Prolactin replacement in adult dwarf mice does not reverse the deficit in tuberoinfundibular dopaminergic neuron number

MicroRNA-7a2 Regulates Prolactin in Developing Lactotrophs and Prolactinoma Cells

Prolactin production is controlled by a complex and temporally dynamic network of factors. Despite this tightly coordinated system, pathological hyperprolactinemia is a common endocrine disorder that is often not understood, thereby highlighting the need to expand our molecular understanding of lactotroph cell regulation. MicroRNA-7 (miR-7) is the most highly expressed miRNA family in the pituitary gland and the loss of the miR-7 family member, miR-7a2, is sufficient to reduce prolactin gene expression in mice. Here, we used conditional loss-of-function and gain-of-function mouse models to characterize the function of miR-7a2 in lactotroph cells. We found that pituitary miR-7a2 expression undergoes developmental and sex hormone-dependent regulation. Unexpectedly, the loss of mir-7a2 induces a premature increase in prolactin expression and lactotroph abundance during embryonic development, followed by a gradual loss of prolactin into adulthood. On the other hand, lactotroph development is delayed in mice overexpressing miR-7a2. This regulation of lactotroph function by miR-7a2 involves complementary mechanisms in multiple cell populations. In mouse pituitary and rat prolactinoma cells, miR-7a2 represses its target Raf1, which promotes prolactin gene expression. These findings shed light on the complex regulation of prolactin production and may have implications for the physiological and pathological mechanisms underlying hyperprolactinemia.

Prolactin induces tuberoinfundibular dopaminergic neurone differentiation in Snell dwarf mice if administered beginning at 3 days of age

The hypothalamic tuberoinfundibular dopaminergic (TIDA) neurones secrete dopamine, which inhibits prolactin secretion. TIDA neurone numbers are deficient in Ames (df/df) and Snell (dw/dw) dwarf mice, which lack prolactin, growth hormone and thyroid-stimulating hormone. Prolactin therapy initiated before 21 days maintains normal-sized TIDA neurone numbers in df/df mice and, when initiated as early as 7 days, maintains the maximum TIDA neurone numbers observed in dw/dw development, which are decreased compared to those in normal mice. The present study investigated the effect of prolactin dose and species on TIDA neurone development. Snell dwarf and normal mice were treated with saline, 5 microg of ovine prolactin (oPRL), 50 microg of oPRL, or 50 microg of recombinant mouse prolactin (rmPRL) beginning at 3 days of age. Brains were analysed at 45 days using catecholamine histofluorescence, and immunohistochemistry for tyrosine hydroxylase or bromodeoxyuridine. Normal mice had greater (P <or= 0.01) TIDA neurones than dw/dw, regardless of treatment. TIDA neurones in 50 microg oPRL-treated dw/dw mice were greater (P <or= 0.05) than those in 5 microg oPRL- and rmPRL-treated dw/dw mice, which were greater (P <or= 0.01) than those in saline-treated dw/dw mice. Fifty microgram oPRL-treated dw/dw mice also had greater (P < 0.01) TIDA neurone numbers than the maximum numbers observed in untreated dw/dw mice development. Among saline, 5 microg oPRL and 50 microg oPRL treatments, but not rmPRL, A14 neurone numbers were higher (P <or= 0.01) in normal compared to in dw/dw mice. The mechanism of TIDA neurone recruitment was investigated using bromodeoxyuridine (BrdU) treatment at intervals after 21 days. Mice treated with rmPRL, but not oPRL, had increased BrdU incorporation in the periventricular area surrounding the third ventricle and median eminence and in the arcuate nucleus. The data obtained in the present study indicate that oPRL, but not rmPRL, when given at a high enough dose, induces TIDA neurone differentiation in dw/dw mice. This supports neurotrophic effects of prolactin on TIDA neurones in early postnatal development that extends beyond maintenance of the cell population.

Long-term, homologous prolactin, administered through ectopic pituitary grafts, induces hypothalamic dopamine neuron differentiation in adult Snell dwarf mice

Pituitary prolactin (PRL) secretion is inhibited by dopamine (DA) released into the portal circulation from hypothalamic tuberoinfundibular DA (TIDA) neurons. Ames (df/df) and Snell (dw/dw) dwarf mice lack PRL, GH, and TSH, abrogating feedback and resulting in a reduced hypophysiotropic TIDA population. In Ames df/df, ovine PRL administration for 30 d during early postnatal development increases the TIDA neuron number to normal, but 30 d PRL treatment of adult df/df does not. The present study investigated the effects of homologous PRL, administered via renal capsule pituitary graft surgery for 4 or 6 months, on hypothalamic DA neurons in adult Snell dw/dw mice using catecholamine histofluorescence, tyrosine hydroxylase immunocytochemistry, and bromodeoxyuridine immunocytochemistry. PRL treatment did not affect TIDA neuron number in normal mice, but 4- and 6-month PRL-treated dw/dw had significantly increased (P < or = 0.01) TIDA (area A12) neurons compared with untreated dw/dw. Snell dwarfs treated with PRL for 6 months had more (P < or = 0.01) TIDA neurons than 4-month PRL-treated dw/dw, but lower (P < or = 0.01) numbers than normal mice. Periventricular nucleus (area A14) neuron number was lower in dwarfs than in normal mice, regardless of treatment. Zona incerta (area A13) neuron number was unchanged among phenotypes and treatments. Prolactin was unable to induce differentiation of a normal-sized A14 neuron population in dw/dw. Bromodeoxyuridine incorporation was lower (P < or = 0.01) in 6-month PRL-treated normal mice than in 6-month PRL-treated dwarfs in the subventricular zone of the lateral ventricle and in the dentate gyrus, and lower (P < or = 0.05) in 4-month untreated dwarfs than in 4-month untreated normal mice in the median eminence and the periventricular area surrounding the third ventricle. Thus, a PRL-sensitive TIDA neuron population exists in adult Snell dwarf mice when replacement uses homologous hormone and/or a longer duration. This finding indicates that there is potential for neuronal differentiation beyond early developmental periods and suggests plasticity within the mature hypothalamus.

Postnatal regression of hypothalamic dopaminergic neurons in prolactin-deficient Snell dwarf mice

Both Snell (Pit-1(dw) or (dwj), dw/dw) and Ames (Prophet of Pit-1(df), df/df) dwarf mice fail to produce prolactin (PRL) as well as GH due to deficient transcription factor Pit-1 activity and have reduced numbers of hypothalamic PRL-inhibiting area A12 tuberoinfundibular dopaminergic (TIDA) neurons. It has been reported that the TIDA deficit in Ames dwarf mice develops postnatally as a reduction in number after an initial increase that is comparable to that of normal siblings. The present study was designed to characterize A12 TIDA neuronal development in the Snell dwarf (dw/dw) compared with littermate normal mice. Brains of normal (DW/?) and dw(j)/dw(j) mice were examined at 7, 14, 21, 30, and > or = 60 postnatal days (d) by catecholamine fluorescence and quantification of neuron number after tyrosine hydroxylase immunostaining in dopaminergic (DA) areas A12, A13 (medial zona incerta), and A14 (periventricular nucleus). Fluorescence was less in dw/dw than in DW/? A12 perikarya and median eminence but was not reduced in other DA areas, such as substantia nigra, at all ages; A12 fluorescence was virtually absent in Snell dwarf adults. Numbers of TIDA neurons were comparable in normal and Snell dwarf mice at 7 d. In normal (DW/?) mice, A12 neurons increased in number to adult levels at 14 d and were significantly higher than in Snell dwarf (dw/dw) mice at 14 d (P < 0.05) and at subsequent ages (P < 0.01). In Snell dwarf mice, numbers of A12 neurons did not differ at 7, 14, and 21 d, decreased at 30 d (P < 0.05), and reached, at 60 d, 23% of the population in normal sibling mice (P < 0.01 compared with earlier ages). Neuron numbers in nonhypophysiotropic DA area A13 did not vary with age or phenotype. In A14, cell number was higher in both phenotypes at 14 d (P < 0.05 for DW/?; P < 0.01 for dw/dw); neuron number was lower in dw/dw than in DW/? mice at 30 d (P < 0.05) and 60 d (P < 0.01). Thus, compared with normal mice of the same strain, the A12 deficit is more severe in Snell (dw/dw) than in Ames (df/df) dwarf hypothalamus (48% of DF/?), as previously reported, and develops as a decline from the population present at 7 d rather than first increasing. A reduction in A14 neuron number also occurs in the Snell dwarf. Treatment of DW/dw- and dw/dw-containing litters with ovine PRL (50 mug/d, ip), beginning at 12 or 7 d and continuing until 42 d, resulted in TIDA neuron numbers in Snell dwarfs that were lower than those in normal siblings (P < 0.01 for both) but were higher than in untreated adult dwarfs and comparable to the TIDA population size in dwarfs at 7 d, indicating that PRL maintained this maximal number and prevented TIDA neuron dedifferentiation, which occurs in dwarf postnatal development.

Growth hormone-releasing hormone-producing and dopaminergic neurones in the mouse arcuate nucleus are independently regulated populations

Differentiation of hypophysiotropic neurones that regulate the secretion of growth hormone (GH) and prolactin is influenced by GH and prolactin. Genetic GH and prolactin deficiency in mutant rodent models such as the Ames dwarf (df/df) mouse results in an increase in the number of GH-stimulatory GH-releasing hormone (GHRH) neurones and a reduction of prolactin-inhibitory tuberoinfundibular dopaminergic (TIDA) neurones in the arcuate nucleus during postnatal development. The present study tested the hypothesis that these concomitant changes in numbers of tyrosine hydroxylase (TH)- and GHRH-immunoreactive neurones in df/df hypothalamus might represent a neuronal population of fixed number that undergoes a partial change in phenotype during postnatal development. To evaluate this possibility, the postnatal reduction of the df/df TIDA population was prevented by administering prolactin neonatally to preserve TH phenotype; dwarf and normal sibling mice were treated with daily injections of ovine prolactin or vehicle starting at postnatal day 12 and continuing for 30 days. Following this treatment, numbers of arcuate neurones containing GHRH or TH, or both, were quantified using immunocytochemistry. It was hypothesized that prolactin preservation of TH-immunoreactive cell number would be accompanied by either a decrease in the GHRH-producing population or an increase in numbers of cells producing both TH and GHRH. In prolactin-treated normal (DF/df) mice, numbers of arcuate TH-immunoreactive neurones were similar to those in vehicle-treated normals. Numbers of TH-positive neurones in prolactin-treated dwarfs were higher than in vehicle-treated dwarfs, and did not differ from numbers in DF/df. Numbers of GHRH-immunoreactive cells in vehicle-treated df/df were higher than in vehicle-treated DF/df, and were not different in prolactin-treated groups of either dwarf or normal mice. Neurones containing both TH and GHRH constituted 15% of the TH population, and 76% of the GHRH population, in control normal mice; in control dwarfs, double-labelled cells were 9.3% of TH and 9.9% of GHRH. Numbers of cells immunoreactive for both TH and GHRH were not affected by prolactin treatment in either mouse type. These results demonstrate that the increase in number of GHRH-expressing neurones in the df/df arcuate nucleus does not occur at the expense of the TH phenotype, and that this increase is not influenced by prolactin feedback. Although coexpression of TH and GHRH in a subpopulation indicates that TIDA and GHRH populations are not exclusive, they appear to be influenced independently by prolactin and GH signals during development.

Stimulatory effect of human, but not bovine, growth hormone expression on numbers of tuberoinfundibular dopaminergic neurons in transgenic mice

Mice transgenic for heterologous and ectopic GH expression serve as models for studying the feedback effects of elevated nonregulated GH on hypothalamic hypophysiotropic neurons as well as on peripheral function. For example, hypothalamic somatostatin expression has been shown to be increased markedly in mice bearing either bovine (b) or human (h) GH transgenes. Human, but not bovine, GH has lactogenic properties in mice, and appears to stimulate PRL-inhibiting tuberoinfundibular dopaminergic (TIDA) neurons. The present study was designed to determine the effect of a lifelong excess of hGH on dopamine (DA) expression in and numbers of TIDA neurons. Male mice of four transgenic lines were examined. The transgenic animals bore constructs of either bGH or hGH fused to either metallothionein (MT) or phosphoenolpyruvate carboxykinase (PEPCK) promoters; brains of transgenic mice were compared morphologically with those of nontransgenic littermates. Formaldehyde-induced catecholamine histofluorescence and tyrosine hydroxylase (TH) immunocytochemistry were examined in alternate brain sections; cell number was quantified for TIDA neurons (area A12) and a nonhypophysiotropic diencephalic DA area, the medial zona incerta (A13). Body weights were higher (P < 0.01) in PEPCK-GH than in MT-GH transgenic mice, as were serum levels of heterologous GH in those lines. In MT-hGH, but not MT-bGH or PEPCK-bGH, transgenic mice, A12 perikaryal fluorescence was enhanced, and ME fluorescence was reduced compared with those in control animals. The reduced ME DA is likely to reflect stimulation of TIDA neurons, because A12 TH-immunoreactive neuron number was increased by 34% in MT-hGH mice compared with that in controls (P < 0.05). In mice bearing the PEPCK-hGH construct, A12 TH neuron number was increased 47% (P < 0.001) compared with that in littermate controls. There were no differences in A13 cell number among animals, and A12 cell numbers in mice expressing bGH did not differ from control values. These results suggest that although extremely high levels of circulating bGH do not stimulate TIDA neurons, lifelong high levels of hGH have a stimulatory and graded effect on developmental differentiation of these cells for TH and DA production, supporting the concept of PRL as a trophic factor for TIDA neurons.