Current understanding of the mechanism of action of classic and newer antidepressant drugs

Docking studies on monoamine oxidase-B inhibitors: Estimation of inhibition constants (Ki) of a series of experimentally tested compounds

Monoamine oxidase (EC1.4.3.4; MAO) is a mitochondrial outer membrane flavoenzyme that catalyzes the oxidation of biogenic amines. It has two distinct isozymic forms designated MAO-A and MAO-B, each displaying different substrate and inhibitor specificities. They are the well-known targets for antidepressant and neuroprotective drugs. Elucidation of the X-ray crystallographic structure of MAO-B has opened the way for molecular modeling studies. A series of experimentally tested (1-10) model compounds has been docked computationally to the active site of the MAO-B enzyme. The AutoDock 3.0.5 program was employed to perform automated molecular docking. The free energies of binding (DeltaG) and inhibition constants (K(i)) of the docked compounds were calculated by the Lamarckian Genetic Algorithm (LGA) of AutoDock 3.0.5. Excellent to good correlations between the calculated and experimental K(i) values were obtained.

Inhibition of amine oxidases activity by 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives

A novel series of 1-acetyl-3,5-diphenyl-4,5-dihydro-(1H)-pyrazole derivatives have been synthesised and investigated for the ability to inhibit selectively monoamine oxidases, swine kidney oxidase, and bovine serum amine oxidase. The newly synthesised compounds 1-6 proved to be reversible and non-competitive inhibitors of all types of the assayed amine oxidases. Compounds inhibit monoamine oxidases potently, displaying low I(50) values of particular interest. In particular 1-acetyl-3-(2,4-dihydroxyphenyl)-5-(3-methylphenyl)-4,5-dihydro-(1H)-pyrazole 6 showed to be a potent monoamine oxidase inhibitor with a K(i) of about 10(-8)M. Further insights in the theoretical evaluation of the possible interactions between the compounds and monoamine oxidase B have been developed through a computational approach.

The SSRIs drug Fluoxetine, but not the noradrenergic tricyclic drug Desipramine, improves memory performance during acute major depression

Accumulating evidence suggests that noradrenergic and serotonergic drugs are equally effective in ameliorating the depressive symptoms of major depression. Major depression is associated also with memory impairments, but the comparative effects of the antidepressant drugs on memory are not clear. We previously found that serotonergic neurotransmission is of particular importance for some aspects of episodic memory. We set out to test whether treatment with the selective serotonergic drug Fluoxetine (Prozac) would be advantageous in this respect over treatment with the selective noradrenergic tricyclic antidepressant drug Desipramine (Deprexan). Seventeen patients with major depressive episode, randomly assigned for treatment with either Fluoxetine (n = 8) or Desipramine (n = 9), were assessed for their clinical situation and for memory performance at the beginning of treatment, after 3 weeks, and after 6 weeks of pharmacological treatment. We found that although clinically both drugs were equally effective, the improvement of memory performance in the Fluoxetine-treated patients was significantly greater compared with that of the Desipramine-treated patients. The results support the role of serotonin in memory. More studies in larger samples of patients are required, but it may be that in cases where memory impairment is a major symptom, it would be beneficial to consider serotonergic antidepressant drugs for treatment. Furthermore, in cases where, for various reasons, the treatment of choice is noradrenergic, it may be worthwhile to consider a supplementary serotonergic drug to improve memory deficits.

Effects of fluvoxamine treatment on the in vivo binding of [F-18]FESP in drug naive depressed patients: a PET study

This study investigates the effect of chronic treatment with Fluvoxamine, a potent and specific serotonin reuptake sites inhibitor (SSRI), on 5HT(2) serotonin and D(2) dopamine receptors in the brain of drug naive unipolar depressed patients. Drug effect was evaluated in different cortical areas and in the basal ganglia by positron emission tomography (PET) and fluoro-ethyl-spiperone ([(18)F]FESP), an high affinity 5HT(2) serotonin and D(2) dopamine receptors antagonist. Patients underwent a PET study at recruitment and after clinical response to Fluvoxamine treatment. Nine of the 15 patients recruited completed the study. Fluvoxamine treatment significantly improved clinical symptoms and modified [(18)F]FESP binding in the frontal and occipital cortex of all of the nine patients who completed the study; in these regions a mean 31% increase in the in vivo [(18)F]FESP binding was found (P < 0.01). On the contrary, no significant changes in the in vivo [(18)F]FESP binding were found in the basal ganglia where [(18)F]FESP binds mainly to D(2) dopamine receptors. Chronic treatment with Fluvoxamine significantly increases the in vivo binding of [(18)F]FESP in the frontal and occipital cortex of drug naive unipolar depressed patients. The increase of the in vivo binding of [(18)F]FESP may reflect a modification in 5HT(2) binding capacity secondary to changes in cortical serotonin activity.

Conventional and new antidepressant drugs in the elderly

Depression in the elderly is nowadays a predominant health care problem, mainly due to the progressive aging of the population. It results from psychosocial stress, polypathology, as well as some biochemical changes which occur in the aged brain and can lead to cognitive impairments, increased symptoms from medical illness, higher utilization of health care services and increased rates of suicide and nonsuicide mortality. Therefore, it is very important to make an early diagnosis and a suitable pharmacological treatment, not only for resolving the acute episode, but also for preventing relapse and enhancing the quality of life. Age-related changes in pharmacokinetics and in pharmacodynamics have to be kept into account before prescribing an antidepressant therapy in an old patient. In this paper some of the most important and tolerated drugs in the elderly are reviewed. Tricyclic antidepressants have to be used carefully for their important side effects. Nortriptyline, amytriptiline, clomipramine and desipramine as well, seem to be the best tolerated tricyclics in old people. Second generation antidepressants are preferred for the elderly and those patients with heart disease as they have milder side effects and are less toxic in overdose and include the so called atypicals, such as selective serotonin reuptake inhibitors, serotonin noradrenalene reuptake inhibitors and noradrenaline reuptake inhibitors. Monoamine oxidase (MAO) inhibitors are useful drugs in resistant forms of depression in which the above mentioned drugs have no efficacy; the last generation drugs (reversible MAO inhibitors), such as meclobemide, seem to be very successful. Mood stabilizing drugs are widely used for preventing recurrences of depression and for preventing and treating bipolar illness. They include lithium, which is sometimes used especially to prevent recurrence of depression, even if its use is limited in old patients for its side effects, the anticonvulsants carbamazepine and valproic acid. Putative last generation mood stabilizing drugs include the dihydropyridine L-type calcium channel blockers and the anticonvulsants phenytoin, lamotrigine, gabapentin and topiramate, which have unique mechanisms of action and also merit further systematic study. Psychotherapy is often used as an adjunct to pharmacotherapy, while electroconvulsant therapy is used only in the elderly patients with severe depression, high risk of suicide or drug resistant forms.

Platelet alpha2A-adrenoceptor function in major depression: Gi coupling, effects of imipramine and relationship to treatment outcome

Studies suggest alpha2A-adrenoceptors (alpha(2A)AR) dysregulation in major depressive disorder (MDD). Platelet alpha(2A)ARs exist in high- and low-conformational states that are regulated by Gi protein. Although alpha(2A)AR coupling to Gi protein plays an important role in signal transduction and is modulated by antidepressants, it has not been previously investigated. Alpha2AR density in the high- and low-conformational states, agonist affinity and coupling efficiency were investigated in 27 healthy control subjects, 23 drug-free MDD patients and 16 patients after imipramine treatment using [3H]yohimbine saturation and norepinephrine displacement of [3H]yohimbine binding experiments. Coupling measures were derived from NE-displacement experiments. Patients had significantly higher alpha(2A)AR density, particularly in the high-conformational state, than control subjects. Coupling indices were normal in patients. High pre-treatment agonist affinity to the receptor in the high-conformational state and normal coupling predicted positive treatment outcome. Decreased coupling to Gi predicted a negative treatment outcome. Imipramine induced uncoupling (-11%) and redistribution of receptor density in treatment responders only, but had no effect on alpha(2A)AR coupling or density in treatment non-responders. Increased alpha(2A)AR density may represent a trait marker in MDD. The results provide indirect evidence for abnormal protein kinase A (PKA) and protein kinase C (PKC) in MDD which may be pursued in future investigations.

A reversible monoamine oxidase A inhibitor, befloxatone: structural approach of its mechanism of action

Experimental and theoretical physico-chemical methods were used to investigate the interaction between several reversible monoamine oxidase A inhibitors in the oxazolidinone series and the active site of the enzyme. Phenyloxazolidinones include toloxatone and analogues, among which befloxatone was selected as drug candidate for the treatment of depression. Identification of the forces responsible for the crystal cohesion of befloxatone reveals functional groups that could interact with monoamine oxidase. Calculation of electronic properties of those compounds using ab initio molecular orbital methods lead to a description of the mode of interaction between befloxatone and the cofactor of the enzyme. Electronic absorption spectroscopy measurements confirm the hypothesis of a privileged interaction of phenyloxazolidinone-type inhibitors with the flavin cofactor of MAO. Additional sites of interaction with the protein core of MAO A are also examined with regard to the primary structure of the enzyme. As a result of this work, a model is proposed for the reversible inhibition of MAO A by befloxatone via long distance, reversible interactions with the flavin adenine dinucleotide (FAD) cofactor of the enzyme and with specific amino acids of the active site. This model is partially corroborated by experimental evidence and should be helpful in designing new potent inhibitors of monoamine oxidase.

Serum levels of clomipramine and desmethylclomipramine and clinical improvement in panic disorder

Several placebo-controlled trials have shown the efficacy of clomipramine (CMI) in panic disorder. However, none has investigated the relationship between CMI, and desmethylclomipramine (DCMI) plasma levels, and outcome. In this trial, 41 patients meeting the DSM-III-R criteria for panic disorder with/without agoraphobia received 50-200 mg of CMI daily in a single-blind, flexible dose regimen for 14 weeks. At the end of treatment, 97% of the patients were free of panic attacks. Patients were classified into two groups of improvement according to the panic symptom items of the 'Patient-Rated Anxiety Scale'. A repeated-measures analysis of variance suggested a significant association between outcome and serum DCMI level/daily dose ratio as well as total serum level/daily dose. Patients with intense improvement showed DCMI and total serum levels lower than those with moderate improvement. The results indicate the importance of monitoring clomipramine and desmethylclomipramine serum levels in this disorder.

Dysthymia: clinical picture, extent of overlap with chronic fatigue syndrome, neuropharmacological considerations, and new therapeutic vistas

Dysthymia, as defined in the American Psychiatric Association and International Classification of Mental Disorders, refers to a prevalent form of subthreshold depressive pathology with gloominess, anhedonia, low drive and energy, low self-esteem and pessimistic outlook. Although comorbidity with panic, social phobic, and alcohol use disorders has been described, the most significant association is with major depressive episodes. Family history is loaded with affective, including bipolar, disorders. The latter finding explains why dysthymia, especially when onset is in childhood, can lead to hypomanic switches, both spontaneously and upon pharmacologic challenge in as many as 30%. Indeed, antidepressants from different classes -tricyclic antidepressants (TCAs), monoamine oxidase inhibitors (MAOIs), reversible inhibitors of monoamine oxidase A (RIMAs), selective serotonin-reuptake inhibitors (SSRIs) and, more recently, amisulpride, and spanning noradrenergic, serotonergic as well as dopaminergic mechanisms of action - have been shown to be effective against dysthymia in an average of 65% of cases. This is a promising development because social and characterologic disturbances so pervasive in dysthymia often, though not always, recede with continued pharmacotherapy beyond acute treatment. Despite symptomatic overlap of dysthymia with chronic fatigue syndrome - especially with respect to the cluster of symptoms consisting of low drive, lethargy, lassitude and poor concentration - neither the psychopathologic status, nor the pharmacologic response profile of the latter syndrome is presently understood. Chronic fatigue today is where dysthymia was two decades ago. We submit that the basic science - clinical paradigm that has proven so successful in dysthymia could, before too long, crack down the conundrum of chronic fatigue as well. At a more practical level, we raise the possibility that a subgroup within the chronic fatigue group represents a variant of dysthymia.

Chronic administration of imipramine and citalopram alters the expression of NMDA receptor subunit mRNAs in mouse brain. A quantitative in situ hybridization study

Chronic administration of antidepressants produces region-specific adaptive changes in the radioligand binding properties of N-methyl-D-aspartate (NMDA) receptors. We hypothesized that this effect of chronic antidepressant administration was owing to an alteration in NMDA receptor subunit composition. This hypothesis was examined using in situ hybridization with [35S]-labeled riboprobes to quantify the impact of chronic (16 d) injection with either imipramine (15 mg/kg) or citalopram (20 mg/kg) on the levels of transcripts encoding NMDA receptor subunits in mouse brain. These antidepressants altered the levels of mRNA encoding the zeta-subunit in a parallel fashion, with both drugs either reducing transcript levels (e.g., in the cortex, cerebellum, thalamus, and striatum) or producing no substantial effects (e.g., hippocampus). In contrast, these antidepressants often produced distinct, region-specific effects on mRNA levels encoding the epsilon family of subunits. For example, citalopram treatment produced widespread reductions in epsilon 1-subunit mRNA levels (e.g., in frontal cortex, CA2 of hippocampus, and amygdala), whereas imipramine reduced levels of this transcript only in the amygdala. Conversely, imipramine treatment produced widespread reductions in epsilon 2-subunit mRNA levels (e.g., in cortex, CA1-4 of hippocampus, and amygdala), whereas the effects of citalopram on levels of this transcript were largely restricted to amygdala. These findings indicate that long-term antidepressant treatment produces region-specific changes in expression of transcripts for NMDA receptor subunits, presumably altering NMDA receptor composition. Because subunit composition determines the physiological and pharmacological properties of NMDA receptors, these changes may play a critical role in the therapeutic actions of structurally diverse antidepressants.