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      GABA B1 Knockout Mice Reveal Alterations in Prolactin Levels, Gonadotropic Axis, and Reproductive Function

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          γ-Aminobutyric acid (GABA) has been implicated in the control of hypophyseal functions. We evaluated whether the constitutive loss of functional GABA<sub>B</sub> receptors in GABA<sub>B1</sub> knockout (GABA<sub>B1</sub><sup>–/–</sup>) mice alters hormonal levels, under basal and stimulated conditions, and reproductive function. The serum hormone levels were measured by radioimmunoassay, the estrous cyclicity was evaluated by vaginal lavages, and the mating behavior was determined by the presence of vaginal plugs. A moderate hyperprolactinemic condition was observed, in which prolactin increase and thyroid-stimulating hormone decrease were similar between genotypes. Basal luteinizing hormone (LH), follicle-stimulating hormone, thyroid-stimulating hormone, and growth hormone levels were similar between genotypes in each sex. Analysis of the gonadotropin axis revealed no differences in puberty onset between female genotypes. In con trast, the estrous cyclicity was significantly disrupted in GABA<sub>B1</sub><sup>–/–</sup> female mice, showing significantly extended periods in estrus and shortened periods in proestrus. Reproduction was significantly compromised in GABA<sub>B1</sub><sup>–/–</sup> females, with a significantly lower proportion of mice (37.5%) getting pregnant during the first 30 days of mating as compared with wild-type controls (87.5%). Moreover, only 14% of vaginal plug positive GABA<sub>B1</sub><sup>–/–</sup> females had successful pregnancies as compared with 75% in the controls. In addition, the postovariectomy LH rise was significantly advanced in GABA<sub>B1</sub><sup>–/–</sup> mice, while the response to estradiol feedback was similar in both genotypes. In conclusion, our endocrine analysis of GABA<sub>B1</sub><sup>–/–</sup> mice reveals that GABA<sub>B</sub> receptors are involved in the regulation of basal prolactin titers. Moreover, the hypothalamic-hypophyseal-ovarian axis is seriously disturbed, with alterations in cyclicity, postcastration LH increase, and fertility indexes. The molecular mechanism underlying these hormonal disturbances remains to be addressed.

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          GABA(B)-receptor subtypes assemble into functional heteromeric complexes.

          B-type receptors for the neurotransmitter GABA (gamma-aminobutyric acid) inhibit neuronal activity through G-protein-coupled second-messenger systems, which regulate the release of neurotransmitters and the activity of ion channels and adenylyl cyclase. Physiological and biochemical studies show that there are differences in drug efficiencies at different GABA(B) receptors, so it is expected that GABA(B)-receptor (GABA(B)R) subtypes exist. Two GABA(B)-receptor splice variants have been cloned (GABA(B)R1a and GABA(B)R1b), but native GABA(B) receptors and recombinant receptors showed unexplained differences in agonist-binding potencies. Moreover, the activation of presumed effector ion channels in heterologous cells expressing the recombinant receptors proved difficult. Here we describe a new GABA(B) receptor subtype, GABA(B)R2, which does not bind available GABA(B) antagonists with measurable potency. GABA(B)R1a, GABA(B)R1b and GABA(B)R2 alone do not activate Kir3-type potassium channels efficiently, but co-expression of these receptors yields a robust coupling to activation of Kir3 channels. We provide evidence for the assembly of heteromeric GABA(B) receptors in vivo and show that GABA(B)R2 and GABA(B)R1a/b proteins immunoprecipitate and localize together at dendritic spines. The heteromeric receptor complexes exhibit a significant increase in agonist- and partial-agonist-binding potencies as compared with individual receptors and probably represent the predominant native GABA(B) receptor. Heteromeric assembly among G-protein-coupled receptors has not, to our knowledge, been described before.
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            Use of the thyroid hormone analogue eprotirome in statin-treated dyslipidemia.

            Dyslipidemia increases the risk of atherosclerotic cardiovascular disease and is incompletely reversed by statin therapy alone in many patients. Thyroid hormone lowers levels of serum low-density lipoprotein (LDL) cholesterol and has other potentially favorable actions on lipoprotein metabolism. Consequently, thyromimetic drugs hold promise as lipid-lowering agents if adverse effects can be avoided. We performed a randomized, placebo-controlled, double-blind, multicenter trial to assess the safety and efficacy of the thyromimetic compound eprotirome (KB2115) in lowering the level of serum LDL cholesterol in patients with hypercholesterolemia who were already receiving simvastatin or atorvastatin. In addition to statin treatment, patients received either eprotirome (at a dose of 25, 50, or 100 microg per day) or placebo. Secondary outcomes were changes in levels of serum apolipoprotein B, triglycerides, and Lp(a) lipoprotein. Patients were monitored for potential adverse thyromimetic effects on the heart, bone, and pituitary. The addition of placebo or eprotirome at a dose of 25, 50, or 100 microg daily to statin treatment for 12 weeks reduced the mean level of serum LDL cholesterol from 141 mg per deciliter (3.6 mmol per liter) to 127, 113, 99, and 94 mg per deciliter (3.3, 2.9, 2.6, and 2.4 mmol per liter), respectively, (mean reduction from baseline, 7%, 22%, 28%, and 32%). Similar reductions were seen in levels of serum apolipoprotein B, triglycerides, and Lp(a) lipoprotein. Eprotirome therapy was not associated with adverse effects on the heart or bone. No change in levels of serum thyrotropin or triiodothyronine was detected, although the thyroxine level decreased in patients receiving eprotirome. In this 12-week trial, the thyroid hormone analogue eprotirome was associated with decreases in levels of atherogenic lipoproteins in patients receiving treatment with statins. ( number, NCT00593047.) 2010 Massachusetts Medical Society
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              International Union of Pharmacology. XXXIII. Mammalian gamma-aminobutyric acid(B) receptors: structure and function.

               N G Bowery (2002)
              The gamma-aminobutyric acid(B) (GABA(B)) receptor was first demonstrated on presynaptic terminals where it serves as an autoreceptor and also as a heteroreceptor to influence transmitter release by suppressing neuronal Ca(2+) conductance. Subsequent studies showed the presence of the receptor on postsynaptic neurones where activation produces an increase in membrane K(+) conductance and associated neuronal hyperpolarization. (-)-Baclofen is a highly selective agonist for GABA(B) receptors, whereas the established GABA(A) receptor antagonists, bicuculline and picrotoxin, do not block GABA(B) receptors. The receptor is G(i)/G(o) protein-coupled with mixed effects on adenylate cyclase activity. The receptor comprises a heterodimer with similar subunits currently designated 1 and 2. These subunits are coupled via coiled-coil domains at their C termini. The evidence for splice variants is critically reviewed. Thus far, no unique pharmacological or functional properties have been assigned to either subunit or the variants. The emergence of high-affinity antagonists for GABA(B) receptors has enabled a synaptic role to be established. However, the antagonists have generally failed to establish the existence of pharmacologically distinct receptor types within the GABA(B) receptor class. The advent of GABA(B1) knockout mice has also failed to provide support for multiple receptor types.

                Author and article information

                S. Karger AG
                May 2006
                31 May 2006
                : 82
                : 5-6
                : 294-305
                aInstituto de Biología y Medicina Experimental-CONICET y bFacultad de Medicina, Universidad de Buenos Aires, Buenos Aires, Argentina; cDepartment of Clinical-Biological Sciences, University of Basel, Basel, Switzerland
                93128 Neuroendocrinology 2005;82:294–305
                © 2005 S. Karger AG, Basel

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                Page count
                Figures: 7, Tables: 2, References: 65, Pages: 12
                Original Paper


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