3H-Mianserin binding in calf caudate: possible involvement of serotonin receptors in anti-depressant drug action

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.

Partial purification of [3H]mianserin binding sites

Frontal cortex membranes were solubilized with digitonin, prelabelled with 4 nM [3H]mianserin, and partially purified by isoelectric focussing. Bound [3H]mianserin separated from free [3H]mianserin as a single radioactive peak with a pI value of 5.03. A 14-fold purification was achieved. Focussing in the presence of 1 microM ketanserin (R 41468) or 1 microM chlorpyramine (a histamine H1-antagonist) or 10 microM spiperone (an S2-antagonist) completely abolished the peak of radioactivity at pH 5. Pharmacological characterization of the eluted radioactive peak revealed both histaminergic and serotonergic binding properties. The data suggest that the serotonergic and histaminergic components of [3H]mianserin binding to frontal cortex are not separable by solubilization, gel filtration or isoelectric focussing.

Chronic antidepressant therapy and associated changes in central monoaminergic receptor functioning

Acutely administered antidepressants possess a multiplicity of pharmacological actions. However, the fact that agents possessing similar pharmacological actions are devoid of antidepressant activity, together with the lack of correlation between doses required for acute pharmacological effects and clinical efficacy, suggest that the mechanism(s) of action of antidepressants cannot be directly attributed to the acute pharmacological properties of the drugs. The lag phase in onset of clinical effectiveness emphasizes the importance of adaptive changes following chronic antidepressant administration. A rapidly accelerating trend in attempting to delineate the precise molecular mechanisms of action of antidepressants is the shift in emphasis following chronic antidepressant therapies from alterations in uptake, storage, synthesis and release of neurotransmitters to adaptive changes in receptor functioning. These adaptations occur both pre- and postsynaptically. Examples of the former are alpha 2 and DA presynaptic receptors, both being down-regulated by certain forms of chronic antidepressant therapy. The fact that the NE-coupled adenylate cyclase system in rat brain slices is down-regulated by tricyclics, atypical antidepressants, MAO inhibitors and ECT emphasizes the importance of the system. Electrophysiological and behavioral studies point to the up-regulation of central alpha 1 and 5-HT receptor functioning following long-term antidepressant therapy. In contrast to the beta-adrenoceptor, these findings cannot be correlated with data from radioligand binding studies. In general central alpha 1-adrenoceptor binding remains unaltered. This is also true for 5-HT1 binding whereas cortical 5-HT2 binding is both increased and decreased depending on the type of antidepressant therapy being investigated. The relationship of these adaptive changes to the clinical efficacy of antidepressants in man is not clear since there is generally a lack of good models for studying human central receptor functioning. A review of current data from animal studies would tend to disfavour the view that all forms of antidepressant therapy possess a common mechanism of action. Perhaps multiple intervention sites exist. The introduction and evaluation of agents possessing a specificity of pharmacological action will undoubtedly aid psychotherapeutic research. The knowledge that peptides and 'classical' neurotransmitters can co-exist in the same neurone will undoubtedly generate studies of the significance and importance of the co-transmitter function of peptides in the mechanisms of action of antidepressant therapies.

[3H]Mianserin binding to solubilized membranes of frontal cortex

[3H]Mianserin binding sites were characterized in membranes of dog frontal cortex and solubilized with digitonin. The solubilized [3H]mianserin binding sites retained the binding characteristics of the membrane preparation. The dissociation constant (KD) for the soluble material was 2.2 nM which compared closely to that of the membrane-bound site (1.1 nM). Drug binding profiles indicated that the solubilized sites were both serotonergic and histaminergic in nature. Drugs active both H1- and S2-receptors potently displaced [3H]mianserin with Hill slopes close to 1. More selective serotonergic drugs such as LSD and spiperone competed for [3H]mianserin binding with shallow displacement curves. Of the neurotransmitters tested, serotonin was the most potent for both preparations. Because all the components of [3H]mianserin binding co-solubilize, the soluble material can be used to isolate individual sites.

Use of [3H]spiperone for labelling dopaminergic and serotonergic receptors in bovine caudate nucleus

[3H]Spiperone binding has been used to study neurotransmitter receptors in bovine caudate nucleus in displacement and saturation binding experiments. Displacement curves for several antagonists are biphasic and can be analysed into contributions from dopaminergic and serotonergic sites. Antagonist binding at each class of sites follows the simple mass action equations for binding at a homogeneous set of sites (slope factors close to unity). Agonist displacement curves also indicate complex behaviour, but agonist binding to the dopaminergic sites alone exhibits heterogeneous properties (slope factors less than unity). Saturation binding experiments have been conducted on each class of site, defining dopaminergic binding of [3H]spiperone as that binding displaced by 0.1 mM-dopamine and serotonergic binding as that displaced by 0.3 microM-mianserin. In each case, a single class of binding sites was detected: the binding parameters derived in this way have been used to calculate the proportions of the two classes of binding site observed in displacement experiments. Good agreement was obtained between calculated and observe values.