Noradrenaline release from cultured mouse postganglionic sympathetic neurons: autoreceptor-mediated modulation

The possible existence of alpha2-autoreceptors, P2-autoreceptors, and adenosine A1- or A2A-receptors was studied in cultured thoracolumbar postganglionic sympathetic neurons from mice. The cells were preincubated with [3H]noradrenaline and then superfused. The selective alpha2-adrenoceptor agonist UK 14,304 reduced the electrically evoked overflow of tritium. When the cultures were stimulated by trains of increasing pulse number, ranging from a single pulse to 72 pulses at 3 Hz, the concentration-inhibition curve of UK 14,304 was shifted progressively to the right and the maximal inhibition obtainable became progressively smaller. Six alpha-adrenoceptor antagonists shifted the concentration-inhibition curve of UK 14,304 in a parallel manner to the right. Neither ATP (3-300 microM), adenosine (0.01-100 microM), the selective A1-receptor agonist cyclopentyladenosine (1-1,000 nM), nor the selective A2A-receptor agonist CGS-21680 (1-10,000 nM) changed the basal or the electrically evoked overflow of tritium. It is concluded that the cultured neurons possess presynaptic, release-inhibiting alpha2-autoreceptors. As in intact tissues, the effectiveness of presynaptic alpha2-adrenergic inhibition depends on the "strength" of the releasing stimulus. The pK(D) values of the six antagonists against UK 14,304 indicate that the autoreceptors belong to the pharmacological alpha2D and hence the genetic alpha(2A/D) subtype of alpha2-adrenoceptor. Neither P2-autoreceptors nor receptors for adenosine, the degradation product of ATP, were detected.

Modulation of sibutramine-induced increases in extracellular noradrenaline concentration in rat frontal cortex and hypothalamus by alpha2-adrenoceptors

1. The effects of sibutramine (0.25 - 10 mg kg-1 i.p.) on extracellular noradrenaline concentration in the frontal cortex and hypothalamus of freely-moving rats were investigated using microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of sibutramine in these brain areas was also determined. 2. Sibutramine induced an increase in extracellular noradrenaline concentration, the magnitude of which paralleled dose, in both brain areas. In the cortex, this increase was gradual and sustained, whereas in the hypothalamus it was more rapid and of shorter duration. 3. In both the cortex and hypothalamus, pretreatment of rats with the alpha2-adrenoceptor antagonist RX821002 (3 mg kg-1 i.p.) potentiated increases in the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i. p.), by 7 and 10 fold respectively. RX821002 also reduced the latency of sibutramine to reach its maximum effect in the cortex, but not in the hypothalamus. 4. Infusion of RX821002 (1 microM) via the probe increased the accumulation of extracellular noradrenaline induced by sibutramine (10 mg kg-1 i.p.) in both brain areas. In the hypothalamus, the effects of RX821002 on the accumulation of noradrenaline induced by sibutramine were 2 fold greater than those in the cortex. 5. These findings support evidence that sibutramine inhibits the reuptake of noradrenaline in vivo, but that the accumulation of extracellular noradrenaline is limited by noradrenergic activation of presynaptic alpha2-adrenoceptors. Furthermore, the data suggest that terminal alpha2-adrenoceptors in the hypothalamus exert a greater inhibitory effect over the control of extracellular noradrenaline accumulation than do those in the cortex.

Electrophysiologic effects of agmatine on pacemaker cells in sinoatrial node of rabbits

AIM

To study the electrophysiologic effects of agmatine (Agm) on pacemaker cells in sinoatrial (SA) node.

METHODS

Parameters of action potential (AP) in SA node were recorded using intracellular microelectrode technique.

RESULTS

Agm not only slowed down the amplitude of action potential (APA), maximal rate of depolarization (Vmax), velocity of diastolic (phase 4) depolarization (VDD), and rate of pacemaker firing (RPF), but also prolonged 90% duration of action potential (APD90) in a concentration-dependent manner. The effects of Agm (10 mmol.L-1) could be blocked completely by pretreatment with idazoxan (0.15 mmol.L-1), an alpha 2-adrenergic receptor (alpha 2-AR) and imidazoline receptor (IR) antagonist. Pretreatment with NG-nitro-L-arginine methyl ester (L-NAME, 1 mmol.L-1), an NOS inhibitor, did not affect the electrophysiologic effects of Agm on pacemaker cells in SA node. Elevation of Ca2+ concentration (5 mmol.L-1) in perfusate antagonized the effects of Agm (10 mmol.L-1). Lemakalim (Lem, 30 mumol.L-1), an opener of ATP-sensitive potassium channels, partially inhibited the prolonging effect of Agm on repolarization.

CONCLUSION

The electrophysiologic effects of Agm on pacemaker cells in SA node were likely attributed to the reduction in calcium influx and potassium efflux and mediated by alpha 2-AR and IR.

Effect of the alpha 2 adrenoreceptor antagonist, idazoxan, on motor disabilities in MPTP-treated monkey

1. The motor effect of the alpha 2 adrenoreceptor antagonist, idazoxan, was compared to that of L-dopa in MPTP-treated monkeys. 2. Idazoxan 2.0 mg/kg improved parkinsonian motor abnormalities which was comparable to the effects of a minimal effective dose of L-dopa. 3. At 2.0 and 5.0 mg/kg, the parkinsonian rigidity was the item most frequently alleviated by idazoxan (respectively 63.6% and 68.2%). 4. These findings provide support for the therapeutic utility of alpha 2 antagonists in the treatment of Parkinson's disease.

Enhanced cortical dopamine output and antipsychotic-like effects of raclopride by alpha2 adrenoceptor blockade

Clozapine exerts superior clinical efficacy and markedly enhances cortical dopamine output compared with classical antipsychotic drugs. Here the alpha2 adrenoceptor antagonist idazoxan was administered to rats alone or in combination with the D2/3 dopamine receptor antagonist raclopride. Dopamine efflux in the medial prefrontal cortex and conditioned avoidance responding were analyzed. Idazoxan selectively potentiated the cortical output of dopamine and augmented the suppression of conditioned avoidance responding induced by raclopride. These results challenge basic assumptions underlying the dopamine hypothesis of schizophrenia and provide insight into clozapine's mode of action.

Adrenergic alpha2C-receptors reside in rat striatal GABAergic projection neurons: comparison of radioligand binding and immunohistochemistry

We have investigated the distribution of alpha2c-adrenergic receptors in the rat striatum and characterized the striatal neuron types expressing these receptors. Sequential double-labelled immunocytochemistry was performed with a polyclonal antibody against rat alpha2c-adrenoceptors and antibodies against GABA, Calbindin-D28k, parvalbumin and calretinin. The subregional distribution of alpha2c-adrenoceptor binding sites in the striatum was also quantitatively investigated using selective radioligands. Almost all lightly stained striatal GABAergic neurons, with the morphological characteristics of medium-sized spiny projection neurons (94% of GABAergic cells counted), contained alpha2c-adrenoceptor-immunoreactive structures. Intensely labelled GABAergic inteneurons (6%) were devoid of alpha2c-adrenoceptor immunoreactivity. The co-localization of calbindin- and alpha2c-adrenoceptor immunoreactivity in the majority of the cells confirmed the presence of alpha2c-adrenoceptors in the population of medium-sized spiny neurons. Furthermore, the alpha2c-adrenoceptor/calbindin double-labelling disclosed the existence of three neuronal subsets in the matrix compartment of the striatum: a large proportion (83%) of double-labelled neurons, a population of neurons (8%) that exhibited only alpha2c-adrenoceptor immunoreactivity without calbindin immunoreactivity, and a population of neurons (9%) immunoreactive for calbindin, but lacking alpha2c-adrenoceptors. In addition, alpha2c-adrenoceptor immunolabelled neurons were observed in calbindin-free striatal patches. Parvalbumin- and calretinin-positive neurons never displayed alpha2c-adrenoceptor immunoreactivity, confirming that striatal GABAergic interneurons are devoid of alpha2c-adrenoceptors. The present findings indicate that alpha2c-adrenoceptors are localized in GABAergic medium-sized spiny projection neurons but not in interneurons of the rat striatum, and that they may modulate both the direct and indirect pathways of the basal ganglia, as well as participate in the regulation of mesencephalic dopaminergic neurons.

Locus coeruleus neuronal activity and noradrenaline availability in the frontal cortex of rats chronically treated with imipramine: effect of alpha 2-adrenoceptor blockade

BACKGROUND

Previous studies indicate a reduced feedback inhibition of brain noradrenaline (NA) neurons in the locus coeruleus (LC) during chronic administration of antidepressants which inhibit the NA reuptake mechanism due to functional downregulation of somatodendritic alpha 2-adrenoceptors in the LC. Therefore, we have here studied the LC neuronal responsiveness to administration of the alpha 2-adrenoceptor antagonist idazoxan (IDA) after both short-term and long-term imipramine (IMI) administration.

METHODS

Rats were treated for different periods with systemic IMI. In these rats, basal activity of central noradrenergic function and the effect of IDA was assessed by means of extracellular single-cell recording from LC neurons and in vivo microdialysis of extracellular NA levels in the frontal cortex (FC).

RESULTS

The average firing rate of LC neurons was significantly reduced in rats by short-term IMI treatment compared with long-term treatment. The output of NA in the FC of all IMI-treated animals was significantly increased compared with saline-treated rats. Moreover, the enhancing effect of IDA on both the firing rate of LC neurons and the cortical NA output was larger in rats after long-term treatment with IMI than after short-term administration.

CONCLUSIONS

Our results clearly support the notion of development of functional downregulation of alpha 2-autoreceptors on LC neurons during chronic administration of NA reuptake inhibiting antidepressants. Moreover, the data suggest that addition of alpha 2-adrenoceptor antagonists may augment the clinical effect of such drugs in major depression.

The effects of idazoxan on reaction times, eye movements and the mood of healthy volunteers and patients with upper respiratory tract illnesses

An experiment was carried out to determine whether idazoxan, a drug which increases the turnover of central noradrenaline, removes the malaise (reduced alertness, slower psychomotor performance) associated with upper respiratory tract illness (URTI). Eighty-one volunteers were tested when healthy and 17 returned to the laboratory when they developed URTIs. Those who remained healthy were then recalled as a control group. Volunteers were tested before and after receiving either idazoxan (40mg) or a lactose placebo. Idazoxan removed the URTI-induced slowing in a simple reaction time task and this group performed at a comparable level to the healthy group. No significant stimulant effect of idazoxan was found in the healthy subjects. The results suggest that at least part of the malaise induced by URTIs may reflect reductions in central noradrenaline and that this can be reversed by compounds such as idazoxan.

The antinociceptive effect of the mu-opioid fentanyl is reduced in the presence of the alpha(2)-adrenergic antagonist idazoxan in inflammation

Interactions between alpha(2)-adrenergic and mu-opioid systems play an important role in the modulation of hyperalgesic states. The antinociceptive effects of alpha(2)-adrenergic agonists and mu-opioids are potentiated when co-administered; however, attempts to induce cross reversal of the antinociceptive effects of alpha(2)-adrenergic and mu-opioid systems have produced contradictory results. We have studied the possible endogenous tonic control of the alpha(2)-adrenergic systems in the modulation of pain in inflammation, and the interactions between the two antinociceptive systems in rat spinal cord nociceptive reflexes activated by both natural and electrical stimulation. The facilitatory actions of the alpha(2)-adrenergic antagonist idazoxan were compared in control rats and in animals with carrageenan-induced paw inflammation. The antinociceptive effect of the mu-opioid fentanyl was tested alone and in the presence of idazoxan. In agreement with some previous observations, idazoxan i.v. produced no change in responses to natural and electrical stimulation in normal animals. In animals with inflammation, idazoxan only induced facilitation of responses evoked by noxious thermal stimulation but not by mechanical or electrical stimulation. Fentanyl reduced the responses to either stimuli with lower potency in the presence of idazoxan, but only in animals with inflammation. Its dose-response curve was shifted to the right between 1.8- and 3. 5-fold depending on the stimulus used. It is concluded that the increase of thermal responses by idazoxan in animals with inflammation is probably due to changes in the peripheral blood flow. Nevertheless, since an interaction with mu-opioids is clear in inflammation, endogenous alpha(2)-adrenergic systems play an important role in the modulation of the effectiveness of opioids during inflammation.

The alpha2-adrenergic receptor antagonist idazoxan reduces dyskinesia and enhances anti-parkinsonian actions of L-dopa in the MPTP-lesioned primate model of Parkinson's disease

Dopamine replacement therapy in patients with Parkinson's disease is plagued by the emergence of abnormal involuntary movements known as L-dopa-induced dyskinesias. It has been demonstrated that yohimbine can reduce L-dopa-induced dyskinesia in the MPTP-lesioned primate model of Parkinson's disease. Yohimbine is, among other things, an alpha-adrenergic receptor antagonist. In this study, we demonstrate that the selective and potent alpha2-adrenergic receptor antagonist idazoxan reduces L-dopa-induced dyskinesia in the MPTP-lesioned marmoset model of Parkinson's disease. The alpha2-adrenergic receptor antagonists rauwolscine and yohimbine also reduce L-dopa-induced dyskinesia. Furthermore, we demonstrate that coadministration of idazoxan with L-dopa can provide an anti-parkinsonian action more than twice the length of that seen with L-dopa alone. However, idazoxan as a monotherapy displayed no anti-parkinsonian actions. We propose that idazoxan in combination with L-dopa may provide a novel approach to the treatment of Parkinson's disease that will not only reduce the dyskinetic side effects, but extend the anti-parkinsonian actions of L-dopa. Idazoxan, as an adjunct to dopamine replacement, may prove useful in the treatment of parkinsonian patients at all stages of disease progression.

Rapid recovery of self-stimulation responding from depression by clozapine is prevented by the alpha 2-adrenoceptor agonist, clonidine

Typical and atypical antipsychotic agents were tested on rats responding for variable-interval electrical stimulation of the ventral tegmental area (VTA). The typical neuroleptics, chlorpromazine and haloperidol, led to prolonged depression of responding lasting at least 4 h, whereas response rates after similarly effective doses of the atypical agents, clozapine and risperidone, recovered to control levels in the same period. The role of alpha 2-adrenoceptors in producing these differences was investigated by administering clozapine together with an alpha 2-adrenoceptor agonist, clonidine, and chlorpromazine together with an alpha 2-adrenoceptor antagonist, idazoxan. The addition of clonidine extended the response-depressant activity of clozapine, resulting in prolonged depression comparable to that produced by chlorpromazine or haloperidol. Conversely, the addition of idazoxan to chlorpromazine shortened the duration of chlorpromazine's suppressant action to a level comparable to that of clozapine or risperidone. These results suggest that the brevity of clozapine's effects on operant behaviour (a feature which may be related to its reduced liability to extrapyramidal side-effects) may be a consequence of its alpha 2-adrenoceptor antagonist properties.

Cerebral glucose metabolic and plasma catecholamine responses to the alpha(2) adrenoceptor antagonist ethoxyidazoxan given to healthy volunteers

RATIONALE

Methods that test for the central effects of alpha(2)-adrenoceptor antagonists can facilitate the clinical development of such compounds. Recently we evaluated the effects of idazoxan (IDX), an alpha(2)-adrenoceptor antagonist with high affinity for imidazoline sites, on a variety of measures potentially sensitive to blockade of alpha(2)-adrenoceptors including regional brain glucose metabolic rate.

OBJECTIVE

To test whether these effects on brain metabolic rate could have been mediated by imidazoline binding sites, single dose challenges of 9 or 12 mcg/kg ethoxyidazoxan (ETX; an alpha(2)-adrenoceptor antagonist which does not bind to imidazoline sites) were given to healthy male volunteers.

METHODS

The effects on brain glucose metabolism, blood pressure, catecholamines, and behavior were assessed.

RESULTS

Blood pressure increased 10-15% after both doses. Plasma catecholamines increased 2- to 2.5-fold and responses were dose dependent. There was no evidence of either dose being anxiogenic. Both doses of ETX produced diffuse increases in brain glucose metabolism.

CONCLUSIONS

Brain glucose metabolic responses were more widespread and monotonic than we had observed with IDX. ETX also produced robust increases in glucose metabolism in cerebellum. While we were unable to exclude the possibility that some of the brain metabolic responses we had observed with IDX were mediated by imidazoline sites, ETX may be sufficiently distinct from IDX in alpha(2)-adrenoceptor affinity that differences in acute metabolic responses occurred.

Characterization of alpha2 adrenergic receptor subtypes in human ocular tissue homogenates

PURPOSE

To determine the predominant alpha2 adrenergic receptor subtypes present in the human eye.

METHODS

Saturation- and competition-receptor- binding experiments were performed with the radioligand [3H]RX821002 in human ciliary body, retinal pigmented epithelium-choriocapillaris, iris, and neurosensory retina. The affinities of various adrenergic antagonists in these ocular tissues were compared with their affinities for the cloned alpha2A, alpha2B, and alpha2C adrenergic receptor subtypes.

RESULTS

The density of alpha2 adrenergic receptors was highest in the iris (440 femtomoles/mg protein), lowest in the neurosensory retina (14 femtomoles/mg protein), and intermediate in the other two tissues (approximately 90 fmol/mg protein). The drug affinities in all four human ocular tissues were highly correlated (correlation coefficients between 0.94 and 0.97) with the affinities for the human alpha2A adrenergic receptor subtype and poorly correlated (correlation coefficients between 0.15 and 0.66) with the alpha2B and alpha2C subtypes.

CONCLUSIONS

In agreement with previous studies in several animal species, the alpha2 adrenergic receptors in the human ciliary body, retinal pigmented epithelium-choriocapillaris, iris, and neurosensory retina are predominately of the alpha2A subtype.

Electrochemical and electrophysiological characterization of neurotransmitter release from sympathetic nerves supplying rat mesenteric arteries

1. Characteristic features of noradrenaline (NA) and adenosine 5'-triphosphate (ATP) release from postganglionic sympathetic nerves in rat small mesenteric arteries in vitro have been investigated on an impulse-by-impulse basis. NA release was measured using continuous amperometry and ATP release was monitored by intracellular recording of excitatory junction potentials (e.j.ps). 2. Electrical stimuli evoked transient increases in oxidation current. During trains of ten stimuli at 0.5 - 4 Hz there was a depression in the amplitude of oxidation currents evoked following the first stimulus in the train. 3. The neuronal NA uptake inhibitor, desmethylimipramine (1 microM), increased the amplitude of the summed oxidation current evoked by ten stimuli at 1 Hz and slowed the decay of oxidation currents evoked by trains of ten stimuli at 1 and 10 Hz. 4. The alpha2-adrenoceptor antagonist, idazoxan (1 microM), increased the amplitudes of the oxidation currents evoked during trains of ten stimuli at 0.5 - 10 Hz but had no effect on the oxidation currents evoked by the first stimulus in the train. 5. Idazoxan (1 microM) increased the amplitude of all e.j.ps evoked during trains of stimuli at 0.5 and 1 Hz. In addition, the facilitatory effect of idazoxan on e.j.ps was significantly greater than that on oxidation currents. 6. The findings indicate that NA release from sympathetic nerves supplying small mesenteric arteries is regulated by activation of presynaptic alpha2-adrenoceptors and that clearance of released NA in this tissue depends, in part, upon neuronal uptake. The different effects of idazoxan on the oxidation currents and e.j.ps may indicate that the release of NA and ATP is differentially modulated.

Noradrenaline and mixed alpha 2-adrenoceptor/imidazoline-receptor ligands: effects on sodium intake

The effect of noradrenaline, and mixed ligands to alpha 2-adrenoceptors (alpha 2-AR) and imidazoline receptors (IR), injected intracerebroventricularly (i.c.v.), on sodium intake of sodium depleted rats, was tested against idazoxan, a mixed antagonist ligand to alpha 2-AR and IR. The inhibition of sodium intake induced by noradrenaline (80 nmol) was completely reversed by idazoxan (160 and 320 nmol) injected i.c.v. The inhibition of sodium intake induced by mixed ligands to alpha 2-AR and IR, UK14,304, guanabenz and moxonidine, was antagonized from 50 to 60% by idazoxan i.c.v. The results demonstrate that noradrenaline, a non-ligand for IR, acts on alpha 2-AR inhibiting sodium intake. The possibility that either alpha 2-AR or IR mediate the effect of mixed agonists on sodium intake remains an open question.

Determination of the somatodendritic alpha2-adrenoceptor subtype located in rat locus coeruleus that modulates cortical noradrenaline release in vivo

The regulation of extracellular noradrenaline levels in the cingulate cortex by somatodendritic alpha2-adrenoceptors located in the locus coeruleus was evaluated in the rat by using dual-probe microdialysis. The concentration of noradrenaline in the cingulate cortex was decreased (37%-40%) by administration into the locus coeruleus (1microM) of the agonists clonidine and UK14304 (bromoxidine), whereas it was increased by similar administration of the nonselective antagonist RX821002 (2-methoxyidazoxan) (+ 103%) and the selective alpha2A-adrenoceptor antagonist BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidaz ole) (+ 148%). The selective alpha2B/C-adrenoceptor antagonist ARC239 (2-[2[4-(o-methoxyphenyl)piperazin-1-yl]ethyl]-4,4-dimethyl-1,3-(2 H,4H)-isoquinolimedione) did not induce changes. In the presence of BRL44408, the effects of clonidine and UK14304 were abolished, but they were not modified in the presence of ARC239. The data demonstrate that noradrenaline release in terminal areas is tonically modulated by somatodendritic alpha2A-adrenoceptors.

Comparison of changes in the extracellular concentration of noradrenaline in rat frontal cortex induced by sibutramine or d-amphetamine: modulation by alpha2-adrenoceptors

1. The effects of sibutramine (0.25 - 10 mg kg-1, i.p.) on extracellular noradrenaline concentration in the frontal cortex of halothane-anaesthetized rats were compared with those of d-amphetamine (1 - 3 mg kg-1, i.p.) using in vivo microdialysis. The role of presynaptic alpha2-adrenoceptors in modulating the effects of these drugs on extracellular noradrenaline concentration were also investigated by pretreating rats with the selective alpha2-adrenoceptor antagonist, RX821002. 2. Sibutramine induced a gradual and sustained increase in extracellular noradrenaline concentration. The dose-response relationship was described by a bell-shaped curve with a maximum effect at 0.5 mg kg-1. In contrast, d-amphetamine induced a rapid increase in extracellular noradrenaline concentration, the magnitude of which paralleled drug dose. 3. Pretreatment with the alpha2-adrenoceptor antagonist, RX821002 (dose 3 mg kg-1, i.p.) increased by 5 fold the accumulation of extracellular noradrenaline caused by sibutramine (10 mg kg-1) and reduced the latency of sibutramine to reach its maximum effect from 144 - 56 min. 4. RX821002-pretreatment increased by only 2.5 fold the increase in extracellular noradrenaline concentration caused by d-amphetamine alone (10 mg kg-1) and had no effect on the latency to reach maximum. 5. These findings support evidence that sibutramine acts as a noradrenaline uptake inhibitor in vivo and that the effects of this drug are blunted by indirect activation of presynaptic alpha2-adreno-ceptors. In contrast, the rapid increase in extracellular noradrenaline concentration induced by d-amphetamine is consistent with this being mainly due to an increase in Ca2+-independent release of noradrenaline.

Idazoxan potentiates rather than antagonizes some of the cognitive effects of clonidine

Several investigations have revealed substantial influences of pharmacological manipulation of central noradrenergic activity upon performance in cognitive tests sensitive to frontal lobe dysfunction. They suggest a significant role for the noradrenergic coeruleo-cortical projection in cognitive function but conflicting findings and the complex pharmacology of adrenoceptor agents make it difficult to be precise about underlying mechanisms. In order to clarify these we have compared the effects of an alpha1/alpha2-adrenoceptor agonist, clonidine, an alpha2-adrenoceptor antagonist, idazoxan, and these agents in combination. Three groups of healthy volunteers were used to investigate the effects of these noradrenergic manipulations upon performance of tasks from the CANTAB test battery known to be sensitive to frontal lobe dysfunction. Previously reported effects of clonidine upon sustained visual attention and upon session-to-session improvement were replicated. Furthermore, idazoxan inhibited the hypotensive effect of clonidine. Idazoxan had no overall effect on performance of any of the tests but did inhibit session-to-session improvement in performance of a planning task, attentional set shifting and sustained visual attention. Rather than leading to the anticipated mutual antagonism of effects, combining clonidine and idazoxan led to a wider and more striking range of cognitive impairments. These results are discussed alongside findings which support a role for imidazoline (I1) receptors in blood pressure control, where clonidine and idazoxan are antagonistic, and evidence of less potent antagonism at somato-dendritic alpha2-adrenoceptors in the locus coeruleus.

Effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline in the rat frontal cortex: augmentation via 5-HT1A receptor antagonism

Venlafaxine is a novel serotonin/noradrenaline reuptake inhibitor (SNRI) which has been shown clinically to be an effective antidepressant (AD) with a faster onset of action than serotonin specific reuptake inhibitors (SSRI). Preclinically, venlafaxine has been shown to potently inhibit dorsal raphe neuronal (DRN) firing through a 5-HT1A mediated mechanism, in a similar manner to SSRIs. Here we demonstrate the acute neurochemical effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline (NA) from the rat frontal cortex using in vivo microdialysis. Administration of venlafaxine (3-50 mg/kg s.c.) resulted in a significant dose-dependent increase in extracellular NA, but produced no significant increase in 5-HT concentrations. Combination treatment with the selective 5-HT1A antagonist WAY100635 produced a dose-dependent augmentation of venlafaxine-induced (3-30 mg/kg s.c) extracellular 5-HT concentrations, but had no further effect on NA above that produced by venlafaxine alone. WAY100635, at doses as low as 0.03 mg/kg s.c., maintained this potentiation effect. The beta-adrenergic/5-HT1A receptor antagonist (+/-)pindolol and the selective 5-HT1B/D antagonist GR127935 produced no significant augmentation of venlafaxine-induced changes in either 5-HT or NA. Using the alpha1 and alpha2-adrenoceptor antagonists, prazosin and idazoxane, we also demonstrate the role of the alpha-adrenoceptors in the augmentation of venlafaxine-induced changes. The possible mechanisms underlying venlafaxines improved clinical AD action and the potential for further enhancement of this SNRIs clinical effects are discussed.

Influence of agmatine in adaptation of cAMP signal transduction system of opiate receptors

AIM

To observe attenuative effects of agmatine on opiate desensitization and substance dependence.

METHODS

Guanosine 5'-O-(3-[35S] thiotriphosphate) ([35S]GTTP) binding and cellular cyclic AMP (cAMP) level were determined by radioligand binding assay and radioimmunoassay in NG108-15 cells, respectively.

RESULTS

Agmatine increased stimulative action of opioids on [35S]GTTP binding by about 35% and inhibitory effects of opioids on cellular cAMP concentration by about 114.3% in NG108-15 cells pretreated with opioids. On the other hand, it also inhibited cAMP over-shooting by 214.9% of morphine substance dependent cells precipitated by naloxone compared with that of control. These effects of agmatine were antagonized by idazoxan in a concentration-dependent manner.

CONCLUSION

Agmatine reversed the formative process of adaptation in cAMP signal transduction cascade.

Protection by imidazol(ine) drugs and agmatine of glutamate-induced neurotoxicity in cultured cerebellar granule cells through blockade of NMDA receptor

This study was designed to assess the potential neuroprotective effect of several imidazol(ine) drugs and agmatine on glutamate-induced necrosis and on apoptosis induced by low extracellular K+ in cultured cerebellar granule cells. Exposure (30 min) of energy deprived cells to L-glutamate (1-100 microM) caused a concentration-dependent neurotoxicity, as determined 24 h later by a decrease in the ability of the cells to metabolize 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazoliumbromide (MTT) into a reduced formazan product. L-glutamate-induced neurotoxicity (EC50=5 microM) was blocked by the specific NMDA receptor antagonist MK-801 (dizocilpine). Imidazol(ine) drugs and agmatine fully prevented neurotoxicity induced by 20 microM (EC100) L-glutamate with the rank order (EC50 in microM): antazoline (13)>cirazoline (44)>LSL 61122 [2-styryl-2-imidazoline] (54)>LSL 60101 [2-(2-benzofuranyl) imidazole] (75)>idazoxan (90)>LSL 60129 [2-(1,4-benzodioxan-6-yl)-4,5-dihydroimidazole](101)>RX82 1002 (2-methoxy idazoxan) (106)>agmatine (196). No neuroprotective effect of these drugs was observed in a model of apoptotic neuronal cell death (reduction of extracellular K+) which does not involve stimulation of NMDA receptors. Imidazol(ine) drugs and agmatine fully inhibited [3H]-(+)-MK-801 binding to the phencyclidine site of NMDA receptors in rat brain. The profile of drug potency protecting against L-glutamate neurotoxicity correlated well (r=0.90) with the potency of the same compounds competing against [3H]-(+)-MK-801 binding. In HEK-293 cells transfected to express the NR1-1a and NR2C subunits of the NMDA receptor, antazoline and agmatine produced a voltage- and concentration-dependent block of glutamate-induced currents. Analysis of the voltage dependence of the block was consistent with the presence of a binding site for antazoline located within the NMDA channel pore with an IC50 of 10-12 microM at 0 mV. It is concluded that imidazol(ine) drugs and agmatine are neuroprotective against glutamate-induced necrotic neuronal cell death in vitro and that this effect is mediated through NMDA receptor blockade by interacting with a site located within the NMDA channel pore.

Central moxonidine on water and NaCl intake

In this study we investigated: (a) the effects of intracerebroventricular (i.c.v.) injections of moxonidine (an alpha2-adrenergic and imidazoline receptor agonist) on the ingestion of water and NaCl induced by 24 h of water deprivation; (b) the effects of i.c.v. injection of moxonidine on central angiotensin II (ANG II)- and carbachol-induced water intake; (c) the effects of the pre-treatment with i.c.v. idazoxan (an alpha2-adrenergic and imidazoline receptor antagonist) and RX 821002 (a selective alpha2-adrenergic antagonist) on the antidipsogenic action of central moxonidine. Male Holtzman rats had stainless steel cannulas implanted in the lateral cerebral ventricle. Intracerebroventricular injection of moxonidine (5 and 20 nmol/1 microl) reduced the ingestion of 1.5% NaCl solution (4.1 +/- 1.1 and 2.9 +/- 2.5 ml/2 h, respectively vs. control = 7.4 +/- 2.1 ml/2 h) and water intake (2.0 +/- 0.6 and 0.3 +/- 0.2 ml/h, respectively vs. control = 13.0 +/- 1.4 ml/h) induced by water deprivation. Intracerebroventricular moxonidine (5 nmol/1 microl) also reduced i.c.v. ANG II-induced water intake (2.8 +/- 0.9 vs. control = 7.9 +/- 1.7 ml/1 h) and i.c.v. moxonidine (10 and 20 nmol/1 microl) reduced i.c.v. carbachol-induced water intake (4.3 +/- 1.7 and 2.1 +/- 0.9, respectively vs. control = 9.2 +/- 1.0 ml/1 h). The pre-treatment with i.c.v. idazoxan (40 to 320 nmol/1 microl) abolished the inhibitory effect of i.c.v. moxonidine on carbachol-induced water intake. Intracerebroventricular idazoxan (320 nmol/1 microl) partially reduced the inhibitory effect of moxonidine on water deprivation-induced water intake and produced only a tendency to reduce the antidipsogenic effect of moxonidine on ANG II-induced water intake. RX 821002 (80 and 160 nmol/1 microl) completely abolished the antidipsogenic action of moxonidine on ANG II-induced water intake. The results show that central injections of moxonidine strongly inhibit water and NaCl ingestion. They also suggest the involvement of central alpha2-adrenergic receptors in the antidipsogenic action of moxonidine.

Alpha2-adrenoceptor antagonists evoke endothelium-dependent and -independent relaxations in the isolated rat aorta

This study was designed to determine whether the alpha2-adrenoceptor antagonists idazoxan, yohimbine, and rauwolscine cause endothelium-dependent and -independent responses in the rat aorta. Rings of rat aorta, with and without endothelium, were suspended for the measurement of isometric force in modified Krebs-Ringer bicarbonate solution (37 degrees C; aerated with 95% O2 and 5% CO2). The alpha2-adrenoceptor antagonists, in the concentration range of 10(-8)-10(-6) M, relaxed phenylephrine-contracted rings with, but not those without endothelium. alpha2-Adrenoceptor antagonists (3 x 10(-6) M for 1 min) increased the accumulation of cyclic guanosine monophosphate (cGMP) about twofold in the aortas with endothelium. The relaxation and the increased cGMP induced by alpha2-antagonists were attenuated by methylene blue (10(-6) M) and N(G)-nitro-L-arginine (L-NA, 3 x 10(-5) M), whereas propranolol (10(-6) M) did not affect the relaxation. In concentrations >10(-6) M, alpha2-adrenoceptor antagonists relaxed the rat aorta without endothelium. The endothelium-independent relaxation by alpha2-adrenoceptor antagonists was abolished by increased external K+ and reduced significantly by tetraethylammonium (TEA, 10(-2) M, a Ca2+-dependent K+ channel blocker), but not inhibited by glibenclamide (10(-5) M, an ATP-sensitive K+ channel blocker). In the rabbit aorta, only endothelium-independent relaxations were observed with alpha2-adrenoceptor antagonists in the concentration range of 10(-8)-10(-5) M, and these relaxations were not antagonized by TEA. These results suggest that alpha2-adrenoceptor antagonists relax the rat aorta through endothelium-dependent mechanism at lower concentrations and endothelium-independent mechanisms at higher concentrations. The endothelium-dependent relaxations are likely to be mediated by the endothelium-derived relaxing factor (EDRF)/NO pathway because they are associated with the accumulation of cGMP, whereas the endothelium-independent relaxations may be caused by the opening of potassium channels in the vascular smooth muscle.

Imidazolines and the pancreatic B-cell. Actions and binding sites

Stimulation of insulin secretion by imidazoline compounds displays variable characteristics. Phentolamine (10-100 microM) increased secretion of perifused mouse islets at nonstimulatory glucose concentrations (5 mM) and even in the absence of glucose. Idazoxan (20-100 microM) elicited a moderate increase in insulin secretion, which required the presence of a stimulatory glucose concentration (10 mM). Phentolamine is therefore a stimulator of secretion in its own right, whereas idazoxan may be termed an enhancer of secretion. Both compounds inhibited the activity of ATP-dependent K+ channels in inside-out patches from B-cells; however, idazoxan achieved only an incomplete block. Both compounds depolarized the B-cell plasma membrane to an extent that permitted the opening of voltage-dependent Ca2+ channels (-40 to -30 mV). An increase in cytoplasmic Ca2+ concentration was induced by phentolamine and much less so by idazoxan. Activation of protein kinase C, a possible mechanism to amplify Ca(2+)-induced secretion, could not be verified for phentolamine. It thus appears that stimulation of insulin secretion by phentolamine is due to its blocking effect on KATP channels, which may be the correlate of non-adrenergic imidazoline binding sites which were characterized in insulin-secreting HIT cells. Whether incomplete closure of KATP channels by idazoxan or additional effects are responsible for the requirement of high glucose to stimulate secretion remains to be clarified.