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Endocrinology Vol. 140, No. 11 5348-5355
Copyright © 1999 by The Endocrine Society

ARTICLES

Pituitary Lactotroph Adenomas Develop after Prolonged Lactotroph Hyperplasia in Dopamine D2 Receptor-Deficient Mice1

Sylvia L. Asa, Michele A. Kelly, David K. Grandy and Malcolm J. Low

Department of Pathology and Laboratory Medicine (S.L.A.), Mount Sinai Hospital, University of Toronto, Toronto, Ontario, M5G 1X5 Canada; and Department of Physiology and Pharmacology (D.K.G.) and Vollum Institute (M.A.K., M.J.L.) Oregon Health Sciences University, Portland, Oregon 97210

Address all correspondence and requests for reprints to: Dr. Sylvia L. Asa, Department of Pathology and Laboratory Medicine, Mount Sinai Hospital 600 University Avenue, Toronto, Ontario, Canada M5G 1X5. E-mail: sasa{at}mtsinai.on.ca


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Note Added in Proof
 References
 
Tuberoinfundibular dopamine tonically inhibits PRL expression and secretion from the pituitary gland by the activation of dopamine D2 receptors (D2R) localized on lactotrophs. Mutant female mice that lack D2Rs have persistent hyperprolactinemia but also develop extensive hyperplasia of pituitary lactotrophs and peliosis of the adenohypophysis at 9 to 12 months of age, while age-matched male D2R-deficient mice have no morphologic adenohypophysial lesion. We now report that both female and male D2R-deficient mice 17 to 20 months of age develop pituitary lactotroph adenomas. Of 12 aged female mice examined, all developed monohormonal PRL-immunoreactive neoplasms that had a characteristic juxtanuclear Golgi pattern of PRL staining and loss of the reticulin fiber network. Several of these adenomas were 50-fold larger than normal glands with marked suprasellar extension and invasion of brain but no gross evidence of distant metastases. They also had striking peliosis that was more marked than the lesion seen in the hyperplastic pituitaries of the younger females. These findings demonstrate that a chronic loss of neurohormonal dopamine inhibition promotes the hyperplasia-neoplasia sequence in adenohypophysial lactotrophs. Our results are analogous to previous data indicating that protracted stimulation of adenohypophysial cells by hormones or growth factors results in proliferation with initial hyperplasia followed by the development of neoplasia. Six aged male D2R-deficient mice had slightly enlarged anterior pituitaries similar in size to normal female glands. However, each case exhibited multifocal, microscopic lactotroph adenomas with strong nuclear immunoreactivity for estrogen receptors and Pit-1 transcription factor. The unexpected development of adenomas in males without preexisting or concomitant hyperplasia suggests that prolonged loss of dopamine inhibition may also cause neoplasia by distinct cellular mechanisms in male and female animals.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Note Added in Proof
 References
 
HYPOTHALAMIC HORMONES REGULATE hormone synthesis and release by adenohypophysial cells and some pituitary tumors. It has been suggested that increased stimulation or decreased inhibition by the hypothalamic peptides that physiologically regulate the hormonal activity of adenohypophysial cells may also play a role in the development of pituitary adenomas (1). Pituitary lactotroph adenomas have been attributed to a lack of dopaminergic inhibition mediated by the D2 subtype of dopamine receptors. The etiologic role of decreased hypothalamic inhibition was supported by the documentation of vascular changes including arteriogenesis in lactotroph adenomas. This finding led to speculation that neovascularization from the systemic circulation, which has negligible levels of dopamine, allows lactotrophs to escape dopaminergic tonic inhibition (2). The pituitaries of patients bearing lactotroph tumors have been reported to have associated hyperplasia of lactotrophs in some cases (3, 4, 5). This finding has been interpreted as reflective of an underlying hormone-regulatory imbalance as the cause of lactotroph adenomas.

There is only limited evidence for a direct role of tuberoinfundibular dopamine acting on D2 receptors in normal adenohypophysial cells other than lactotrophs. Dopamine can directly stimulate GH release from cultured rat primary pituitary cultures, an effect apparently mediated by D1 receptors (6, 7). Human tumor cells derived from GH-producing adenomas often exhibit decreased GH release in response to dopamine; this effect is mimicked by D2R agonists and blocked by D2R antagonists (8). Concomitant elevation of serum PRL in patients with acromegaly and mixed GH- and PRL-producing adenomas predicts a more robust reduction of GH by D2R agonists (9). These findings may be explained by the expression of classical D2R pharmacological responses in mammosomatotrophs but not pure tumoral somatotrophs derived from cultured human adenomas (10). Moreover, the rat mammosomatotroph GH3 cell line can be induced to express D2R binding sites by treatment of the cells with nerve growth factor (11). Together these studies suggest that D2R expression in somatotroph adenomas, in contrast to lactotroph adenomas, is secondary to tumor induction or indicative of the tumor origin from a precursor mammosomatotroph cell.

To better understand the relationship between functionally reduced dopamine inhibition of pituitary function and the development of pituitary tumors, we studied mutant mice with a targeted disruption of the gene encoding the D2R. D2R-deficient mice exhibit hyperprolactinemia due to derepression of the adenohypophysial target cells of dopamine (12). Using this animal model, we studied the effect of primary D2R gene underexpression and the consequent protracted lack of dopamine signaling by the D2R. In a previous study, we suggested that a lack of dopaminergic inhibition resulted in lactotroph hyperplasia in female mice studied at one year of age (12). We now report the consistent development of pituitary adenomas in mice of both sexes at 17 to 20 months of age. These tumors are composed only of lactotrophs with no detectable mammosomatotroph component. These results provide strong evidence that sustained lack of dopamine neurohormonal inhibition promotes the development of neoplasia in the pituitary.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Note Added in Proof
 References
 
Generation of mutant mice and characterization of D2 receptors
D2R-deficient mice, official strain designation Drd2tm1low by the Induced Mutant Resource at The Jackson Laboratory (Bar Harbor, ME), were generated in our laboratory as described previously (12). Briefly, a targeting vector for homologous recombination was constructed to delete nucleotide sequences in exon 8 of the mouse D2dr gene that encode the last two transmembrane domains, the third extracellular loop, and the intracytoplasmic carboxyl-terminal tail of the receptor. Correctly targeted D3 embryonic stem cells (129/SvPas) were microinjected into C57BL/6J blastocysts. Mice were genotyped by a combination of Southern blot or PCR analysis of genomic DNA. Homozygous D2R-deficient mice have no detectable D2R binding sites in striatal membranes (12), no detectable D2R-like immunoreactivity in the brain using antipeptide antisera raised against three separate receptor domains (unpublished data), no further increase in serum PRL after administration of the D2R antagonist haloperiol (12), and an absence of locomotor responses to selective D2R antagonists or agonists (13) and unpublished data).

Aged mice used for these studies included both F2 hybrid animals on a mixed genetic background (129/Sv, C57BL/6) and N5 incipient congenic animals generated by backcrossing the mutant allele on the C57BL/6J background for five consecutive generations (13). F2 and N5 D2R +/+ siblings from the mutant colonies, wild-type 129/SvEvTac mice (Taconic Farms, Inc. Germantown, NY) and wild-type C57BL/6J mice (The Jackson Laboratory) served as controls. Mice were separated by sex, housed in groups of 4 to 5 with mixed genotypes, and had ad libitum access to water and standard rodent chow. The experimental protocols were approved by the local Institutional Animal Care and Use Committee and conducted in accordance with the Public Health Service Policy on Humane Care and Use of Laboratory Animals.

Morphologic methods
D2R-deficient mice, 12 females and 6 males aged 17 to 20 months of age, and age- and sex-matched controls were killed by decapitation. At autopsy, the pituitaries were removed and weighed; the other organs were carefully inspected.

For light microscopy, portions of each pituitary were fixed in buffered formalin and embedded in paraffin; sections 4–5 µm thick were stained with hematoxylin and eosin and with the Gordon-Sweet silver method to demonstrate the reticulin fiber network.

Immunocytochemical stains to localize adenohypophysial hormones were performed using the streptavidin-biotin-peroxidase complex technique. Primary antisera directed against rat pituitary hormones were used at the specified dilutions with overnight incubations: GH 1:2500, PRL 1:2500, ß-TSH 1:3000, ß-FSH 1:600, ß-LH 1:2500 (National Hormone and Pituitary Program, Rockville, MD) and ACTH prediluted preparation further diluted 1:20 (DAKO Corp., Carpinteria, CA). The reaction products were visualized with the Ultrastreptavidin kit (Signet Laboratories, Inc., Dedham, MA) and 3,3'-diaminobenzidine (DAB). Rabbit polyclonal antisera directed against Pit-1 (Berkeley Antibody Co., Richmond, CA) and SF-1 (Upstate Biotechnology, Inc., Lake Placid, NY), both diluted 1:400, and a monoclonal antibody against ER{alpha} (clone 6F11, Novocastra, Newcastle-Upon-Tyne, UK) diluted 1:70 were applied for 60 min after microwave antigen retrieval and the reaction was detected with the ABC Elite kit (Vector Laboratories, Inc. Burlingame, CA) and DAB.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Note Added in Proof
 References
 
Gross appearance
The aged D2R-deficient mice appeared similar to age-matched controls in general health. There continued to be no obvious neurological impairment in the mutant mice, based on direct observation of locomotion in their home cages, as reported previously in younger animals (13). Male N5 congenic C57BL/6J D2R-deficient mice were noticeably smaller than their wild-type siblings (Table 1Go). Absolute testicular weight was lower in the mutant males, but relative to body weight was significantly greater compared with controls (6.5 ± 0.2 and 5.3 ± 0.4 mg/g, mean ± SEM, P = 0.02 by Student’s t test). Weights of the androgen-sensitive preputial and bulbourethral glands were the same in D2R-deficient and wild-type males (Table 1Go). Both male and female N5 congenic mutants could be distinguished visually from wild-type mice by subtle changes in their pelage consisting of an altered luster in terminal hairs, decreased guard hairs, and decreased amounts of the normally hypopigmented hairs on the perineum and surrounding the nipples. A single 18-month-old female D2R-deficient mouse was killed because of acute weight loss and a deformed cranium suggestive of an underlying massive intracranial tumor; this was in fact one of the largest pituitary adenomas that displaced the intracranial contents sufficiently to cause the bulge in the cranium.


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  		Table 1. Male reproductive organs in 12- to 20-month-old D2 receptor knockout (KO) and wild-type (WT) mice

 
All 12 female D2R-deficient mice, 17 to 18 months of age when killed, had markedly enlarged and congested pituitary glands extending upwards out of the sella turcica (Fig. 1aGo). Grossly, the neural and intermediate lobes were intact and were of normal size; the pituitary enlargement was attributable to multiple nodules in the adenohypophysis. Several tumors extended to involve the adjacent brain. The weight of the pituitaries varied from 12 to 176 mg. These data contrast with the 2- to 3-fold enlargement characteristic of the pure hyperplasia stage in 9–15 month old mutant females (Table 2Go). The largest tumor infiltrated between the cerebellum and the colliculi and was visible at the surface; however, it was easily separated from the brain with blunt dissection.



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  		Figure 1. Gross and histologic appearance of pituitary adenomas in D2R-deficient female mice. a, The pituitaries of two 18-month-old D2R-deficient female mice (left) are enlarged and congested compared with normal pituitary of an age-matched control female mouse (right). b, The pituitary of a 17-month-old D2R-deficient female mouse is grossly enlarged and markedly distorted. The posterior and intermediate lobes (arrow) are normal but compressed; the surrounding mass contains abundant blood. (H&E stain; magnification, x10.) c, The tumor nodule in the adenohypohysis of a D2R-deficient female mouse has striking peliosis, characterized by dilated blood-filled spaces that lack endothelial lining. There is also deposition of hemosiderin. (H&E stain; magnification, x30.) d, The polygonal tumor cells form cords and solid nests. They have chromophobic cytoplasm and occasional binucleate forms are seen (arrow). (H&E stain; magnification, x150.) e, Scattered tumor cells have marked nuclear pleomorphism. (H&E stain; magnification, x200.) f, At the periphery of the pituitary of a 17-month-old female mouse, the reticulin stain shows the distorted pattern of the surrounding hyperplasia. The nodules exhibit total breakdown of the reticulin fiber network, diagnostic of adenoma. (Gordon Sweet silver stain; magnification, x30).

 

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  		Table 2. Pituitary gland weights (mg) in female D2 receptor knockout (KO) and wild-type (WT) mice

 
The six aged D2R-deficient males had minimally enlarged, hyperemic pituitary anterior lobes with grossly normal posterior and intermediate lobes. The approximate weights of these glands were 2 to 3 mg, similar to normal female pituitaries or about twice the size of normal male glands.

Pathological features of the adenohypophyses were the same in F2 hybrid and N5 congenic D2R-deficient mice so no further distinction is presented here.

Histologic findings
In all 12 D2R-deficient females aged, 17 to 18 months, histologic examination showed markedly abnormal adenohypophysial morphology with histologic evidence of pituitary tumors (Fig. 1bGo). The posterior lobes were identified as normal neurohypophyses, and the intact intermediate lobes were of normal size and architecture. In contrast, the anterior lobes were grossly enlarged and somewhat nodular with marked peliosis, characterized by pools of extravasated blood cells that lacked endothelial lining (Fig. 1cGo). At the periphery and between nodules, the tissue was similar to that seen previously in 9- to 12-month-old mice as reported elsewhere (12); the acinar structure was slightly enlarged, and there was less prominent peliosis. The large nodules were lobulated tumors that had the appearance of multifocal neoplasms. They were composed of sheets and cords of monomorphous cells with occasional binucleate cells (Fig. 1dGo) and scattered mitotic figures. Some tumors showed marked nuclear pleomorphism (Fig. 1eGo). The tumor cells were polygonal with chromophobic cytoplasm that sometimes harbored pale juxtanuclear globular structures. The Gordon-Sweet silver stain documented the presence of a distorted reticulin network at the periphery of the adenomas with absence of reticulin fibers within the tumors (Fig. 1fGo). These features fulfilled the criteria for the diagnosis of adenoma (14).

All six male D2R-deficient mice aged 17–20 months had small multifocal nodules (Fig. 2aGo) with breakdown of the reticulin fiber network (Fig. 2bGo), indicating the development of multiple adenomas.



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  		Figure 2. Histology and immunohistochemistry of pituitary adenomas in D2R-deficient male mice. a, The pituitary of a 17-month-old D2R-deficient male mouse is not grossly enlarged, but the lateral lobe contains a small adenoma with peliosis. (H&E stain; magnification, x60.) b, The reticulin pattern in the pituitary of a 17-month-old male D2R-deficient mouse has a nodule with total breakdwon of the reticulin fiber network that is diagnostic of adenoma. (Gordon Sweet silver stain; magnification, x60.) c, The pituitary adenoma cells have a strong juxtanuclear Golgi pattern of PRL immunoreactivity characteristic of sparsely granulated lactotrophs. (Avidin-biotin-peroxidase complex technique; magnification, x200.) d, The tumor cells have strong nuclear reactivity for Pit-1. (Avidin-biotin-peroxidase complex technique; magnification, x170.) e, The tumor cells exhibit nuclear staining for ER. (Avidin-biotin-peroxidase complex technique; magnification, x170.)

 
Immunohistochemistry
In the adenohypophyses of the female D2R-deficient mice, the majority of tumor cells in all adenomas showed strong immunoreactivity for PRL with two patterns (Fig. 3aGo). The majority of the cells had a juxtanuclear globular staining pattern indicating the Golgi region; this is characteristic of sparsely granulated lactotroph adenomas. Scattered tumor cells had a densely granulated morphology with PRL immunoreactivity throughout the cytoplasm. The tumor cells also exhibited strong nuclear reactivity for Pit-1 (Fig. 3bGo) as well as estrogen receptor {alpha} (ER) (Fig. 3cGo) and were negative for SF-1 (not shown).



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  		Figure 3. Immunohistochemistry of pituitary adenomas in D2R-deficient female mice. a, The pituitary adenoma cells have a strong juxtanuclear Golgi pattern of PRL immunoreactivity characteristic of sparsely granulated lactotrophs. (Avidin-biotin-peroxidase complex technique; magnification, x100.) b, The tumor cells have strong nuclear reactivity for Pit-1. (Avidin-biotin-peroxidase complex technique; magnification, x100.) c, The tumor cells exhibit variable nuclear staining for ER. (Avidin-biotin-peroxidase complex technique; magnification, x170.) d, Scattered cells in the hyperplastic pituitary around the adenomas are immunoreactive for ACTH, as is the intermediate lobe (bottom right), but the adenoma is negative. (Avidin-biotin-peroxidase complex technique; magnification, x100.)

 
GH, ACTH, ß-TSH, ß-FSH, and ß-LH were detected in cells within the hyperplastic areas but not within the tumors; the cells containing these hormones were of normal size and shape. ACTH immunoreactivity was also present in the intermediate lobes and remained entirely normal in intensity and distribution (Fig. 3dGo).

The pituitaries of the six male D2R-deficient mice had microscopic foci of lactotroph adenomas with juxtanuclear PRL staining (Fig. 2cGo) and nuclear Pit-1 (Fig. 2dGo) and ER reactivity (2e). The nodules were interspersed within otherwise unremarkable adenohypophysis that had the usual distribution of cells containing GH, ACTH, ß-TSH, ß-FSH, and ß-LH (not shown).

Control animals
Sibling D2R+/+ mice from the mutant colonies, wild-type 129/SvEvTac mice and C57BL/6J mice served as controls in this study; in all control mice aged 17–18 months, there was no evidence of adenohypophysial hyperplasia or neoplasia on examination by histology and immunohistochemistry (not shown). There was no evidence of peliosis in the pituitaries of these animals.


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Note Added in Proof
 References
 
For many years there has been controversy regarding the basis of pituitary tumorigenesis. The putative role of hypothalamic stimulation in pituitary tumor development has received support from a substantial body of evidence. Hypothalamic stimulating hormones can cause proliferation of their target adenohypophysial cells (1), and adenohypophysial hyperplasia is well documented as a consequence of chronic stimulation in patients with extrahypothalamic tumors secreting hormones such as GHRH or CRH (1). In vitro, human pituitary adenoma cells are known to respond to stimulation by hypothalamic stimulatory hormones, indicating the presence of appropriate receptors on these tumors. Moreover, transgenic mice overexpressing GHRH have proven that prolonged chronic GHRH overstimulation alone can result in tumor formation (15, 16).

The pathogenesis of lactotroph adenomas may involve defective inhibition by hypothalamic dopamine or excessive stimulation by a putative PRL-releasing factor such as TRH, vasoactive intestinal peptide (VIP) (17), or other novel peptides (18, 19). Lactotroph proliferation during pregnancy (3, 20) has been attributed to estrogen which, in rodents, causes pituitary lactotroph hyperplasia and adenoma (21). Recent evidence in galanin knockout mice suggests that lactotroph-derived galanin is necessary for the proliferative response to estrogen (22). Oral contraceptives have been implicated in growth of some lactotroph adenomas (23) and were thought to be responsible for an increase in the incidence of lactotroph adenomas in the late 1970s; however, it is more likely that the increased incidence reflects awareness of the entity due to the discovery of PRL. Although high doses of estrogen undoubtedly stimulate lactotrophs, and a few lactotroph adenomas may grow during pregnancy (24), these tumors are not more numerous or larger during gestation (20) and there is little evidence that low dose oral contraceptives play a significant role in pituitary tumor development.

Because lactotrophs are regulated by tonic dopaminergic inhibition mediated predominantly by the D2R (25, 26), loss or reduction of hypothalamic inhibition is a plausible etiologic factor in the development of lactotroph adenomas. Activation of the D2R results in altered cAMP production, potassium and calcium channel fluxes, phosphatidyl inositol turnover, and intracellular calcium concentrations (27). While most tumors can be functionally suppressed and reduced in size by dopamine (23, 24), the dopaminergic resistance that is found in some of these tumors may implicate absent dopamine receptors or altered receptor-coupled signal transduction as an etiological factor (28, 29). Thus far, however, investigation of the D2R gene has revealed it to be structurally intact in human lactotroph adenomas (30). More detailed characterization of the D2R and associated second messenger pathways in tumors from patients with variable dopamine sensitivity is required to resolve this matter.

A role of dopamine in the proliferation of lactotrophs has previously been investigated pharmacologically using D2R antagonists in the estrogen-sensitive Wistar rat. Acute treatment with either sulpiride or estradiol alone caused small increases in the mitotic index of lactotrophs, whereas the same treatments in combination produced a synergistic effect (31). Sulpiride’s action on mitotic index was blocked by an antiestrogen, whereas the estrogen effect was blocked with a dopaminergic agonist, suggesting that both an inhibition of D2R function and estrogen actions are important for stimulating lactotroph proliferation. Similar short-term effects have been reported (32) while the combined treatment of rats with a neuroleptic and estrogen for 60 days was similar to estrogen alone (33). These and similar studies have not determined a cellular mechanism to explain the interaction between dopamine and estrogen effects on lactotroph mitogenesis, and to our knowledge, there are no published reports of chronic neuroleptic treatment in rats causing the hyperplasia/neoplasia sequence that we have observed in the D2R-deficient mice.

Dopamine may also be important in the regulation of GH, TSH, and even gonadotropins (25). A proportion of patients with GH-producing adenomas respond to therapy with dopaminergic agonists (34). Thyrotroph adenomas has been shown to have abnormal but highly variable responses to dopamine (35, 36). However, there is no evidence of alterations in the D2R gene in adenomas that secrete GH or TSH (30). Gonadotroph adenomas may also respond to dopaminergic inhibition (37, 38, 39). It appears, however, that dopamine is not critical for the regulation of these adenohypophysial cell types because our animals have no detectable morphologic alterations of somatotrophs, thyrotrophs, or gonadotrophs.

Hormonal stimulation may play a role in the development of several neoplasms (40) including carcinomas of breast, endometrium, and prostate (41, 42, 43). Among endocrine organs, thyroid tumors are thought to be TSH dependent (44), some tumors of adrenal cortex are thought to be dependent on ACTH stimulation (45), and chronic overstimulation has been implicated in the formation and growth of gonadal neoplasms (46, 47). Loss of feedback inhibition may account for the development of parathyroid adenomas in tertiary hyperparathyroidism (48) and of some pituitary adenomas composed of corticotrophs, thyrotrophs, or gonadotrophs (14); the complex regulation of these cells raises the possibility that their proliferation is also modulated by sustained hormonal stimulation.

The present study provides the first evidence that disruption of a single neurohormone receptor gene leads to tumor formation; it is also the first demonstration that loss of hormonal inhibition alone can result in frank tumor formation. It should be stressed, however, that other pathogenetic factors are almost certainly necessary for neoplastic transformation in these animals. According to the multistep theory of carcinogenesis (49), an irreversible initiating event is required to permanently alter the genome of a cell and predispose it to the neoplastic phenotype, whereas promotion, long-term stimulation of cell proliferation, is necessary for an initiated cell to express the transformed phenotype. Lack of the D2R likely encourages or permits the development of tumors by increasing the population of proliferating cells that are susceptible to oncogenic factors or mutation. This proliferation may be mediated by other factors, such as various growth factors that are known to exist in the anterior pituitary (1). This and previous models of hormone- or growth factor-mediated tumorigenesis in the pituitary (15, 16, 50) all have in common the development of multifocal neoplasms in older animals after prolonged hyperplasia. In the human, the situation is probably analogous in that the majority of pituitary adenomas develop from transformed cells that are dependent on hormonal and/or growth factor stimulation for tumor progression (1). The evidence for the tumorigenic potential of hormones and growth factors in humans and in these animal models indicates that they stimulate hyperplasia and, after prolonged exposure, true adenoma development occurs. This is readily interpreted as the result of genetic alteration of cells in a fertile environment; proliferating cells would be at increased risk of genetic alteration during mitosis and manifestation of genetic alterations would be precipitated by the growth stimulus. However, the identity of the gene(s) responsible for transformation remains uncertain (1), and further analyses of these tumors may yield information in this regard.

The absence of associated lactotroph hyperplasia in aged male mice with microscopic adenomas raises the possibility that a different series of cellular and genetic events occur secondary to the loss of D2R signaling in male and female mice, but both pathways eventually result in neoplasia. Alternatively, because the age of onset of histopathologically identified adenomas is similar in males and females, the hyperplastic lesion of lactotrophs present only in females could be due to gender-specific hormonal and growth factor responses that are actually independent of the events causing adenomas. Future studies should also distinguish between these models of pituitary neoplasia in the D2R-deficient mice.


   Note Added in Proof
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Note Added in Proof
References
 
Serum PRL levels ranged to greater than 10,000 ng/ml from female D2R-deficient mice with large adenomas, and there was a direct correlation between pituitary weight and serum PRL. Two male D2R-deficient mice with microscopic lactotroph adenomas had elevated serum prolactin levels of 303 and 324 ng/ml.


   Acknowledgments
 
The authors acknowledge the technical assistance of Kelvin So, Sophie Lapostolle, and Shane Hentges.


   Footnotes
 
1 This work was supported by Grants of the Medical Research Council of Canada (MT 14463, MT 14404) and the NIH (DA 12062). Back

Received April 20, 1999.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Note Added in Proof
 References
 

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