Idazoxan (RX 781094) selectively antagonizes α2-adrenoceptors on rat central neurons

Freedman, Jonathan E.; Aghajanian, George K.

doi:10.1016/0014-2999(84)90618-6

Cited by 147 publications

(57 citation statements)

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“…The ␣ 2 -adrenoceptor located on the cell body of LC NE neurons is important in the negative feedback regulation of the firing activity of these neurons (Freedman and Aghajanian, 1984;Mateo et al, 1998). To determine whether the suppression of firing of NE neurons in rats treated with YM992 for 2 days was due to an increased activation of ␣ 2 -adrenoceptors, systemic injections of the selective ␣ 2 -adrenoceptor antagonist idazoxan (1 mg kg Ϫ1 i.v.)…”

Section: Resultsmentioning

confidence: 99%

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Szabo¹,

Blier²

2002

J Pharmacol Exp Ther

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YM992 [(S)-2-[[(7-fluoro-4-indanyl)oxy]methyl]morpholine monohydrochloride] is a selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitor (SSRI) and a potent 5-HT 2A antagonist. The aim of the present study was to assess, using in vivo extracellular unitary recordings, the effect of acute and sustained administration of YM992 (40 mg kg Ϫ1 day Ϫ1 s.c., using osmotic minipumps) on the spontaneous firing activity of locus coeruleus (LC) norepinephrine (NE) neurons. Acute intravenous injection of YM992 (4 mg kg Ϫ1 ) significantly decreased NE neuron firing activity by 29% and blocked the inhibitory effect of a subsequent injection of the 5-HT 2 agonist DOI [1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride]. A 2-day treatment with YM992 decreased the firing rate of NE neurons by 66%, whereas a partial recovery was observed after a 7-day treatment and a complete one after a 21-day treatment. Following the injection of the ␣ 2 -adrenoceptor antagonist idazoxan (1 mg kg -1 i.v.), NE neuron firing was equalized in controls and 2-day YM992-treated rats. This put into evidence an increased degree of activation of ␣ 2 -adrenergic autoreceptors in the treated rats. The suppressant effect of the ␣ 2 -adrenoceptor agonist clonidine was significantly decreased in longterm YM992-treated rats. The recovery of LC firing activity after long-term YM992 administration could thus be explained by a decreased sensitivity of ␣ 2 -adrenergic autoreceptors. Sustained SSRI administration leads to a gradual reduction of the firing activity of NE neurons during long-term administration, whereas YM992 produced opposite effects. The exact basis for the increased synaptic availability of NE by YM992 remains to be elucidated. This NE activity, resulting from 5-HT reuptake inhibition plus 5-HT 2A receptor antagonism, might confer additional benefits in affective and anxiety disorders.The norepinephrine (NE) and the serotonin (5-hydroxytryptamine; 5-HT) systems have both been implicated in anxiety and affective disorders. Although the etiopathology of these two disorders remains enigmatic, greater knowledge exists pertaining to the interactions/alterations of these monoaminergic systems during antidepressant drug treatment. It is well established that locus coeruleus (LC) NE neurons modulate the 5-HT system, and evidence is accumulating for a major influence of 5-HT on the NE system (see Haddjeri et al., 1997;Kaehler et al., 1999). The LC receives dense 5-HT projections coming from dorsal raphe and pericoerulear 5-HT neurons (Aston-Jones et al., 1991;Kaehler et al., 1999), which exert an inhibitory role (Léger and Descarries, 1978;Segal, 1979). This is supported by the observation that lesioning 5-HT neurons with a 5-HT neurotoxin produced a marked elevation of firing rate of NE neurons (Haddjeri et al., 1997). Long-term, but not acute or short-term (2-day) administration of SSRIs decreases the spontaneous firing activity of LC NE neurons in the rat (Béïque et al.,1998;Szabo et al., 2000;Szabo and Blier, 2001a;Grant a...

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Section: Resultsmentioning

confidence: 99%

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Szabo¹,

Blier²

2002

J Pharmacol Exp Ther

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“…This inhibitory action of clonidine is believed to result from the activation of a 2 -adrenergic autoreceptors on the cell body and terminals of NE neurons, thereby decreasing the endogenous NE input to excitatory a 1 -adrenoceptors on 5-HT neurons in the DR nucleus (Svensson et al, 1975;Baraban and Aghajanian, 1980;Clement et al, 1992). Accordingly, acute administration of the a 2 -adrenergic antagonists idazoxan and mirtazapine increase the spontaneous firing activity of DR 5-HT neurons and antagonize the suppressant effect of clonidine on these neurons (Freedman and Aghajanian, 1984;Garrat et al, 1991;Clement et al, 1992;Haddjeri et al, 1996). Furthermore, the enhancing effect of mirtazapine on the firing activity of 5-HT neurons is abolished by lesioning NE neurons, indicating that this effect of mirtazapine is mediated via NE neurons (Haddjeri et al, 1996).…”

Section: Introductionmentioning

confidence: 99%

Electrophysiological Evidence for the Tonic Activation of 5-HT1A Autoreceptors in the Rat Dorsal Raphe Nucleus

Haddjeri

Lavoie

Blier

2004

Neuropsychopharmacol

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Serotonin (5-hydroxytryptamine, 5-HT) and norepinephine (NE) neurons have reciprocal connections. These may thus interfere with anticipated effects of selective pharmacological agents targeting these neurons. The main goal of the present study was to assess whether the somatodendritic 5-HT 1A autoreceptor is tonically activated by endogenous 5-HT in anesthetised rats, using in vivo extracellular unitary recordings. In rats with their NE neurons lesioned using 6-hydroxydopamine (6-OHDA) and in controls administered the NE reuptake inhibitor desipramine to suppress NE neuronal firing, the a 2 -adrenoceptor agonist clonidine no longer inhibited 5-HT neuron firing, therefore indicating the important modulation of the firing activity of 5-HT neurons by NE neurons. In control rats, the administration of the potent and selective 5-HT 1A receptor antagonist WAY 100,635 ((N-{2-[4(2-methoxyphenyl)-1-piperazinyl]ethyl}-N-(2-pyridinyl)cyclohexanecarboxamide trihydroxychloride) (100 mg/kg, i.v.) did not modify the spontaneous firing activity of 5-HT neurons, but in NE-lesioned rats using either 6-OHDA or DSP-4, WAY 100,635 produced a mean firing increase of 80 and 69%, respectively. When desipramine and D-amphetamine were used in control rats to prevent alterations in the availability of NE in the dorsal raphe, again WAY 100,635 produced a significant disinhibition of the firing of 5-HT neurons (83 and 53%, respectively). These data support the notion that the NE system tonically activates the firing activity of 5-HT neurons. When the fluctuations of the function of NE neurons normally produced by WAY 100,635 were prevented, a tonic activation of 5-HT 1A autoreceptors by endogenous 5-HT was unmasked.

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“…Idazoxan was introduced as a competitive a2-adrenoceptor antagonist (Freedman & Aghajanian, 1984;Dabire, 1986). Recently idazoxan has been found to label specifically nonadrenoceptor sites (Michel et al, 1989;Ernsberger et al, 1990) and has been used as a tool to identify imidazoline receptors Reis et al, 1992).…”

Section: Resultsmentioning

confidence: 99%

“…The increase in urine flow rate and sodium excretion may also have been mediated by a2-adrenoceptor antagonism since idazoxan has a fairly high affinity for this receptor (Freedman et al, 1984;Dabire, 1986). However, antagonism of endogenous x2-adrenoceptor function in the kidney would be expected to decrease urine flow rate secondary to a decrease in free water clearance (Blandford & Smyth, 1988b;Gellai, 1990).…”

Section: Resultsmentioning

confidence: 99%

Antagonism by idazoxan at low dose but not high dose, of the natriuretic action of moxonidine

Allan

Penner

Smyth

1996

British J Pharmacology

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1 Recent studies concerning the imidazoline receptor have utilized idazoxan as a specific imidazoline receptor antagonist. The aim of the present study was to describe the in vivo effects of various doses of idazoxan on renal function, in the presence and absence of moxonidine, an I, imidazoline receptor agonist. 2 In anaesthetized, unilaterally nephrectomized (7 to 10 days) Sprague Dawley rats, an intrarenal infusion of moxonidine (3 nmol kg-' min-') increased urine flow rate, sodium excretion and osmolar clearance without altering free water clearance. Pretreatment with intravenous idazoxan at 0.1 and 0.3 mg kg-' produced a dose-related decrease in the renal actions of moxonidine. However, a higher dose of idazoxan (1 mg kg-') was not as effective as the 0.3 mg kg-' dose in blocking the effects of moxonidine.3 In a separate series of experiments, the direct renal actions of idazoxan alone were investigated. Idazoxan at 0.3 mg kg-' failed to alter urine flow rate and sodium excretion. However, idazoxan at 1 mg kg-' produced a significant increase in urine flow rate and sodium excretion in association with an increase in osmolar clearance. 4 These results do not prove but are consistent with low doses of idazoxan antagonizing the sites stimulated by moxonidine (renal imidazoline receptors). However, at higher doses, idazoxan may function as a partial agonist and/or interact with other receptors to increase urine flow rate, independent of imidazoline receptor blockade. These studies underscore the importance of the dose of idazoxan administered when this antagonist is used as a tool to investigate imidazoline receptors.

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Idazoxan (RX 781094) selectively antagonizes α2-adrenoceptors on rat central neurons

Cited by 147 publications

References 17 publications

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Electrophysiological Evidence for the Tonic Activation of 5-HT1A Autoreceptors in the Rat Dorsal Raphe Nucleus

Antagonism by idazoxan at low dose but not high dose, of the natriuretic action of moxonidine

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Idazoxan (RX 781094) selectively antagonizes α2-adrenoceptors on rat central neurons

Cited by 147 publications

References 17 publications

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Electrophysiological Evidence for the Tonic Activation of 5-HT1A Autoreceptors in the Rat Dorsal Raphe Nucleus

Antagonism by idazoxan at low dose but not high dose, of the natriuretic action of moxonidine

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Product

Resources

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Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons

Effects of Serotonin (5-Hydroxytryptamine, 5-HT) Reuptake Inhibition Plus 5-HT_2A Receptor Antagonism on the Firing Activity of Norepinephrine Neurons