Interaction between mianserin and clonidine at alpha 2-adrenoceptors

Measurement of 3-methoxy-4-hydroxyphenylglycol (MHPG) in mouse brain by h.p.l.c. with electrochemical detection, as an index of noradrenaline utilisation and presynaptic alpha 2-adrenoceptor function

1. A novel method for measurement of 3-methoxy-4-hydroxyphenylglycol (MHPG) in mouse brain by use of high performance liquid chromatography (h.p.l.c.) with electrochemical detection is described. This technique incorporates an ethyl acetate purification procedure and uses 3-hydroxy-4-methoxyphenylglycol (iso-MHPG) as the internal standard. 2. Inhibition of monoamine oxidase by injection of tranylcypromine (5 and 10 mg kg-1) or pargyline (50 and 100 mg kg-1) markedly decreased brain MHPG concentrations. After injection of the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine (200 mg kg-1), there were time-dependent linear decreases in the concentrations of noradrenaline and MHPG in mouse brain. In addition, a very good correlation (r = 0.95, n = 30; P less than 0.001) was found between the concentrations of noradrenaline and MHPG present in the brains of the same mice after alpha-methyl-p-tyrosine treatment. 3. Mouse brain MHPG concentrations were dose-dependently reduced after administration of the alpha 2-adrenoceptor agonist, clonidine (1-3000 micrograms kg-1), and elevated by the antagonists, idazoxan (1 and 5 mg kg-1), and yohimbine (1 and 5 mg kg-1). Intracerebroventricular injection of the alpha 1-adrenoceptor agonist, phenylephrine (5-50 micrograms) dose-dependently increased MHPG levels. The alpha 1-adrenoceptor antagonist, prazosin, had no effect at the moderate dose of 1 mg kg-1, but increased MHPG concentrations at 5 mg kg-1. The beta-adrenoceptor agonist, clenbuterol (10-1000 micrograms kg-1) and the antagonist, pindolol (1 and 5 mg kg-1), were both without effect. 4. The decrease in brain MHPG concentrations induced by clonidine (100 micrograms kg-1) was prevented by prior injection of 1 mg kg-1 of idazoxan or yohimbine, but not by prazosin or pindolol. 5. MHPG levels were decreased after administration of the noradrenaline reuptake inhibitor desipramine (5 and 10 mg kg-1) and the non-selective monoamine reuptake inhibitors, sibutramine HCl (BTS 54 524; 1 and 3 mg kg-1) and amitryptyline (5 mg kg-1). However, the selective 5-hydroxytryptamine reuptake inhibitor, zimeldine (5 and 10 mg kg-1), was without effect. Dexamphetamine (1 and 5 mg kg-1) and methamphetamine (1 and 5 mg kg-1) both decreased brain MHPG concentrations in a dose-related fashion. 6. Overall the data show that MHPG can be used as a functional index of both presynaptic alpha 2-adrenoceptor activity and noradrenaline turnover and utilisation.

Effect of clonidine and yohimbine on sleep in man: polygraphic study and EEG analysis by normalized slope descriptors

An all-night polygraphic study of sleep was carried out in 6 healthy volunteers acutely medicated with clonidine (225 micrograms), yohimbine (15 mg), a combination of both drugs or with placebo at random order under simple blind conditions. The conventional scoring of sleep stages was extended by an on-line analysis of the EEG according to Hjorth. Clonidine, an agonist at alpha2 receptors, induced increases of the stages 2 and 3 + 4 but a decrease of REM sleep; the reduction of the parameters mobility and complexity during non-REM sleep signified synchronization of the EEG. Yohimbine, an antagonist at the same receptor sites, enhancing noradrenaline release, increased stage 1 and REM sleep, but decreased the stages 2 and 3 + 4; increases of mobility during REM and non-REM sleep signified desynchronization. The combined treatment with clonidine + yohimbine reduced REM sleep, but not as extensively as clonidine alone; the Hjorth parameters were without significant alterations compared to placebo. The physiological hypotonia was enhanced by clonidine and the combined treatment with clonidine + yohimbine. These results favour the hypothesis of a noradrenergic facilitation of REM sleep.

Adrenoreceptor interactions of the enantiomers and metabolites of mianserin: are they responsible for the antidepressant effect?

Mianserin is a tetracyclic antidepressant whose postulated mechanism of action involves release of noradrenaline mediated via cortical alpha 2-adrenergic autoreceptor blockade. This property resides stereoselectively in the S(+)-enantiomer of mianserin, which is also more potent in behavioural tests indicative of antidepressant activity, and in the reversal of clonidine-induced effects. Cortical receptor binding studies have indicated that although a similar stereoselectivity prevails for the inhibition of both alpha 2-binding (clonidine) and alpha 1-binding (prazosin), mianserin and its enantiomers are more potent antagonists at alpha 2- than at alpha 1-binding sites. However, no stereoselectivity is apparent for the antagonism of cortical alpha 2-heteroreceptors controlling serotonin release. Following chronic administration, (+/-)- and S(+)-mianserin, but not the R(-)-enantiomer, produce functional supersensitivity at alpha 2-autoreceptors which is unaccompanied by changes in clonidine binding. Neither mianserin nor its enantiomers alter the sensitivity of alpha 2-heteroreceptors following chronic administration. Like mianserin and its S(+)-enantiomer, but unlike R(-)-mianserin and the 8-hydroxy metabolite, the desmethyl metabolite inhibits noradrenaline uptake in vitro. 8-hydroxymianserin and, to a lesser extent, desmethylmianserin release noradrenaline from cortical slices via alpha 2-autoreceptor antagonism, but only the 8-hydroxy metabolite blocks alpha 2-autoreceptors and alpha 2-heteroreceptors in synaptosomal preparations. It is likely that S(+)-mianserin, desmethylmianserin, and 8-hydroxymianserin contribute substantially to the overall facilitating effect of mianserin on noradrenergic transmission in vivo. As yet it is unclear whether this effect is exclusively responsible for the antidepressant activity of mianserin or whether the stereoselectivity also shown by the mianserin enantiomers towards serotonin receptors plays a complementary role.

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.