the pituitary was brain-derived neurotrophic factor (BDNF), which was observed in the anterior and intermediate lobes. Several transcripts of the putative receptor for BDNF, trkB, were present in the anterior and posterior lobes of the pituitary. A low amount of trkC mRNA was found in both the anterior and the intermediate lobe. Dexamethasone treatment decreased both BDNF and trkB mRNA levels in the anterior lobe of the pituitary. Adrenalectomy had no effect on trkB expression, but it decreased BDNF mRNA levels in comparison to the control animals. This effect could not be reversed by dexamethasone substitution, suggesting that BDNF, mRNA levels may be regulated not only by glucocorticoids but also by other adrenal hormones. These results demonstrate that BDNF, trkB and trkC are expressed in the pituitary gland and that glucocorticoids and possibly other adrenal hormones may modulate pituitary functions by regulating the expression of neurotrophic factors and their receptors. Whether BDNF acts as a secreted hormone, a trophic factor, or has autocrine/paracrine functions within the pituitary through its receptor, trkB, remains to be studied.

Nerve growth factor receptor in neural lobe of rat pituitary gland: immunohistochemical localization, biochemical characterization and regulation

Nerve growth factor-receptor immunoreactivity was detected in the neural lobe of the pituitary gland in developing and adult rats of both sexes. The presence of nerve growth factor receptor in the neural lobe was further verified by a quantitative 125I-nerve growth factor/crosslink/immunoprecipitation assay and subsequent visualization by SDS-PAGE autoradiography. Nerve growth factor-receptor immunoreactivity was detected in the neural lobe of postnatal 5-day-old rats, had increased by 2 months and was much higher in 1-year-old rats. In 2-month-old rats, no immunoreactivity was observed in anterior or intermediate lobes. Pituitary stalk transection in young adult rats greatly increased the expression of nerve growth factor-receptor immunoreactivity in the neural lobe, although the staining pattern remained the same. This increase began 3 days after surgery, and reached peak levels at approximately 15 days. Other physiological or non-physiological changes did not alter the nerve growth factor-receptor immunoreactivity in the neural lobe; these changes included dehydration, pregnancy and lactation, castration of male rats, bilateral superior cervical ganglionectomy and intraventricular injection of colchicine. Intravenously injected 125I-nerve growth factor was specifically accumulated in both normal and denervated neural lobe. Nerve growth factor-receptor immunohistochemical electron microscopy showed that the receptor-positive cells are fusiform and found both inside and outside the basal lamina that delimits the neural lobe parenchyma. Based upon the anatomical localization, morphology and response to axotomy, we identify, at least the perivascular component, as microglia. These data suggest a role for nerve growth factor and/or nerve growth factor receptor in microglial function.