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M J Low

Publications and source records attributed to M J Low.

At least 55 records · Page 3Linked to original sources
PubMed1999

Dopamine D2 receptor-deficient mice exhibit decreased dopamine transporter function but no changes in dopamine release in dorsal striatum.

Presynaptic D2 dopamine (DA) autoreceptors, which are well known to modulate DA release, have recently been shown to regulate DA transporter (DAT) activity. To examine the effects of D2 DA receptor deficiency on DA release and DAT activity in dorsal striatum, we used mice genetically engineered to have two (D2+/+), one (D2+/-), or no (D2-/-) functional copies of the gene coding for the D2 DA receptor. In vivo microdialysis studies demonstrated that basal and K+-evoked extracellular DA concentrations were similar in all three genotypes. However, using in vivo electrochemistry, the D2-/- mice were found to have decreased DAT function, i.e., clearance of locally applied DA was decreased by 50% relative to that in D2+/+ mice. In D2+/+ mice, but not D2-/- mice, local application of the D2-like receptor antagonist raclopride increased DA signal amplitude, indicating decreased DA clearance. Binding assays with the cocaine analogue [3H]WIN 35,428 showed no genotypic differences in either density or affinity of DAT binding sites in striatum or substantia nigra, indicating that the differences seen in DAT activity were not a result of decreased DAT expression. These results further strengthen the idea that the D2 DA receptor subtype modulates activity of the striatal DAT.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1999

Pituitary lactotroph adenomas develop after prolonged lactotroph hyperplasia in dopamine D2 receptor-deficient mice.

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.

Adrenocorticotropic Hormone↗
PubMed1999

LbetaT2 gonadotroph cells secrete follicle stimulating hormone (FSH) in response to active A.

Secretion of luteinizing hormone in response to gonadotropin releasing hormone (GnRH) has been described in the recently developed LbetaT2 gonadotroph cell line. We evaluated the expression of follicle stimulating hormone (FSH)beta mRNA and secretion of FSH from LbetaT2 cells in response to GnRH and activin A. LbetaT2 cells were treated with activin A in doses from 0 to 50 ng/ml, with or without a daily 10 nM GnRH pulse, or with GnRH alone. FSH secretion was stimulated over 6-fold by concomitant GnRH and activin A in a dose-responsive fashion at 72 h of treatment. FSHbeta mRNA was detectable by ribonuclease protection assay only in cells treated with activin A with or without GnRH. The demonstration of FSHbeta gene expression in LbetaT2 cells further validates these cells as mature, differentiated gonadotrophs and as an important tool for the study of gonadotroph physiology.

Activins↗
PubMed1998

Authentic cell-specific and developmentally regulated expression of pro-opiomelanocortin genomic fragments in hypothalamic and hindbrain neurons of transgenic mice.

The pro-opiomelanocortin (POMC) gene is expressed in a subset of hypothalamic and hindbrain neurons and in pituitary melanotrophs and corticotrophs. POMC neurons release the potent opioid beta-endorphin and several active melanocortins that control homeostasis and feeding behavior. POMC gene expression in the CNS is believed to be controlled by distinct cis-acting regulatory sequences. To analyze the transcriptional regulation of POMC in neuronal and endocrine cells, we produced transgenic mice carrying POMC27*, a transgene containing the entire 6 kb of the POMC transcriptional unit together with 13 kb of 5' flanking regions and 8 kb of 3' flanking regions. POMC27* was tagged with a heterologous 30 bp oligonucleotide in the third exon. In situ hybridization studies showed an accurate cell-specific pattern of expression of POMC27* in the arcuate nucleus and the pituitary. Hypothalamic mRNA-positive neurons colocalized entirely with beta-endorphin immunoreactivity. No ectopic transgenic expression was detected in the brain. Deletional analyses demonstrated that neuron-specific expression of POMC transgenes required distal 5' sequences localized upstream of the pituitary-responsive proximal cis-acting elements that were identified previously. POMC27* exhibited a spatial and temporal pattern of expression throughout development that exactly paralleled endogenous POMC. RNase protection assays revealed that POMC27* expression mimicked that of POMC in different areas of the CNS and most peripheral organs with no detectable ectopic expression. Hormonal regulation of POMC27* and POMC was identical in the hypothalamus and pituitary. These results show that distal 5' sequences of the POMC gene located between -13 and -2 kb target expression into the CNS of transgenic mice in a precise neuron-specific, developmentally and hormonally regulated manner.

Animals↗
PubMed1998

Locomotor activity in D2 dopamine receptor-deficient mice is determined by gene dosage, genetic background, and developmental adaptations.

Locomotor activity is a polygenic trait that varies widely among inbred strains of mice (). To characterize the role of D2 dopamine receptors in locomotion, we generated F2 hybrid (129/Sv x C57BL/6) D2 dopamine receptor (D2R)-deficient mice by gene targeting and investigated the contribution of genetic background to open-field activity and rotarod performance. Horizontal activity of D2R-/- mice was approximately half that of drug-naive, strain-matched controls but was significantly greater than haloperidol-treated controls, which were markedly hypokinetic. Wild-type 129/SvEv and C57BL/6 mice with functional D2 receptors had greater interstrain differences in spontaneous activity than those among the F2 hybrid mutants. Incipient congenic strains of D2R-deficient mice demonstrated an orderly gene dosage reduction in locomotion superimposed on both extremes of parental background locomotor activity. In contrast, F2 hybrid D2R-/- mice had impaired motor coordination on the rotarod that was corrected in the congenic C57BL/6 background. Wild-type 129/SvEv mice had the poorest rotarod ability of all groups tested, suggesting that linked substrain 129 alleles, not the absence of D2 receptors per se, were largely responsible for the reduced function of the F2 hybrid D2R-/- and D2R+/- mice. Neurochemical and pharmacological studies revealed unexpectedly normal tissue striatal monoamine levels and no evidence for supersensitive D1, D3, or D4 dopamine receptors in the D2R-/- mice. However, after acute monoamine depletion, akinetic D2R+/- mice had a significantly greater synergistic restoration of locomotion in response to SKF38393 and quinpirole compared with any group of D2R+/+ controls. We conclude that D2R-deficient mice are not a model of Parkinson's disease. Our studies highlight the interaction of multiple genetic factors in the analysis of complex behaviors in gene knock-out mice.

Adaptation, Physiological↗
PubMed1998

Alcohol preference and sensitivity are markedly reduced in mice lacking dopamine D2 receptors.

Although dopaminergic transmission has been strongly implicated in alcohol self-administration, the involvement of specific dopamine receptor subtypes has not been well established. We studied the ethanol preference and sensitivity of D2-receptor-deficient mice to directly evaluate whether dopamine D2 receptors contribute to alcohol (ethanol) consumption. We report a marked aversion to ethanol in these mice, relative to the high preference and consumption exhibited by wild-type littermates. Sensitivity to ethanol-induced locomotor impairment was also reduced in these mutant mice, although they showed a normal locomotor depressant response to the dopamine D1 antagonist SCH-23390. These data demonstrate that dopamine signaling via D2 receptors is an essential component of the molecular pathway determining ethanol self-administration and sensitivity.

Alcohol Drinking↗
PubMed1998

Simian virus 40 T antigen-induced gonadotroph adenomas: a model of human null cell adenomas.

The cell of origin of human null cell pituitary adenomas is disputed. Although these tumors, by definition, do not produce any of the anterior pituitary hormones in vivo, they have been shown to express gonadotropin subunit genes, release gonadotropin hormones in vitro, and express the gonadotroph-associated transcription factor steroidogenic factor-1. However, they demonstrate variable responses to releasing hormones in vitro, raising questions about their origin from differentiated gonadotrophs or pluripotent stem cells. In this set of experiments, transgenic mice carrying a temperature-sensitive mutant (TSA58) of simian virus 40 T antigen driven by human FSHbeta regulatory elements were produced. These animals developed slow growing, multifocal pituitary nodules that demonstrated secretion of FSH with serum FSH levels 10-fold higher in male transgenic animals and 5-fold higher in female transgenic animals than those in nontransgenic controls. Anterior pituitary pathology progressed from diffuse gonadotroph hyperplasia to nodular adenomas with persistent, but decreasing, immunoreactivity for FSHbeta and LHbeta. Ultrastructural characteristics of the tumors were identical to those of human null cell adenomas. These results support the hypothesis that human null cell adenomas are derived from gonadotrophs and provide an animal model for further study of this disease.

Adenoma↗
PubMed1998

Genetic rescue of follicle-stimulating hormone beta-deficient mice.

FSH is an alpha:beta heterodimeric pituitary glycoprotein that shares a common alpha-subunit with LH and TSH. To study the role of FSH in mammalian reproduction, we have previously generated an FSH-deficient mouse model using embryonic stem (ES) cell technology by introducing a null mutation in the unique FSHbeta gene. Male mice deficient in FSH are fertile despite their small testes and reduced sperm number and motility. In contrast, FSH-deficient female mice are infertile due to a block in folliculogenesis at the preantral stage. In this set of experiments, we have rescued the mutant phenotypes of FSHbeta-deficient mice by two genetic strategies. In the type I rescue mice, we introduced into the FSHbeta-deficient background a 10-kb human FSHbeta transgene that is selectively expressed in pituitary gonadotropes. The presence of this transgene [and thus the interspecies hybrid (i.e. mouse alpha:human FSHbeta hormone)] in the background of mouse FSHbeta deficiency completely restored the testis size, sperm number, and motility defects to levels comparable to those seen in control male mice. All of the mouse FSHbeta-deficient female mice carrying this human FSHbeta transgene resumed normal folliculogenesis, were fertile and delivered normal size litters. In the type II rescue mice, human FSH (human alpha:human FSHbeta) was ectopically produced from multiple tissues in the mutant background using a mouse metallothionein-I promoter. Whereas ectopic production of human FSH completely rescued the mouse FSHbeta-deficient male mice, only 3 out of 10 mouse FSHbeta-deficient females bearing these human FSH transgenes were fertile and carried their pregnancies to term and parturition. We conclude that the resultant phenotypes due to a genetic deficiency of mouse FSHbeta can be corrected by appropriate expression of human FSH transgenes and that human FSHbeta gene regulation and function in the mouse pituitary are indistinguishable from the endogenous mouse FSHbeta gene.

Animals↗
PubMed1998

Intravenous self-administration of ethanol in beta-endorphin-deficient mice.

Extensive research on both human alcoholics and in animal models of alcoholism has implicated the release of endogenous opioids in the consumption of ethanol. Various experiments using opioid antagonists have indicated that these drugs cause both humans and animals to decrease their consumption of ethanol. However, it remains unclear exactly which of the endogenous opioids mediates the rewarding effects of ethanol. The present experiment used intravenous self-administration of ethanol to determine whether beta-endorphin (BE)-deficient mice differed from wild-type (WT) mice in ethanol self-administration. The BE-deficient mice completely lack BE, but are otherwise similar to the WT mice. By using intravenous self-administration, we were able to rule out any ability of BE to mediate differences in ethanol consumption via palatability factors alone. Both types of mice were 7 generations backcrossed onto a C57BL/6J inbred strain background. During nine daily, 2-hr free-operant sessions, 14 BE-deficient and 17 WT mice could nosepoke for 75 mg/kg ethanol infusions delivered intravenously on an fixed-ratio 3 schedule with a 2-sec time-out after each reinforcer delivery. Reinforcer delivery occurred following nosepokes in only one of two holes. Contrary to what was expected, BE-deficient mice acquired selective operant responding for ethanol, whereas WT mice did not. Although the two genotypes did not differ in either operant or locomotor behavior during the first session, by the end of the nine sessions, BE deficient mice were reliably nosepoking for ethanol, whereas WT mice were not. These findings may indicate that BE is not essential for the postingestive reinforcing effects of ethanol in these animals.

Alcoholism↗
PubMed1997

Exocrine gland dysfunction in MC5-R-deficient mice: evidence for coordinated regulation of exocrine gland function by melanocortin peptides.

The effects of pituitary-derived melanocortin peptides are primarily attributed to ACTH-mediated adrenocortical glucocorticoid production. Identification of a widely distributed receptor for ACTH/MSH peptides, the melanocortin-5 receptor (MC5-R), suggested non-steroidally mediated systemic effects of these peptides. Targeted disruption of the MC5-R produced mice with a severe defect in water repulsion and thermoregulation due to decreased production of sebaceous lipids. High levels of MC5-R was found in multiple exocrine tissues, including Harderian, preputial, lacrimal, and sebaceous glands, and was also shown to be required for production and stress-regulated synthesis of porphyrins by the Harderian gland and ACTH/MSH-regulated protein secretion by the lacrimal gland. These data show a requirement for the MC5-R in multiple exocrine glands for the production of numerous products, indicative of a coordinated system for regulation of exocrine gland function by melanocortin peptides.

Animals↗
PubMed1997

Mice lacking dopamine D4 receptors are supersensitive to ethanol, cocaine, and methamphetamine.

The human dopamine D4 receptor (D4R) has received considerable attention because of its high affinity for the atypical antipsychotic clozapine and the unusually polymorphic nature of its gene. To clarify the in vivo role of the D4R, we produced and analyzed mutant mice (D4R-/-) lacking this protein. Although less active in open field tests, D4R-/- mice outperformed wild-type mice on the rotarod and displayed locomotor supersensitivity to ethanol, cocaine, and methamphetamine. Biochemical analyses revealed that dopamine synthesis and its conversion to DOPAC were elevated in the dorsal striatum from D4R-/- mice. Based on these findings, we propose that the D4R modulates normal, coordinated and drug-stimulated motor behaviors as well as the activity of nigrostriatal dopamine neurons.

3,4-Dihydroxyphenylacetic Acid↗
PubMed1997

Pituitary lactotroph hyperplasia and chronic hyperprolactinemia in dopamine D2 receptor-deficient mice.

Dopamine secreted from hypophysial hypothalamic neurons is a principal inhibitory regulator of pituitary hormone secretion. Mice with a disrupted D2 dopamine receptor gene had chronic hyperprolactinemia and developed anterior lobe lactotroph hyperplasia without evidence of adenomatous transformation. Unexpectedly, the mutant mice had no hyperplasia of the intermediate lobe melanotrophs. Aged female D2 receptor -/- mice developed uterine adenomyosis in response to prolonged prolactin exposure. These data reveal a critical role of hypothalamic dopamine in controlling pituitary growth and support a multistep mechanism for the induction and perpetuation of lactotroph hyperplasia, involving the lack of dopamine signaling, a low androgen/estrogen ratio, and a final autocrine or paracrine "feed-forward" stimulation of mitogenesis, probably by prolactin itself.

Animals↗
PubMed1997

Two distinct pituitary cell lines from mouse intermediate lobe tumors: a cell that produces prolactin-regulating factor and a melanotroph [seecomments].

The intermediate lobe (IL) of the pituitary produces a PRL-regulating factor (PRF). Targeted tumorigenesis, using the POMC promoter ligated to SV40 large T antigen (Tag), generated transgenic mice that develop IL tumors with PRF activity. Our goal was to establish and characterize a PRF-producing cell line. Two cell lines, which differ markedly in size and morphology, were independently developed from IL tumors and designated mIL5 and mIL39. These cells are transformed, as judged by rapid proliferation, low serum requirements, and generation of secondary tumors in nude mice. RT-PCR revealed that mIL39, but not mIL5 cells, express POMC and dopamine D2 receptors, typical of a melanotroph phenotype. Although mIL5 cells originated from an IL tumor, they do not express messenger RNA for SV40 Tag. The bioassay for PRF used GH3 cells stably transfected with the PRL promoter ligated to a luciferase reporter gene (GH3/luc). Coculture of mIL5 with GH3/luc cells induced cell-density dependent increases in PRL gene expression and release, whereas mIL39 cells showed negligible PRF activity. Incubation of GH3/luc cells with conditioned media from mIL5, but not mIL39 cells, stimulated PRL gene expression and release up to 10-fold. Coculture of mIL5 cells with primary rat anterior pituitary cells stimulated PRL, but not GH, release. Fractionation of mIL5 cell extracts by reverse phase HPLC resolved PRF activity into one major and one minor peak. In conclusion, we have developed two novel and distinct cell lines from mouse intermediate lobe tumors. The first reported melanotroph cell line, mIL39, could provide a valuable model for studying dopaminergic regulation of POMC gene expression and release. In contrast, the mIL5 cells do not express POMC, D2 receptors, or SV40 Tag and appear to have been immortalized by a spontaneous mutation(s). These cells produce and secrete a potent PRF and could be used for the purification and biochemical characterization of PRF.

Animals↗
PubMed1996

Germ cell defects and hematopoietic hypersensitivity to gamma-interferon in mice with a targeted disruption of the Fanconi anemia C gene.

Fanconi anemia (FA) is an autosomal recessive chromosome instability syndrome characterized by progressive bone marrow (BM) failure, skeletal defects, and increased susceptibility to malignancy. FA cells are hypersensitive to DNA cross-linking agents, oxygen and have cell cycle abnormalities. To develop an animal model of the disease we generated mice homozygous for a targeted deletion of exon 9 of the murine FA complementation group C gene (fac). Mutant mice had normal neonatal viability and gross morphology, but their cells had the expected chromosome breakage and DNA cross-linker sensitivity. Surprisingly, male and female mutant mice had reduced numbers of germ cells and females had markedly impaired fertility. No anemia was detectable in the peripheral blood during the first year of life, but the colony forming capacity of marrow progenitor cells was abnormal in vitro in mutant mice. Progenitor cells from fac knock-out mice were hypersensitive to interferon gamma. This previously unrecognized phenotype may form the basis for BM failure in human FA.

Animals↗
PubMed1996

Absence of opioid stress-induced analgesia in mice lacking beta-endorphin by site-directed mutagenesis.

A physiological role for beta-endorphin in endogenous pain inhibition was investigated by targeted mutagenesis of the proopiomelanocortin gene in mouse embryonic stem cells. The tyrosine codon at position 179 of the proopiomelanocortin gene was converted to a premature translational stop codon. The resulting transgenic mice display no overt developmental or behavioral alterations and have a normally functioning hypothalamic-pituitary-adrenal axis. Homozygous transgenic mice with a selective deficiency of beta-endorphin exhibit normal analgesia in response to morphine, indicating the presence of functional mu-opiate receptors. However, these mice lack the opioid (naloxone reversible) analgesia induced by mild swim stress. Mutant mice also display significantly greater nonopioid analgesia in response to cold water swim stress compared with controls and display paradoxical naloxone-induced analgesia. These changes may reflect compensatory upregulation of alternative pain inhibitory mechanisms.

Amino Acid Sequence↗
PubMed1995

DNA elements with AT-rich core sequences direct pituitary cell-specific expression of the pro-opiomelanocortin gene in transgenic mice.

Corticotrophs are the first fully differentiated cells to appear in the anterior pituitary during organogenesis and are distinguished by pro-opiomelanocortin (POMC) gene expression. Earlier studies in our laboratory defined three DNA regions (sites 1, 2 and 3) within promoter sequences at the 5'-end of the rat POMC gene (-323/-34) that cooperatively targeted cell-specific gene expression to corticotrophs and melanotrophs in transgenic mice. In this study we analysed the DNA-nuclear protein interactions underlying this functional activity. We demonstrated that the transcriptional activator SP1 interacts with GC-rich regions in sites 1 (-146/-136) and 2 (-201/-192) and an unidentified protein, which we call PP1 (putative pituitary POMC1), interacts with AT-rich regions in sites 2 (-202/-193) and 3 (-262/-253). The PP1-binding activity appears to be specific to cells that express the POMC gene because it was detected in nuclear extracts prepared from AtT20 corticotroph cells and mouse melanotroph tumours but not from GH4 pituitary tumour cells, HeLa cells or liver. Site-directed mutagenesis of core binding sequences demonstrated that PP1 is required for the correct cell-specific expression of the POMC gene in the pituitary gland of transgenic mice and SP1 appears to support such an expression. The best core binding sequence for PP1 is TAAT, a possible transcription factor homeodomain contact site. However, PP1 is distinct from Brn 3.0, a POU protein that also binds to site 3. We conclude that PP1 is a transcriptional activator for pituitary-specific POMC gene expression.

Animals↗
PubMed1995

Cloning of the mouse gonadotropin beta-subunit-encoding genes, I. Structure of the follicle-stimulating hormone beta-subunit-encoding gene.

More than 16kb of genomic sequence encompassing the mouse follicle-stimulating hormone beta subunit (FSH beta)-encoding gene was isolated from a 129SvEv mouse genomic library. Comparisons of the nucleotide and deduced amino-acid sequences to the FSH beta sequences of other mammalian species confirm an evolutionarily conserved role of this important protein in reproduction.

Amino Acid Sequence↗