Modulation of dopaminergic transmission by alpha-noradrenergic agonists and antagonists: evidence for antidopaminergic properties of some alpha antagonists

Differential behavioural effect of quinpirole in neuroleptic-pretreated rats - role of alpha(1)-adrenoceptor

This paper presents the effect of 14-day intraperitoneal (i.p.) neuroleptic treatment on the behavioural response of Wistar rats to (-)-quinpirole hydrochloride (3 mg/kg, i.p.) administered 24 h after the last neuroleptic dose. Chlorpromazine hydrochloride (10 mg/kg), haloperidol (2 mg/kg) or (+/-)-sulpiride (100 mg/kg) increased the effect of quinpirole; however, there were qualitative and quantitative differences between the neuroleptics. Chlorpromazine and haloperidol, but not sulpiride, pretreatment enhanced quinpirole-induced locomotor hyperactivity. Prazosin (0.5 mg/kg, i.p. ) given to chlorpromazine-treated rats 1 h before quinpirole attenuated the quinpirole-induced hyperlocomotion. In chlorpromazine-pretreated rats, quinpirole elicited defensive aggressive behaviour with vocalization, copulatory attempts, intense rearing and head-down sniffing. When prazosin was given before quinpirole, head-down sniffing and object-directed oral activity were mainly observed. In haloperidol-pretreated rats, quinpirole induced intense head-down sniffing, rearing, grooming and object-directed oral activity. In sulpiride-pretreated rats, quinpirole induced intense head-down sniffing, grooming and object-directed oral activity. The results of the study suggest that differences in the behavioural expression of dopamine D(2) receptor supersensitivity induced by neuroleptics may be, at least in part, caused by concurrent stimulation of alpha(1)-adrenoceptors.

Alpha2-adrenergic control of dopamine overflow and metabolism in mouse striatum

The effects of the alpha2-adrenoceptor drugs, medetomidine and atipamezole, on dopamine overflow evoked by low (6 Hz-10 s) and high (50 Hz-4 s) frequency electrical stimulation of the median forebrain bundle were studied in striatum of BALB/C mice anaesthetized with chloral hydrate with fast in vivo voltammetry techniques. The effects of these drugs on the basal concentrations of dopamine metabolites were also investigated by means of differential pulse voltammetry. Medetomidine dose dependently decreased dopamine overflow in nucleus accumbens in the dose range 5-100 microg/kg, s.c. This effect was seen only at low frequency stimulation and reached 85% at a dose of 100 microg/kg. Medetomidine also decreased the basal concentration of striatal homovanillic acid. This effect did not exceed 35%. Atipamezole antagonized the inhibitory effects of medetomidine on the dopamine overflow. but showed no effect itself. We suggest that alpha2-adrenoceptors in dopaminergic terminal fields in the mouse striatum are involved in the regulation of dopamine release at physiological stimulation frequencies.

Some unusual effects of alpha 2-adrenergic drugs on cortical high voltage spindles in rats

The dose-response curves for a number of alpha-adrenergic drugs were investigated to estimate a possible role of the alpha 2/alpha 1 selectivity of these drugs on the incidence of cortical high voltage spindles (HVS), reflecting level of vigilance. The alpha 2 antagonists yohimbine (0.25-4 mg/kg) and idazoxan (0.5-4 mg/kg), but not atipamezole induced a biphasic effect on the incidence of HVS in rats. This effect of relatively small doses of yohimbine and idazoxan should be taken into consideration when using these drugs as alpha 2 antagonists in behavioral and neurophysiological tests. On the other hand the linearity of the dose-response curve for atipamezole (0.01-4 mg/kg) indicates that this drug is a good candidate for use in such tests.

Dose-response relationship for oral idazoxan effects in healthy human subjects: comparison with oral yohimbine

The effects of oral administration of the alpha 2 adrenergic receptor antagonists idazoxan (20 mg, 40 mg, 80 mg) and yohimbine (20 mg) were compared using a placebo-controlled within-subjects design. Healthy subjects completed 5 test days during which medication effects on mood and anxiety states, physiologic indices, plasma cortisol levels, and plasma levels of the norepinephrine metabolite 3-methoxy-4-hydroxy-phenylethylene glycol (MHPG) were assessed. Idazoxan dose-dependently increased plasma MHPG, plasma cortisol, systolic and diastolic blood pressure, and Panic Attack Symptom Scale scores in healthy subjects. Overall, yohimbine and idazoxan produced a similar pattern of behavioral and neuroendocrine responses. Since idazoxan possesses relatively greater receptor specificity compared to yohimbine, it may be a more useful alpha 2 antagonist in humans.

Presynaptic alpha 2-adrenoceptor function and sleep in man: studies with clonidine and idazoxan

The effects of an alpha 2-adrenoceptor agonist, clonidine and an antagonist, idazoxan, were studied on nocturnal sleep in man. Clonidine increased non-rapid eye movement sleep and idazoxan reduced slow wave sleep and increased awake activity. Changes in the continuity of sleep with clonidine were similar to, and those with idazoxan opposite to, the effects of maprotiline, an inhibitor of the uptake of noradrenaline, used as an active control. These findings support the previous conclusion that raised levels of noradrenaline in the synapse, after inhibition of uptake, lead to increased presynaptic inhibition of release of transmitter in man. However, all three drugs decreased rapid eye movement (REM) sleep and the ratio of REM to nonREM sleep and this is believed to be due to a non-specific upset of the balance of influences which control the appearance of REM sleep.

Specific α2-adrenoreceptor antagonists induce behavioural activation in the rat

The behavioural effects of the specific and selective α(2)-adrenoreceptor antagonists, idazoxan, efaroxan and RX811059, have been investigated in the rat. All three drugs induced periods of behavioural activation characterized by increased locomotion and exploration (rearing and hole dipping). However, these effects were only apparent in animals which were fully habituated to their environments and thus displayed low baseline activity. The behaviour observed lay within the normal range of activity and was not apparent under conditions when exploration was stimulated such as in a novel environment. α( 2)-Adrenoreceptor antagonist- induced activation was a weak response when compared with the intense and prolonged hyperactivity, in both novel and non-novel environments, induced by the amine releaser D- amphetamine. Possible mechanisms involving a direct action of noradrenaline at postsynaptic α( 1)-adrenoreceptors (subsequent to enhanced presynaptic α(2)-receptor feedback blockade) or an indirect action of α(2)-antagonists on dopamine function in mesolimbic pathways are discussed.

Effects of clonidine and yohimbine, alone and in combination with morphine, on supraspinal analgesia

Morphine raised the threshold for escape from aversive electrical stimulation, delivered to the mesencephalic reticular formation. Clonidine, given alone, had no effect; however, when administered with morphine it blocked the analgesic effect of morphine. Conversely, clonidine, but not morphine, increased the latency to respond to the aversive stimulation, suggesting that clonidine may not have analgesic properties but may merely impair the ability of the animal to respond to the nociceptive stimulation. Yohimbine produced hyperalgesia and also blocked the effect of morphine. These findings are similar to those seen with dopamine agonists and may be related to the effects of yohimbine on the release of dopamine.

Clonidine modulates dopamine cell firing in rat ventral tegmental area

The effect of clonidine (5-20 micrograms/kg i.v.) on the activity of single, identified dopamine neurons in the ventral tegmental area of the mesencephalon was studied in chloral hydrate-anesthetized male rats. Clonidine regularized cell firing without affecting the firing rate of the neurons. This effect was blocked by idazoxan (0.5 mg/kg i.v.) or yohimbine (1.0 mg/kg i.v.), but not by phentolamine (1.0 mg/kg i.v.), indicating that clonidine acts at central alpha 2-adrenoceptors. Idazoxan or yohimbine alone produced deregularization and excitation of cell firing. Pretreatment with reserpine (5 mg/kg s.c.) 4 h before the experiment abolished the neuromodulatory effect of clonidine. Thus, the regularization of ventral tegmental area dopamine cell firing by clonidine is indirect and dependent on endogenous monoamines in brain, and, in principle, a tonic adrenergic control of DA cell firing pattern is indicated. The regularization of DA cell activity produced by clonidine may underlie certain therapeutic neuropsychiatric actions of the drug.

Yohimbine's anxiogenic action: evidence for noradrenergic and dopaminergic sites

Yohimbine (2.5 or 4 mg/kg) reduced the percentage of open arm entries and the percentage of time spent on the open arms displayed by rats on an elevated plus-maze indicating anxiogenic activity. These effects were reversed by the alpha 2-adrenoceptor agonist clonidine (0.01 mg/kg) and by the dopamine receptor agonist apomorphine (0.57 mg/kg). The following failed to reverse the effects of yohimbine: the selective alpha 2-adrenoceptor agonists, guanfacine (0.25 and 1 mg/kg), B-HT920 (0.025 and 0.1 mg/kg), B-HT933 (1 and 10 mg/kg); the beta-blocker propranolol (2.5 and 10 mg/kg); the alpha 1-adrenoceptor agonist phenylephrine; the D1 agonist SK&F 38393 (5 and 10 mg/kg) and the D2 agonist LY 171555 (0.5 and 1 mg/kg). Therefore, it is unlikely that activity at only the alpha 1, alpha 2, beta, D1 or D2 sites can entirely account for the anxiogenic actions of yohimbine in the elevated plus-maze. Evidence that clonidine affects the dopaminergic system and that apomorphine affects the noradrenergic system suggest that yohimbine may produce its anxiogenic response by activity on both the noradrenergic and dopaminergic systems.

Alpha 1- and alpha 2-adrenoreceptor antagonists differentially influence locomotor and stereotyped behaviour induced by d-amphetamine and apomorphine in the rat

The importance of dopamine (DA) in mediating locomotor, exploratory and stereotyped behaviour in rodents is well established. Evidence also indicates a modulatory role for noradrenaline (NA) although, due to nonspecificity. of previously available agents, a precise role remains undefined. The effects of the specific and selective alpha-adrenoreceptor antagonists idazoxan (alpha 2) and prazosin (alpha 1) on behaviour induced by amphetamine and apomorphine have been investigated in the rat. d-Amphetamine (2 mg/kg) induced hyperactive locomotion and exploration. Pretreatment with prazosin (1 mg/kg) markedly reduced these responses. In contrast, pretreatment with idazoxan (20 mg/kg) only marginally altered d-amphetamine hyperactivity. Apomorphine (0.5 mg/kg) induced biphasic locomotor and exploratory activity. Neither alpha-antagonist affected the initial burst of activity (60 min), although prazosin inhibited whereas idazoxan potentiated the secondary phase (90-180 min). At higher dosage, amphetamine (6 mg/kg) and apomorphine (2 mg/kg) induced stereotyped behaviours. Prazosin pretreatment enhanced stereotyped gnawing and decreased sniffing and locomotion, whereas idazoxan increased locomotion and decreased amphetamine-induced mouth movements. These data indicate that DA-induced locomotor and stereotyped behaviours are differentially influenced (in opposite directions) by both alpha1- and alpha 2-adrenoreceptor antagonists. NA may thus modulate the expression and character of behaviour by influencing DA function in certain brain areas.

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat of the Okamoto strain (SHR). II. The central nervous system (CNS)

Steady state levels of monoamine neurotransmitters were examined in SHR, a genetic model of hypertension and compared to its normotensive control (WKY). SHR and WKY were also challenged with alpha 2-adrenergic antagonists, (yohimbine, YOH, idazoxan) or an alpha 1-antagonist (prazosin) and alterations in CNS monoamine metabolism evaluated. SHR were found to have elevated levels of NE and 5-HT in a number of brain regions involved in cardiovascular control when compared to WKY. DA levels and metabolism were also altered in the SHR. Blockade of alpha 2-adrenoceptors and other direct and indirect actions of YOH exacerbated the abnormalities in central monoaminergic neurotransmission in SHR. Significant decreases in NE content were produced by YOH or idazoxan treatment in both SHR and WKY, presumably the result of the inhibition of alpha 2-adrenoceptor medicated presynaptic control of NE release. YOH treatment abolished the differences in steady state levels of NE between SHR and WKY, however, idazoxan did not. YOH administration resulted in significant increases in DA and 5-HT in a number of brain regions of both SHR and WKY. Idazoxan or prazosin produced few changes in DA and 5-HT metabolism except for increases in DA content in the spinal cord and brainstem of SHR given idazoxan. The YOH-induced increases in DA and 5-HT content of SHR were of a greater magnitude than the WKY in several brain regions. DOPAC levels were significantly elevated by YOH in both WKY and SHR, reflecting the antidopaminergic properties of YOH. 5-HIAA content was significantly reduced by YOH in a number of brain regions in both SHR and WKY, however, this effect was attenuated in several brain regions in SHR. The results of the present study demonstrate the multifarious nature of the alterations in CNS monoamine metabolism in SHR.

Pharmacological profile of a new potent and specific alpha 2-adrenoceptor antagonist, L-657,743

L-657,743,(2S,12bS)1',3'-dimethylspiro(1,3,4,5',6,6',7,12 b-octahydro-2H- benzo[b]furo[2,3-a]quinolizine)-2,4'-pyrimidin-2'-one, was tested in several in vitro and in vivo models for alpha 2-adrenoceptor antagonism. L-657,743 exhibited a high affinity (less than or equal to 1 nM) for alpha 2-adrenoceptors labelled by [3H] rauwolscine or [3H]clonidine with a 240-fold selectivity versus alpha 1-adrenoceptors labelled by [3H]prazosin. L-657,743 was a potent, selective, and competitive alpha 2-adrenoceptor antagonist in the rat isolated vas deferens (pA2 = 9.3 vs. clonidine; pA2 = 7.1 vs methoxamine). In vivo, L-657,743 potently blocked clonidine-induced mydriasis in the rat and stimulated cerebrocortical norepinephrine synthesis, two indices of central alpha 2-adrenoceptor antagonism. L-657,743 exhibited a comparatively low affinity for several monoamine receptor subtypes (D1, D2, 5-HT1, 5-HT2) in radioligand binding assays in vitro and a comparatively low potency to alter the synthesis of brain DA and 5-HT in vivo indicating a marked alpha 2-specificity versus other monoamine receptor mechanisms. Compared to yohimbine, L-657,743 had considerably higher alpha 2-antagonist potency and alpha 2/alpha 1 selectivity and was significantly more alpha 2-specific (i.e., vs. DA, 5-HT receptors).

Yohimbine-induced alterations of monoamine metabolism in the spontaneously hypertensive rat. I. The peripheral nervous system

The effects of alpha 2 adrenoreceptor blockade with YOH on blood pressure, plasma catecholamines and norepinephrine (NE) stores in kidney, adrenal and spleen of spontaneously hypertensive rats of the Okamoto strain (SHR) and Wistar-Kyoto (WKY) control animals were examined. YOH administration resulted in a significant (p less than 0.001) reduction in arterial pressure in both SHR and WKY. Plasma NE and EPI were significantly (p less than 0.05) elevated by YOH treatment in both SHR and WKY, but SHR exhibited a significantly (p less than 0.05) greater percent increase in plasma NE than WKY. YOH produced significant decreases in splenic NE content in both SHR and WKY but reduced renal NE content in the SHR only. SHR had significantly higher basal renal NE and DA content and fewer NE uptake (3H-desmethylimipramine binding) sites (p less than 0.05) than WKY. Treatment of SHR or WKY with either the alpha 2-adrenergic antagonist, idazoxan, or the alpha 1-antagonist, prazosin, failed to significantly alter renal NE levels from those found after saline injection. The enhanced YOH-induced renal NE depletion in SHR suggests an alteration in the presynaptic control of NE release in the genetically hypertensive rat, however, the effects of YOH in the SHR may be mediated by mechanisms unrelated to alpha 2-adrenergic receptors.

CGS 10746B: an atypical antipsychotic candidate that selectively decreases dopamine release at behaviorally effective doses

CGS 10746B, a benzothiadiazepine, has a behavioral profile in mice and monkeys similar to the atypical antipsychotic clozapine. Unlike clozapine, CGS 10746B suppresses dopamine neuron firing rates and, when administered at behaviorally effective doses by the oral or intraperitoneal route, decreases neostriatal dopamine release without changing dopamine metabolism or occupying D2 receptors. CGS 10746B is the first atypical antipsychotic candidate that selectively decreases dopamine release.

Discriminative stimulus properties of clonidine: substitution by ergot derivatives

Bromocriptine, lergotrile and lisuride but not apomorphine, ergonovine or lysergic acid diethylamide (LSD) mimicked clonidine in rats trained to discriminate this compound (0.02 mg/kg) from saline. BC 105, ketanserin and haloperidol failed to block the clonidine cue. Yohimbine was an effective antagonist of clonidine but not of the lisuride substitution for clonidine. Since dopamine does not appear to be involved in the stimulus effects of clonidine, the behavioral similarities between clonidine and some ergots may be related to alpha-adrenoceptor stimulation; however, the role of blood pressure changes in the stimulus effects of all of these compounds should not be overlooked.

Evidence of norepinephrine-mediated suppression of para-chlorophenylalanine-induced muricidal behavior

Rats were made aggressive (muricide behavior) by p-chlorophenylalanine (p-CPA) treatment. The aggressive behavior was inhibited by the electrical stimulation of the locus coeruleus (LC) or by the administration of yohimbine (20 mg/kg i.p.). Both types of inhibition were blocked by the administration of low doses of clonidine (0.15 mg/kg i.p.). These results suggest the involvement of the adrenergic system in modulating muricidal behavior induced by p-CPA.

Postsynaptic alpha 2-noradrenergic receptors mediate feeding induced by paraventricular nucleus injection of norepinephrine and clonidine

This study examines the feeding response induced by hypothalamic noradrenergic stimulation, in terms of the type and synaptic position of its mediating receptor. Tests with norepinephrine or the alpha 2 receptor agonist clonidine, injected into the area of the paraventricular nucleus (PVN), revealed a potent feeding response in satiated animals. This response by either agonist was blocked, in a dose-dependent fashion, by local injection of the alpha 2-noradrenergic antagonists, rauwolscine and yohimbine. It was also blocked by the general antagonist, phentolamine. In contrast, it was unaffected by hypothalamic injection of the alpha 1-noradrenergic antagonists, prazosin and corynanthine. These results indicate that feeding elicited by noradrenergic stimulation in the region of the PVN is mediated through alpha 2-type receptors. These alpha 2 receptors appear to be located postsynaptically, since the effectiveness of clonidine in eliciting eating was undisturbed by prior injection of the catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine.

Clonidine sensitizes mice for apomorphine-induced stereotypic gnawing: antagonism by neuroleptics and cholecystokinin-like peptides

In mice sensitized for apomorphine by either scopolamine or teflutixol, clonidine antagonized the antistereotypic effect of ceruletide and haloperidol. The same effect of clonidine occurred in normal mice with methylphenidate-induced gnawing. In naive mice, clonidine alone had a sensitizing effect for the action of apomorphine leading to wire-gnawing. Yohimbine and rauwolscine (but not corynanthine) antagonized this effect of clonidine. The gnawing-inducing effect of methylphenidate was also enhanced by clonidine but not to the same extent as that of apomorphine. The stereotypic effect of apomorphine (in mice sensitized by either scopolamine or clonidine) was antagonized by yohimbine and rauwolscine but not by corynanthine. Apomorphine-induced wire gnawing was used as test of the antistereotypic effect of haloperidol, trifluoperazine, teflutixol, CCK-8, ceruletide and 8 related peptides. Ceruletide and 2 of its analogues were more potent than the neuroleptics; CCK-8 was 7 times less active than ceruletide. In conclusion, clonidine sensitized mice for the stereotypic effect (wire-gnawing) of apomorphine and methylphenidate. The clonidine-apomorphine effect permits the estimation of antistereotypic effects.

Comparison of the effects of recently developed alpha 2-adrenergic antagonists with yohimbine and rauwolscine on monoamine synthesis in rat brain

The effects of two recently developed alpha 2-adrenergic antagonists, RX 781094 and WY 26703, on the synthesis of norepinephrine (NE), dopamine (DA) and serotonin (5-HT) in rat brain were compared to those of yohimbine, its diastereoisomer rauwolscine, and mianserin. Intraperitoneal administration of these compounds increased cortical NE synthesis with the potency order: yohimbine, RX 781094, WY 26703 greater than rauwolscine greater than mianserin. Within a similar dose range, yohimbine, rauwolscine and WY 26703 also stimulated striatal DA synthesis and decreased hypothalamic 5-HT synthesis, while RX 781094 and mianserin were very weak or inactive. Yohimbine and the structurally-related WY 26703 were also active as DA antagonists in the gamma-butyrolactone model for DA autoreceptor function. Based on the drug-induced changes in monoamine synthesis as indication of receptor-mediated events, RX 781094 has greater selectivity as an alpha 2-antagonist than compounds structurally related to yohimbine.

Discrimination of neuroleptics by means of their interaction with amfonelic acid: an attempt to characterize the test

The non-amphetamine stimulant amfonelic acid (AFA) is known to enhance the effects of haloperidol, trifluoperazine and spiperone but not those of the atypical neuroleptics clozapine, thioridazine, and sulpiride, on the striatal levels of 3,4-dihydroxyphenylacetic acid. Consequently, the interaction between neuroleptics and AFA has been proposed as a test to discriminate typical and atypical neuroleptics. In this study, these findings were confirmed. Essentially the same results were obtained when the levels of homovanillic acid were measured. However, striatal dopamine levels were decreased similarly by combinations of AFA with typical and atypical neuroleptics. A comparison of the results with 17 neuroleptics with their reported clinical liability to cause extrapyramidal symptoms supported the idea that the test may discriminate drugs with a better ratio of therapeutic vs side effects. A series of antidepressants and alpha-noradrenergic antagonists possessing antidopaminergic properties was found to perform like atypical neuroleptics, i.e. their effects on dopamine metabolites were not enhanced by AFA.

B-HT 958--an antagonist at alpha 2-adrenoceptors and an agonist at dopamine autoreceptors in the brain

B-HT 958 (2-amino-6-(p-chlorobenzyl)-4H-5,6,7,8-tetrahydrothiazolo[5,4-d]az epine), a compound chemically related to clonidine-like drugs of the azepine type, was described previously as a mixed agonist-antagonist at peripheral alpha 2-adrenoceptor sites. In the present experiments the actions of B-HT 958 on brain noradrenergic and dopaminergic mechanisms were examined using behavioural, pharmacological and biochemical methods. (i) In the dog, intracisternally injected B-HT 958 (300 micrograms/kg) abolished the reflex bradycardia facilitated by the alpha 2-agonist B-HT 920 (10 micrograms/kg i.ci.). (ii) In the whole mouse brain as well as in the rat hypothalamus and neocortex, but not in the hippocampus, amygdala and nucleus accumbens B-HT 958 (5-20 mg/kg s.c.) accelerated the alpha-methyltyrosine (alpha-MT)-induced disappearance of noradrenaline. (iii) B-HT 958 decreased in a dose-dependent manner the exploratory activity of mice (ED50: 6.3 mg/kg s.c.) and slightly enhanced the motor activity of reserpine-treated mice only in high doses. (iv) B-HT 958 (20 mg/kg s.c.) lowered the level of homovanillic acid in the striatum and nucleus accumbens and dose dependently (1-20 mg/kg) slowed the alpha-MT-induced disappearance of dopamine in these 2 brain regions of the rat as well as in the whole mouse brain. (v) The gamma-butyrolactone-induced increase in DOPA levels was effectively antagonized by B-HT 958 (1 and 5 mg/kg s.c.) in the rat striatum and nucleus accumbens. These behavioural, pharmacological and biochemical observations indicate that B-HT 958 possesses central alpha-adrenoceptor blocking activity and exerts a strong agonistic effect on brain dopamine autoreceptors.

The effectiveness of yohimbine in blocking rat central dopamine autoreceptors in vivo

The influence of various alpha-adrenoceptor antagonists (10 mg/kg i.p.) upon the rate of turnover of dopamine (DA) in the rat brain was investigated. Taking the levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) as a measure of the rate of DA turnover, it was found that prazosin and phenoxybenzamine decreased, whereas piperoxane and yohimbine increased the turnover rate both in the corpus striatum and in the tuberculum olfactorium. Azapetine, phentolamine and tolazoline as well as the beta-adrenoceptor antagonist propranolol were without a significant effect, whereas the DA antagonist haloperidol increased DOPAC and HVA levels and decreased the levels of DA itself. The possibility that the yohimbine-induced increase in the DA turnover rate was produced by a direct blockade of DA autoreceptors, was investigated under conditions where influences other than those elicited via DA autoreceptors are thought to be eliminated, i.e. in rats treated with reserpine or gamma-butyrolactone (GBL). In rats that were pretreated with reserpine, yohimbine (10 mg/kg i.p.) was found to be ineffective in antagonizing the reduction of the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition, that was produced by the DA agonist apomorphine (2.0 mg/kg i.p.). In rats pretreated with reserpine, yohimbine (10 mg/kg i.p.) was also ineffective in antagonizing the reduction of the DOPAC and HVA levels produced by apomorphine (2.0 mg/kg i.p.), but it was effective in antagonizing the reduction of the HVA level that was produced by the selective DA autoreceptor agonist N,N-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT, 1.0 mg/kg i.p.).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of idazoxan on catecholamine systems in rat brain

Experiments have been performed to assess the potency of idazoxan (RX 781094) at alpha and beta-adrenoceptors and dopamine receptors and on catecholamine uptake processes in rat brain. The effects of idazoxan on the turnover rates of noradrenaline and dopamine have been determined. Radioligand binding studies with cerebral cortex membranes have demonstrated that idazoxan exhibits 46-fold selectivity for alpha 2-adrenoceptors labelled by (3H)-idazoxan (Mean Ki +/- S.E.M. = 3.1 +/- 0.4 nM) compared with alpha 1-adrenoceptors labelled by (3H)-prazosin (Mean Ki +/- S.E.M. = 142 +/- 27 nM). Under the same conditions, yohimbine showed 6-fold selectivity for alpha 2-adrenoceptors. Idazoxan had low affinity for beta-adrenoceptors labelled by (3H)-dihydroalprenolol (IC50 value greater than 10 microM), for dopamine receptors labelled by (3H)-domperidone (IC50 value greater than 20 microM), for the (3H)-noradrenaline uptake site in rat hypothalamus (IC50 = 31 microM) and for the (3H)-dopamine uptake site in rat striatum (IC50 value approximately 800 microM). In rats treated with alpha-methyl-p-tyrosine, idazoxan (10-80 mg/kg, po) produced a marked increase (63% at 10, 217% at 20 mg/kg, po) in the apparent rate of turnover of noradrenaline in rat cortex/striatum, without affecting the rate of turnover of dopamine. This was in contrast to yohimbine (5-20 mg/kg, po) which increased the turnover rates of both catecholamines. In the absence of alpha-methyl-p-tyrosine, idazoxan (5-40 mg/kg, po) produced a dose related increase in the MHPG concentration and a small (20-30%) reduction in the steady state concentration of NA; the duration of the reduction was dose-related. DA steady state concentrations were unaffected. Idazoxan is a new selective alpha 2-adrenoceptor antagonist which should prove a valuable investigative tool in neurochemical studies and which may be a useful clinical agent in the management of the affective disorders.

Interactions of neuroleptic compounds at alpha 2-adrenergic receptor affinity states in bovine caudate nucleus

Several recent studies have suggested a connection between neuroleptics and/or dopamine systems, and alpha 2-adrenergic receptors. Competition of a variety of neuroleptics at specific alpha 2-adrenergic binding sites (labeled with [3H]rauwolscine or [3H]p-aminoclonidine) in bovine caudate and cortex was examined. Few of the compounds were potent competitors, with (+)-butaclamol, alpha-flupenthixol, clozapine and pimozide the most potent (IC50 range 100-600 nM), while triflupromazine, mesoridazine, and haloperidol were the least potent (IC50 range 5 000-10 000 nM). These findings indicate that few, if any, of the examined neuroleptics have specific, high-affinity interactions with the multiple affinity states of the alpha 2-adrenergic receptor.