Presynaptic alpha-adrenoceptor blocking properties among tri- and tetra-cyclic antidepressant drugs

Alpha-adrenoceptors involved on the cardiovascular response induced by mianserin in the pithed rat

1. The effects of the antidepressant drug mianserin on the cardiovascular responses induced by preganglionic electrical stimulation, and i.v. infusion of the adrenergic agonists noradrenaline (NA, alpha 1 and alpha 2), phenylephrine (alpha 1) and xylazine (alpha 2) in the pithed normotensive rat were studied. 2. Mianserin inhibited in a dose-dependent manner the pressor effect caused by electrical stimulation of spinal cord (Th7-Th9) and the infusion of NA, phenylephrine and xylazine. Cocaine increased the inhibitory effect of mianserin on the pressor effect caused by electrical stimulation and NA. 3. Mianserin blocked the xylazine-induced inhibition of cardiac nerve stimulation effect. 4. These results suggest that mianserin blocks the NA uptake, and it is more effective in blocking presynaptic alpha 2- than postsynaptic alpha-adrenoceptors.

The effects of imipramine, mianserin and trazodone on the chronotropic, inotropic and coronary vascular responses in the isolated perfused rat heart

1. The cardiovascular effects of the tricyclic antidepressant imipramine and two second generation antidepressants mianserin, a tetracyclic, and trazodone, a triazolopyridine derivative, were investigated in the isolated perfused rat heart. 2. Imipramine caused cardiac slowing and a negative inotropic effect at 2.5 microM after 30 min of perfusion. Conversely mianserin and trazodone had no effect on heart rate at 5 microM with inotropic state remaining above control values after 30 min of perfusion. 3. Varying effects on coronary flow, which appear to correlate well with the documented receptor actions of each drug, were demonstrated. Imipramine caused a decrease in coronary flow at 1.25 and 2.5 microM, followed by an increase at 10 microM. Mianserin decreased coronary flow at all concentrations between 1 and 20 microM. Trazodone elicited a marked elevation in coronary flow over the dose range of 2.5 to 250 microM. 4. The results in this model suggest that although the second generation agents appear to cause less cardiodepression all three agents elicit quantitatively different coronary vascular responses.

Chronic inhibition of monoamine oxidase reduces noradrenaline release in rat vas deferens and anococcygeus muscle

Rats were treated once (acute) or once daily for 21 days (chronic) with clorgyline (2 mg/kg) or nialamide (50 mg/kg). (-)Deprenyl (1 mg/kg) was given for 21 days. One day after the last injection, vas deferens and anococcygeus muscles were removed and noradrenaline stores labelled with 3H-noradrenaline. Efflux of total tritium following electrical field stimulation was decreased by both acute and chronic treatment with clorgyline and nialamide, as well as by chronic treatment with deprenyl. Total tritium release from anococcygeus muscle was reduced by both acute and chronic treatment with clorgyline. Fractionation of the effluent showed that release of both free noradrenaline and metabolites was decreased by MAO inhibitor treatment in vivo, but this effect was not reproduced by in vitro incubation of the vas deferens with clorgyline (1 microM). By contrast to the effect of electrical field stimulation, release of 3H-noradrenaline induced by veratrine was increased by chronic treatment with both clorgyline and nialamide. Release of total tritium by depolarising concentrations of KCl was also increased by chronic clorgyline treatment. These results could be explained by a proportionally greater release of tritium from a cytoplasmic compartment following veratrine and KCl than electrical stimulation, since MAO inhibition increases cytoplasmic noradrenaline levels. Alternatively, release by electrical stimulation may be affected to a greater extent by presynaptic alpha 2-adrenoceptors, and presynaptic receptors may be stimulated by increased synaptic levels of free noradrenaline following MAO inhibition.

Antidepressant drugs: imipramine, mianserin and trazodone

The advent of newer antidepressant drugs (second generation) during the past two decades has provided an alternative to the use of tricyclic antidepressants in the alleviation of depression. These antidepressants have not been proven to be superior in the therapy of depression to the tricyclic antidepressants but they have been reported to cause fewer cardiac effects. Most of the reported adverse cardiac reactions elicited by antidepressant drugs are based on observations from clinical studies. The possible underlying mechanisms by which these adverse reactions arise have for the large part been proposed on the basis of clinical findings which have been extrapolated back to the known pharmacological actions of such drugs. There is a paucity of hard experimental data in this respect.

Reduced peripheral presynaptic adrenoceptor sensitivity following chronic antidepressant treatment in rats

Responses of the isolated vas deferens of the rat to clonidine (inhibition of contractions to field stimulation at 0.1 Hz) and noradrenaline (contraction of longitudinal muscle) were determined after one or 21 daily injections of the animals with desmethylimipramine, clorgyline, selegiline or tranylcypromine. Desmethylimipramine (10 mg kg-1) and clorgyline (2 mg kg-1) increased the clonidine EC50 in the isolated vas deferens after 21 but not after one daily injection(s). Tranylcypromine (5 mg kg-1) increased clonidine EC50 after both one and 21 injections and selegiline (1 mg kg-1) did not affect clonidine EC50 after either one or 21 injections. Only desmethylimipramine had a significant effect on noradrenaline responsiveness, producing an inconsistent decrease in EC50 with a consistent increase in maximum contractile response after 21 but not after one daily injection(s). Clorgyline (10(-5) M) increased the contractile response of the isolated vas deferens to field stimulation and antagonized the inhibitory effect of clonidine when added directly to the isolated tissue preparation. Neither clorgyline (5 mg kg-1), selegiline (1 mg kg-1) nor tranylcypromine (2.5 mg kg-1) affected significantly the inhibitory response to clonidine (1 microgram kg-1) on contractions of in situ sympathetically stimulated vas deferens in pithed rats. These results show that down-regulation of alpha 2-presynaptic adrenoceptors by chronic treatment with desmethylimipramine and clorgyline occurs in peripheral organs as well as in the central nervous system.

The effect of mianserin on alpha-2 adrenergic receptor function in depressed patients

Recent clinical investigations have shown that long term treatment with the tricyclic antidepressants desipramine and amitriptyline reduces the sensitivity of the alpha-2 adrenergic autoreceptor. In order to determine whether the tetracyclic antidepressant mianserin also has this action, the effect of clonidine, an alpha-2 adrenergic receptor agonist, on plasma levels of the norepinephrine metabolite 3-methoxy-4- hydroxyphenlethyleneglycol (MHPG), blood pressure, and patient-rated sedation were measured in fifteen depressed patients before and during mianserin treatment. Postsynaptic alpha-2 adrenergic receptor function was assessed by measuring the growth hormone response to clonidine before and during treatment. Mianserin had little or no effect on the ability of clonidine to lower plasma MHPG and blood pressure, and to increase sedation and growth hormone secretion. The findings of this investigation indicates that long term mianserin treatment does not produce significant subsensitivity of the alpha-2 adrenergic receptor and suggests that a reduction in alpha-2 adrenergic autoreceptor sensitivity is not a necessary action for all effective antidepressant treatments.

Effect of destruction of central noradrenergic and serotonergic nerve terminals by systemic neurotoxins on the long-term effects of antidepressants on beta-adrenoceptors and 5-HT2 binding sites in the rat cerebral cortex

The dependence of intact noradrenergic and serotonergic nerve terminals for the decrease in the number of beta-adrenoceptors and 5-HT2 binding sites in the cerebral cortex produced by long-term treatment of rats with antidepressant drugs was examined. Noradrenergic nerve terminals were destroyed with the selective noradrenaline neurotoxin DSP4, and serotonergic nerve terminals were destroyed with p-chloroamphetamine (PCA). It was found that lesioning of the noradrenergic nerve terminals abolished the decrease in beta-adrenoceptors produced by desipramine, mianserin and zimeldine and partially antagonized that of the beta-adrenoceptor agonist clenbuterol. PCA pretreatment did not antagonize the long-term effects on the beta-adrenoceptor produced by these compounds. Lesioning of serotonergic nerve terminals affected the down-regulation of 5-HT2 binding sites produced by long-term treatment with mianserin, desipramine and amiflamine. DSP4 pretreatment partially abolished the down-regulation of 5-HT2 binding sites produced by long-term treatment with desipramine, while the effects of mianserin and amiflamine were unaffected by pretreatment with DSP4.

Presynaptic alpha 2 -adrenergic stimulation leads to growth hormone release in the dog

In previous studies we have shown that the alpha 2 -adrenergic receptor agonist clonidine (CLON) releases growth hormone (GH) in conscious dogs, an effect abolished by the selective alpha 2-receptor antagonist yohimbine (YOH) and by reserpine, but not by the alpha 1-receptor antagonist prazosin (1). In the present work intravenous (iv) administration of CLON in conscious dogs evoked a dose-related rise in plasma GH at doses of 2-8 /micrograms/Kg, but not at 16 and 32 /micrograms/Kg. Acute pretreatment with the selective inhibitor of norepinephrine (NE) synthesis, DU-18288, or with a potent antagonist of presynaptic alpha 2-receptors, mianserin abolished the GH rise induced by CLON (4 /micrograms/Kg iv). In contrast, a 10-day-pretreatment with YOH greatly enhanced the GH-releasing effect of CLON (2 /micrograms/Kg iv). In all these data indicate that in the dog: 1) CLON induces GH release via activation of alpha 2-adrenergic receptors; 2) these receptors are likely located on presynaptic sites [experiments with reserpine (1), DU-18288, mianserin, dose-response curve with CLON 2-32/micrograms/kg iv]; 3) the adrenergic receptors involved in GH release exhibit supersensitivity upon (YOH-induced) chronic pharmacologic denervation. In view of the inhibitory action of presynaptic alpha 2-adrenergic receptors (autoreceptors) on NE function, it may be envisioned that in the dog noradrenergic activation is inhibitory and not stimulatory to GH release.

Effects of mianserin on noradrenergic mechanisms

Mianserin increased stimulation-induced release of noradrenaline from guinea-pig atria and brain cortex slices. The increase still occurred when neuronal reuptake of noradrenaline was blocked by cocaine. Mianserin blocked the neuronal uptake of 3H-noradrenaline only in high concentrations (1 microM in atria produced 40% block; 10 microM in cortex produced 57% block). Lower concentrations (0.01 microM in atria; 1 microM in cortex) significantly increased stimulation-induced release of noradrenaline. It is concluded that increases in transmitter release from noradrenergic nerves produced by mianserin can be attributed to blockade of the alpha 2-adrenoceptors involved in autoinhibitory feedback mechanism at nerve terminals, rather than to blockade of neuronal reuptake of noradrenaline. This effect on noradrenergic transmission in the central nervous system could explain the antidepressant actions of mianserin in accordance with the hypothesis of Schildkraut.

Possible subsensitization of alpha 2-adrenergic receptors by chronic monoamine oxidase inhibitor treatment in psychiatric patients

Clonidine was administered to nine psychiatric patients before and after chronic treatment (3 to 4 weeks) with clorgyline, a selective monoamine oxidase type A inhibitor with antidepressant efficacy. The hypotensive response to clonidine, believed to be mediated by brain alpha 2-adrenergic receptors, was significantly attenuated by chronic but not acute (2 to 3 days) clorgyline treatment, with a time course similar to the onset of its clinical efficacy. This study supports the hypothesis that subsensitization of alpha 2-adrenergic receptors plays an important role in clorgyline's antidepressant effects and may constitute a key contribution to the mode of action of other antidepressant treatments as well.

Interaction between mianserin and clonidine at alpha 2-adrenoceptors

The purpose of the present study was to characterize the effects of mianserin at alpha 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 alpha 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.

Effects of antidepressant drugs on different receptors in the brain

Radioligand receptor binding techniques were used to characterize the effects of different structural types of antidepressant drugs on neurotransmitter receptors. The tricyclic antidepressants more or less potently inhibited the binding to rat brain preparations of several different radiolabelled ligands [3H]WB4101, [3H]QNB, [3H]-d-LSD, [3H]mepyramine). The potency of the nontricyclic antidepressants varied greatly. Mianserin, potently displaced [3H]mepyramine, [3H]d-LSD and [3H]WB4101 while it was very weak on [3H]QNB-binding. Nomifensine and the specific 5-HT uptake inhibitors zimelidine and alaproclate had very low affinity for these receptors. All the antidepressants tested were practically devoid of activity on [3H]DHA binding, [3H]spiroperidol binding, [3H]flunitrazepam binding, [3H]muscimol binding and [3H]naloxone binding. The implications of these findings for biogenic amine theories of affective disorders are discussed.

Pre- and postsynaptic alpha-adrenoceptor antagonism by neuroleptics in vivo

Different types of neuroleptics were studied for pre- and postsynaptic alpha-adrenoceptor antagonism in pithed rats using the blood pressure effect of clonidine as a measure of postsynaptic alpha-adrenoceptor activation and the depression of the heart rate response to electrical stimulation as a measure of presynaptic alpha-adrenoceptor activation. Yohimbine (0.1 mg/kg) was more active at pre- than postsynaptic alpha-adrenoceptors while the reverse was found with prazosin (0.02-5 mg/kg). Phentolamine (1-5 mg/kg) on the other hand was very active at both receptors. Cocaine 1-5 mg/kg had no effect on these responses indicating that noradrenaline uptake inhibition presumably does not interfere with the revaluation of the alpha-adrenoceptor antagonistic effect of the neuroleptics. Melperone, haloperidol, thioridazine and flupenthixol (0.15 mg/kg) were more selective antagonists at postsynaptic alpha-adrenoceptors than prazosin. Clozapine, chlorprothixene showed preferential presynaptic (0.15 mg/kg) were antagonists at both types of receptors. Chlorprothixene showed preferential presynaptic alpha-antagonism of high potency. Chlorprothixene was the only neuroleptic drug which like phentolamine (1-5 mg/kg) gave complete presynaptic alpha-antagonism. These results indicate widely different selectivity of neuroleptics for pre- and postsynaptic alpha-adrenoceptors on peripheral sympathetic nerves. It is suggested that neuroleptics with presynaptic alpha-adrenoceptor antagonism may enhance the activity of Beta-adrenergic systems indirectly both in peripheral organs like the heart, and within the central nervous system.

Anxiolytics antagonize yohimbine's discriminative stimulus properties

Male Sprague-Dawley rats were trained to discriminate 3.2 mg/kg yohimbine HCl from saline in a two-lever operant procedure. Generalization tests indicated that piperoxane, another alpha 2-adrenergic blocker with anxiogenic properties in humans, produces yohimbine-like discriminati-effects.. In contrast to yohimbine and piperoxane, many other agents were discriminated as vehicle, including corynanthine, raubasine, phentolamine, prazosin, WB-4101, mianserin, tolazoline, and mezilamine. Diazepam caused a dose-related antagonism of yohimbine's stimulus properties. A partial antagonism of yohimbine cueing was also obtained with meprobamate, phenobarbital, chlordiazepoxide, and clonazepam. These results suggest that yohimbine discrimination in rats may be a useful model for detecting agents with anxiolytic activity.

Differences in presynaptic alpha-blockade, noradrenaline uptake inhibition, and potential antidepressant activity between (+)- and (-)mianserin

The effect of racemic mianserin on K+-evoked tritium release from rat brain cortex slices previously incubated with 3H-L-noradrenaline was studied. Racemic mianserin (10(-9)--10(-5) M) increased stimulation-induced release dose-dependently. As methysergide, metiamide, and cyproheptadine failed to do so, it was concluded that this effect was probably not caused by the antihistamine or antiserotonin activity of racemic mianserin, but due to its alpha-adrenolytic effect. Evaluation of the effects of the enantiomers (+)(S)mianserin and (-)(R)mianserin showed that the alpha-adrenolytic effect resided in the (+)isomer, whereas the (-)isomer was inactive at a concentration of 10(-6) M. Inhibition of noradrenaline into rat hypothalamic synaptosomes also showed stereospecificity in that (+)mianserin was about 300-times more active than(-)mianserin. Inhibition of rat muricidal behavior, a test for potential antidepressant activity, showed a similar dissociation in the effects of the two enantiomers, in that (+)mianserin was active, whereas (-)mianserin was not.