Novel form of long-term synaptic depression in rat hippocampus induced by activation of alpha 1 adrenergic receptors. (49/223)

Neurons located in the locus coeruleus project to hippocampus and provide noradrenergic innervation necessary for hippocampal-dependent learning and memory. The mechanisms underlying the function of norepinephrine (NE) in memory processing are unknown but likely reside in the ability of NE to modulate the efficacy of glutamate synaptic transmission via activation of G-protein-coupled adrenergic receptors. Here we show that application of NE to rat hippocampal slices in vitro induces a long-term depression (LTD) of synaptic transmission at excitatory CA3-CA1 synapses that persists for >/=40 min after agonist washout. This LTD, which we refer to as NE LTD, is mediated by activation of alpha1 adrenergic receptors because the alpha1 agonist methoxamine can induce LTD at the same magnitude as that induced with the nonselective adrenergic agonist NE. Furthermore, NE LTD induced by either NE or methoxamine is blocked with the alpha1 receptor antagonist, prazosin, but is unaffected by antagonists of alpha2 and beta receptors. This plasticity persists in the presence of the GABA(A) receptor antagonist bicuculline, indicating that adrenergic modulation of GABA(A) receptor-mediated transmission does not underlie NE LTD. Induction of NE LTD requires presynaptic activity during agonist application and postsynaptic activation of N-methyl-d-aspartate receptors, fulfilling Hebbian criteria of coincident pre- and postsynaptic activity. The expression of NE LTD is likely to be postsynaptic because paired-pulse facilitation ratios during NE LTD expression are not different from baseline, similar to LTD induced by low-frequency stimulation. Thus we report the identification and characterization of a novel Hebbian form of LTD in hippocampus that is induced after activation of alpha1 adrenergic receptors. This plasticity may be a mechanism by which the adrenergic system participates in normal cognitive function.  (+info)

Catecholamine-induced vascular wall growth is dependent on generation of reactive oxygen species. (50/223)

Alpha1-adrenoceptor-dependent proliferation of vascular smooth muscle cells (VSMCs) is strongly augmented by vascular injury, and may contribute to intimal growth and lumen loss. Because reactive oxygen species (ROS) are increased by injury and have been implicated as second messengers in proliferation of VSMCs, we investigated the role of ROS in catecholamine-induced VSMC growth. Rat aortae were isolated 4 days after balloon injury, maintained in organ culture under circumferential wall tension, and exposed to agents for 48 hours. The antioxidants N-acetylcysteine (NAC, 10 mmol/L) and Tiron (5 mmol/L) and the flavin-inhibitor diphenylene iodonium (DPI, 20 micromol/L) abolished norepinephrine-induced increases in protein synthesis and DNA content in media. In aortic sections, norepinephrine augmented ROS production (dihydroethidium confocal microscopy), which was dose-dependently inhibited by NAC, Tiron, and DPI. In cultured VSMCs, phenylephrine caused time- and dose-dependent ROS generation (aconitase activity), had similar efficacy to thrombin (1 U/mL), and was eliminated by the superoxide dismutase (SOD) mimetic Mn-(III)-tetrakis-(4-benzoic-acid)-porphyrin-chloride (200 micromol/L) and Tiron. Phenylephrine-induced ROS production and increases in DNA and protein content were blocked by prazosin (0.3 micromol/L) and abolished in p47phox-/- cells. PEG-SOD (25 U/mL) had little effect, whereas PEG-catalase (50 U/mL) eliminated phenylephrine-induced proliferation in VSMCs. DPI (10 micromol/L) and apocynin (30 micromol/L) abolished phenylephrine-stimulated mitogenesis, whereas inhibitors of other intracellular ROS sources had not effect. Furthermore, PE increased p47phox expression (RT-PCR). These data demonstrate that the trophic effect of catecholamines on vascular wall cells is dependent on a ROS-sensitive step that we hypothesize consists of activation of the NAD(P)H-dependent vascular oxidase.  (+info)

Agonist-induced activation of matrix metalloproteinase-7 promotes vasoconstriction through the epidermal growth factor-receptor pathway. (51/223)

Matrix metalloproteinase (MMP)-dependent shedding of heparin-binding epidermal growth factor (HB-EGF) and subsequent activation of the EGF receptor (EGFR) in the cardiovasculature is emerging as a unique mechanism signaling growth effects of diverse G protein-coupled receptors (GPCRs). Among these GPCRs are adrenoceptors and angiotensin receptors that contribute to the pathogenesis of hypertension through their vasoconstrictive and growth effects. Focusing on alpha(1b)-adrenoceptors, we suggest here that MMP-dependent activation of the EGFR promotes vasoconstriction as well as growth. We identified MMP-7 as a major HB-EGF sheddase in rat mesenteric arteries and alpha(1b)-adrenoceptors, angiotensin receptors, and hypertension-stimulated MMP-7 activity. Adrenoceptors stimulated EGFR autophosphorylation in arteries, and this transactivation was opposed by the MMP-7 inhibitor GM6001 as well as MMP-7-specific antibodies. In isolated microperfused arteries, blockade of EGFR transactivation with inhibitors of the EGFR (AG1478 and PD153035), HB-EGF (CRM197 and neutralizing antibodies), or MMPs (doxycycline) inhibited adrenergic vasoconstriction. In spontaneously hypertensive rats but not in normotensive rats, the inhibition of MMPs with doxycycline (19.2 mg/d from week 7 until week 12) reduced systolic blood pressure and attenuated HB-EGF shedding in the mesenteric arteries. These findings suggest a previously unknown mechanism of vasoregulation whereby agonists of certain GPCRs (such as adrenoceptors and angiotensin receptors) activate MMPs (such as MMP-7) that shed EGFR ligands (such as HB-EGF), which then activate the EGFR, thereby promoting vasoconstriction as well as growth. Because this mechanism is triggered by agonists typically overexpressed in hypertension, its blockade may have therapeutic potential for simultaneously inhibiting pathological vasoconstriction and growth in hypertensive disorders.  (+info)

The effects of phenylephrine on pupil diameter and accommodation in rhesus monkeys. (52/223)

PURPOSE: Phenylephrine is used to dilate the iris through alpha-adrenergic stimulation of the iris dilator muscle. Sympathetic stimulation of the ciliary muscle is believed to be inhibitory, decreasing accommodative amplitude. Investigations in humans have suggested some loss of functional accommodation after phenylephrine. It is unclear whether this loss is due to direct action of phenylephrine on the ciliary muscle or to secondary optical factors associated with mydriasis. The purpose of this study was to determine whether phenylephrine affects Edinger-Westphal (EW)-stimulated accommodation in rhesus monkeys. METHODS: The time course for maximum mydriasis was determined by videographic pupillography after phenylephrine instillation in 10 normal rhesus monkeys. Static and dynamic EW-stimulated accommodative responses were studied in five iridectomized rhesus monkeys before and after phenylephrine instillation. Accommodative amplitude was measured with a Hartinger coincidence refractometer. Dynamic accommodative responses were measured with infrared photorefraction, and functions were fitted to the data to determine peak velocity versus accommodative response relationships. RESULTS: The maximum dilated pupil diameter of 8.39 +/- 0.23 mm occurred 15 minutes after administration of phenylephrine. In iridectomized monkeys, postphenylephrine accommodative amplitudes were similar to prephenylephrine amplitudes. Dynamic analysis of the accommodative responses showed linear peak velocity versus accommodative amplitude relationships that were not statistically different before and after phenylephrine. CONCLUSIONS: alpha-Adrenergic stimulation causes a strong pupil dilation in noniridectomized monkey eyes but does not affect EW-stimulated accommodative amplitude or dynamics in anesthetized, iridectomized rhesus monkeys.  (+info)

Inhibition of alpha-adrenergic vasoconstriction in exercising human thigh muscles. (53/223)

The mechanisms underlying metabolic inhibition of sympathetic responses within exercising skeletal muscle remain incompletely understood. The aim of the present study was to test whether alpha(2)-adrenoreceptor-mediated vasoconstriction was more sensitive to metabolic inhibition than alpha(1)-vasoconstriction during dynamic knee-extensor exercise. We studied healthy volunteers using two protocols: (1) wide dose ranges of the alpha-adrenoreceptor agonists phenylephrine (PE, alpha(1) selective) and BHT-933 (BHT, alpha(2) selective) were administered intra-arterially at rest and during 27 W knee-extensor exercise (n= 13); (2) flow-adjusted doses of PE (0.3 microg kg(-1) l(-1)) and BHT (15 microg kg(-1) l(-1)) were administered at rest and during ramped exercise (7 W to 37 W; n= 10). Ultrasound Doppler and thermodilution techniques provided direct measurements of femoral blood flow (FBF). PE (0.8 microg kg(-1)) and BHT (40 microg kg(-1)) produced comparable maximal reductions in FBF at rest (-58 +/- 6 versus-64 +/- 4%). Despite increasing the doses, PE (1.6 microg kg(-1) min(-1)) and BHT (80 microg kg(-1) min(-1)) caused significantly smaller changes in FBF during 27 W exercise (-13 +/- 4 versus-3 +/- 5%). During ramped exercise, significant vasoconstriction at lower intensities (7 and 17 W) was seen following PE (-16 +/- 5 and -16 +/- 4%), but not BHT (-2 +/- 4 and -4 +/- 5%). At the highest intensity (37 W), FBF was not significantly changed by either drug. Collectively, these data demonstrate metabolic inhibition of alpha-adrenergic vasoconstriction in large postural muscles of healthy humans. Both alpha(1)- and alpha(2)-adrenoreceptor agonists produce comparable vasoconstriction in the resting leg, and dynamic thigh exercise attenuates alpha(1)- and alpha(2)-mediated vasoconstriction similarly. However, alpha(2)-mediated vasoconstriction appears more sensitive to metabolic inhibition, because alpha(2) is completely inhibited even at low workloads, whereas alpha(1) becomes progressively inhibited with increasing workloads.  (+info)

beta-Adrenoceptors potentiate alpha1-adrenoceptor-mediated inotropic response in rat left atria. (54/223)

AIM: To investigate whether stimulation of beta-adrenoceptor (AR) and its subtypes augment alpha1-AR-evoked positive inotropic response and inositol phosphate (InsP) accumulation in isolated rat left atria. METHODS: Inotropic response was determined by contractile function experiment in isolated electrically driven rat left atria. 3H-InsP accumulations were measured by 3H-inositol incorporation and column chromatography. RESULTS: (1) Stimulation of alpha1-AR by phenylephrine (PE) or norepinephrine (NE) in the presence of propranolol (Prop) evoked positive inotropic response and 3H-InsP accumulations, while stimulation of beta-AR by isoprenaline (ISO) or NE in the presence of phentolamine (Phen) only evoked positive inotropic response, but not 3H-InsP accumulations. (2) Simultaneous stimulation of alpha1- and beta-AR by NE or ISO plus PE significantly shifted the concentration-dependent inotropic response curves and 3H-InsP accumulation curves to the left and upward compared with individual alpha1-AR stimulation by PE or NE in the presence of Prop. (3) In the presence of ICI118551 (selective beta2-AR antagonist) or CGP12177 (selective beta1-AR antagonist), stimulation of either beta1- or beta2-AR did not change alpha1-AR-evoked inotropic response and 3H-InsP accumulations. CONCLUSION: Stimulation of beta1-AR and beta2-AR potentiates alpha1-AR-mediated positive inotropic response and InsP accumulation in isolated rat left atria.  (+info)

Protein kinase Czeta regulates phospholipase D activity in rat-1 fibroblasts expressing the alpha1A adrenergic receptor. (55/223)

BACKGROUND: Phenylephrine (PHE), an alpha1 adrenergic receptor agonist, increases phospholipase D (PLD) activity, independent of classical and novel protein kinase C (PKC) isoforms, in rat-1 fibroblasts expressing alpha1A adrenergic receptors. The aim of this study was to determine the contribution of atypical PKCzeta to PLD activation in response to PHE in these cells. RESULTS: PHE stimulated a PLD activity as demonstrated by phosphatidylethanol production. PHE increased PKCzeta translocation to the particulate cell fraction in parallel with a time-dependent decrease in its activity. PKCzeta activity was reduced at 2 and 5 min and returned to a sub-basal level within 10-15 min. Ectopic expression of kinase-dead PKCzeta, but not constitutively active PKCzeta, potentiated PLD activation elicited by PHE. A cell-permeable pseudosubstrate inhibitor of PKCzeta reduced basal PKCzeta activity and abolished PHE-induced PLD activation. CONCLUSION: alpha1A adrenergic receptor stimulation promotes the activation of a PLD activity by a mechanism dependent on PKCzeta; Our data also suggest that catalytic activation of PKCzeta is not required for PLD stimulation.  (+info)

Characterization of G proteins involved in activation of nonselective cation channels and arachidonic acid release by norepinephrine/alpha1A-adrenergic receptors. (56/223)

We demonstrated recently that norepinephrine activates Ca2+ -permeable nonselective cation channels (NSCCs) in Chinese hamster ovary cells stably expressing alpha1A-adrenergic receptors (CHO-alpha1A). Moreover, extracellular Ca2+ through NSCCs plays essential roles in norepinephrine-induced arachidonic acid release. The purpose of the present study was to identify the G proteins involved in the activation of NSCCs and arachidonic acid release by norepinephrine. For these purposes, we used U73122, an inhibitor of phospholipase C (PLC), and dominant negative mutants of G12 and G13 (G12G228A and G13G225A, respectively). U73122 failed to inhibit NSCCs activation by norepinephrine. The magnitudes of norepinephrine-induced extracellular Ca2+ influx in CHO-alpha1A microinjected with G13G225A were smaller than those in CHO-alpha1A. In contrast, the magnitudes of norepinephrine-induced extracellular Ca2+ influx in CHO-alpha1A microinjected with G12G228A were similar to those in CHO-alpha1A. In addition, neither a Rho-associated kinase (ROCK) inhibitor nor a phosphoinositide 3-kinase inhibitor affected norepinephrine-induced extracellular Ca2+ influx. G13G225A, but not G12G228A, also inhibited arachidonic acid release partially. These results demonstrate that 1) the Gq/PLC-pathway is not involved in NSCCs activation by norepinephrine, 2) G13 couples with CHO-alpha1A and plays important roles for norepinephrine-induced NSCCs activation, 3) neither ROCK- nor PI3K-dependent cascade is involved in NSCCs activation, and 4) G13 is involved in norepinephrine-induced arachidonic acid release in CHO-alpha1A.  (+info)