Pharmacodynamic studies on mianserin and its interaction with clonidine

Summary There is evidence that clonidine's hypotensive effect is reduced by the concurrent administration of tricyclic antidepressants. It has been proposed that this results from an interaction at ₂-receptors in the brain stem where clonidine acts as a relatively selective agonist and the tricyclic antidepressants as antagonists. Mianserin is an antidepressant with a tetracyclic structure and, although it has been reported to cause less cardiovascular disturbance, there is evidence that it also has -adrenoceptor blocking effects. This study in 6 normotensive healthy male volunteers was designed to investigate a possible interaction between clonidine and the antidepressant mianserin. Administration of the first dose of 20 mg mianserin was associated with acute cardiovascular effects, notably transient postural hypotension, but no significant disturbance of heart rate or blood pressure was detected after 3 days continuous treatment with mianserin 20 mg tid. Following pre-treatment with mianserin or placebo the responses to a single oral dose of 300 µg clonidine were then assessed. The combination of mianserin and clonidine was not associated with any attenuation of clonidine's hypotensive effect, erect or supine, but there was significant attenuation of clonidine's supine bradycardic effect. There was no evidence that mianserin interfered with the ability of clonidine to diminish salivary flow, cause sedation, and reduce catecholamine output, but it was noted that mianserin itself had a very pronounced sedative effect. Mianserin alone had no significant effect on salivary flow. This short term study demonstrates that mianserin does not significantly interfere with the responses to a single oral dose of clonidine.     

Interaction between mianserin and clonidine at α 2-Adrenoceptors

Summary The purpose of the present study was to characterize the effects of mianserin at ₂-adrenoceptors. Firstly, the action of mianserin on postganglionic sympathetic fibres has been studied using the tachycardia induced by stimulation of the cardiac nerve in dogs. Mianserin increased this tachycardia, but could not prevent the inhibitory effect of clonidine in this model. However, an antagonistic effect of mianserin against clonidine was observed when animals were pretreated with desipramine. Secondly, mianserin antagonized the inhibitory effect of clonidine on the electrically stimulated guinea-pig ileum. In high concentrations, mianserin reduced both electrically and acetylcholine induced contractions. Thirdly, mianserin antagonised the sleep induced by clonidine in chickens.These results are consistent with ₂-adrenoceptor blocking properties of mianserin in peripheral noradrenergic fibres in dogs, in cholinergic fibres in guinea-pig ileum and in the central nervous system in chickens.     

Chronic effects of miaserin on noradrenaline metabolism in the rat brain: Evidence for a pre-synaptic α-adrenolytic action in vivo

Chronic administration of the pre-synaptic -adrenoreceptor agonist clonidine decreases the concentration of the extra-neuronal metabolite of noradrenaline normetanephrine in the amygdaloid cortex and increases it in the mid-brain. Conversely, blockade of these pre-synaptic receptors by yohimbine increases the normetanephrine concentration in the amygdaloid cortex and decreases it in the mid-brain. Mianserin had a qualitatively similar action to that of yohimbine. When given clinically to rats in combination with clonidine, mianserin antagonizes both the depression of behaviour of the rats in the open field apparatus and also the effects of the -agonist in reducing the concentration of normetanephrine in the amygdaloid cortex. It thus appears that the chronic effects of mianserin are due to an increase in noradrenaline release as a consequence of the inhibition of pre-synaptic -adrenoreceptors.     

Binding of antidepressants to human brain receptors: focus on newer generation compounds

Using radioligand binding assays and postmortem normal human brain tissue, we obtained equilibrium dissociation constants (K_ds) for 17 antidepressants and two of their metabolites at histamine H₁, muscarinic, ₁-adrenergic, ₂-adrenergic, dopamine D₂, serotonin 5-HT_1A, and serotonin 5-HT₂ receptors. Several newer antidepressants were compared with older drugs. In addition, we studied some antimuscarinic, antiparkinson, antihistamine, and neuroleptic compounds at some of these receptors. For the antidepressants, classical tricyclic antidepressants were the most potent drugs at five of the seven receptors (all but ₂-adrenergic and 5-HT_1A receptors). The chlorophenylpiperazine derivative antidepressants (etoperidone, nefazodone, trazodone) were the most potent antidepressants at ₂-adrenergic and 5-HT_1A receptors. Of ten antihistamines tested, none was more potent than doxepin at histamine H₁ receptors. At muscarinic receptors antidepressants and antihistamines had a range of potencies, which were mostly weaker than those for antimuscarinics. From the in vitro data, we expect adinazolam, bupropion, fluoxetine, sertraline, tomoxetine, and venlafaxine not to block any of these five receptors in vivo. An antidepressant's potency for blocking a specific receptor is predictive of certain side effects and drug-drug interactions. These studies can provide guidelines for the clinician in the choice of antidepressant.     

Effects of antidepressant drugs and electroconvulsive shock on pre- and postsynaptic α2-adrenoceptor function in the brain: rapid down-regulation by sibutramine hydrochloride

Clonidine (0.1 mg/kg IP)-induced hypoactivity and mydriasis responses were respectively used as functional indices of pre- and postsynaptic ₂-adrenoceptors in mouse brain. A single injection of various antidepressant drugs had no effect on either response when measured 24 h later. However, 14 days' treatment with sibutramine HCl (3 mg/kg IP), dothiepin (50 mg/kg IP), amitriptyline (10 mg/kg IP), desipramine (10 mg/kg IP) or tranylcypromine (10 mg/kg IP) markedly attenuated both clonidine-induced hypoactivity and mydriasis. Repeated administration of zimeldine (10 mg/kg IP), mianserin (10 mg/kg IP) or clenbuterol (5 mg/kg IP) had no effect on either response. Subchronic treatment with sibutramine HCl (3 mg/kg IP; 3 days) also attenuated pre- and postsynaptic ₂-adrenoceptor function. Five ECS (200 V, 2 s) spread over 10 days, but not a single shock, reduced the hypoactivity and mydriasis responses to clonidine. Together, the results indicate that pre- and postsynaptic ₂-adrenoceptor function is attenuated by repeated treatment with those antidepressants which acutely increase synaptic levels of noradrenaline. These adrenergic receptor populations are also desensitized by ECS, although this effect is probably mediated via a different mechanism. Finally, the rapid down-regulation observed with sibutramine HCl is not confined to-adrenoceptors alone, because pre- and postsynaptic ₂-adrenoceptor function is also attenuated by 3 days of treatment with this novel antidepressant drug.     

In vivo electrophysiological evidence for tonic activation by endogenous noradrenaline of α2-adrenoceptors on 5-hydroxytryptamine terminals in the rat hippocampus

Summary The activation of ₂-adrenergic heteroreceptors was studied by comparing the effectiveness of the electrical stimulation of the ascending 5-HT pathway in suppressing the firing activity of CA₃ dorsal hippocampus pyramidal neurons prior to, and following, the intravenous administration of noradrenergic agents. Desipramine (2 mg/kg), a selective noradrenaline reuptake blocker, reduced the efficacy of the stimulation; this effect was reversed by the ₂-adrenoceptor antagonists yohimbine (0.5 mg/kg) and (-)mianserin (0.5 mg/kg), but not by idazoxan (0.5 mg/kg), an adrenoceptor antagonist with preferential affinity for the imidazoline recognition sites. Low doses of the ₂-adrenoceptor agonist clonidine (2 and 10 g/kg) enhanced the efficacy of the stimulation, while high doses (100 and 400 g/kg) reduced it. These incremental and decremental effects of clonidine were reversed by 0.1 and 1 mg/kg of yohimbine, respectively. The enhancing effect of the low dose of clonidine (10 g/kg) was abolished in rats pretreated with the noradrenaline neurotoxin 6-hydroxydopamine. However, the inhibitory effect of a high dose of clonidine (100 g/kg) was unaltered by this pretreatment. These results indicate that low doses of clonidine preferentially activate ₂-adrenergic autoreceptors on the noradrenaline neurons resulting in a reduction of the tonic inhibitory effect of endogenous noradrenaline on 5-HT neurotransmission, while higher doses of clonidine would decrease 5-HT neurotransmission through the direct activation of ₂-adrenergic heteroreceptors on 5-HT terminals. Furthermore, the selective ₂-adrenergic heteroreceptors antagonist(-)mianserin (0.5 mg/kg) increased by itself the efficacy of 5-HT neurotransmission, an effect not observed with yohimbine and idazoxan. Taken together, these results suggest that, in vivo, the ₂-adrenoceptors on 5-HT terminals of the rat hippocampus are tonically activated by endogenous noradrenaline and modulate 5-HT release. Correspondence to R. Mongeau at the above address     

A comparison of various antidepressant drugs demonstrates rapid desensitisation of α2-adrenoceptors exclusively by sibutramine hydrochloride

The functional status of presynaptic and post-synaptic ₂-adrenoceptors in murine brain was respectively monitored using the hypoactivity (sedation) and mydriasis (pupil dilatation) responses to clonidine (0.1 mg/kg IP). Both responses were attenuated 24 h after 3 days of injection of sibutramine hydrochloride (3 mg/kg IP). To ascertain whether this property was exclusive to sibutramine, the following antidepressant drugs were also tested for their ability to down-regulate ₂-adrenoceptors rapidly: amitriptyline, doxepin, nomifensine, desipramine, amoxapine, fluoxetine, zimeldine, tranylcypromine and mianserin. When given for 3 or 5 days at the low dose of 3 mg/kg IP, none of the other antidepressants reduced clonidine-induced hypoactivity or mydriasis. Furthermore, increasing the dose of amitriptyline, doxepin, nomifensine, desipramine, amoxapine and tranylcypromine to 10 mg/kg IP did not enable these antidepressants to attenuate the ₂-adrenoceptor-mediated responses after 3 days of treatment. An electroconvulsive shock (ECS; 200 V, 2 s) given once daily attenuated clonidine-induced mydriasis, but not hypoactivity, when administered for 3 days and both responses when administered for 5 days. In conclusion, this comparative study using antidepressant treatments with differing pharmacological modes of action demonstrated that sibutramine was the only drug which rapidly down-regulated pre- and postsynaptic ₂-adrenoceptors. ECS down-regulated postsynaptic ₂-adrenoceptors when given for 3 days, but required 5 days to desensitise both ₂-adrenoceptor populations.     

The α2-adrenoceptor antagonists idazoxan and yohimbine increase rates of DRL responding in rats

Previous studies have reported that antidepressant drugs exert specific effects on responding maintained by DRL schedules of reinforcement, giving rise to increased frequencies of reinforcement. In order to investigate whether the ₂-adrenoceptor antagonist idazoxan would produce similar effects, the actions of this compound were compared with those of yohimbine, imipramine, mianserin and d-amphetamine in rats trained to lever press for food reinforcement on a DRL 60-s schedule. Neither imipramine nor mianserin produced any effects on response rate or reinforcement frequency, except at the highest doses. In contrast, both idazoxan and yohimbine gave rise to dose-related increases in rates of responding and consequent decreases in reinforcement frequencies. Amphetamine also increased responding, but higher doses of this drug produced marked hyperactivity and stereotyped movements which were not observed after idazoxan and yohimbine. Although the present behavioural baseline was not sensitive to antidepressants, it demonstrated an unexpected activity of two ₂-adrenoceptor antagonists which deserves further investigation.     

Modulation by antidepressant drugs of CNS postsynaptic α2-adrenoceptors mediating mydriasis in the rat

Summary The modulation of central postsynaptic ₂-adrenoceptors mediating mydriasis in the pentobarbitoneanaesthetized rat was studied after the acute and short/longterm administration of antidepressant treatments (drugs, electroshock). The acute administration of cyclic antidepressant drugs (2.5 mg/kg, i.v.) resulted in different mydriatic effects (amitriptyline > protriptyline imipramine > clomipramine > nortriptyline > desipramine maprotiline) which were attenuated (17–55%) by idazoxan (1 mg/kg, i.v., 5 min) and reserpine (5 mg/kg, s.c., 18 h) indicating that, besides the well-known anticholinergic properties of some of these drugs, their mydriatic effects are due in part to activation of ₂-adrenoceptors (through endogenous noradrenaline). In contrast, the long-term (7–21 days) but not the short-term (1–4 days) administration of cyclic antidepressant drugs (2.5–10 mg/kg, i. p.), MAO inhibitors (1 mg/kg, i.p.), lithium (20 mg/kg, i.p.) and electroshock (150 mA, 63 Hz, 8 ms during 300 ms) resulted in dose- and time-dependent reductions of the dose-pupillary response curve for clonidine (ED₅₀ increased 1.2–2.0-fold; E _max decreased by 9–29%) which indicated desensitization of postsynaptic ₂-adrenoceptors. In line with these findings, treatment for 7 days with clonidine (0.1–1 mg/kg, i.p.) or idazoxan (3–10 mg/kg, i.p.) led to an opposite modulation (down- and up-regulation) of the dose-pupillary response curve for clonidine. The main results demonstrate that cyclic antidepressant drugs, through indirect mechanisms which involve endogenous noradrenaline, can modulate the sensitivity of brain postsynaptic ₂-adrenoceptors mediating mydriasis in the rat. Send offprint requests to A. Menargues at the above address     

Presynaptic α-adrenoceptors and the action of tricyclic antidepressant drugs in behavioural despair in rats

The influence of the -adrenolytics, yohimbine and phentolamine, given in low doses thought to block presynaptic -adrenoceptors, on the action of the tricyclic antidepressants imipramine, nortriptyline and amitriptyline in the behavioural despair test was investigated in Wistar rats. Antidepressant drugs given in a single dose or for 2 weeks reduced immobility in rats. Yohimbine potentiated the effect of nortriptyline or amitriptyline administered in a single dose and the effect of imipramine and amitriptyline given for 2 weeks. The potentiating effect of phentolamine was observed only in rats receiving a single dose of nortriptyline. Yohimbine given chronically for 3 weeks together with antidepressants counteracted the effect of imipramine and nortriptyline on behavioural despair. Similarly clonidine abolished the reduction of immobility induced by antidepressants given in a single dose. It is concluded that the despair test is a behavioural model which is sensitive to the noradrenergic component of the drugs and that the blockade of presynaptic -adrenoceptors facilitates the action of tricyclic antidepressants in this test.     

Discriminative stimulus effects of the α2-adrenoceptor antagonist idazoxan

Rats were trained to discriminate a dose of the ₂-adrenoceptor antagonist idazoxan (10 mg/kg IP) from saline. The discriminative stimulus produced by idazoxan was dose related and generalised to yohimbine. However, generalisation did not occur with a variety of compounds from other pharmacological categories including the ₁-adrenoceptor agonist cirazoline, the ₂-adrenoceptor antagonist prazosin, and the ₂-adrenoceptor agonist clonidine. The idazoxan stimulus was not antagonised by either prazosin or clonidine, although it was clear that idazoxan antagonised the reductions in response rate produced by clonidine. Dose-related responding on the idazoxan-associated lever was produced by the anxiolytics buspirone and ipsapirone and by their metabolite MJ 13653 (1-PP), which has previously been shown to be an ₂-adrenoceptor antagonist. In general, however, high levels of generalisation occurred with these three compounds only at doses which substantially reduced response rates. These results demonstrate that idazoxan can give rise to a discriminative stimulus which is probably mediated through antagonism at ₂-adrenoceptors although the failure of clonidine to block the idazoxan stimulus is difficult to explain.     

The influence of SK & F 64139, a phenylethanolamine-N-methyltransferase inhibitor,on centrally mediated cardiovascular effects of α-methyldopa and clonidine

Summary The inhibitor of phenylethanolamine-N-methyl transferase (PNMT), SK & F 64139 (7,8-dichloro-1,2,3,4-tetrahydroisoquinoline), when given i.v. (5 mg/kg), did not prevent the decrease of blood pressure induced in conscious spontaneously hypertensive rats (SHR) by -methyldopa (50 mg/kg i.v.). However, i.c.v. administration of SK & F 64139 (5 mg/kg) to conscious SHR reduced both the -methyldopa-and the clonidine-induced hypotension. Bradycardia in response to clonidine was also prevented by i.c.v. SK & F 64139.When the antihypertensive effect of -methyldopa or clonidine was fully established, i.c.v. administration of SK & F 64139 returned blood pressure within a few minutes to the initial value. Similarly, the bradycardia after clonidine was promptly reversed by i.c.v. SK & F 64139.In pithed rats the pressor responses to both methoxamine and clonidine were antagonized by SK & F 64139 suggesting blockade of vascular ₁- and ₂-adrenoceptors by the PNMT inhibitor.Blockade of central -adrenoceptors by SK & F 64139 appears to adequately explain the inhibition of the antihypertensive effects of -methyldopa and clonidine. The present results do not support the claim (Gerkens et al. 1980) that inhibition of the central formation of -methyladrenaline is the mechanism underlying the antagonism of -methyldopa-induced hypotension by SK & F 64139.     

Alpha adrenoceptors in rat brain: Direct identification with prazosin

Summary Tritiated prazosin was used to characterize high affinity binding sites with characteristics similar to ₁ adrenoceptors in rat brain membranes. These sites were compared with ₂ adrenoceptors labeled with tritiated clonidine. The prazosin sites had an association constant of 2 nM^–1 and bound the ligand optimal around pH 7.0. The density of the sites was 300 fmoles per mg of protein; the half time of dissociation of prazosin was 7 min at 30° C. The order or potencies of agonists, determined from binding-inhibition experiments with labeled prazosin, was: naphazoline > clonidine > adrenaline > noradrenaline > phenylephrine > -methylnoradrenaline > dophamine. The order of potencies of antagonists was: prazosin > phenoxybenzamine > phentolamine > clozapine > yohimbine. Sodium ions and divalent cations as well as guanyl nucleotides have little or no effect on the binding of the labeled antagonist. This is in contrast to the binding of the labeled agonist clonidine (Glossmann and Presek, 1979a, 1979b). Labeled prazosin may be a useful tool to characterize ₁ adrenoceptors.This is part of the thesis of R. H. to be presented to the Fachbereich Humanmedizin, Justus Liebig-Universität Giessen, in partial fulfillment of the requirements for a Doctor of Medical Science degreee     

Mianserin: result of a decade of antidepressant research

Mianserin is a new tetracyclic compound being the active principle of the antidepressant drug Tolvon®. Originally synthesized as an antiallergic, the discovery of the antidepressant activity of mianserin was serendipitous.This review describes the historical development of mianserin as the first representative of a second generation of antidepressants. A summary is given of the clinical trials performed leading to a therapeutic profile of an antidepressant of similar efficacy as tricyclic products, however, with fewer and less severe side effects. Unlike tricyclic products mianserin lacks cardiotoxicity resulting in a superior safety also in overdosage.     

Agonist-antagonist properties of fluorescent opioid probes in the guinea-pig myenteric plexus-longitudinal muscle preparation

Summary The behavioral consequences of -adrenoceptor subsensitivity were investigated by determining whether a physiological response that is mediated by -receptors, isoproterenol-induced drinking (IID), would be reduced by subacute antidepressant/ ₂-antagonist treatmentThe coadministration of typical (e.g., imipramine) or atypical (e.g., mianserin) antidepressants with yohimbine or piperoxan twice daily for four consecutive days reduced IID. Both the time course as well as the magnitude of -adrenoceptor subsensitivity could be behaviorally demonstrated. In addition, the reduction in IID observed after coadministration of imipramine with yohimbine was a centrally mediated effect since it was observed after systemic (subcutaneous) and central (intraventricular) administration of isoproterenol. These results provide evidence that IID is an appropriate behavioral model to demonstrate -adrenoceptor subsensitivity following subacute antidepressant/ ₂-antagonist treatment.Paper presented in part at FASEB, St. Louis, MO, USA (Abstract: Fedn Proc 43:941, 1984)     

α-Adrenoceptor-mediated inhibition of noradrenaline release in rabbit brain cortex slices

Summary Brain cortex slices from rabbits were preincubated with [³H]noradrenaline and then superfused and stimulated electrically at 3Hz. In the presence of cocaine 30 M, unlabelled noradrenaline, -methylnoradrenaline, clonidine, oxymetazoline, xylazine and guanabenz decreased, whereas yohimbine, corynanthine, phentolamine, tolazoline and azapetine increased the stimulation-evoked overflow of tritium. Phenylephrine and prazosin had no effect on the evoked overflow except at concentrations that greatly accelerated the basal outflow of tritium. The results indicate that the noradrenergic axons of rabbit brain cortex are endowed with presynaptic -adrenoceptors which are exclusively of the ₂-type. Addition of various concentrations of cocaine, addition of pargyline, or stimulation at different current strengths was used to obtain either a high or a low stimulation-evoked overflow of tritium. Independently of the method used, a low evoked overflow coincided with a large percentage inhibition produced by 0.1M clonidine, whereas a high evoked overflow coincided with a smaller percentage inhibition produced by clonidine. The results indicate that drugs which block the re-uptake of noradrenaline diminish the presynaptic inhibitory effect of -adrenergic agonists by increasing the biophase concentration of released noradrenaline.     

Effects of clonidine and some α-adrenoreceptor blocking agents on avoidance conditioned reflexes in rats: Their interactions and antagonism by atropine

Clonidine (150 g/kg s. c.) depressed avoidance conditioned reflexes in Long-Evans rats. Some -adrenoceptor blocking agents piperoxan, tolazoline, phentolamine, dibenamine and phenoxybenzamine also depressed these reflexes. Yohimbine (1–2 mg/kg) was found to have no significant effect. Atropine (10 to 15 mg/kg i. p.) antagonized the depression of conditioned avoidance reflexes induced by -adrenoceptor blocking agents as well as the depression produced by clonidine.Yohimbine (1–2 mg/kg) reduced the depressing effects of clonidine on avoidance conditioned reflexes. In rats treated with piperoxan (10 mg/kg) the effects of clonidine were reduced. In these rats the depression of conditioned reflexes was less than in rats treated with clonidine or piperoxan alone.These experiments suggest that clonidine depressed avoidance conditioned reflexes in rats by activating central -adrenoceptors. It is proposed that -adrenoceptor blocking agents might act as partial agonists.     

Presynaptic imidazoline receptors and α2-adrenoceptors in the human heart: discrimination by clonidine and moxonidine

The involvement of presynaptic ₂-autoreceptors and imidazoline receptors in the modulation of noradrenaline release was investigated in strips from human atrial appendages preincubated with [³H]noradrenaline and superfused with medium containing desipramine and corticosterone. Electrical impulses were applied transmurally at 2 Hz. The imidazoline derivatives moxonidine and clonidine reduced the evoked tritium overflow in a concentration-dependent manner. Moxonidine was 18-fold more potent than clonidine. The concentration-response curve for moxonidine, but not for clonidine was shifted to the right by the ₂-adrenoceptor antagonist rauwolscine. The apparent pA₂ value of rauwolscine against moxonidine was 8.41. An inhibitory effect was also observed for the imidazoline derivative BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), a mixed ₂-adrenoceptor antagonist/imidazoline receptor agonist; BDF 6143 was about 2-fold more potent than clonidine. Rauwolscine (1 M) did not substantially shift the concentration-response curve of BDF 6143.It is concluded that noradrenaline release in the human atrium is inhibited not only via presynaptic ₂-autoreceptors but also via presynaptic non-I₁, non-I₂ imidazoline receptors. The failure of rauwolscine to antagonize the inhibitory effect of clonidine suggests that clonidine preferentially stimulates the presynaptic imidazoline receptors; the ₂-adrenoceptor component of its action is probably suppressed by an inhibitory interaction between the two receptors. In contrast, moxonidine acts via presynaptic ₂-autoreceptors, leaving the presynaptic imidazoline receptor unaffected.     

No evidence for functional imidazoline receptors on locus coeruleus neurons

₂-Adrenoceptor agonists inhibit the firing of locus coeruleus (LC) neurons. It was recently observed that the ₂-adrenoceptor agonists clonidine, rilmenidine and cirazoline, when injected intravenously in anaestbetized rats pretreated with the irreversible ₂-adrenoceptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), excite the LC. The effect was attributed to activation of 11 imidazoline receptors. The aim of the present experiments was to characterize the direct effect of ₂-adrenoceptor and I₁ imidazoline receptor agonists on LC neurons.Electrical activity of LC neurons was extracellularly recorded in midpontine slices prepared from the rat brain. Concentration-response curves were obtained for the ₂-agonist noradrenaline and the mixed I₁/₂-receptor agonists clonidine, rilmenidine and moxonidine in slices without treatment and in slices treated with 6-chloro-N-methyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SK&F86466) or EEDQ, ₂-adrenoceptor antagonists with low affinity for I₁ and I₂ imidazoline receptors, respectively. All four agonists concentration-dependently reduced the firing rate of the neurons, with full inhibition at higher concentrations. SK&F86466 shifted the concentration-response curves of the agonists to the right; the calculated antagonist dissociation constants are compatible with an effect of the agonists on ₂-adrenoceptors. EEDQ completely prevented the inhibition by the agonists. Neither in SK&F86466- nor in EEDQ-treated slices was an excitation by clonidine, rilmenidine and moxonidine observed.We conclude that the LC neurons do not possess functional I₁ (and also no I₂) imidazoline receptors. The effects of noradrenaline, clonidine, rilmenidine and moxonidine on the neurons can be fully explained with an interaction with inhibitory ₂-adrenoceptors (probably of the _2D subtype). The excitation of the LC by imidazoline receptor agonists under in vivo conditions, hence, is not a direct effect on the neurons of the LC.     

Uptake inhibitors do not change the effect of imidazoline α2-adrenoceptor agonists on transmitter release evoked by single pulse stimulation in mouse vas deferens

Summary The present study was undertaken to compare the presynaptic interaction of neuronal noradrenaline uptake inhibitors with imidazoline and phenylethylamine ₂adrenoceptor agonists under two different conditions: at low and high noradrenaline concentrations in the biophase.Isolated mouse vasa deferentia were stimulated with trains of 7 pulses given at 0.2 Hz and the inhibition by the ₂-adrenoceptor agonists clonidine, -methylnoradrenaline, and UK-14,304 of twitch responses was measured in the absence and in the presence of either cocaine (12 mol/l) or desipramine (40 nmol/l). The effects were determined for the first (equivalent to single pulse stimulation) and the last stimulus of each train. Both uptake inhibitors antagonized the presynaptic inhibitory effects of imidazolines (clonidine and UK-14,304) on the last twitch; the effects on the first twitch remained unchanged. In contrast, the uptake inhibitors potentiated the inhibitory effect of the phenylethylamine (-methylnoradrenaline) on both the first and the last twitches.These results support the view that the concentration of noradrenaline in the biophase plays a decisive role in the inhibition by a₂-adrenoceptor agonists of the electrically evoked release of noradrenaline. Agonists that are not substrates of neuronal uptake (i.e., clonidine, UK-14,304) become less effective when noradrenaline is present in the biophase while substrates of neuronal uptake (i. e., -methylnoradrenaline) do not. The results argue against the hypothesis that uptake inhibitors interact directly with presynaptic ₂-adrenoceptors or act at some link between uptake and receptor sites. Send offprint requests to S. Guimarães at the above address