Locus coeruleus neuronal activity and noradrenaline availability in the frontal cortex of rats chronically treated with imipramine: effect of alpha 2-adrenoceptor blockade

Behavioral and neurochemical interactions of the tricyclic antidepressant drug desipramine with L-DOPA in 6-OHDA-lesioned rats. Implications for motor and psychiatric functions in Parkinson's disease

RATIONALE

The pharmacological effects of antidepressants in modulating noradrenergic transmission as compared to serotonergic transmission in a rat model of Parkinson's disease under chronic L-DOPA therapy are insufficiently explored.

OBJECTIVES

The aim of the present study was to investigate the effect of the tricyclic antidepressant desipramine administered chronically alone or jointly with L-DOPA, on motor behavior and monoamine metabolism in selected brain structures of rats with the unilateral 6-OHDA lesion.

METHODS

The antiparkinsonian activities of L-DOPA and desipramine were assessed behaviorally using a rotation test and biochemically based on changes in the tissue concentrations of noradrenaline, dopamine and serotonin and their metabolites, evaluated separately for the ipsi- and contralateral motor (striatum, substantia nigra) and limbic (prefrontal cortex, hippocampus) structures of rat brain by HPLC method.

RESULTS

Desipramine administered alone did not induce rotational behavior, but in combination with L-DOPA, it increased the number of contralateral rotations more strongly than L-DOPA alone. Both L-DOPA and desipramine + L-DOPA significantly increased DA levels in the ipsilateral striatum, substantia nigra, prefrontal cortex and the ipsi- and contralateral hippocampus. The combined treatment also significantly increased noradrenaline content in the ipsi- and contralateral striatum, while L-DOPA alone decreased serotonin level on both sides of the hippocampus.

CONCLUSIONS

The performed analysis of the level of monoamines and their metabolites in the selected brain structures suggests that co-modulation of noradrenergic and dopaminergic transmission in Parkinson's disease by the combined therapy with desipramine + L-DOPA may have some positive implications for motor and psychiatric functions but further research is needed to exclude potential negative effects.

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.

Genetic Markers for Later Remission in Response to Early Improvement of Antidepressants

Planning subsequent treatment strategies based on early responses rather than waiting for delayed antidepressant action can be helpful. We identified genetic markers for later non-remission in patients exhibiting poor early improvement using whole-exome sequencing data of depressive patients treated in a naturalistic manner. Among 1000 patients, early improvement at 2 weeks (reduction in Hamilton Depression Rating Scale [HAM-D] score ≥ 20%) and remission at 12 weeks (HAM-D score ≤ 7) were evaluated. Gene- and variant-level analyses were conducted to compare patients who did not exhibit early improvement and did not eventually achieve remission (n = 126) with those who exhibited early improvement and achieved remission (n = 385). Genes predicting final non-remission in patients who exhibited poor early improvement (COMT, PRNP, BRPF3, SLC25A40, and CGREF1 in males; PPFIBPI, LZTS3, MEPCE, MAP1A, and PFAS in females; ST3GAL5 in the total population) were determined. Among the significant genes, variants in the PRNP (rs1800014), COMT (rs6267), BRPF3 (rs200565609), and SLC25A40 genes (rs3213633) were identified. However, interpretations should be made cautiously, as complex pharmacotherapy involves various genes and pathways. Early detection of poor early improvement and final non-remission based on genetic risk would be helpful for decision-making in a clinical setting.

In vivo electrophysiological recordings of the effects of antidepressant drugs

Antidepressant drugs are a standard biological treatment for various neuropsychiatric disorders, yet relatively little is known about their electrophysiologic and synaptic effects on mood systems that set moment-to-moment emotional tone. In vivo electrical recording of local field potentials (LFPs) and single neuron spiking has been crucial for elucidating important details of neural processing and control in many other systems, and yet electrical approaches have not been broadly applied to the actions of antidepressants on mood-related circuits. Here we review the literature encompassing electrophysiologic effects of antidepressants in animals, including studies that examine older drugs, and extending to more recently synthesized novel compounds, as well as rapidly acting antidepressants. The existing studies on neuromodulator-based drugs have focused on recording in the brainstem nuclei, with much less known about their effects on prefrontal or sensory cortex. Studies on neuromodulatory drugs have moreover focused on single unit firing patterns with less emphasis on LFPs, whereas the rapidly acting antidepressant literature shows the opposite trend. In a synthesis of this information, we hypothesize that all classes of antidepressants could have common final effects on limbic circuitry. Whereas NMDA receptor blockade may induce a high powered gamma oscillatory state via direct and fast alteration of glutamatergic systems in mood-related circuits, neuromodulatory antidepressants may induce similar effects over slower timescales, corresponding with the timecourse of response in patients, while resetting synaptic excitatory versus inhibitory signaling to a normal level. Thus, gamma signaling may provide a biomarker (or “neural readout”) of the therapeutic effects of all classes of antidepressants.

α2-Adrenoceptors in the treatment of major neuropsychiatric disorders

Presynaptic autoreceptors mediate a retrograde transfer of information by a negative feedback mechanism mediated by the transmitter of the neuron, and fulfill an autoregulatory function in neurotransmission in the peripheral and central nervous system (CNS). Starting with norepinephrine (NE), it was later reported that an autoreceptor-mediated negative feedback mechanism exists for other neurotransmitters, including dopamine (DA), serotonin, acetylcholine, histamine, GABA, and glutamate. This feedback mechanism regulates calcium-dependent transmitter release and synthesis through terminal presynaptic autoreceptors, while the firing rate of the neuron is regulated through somatodendritic autoreceptors.

Modulation of OCT3 expression by stress, and antidepressant-like activity of decynium-22 in an animal model of depression

The organic cation transporter-3 (OCT3) is a glucocorticoid-sensitive uptake mechanism that has been shown to regulate the bioavailability of monoamines in brain regions that are implicated in the pathophysiology of depression. In the present study, the relative impacts of acute stress alone and acute stress with a history of repeated stress (chronic+acute) were evaluated in two strains of rats: the stress-vulnerable Wistar-Kyoto (WKY) strain and the somewhat more stress-resilient Long-Evans (LE) strain. OCT3 mRNA was significantly upregulated in the hippocampus of LE rats 2h after exposure to acute restraint stress, but not in acutely-restrained rats with a history of repeated social defeat stress. WKY rats exhibited a very different pattern. OCT3 mRNA was unaffected by acute restraint stress alone but was robustly upregulated after repeated+acute stress. There was also a corresponding increase in cytosolic OCT3 protein following repeated+acute stress in WKY rats 3h after presentation of the acute stressor. These results are consistent with the hypothesis that altered expression of the OCT3 may play a role in stress coping, and strain differences in regulation of this expression may contribute to differences in physiological and behavioral responses to stress. Furthermore, the OCT3 inhibitor, decynium 22 (1 and 10μg/kg, i.p.) reduced immobility of WKY rats, but not that of LE rats, in the forced swim test, suggesting that blockade of the OCT3 has antidepressant-like effects. Since WKY rats also appear to be resistant to the behavioral effects of traditional antidepressants, this also suggests that OCT3 antagonism may be an alternative therapeutic strategy for the treatment of depression in individuals who do not respond to conventional antidepressants.

α2 adrenergic receptor dysregulation in depressive disorders: implications for the neurobiology of depression and antidepressant therapy

Dysfunction in noradrenergic neurotransmission has long been theorized to occur in depressive disorders. The α2 adrenergic receptor (AR) family, as a group of key players in regulating the noradrenergic system, has been investigated for involvement in the neurobiology of depression and mechanisms of antidepressant therapies. However, a clear picture of the α2ARs in depressive disorders has not been established due to the existence of apparently conflicting findings in the literature. In this article, we report that a careful accounting of methodological differences within the literature can resolve the present lack of consensus on involvement of α2ARs in depression. In particular, the pharmacological properties of the radioligand (e.g. agonist versus antagonist) utilized for determining receptor density are crucial in determining study outcome. Upregulation of α2AR density detected by radiolabeled agonists but not by antagonists in patients with depressive disorders suggests a selective increase in the density of high-affinity conformational state α2ARs, which is indicative of enhanced G protein coupling to the receptor. Importantly, this high-affinity state α2AR upregulation can be normalized with antidepressant treatments. Thus, depressive disorders appear to be associated with increased α2AR sensitivity and responsiveness, which may represent a physiological basis for the putative noradrenergic dysfunction in depressive disorders. In addition, we review changes in some key α2AR accessory proteins in depressive disorders and discuss their potential contribution to α2AR dysfunction.

An extension of hypotheses regarding rapid-acting, treatment-refractory, and conventional antidepressant activity of dextromethorphan and dextrorphan

It was previously hypothesized that dextromethorphan (DM) and dextrorphan (DX) may possess antidepressant properties, including rapid and conventional onsets of action and utility in treatment-refractory depression, based on pharmacodynamic similarities to ketamine. These similarities included sigma-1 (σ(1)) agonist and NMDA antagonist properties, calcium channel blockade, muscarinic binding, serotonin transporter (5HTT) inhibition, and μ receptor potentiation. Here, six specific hypotheses are developed in light of additional mechanisms and evidence. Comparable potencies to ketamine for DM and DX are detailed for σ(1) (DX>DM>ketamine), NMDA PCP site (DX>ketamine>DM), and muscarinic (DX>ketamine>>>>DM) receptors, 5HTT (DM>DX≫ketamine), and NMDA antagonist potentiation of μ receptor stimulation (DM>ketamine). Rapid acting antidepressant properties of DM include NMDA high-affinity site, NMDR-2A, and functional NMDR-2B receptor antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation, μ potentiation, and 5HTT inhibition), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, μ potentiation, and 5HTT inhibition), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Those for dextrorphan include NMDA high-affinity site and NMDR-2A antagonism, σ(1) stimulation, putative mTOR activation (by σ(1) stimulation and ß adrenoreceptor stimulation), putative AMPA receptor trafficking (by mTOR activation, PCP antagonism, σ(1) stimulation, ß stimulation, and μ antagonism), and dendritogenesis, spinogenesis, synaptogenesis, and neuronal survival by NMDA antagonism and σ(1) and mTOR signaling. Conventional antidepressant properties for dextromethorphan and dextrorphan include 5HTT and norepinephrine transporter inhibition, σ(1) stimulation, NMDA and PCP antagonism, and possible serotonin 5HT1b/d receptor stimulation. Additional properties for dextromethorphan include possible presynaptic α(2) adrenoreceptor antagonism or postsynaptic α(2) stimulation and, for dextrorphan, ß stimulation and possible muscarinic and μ antagonism. Treatment-refractory depression properties include increased serotonin and norepinephrine availability, PCP, NMDR-2B, presynaptic alpha-2 antagonism, and the multiplicity of other antidepressant receptor mechanisms. Suggestions for clinical trials are provided for oral high-dose dextromethorphan and Nuedexta (dextromethorphan combined with quinidine to block metabolism to dextrorphan, thereby increasing dextromethorphan plasma concentrations). Suggestions include exclusionary criteria, oral dosing, observation periods, dose-response approaches, and safety and tolerability are considered. Although oral dextromethorphan may be somewhat more likely to show efficacy through complementary antidepressant mechanisms of dextrorphan, a clinical trial will be more logistically complex than one of Nuedexta due to high doses and plasma level variability. Clinical trials may increase our therapeutic armamentarium and our pharmacological understanding of treatment-refractory depression and antidepressant onset of action.

Desipramine selectively potentiates norepinephrine-elicited ERK1/2 activation through the α2A adrenergic receptor

The precise physiological effects of antidepressant drugs, and in particular their actions at non-monoamine transporter targets, are largely unknown. We have recently identified the tricyclic antidepressant drug desipramine (DMI) as a direct ligand at the α(2A) adrenergic receptor (AR) without itself driving heterotrimeric G protein/downstream effector activation [5]. In this study, we report our novel finding that DMI modulates α(2A)AR signaling in response to the endogenous agonist norepinephrine (NE). DMI acted as a signaling potentiator, selectively enhancing NE-induced α(2A)AR-mediated ERK1/2 MAPK signaling. This potentiation of ERK1/2 activation was observed as an increase in NE response sensitivity and a prolongation of the activation kinetics. DMI in a physiologically relevant ratio with NE effectively turned on ERK1/2 signaling that is lacking in response to physiological NE alone. Further, the DMI-induced ERK1/2 potentiation relied on heterotrimeric G(i/o) proteins and was arrestin-independent. This modulatory effect of DMI on NE signaling provides novel insight into the effects of this antidepressant drug on the noradrenergic system which it regulates, insight which enhances our understanding of the therapeutic mechanism for DMI.

Antidepressant effects of estrogens: a basic approximation

The use of estrogenic compounds as antidepressants or as coadjuvants to facilitate the effect of antidepressants has reported controversial results, suggesting that many factors could influence their actions. This review analyzes, from a basic research perspective, the possible factors that may underlie the antidepressant action of estrogens alone or in combination. The possible mechanisms of action of estrogens alone and in combination with the selective serotonin reuptake inhibitor, fluoxetine, the selective noradrenaline reuptake inhibitor, desipramine, and the mixed serotonin/noradrenaline reuptake inhibitor, venlafaxine are reviewed, focusing on monoaminergic systems and estrogen receptors as main targets. The antidepressant effect of estrogens depends on the type of estrogen, treatment duration, doses, sex, time after ovariectomy, and age. Estrogens potentiate the antidepressant-like action of fluoxetine, venlafaxine, and desipramine and drastically shorten their latency of action. The antidepressant-like effect of estrogens alone or in combination with antidepressants seems to be mediated by monoaminergic and classic estrogen receptors, as WAY100635, an antagonist to the serotonin 1A receptor, idaxozan, an antagonist to alpha2 adrenergic receptors, and RU 58668, an estrogen receptor antagonist, blocked their antidepressant-like effect. In conclusion, estrogens produce antidepressant-like actions by themselves and importantly facilitate the action of clinically used antidepressants.

A randomized controlled trial of imipramine in patients with irritable bowel syndrome

AIM: To study the efficacy of low-dose imipramine in relieving symptoms associated with the irritable bowel syndrome (IBS).

METHODS: A randomized, double-blind trial of 25 mg imipramine vs matched placebo for 12 wk was performed. Doubling the dose was allowed once at week 2 in case of an unsatisfactory early response. Primary efficacy variables were subjective global symptom relief and quality of life (QoL) using SF-36 at week 12.

RESULTS: One hundred and seven patients were enrolled by advertisement or referral by general practitioners and 56 (31 imipramine: 25 placebo) completed the 16-wk study. Baseline characteristics were comparable. A high overall dropout rate was noted in the imipramine and placebo arms (47.5% vs 47.9%, P > 0.05), a mean of 25.0 and 37.4 d from enrollment, respectively (P < 0.05). At the end of 12 wk, there was a significant difference in global symptom relief with imipramine over placebo (per-protocol: 80.6% vs 48.0%, P = 0.01) and a trend on intent-to-treat (ITT) analysis (42.4% vs 25.0%, P = 0.06). This improvement was evident early and persisted to week 16 (P = 0.024 and 0.053 by per-protocol and ITT analyses, respectively). Mean cumulative and component-specific SF-36 scores improved in the imipramine group only (per-protocol, P < 0.01). Drug-related adverse events leading to patient dropout were more common in the imipramine group (25.4% vs 12.5%, P > 0.05).

CONCLUSION: Imipramine may be effective in the treatment of IBS patients and is associated with improved QoL. Careful patient selection, initiation of a low dose with gradual escalation and monitoring for side effects may result in an improved therapeutic response.

Antidepressant drugs with differing pharmacological actions decrease activity of locus coeruleus neurons

Previous studies suggest that all effective antidepressant (AD) drugs decrease activity of locus coeruleus (LC) neurons. However, little data exist regarding blood levels of drug in these studies, and what data do exist suggest blood levels might have been very high. To assess whether decreased LC activity is produced by drugs that selectively block reuptake for either norepinephrine or serotonin at therapeutically relevant blood levels, effects of chronic administration of desipramine, paroxetine, and escitalopram on LC activity were measured across a range of doses and blood levels of drug. Further, effects of a range of doses of mirtazapine were examined; in that mirtazapine blocks alpha2 adrenergic receptors, it might be anticipated to increase rather than decrease LC activity. Finally, to begin to assess whether the response of LC to ADs was specific to these drugs, effects of four non-AD drugs (single dose) were measured. Drugs were administered via osmotic minipump for 14 d. Electrophysiological recording of LC activity (assessment of both spontaneous firing rate and sensory-evoked 'burst' firing) then took place under isoflurane anaesthesia on the last day of drug treatment. The blood level of drugs present at the end of the recording session was also measured. All AD drugs tested decreased LC spontaneous and sensory-evoked 'burst' firing, and this was observed across a wide range of blood levels for the drugs. Non-AD drugs did not decrease LC activity. The findings of this investigation continue to support the possibility that all effective AD drugs decrease LC activity.

Reduced stress-sensitivity or increased reward experience: the psychological mechanism of response to antidepressant medication

Depression has often been associated with increased negative affect reactivity to stress (Stress-Sensitivity) and reduced capacity to experience pleasure or positive affect (Reward Experience). To date, no studies have prospectively examined changes in Stress-Sensitivity and Reward Experience following antidepressant treatment. The sample included 83 depressed patients and 22 healthy controls. A randomized controlled trial was carried out with patients receiving either imipramine or placebo for 6 weeks. At baseline and 6 weeks, patients and controls participated in an Experience Sampling procedure, prospectively measuring ecologically valid daily life appraisals of activities and mood states. The course of depression was assessed with the Hamilton Depression Rating Scale (HDRS). Multilevel linear regression analyses showed that patients had higher negative and lower positive appraisals of activities than controls. In addition, patients showed increased Stress-Sensitivity (negative affect reactivity to negatively appraised activities). Treatment with imipramine decreased Stress-Sensitivity and increased Reward Experience (positive affect reactivity to positively appraised activities). Changes in Stress-Sensitivity and Reward Experience were in part reducible to changes in the process of activity appraisal itself. However, increase in Reward Experience, but not decrease in Stress-Sensitivity, discriminated between patients who responded and those who did not, independent of changes in the process of activity appraisal itself. Response to treatment in depression may be conditional on restoration of hedonic capacity, the cerebral substrate of which requires further study in relation to antidepressant response. A search for (synergistic) antidepressant therapies specifically targeting ability to experience reward may be warranted.

Postsynaptic alpha-2 adrenergic receptors are critical for the antidepressant-like effects of desipramine on behavior

The antidepressant desipramine inhibits the reuptake of norepinephrine (NE), leading to activation of both pre- and postsynaptic adrenergic receptors, including alpha-1, alpha-2, beta-1, and beta-2 subtypes. However, it is not clear which adrenergic receptors are involved in mediating its antidepressant effects. Treatment of mice with desipramine (20 mg/kg, i.p.) produced an antidepressant-like effect, as evidenced by decreased immobility in the forced-swim test; this was antagonized by pretreatment with the alpha-2 adrenergic antagonist idazoxan (0.1-2.5 mg/kg, i.p.). Similarly, idazoxan, administered peripherally (0.5-2.5 mg/kg, i.p.) or centrally (1-10 microg, i.c.v.), antagonized the antidepressant-like effect of desipramine in rats responding under a differential-reinforcement-of-low-rate (DRL) 72-s schedule, ie, decreased response rate and increased reinforcement rate. By contrast, pretreatment with the beta-adrenergic antagonists propranolol and CGP-12177 or the alpha-1 adrenergic antagonist prazosin did not alter the antidepressant-like effect of desipramine on DRL behavior. The lack of involvement of beta-adrenergic receptors in mediating the behavioral effects of desipramine was confirmed using knockout lines. In the forced-swim test, the desipramine-induced decrease in immobility was not altered in mice deficient in beta-1, beta-2, or both beta-1 and beta-2 adrenergic receptors. In addition, desipramine (3-30 mg/kg) produced an antidepressant-like effect on behavior under a DRL 36-s schedule in mice deficient in both beta-1 and beta-2 adrenergic receptors. As antagonism of presynaptic alpha-2 adrenergic receptors facilitates NE release, which potentiates the effects of desipramine, the present results suggest that postsynaptic alpha-2 adrenergic receptors play an important role in its antidepressant effects.

Enhanced norepinephrine output during long-term desipramine treatment: a possible role for the extraneuronal monoamine transporter (SLC22A3)

To study the delay (2-6 weeks) between initial administration of norepinephrine reuptake inhibitor antidepressants and onset of clinical antidepressant action, we examined the effects of desipramine treatment on urinary and plasma catecholamines and their metabolites during the initial 6 weeks of treatment in depressed patients. Catecholamines and metabolites in 24-h urine collections and 8:00 a.m. plasma samples were measured at baseline and after 1, 4, and 6 weeks of desipramine treatment. Desipramine treatment produced significant increases in urinary norepinephrine (NE) and normetanephrine (NMN) and plasma NE at Weeks 4 and 6, but not at Week 1. The ratio of urinary NE/NMN was increased at Weeks 4 and 6, suggesting a reduction in the metabolism of NE to NMN at extraneuronal sites by Weeks 4 and 6. The increases in urinary NE and NMN and plasma NE at Weeks 4 and 6 of desipramine treatment were associated with a reduction in the conversion of NE to NMN. This would be compatible with a blockade of the extraneuronal monoamine transporter (organic cation transporter 3; SLC22A3) by NMN. Inhibition of the extraneuronal monoamine transporter may be an important component in the clinical pharmacology of the norepinephrine reuptake inhibitor antidepressant drugs, such as desipramine.

Blockade of autoreceptor-mediated inhibition of norepinephrine release by atipamezole is maintained after chronic reuptake inhibition

The role of alpha(2)-adrenergic autoreceptor desensitization in the delayed onset of antidepressant efficacy of selective norepinephrine (NE) reuptake inhibitors is unclear. Using the alpha(2)-antagonist yohimbine, we showed previously that chronic treatment with desipramine (DMI) did not alter autoreceptor-mediated inhibition of NE release in the cortex. However, yohimbine may have non-selective effects that could confound this interpretation. Thus, using microdialysis, we measured acute effects of the highly selective alpha(2)-antagonist atipamezole on NE release in the prefrontal cortex following chronic DMI treatment, after 0-8 d washout. Atipamezole induced a similar elevation of extracellular NE in all treatment groups, indicating no change in autoreceptor function. Further, the effect was most rapid in DMI-treated rats with 0- and 2-d washout, suggesting that autoreceptor-mediated inhibition was most prominent when NE levels were highest. This provides further evidence that autoreceptor-mediated inhibition of NE neurotransmission remains functional after chronic DMI treatment, arguing against the hypothesis that desensitization of alpha(2)-autoreceptors accounts for the delayed onset of action of selective NE reuptake inhibitors.

Behavior-dependent modulation of hippocampal EEG activity by the selective norepinephrine reuptake inhibitor reboxetine in rats

Both active wakefulness and rapid eye movement sleep (REM) give rise to rhythmic synchronized hippocampal field oscillations, known as theta activity. Antidepressant drugs, including norepinephrine re-uptake inhibitors are proven to diminish REM sleep, and REM sleep-related hippocampal theta oscillation. Since reboxetine, a highly selective norepinephrine re-uptake inhibitor has been shown to block REM sleep, but induce or facilitate hippocampal theta activity in anesthetized rats, the current study investigated the effects of reboxetine on stage- and behavior dependent theta activity. Polysomnographic recordings, which included hippocampal field potentials at the hippocampal fissure, were carried out in rats for 8 h during the light phase of the circadian cycle. Theta rhythm was analyzed during three different behavioral conditions: REM sleep, during motor activity in a familiar environment, and during exploration in a novel environment. We found that, compared with REM sleep, theta power was relatively low during periods of active wakefulness when the animal was in the familiar home cage, but considerably increased during exploration in a novel environment. Reboxetine suppressed sleep and thus abolished REM sleep-related hippocampal theta rhythm, attenuated theta in the familiar environment, and significantly enhanced theta oscillations associated with exploratory behavior. Our findings demonstrate a state- and behavior-dependent modulation of hippocampal theta activity by reboxetine, providing further evidence for a prominent role of norepinephrine in arousal and focused or selective attention.

Associations of a regulatory polymorphism of monoamine oxidase-A gene promoter (MAOA-uVNTR) with symptoms of depression and sleep quality

OBJECTIVE

To examine the relationships among the variable number of tandem repeats in the monoamine oxidase-A linked polymorphic region allelic variation (MAOA-uVNTR) and the symptoms of depression and sleep quality. The monoamine oxidase-A (MAOA) gene, which plays a vital role in degradation of neurotransmitters such as serotonin, norepinephrine, and dopamine, contains a polymorphism in its promoter region (MAOA-uVNTR) that affects transcriptional efficiency. MAOA-uVNTR genotype has been associated with both psychological and physical measures.

METHODS

The sample consisted of 74 males enrolled in a case/control study of caregivers for relatives with dementia. Age- and race-adjusted linear regression models were used to examine the association between low versus high MAOA-uVNTR activity alleles, symptoms of depression (Center for Epidemiological Studies of Depression), and sleep quality ratings (Pittsburgh Sleep Quality Index).

RESULTS

MAOA-uVNTR alleles associated with less transcriptional activity were related to increased symptoms of depression (p < .04; Cohen's d = 0.52) and poorer sleep quality (p < .04; Cohen's d = 0.31).

CONCLUSIONS

Individuals with less active MAOA-uVNTR alleles may be at increased risk for depressive symptoms and poor sleep.

In Vivo Effect of Venlafaxine on Locus Coeruleus Neurons: Role of Opioid, α2-Adrenergic, and 5-Hydroxytryptamine1A Receptors

The locus coeruleus (LC) is involved in several neural pathways responsible for some somatic and emotional processes, such as pain and depression; its activity is regulated by several receptors, such as opioid, alpha(2)-adrenergic, and 5-hydroxytryptamine (5-HT)(1A) receptors. The present study investigates the in vivo effects of venlafaxine, an antidepressant with analgesic properties, on locus coeruleus neurons, and its modulation by opioid, alpha(2)-adrenergic, and 5-HT(1A) receptors. The results show that acute administration of venlafaxine produced a dose-dependent, complete inhibition of LC activity. This inhibitory effect was not reversed by the opioid receptor antagonist naloxone, but subsequent administration of idazoxan, an alpha(2)-adrenoceptor antagonist, did reverse it. The preadministration of the 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) (1 and 40 microg/kg) significantly enhanced the venlafaxine inhibitory effect, decreasing the ED(50) by 56 and 44%, respectively. A 14-day treatment with venlafaxine (40 mg/kg/day) induced a suppression of the firing activity of LC neurons. In these treated animals, venlafaxine produced an inhibitory effect similar to that in nontreated animals. This inhibitory effect was not reversed by naloxone, but it was reversed by idazoxan. In addition, the preadministration of 8-OH-DPAT (40 microg/kg) significantly enhanced the venlafaxine effect, decreasing the ED(50) by 60%. These results suggest that the effect of venlafaxine on LC neurons is modulated by alpha(2)-adrenergic and 5-HT(1A) receptors, and not by opioid receptors. These data could contribute to the further understanding of the antidepressant and analgesic mechanism of action of venlafaxine.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Idazoxan increases perforant path-evoked EPSP slope paired pulse inhibition and reduces perforant path-evoked population spike paired pulse facilitation in rat dentate gyrus

Norepinephrine, acting via beta-adrenoceptors, enhances the perforant path-evoked potential in dentate gyrus. Using systemic idazoxan to increase norepinephrine, and paired perforant path pulses to probe early inhibition, previous investigators reported that idazoxan increased initial spike amplitude and increased somatic feedback inhibition. Here, feedback inhibition was re-examined in idazoxan-treated (5 mg/kg) rats under urethane anesthesia. To control for initial increased spike amplitude after idazoxan, evoked potentials were matched, pre- and post-idazoxan, on initial population spike. Input-output current profiles were also compared pre- and post-idazoxan. Saline- and timolol-filled micropipettes permitted evaluation of a contribution of local beta-adrenoceptors. As previously observed, initial spike amplitude was potentiated by idazoxan. Comparable spike potentiation was not seen on the timolol micropipette. Paired pulse inhibition of spike amplitude apparently increased, but input-output curve comparisons revealed a loss of feedback facilitation rather than an increase in feedback inhibition. Initial EPSP slopes were depressed after idazoxan in input-output curve data. EPSP slope feedback ratios were significantly reduced following idazoxan. These data suggest idazoxan has multiple effects on perforant path input to the dentate gyrus. Spike potentiation following idazoxan has previously been shown to depend on intact norepinephrine input. Here, the reduction in spike potentiation on the timolol pipette is consistent with other evidence that norepinephrine-mediated potentiation of the perforant path-evoked potential is dependent on local beta-adrenoceptor activation. The input-output data suggest a decrease in feedback facilitation after idazoxan is likely to account for the apparent increase in feedback inhibition previously reported. Decreased EPSP slope ratios with similar paired pulse intervals have been reported in novel environments. Since exposure to novel environments activates locus coeruleus neurons, norepinephrine may mediate the change in EPSP slope inhibition reported in awake rats. In summary, these results are consistent with the hypothesis that idazoxan potentiates granule cell responses to perforant path input in the dentate gyrus via increases in norepinephrine that lead to beta-adrenoceptor activation, and, further, that idazoxan reduces paired pulse feedback spike facilitation and enhances EPSP slope, but not spike, feedback inhibition.

Comparing sertindole to other new generation antipsychotics on preferential dopamine output in limbic versus striatal projection regions: Mechanism of action

The effects of acute administration of sertindole on DA output were examined in the shell part of the nucleus accumbens (NACS) and the striatum (STR), areas which are associated with limbic functions and motor control, respectively, by using in vivo differential normal pulse voltammetry in rats. The effect of sertindole was compared to those obtained with the reference antipsychotic drugs clozapine and haloperidol, new generation antipsychotics represented by risperidone, olanzapine, ziprasidone, quetiapine, and aripiprazole, as well as, with those of preferential D2/3, D4, 5-HT1A, 5-HT2A, 5-HT2C, alpha1, and alpha2 receptor ligands. In similarity with the new generation antipsychotics, sertindole preferentially increase DA output in the NACS as compared to the STR whereas the opposite was true for haloperidol. The regional specific effect of the partial D2 receptor agonist aripiprazole was mainly driven by a decrease in striatal rather that by an increase in accumbal DA output. The selective 5-HT2A and D4 receptor antagonists MDL100,151 and Lu 38-012, respectively, both preferentially increased DA output in the NACS. Thus, the present results are in line with the hypothesis that 5-HT2A receptor antagonism is of importance for the observed limbic selectivity of new generation antipsychotics and, in turn, to their favorable clinical profile especially as regards extrapyramidal side effects (EPS) liability. For some compounds, blockade of D4 receptors may also play a role in this regard.

Idazoxan activates rat forebrain glycogen phosphorylase in vivo: A histochemical study

In vitro experiments show norepinephrine activates glycogen phosphorylase and glycogenolysis in forebrain glia. The present study used idazoxan (5 mg/kg) to elevate NE in vivo and examined patterns of active (aGP) and total (tGP) glycogen phosphorylase reactivity in selected neocortical, hippocampal, diencephalic, and striatal sites using a histochemical method. In somatosensory neocortex, aGP reactivity was highest in Layer 4 with consistent reactivity in the barrel fields in vehicle-treated brains. In the hippocampus, the stratum lacunosum moleculare was highly reactive, while cell layers were least reactive. The dentate gyrus and CA3 were more reactive for aGP than CA1. In the diencephalon, the medial habenula was most reactive followed by the reticular nucleus of the thalamus. In the striatum, globus pallidus was most reactive. Reactivity patterns for tGP were similar to those for aGP, but more intense. The neocortex had the highest overall reactivity for tGP. An estimate of the percentage of aGP relative to tGP suggested the regions sampled had similar levels of median basal activation (approximately 65%). Idazoxan increased aGP reactivity in all regions of the neocortex assessed (layers 3-6 of primary and secondary somatosensory cortex and the barrel fields). The neuropil layers, but not the cell layers, of hippocampus were more reactive following idazoxan treatment. Idazoxan also increased aGP reactivity in the laterodorsal, paraventricular, and reticular nuclei of the thalamus. The largest idazoxan-induced changes, as an estimated percentage of tGP, occurred in the hippocampus (approximately 16% for stratum lacunosum moleculare and for CA1 stratum oriens). Increases ranged from approximately 3 to 6% in neocortex and were less than 3% in the diencephalic and striatal areas. These effects of idazoxan are consistent with a role for norepinephrine in activating forebrain glycogenolyis in vivo and supporting increased brain metabolism. They contrast with earlier evidence showing that idazoxan reduces 2-deoxyglucose uptake in these brain areas. Idazoxan, and norepinephrine, may preferentially recruit glycolytic over oxidative metabolism in the rat forebrain.

The catechol-O-methyltransferase Val108/158Met polymorphism affects short-term treatment response to mirtazapine, but not to paroxetine in major depression

The catechol-O-methyltransferase (COMT) is a major degrading enzyme in the metabolic pathways of catecholaminergic neurotransmitters such as dopamine and norepinephrine. This study investigated whether the functionally relevant Val(108/158)Met gene variant is associated with differential antidepressant response to mirtazapine and/or paroxetine in 102 patients with major depression (DSM-IV criteria) participating in a randomized clinical trial with both drugs. In patients treated with mirtazapine, but not paroxetine, allelic variations in the COMT gene were associated with differential response. COMT(VAL/VAL) and COMT(VAL/MET) genotype carriers showed a better response than COMT(MET/MET)-bearing patients in the mirtazapine group. Moreover, carriers of the COMT(VAL/VAL) or COMT(VAL/MET) genotype had significantly greater HAMD-17 (Hamilton Rating Scale for Depression 17 item version) score reductions than COMT(MET/MET) homozygotes from week 2 to 6, respectively, in the mirtazapine group. Time course of response and antidepressant efficacy of mirtazapine, but not paroxetine, seem to be influenced in a clinically relevant manner by this allelic variation within the COMT gene.

The role of alpha1- and alpha2-adrenoreceptors on venlafaxine-induced elevation of extracellular serotonin, noradrenaline and dopamine levels in the rat prefrontal cortex and hippocampus

The role of adrenergic alpha1- and alpha2-adrenoreceptors in augmentation of venlafaxine-induced elevation of extracellular serotonin (5-HT),noradrenaline (NA) and dopamine (DA) levels in the rat prefrontal cortex (PFC) and hippocampus (HIPP) was studied by in vivo microdialysis in anaesthetized rats. The alpha1-adrenoreceptor antagonist prazosin given alone (0.3 mg/kg, s.c.) induced only a moderate reduction of hippocampal 5-HT and NA levels. The alpha2-adrenoreceptor antagonist idazoxan (1.5 mg/kg, s.c.) causes moderate increases in the levels of 5-HT and DA in the PFC. The mixed 5-HT and NA reuptake inhibitor venlafaxine (10 mg/kg, i.p.) increased the efflux of 5-HT, NA and DA almost equally, to approximately 200% of the control levels in the PFC. The levels of 5-HT increased to 310%, an effect approximately twice the effect on NA in the HIPP. Venlafaxine also produced a moderate increase in DA levels in the PFC but had no effect in the HIPP. Pre-treatment with prazosin caused a significant attenuation of the venlafaxine induced 5-HT effect in the PFC, and a moderate increase in DA levels in the HIPP. Prazosin had no significant effect on the venlafaxine-induced increase of the NA levels in PFC or HIPP. A combined treatment of venlafaxine with idazoxan increased the venlafaxine NA and DA effects in PFC by a factor of two and resulted in a very robust five-fold augmentation of NA and DA concentrations in the HIPP. In summary, idazoxan was found to produce a potent enhancement of the venlafaxine effect to increase extracellular NA and DA levels in the PFC and, in particular, in the HIPP. Idazoxan had no effect on venlafaxine-induced elevation of extracellular 5-HT levels in either PFC or HIPP and prazosin induced a decrease of 5-HT in the PFC. The present data suggest that blockade of alpha2-adrenoreceptors may play an important role in augmentation of the effects of mixed monoamine reuptake inhibitors.

Role of presynaptic alpha2-adrenoceptors in antidepressant action: recent findings from microdialysis studies

The therapeutic effect of an antidepressant drug takes at least 2 to 3 weeks to develop and a significant proportion of patients have no or only partial benefit regardless of the class of antidepressant used. Research into the neurobiological basis of antidepressant action has suggested new strategies to improve the antidepressant effect. Recent microdialysis studies show that hypofunction of the presynaptic autoreceptors enhances the increase of extracellular serotonin (5-HT) induced by selective serotonin reuptake inhibitors (SSRIs) so it has been suggested that the antidepressant effect may be speeded up by blockade of the autoreceptors. The similarity between the synaptic mechanisms controlling serotonergic and noradrenergic transmission has stimulated preclinical research into the role of presynaptic alpha(2)-adrenoceptors in the effect of noradrenaline (NA) reuptake inhibitors (NRIs) on NA availability at central synapses. The microdialysis studies reviewed here indicate that NRIs including desipramine, reboxetine and atomoxetine, the mixed 5-HT/NA reuptake inhibitors sibutramine, duloxetine, venlafaxine or the NA/DA reuptake inhibitor amineptine, increased extracellular NA in various regions of the rat brain. The effect was enhanced by chronic treatment and even more by the co-administration of alpha(2)-adrenoceptor antagonists. The results support the theory that desensitization of the alpha(2)-adrenoceptor contributes to enhancing the effect of NRIs seen after chronic administration and may account for the slow onset of the antidepressant effect. Finally, they suggest that co-administration of an alpha(2)-adrenoceptor antagonist may improve the therapeutic effect of NRI.

New insights into the mechanisms of antidepressant therapy

Depressive disorders are among the most frequent psychiatric diseases in the Western world with prevalence numbers between 9% and 18%. They are characterized by depressed mood, a diminished interest in pleasurable activities, feelings of worthlessness or inappropriate guilt, decrease in appetite and libido, insomnia, and recurrent thoughts of death or suicide. Among other findings, reduced activity of monoaminergic neurotransmission has been postulated to play a role in the pathogenesis of depression. Consistent with this hypothesis, most antidepressive drugs exert their action by elevating the concentration of monoamines in the synaptic cleft. However, it is not the enhancement of monoaminergic signaling per se, but rather long-term, adaptive changes that may underlie the therapeutic effect. These include functional and structural changes that are discussed later. In addition, in the last years, evidence has emerged that remissions induced in patients using lithium or electroconvulsive therapy are accompanied by structural changes in neuronal networks thereby affecting synaptic plasticity in various regions of the brain.

Evidence that the metabotropic glutamate receptor 5 antagonist MPEP may act as an inhibitor of the norepinephrine transporter in vitro and in vivo

The mechanisms through which blockade of metabotropic glutamate receptors 5 (mGluR5) results in anxiolytic and antidepressant effects are currently unknown. In the present study, we therefore hypothesized that the anxiolytic- and antidepressant-like profile of the noncompetitive mGluR5 receptor antagonist 2-ethyl-6-(phenylethynyl)-pyridine (MPEP) may be mediated by inhibition of the norepinephrine transporter (NET). Accordingly, we first examined the potency of MPEP to bind to or inhibit uptake at the NET as well as the dopamine and serotonin transporters (DAT and SERT, respectively). We also examined the simultaneous in vivo effects of MPEP and desipramine (DMI) on both NE-like oxidation current in the amygdala (AMY) and cell firing in the locus coeruleus (LC) by means of differential pulse voltammetry (DPV) coupled with electrophysiology. MPEP completely displaced the binding of [3H]-nisoxetine on human NET with a pKi of 6.63 +/- 0.02. In addition, MPEP was able to inhibit [3H]-NE uptake in LLCPK cells expressing human NET, with a pIC50 of 5.55 +/- 0.09. In vivo DPV data revealed that both MPEP (30 mg/kg i.p.) and DMI (10 mg/kg i.p.) significantly increased NE-like voltammetric responses levels in the AMY, whereas both compounds also significantly decreased cell firing monitored concomitantly from the second microelectrode in the LC. Collectively, the results of the present study provide potential new mechanisms through which MPEP exerts its anxiolytic and antidepressant effects.

Cortical alpha-adrenoceptor downregulation by tricyclic antidepressants in the rat brain

The aim of the present study was to examine the effect of chronic tricyclic antidepressants (TCAs) treatment on the density of alpha-adrenoceptors in the rat brain. Density of alpha1- and alpha2-adrenoceptors was measured in cortex and hippocampus of rats treated with imipramine (IMI, 5mg/kg body weight), desipramine (DMI, 10mg/kg body weight), clomipramine (CMI, 10mg/kg body weight) and amitriptyline (AMI, 10mg/kg body weight), for 40 days, using [3H]prazosin and [3H]clonidine, respectively. The density of cortical alpha1-adrenoceptors was significantly decreased with IMI (46%), DMI (21%), CMI (50%) and AMI (67%) treatment, without altering the affinity of the receptor. The density of cortical alpha2-adrenoceptors was also significantly decreased with DMI (69%), CMI (81%) and AMI (80%) treatment, without affecting the affinity for [3H]clonidine. The density of hippocampal alpha1-adrenoceptors was significantly decreased only with AMI treatment (47%), without affecting the affinity for [3H]prazosin. However, no change in hippocampal alpha2-adrenoceptor density was observed with any of these TCAs. The results suggest that chronic antidepressant (AD) treatment downregulates the cortical, but not hippocampal, alpha1- and alpha2-adrenoceptors in rat brain. The region-specific downregulation of alpha1- and alpha2-adrenoceptors density, which occur after prolonged AD treatment, may underline the therapeutic mechanism of action.

Regulatory effects of reboxetine treatment alone, or following paroxetine treatment, on brain noradrenergic and serotonergic systems

When patients do not respond to an initial antidepressant, one clinical approach is to switch to an agent in a different pharmacological class. However, few studies have examined the neurochemical consequences of this practice. To study this, we examined changes in binding sites in rat brain for norepinephrine (NET) and serotonin transporters (SERT), alpha1, alpha2, and beta1 adrenergic receptors after chronic administration of paroxetine (PRX), reboxetine (RBX), or PRX followed by RBX. We also examined the effects of these treatments on mRNA expression for tyrosine hydroxylase (TH). RBX treatment for 3 weeks reduced NET binding significantly, by approximately 40% in terminal field areas, and 6 weeks of RBX reduced it even more, by approximately 60%. RBX treatment for 3 and 6 weeks reduced beta1 adrenergic receptor-binding sites equally, by 50-60%. At no time did RBX treatment reduce SERT-binding sites. PRX treatment had no effect on beta1 adrenergic or NET-binding sites, but reduced SERT-binding sites by 75-80%. Neither treatment altered mRNA for TH, alpha1, or alpha2 adrenergic receptor-binding sites. When 3 weeks of RBX treatment followed 3 weeks of PRX treatment, NET-binding sites were reduced to the same extent as measured after 6 weeks of RBX treatment alone, indicating that PRX pretreatment may have 'primed' the subsequent regulatory effect of RBX on the NET. Thus, pretreatment of rats with PRX actually enhanced at least one regulatory effect of RBX treatment on the noradrenergic system, and did not interfere with any other pharmacological effect caused by RBX treatment.

The neurobiology and control of anxious states

Fear is an adaptive component of the acute "stress" response to potentially-dangerous (external and internal) stimuli which threaten to perturb homeostasis. However, when disproportional in intensity, chronic and/or irreversible, or not associated with any genuine risk, it may be symptomatic of a debilitating anxious state: for example, social phobia, panic attacks or generalized anxiety disorder. In view of the importance of guaranteeing an appropriate emotional response to aversive events, it is not surprising that a diversity of mechanisms are involved in the induction and inhibition of anxious states. Apart from conventional neurotransmitters, such as monoamines, gamma-amino-butyric acid (GABA) and glutamate, many other modulators have been implicated, including: adenosine, cannabinoids, numerous neuropeptides, hormones, neurotrophins, cytokines and several cellular mediators. Accordingly, though benzodiazepines (which reinforce transmission at GABA(A) receptors), serotonin (5-HT)(1A) receptor agonists and 5-HT reuptake inhibitors are currently the principle drugs employed in the management of anxiety disorders, there is considerable scope for the development of alternative therapies. In addition to cellular, anatomical and neurochemical strategies, behavioral models are indispensable for the characterization of anxious states and their modulation. Amongst diverse paradigms, conflict procedures--in which subjects experience opposing impulses of desire and fear--are of especial conceptual and therapeutic pertinence. For example, in the Vogel Conflict Test (VCT), the ability of drugs to release punishment-suppressed drinking behavior is evaluated. In reviewing the neurobiology of anxious states, the present article focuses in particular upon: the multifarious and complex roles of individual modulators, often as a function of the specific receptor type and neuronal substrate involved in their actions; novel targets for the management of anxiety disorders; the influence of neurotransmitters and other agents upon performance in the VCT; data acquired from complementary pharmacological and genetic strategies and, finally, several open questions likely to orientate future experimental- and clinical-research. In view of the recent proliferation of mechanisms implicated in the pathogenesis, modulation and, potentially, treatment of anxiety disorders, this is an opportune moment to survey their functional and pathophysiological significance, and to assess their influence upon performance in the VCT and other models of potential anxiolytic properties.

Antidepressant-like effect of different estrogenic compounds in the forced swimming test

The present study evaluated the possible antidepressant-like action of the natural estrogen 17beta-estradiol (E(2), 2.5-10 microg/rat), the synthetic steroidal estrogen ethinyl-estradiol (EE(2), 1.25-10.0 microg/rat), and the nonsteroidal synthetic estrogen, diethyl-stilbestrol (DES, 0.25-1.0 mg/rat) in ovariectomized adult female Wistar rats using the forced swimming test (FST). The behavioral profile induced by the estrogens was compared with that induced by the antidepressants fluoxetine (FLX, 2.5-10 mg/kg) and desipramine (DMI, 2.5-10 mg/kg). In addition, the temporal course of the antidepressant-like action of the estrogenic compounds was analyzed. FLX and DMI induced an antidepressant-like effect characterized by a reduced immobility and increased swimming for FLX and decreased immobility and increased climbing for DMI. Both E(2) and EE(2) produced a decrease in immobility and an increase in swimming, suggesting an antidepressant-like action. DES did not affect the responses in this animal model of depression at any dose tested. The time course analysis of the actions of E(2) (10 microg/rat) and EE(2) (5 microg/rat) showed that both compounds induced an antidepressant-like effect observed 1 h after their injection lasting for 2-3 days.

In vivo tonic modulation of the noradrenaline release in the rat cortex by locus coeruleus somatodendritic alpha(2)-adrenoceptors

The regulation of noradrenaline release in the rat cingulate cortex by somatodendritic alpha(2)-adrenoceptors placed in the locus coeruleus was evaluated by dual-probe microdialysis. The alpha(2)-adrenoceptor antagonists BRL44408 (2-[2H-(1-methyl-1,3-dihydroisoindole)methyl]-4,5-dihydroimidazole), RS79948 ((8,12,13)-decahydro-3methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]-naphthyridine) and RX821002 (2-methoxyidazoxan) administered by reverse dialysis into the locus coeruleus increased concentration-dependently (0.01-100 microM) noradrenaline release in the cortex (maximal effects 170+/-30%, 543+/-17%, 195+/-26%, respectively). Administration of the alpha(2)-adrenoceptor antagonist idazoxan increased at lower (0.1-10 microM) but decreased at the highest dose (100 microM) noradrenaline in the cortex. These data demonstrate that somatodendritic alpha(2)-adrenoceptors in the locus coeruleus exert an inhibitory tonic modulation on noradrenaline release in noradrenergic terminal areas.

Acute and chronic effects of desipramine and clorgyline on alpha(2)-adrenoceptors regulating noradrenergic transmission in the rat brain: a dual-probe microdialysis study

1. The effects of desipramine (3 mg kg(-1) i.p.) and clorgyline (1 mg kg(-1) i.p.) on extracellular noradrenaline (NA) in the locus coeruleus (LC) and cingulate cortex were assessed in freely-moving rats by dual-probe microdialysis. Functional activities of alpha(2)-adrenoceptors regulating NA release in the LC and cingulate cortex were determined by systemic (0.3 mg kg(-1) i.p.) or local (0.1 - 100 microM) clonidine administration. 2. Extracellular NA was increased in the LC and cingulate cortex following acute desipramine but not clorgyline treatment. Systemic clonidine decreased NA similarly in desipramine-, clorgyline-, and saline-treated animals, in both brain areas. 3. Long-term (twice daily, 14 days) but not short-term (twice daily, 7 days) desipramine, and long-term clorgyline (once daily, 21 days) treatments increased NA (3 fold) in cingulate cortex but not in the LC. Following long-term treatments, responses of NA to systemic clonidine were attenuated in the LC and cingulate cortex. 4. Clonidine perfusion by reverse dialysis into the cingulate cortex decreased local NA (-55 +/- 9%). The effect was attenuated by long-term desipramine (-31 +/- 9%) and clorgyline (-10 +/- 2%) treatments. 5. Clonidine perfusion by reverse dialysis into the LC decreased NA in the LC (-89 +/- 2%) and in cingulate cortex (-52 +/- 12%). This effect was attenuated in the LC following long-term desipramine (-72 +/- 4%) and clorgyline (-62 +/- 12%) treatments but it was not modified in the cingulate cortex (-57 +/- 10% and -68 +/- 6%, respectively). 6. These findings demonstrate that chronic desipramine or clorgyline treatments increase NA in noradrenergic terminal areas and desensitize alpha(2)-adrenoceptors modulating local NA release at somatodendritic and terminal levels. However, somatodendritic alpha(2)-adrenoceptors that control LC firing activity are not desensitized.