Effects of long-term treatment with desipramine on microtubule proteins in rat cerebral cortex.

The molecular mechanism of the action of antidepressants beyond the receptor level has not yet been elucidated. We have investigated the effects of long-term treatment with desipramine on the phosphorylation state of microtubule-associated protein 2 (MAP2) and microtubule assembly in the rat cerebral cortex. Phosphorylation of MAP2 was detected by immunoblotting after immunoprecipitation of MAP2 in the soluble fraction. The degree of phosphorylation of serine residues of MAP2 was significantly increased after chronic administration of desipramine without changes in the total concentration of MAP2. Microtubule assembly in crude brain extracts was monitored in terms of changes in turbidity measured at 350 nm using a spectrophotometer. Chronic but not acute treatment with desipramine inhibited microtubule assembly, assayed in the presence of a phosphatase inhibitor, calyculin A, whereas the inhibition was completely nullified in the absence of calyculin A. Desipramine had no direct effect on microtubule assembly in vitro. These results raise the possibility that the changes in the degree of phosphorylation of MAP2 and microtubule assembly represent intracellular modifications involved in functional changes elicited by long-term treatment with desipramine.

Involvement of protein kinase in the regulation of beta-adrenergic receptors by antidepressants.

The role of serotonin (5-HT) and protein kinases in the regulation of the beta-receptor induced by typical and atypical antidepressants was investigated. Treatment with either mianserin or maprotiline for seven days produced a significant decrease in the beta-receptor density measured 6h after the last dose. The reduction in beta-receptors disappeared within 24h. However, combined treatment of mianserin or maprotiline with either fluoxetine or 5-hydroxytryptophan significantly decreased beta-receptors even 24h after the last dose. Following treatment with p-chlorophenylalanine the reduction in beta-receptors induced by desipramine was reversible within 24h. These results demonstrate that an increase in the synaptic 5-HT availability may contribute to the prolongation of the beta-receptor down-regulation by antidepressants. The intraventricular infusion of 12-0-tetradecanoyl-phorbol-13-acetate (TPA), a potent protein kinase C (PK-C) activator for seven days caused a significant decrease in beta-receptors, while forskolin or dibutyryl cyclic adenosine monophosphate had no influence on the receptor. The TPA-induced and desipramine-induced decreases in beta-receptors were not additive, suggesting that a similar mechanism is involved. The infusion of the PK-C inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) caused an inhibition of the desipramine-induced decrease in beta-receptors. It is suggested that the beta-receptor down-regulation by antidepressants might be also regulated by PK-C through 5-HT-Ca2+-inositol-phospholipid systems.

Role of serotonin in the regulation of beta-adrenoceptors by antidepressants.

The role of serotonin (5-HT) in the regulation of the beta-receptor density induced by long-term treatment with typical and atypical antidepressants was examined. Treatment with either mianserin (15 mg/kg, twice daily) or maprotiline (10 mg/kg, twice daily) for 7 days caused a significant decrease in the beta-receptor density, measured 6 h after the last dose, without a change in affinity. The reduction in beta-receptors disappeared rapidly (within 24 h). However, treatment with mianserin or maprotiline combined with fluoxetine, a selective 5-HT uptake inhibitor, significantly decreased the beta-receptor density even 24 h after the last dose. The combined administration of mianserin and 5-hydroxytryptophan (5-HTP) mimicked the effect of the combination with fluoxetine. Following pretreatment with p-chlorophenylalanine (PCPA) for 6 days, desipramine treatment for 3 days significantly decreased the beta-receptors 6 h after the last dose but this desipramine-induced decrease in beta-receptors was rapidly reversible (within 24 h). These results demonstrate that while intrasynaptic 5-HT levels are not a factor in the decrease in beta-receptors, they do play an important role in the preservation of the down-regulated state of the beta-receptor caused by antidepressants from rapid reversibility.

The characterization of [3H]sulpiride binding sites in rat striatal membranes.

The characteristics of [3H]sulpiride binding to the D2 dopamine receptor in rat striatal membranes were examined under several conditions. In the absence of sodium ions, the specific binding of [3H]sulpiride could not be detected. In the direct binding experiment, at 25 degrees C, the affinity of [3H]sulpiride for D2 receptors was increased in a dose-dependent manner of sodium ions whereas magnesium ions have opposite effects on an affinity of [3H]sulpiride binding. The lowering incubation temperature (4 degrees C) also produced a further increase in affinity of the ligand. Under all conditions, [3H]sulpiride labeled a single homogenous site of the receptor. On the other hand, the result from quantitative analysis of agonist/[3H]sulpiride competition curves indicated an existence of high (RH) and low (RL) affinity states for agonists and the proportion of two-affinity states was modulated by guanosine triphosphate (GTP), magnesium ions and lowering temperatures. GTP, together with sodium ions, caused a full conversion of RH to RL, while an increase in the affinity for agonists with a partial conversion of RL to RH could be induced by magnesium ions at 25 degrees C. At a lower (4 degrees C) temperature, the agonist competition curve indicated an existence of a single agonist low-affinity state (RL) and then, the effects of GTP and magnesium ions in the agonist affinity observed at 25 degrees C were abolished. These results can be incorporated into a two-step, ternary complex model involving an inhibitory guanine nucleotide binding protein for the agonist and antagonist interaction with D2 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Solubilization and characterization of rat brain alpha 2-adrenergic receptor.

alpha 2-Adrenergic receptors labelled by [3H]-clonidine (alpha 2-agonist) can be solubilized from the rat brain in a form sensitive to guanine nucleotides with a zwitterionic detergent, 3-[3-(cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS). About 40% of the original [3H]CLO binding sites in the membranes were solubilized with 6 mM CHAPS. Separation of the soluble [3H]CLO-bound complex was performed by the vacuum filtration method using polyethylenimine-treated GF/B filters. Solubilized [3H]CLO binding sites retained the same pharmacological characteristics of membrane-bound alpha 2-adrenergic receptors. Scatchard plots of [3H]CLO binding to solubilized alpha 2-receptors were curvilinear, indicating the existence of the two distinct binding components. Solubilized receptors were eluted as a single peak from Bio-Gel A-1.5 m column with a Stokes radius of 6.6 nm. The isoelectric point was 5.6-5.8. Regulations of the receptor binding by guanine nucleotides, monovalent cations, and sulfhydryl-reactive agents were maintained intact in the soluble state, whereas those by divalent cations were lost. The apparent retention of receptors and guanine nucleotide binding regulatory component(s) in the soluble state may allow a investigation of the regulation mechanisms of the brain alpha 2-adrenergic receptor system at the molecular level.

Initial detection of [3H]clonidine binding to solubilized rat brain alpha 2-adrenergic receptors: regulation by guanine nucleotides.

Alpha 2-adrenergic receptors labeled by [3H]clonidine (alpha 2-agonist) can be solubilized from the rat brain with a zwitterionic detergent, 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS). CHAPS-solubilized receptors have the same characteristics of membrane-bound alpha 2-receptors in the brain, and the regulation of receptor binding by guanine nucleotides is retained in the soluble state.

Quantitative analysis of rat brain alpha 2-receptors discriminated by [3H]clonidine and [3H]rauwolscine.

Quantitative analysis of direct ligand binding of both [3H]clonidine and [3H]rauwolscine to the rat cerebral cortex alpha 2-receptors indicates the existence of two affinity states of the same receptor populations. In the presence of Mn2+, the high affinity state of [3H]clonidine binding was increased, whereas the high affinity state of [3H]rauwolscine binding was reduced. By contrast, GTP in micromolar ranges caused a decrease of the agonist high affinity state and an increase of the antagonist high affinity state. The total receptor sites and the respective separate affinities for both radioligands were approximately equal to their control values under all conditions, indicating that Mn2+ and GTP modulate the proportion of the two affinity states of the receptor. These results can be incorporated into a two-step, ternary complex model involving a guanine nucleotide binding protein (N protein) for the agonist and antagonist interaction with the alpha 2-receptor. Furthermore, the effects of GTP on the interaction of both ligands with the two affinity states can be mimicked by EDTA. It is suggested that divalent cations induce the formation of the receptor-N protein binary complex showing high affinity for agonists and low affinity for antagonists.

Interaction of neuroleptics and antidepressants with rat brain alpha 2-receptors: a possible relationship between alpha 2-receptor antagonism and antidepressant action.

The relative potencies of a variety of neuroleptic drugs and antidepressant agents in competing for the binding of 3H-clonidine and 3H-yohimbine to alpha 2-adrenergic receptors of the rat cerebral cortex were quantified. The rank order of potencies of neuroleptics tested in competing for both ligand bindings is: clocapramine, carpipramine much greater than perphenazine, fluphenazine, alpha,beta-flupentixol greater than propericiazine, levomepromazine greater than chlorpromazine, pimozide greater than moperone much greater than haloperidol, sulpiride. Among the antidepressants, mianserin is the most potent antidepressant. Amitriptyline has a substantial affinity, while desipramine, imipramine, and clomipramine are the least potent. It is concluded that a number of neuroleptics and antidepressant agents display a potent or moderate affinity to alpha 2-receptor sites. These findings support the suggestion that some neuroleptics enhance the release of norepinephrine (NE) mainly by acting on presynaptic alpha 2-receptors, and that the alpha 2-receptor blocking property may have an important role in the mechanism of the antidepressant effect of some neuroleptic compounds.