Modulation of dialysate levels of dopamine, noradrenaline, and serotonin (5-HT) in the frontal cortex of freely-moving rats by (-)-pindolol alone and in association with 5-HT reuptake inhibitors: comparative roles of beta-adrenergic, 5-HT1A, and 5-HT1B receptors.
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(-)-Pindolol, which possesses significant affinity for 5-HT1A, 5-HT1B, and beta 1/2-adrenergic receptors (AR)s, dose-dependently increased extracellular levels of dopamine (DA) and noradrenaline (NAD) versus 5-HT, in dialysates of the frontal cortex (FCX), but not accumbens and striatum, of freely-moving rats. In distinction, the preferential beta 1-AR antagonist, betaxolol, and the preferential beta 2-AR antagonist, ICI118,551, did not increase basal levels of DA, NAD, or 5-HT. Further, they both dose-dependently and markedly blunted the influence of (-)-pindolol upon DA and NAD levels. The selective 5-HT1A receptor antagonist, WAY100,635, slightly attenuated the (-)-pindolol-induced increase in DA and NAD levels, while the selective 5-HT1B antagonist, SB224,289, was ineffective. These data suggest that (-)-pindolol facilitates frontocortical dopaminergic (and adrenergic) transmission primarily by activation of beta 1/2-ARs and, to a lesser degree, by stimulation of 5-HT1A receptors, whereas 5-HT1B receptors are not involved. (-)-Pindolol potentiated the increase in FCX levels of 5-HT elicited by the 5-HT reuptake inhibitors, fluoxetine and duloxetine, and also enhanced their ability to elevate FCX levels of DA--though not of NAD. In contrast to (-)-pindolol, betaxolol and ICI118,551 did not affect the actions of fluoxetine, whereas both WAY100,635 and SB224,289 potentiated the increase in levels of 5-HT--but not DA or NAD levels--elicited by fluoxetine. In conclusion, (-)-pindolol modulates, both alone and together with 5-HT reuptake inhibitors, dopaminergic, adrenergic, and serotonergic transmission in the FCX via a complex pattern of actions at beta 1/2-ARs, 5-HT1A, and 5-HT1B receptors. These findings have important implications for clinical studies of the influence of (-)-pindolol upon the actions of antidepressant agents. (+info)
Constitutively active mutants of the beta1-adrenergic receptor.
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We provide the first evidence that point mutations can constitutively activate the beta(1)-adrenergic receptor (AR). Leucine 322 of the beta(1)-AR in the C-terminal portion of its third intracellular loop was replaced with seven amino acids (I, T, E, F, C, A and K) differing in their physico-chemical properties. The beta(1)-AR mutants expressed in HEK-293 cells displayed various levels of constitutive activity which could be partially inhibited by some beta-blockers. The results of this study might have interesting implications for future studies aiming at elucidating the activation process of the beta(1)-AR as well as the mechanism of action of beta-blockers. (+info)
Effects of betaxolol on light responses and membrane conductance in retinal ganglion cells.
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PURPOSE: To examine the physiological effects of betaxolol, a beta1-adrenergic receptor blocker commonly used in the treatment of glaucoma, on retinal ganglion cells and to evaluate its potential to elicit responses consistent with a neuroprotective agent against ganglion cell degeneration. METHODS: Single-unit extracellular recording, electroretinogram (ERG), intracellular and whole-cell patch-clamp recording techniques were made from flatmounted, isolated retina, superfused eyecup, and living retinal slice preparations of the larval tiger salamander. RESULTS: Bath application of 20 microM betaxolol reduced the glutamate-induced increase of spontaneous spike rate in retinal ganglion cell by approximately 30%. The glutamate-induced postsynaptic current recorded under voltage-clamp conditions was reduced by 50 microM betaxolol, and the difference current-voltage (I-V) relation (I(Control)-I(betaxolol)) was N-shaped and AP5-sensitive, characteristic of N-methyl-D-aspartate receptor-mediated current. Application of 50 microM betaxolol reversibly reduced the voltage-gated sodium and calcium currents by approximately one third of their peak amplitudes. The times-to-action of betaxolol on ganglion cells are long (15-35 minutes for 20-50 microM betaxolol), indicative of modulation through slow biochemical cascades. Betaxolol, up to 100 microM, exerted no effects on horizontal cells or the ERG, suggesting that the primary actions of this beta1 blocker are restricted to retinal ganglion cells. CONCLUSIONS: These physiological experiments provide supporting evidence that betaxolol acts in a manner consistent with preventing retinal ganglion cell death induced by elevated extracellular glutamate or by increased spontaneous spike rates under pathologic conditions. The physiological actions of betaxolol lead to reducing neurotoxic effects in ganglion cells, which are the most susceptible retinal neurons to glutamate-induced damages under ischemic and glaucomatous conditions. Therefore, betaxolol has the potential to be a neuroprotective agent against retinal degeneration in patients with disorders mediated by such mechanisms. (+info)
Betaxolol, a beta(1)-adrenoceptor antagonist, reduces Na(+) influx into cortical synaptosomes by direct interaction with Na(+) channels: comparison with other beta-adrenoceptor antagonists.
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Betaxolol, a beta(1)-adrenoceptor antagonist used for the treatment of glaucoma, is known to be neuroprotective in paradigms of ischaemia/excitotoxicity. In this study, we examined whether betaxolol and other beta-adrenoceptor antagonists interact directly with neurotoxin binding to sites 1 and 2 of the voltage-sensitive sodium channel (Na(+) channel) in rat cerebrocortical synaptosomes. Betaxolol inhibited specific [(3)H]-batrachotoxinin-A 20-alpha-benzoate ([(3)H]-BTX-B) binding to neurotoxin site 2 in a concentration-dependent manner with an IC(50) value of 9.8 microM. Comparison of all the beta-adrenoceptor antagonists tested revealed a potency order of propranolol>betaxolol approximately levobetaxolol>levobunolol approximately carteolol>/=timolol>atenolol. None of the drugs caused a significant inhibition of [(3)H]-saxitoxin binding to neurotoxin receptor site 1, even at concentrations as high as 250 microM. Saturation experiments showed that betaxolol increased the K(D) of [(3)H]-BTX-B binding but had no effect on the B(max). The association kinetics of [(3)H]-BTX-B were unaffected by betaxolol, but the drug significantly accelerated the dissociation rate of the radioligand. These findings argue for a competitive, indirect, allosteric mode of inhibition of [(3)H]-BTX-B binding by betaxolol. Betaxolol inhibited veratridine-stimulated Na(+) influx in rat cortical synaptosomes with an IC(50) value of 28. 3 microM. Carteolol, levobunolol, timolol and atenolol were significantly less effective than betaxolol at reducing veratridine-evoked Na(+) influx. The ability of betaxolol to interact with neurotoxin site 2 of the Na(+) channel and inhibit Na(+) influx may have a role in its neuroprotective action in paradigms of excitotoxicity/ischaemia and in its therapeutic effect in glaucoma. (+info)
Effects of glaucoma medications on the cardiorespiratory and intraocular pressure status of newly diagnosed glaucoma patients.
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AIMS: To evaluate the short term cardiovascular, respiratory, and intraocular pressure (IOP) effects of four glaucoma medications in newly diagnosed glaucoma patients. METHODS: 141 newly diagnosed glaucoma patients were recruited and underwent a full ocular, cardiovascular, and respiratory examination, including an electrocardiogram (ECG) and spirometry. They were prescribed one of four topical glaucoma medications and reviewed 3 months later. One eye of each patient was randomly chosen for analysis, performed using analysis of variance and the chi(2) test. RESULTS: Latanoprost had the greatest mean IOP lowering effect in both the primary open angle glaucoma (POAG) (p = 0.005) and the "presumed" normal tension glaucoma (NTG) groups (p = 0.33), reducing the IOP by 8.9 mm Hg and 4.1 mm Hg respectively. Timolol was associated with lowered pulse rates and reductions in the spirometry measurements. 41% of patients using brimonidine complained of systemic side effects and over 55% of patients using betaxolol complained of ocular irritation. 28% of patients required an alteration in their glaucoma management. CONCLUSIONS: Latanoprost appears to be a useful primary treatment for glaucoma patients, in view of superior IOP control and a low incidence of local and systemic side effects. Timolol causes a reduction in measurements of respiratory function, a concern in view of the potential subclinical reversible airways disease in the elderly glaucoma population. Brimonidine is associated with substantial, unpredictable systemic side effects and betaxolol causes ocular irritation and weak IOP control. Spirometry is advised in all patients receiving topical beta blocker therapy to control their glaucoma. (+info)
Ligand regulation of green fluorescent protein-tagged forms of the human beta(1)- and beta(2)-adrenoceptors; comparisons with the unmodified receptors.
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Stable clones of HEK293 cells expressing either FLAG(TM) epitope-tagged, wild type human beta(1)- and beta(2)-adrenoceptors or C-terminally green fluorescent protein (GFP)-tagged forms of these receptors were established. The binding affinity of [(3)H]-dihydroalprenolol and other ligands was little affected by addition of GFP to the C-terminal of either receptor. Isoprenaline induced the internalisation of both beta(1)-adrenoceptor-GFP and beta(2)-adrenoceptor-GFP and following removal of the agonist both constructs were able to recycle to the cell surface. The extent of internalisation of beta(2)-adrenoceptor-GFP produced by isoprenaline was substantially greater than for beta(1)-adrenoceptor-GFP. C-terminal addition of GFP slowed markedly the rate of internalization of both the beta(1)-adrenoceptor and the beta(2)-adrenoceptor in response to isoprenaline. Sustained exposure to isoprenaline (24 h) produced substantially greater levels of downregulation of native beta(2)-adrenoceptor compared to beta(2)-adrenoceptor-GFP although both were equally effectively removed from the plasma membrane. Sustained exposure to isoprenaline resulted in a large fraction of beta(2)-adrenoceptor-GFP becoming trapped in internal vesicles/lysosomes but not degraded. Even after sustained exposure to isoprenaline a significant fraction of beta(1)-adrenoceptor-GFP remained at the cell surface. These results indicate that although GFP tagging of beta-adrenoceptors can provide qualitative visual patterns of agonist-induced receptor trafficking and regulation in HEK293 cells the quantitative details vary markedly from those obtained with the unmodified receptors. (+info)
Topical ophthalmic beta blockers may cause release of histamine through cytotoxic effects on inflammatory cells.
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AIM: To evaluate the effects of beta blockers used in ophthalmology on the release of histamine from mixed cell preparations containing human leucocytes and basophils. METHODS: A mixed leucocyte and basophil preparation was obtained from venous blood of healthy non-atopic volunteers. Cell preparations were then incubated with betaxolol, metipranolol, timolol, or carteolol. After incubation for 1 hour the histamine content of the supernatant was analysed by automated fluorometric analysis. Cell viability was tested by measuring lactate dehydrogenase (LDH) concentrations. RESULTS: Betaxolol and metipranolol in concentrations between 10(-2) M and 10(-3) M liberated histamine from human blood cells in a dose dependent manner. Carteolol and timolol had no effect on histamine at these concentrations. At the same concentrations LDH was also detected in the supernatants of cell suspensions incubated with metipranolol or betaxolol. CONCLUSIONS: Betaxolol and metipranolol induce substantial histamine release from human leucocytes, probably as a result of their cytotoxic effect. (+info)
Detection of receptor ligands by monitoring selective stabilization of a Renilla luciferase-tagged, constitutively active mutant, G-protein-coupled receptor.
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The wild-type beta2-adrenoceptor and a constitutively active mutant of this receptor were C-terminally tagged with luciferase from the sea pansy Renilla reniformis. C-terminal addition of Renilla luciferase did not substantially alter the levels of expression of either form of the receptor, the elevated constitutive activity of the mutant beta2-adrenoceptor nor the capacity of isoprenaline to elevate cyclic AMP levels in intact cells expressing these constructs. Treatment of cells expressing constitutively active mutant beta2-adrenoceptor-Renilla luciferase with antagonist/inverse agonist ligands resulted in upregulation of levels of this polypeptide which could be monitored by the elevated luciferase activity. The pEC50 for ligand-induced luciferase upregulation and ligand affinity to bind the receptor were highly correlated. Similar upregulation could be observed following sustained treatment with agonist ligands. These effects were only observed at a constitutively active mutant of the beta2-adrenoceptor. Co-expression of the wild-type beta2-adrenoceptor C-terminally tagged with the luciferase from Photinus pyralis did not result in ligand-induced upregulation of the levels of activity of this luciferase. Co-expression of the constitutively active mutant beta2-adrenoceptor-Renilla luciferase and an equivalent mutant of the alpha1b-adrenoceptor C-terminally tagged with green fluorescent protein allowed pharmacological selectivity of adrenoceptor antagonists to be demonstrated. This approach offers a sensitive and convenient means, which is amenable to high throughput analysis, to monitor ligand binding to a constitutively active mutant receptor. As no prior knowledge of receptor ligands is required this approach may be suitable to identify ligands at orphan G protein-coupled receptors. (+info)