Further studies on alpha 2-adrenoceptor subtypes involved in the modulation of [3H]noradrenaline and [3H]5-hydroxytryptamine release from rat brain cortex synaptosomes

Calcium-dependent inhibition of synaptosomal serotonin transport by the alpha 2-adrenoceptor agonist 5-bromo-N-[4,5-dihydro-1H-imidazol-2-yl]-6-quinoxalinamine (UK14304)

Termination of serotonergic transmission is the function of the plasma membrane 5-hydroxytryptamine (serotonin, 5-HT) transporter (SERT), which is also a high-affinity target in vivo for antidepressants, amphetamines, and cocaine. Studies show that SERT is regulated by protein kinase- and phosphataselinked pathways. In contrast, receptor-linked modulation of SERT is only minimally defined. Because noradrenergic stimulation is reported to influence 5-HT release, we explored possible presynaptic adrenoceptor-mediated regulation of SERT. In mouse forebrain synaptosomes, alpha2-adrenoceptor agonists, particularly 5-bromo-N-[4,5-dihydro-1H-imidazol-2-yl]-6-quinoxalinamine (UK14304), triggered a concentration- and time-dependent decrease in 5-HT transport. In contrast, 5-HT uptake was unaffected by pharmacological alpha1-adrenoceptor activation. Kinetically, UK14304 significantly decreased the apparent substrate affinity, Km without altering transport capacity, Vmax. At concentrations of UK14304 supporting maximal inhibition of SERT in synaptosomes, no effect on SERT in transfected cells was observed, suggesting that UK14304 acts indirectly to reduce SERT activity. The effect of UK14304 on 5-HT uptake was not shared by other Na+ and Cl--dependent transporters. UK14304-mediated inhibition of SERT function was yohimbine-sensitive, as was inhibition triggered by norepinephrine, and was abolished in the absence of added Ca2+. Moreover, UK14304 effects were attenuated by voltage-sensitive Ca2+ channel antagonists, consistent with a role for Ca2+ in UK14304 effects. In agreement with altered 5-HT transport activity in vitro, in vivo chronoamperometry studies revealed that UK14304 significantly prolonged 5-HT clearance. Our findings suggest that UK14304 modulates SERT function in vitro and in vivo via signaling pathways, possibly supported by an influx of Ca2+ through voltage-sensitive Ca2+ channels.

Expression of catecholaminergic mRNAs in the hyperactive mouse mutant coloboma

The SNAP-25 deficient mouse mutant coloboma (Cm/+) is an animal model for investigating the biochemical basis of locomotor hyperactivity. The spontaneous hyperactivity exhibited by coloboma is three times greater than control mice and is a direct result of the SNAP-25 deletion. SNAP-25 is a presynaptic protein that regulates exocytotic neurotransmitter release; coloboma mice express only 50% of normal protein concentrations. Previous research has determined that there is an increase in the concentration of norepinephrine but a decrease in dopamine utilization in the striatum and nucleus accumbens of coloboma mice. In situ hybridization analysis revealed that there were corresponding increases in tyrosine hydroxylase (TH) mRNA expression in noradrenergic cell bodies of the locus coeruleus of Cm/+ mice. In contrast, TH mRNA expression in substantia nigra appeared normal in the mutant mouse. alpha(2)-Adrenergic receptors are important modulators of central noradrenergic function and dopamine release. In situ hybridization data revealed that alpha(2A)-adrenergic receptor mRNA expression is upregulated in Cm/+ mice. These results suggest an underlying abnormality in noradrenergic regulation in this hyperactive mouse mutant.

Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents

The frontal cortex (FCX) plays a key role in processes that control mood, cognition and motor behaviour, functions which are compromised in depression, schizophrenia and other psychiatric disorders. In this regard, there is considerable evidence that a perturbation of monoaminergic input to the FCX is involved in the pathogenesis of these states. Correspondingly, the modulation of monoaminergic transmission in the FCX and other corticolimbic structures plays an important role in the actions of antipsychotic and antidepressant agents. In order to further understand the significance of monoaminergic systems in psychiatric disorders and their treatment, it is essential to characterize mechanisms underlying their modulation. Within this framework, the present commentary focuses on our electrophysiological and dialysis analyses of the complex and reciprocal pattern of auto- and heteroreceptor mediated control of dopaminergic, noradrenergic and serotonergic transmission in the FCX. The delineation of such interactions provides a framework for an interpretation of the influence of diverse classes of antidepressant agent upon extracellular levels of dopamine, noradrenaline and serotonin in FCX. Moreover, it also generates important insights into strategies for the potential improvement in the therapeutic profiles of antidepressant agents.

Effects of clonidine and alpha-adrenoceptor antagonists on motor activity in DSP4-treated mice II: interactions with apomorphine

Adult mice were administered either the noradrenaline (NA) neurotoxin, N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) or distilled water (control), 10-12 days before motor activity testing, and 6 h before testing all the mice were administered reserpine (10 mg/kg), the monoamine-depleting agent. The interactive effects of (I) clonidine, the alpha(2)-adrenoceptor agonist, with the dopamine (DA) agonist, apomorphine, and the alpha(2)-antagonist, yohimbine, and (II) with either yohimbine or the alpha(1)-antagonist, prazosin, upon motor behaviour in activity test chambers were studied in reserpinized DSP4-treated and control mice. It was shown that apomorphine (3 mg/kg) increased locomotor and total activity in both reserpinized DSP4-treated and control mice but the effect was attenuated in the DSP4 mice. Co-administration of clonidine (3 mg/kg) with apomorphine potentiated the effects of apomorphine on motor activity and this effect was enhanced markedly by DSP4 pretreatment. Yohimbine (10 mg/kg) antagonized the motor activity-stimulating effects of apomorphine in both DSP4-treated and control mice. Co-administration of clonidine with apomorphine, following yohimbine, restored motor activity levels to those obtained in the absence of yohimbine and this effect upon locomotor activity was enhanced by DSP4 pretreatment. The effects of clonidine on motor activity were enhanced by NA-denervation. Prazzosin (3 mg/kg) enhanced the locomotor activity of both reserpinized DSP4-treated and control mice after the initial 30-min period but was not affected by DSP4 treatment. Analysis of post-decapitation convulsions (PDCs) indicated loss of the reflex by DSP4 pretreatment. Reserpine pretreatment abolished the initial, exploratory phase (30 min) of motor activity. These results demonstrate interactions between NA and DA systems that may bear eventual relevance to neurologic disorders such as parkinsonism.

Mirtazapine enhances frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission by blockade of alpha2-adrenergic and serotonin2C receptors: a comparison with citalopram

Mirtazapine displayed marked affinity for cloned, human alpha2A-adrenergic (AR) receptors at which it blocked noradrenaline (NA)-induced stimulation of guanosine-5'-O-(3-[35S]thio)-triphosphate ([35S]-GTPgammaS) binding. Similarly, mirtazapine showed high affinity for cloned, human serotonin (5-HT)2C receptors at which it abolished 5-HT-induced phosphoinositide generation. Alpha2-AR antagonist properties were revealed in vivo by blockade of UK-14,304-induced antinociception, while antagonist actions at 5-HT2C receptors were demonstrated by blockade of Ro 60 0175-induced penile erections and discriminative stimulus properties. Mirtazapine showed negligible affinity for 5-HT reuptake sites, in contrast to the selective 5-HT reuptake inhibitor, citalopram. In freely moving rats, in the dorsal hippocampus, frontal cortex (FCX), nucleus accumbens and striatum, citalopram increased dialysate levels of 5-HT, but not dopamine (DA) and NA. On the contrary, mirtazapine markedly elevated dialysate levels of NA and, in FCX, DA, whereas 5-HT was not affected. Citalopram inhibited the firing rate of serotonergic neurons in dorsal raphe nucleus, but not of dopaminergic neurons in the ventral tegmental area, nor adrenergic neurons in the locus coeruleus. Mirtazapine, in contrast, enhanced the firing rate of dopaminergic and adrenergic, but not serotonergic, neurons. Following 2 weeks administration, the facilitatory influence of mirtazapine upon dialysate levels of DA and NA versus 5-HT in FCX was maintained, and the influence of citalopram upon FCX levels of 5-HT versus DA and NA was also unchanged. Moreover, citalopram still inhibited, and mirtazapine still failed to influence, dorsal raphe serotonergic neurons. In conclusion, in contrast to citalopram, mirtazapine reinforces frontocortical dopaminergic and corticolimbic adrenergic, but not serotonergic, transmission. These actions reflect antagonist properties at alpha2A-AR and 5-HT2C receptors.

Pilocarpine treatments differentially affect alpha2-adrenoceptors which modulate the firing rate of locus coeruleus neurones and the synthesis and release of noradrenaline in rat brain

We have investigated the effect of treatments with the muscarinic acetylcholine receptor agonist, pilocarpine, on the sensitivity of central alpha2-adrenoceptors that regulate the firing activity of rat locus coeruleus, the tyrosine hydroxylase activity in the rat cortex, hippocampus and hypothalamus, and the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. Short-term (4 days), but not acute, treatment with pilocarpine caused a small but statistically significant increase in the inhibitory effect of the alpha2-adrenoceptor agonist clonidine on the firing rate of locus coeruleus neurones, with a decrease in the ED50 of 290% (P<0.001). However, no change in the effect of clonidine on the locus coeruleus was observed after longer pilocarpine (11 days) treatment. In the rat cerebral cortex, but not in hippocampus or hypothalamus, chronic (19 days) treatment with pilocarpine caused a decrease in the inhibitory effect of clonidine on tyrosine hydroxylase activity (55%, P<0.05), but did not change the stimulatory effect of the alpha2-adrenoceptor antagonist idazoxan. Moreover, treatments (4, 11 and 19 days) with pilocarpine did not alter the inhibitory effect of clonidine [10(-8)-10(-5) M] on the K(+)-evoked release of [3H]noradrenaline from rat cortical and hippocampal synaptosomes. These results indicate that administration of pilocarpine slightly potentiates some but not all the functional responses mediated by brain presynaptic alpha2-adrenoceptors. In conclusion, these results do not support the hypothesis that chronic treatments with pilocarpine lead to a suitable model of alpha2-adrenoceptor supersensitivity.

Alpha2A- but not alpha2B/C-adrenoceptors modulate noradrenaline release in rat locus coeruleus: voltammetric data

In this study, we used subtype-selective antagonists to determine the subtype of alpha2-adrenoceptor controlling noradrenaline release in rat locus coeruleus. Noradrenaline release was measured in locus coeruleus slices using fast cyclic voltammetry at carbon fibre microelectrodes. On long stimulation trains (40 pulses, 20 Hz), the alpha2A-adrenoceptor selective antagonist BRL 44408 (2-[2H-(1-methyl-1,3-dihydroisoindole) methyl]-4,5-dihydroimidazole) at 100 nM and 1 microM significantly increased stimulated noradrenaline release, whereas the alpha2B/C-selective antagonist ARC 239 (2-[2[4-(o-methoxyphenyl)piperazin-1-yl] ethyl]-4,4dimethyl-1,3-(2H,4H)-isoquinolinedione) at 50 and 500 nM had no effect. On short stimuli (20 pulses, 200 Hz), the non-specific alpha2-adrenoceptor agonist dexmedetomidine (10 nM) significantly decreased noradrenaline release, an effect reversed by BRL 44408 (1 microM) but not by ARC 239 (500 nM). These data demonstrate that autoreceptor control of noradrenaline release in the locus coeruleus is mediated by alpha2A but not alpha2B/C-adrenoceptors.

Simultaneous quantification of serotonin, dopamine and noradrenaline levels in single frontal cortex dialysates of freely-moving rats reveals a complex pattern of reciprocal auto- and heteroreceptor-mediated control of release

In the present study, a novel and exceptionally sensitive method of high-performance liquid chromatography coupled to coulometric detection, together with concentric dialysis probes, was exploited for an examination of the role of autoreceptors and heteroceptors in the modulation of dopamine, noradrenaline and serotonin levels in single samples of the frontal cortex of freely-moving rats. The selective D3/D2 receptor agonist, CGS 15855A [(+/-)-trans-1,3,4,4a,5,10b-hexahydro-4-propyl-2H-[1]benzopyrano[3 ,4-b]-pyridin-9-ol], and antagonist, raclopride, respectively decreased (-50%) and increased (+60%) levels of dopamine without significantly modifying those of serotonin and noradrenaline. The selective alpha2-adrenergic receptor agonist, dexmedetomidine, markedly decreased noradrenaline levels (-100%) and likewise suppressed those of serotonin and dopamine by -55 and -45%, respectively. This effect was mimicked by the preferential alpha2-adrenergic receptor agonist, guanabenz (-100%, -60% and -50%). Furthermore, the alpha2-adrenergic receptor antagonist, RX 821,002 [2(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline], and the preferential alpha2A-adrenergic receptor antagonist, BRL 44408 [2-(2H-(1-methyl-1,3-dihydroisoindole)methyl)-4,5-dihydroimidaz ole], both evoked a pronounced elevation in levels of noradrenaline (+212%, +109%) and dopamine (+73%, +85%). In contrast, the preferential alpha(2B/2C)-adrenergic receptor antagonist, prazosin, did not modify noradrenaline and dopamine levels. RX 821,002 and BRL 44408 did not significantly modify levels of serotonin, whereas prazosin decreased these levels markedly (-55%), likely due to its alpha1-adrenergic receptor antagonist properties. The selective serotonin-1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), reduced serotonin levels (-65%) and increased those of dopamine and noradrenaline by +100%), and +175%, respectively. The selective serotonin-1A antagonist, WAY 100,635 [N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl)cyclo- hexanecarboxamide], which had little affect on monoamine levels alone, abolished the influence of 8-OH-DPAT upon serotonin and dopamine levels and significantly attenuated its influence upon noradrenaline levels. Finally, the selective serotonin-1B agonist, GR 46611 [3-[3-(2-dimethylaminoethyl)-1H-indol-5-yl]-N-(4-methoxybenzyl)acrylamid e], decreased serotonin levels (-49%) and the serotonin-1B antagonist, GR 127,935 [N-[4-methoxy-3-(4-methylpiperazin-1-yl)phenyl]-2'-methyl-4'-(5-me thyl-1,2,4-oxadiazol-3-yl)-biphenyl-4-carboxamide], which did not significantly modify serotonin levels alone, abolished this action of GR 46611. Levels of dopamine and noradrenaline were not affected by GR 46611 or GR 127,935. In conclusion, there is a complex pattern of reciprocal autoreceptor and heteroceptor control of monoamine release in the frontal cortex. Most notably, activation of alpha2-adrenergic receptors inhibits the release of noradrenaline, dopamine and serotonin in each case, while stimulation of serotonin-1A receptors suppresses serotonin, yet facilitates noradrenaline and dopamine release. In addition, dopamine D2/D3 autoreceptors restrain dopamine release while (terminal-localized) serotonin-1B receptors reduce serotonin release. Control of serotonin release is expressed phasically and that of noradrenaline and dopamine release tonically.

alpha2-Adrenoceptor antagonists reverse the 5-HT2 receptor antagonist suppression of head-twitch behavior in mice

The alpha2-adrenoceptor agonist clonidine, as well as 5-HT2 receptor antagonists, reportedly suppress 5-HT2 receptor-mediated head-twitch behavior. We investigated the effect of alpha2-adrenoceptor antagonists on the suppressive action of 5-HT2 receptor antagonists in mice pretreated with the noradrenaline toxin 6-hydroxydopamine (6-OHDA) or the 5-HT synthesis inhibitor p-chlorophenylalanine (p-CPA). In normal mice, idazoxan (0.08-0.2 mg/kg, IP) or yohimbine (0.2-2.0 mg/kg, IP), both alpha2-adrenoceptor antagonists, had no effect on the head-twitch response caused by 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 16 mg/kg, IP), but idazoxan significantly enhanced the response at 0.5 mg/kg. On the other hand, these alpha2-adrenoceptor antagonists, at doses that had no effect on the basal number of head-twitches (idazoxan 0.2 mg/kg and yohimbine 0.5 mg/kg), significantly attenuated not only the suppressive effect of clonidine (0.01 mg/kg, IP) on head-twitch response but also that of the 5-HT2 receptor antagonist ritanserin (0.03 mg/kg, IP). Moreover, idazoxan (0.2 mg/kg) also significantly reversed the inhibition by 0.01 mg/kg (IP) ketanserin, a selective 5-HT2 receptor antagonist. Pretreatment with 6-OHDA plus nomifensine but not with p-CPA significantly attenuated the effect of idazoxan (0.2-0.5 mg/kg) on the ritanserin inhibition of the head-twitch response. Prazosin, an alpha1-adrenoceptor antagonist, dose-dependently suppressed the response, and the effect of prazosin (1.25 mg/kg) was significantly attenuated by 0.5 mg/kg idazoxan. These results indicate that endogenous noradrenaline is involved in the apparent antagonistic interaction between selective alpha2-adrenoceptor antagonists and 5-HT2 receptor antagonists in the head-twitch response, and suggest that noradrenaline stimulation of alpha1-adrenoceptors may be involved in this apparent antagonism.

A re-evaluation of the role of alpha 2-adrenoceptors in the anxiogenic effects of yohimbine, using the selective antagonist delequamine in the rat

1. The acute behavioural effects of the alpha2-adrenoceptor antagonists, yohimbine, idazoxan and delequamine (RS-15385-197) were compared in two tests of exploratory behaviour in the rat, operated in tandem. These were the elevated X-maze test (5 min) and a modified holeboard test (12 min), which comprised a holeboard arena with a small roof in one corner as a 'refuge'. Rats were first placed into this corner, thus enabling measurements of initial emergence latency and the number of forays. The experiments were always done with a concomitant vehicle control group, with 10-12 rats per group, and with the treatment blinded. 2. In order to validate the tests, the effects of representatives of four classes of psychoactive agents were examined, viz. picrotoxin (anxiogenic), chlordiazepoxide (anxiolytic), (+)-amphetamine (stimulant) and diphenhydramine (sedative). The modified holeboard tended to be more sensitive than the measurement of total arm entries in the elevated X-maze at detecting drug effects on exploratory behaviour, but unlike the X-maze it could not clearly identify each class of agent. Thus, picrotoxin (5 mg kg(-1), i.p.) reduced total arm entries and open arm exploration in the X-maze (P<0.02) and suppressed most measures of activity in the holeboard (P<0.05); chlordiazepoxide (7.5 mg kg(-1), i.p.) increased total arm entries and open arm exploration (P<0.02) in the X-maze, without clear-cut effects in the holeboard; (+)-amphetamine (1 mg kg(-1), i.p.) had no significant effects in the X-maze, but increased most holeboard activities (P<0.05), and diphenhydramine (30 mg kg(-1), i.p.) reduced total arm entries in the X-maze (P<0.002) and hole exploration in the holeboard (P<0.05).

Presynaptic alpha 2A-adrenoceptors regulate the 3H-noradrenaline secretion in the guinea-pig urethra

The subtype of the functional presynaptic autoreceptor in adrenergic nerves of the male guinea-pig urethra was pharmacologically characterised. The urethra was incubated with 3H-noradrenaline and superfused with Tyrode solution in vitro and the fractional secretion of 3H-noradrenaline evoked by 300 electrical shocks at 5 Hz was measured. alpha-Adrenoceptor antagonists enhanced the 3H-noradrenaline secretion. The effects of BRL 44408 ((+/-)-2-[(4,5-dihydro-1H-imidazol-2yl)methyl]-2,3- dihydro-1-methyl-1H-isoindole), BRL 41992 ((-)-1,2-dimethyl-2,3,9,13b-tetrahydro-1H-dibenzo[c,f]imidazo[1,5- a] azepine), and WB-4101 (2-([2,6-dimethoxyphenoxyethyl]aminomethyl)-1,4- benzodioxane) were tested. The rank order of apparent EC50 values was BRL 44408 < BRL 41992 < WB-4101, and correlated best with constants for the alpha 2A-C10/alpha 2A adrenoceptor subtype. The results suggest that 3H-noradrenaline secretion in guinea-pig urethra is regulated by a presynaptic adrenoceptor of the alpha 2A subtype.

Modulatory effects of magnolol on potassium-stimulated 5-hydroxytryptamine release from rat cortical and hippocampal slices

Magnolol, a phenolic constituent of magnolia bark, is a known central nervous system depressant. To examine the possibility that magnolol may elicit its depressant effect by modulating central serotonergic activity, its effect on 35 mM K(+)-stimulated 5-[3H]HT release from rat hippocampal and frontal cortical slices were examined. Inclusion of magnolol (1-100 microM) had no effect on 5-HT release in hippocampal slices but elicited a dose-related inhibition on 5-HT release from cortical slices. The inhibitory effect of magnolol on K(+)-stimulated 5-HT release from the cortex was not affected by either antagonists (metergoline, propranolol, and cyproheptadine) (0.01-10 microM) of various 5-HT receptor subtypes or the voltage-dependent sodium channel blocker tetrodotoxin (1 microM). It is concluded that the suppression of brain 5-HT release by magnolol is site-specific, and the suppression of cortical 5-HT release by magnolol is not via the 5-HT autoreceptors at the 5-HT terminals.