Assessing substrates underlying the behavioral effects of antidepressants using the modified rat forced swimming test

Selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed antidepressant class today and exert their antidepressant-like effects by increasing synaptic concentrations of serotonin (5-HT). The rat forced swim test (FST) is the most widely used animal test predictive of antidepressant action. Procedural modifications recently introduced by our laboratory have enabled SSRI-induced behavioral responses to be measured in the modified FST. The use of this model to understand the pharmacological and physiological mechanisms underlying the role of 5-HT in the behavioral effects of antidepressant drugs is reviewed. Although all antidepressants reduced behavioral immobility, those antidepressants that increase serotonergic neurotransmission predominantly increase swimming behavior whereas those that increase catacholaminergic neurotransmission increase climbing behavior. The 5-HT(1A), 5-HT(1B/1D) and 5-HT(2C) receptors are the 5-HT receptors most important to the therapeutic effects of SSRIs, based on extensive evaluation of agonists and antagonists of individual 5-HT receptor subtypes. Studies involving chronic administration have shown that the effects of antidepressants are augmented following chronic treatment. Other studies have demonstrated strain differences in the response to serotonergic compounds. Finally, a physiological model of performance in the rat FST has been proposed involving the regulation of 5-HT transmission by corticotropin releasing factor (CRF).     

Clinical studies of monoamine receptors in the affective disorders and receptor changes with antidepressant treatment

Pre-clinical and clinical studies suggest that the responsiveness of monoamine and cholinergic receptors may be altered in the affective disorders and that antidepressants may modify the sensitivity of these receptors. The growth hormone response to clonidine is reduced in depressed patients compared to controls according to several independent studies, suggesting that post-synaptic alpha 2-adrenergic receptors may be less responsive in depressed patients. The cortisol response to clonidine is enhanced in depressed patients compared to controls in our study raising the possibility that cortisol hypersecretion in depressed patients may be related to noradrenergic dysfunction. The hypotensive response to clonidine is blunted in patients on chronic antidepressant treatment with either clorgyline or desipramine suggesting that pre-synaptic alpha 2-adrenergic receptors may subsensitize with chronic antidepressant treatment. The prolactin increase in response to fenfluramine is less in depressed patients compared to controls suggesting decreased functional activity of the serotonergic system in depression. Platelet alpha 2-adrenergic receptor number as measured by tritiated dihydroergocriptine (3H-DHE) binding is increased in depressed patients compared to controls, while cyclic 3'-5' adenosine monophosphate (cAMP) production in response to prostaglandin E1 (PGE1) and norepinephrine (NE) inhibition of PGE1-stimulated cAMP production are reduced in the platelets of depressed patients. Thus, it is not clear that increased 3H-DHE binding reflects increased functional responsiveness and might in fact be compensatory to decreases in functional responses of alpha 2-adrenergic receptors.     

Receptor sensitivity and the mechanism of action of antidepressant treatment. Implications for the etiology and therapy of depression

Considerable evidence suggests that the acute effects of antidepressant treatments on brain norepinephrine (NE) and serotonin (5-HT) systems cannot account fully for their delayed therapeutic action. This review evaluates the effects of long-term antidepressant treatment on biogenic amine metabolism and on various indexes of presynaptic and postsynaptic receptor function. In contrast to variable effects on NE and 5-HT turnover and on presynaptic receptor sensitivity almost all long-term antidepressant treatments produce consistent alterations in a number of measures of postsynaptic amine receptor sensitivity. Long-term treatment has been found to reduce beta-adrenergic sensitivity while enhancing responses to serotonergic and alpha-adrenergic stimulation, suggesting that modulation of receptor sensitivity may be a mechanism of action common to tricyclic antidepressants, "atypical" antidepressants, monoamine oxidase inhibitors, and electroconvulsive therapy. These findings provide support for hypotheses of amine receptor abnormalities in depression and indicate the need for expanded studies of amine receptor function in patients.     

Impact of substance P receptor antagonism on the serotonin and norepinephrine systems: relevance to the antidepressant/anxiolytic response

Substance P (neurokinin-1 [NK1]) receptor antagonists appear to be effective antidepressant and anxiolytic agents, as indicated in 3 double-blind clinical trials. In laboratory animals, they promptly attenuate the responsiveness of serotonin (5-hydroxytryptamine [5-HT]) and norepinephrine (NE) neurons to agonists of their cell-body autoreceptors, as is the case for some antidepressant drugs that are currently in clinical use. Long-term, but not subacute, antagonism of NK1 receptors in rats increases 5-HT transmission in the hippocampus, a property common to all antidepressant treatments tested thus far. This enhancement seems to be mediated by a time-dependent increase in the firing rate of 5-HT neurons. Mice with the NK1 receptor deleted from their genetic code also have an increased firing rate of 5-HT neurons. Taken together, these observations strongly suggest that NK1 antagonists could become a new class of antidepressant and anxiolytic agents.     

Molecular and cellular mechanisms of antidepressant drugs

The mechanism of action of the antidepressants was reviewed through the summer of 1995. Antagonism of monoamine transport is the primary cellular action associated with many antidepressant medications. However, an increased synaptic concentration of monoamines is not the actual mechanism of antidepressant effects as suggested by the time lapse between treatment initiation and the onset of clinical antidepressant response. Chronic administration of all antidepressants increases the efficiency of 5-HT transmission in the synapse, albeit by different mechanisms. Serotonin transporter antagonists enhance serotonergic neurotransmission by decreasing the functional activity of the 5-HT_1A and 5-HT_1B autoreceptors. Tricyclic antidepressants and electroconvulsive therapy enhance 5-HT neurotransmission by increasing the responsiveness of postsynaptic 5-HT receptors. Monoamine oxidase inhibitors and the 5-HT_1A agonists increase serotonergic function by desensitizing the somatodendritic autoreceptor; the terminal 5-HT autoreceptor is not affected by these agents. Evidence for both enhanced and diminished noradrenergic transmission following antidepressant treatment exists, although the evidence more strongly supports decreased noradrenergic transmission. Ongoing investigations into intracellular adaptations (e.g., steroid receptors, growth factors, etc.) During chronic antidepressant administration offer the promise of furthering our understanding of the mechanism of action of antidepressants. Depression and Anxiety 4:153–159, 1996/1997. © 1997 Wiley-Liss, Inc.     

The experimental and clinical pharmacology of St John's Wort (Hypericum perforatum L.).

Hypericum (St John's Wort) is a plant that has been used for centuries as a medicinal herb. Pre-clinical animals studies suggest that hypericum is effective in three major biochemical systems relevant for antidepressant activity, namely the inhibition of the synaptic re-uptake system for serotonin (5-HT), noradrenaline (NA) and dopamine (DA). It is the only antidepressant capable of inhibiting the re-uptake of 5-HT, NA and DA with similar potencies. The potencies for monoamine re-inhibition and chronic changes in receptors are also consistent with changes seen with known antidepressants. Behavioral studies suggest that hypericum is active in pre-clinical animal models of depression with comparable effects to known antidepressants. Supporting the pre-clinical pharmacology and efficacy, many clinical studies have shown that hypericum has superior efficacy compared to placebo and comparable efficacy to standard antidepressants in the treatment of mild-to-moderate depression. The advantage of hypericum over other antidepressants may result from its favorable side-effect profile. Although pre-clinical and short-term clinical studies demonstrate antidepressant activity, the lack of long-term use and efficacy, and the heterogeneity of patients, interventions, extract preparations from previous clinical studies suggests that more careful and controlled studies are needed to determine the long-term efficacy of hypericum in mild-to-moderate depression.     

Role of norepinephrine in depression

This article reviews the role of norepinephrine (NE) and serotonin (5-HT) in depression and the therapeutic effects of antidepressant drugs from the perspective of human neurotransmitter depletion studies. The data reviewed suggest that both noradrenergic and serotonergic systems are involved in antidepressant action, but the specific impairment that underlies depression is unclear and is likely to vary among patients. Results from neurotransmitter depletion studies in depressed patients who have responded to treatment suggest that, while interactions between NE and 5-HT are likely, neither of these 2 neurotransmitter systems is the final common pathway for the therapeutic effect of antidepressant drugs. NE-selective antidepressant drugs appear to be primarily dependent on the availability of NE for their effects. Likewise, 5-HT-selective antidepressants appear to be primarily dependent on the availability of 5-HT for their effects. Antidepressants that cause effects on both noradrenergic and serotonergic systems-such as mirtazapine-may be dependent on the availability of both neurotransmitters for their effects. Neither 5-HT nor NE depletion induced clinical depression in healthy subjects or worsened depression in unmedicated symptomatic patients with major depression. This finding suggests that the cause of depression is more complex than just an alteration in the levels of 5-HT and/or NE. For some patients, depression may be more directly caused by dysfunction in brain areas or neuronal systems modulated by monoamine systems. We propose that antidepressant drugs may enhance neurotransmission in normal noradrenergic or serotonergic neurons and, through a time-dependent but as yet undiscovered process, restore function to brain areas modulated by monoamine neurons. Future research should focus on understanding the adaptive changes that follow enhancement of synaptic levels of monoamines in neuronal circuits of the frontal cortex, amygdala, and hippocampus. Research investigating the neurobiology of depression may be more informed if the focus is shifted to investigating areas of the brain modulated by monoamine systems rather than the monoamine systems themselves.     

Neuroendocrine aspects of the serotonergic hypothesis of depression

This review examines the role of serotonin (5-HT) in depression. Dysfunction of serotonergic neurons has been implicated as one of the causes of endogenous depression. Since serotonergic neurons innervate the hypothalamus and these neurons send collaterals to several other brain areas, it is possible that hypothalamic sites which control hormone secretion receive the same serotonergic afferents that innervate other limbic areas in the brain. Several investigators have devised neuroendocrine challenge tests measuring the effect of 5-HT agonists on plasma cortisol and prolactin in depressed patients. These tests help to identify dysfunctional 5-HT neurons, and are a "window into the brain." The secretion of cortisol and prolactin is increased predominantly by 5-HT1 receptors. However, changes in 5-HT2 receptors have also been implicated in depression. Results from our laboratory and by others suggest that brain serotonergic neurons stimulate renin and vasopressin secretion by activation of 5-HT2 receptors. Therefore, the renin and vasopressin response to 5-HT agonists should be included in neuroendocrine tests of serotonergic function in affective disorders. Since antidepressants produce a decrease in the density of 5-HT2 receptors, renin and vasopressin could be used to evaluate the antidepressant potential of new drugs.     

Effect of delta-9-tetrahydrocannabinol on behavioral despair and on pre- and postsynaptic serotonergic transmission

Preclinical and clinical studies suggest that direct and indirect cannabinoid agonists, including enhancers of endocannabinoids, engender stress-relieving, anxiolytic and antidepressant effects, mediated by central CB(1) receptors (CB(1)Rs). The effect of the main pharmacologically active principle in cannabis, (-)-trans-Δ(9)-tetrahydrocannabinol (delta-9-THC), on depressive behavior and on the serotonin (5-HT) system, which is implicated in the mechanism of action of antidepressants, has not been extensively clarified. Here, we showed that repeated (5 days), but not single (acute) intraperitoneal (ip) treatment with delta-9-THC (1mg/kg) exerts antidepressant-like properties in the rat forced swim test (FST). This effect was CB(1)R-dependent because it was blocked by the CB(1)R antagonist rimonabant (1mg/kg, ip). Using in vivo electrophysiology, we demonstrated that delta-9-THC modulated dorsal raphe (DR) 5-HT neuronal activity through a CB(1)R-dependent mechanism. Acute intravenous delta-9-THC administration (0.1-1.5mg/kg) elicited a complex response profile, producing excitatory, inhibitory and inert responses of 5-HT neurons. Only excitatory responses were blocked by rimonabant. Finally, repeated but not single delta-9-THC administration (1mg/kg, ip) enhanced tonic 5-HT(1A) receptor activity in the hippocampus, a postsynaptic event commonly elicited by standard antidepressants. These results suggest that delta-9-THC, like other CB(1)R agonists and endocannabinoid enhancers, may possess antidepressant properties at low doses, and could modulate 5-HT transmission in the DR and hippocampus as standard antidepressants such as selective serotonin reuptake inhibitors.     

Antidepressant effects assessed using behavior maintained under a differential-reinforcement-of-low-rate (DRL) operant schedule

Behavior maintained under a differential-reinforcement-of-low-rate (DRL) 72-s operant schedule, which reinforces responses with interresponse times greater than 72 s, exhibits a rather unique sensitivity to antidepressant drugs. Antidepressants from a number of pharmacological classes, including tricyclic antidepressants, selective serotonin or norepinephrine reuptake inhibitors, monoamine oxidase inhibitors, as well as a number of atypical antidepressants and putative antidepressants, reduce response rate and increase reinforcement rate of rats under this schedule. These effects are observed acutely but persist or are augmented with repeated treatment. By contrast, drugs from a number of other psychotherapeutic classes do not, in general, produce similar effects. This includes anxiolytic, sedative, stimulant, opioid, antihistaminic, and anticholinergic drugs, which can produce false positive results in some preclinical tests for antidepressant efficacy. There are conflicting data regarding the utility of DRL behavior for discriminating the effects of antidepressant and antipsychotic drugs. This results in part from methodological differences among studies, but likely also reflects the overlap between the neuropharmacological and clinical effects of some antipsychotic and antidepressant drugs. DRL behavior also has proven useful for identifying neurochemical and neuroanatomical mediators of antidepressant effects on behavior. Consistent with clinical data, it appears that activation of noradrenergic or serotonergic systems provides for parallel means of producing antidepressant-like effects on DRL behavior. Finally, the results of studies using DRL behavior highlight important roles for central beta-1 adrenergic receptors, as well as 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptors, in the mediation of antidepressant-like behavioral effects.     

Neuentwicklungen in der Pharmakotherapie der Depression

In spite of recent progress in the pharmacotherapy of depression, major issues are still unresolved. These include the nonresponse rate of approximately 30% to conventional antidepressant pharmacotherapy, side effects of available antidepressants, and the latency period of several weeks until clinical improvement. Current treatment strategies aim to enhance serotonergic and/or noradrenergic neurotransmission. However, in the meantime, several new pharmacological principles are under investigation with regard to their antidepressant potency. Placebo-controlled, double-blind studies have been performed with 5-HT(1A) receptor agonists and tachykinin receptor antagonists which point towards antidepressant efficacy. While there is some evidence for putative antidepressant properties of various interventions within the hypothalamic-pituitary-adrenal system such as CRH(1) receptor antagonists, steroid synthesis inhibitors, and glucocorticoid receptor antagonists in animal studies, case series, open studies, and small placebo-controlled studies, no definite proof for their antidepressant efficacy has been furnished. Nevertheless, follow-up of new pharmacological strategies is of major importance to provide even better strategies for the clinical management of depression, which also has great socioeconomic impact.     

Putative mechanisms of action of antidepressant drugs in affective and anxiety disorders and pain

An enhancement of neurotransmission of serotonin (5-HT), noradrenaline, or both, underlies the antidepressant response associated with most agents presently available to treat major depression. With respect to the 5-HT system, antidepressant drugs exert immediate effects on some neuronal elements controlling overall transmission, but it is the gradual changes in neuronal responses to such treatments that are ultimately responsible for producing their therapeutic benefits. In major depression, an increase in 5-HT1A transmission is thought to be a crucial determinant of the antidepressant response, whereas an enhancement of 5-HT2 transmission in the orbitofrontal cortex may mediate the therapeutic effect of 5-HT reuptake inhibitors in obsessive-compulsive disorder (OCD). The doses of medication and the durations of treatment necessary to obtain these alterations in 5-HT transmission in various brain structures of laboratory animals are fully consistent with the conditions in the clinic necessary to attenuate symptoms in depression and OCD. It is also possible that the relief of chronic pain produced by some antidepressants may be mediated, in part, by the blockade of peripheral 5-HT2A receptors. These observations emphasize the notion that the 5-HT system is endowed with different adaptive properties in various parts of the body, which, in addition to the multiplicity of 5-HT receptors, makes this chemospecific network important in many disorders.     

5-Hydroxytryptamine and corticosterone in an animal model of depression

1. Rat plasma corticosterone increases during 2 hr restraint stress. The animals then exhibit hypophagia and decreased locomotion occurs on placement 24 hr later in an open field. Repeating the restraint daily for 5-7 days leads to adaptation. Failure to adapt is the depression model which is associated with three factors implicated in the illness ie increased plasma glucocorticoid level, female sex and inadequate 5-HT function as revealed by behavioural response to the agonist 5-methoxy-N,N-dimethyl 5-HT. However, the greater stress induced rise of corticosterone in female rats may reflect a greater response to activation of hypothalamic 5-HT receptors mediating corticoid release. 2. The model responds appropriately to chronic antidepressant pretreatment. Single injections of 5-HT1A agonists (8-OH-DPAT, buspirone, ipsapirone, gepirone) but not of benzodiazepine anxiolytics have similar effects. Therefore, 5-HT1A agonists may have antidepressant activity. Both behavioural and neurochemical evidence indicates that the adaptive effects of 5-HT1A agonists on the depression model are associated with desensitisation of somatodendritic 5-HT1A autoreceptors.     

Decrease in brain serotonin 2 receptor binding in patients with major depression following desipramine treatment: a positron emission tomography study with fluorine-18-labeled setoperone.

BACKGROUND: The neuroreceptor changes involved in therapeutic efficacy of various antidepressants remain unclear. Preclinical studies have shown that long-term administration of various antidepressants causes down-regulation of brain serotonin 2 (5-HT2) receptors in rodents, but it is unknown if similar changes occur following antidepressant treatment in depressed patients. Our purpose, therefore, was to assess the effects of treatment with desipramine hydrochloride on brain 5-HT2 receptors in depressed patients using positron emission tomography (PET) and fluorine-18 (18F)-labeled setoperone. METHODS: Eleven patients who met DSM-IV criteria for major depression as determined by a structured clinical interview for DSM-III-R diagnosis and suitable for treatment with desipramine were recruited. Ten patients underwent a PET scan before and another after 3 to 4 weeks of treatment with desipramine. RESULTS: Eight of the 10 patients responded to desipramine treatment as indicated by more than 50% decrease in Hamilton Depression Rating Scale scores. Depressed patients showed a significant decrease in 5-HT2 receptor binding as measured by setoperone binding in frontal, temporal, parietal, and occipital cortical regions following desipramine treatment. The decrease in 5-HT2 receptor binding was observed bilaterally and was particularly prominent in frontal cortex. CONCLUSIONS: Depressed patients showed a significant reduction in available 5-HT2 receptors in the brain following desipramine treatment, but it is unknown if this change in 5-HT2 receptors is due to clinical improvement or an effect of desipramine that is unrelated to clinical status.     

Reevaluation of the indoleamine hypothesis of depression. Evidence for a reduction of functional activity of central 5-HT systems by antidepressant drugs

Summary The effects of antidepressant drugs on central 5-HT receptor activity were studied in rats and mice. Antidepressant drugs were evaluated for their ability to displace³H-5-HT and³H-d-LSD from membrane binding sites in the dorsal neocortex of ratsin vitro and for their ability to block 5-HTP and d-LSD induced behavioral effects in mice. The degree of blockade of head-twitches in mice produced by the antidepressants was highly correlated with their affinity for³H-d-LSD binding sites. A number of antidepressant drugs such as amitriptyline, nortriptyline, mianserine, doxepine, nomifensine and dibenzepine appear to possess marked 5-HT receptor blocking activity at some types of 5-HT receptors in brain. New antidepressant drugs such as zimelidine, which specifically inhibit 5-HT reuptake and do not block 5-HT receptor sites, may after chronic treatment also reduce the functional activity of 5-HT systems by producing adaptive changes in postsynaptic 5-HT mechanisms. Thus, a new indoleamine hypothesis of depression is presented: the therapeutic action of antidepressant drugs may in part be due to a reduced functional activity of some central 5-HT systems.     

The importance of brain serotonergic receptor mechanisms for the action of antidepressant drugs

The present studies have shown that chronic antidepressant treatment with different types of antidepressant drugs results in adaptive changes in postsynaptic 5-HT receptor mechanisms which appear to result in 5-HT sub- or supersensitivity development depending upon the 5-HT nerve terminal system analyzed. The results underline the heterogeneity of the central 5-HT neuron systems in their responses to chronic antidepressant treatment. In addition, a number of antidepressant drugs have potent, direct actions on 5-HT2 receptors, which may be of clinical relevance.     

Sustained blockade of neurokinin-1 receptors enhances serotonin neurotransmission.

BACKGROUND: Antagonists of neurokinin-1 (NK(1)) receptors, through which substance P acts, have been proposed to belong to a new class of antidepressants with a unique mode of action. It was postulated that they exert this putative therapeutic effect independently of the serotonin (5-HT) neurons. METHODS: The aim of the present study was to assess, using in vivo electrophysiological paradigms, the effects of sustained administration of the nonpeptidic NK(1) antagonist CP-96,345 on the firing activity of rat dorsal raphe 5-HT neurons, the responsiveness of pre- and postsynaptic 5-HT(1A) receptors, and overall 5-HT neurotransmission in the hippocampus. RESULTS: Both short- and long-term treatments with CP-96,345 significantly increased the spontaneous firing activity of dorsal raphe 5-HT neurons, and this increase was associated with an attenuation of somatodendritic 5-HT(1A) autoreceptor responsiveness. In contrast, the inactive enantiomer of CP-96,345 at NK(1) receptors, CP-96,344, did not alter these parameters after short-term administration. Because 5-HT(1A) receptor activation inhibits the firing activity of dorsal hippocampus CA(3) pyramidal neurons, the degree of disinhibition produced by the selective 5-HT(1A) receptor antagonist WAY 100635 was determined to assess the net change in 5-HT neurotransmission. Intravenous injection of WAY 100635 did not disinhibit CA(3) pyramidal neuron firing in rats given saline, CP-96,345 for 2 days, or CP-96,344 for 14 days, but produced a significant enhancement of firing in rats treated with CP-96,345 for 2 weeks. Therefore, only long-term treatment with CP-96,345 enhanced the tonic activation of postsynaptic 5-HT(1A) receptors. CONCLUSIONS: Similar to all other major types of antidepressant treatments, these data indicate that substance P antagonists might alleviate anxiety and major depression, at least in part, by enhancing the degree of activation of some 5-HT receptors in the forebrain.     

Antidepressant-anxiolytic interactions: involvement of the benzodiazepine-GABA and serotonin systems.

1. Recent studies have demonstrated that antidepressant drugs are actually more effective than BZ's in the treatment of anxiety states. The role of two major neurochemical substrates that may be implicated in the anxiolytic activity of antidepressants, the benzodiazepine (BZ)-GABA receptor chloride ionophore complex and central serotonergic pathways, are focused on in this review. 2. A wide range of antidepressants elicit a reduction in BZ receptors and display anxiolytic effects within a conflict paradigm. 3. The anxiolytic activity of antidepressants, however, does not appear to be mediated via the BZ receptor, but possibly via another component of the complex such as the chloride channel-associated with the GABAA receptor. 4. Additionally, as possible candidates for the mechanism of anxiolytic activity of these compounds, results of pharmacological, behavioral and clinical studies point to the importance of serotonin (5-HT)1A receptors and 5-HT transporter sites as targets for the action of antidepressants, triazolobenzodiazepines and anxioselective piperazine derivatives.     

Effect of 5-hydroxytryptophan on serum cortisol levels in major affective disorders. III. Effect of antidepressants and lithium carbonate

Serum cortisol levels were significantly increased following administration of 5-hydroxytryptophan (5-HTP), 200 mg orally, to patients with affective disorders. A three- to five-week period of treatment with lithium carbonate or monoamine oxidase (MAO) inhibitor augmented the mean 5-HTP-induced increase in serum cortisol concentration in manic or depressed patients, respectively: tricyclic antidepressant (TCA) and second-generation antidepressant treatment diminished the mean serum cortisol response in patients with major depression. These results are consistent with the hypothesis that lithium carbonate may enhance serotonin (5-HT) receptor sensitivity, whereas TCA and second-generation antidepressants diminish 5-HT receptor sensitivity. The enhancement of the cortisol response to 5-HTP by MAO inhibitors may be due to decreased metabolism of 5-HT. It is important to assess the effect of thymoleptic drug treatment on various responses to biogenic amine precursors or agonists in patients rather than laboratory animals.     

Central monoamines and their role in major depression

The role of the monoamines serotonin and noradrenaline in mental illnesses including depression is well recognized. All antidepressant drugs in clinical use increase acutely the availability of these monoamines at the synapse either by inhibiting their neuronal reuptake, inhibiting their intraneuronal metabolism, or increasing their release by blocking the alpha(2) auto- and heteroreceptors on the monoaminergic neuron. This acute increase in the amount of the monoamines at the synapse has been found to induce long-term adaptive changes in the monoamine systems that end up in the desensitization of the inhibitory auto- and heteroreceptors including the presynaptic alpha(2) and 5-HT(1B) receptors and the somatodendritic 5-HT(1A) receptors located in certain brain regions. The desensitization of these inhibitory receptors would result in higher central monoaminergic activity that coincides with the appearance of the therapeutic response. These adaptive changes responsible for the therapeutic effect depend on the availability of the specific monoamine at the synapse, as depletion of the monoamines will either reverse the antidepressant effect or causes a relapse in the state of drug-free depressed patient previously treated with antidepressant drugs. Furthermore, blocking the somatodendritic 5-HT(1A) or nerve terminal alpha(2) receptors proved to increase the response rate in the treatment of major and treatment-resistant depression, providing further support to the assumption that the antidepressant effect results from the long-term adaptive changes in the monoamine auto- and heteroregulatory receptors. On the other hand, the chronic treatment with antidepressants resulted in D(2) receptors supersensitivity in the nucleus accumbens. This supersensitivity might play a role in the mechanisms underlying antidepressant induced mood switch and rapid cycling.