Chronic inhibition of monoamine oxidase type A increases noradrenaline release in rat frontal cortex

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Catecholamine autotoxicity. Implications for pharmacology and therapeutics of Parkinson disease and related disorders

Several neurodegenerative diseases involve loss of catecholamine neurons-Parkinson disease is a prototypical example. Catecholamine neurons are rare in the nervous system, and why they are vulnerable in PD and related disorders has been mysterious. Accumulating evidence supports the concept of "autotoxicity"-inherent cytotoxicity of catecholamines and their metabolites in the cells in which they are produced. According to the "catecholaldehyde hypothesis" for the pathogenesis of Parkinson disease, long-term increased build-up of 3,4-dihydroxyphenylacetaldehyde (DOPAL), the catecholaldehyde metabolite of dopamine, causes or contributes to the eventual death of dopaminergic neurons. Lewy bodies, a neuropathologic hallmark of PD, contain precipitated alpha-synuclein. Bases for the tendency of alpha-synuclein to precipitate in the cytoplasm of catecholaminergic neurons have also been mysterious. Since DOPAL potently oligomerizes and aggregates alpha-synuclein, the catecholaldehyde hypothesis provides a link between alpha-synucleinopathy and catecholamine neuron loss in Lewy body diseases. The concept developed here is that DOPAL and alpha-synuclein are nodes in a complex nexus of interacting homeostatic systems. Dysfunctions of several processes, including decreased vesicular sequestration of cytoplasmic catecholamines, decreased aldehyde dehydrogenase activity, and oligomerization of alpha-synuclein, lead to conversion from the stability afforded by negative feedback regulation to the instability, degeneration, and system failure caused by induction of positive feedback loops. These dysfunctions result from diverse combinations of genetic predispositions, environmental exposures, stress, and time. The notion of catecholamine autotoxicity has several implications for treatment, disease modification, and prevention. Conversely, disease modification clinical trials would provide key tests of the catecholaldehyde hypothesis.

Changes in intensity of serotonin syndrome caused by adverse interaction between monoamine oxidase inhibitors and serotonin reuptake blockers

Drug interaction between inhibitors of monoamine oxidase (MAOIs) and selective serotonin (5-hydroxytryptamine, 5-HT) reuptake (SSRIs) induces serotonin syndrome, which is usually mild but occasionally severe in intensity. However, little is known about neural mechanisms responsible for the syndrome induction and intensification. In this study, we hypothesized that the syndrome induction and intensity utilize two different but inter-related mechanisms. Serotonin syndrome is elicited by excessive 5-HT in the brain (presynaptic mechanism), whereas syndrome intensity is attributed to neural circuits involving 5-HT2A and NMDA receptors (postsynaptic mechanism). To test this hypothesis, basal 5-HT efflux and postsynaptic circuits were pharmacologically altered in rats by once daily pretreatment of the MAOI clorgyline for 3, 6, or 13 days. Syndrome intensity was estimated by measuring 5-HT efflux, neuromuscular activity, and body-core temperature in response to challenge injection of clorgyline combined with the SSRI paroxetine. Results showed that the onset of serotonin syndrome is caused by 5-HT efflux exceeding 10-fold above baseline, confirming the presynaptic hypothesis. The neuromuscular and body-core temperature abnormalities, which were otherwise mild in drug-naive rats, were significantly intensified to a severe level in rats pretreated with daily clorgyline for 3 and 6 days but not in rats pretreated for 13 days. The intensified effect was blocked by M100907 and MK-801, suggesting that variation in syndrome intensity was mediated through a 5-HT2A and NMDA receptor-engaged circuit. Therefore, we concluded that pretreatments of MAOI pharmacologically alter the activity of postsynaptic circuits, which is responsible for changes in syndrome intensity.

Blue again: perturbational effects of antidepressants suggest monoaminergic homeostasis in major depression

Some evolutionary researchers have argued that current diagnostic criteria for major depressive disorder (MDD) may not accurately distinguish true instances of disorder from a normal, adaptive stress response. According to disorder advocates, neurochemicals like the monoamine neurotransmitters (serotonin, norepinephrine, and dopamine) are dysregulated in major depression. Monoamines are normally under homeostatic control, so the monoamine disorder hypothesis implies a breakdown in homeostatic mechanisms. In contrast, adaptationist hypotheses propose that homeostatic mechanisms are properly functioning in most patients meeting current criteria for MDD. If the homeostatic mechanisms regulating monoamines are functioning properly in these patients, then oppositional tolerance should develop with prolonged antidepressant medication (ADM) therapy. Oppositional tolerance refers to the forces that develop when a homeostatic mechanism has been subject to prolonged pharmacological perturbation that attempt to bring the system back to equilibrium. When pharmacological intervention is discontinued, the oppositional forces cause monoamine levels to overshoot their equilibrium levels. Since depressive symptoms are under monoaminergic control, this overshoot should cause a resurgence of depressive symptoms that is proportional to the perturbational effect of the ADM. We test this prediction by conducting a meta-analysis of ADM discontinuation studies. We find that the risk of relapse after ADM discontinuation is positively associated with the degree to which ADMs enhance serotonin and norepinephrine in prefrontal cortex, after controlling for covariates. The results are consistent with oppositional tolerance, and provide no evidence of malfunction in the monoaminergic regulatory mechanisms in patients meeting current diagnostic criteria for MDD. We discuss the evolutionary and clinical implications of our findings.

Antidepressant-like pharmacological profile of 3-(4-fluorophenylselenyl)-2,5-diphenylselenophene: Involvement of serotonergic system

This study evaluated the effect of 3-(4-fluorophenylselenyl)-2,5-diphenylselenophene (DPS) in the mouse forced swim test (FST) and tail suspension test (TST), two assays predictive of depressant activity. The involvement of serotonergic system in the effect caused by DPS was studied. The antidepressant-like effect of combined treatment with subeffetive doses of DPS and paroxetine, a selective serotonin reuptake inhibitor (SSRI) was investigated. Further, we verified the possible mechanism responsible for antidepressive-like effect of DPS. The results show that DPS (50 and 100 mg/kg, p.o.) significantly reduced the immobility time during the FST and TST, without accompanying changes in ambulation when assessed in the open-field test. The anti-immobility effect of DPS (50 mg/kg, p.o.) in the FST was prevented by pretreatment of mice with pCPA (100 mg/kg, i.p., once a day for 4 consecutive days, an inhibitor of 5-HT synthesis), WAY 100635 (0.1 mg/kg, s.c., a selective 5-HT1A receptor antagonist), ritanserin (1 mg/kg, i.p., a 5-HT2 receptor antagonist) or ondansetron (1 mg/kg, i.p., a 5-HT3 receptor antagonist). Combined treatment with paroxetine and DPS reduced the immobility time in the FST. DPS at the doses of 10-100 mg/kg did not produce any change in the cerebral activity of MAO-A or MAO-B. DPS at the dose of 50 mg/kg inhibited significantly 5-HT uptake in synaptosomes. These results suggest that DPS produced an antidepressant-like effect in the mouse FST and TST and this effect seems most likely to be mediated through an interaction with serotonergic system, particularly by 5-HT reuptake inhibition.

Long-term administration of monoamine oxidase inhibitors alters the firing rate and pattern of dopamine neurons in the ventral tegmental area

Monoamine oxidase inhibitors (MAOIs) exert their antidepressant action by increasing the function of the serotonin (5-HT), norepinephrine and dopamine (DA) systems. There is, however, limited electrophysiological data on the effects of MAOIs on DA neurons. The effects of 2-d and 21-d administration of three MAOIs were investigated (clorgyline, selective MAOI-A; deprenyl, selective MAOI-B; phenelzine, non-selective MAOI) on the firing activity of DA neurons in the ventral tegmental area using in-vivo electrophysiology in rats. Short-term clorgyline (1 mg/kg) and phenelzine (2.5 mg/kg) was devoid of effect on DA neurons, whereas prolonged administration significantly decreased their firing rate (by 30% and 20%, respectively), number of bursts (by 80% and 45%, respectively), and percentage of spikes occurring in bursts only in clorgyline-treated rats (70%). Deprenyl (0.25 mg/kg) was without effects. DA firing was restored in clorgyline-treated rats by inhibiting 5-HT synthesis using para-chlorophenylalanine (p-CPA; 300 mg/kg. d for three consecutive days). The 5-HT3 antagonist ondansetron (0.5 mg/kg) was devoid of effect in control rats, but completely reversed the alterations of DA neuronal activity in clorgyline-treated rats. An attenuation of DA neuronal activity was thus produced by prolonged blockade of MAOA activity. The absence of effect of MAOA inhibition after subacute administration suggested an indirect mechanism. This was confirmed by the observation that p-CPA antagonized the effects of clorgyline. Since ondansetron completely reversed the effects of clorgyline on DA neuronal activity, the effects of MAOA inhibition appeared to be mediated by 5-HT3 receptors.

From a Parkinson's disease expert: Rasagiline and the future of therapy

John Finberg is a professor of pharmacology at the Faculty of Medicine, Technion - Israel Institute of Technology, home of Israel's two Nobel laureates. He and his colleague Prof. Moussa Youdim were instrumental in the early clinical development of the anti-Parkinson drug rasagiline, which gained UK- and EU-marketing authorization in 2005 and US FDA approval in 2006. In our interview, Finberg reflects on his clinical research to develop rasagiline as a commercial drug and its proposed pharmacological mechanisms of action. Moreover, he elucidates the current state of anti-Parkinson drug discovery and offers direction for future research.

Monoamine oxidase A activity and norepinephrine level in hippocampus determine hyperwheel running in SPORTS rats

An understanding of neurological mechanisms for wheel running by rodents, especially with high exercise activity, would be applicable to a strategy for promotion of exercise motivation in humans. One of several brain regions that are candidates for the regulation of physical exercise is the hippocampus. Here we examined the running activity of Spontaneously-Running-Tokushima-Shikoku (SPORTS) rat, a new animal model for high levels of wheel-running activity, and its relation with the hippocampal norepinephrine (NE) system including the levels of NE, adrenergic receptors, and degradation enzymes for monoamines. In the hippocampus of SPORTS rats, the level of NE in extracellular fluid was augmented, whereas the level in the homogenate of the whole tissue was decreased even for sedentary conditions. Elevated extracellular NE caused downregulation of alpha(2)-adrenergic receptors in the hippocampus of SPORTS rats. Local administration of alpha(2)-adrenergic receptor antagonist yohimbine, but not of alpha(2)-agonist clonidine, into the hippocampus suppressed high running activity in SPORTS rats. The protein expression and the activity levels of monoamine oxidase A (MAOA), a critical enzyme for the degradation of NE, were decreased in the hippocampus of SPORTS rats to increase extracellular NE level. Thus, inhibition of oxidase activity in normal Wistar rats markedly increased wheel-running activity. These results indicate that decreased MAOA activity, elevation of extracellular NE, and alpha(2)-adrenergic receptors in the hippocampus determine the neural basis of the psychological regulation of exercise behavior in SPORTS rats.

Effect of co-administration of subchronic lithium pretreatment and acute MAO inhibitors on extracellular monoamine levels and the expression of contextual conditioned fear in rats

We investigated the effects of clorgyline [a selective MAO (monoamine oxidase inhibitor)-A inhibitor] and lazabemide (a selective MAO-B inhibitor) on extracellular serotonin, dopamine and noradrenaline concentrations in the medial prefrontal cortex after 1-week treatment with subchronic 0.2% or 0.05% Li2CO3 (p.o.) and the effects on expression of contextual conditioned fear, previously reported to be reduced by facilitation of serotonin neurotransmission. As compared to normal diet controls, the subchronic 0.2% Li2CO3 group showed significantly higher levels of extracellular serotonin, but not noradrenaline. No changes were observed in the 0.05% Li2CO3 group. Acute clorgyline (10 mg/kg) treatments combined with subchronic 0.2% Li2CO3 treatments showed significant increases in extracellular serotonin concentrations, but not in dopamine or noradrenaline, as compared with clorgyline treatment alone. There was an additive effect with combined treatment of subchronic 0.2% Li2CO3 and acute clorgyline on the reduction of conditioned freezing, an index of conditioned fear, and this was not observed with subchronic 0.05% Li2CO3. These behavioral data indicate the functional significance of increased extracellular serotonin concentrations due to combined use of a MAO-A inhibitor with subchronic lithium. Effects of lazabemide (10 mg/kg) on extracellular monoamine concentrations and conditioned fear were slight or negligible, and were not affected by subchronic lithium treatment. The present study suggests that lithium augmentation of the antidepressant effect of MAO inhibitors is mediated by additional increases in the extracellular serotonin concentrations induced by MAO-A inhibition and suggests that the anxiolytic action of MAO inhibitors may be enhanced by lithium.

Shared mechanisms of antidepressant and antiepileptic treatments: drugs and devices

Many treatments for the epilepsies and affective disorder share the properties of seizure suppression and mood stabilization. Moreover, affective disorders and the epilepsies appear to share partially similar pathogenic mechanisms. A component of the shared predisposition appears to arise from noradrenergic and serotonergic deficits. Increasing evidence supports the hypothesis that noradrenergic and/or serotonergic elevation is a mechanism of therapeutic benefit shared by most antidepressants and many antiepileptic medications. Medication induced alterations in GABAergic, glutamatergic, and CRH (corticotropin releasing hormone) containing neurons may also contribute to the shared therapeutic properties of antidepressant and antiepileptic medications.

Monoamine oxidase inhibitors reduce conditioned fear stress-induced freezing behavior in rats

The present study examined the acute anxiolytic effects of monoamine oxidase inhibitors on freezing behavior, a putative index of anxiety induced by conditioned fear stress. The selective serotonin 1A receptor agonist tandospirone (0.1-10 mg/kg) inhibited freezing dose dependently. The irreversible, non-selective monoamine oxidase inhibitors tranylcypromine (3 and 15 mg/kg) and phenelzine (30 and 80 mg/kg) reduced freezing significantly. Clorgyline (10 mg/kg, irreversible selective monoamine oxidase A inhibitor), N-(2-aminoethyl)-5-(m-fluorophenyl)-4-thiazole carboxamide (Ro 41-1049) (30 mg/kg, reversible selective monoamine oxidase A inhibitor), selegiline (3 mg/kg, irreversible selective monoamine oxidase B inhibitor) and lazabemide (10 mg/kg, reversible selective monoamine oxidase B inhibitor) had no effect on freezing behavior. However, combined administration of clorgyline (10 mg/kg) and selegiline (3 mg/kg) reduced freezing significantly, as well as combined administration of clorgyline (10 mg/kg) and lazabemide (10 mg/kg), Ro 41-1049 (30 mg/kg) and selegiline (3 mg/kg), or Ro 41-1049 (30 mg/kg) and lazabemide (10 mg/kg). These effects of monoamine oxidase inhibitors on freezing were not due to non-specific motor effects. These results suggest that acute inhibition of both monoamine oxidase A and B reduced anxiety or fear, while inhibition of monoamine oxidase A or B alone failed to reduce anxiety or fear.

Modulation of dihydroxyphenylacetaldehyde extracellular levels in vivo in the rat striatum after different kinds of pharmacological treatment

We recently identified the direct product of dopamine (DA) by monoamine-oxidase (MAO) activity, dihydroxyphenylacetaldehyde (DOPALD) in the trans-striatal dialysate. Based on these findings, in this work, we directly measured the variations in DOPALD levels after various kinds of pharmacological treatment in rat striatal extracellular fluid. Using both reversible and irreversible MAO inhibitors, we found that MAO-A inhibition suppressed, whereas MAO-B inhibition did not modify DOPALD levels in the dialysate. The vesicular DA uptake blocker Ro 4-1284 led to an increase in extracellular DA and DOPALD, whereas the increase in extracellular DA obtained after administration of the plasma membrane DA uptake blocker GBR-12909 occurred without concomitant changes in DOPALD extracellular levels. Microinfusions of DA through the dialysis probe or systemic administration of L-DOPA increased striatal DOPALD to a greater extent compared with other DA metabolites, both in intact and in 6-hydroxydopamine (6-OHDA)-lesioned striatum. This study indicates that the direct product of MAO activity within the rat striatum derives from the activity of the isoenzyme MAO-A. The assay of DOPALD, together with DOPAC, represents a reliable tool to measure directly, in freely moving animals, DA oxidative metabolism. As recent studies have shown that microinfusions of exogenous DOPALD might induce cell death, pharmacological modulation of DOPALD levels might also be relevant for an understanding of the mechanisms involved in DA neurotoxicity.

Striatal dopamine metabolism in monoamine oxidase B-deficient mice: a brain dialysis study

We have studied striatal dopamine (DA) metabolism in monoamine oxidase (MAO) B-deficient mice using brain microdialysis. Baseline DA levels were similar in wild-type and knock-out (KO) mice. Administration of a selective MAO A inhibitor, clorgyline (2 mg/kg), increased DA levels and decreased levels of its metabolites in all mice, but a selective MAO B inhibitor, l-deprenyl (1 mg/ kg), had no effect. Administration of 10 and 50 mg/kg L-DOPA, the precursor of DA, increased the levels of DA similarly in wild-type and KO mice. The highest dose of L-DOPA (100 mg/kg) produced a larger increase in DA in KO than wild-type mice. This difference was abolished by pretreating wild-type mice with l-deprenyl. These results suggest that in mice, DA is only metabolized by MAO A under basal conditions and by both MAO A and B at high concentrations. This is in contrast to the rat, where DA is always metabolized by MAO A regardless of concentration.

Alpha2-adrenoceptor-mediated restraint of norepinephrine synthesis, release, and turnover during immobilization in rats

Stress-related release of norepinephrine (NE) in the brain and periphery probably underlies several neuroendocrine and neurocirculatory responses. NE might influence its own synthesis, release, and turnover, by negative feedback regulation via alpha2-adrenoceptors. We examined central and peripheral noradrenergic function by measuring concentrations of NE, dihydroxyphenylglycol (DHPG), and dihydroxyphenylacetic acid (DOPAC) in hypothalamic paraventricular nucleus (PVN) microdialysate and arterial plasma simultaneously during immobilization (IMMO) in conscious rats. The alpha2-adrenoceptor antagonist yohimbine (YOH) was injected i.p. or perfused locally into the PVN via the microdialysis probe. The i.p. YOH increased plasma NE, epinephrine (EPI), DHPG, dihydroxyphenylalanine, and DOPAC levels by 4.3, 7.3, 2.5, 0.6 and 1.8-fold and PVN microdialysate NE, DHPG, and DOPAC by 1. 2, 0.6 and 0.5-fold. The i.p. YOH also enhanced effects of IMMO on plasma and microdialysate NE, DHPG, and DOPAC. YOH delivered via the PVN microdialysis probe did not affect microdialysate or plasma levels of the analytes at baseline and only slightly augmented microdialysate NE responses to IMMO. The results indicate that alpha2-adrenoceptors tonically restrain NE synthesis, release, and turnover in sympathetic nerves and limit IMMO-induced peripheral noradrenergic activation. In the PVN, alpha2-adrenoceptors do not appear to contribute to these processes tonically and exert relatively little restraint on IMMO-induced local noradrenergic activation.

Stimulation of locus coeruleus neurons by non-I1/I2-type imidazoline receptors: an in vivo and in vitro electrophysiological study

1. Imidazoline binding sites have been reported to be present in the locus coeruleus (LC). To investigate the role of these sites in the control of LC neuron activity, we studied the effect of imidazolines using in vivo and in vitro single-unit extracellular recording techniques. 2. In anaesthetized rats, local (27 pmoles) and systemic (1 mg kg(-1), i.v.) administrations of 2-(2-benzofuranyl)-2-imidazoline (2-BFI), a selective I-imidazoline receptor ligand, increased the firing rate of LC cells (maximal increase: 22+/-5%, P<0.001 and 16+/-7%, P<0.001 respectively). Chronic pretreatment with the irreversible monoamine oxidase inhibitor clorgyline (3 mg kg(-1), i.p., every 12 h for 14 days) abolished this effect. 3. In rat midpontine brain slices containing the LC, bath application (1 mM) of the imidazolines 2-BFI, 2-(4,5-dihydroimidaz-2-yl)-quinoline (BU224), idazoxan, efaroxan, phentolamine and (2-2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX821002) reversibly stimulated LC cells. The maximal effect was approximately 90% except for RX821002 and efaroxan which induced smaller maximal effects (approximately 58% and approximately 35% respectively). Simultaneous application of idazoxan and 2BFI did not lead to additive effects. 4. Bath application of the alpha2-adrenoceptor antagonists, yohimbine (1 - 10 microM) and N-ethoxycarbonyl-1,2-dihydroquinoline (EEDQ) (10 microM), failed to modify LC activity. The irreversible blockade of alpha2-adrenoceptors with EEDQ (10 microM) did not alter the effect of idazoxan or that of efaroxan. Previous application of clorgyline (10 microM) did not modify the excitatory effect of 2-BFI or efaroxan. 5. Changes in the pH of the bathing solution (6.84-7.84) did not influence the effect caused by idazoxan. Bath application of 2-BFI (1 mM) reversed the inhibition induced by diazoxide (300 microM), an ATP-sensitive K+ channel opener, whereas application of glibenclamide (3 microM), an ATP-sensitive K+ channel blocker, partially blocked the effect of 2-BFI. 6. This study shows that imidazoline compounds stimulate the firing rate of LC neurons. This effect is not mediated by alpha2-adrenoceptors nor by I1 or I2-imidazoline receptors but involves a different subtype of imidazoline receptor. Our results indicate that this receptor is located extracellularly and modulates ATP-sensitive K+ channels.

Effects of various stressors on in vivo norepinephrine release in the hypothalamic paraventricular nucleus and on the pituitary-adrenocortical axis

The hypothalamic-pituitary adrenocortical (HPA) system and sympathoneural and adrenomedullary systems are major effector systems that serve to maintain homeostasis during stress. Corticotropin-releasing hormone (CRH) in the paraventricular nucleus (PVN) of the hypothalamus, a determinant of both HPA and autonomic responses to stress, is under the control of many neurotransmitters and neuropeptides. Norepinephrine (NE) potently stimulates CRH neurons in the PVN; however, the physiologic role of NE in stress-induced activation of the HPA is unknown. In the present study we exposed animals to various stressors (immobilization (IMMO), cold (COLD), hemorrhage (HEM), hypoglycemia elicited by insulin administration (INS), pain and tissue damage caused by formalin injection (FORM) and sc injection of physiological saline (SAL), all of which are known to activate the HPA axis. Injection of physiological saline iv was used as a control. In vivo microdialysis was used to assess stressor- and intensity-specific activation of the PVN noradrenergic system, based on measurements of NE, its intraneuronal metabolite dihydroxyphenylglycol (DHPG), and the dopamine metabolite, dihydroxyphenylacetic acid (DOPAC). Simultaneously with microdialysate collections, blood samples were obtained via catheters in the femoral artery to measure plasma ACTH and corticosterone (CORT) levels as dependent measures, to assess stress-induced activation of the HPA axis. At their highest intensities, all the stressors significantly increased levels of PVN microdialysate NE, DHPG, and DOPAC, and plasma ACTH and CORT. PVN NE levels varied across stressors, with IMMO and FORM more potent than INS, COLD, or HEM. INS and HEM evoked proportionately larger plasma ACTH responses than did IMMO, FORM, and COLD. Plasma CORT responses were largest during IMMO, FORM, and HEM. Except for COLD and HEM, there was a strong correlation of plasma ACTH levels with levels of NE, DHPG, and DOPAC in PVN microdialysate. The data suggest that, except for COLD or HEM, there is a strong positive correlation of PVN noradrenergic activation and activity of the HPA axis. With stressors such as IMMO and FORM, NE synthesis, reflected by DOPAC changes, is strongly positively correlated with activity of the HPA axis. Furthermore, the results indicate substantial stressor specificity of PVN catecholaminergic and of HPA responses to different stressors and are inconsistent with a founding tenet of Selye's stress theory, the doctrine of nonspecificity, which defines stress as the nonspecific response of the body to any demand.

Pharmacology of reversible and selective inhibitors of monoamine oxidase type A

The concept of reversible type A monoamine oxidase (MAO-A) inhibitors as effective antidepressant drugs with a minimal side effect profile has been vindicated in practice. Despite this, the pharmacological basis for their actions is unclear. Studies with the irreversible inhibitor clorgyline have shown that chronic but not acute treatment of rats leads to a significant enhancement of noradrenaline release from peripheral sympathetic nerves and cerebral cortex together with a more effective inhibition of MAO-A, as shown by reduction in levels of deaminated metabolites in cortical microdialysis fluid. Reversible inhibitors, however, do not have a cumulative effect on MAO inhibition and may have different effects on noradrenaline release. Reversible inhibitors did not produce the acute reduction in sympathetic nerve activity seen with clorgyline, which may be one factor in explaining their milder side effect profile. Other aspects of the pharmacology of reversible and irreversible selective inhibitors of MAO-A are reviewed.

Effect of long-term administration of antidepressant drugs on the 5-HT3 receptors that enhance the electrically evoked release of [3H]noradrenaline in the rat hippocampus

The present study investigated the effects of various classes of antidepressant drugs (10 mg/kg per day, s.c. during 21 days) on the electrically evoked release of [3H]noradrenaline and on its modulation by the 5-HT3 receptor agonist 2-methyl-5-hydroxy-tryptamine (2-methyl-5-HT) using preloaded rat hippocampal slices. Treatments with either fluoxetine, a selective serotonin (5-HT) reuptake inhibitor, or moclobemide, a reversible type A monoamine oxidase inhibitor, increased the evoked release of [3H]noradrenaline. These two antidepressant treatments did not change, however, the magnitude of the enhancing effect of 2-methyl-5-HT on the electrically evoked release of [3H]noradrenaline. Desipramine produced a much larger increase of the electrically evoked release of [3H]noradrenaline than fluoxetine or moclobemide, and desensitized the 5-HT3 receptors that modulate this release. Trimipramine, which like desipramine has a tricyclic structure but does not block the reuptake of noradrenaline or that of 5-HT, did not increase the evoked release of [3H]noradrenaline and did not desensitize the 5-HT3 receptors that enhance the release of [3H]noradrenaline. Maprotiline, a selective noradrenaline reuptake inhibitor, did not produce the same changes as desipramine, but maprotiline inhibited noradrenaline reuptake to a lesser extent (50%) than desipramine (80%). These results suggest that the high potency noradrenaline reuptake blocker desipramine desensitizes 5-HT3 receptors modulating [3H]noradrenaline release, but that this effect is not common to all antidepressant drugs.

Modification of cerebral cortical noradrenaline release by chronic inhibition of MAO-A

Chronic treatment of rats with clorgyline (1 mg/kg i.p. daily for 21 days) caused a highly significant increase in the concentration of noradrenaline in microdialysate from the frontal cortex of the awake animal. Acute (one injection, 2 mg/kg) or subacute (1 mg/kg daily for 3 days) treatment did not lead to a significant increase in microdialysate noradrenaline. Concentrations of deaminated metabolites (DPHG, MHPG, DOPAC) in the microdialysate decreased with time of treatment, reaching a minimum after 21 days.