Antagonism of the stress-induced increase in cortical norepinephrine output by the selective norepinephrine reuptake inhibitor reboxetine

We have previously shown that long-term treatment of rats with antidepressant drugs that affect the activity of noradrenergic and serotonergic neurons by different mechanisms, inhibits the increase in cortical norepinephrine output induced by stress. With the use of microdialysis, we have now evaluated the effects of reboxetine, an antidepressant drug that selectively inhibits norepinephrine reuptake, on the increase in cortical norepinephrine output elicited in rats by exposure to foot-shock stress or by the acute administration of N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg, i.p.). Foot-shock stress and FG 7142 each induced a marked increase in the cortical extracellular concentration of norepinephrine (+200 and +90%, respectively) in control rats. Long-term treatment with reboxetine (10 mg/kg, i.p., once a day for 21 days) reduced the effect of foot-shock stress and completely antagonized the effect of FG 7142 on cortical norepinephrine output. Our results suggest that changes in the activity of noradrenergic neurons in the cortex might be relevant to the anxiolytic and antidepressant effects of reboxetine.     

Studies on the acute and chronic effects of reboxetine on extracellular noradrenaline and other monoamines in the rat brain

1 The effect of reboxetine, a novel antidepressant drug that potently and selectively inhibits neuronal noradrenaline (NA) uptake, on brain extracellular monoamines was studied by microdialysis. 2 Fifteen mg kg-1 i.p. reboxetine raised extracellular NA in the frontal cortex (by 242%) and dorsal hippocampus (by 240%). 3 Idazoxan (1 mg kg-1 s.c.), given 60 min after 15 mg kg-1 reboxetine, markedly potentiated the effect on extracellular NA in the frontal cortex (by 1580%) and dorsal hippocampus (by 1360%), but had no effect by itself. 4 Twenty-four hours after the last injection of a chronic schedule (15 mg kg-1 i.p. once daily for 14 days) reboxetine had no effect on basal extracellular concentrations of NA in the dorsal hippocampus and a challenge dose of reboxetine (15 mg kg-1) raised extracellular NA similarly in rats treated chronically with reboxetine (by 353%) and saline (by 425%). 5 Ten and 20 microg kg-1 i.p. clonidine dose-dependently reduced hippocampal extracellular NA similarly in rats given chronic reboxetine (by 32% and 57%) and saline (by 42% and 56%). 6 Extracellular concentrations of dopamine and 5-HT in the striatum were similar in rats treated chronically with reboxetine and saline. A challenge dose of reboxetine (15 mg kg-1) had no effect on striatal extracellular dopamine and slightly increased striatal extracellular 5-HT to a similar extent in rats treated chronically with reboxetine (by 137%) and saline (by 142%). 7 The results suggest that combining reboxetine with an alpha2-adrenoceptor antagonist may facilitate its antidepressant activity. Repeated treatment confirmed that reboxetine is fairly selective for the noradrenergic system but provided no evidence of adaptive changes in that system that could facilitate its effect on extracellular NA.     

Noradrenergic antidepressants increase cortical dopamine: potential use in augmentation strategies

Most antidepressant treatments, based on serotonin (5-HT) and/or norepinephrine (NE) transporter blockade, show limited efficacy and slow onset of action, requiring the use of augmentation strategies. Here we report on a novel antidepressant strategy to selectively increase DA function in prefrontal cortex (PFC) without the potential tolerance problems associated to DA transporter blockade. This approach is based on previous observations indicating that extracellular DA in rat medial PFC (mPFC) - but not in nucleus accumbens (NAc) - arises from noradrenergic terminals and is sensitive to noradrenergic drugs. A low dose of reboxetine (3 mg/kg i.p.; NE reuptake inhibitor) non-significantly increased extracellular DA in mPFC. Interestingly, its combined administration with 5 mg/kg s.c. mirtazapine (non-selective α(2)-adrenoceptor antagonist) increased extracellular DA in mPFC (264 ± 28%), but not in NAc. Extracellular NE (but not 5-HT) in mPFC was also enhanced by the combined treatment (472 ± 70%). Repeated (×3) reboxetine + mirtazapine administration produced a moderate additional increase in mPFC DA and markedly reduced the immobility time (-51%) in the forced-swim test. Neurochemical and behavioral effects of the reboxetine + mirtazapine combination persisted in rats pretreated with citalopram (3 mg/kg, s.c.), suggesting its potential usefulness to augment SSRI effects. In situ hybridization c-fos studies were performed to examine the brain areas involved in the above antidepressant-like effects, showing changes in c-fos expression in hippocampal and cortical areas. BDNF expression was also increased in the hippocampal formation. Overall, these results indicate a synergistic effect of the reboxetine + mirtazapine combination to increase DA and NE function in mPFC and to evoke robust antidepressant-like responses.     

Chronic desipramine and fluoxetine differentially affect extracellular dopamine in the rat prefrontal cortex

The effect of chronic administration of desipramine or fluoxetine (10 mg/kg IP once a day for 2 weeks) on extracellular noradrenaline, serotonin and dopamine in the rat prefrontal cortex was studied by transcerebral microdialysis. Chronic desipramine increased extracellular noradrenaline and dopamine by three-fold as compared to saline controls. Acute challenge with 10 mg/kg desipramine increased by more than three-fold extracellular noradrenaline and dopamine in saline controls, but failed further to increase extracellular noradrenaline and dopamine in rats chronically administered desipramine. Chronic fluoxetine more than doubled the extracellular concentrations of serotonin but failed to change the extracellular concentrations of dopamine as compared to saline controls. Challenge with 5 mg/kg fluoxetine while almost doubling extracellular serotonin and dopamine concentrations in saline controls, failed further to increase extracellular serotonin and did not change extracellular dopamine in rats chronically exposed to fluoxetine. In contrast, challenge with 10 mg/kg desipramine normally increased extracellular dopamine in rats chronically exposed to fluoxetine. Therefore, chronic fluoxetine is associated with normal presynaptic dopamine transmission in the prefrontal cortex as a result of tolerance to fluoxetine-induced increase of extracellular dopamine; in contrast, chronic desipramine is associated with an increase of pre-synaptic dopamine transmission in the prefrontal cortex up to a level that cannot be further elevated by acute desipramine challenge. The results suggest that prefrontal cortex dopamine plays a different role in the antidepressant properties, of desipramine and fluoxetine.     

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats

RATIONALE: Although drug discrimination procedures have proven difficult to apply to antidepressant agents, we recently characterized discriminative stimulus properties of the selective serotonin (5-HT) reuptake inhibitor, citalopram, in rats. However, discriminative stimulus properties of selective norepinephrine (NE) reuptake inhibitors remain to be evaluated. OBJECTIVE: We determined the potential discriminative stimulus properties of the highly selective NE reuptake inhibitor and antidepressant, reboxetine. METHODS: Employing a two-lever discrimination procedure, rats were trained to discriminate reboxetine (2.5 mg/kg, IP) from saline. In parallel, the influence of reboxetine (2.5 mg/kg) upon dialysate levels of monoamines in frontal cortex and dorsal hippocampus of freely moving rats was determined. RESULTS: After 54+/-10 training sessions, reboxetine elicited robust stimulus recognition, fully generalizing to itself with an ED50 of 1.2 mg/kg. Two further NE reuptake inhibitors, desipramine (5.3) and maprotiline (1.8), as well as the 5-HT/NE reuptake inhibitor, venlafaxine (1.0), likewise generalized. In contrast, the 5-HT reuptake inhibitors, paroxetine, citalopram and sertraline, and the DA reuptake inhibitors, GBR12935 and bupropion, did not show significant generalization. Reboxetine markedly increased dialysate levels of NE, but not 5-HT, in frontal cortex and hippocampus. Dopamine (DA) levels were also (though less markedly) enhanced in frontal cortex. CONCLUSION: In parallel with an elevation in extracellular levels of NE, the selective NE reuptake inhibitor, reboxetine, elicits a specific discriminative stimulus in rats.     

Inhibition by venlafaxine of the increase in norepinephrine output in rat prefrontal cortex elicited by acute stress or by the anxiogenic drug FG 7142

Venlafaxine is an antidepressant drug that inhibits the reuptake of serotonin and norepinephrine with different efficacies. The effects of repeated administration of this drug on the increase in the extracellular concentration of norepinephrine in the prefrontal cortex, induced by stress or by the anxiogenic drug FG 7142, were studied in freely moving rats. Exposure to foot-shock stress induced a marked increase (+120%) in the extracellular norepinephrine concentration in the prefrontal cortex of control rats. Long-term administration of venlafaxine (10 mg/kg i.p., once a day for 21 days) reduced the effect of stress on norepinephrine output by 75%. This effect of venlafaxine persisted for at least 5 days after discontinuation of drug treatment. Acute administration of FG 7142 (20 mg/kg i.p.), a benzodiazepine receptor inverse agonist, increased norepinephrine output (+90%) in control rats. Chronic treatment with venlafaxine prevented the effect of FG 7142. In contrast, the acute administration of this antidepressant had no effect on the stress- or FG 7142-induced increase in norepinephrine output. These plastic changes in the sensitivity of norepinephrine neurones to foot-shock stress and to an anxiogenic drug may reveal an important neuronal mechanism for the physiological regulation of emotional state. Furthermore, this mechanism might be relevant to the anxiolytic and antidepressant effects of venlafaxine.     

Opioid and monoamine systems mediate the discriminative stimulus of tramadol in rats

We analyzed the ability of the mu opioid peptide receptor ligands morphine and naloxone and several antidepressant drugs that are serotonin (fluoxetine), noradrenaline (reboxetine), mixed serotonin and noradrenaline (milnacipram and venlafaxine), dopamine (nomifensine) reuptake inhibitors, as well as roxindole (a nonselective drug) to substitute for, or alter, tramadol discrimination. Male Wistar rats were trained to discriminate tramadol (20 mg/kg) from saline in a two-choice water-reinforced paradigm. Out of the drugs studied, only morphine substituted for tramadol. In combination experiments, naloxone (0.1-1 mg/kg) attenuated the stimulus effects of tramadol (20 mg/kg) and the substitution evoked by morphine (2 mg/kg). Milnacipram (10 mg/kg) or reboxetine (10 mg/kg) enhanced the effects of tramadol (2.5-10 mg/kg); the other antidepressant drugs used failed to modulate tramadol discrimination. Our results indicate that tramadol can be used as a stimulus cue in rats, and that mu opioid peptide mechanisms are involved in its effects, while noradrenergic uptake inhibitors can enhance tramadol discrimination.     

Mirtazapine-induced corelease of dopamine and noradrenaline from noradrenergic neurons in the medial prefrontal and occipital cortex

The novel antidepressant mirtazapine has been shown to increase extracellular noradrenaline and dopamine in the medial prefrontal cortex. Our previous studies indicate that extracellular dopamine in the cerebral cortex originates largely from noradrenergic terminals, such release being controlled by alpha(2)-adrenoceptors. Because mirtazapine inhibits alpha(2)-adrenoceptors, the possibility that it might corelease dopamine and noradrenaline was investigated. By means of microdialysis, the effect of mirtazapine on extracellular dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and noradrenaline in the medial prefrontal cortex, densely innervated by dopaminergic and noradrenergic neurons, and in the occipital cortex, receiving equal noradrenergic but scarce dopaminergic projections, was compared. Basal extracellular concentration of noradrenaline was similar in both cortices, while dopamine in the occipital cortex was only about 50% lower than in the medial prefrontal cortex, reflecting noradrenergic rather than dopaminergic projections. The intraperitoneal (i.p.) administration of mirtazapine (5 and 10 mg/kg) increased extracellular dopamine, DOPAC and noradrenaline to approximately the same extent in both cortices, an effect totally suppressed by the alpha(2)-adrenoceptors agonist clonidine (0.15 mg/kg, i.p.). To exclude the possibility that mirtazapine-induced increase in dopamine might result from reduced dopamine removal from extracellular space, noradrenaline and dopamine uptake mechanisms were blocked by perfusing 100 microM desipramine into either cortex. The combined i.p. administration of mirtazapine (5 mg/kg) and the local perfusion of desipramine produced an additional increase in extracellular dopamine, DOPAC and noradrenaline in the medial prefrontal cortex and occipital cortex compared with the increase produced by either drug given alone. The results suggest that mirtazapine by inhibiting alpha(2)-adrenoceptors produces a corelease of noradrenaline and dopamine from noradrenergic terminals in the cerebral cortex.     

Fluoxetine, but not other selective serotonin uptake inhibitors, increases norepinephrine and dopamine extracellular levels in prefrontal cortex

RATIONALE: The selective serotonin uptake inhibitor (SSRI) fluoxetine has been shown to not only increase the extracellular concentrations of serotonin, but also dopamine and norepinephrine extracellular concentrations in rat prefrontal cortex. The effect of other SSRIs on monoamine concentrations in prefrontal cortex has not been thoroughly studied. OBJECTIVE: The aim of this study was to compare the ability of five systemically administered selective serotonin uptake inhibitors to increase acutely the extracellular concentrations of serotonin, norepinephrine and dopamine in rat prefrontal cortex. METHODS: The extracellular concentrations of monoamines were determined in the prefrontal cortex of conscious rats using the microdialysis technique. RESULTS: Fluoxetine, citalopram, fluvoxamine, paroxetine and sertraline similarly increased the extracellular concentrations of serotonin from 2- to 4-fold above baseline. However, only fluoxetine produced robust and sustained increases in extracellular concentrations of norepinephrine and dopamine after acute systemic administration. Fluoxetine at the same dose blocked ex vivo binding to the serotonin transporter, but not the norepinephrine transporter, suggesting that the increase of catecholamines was not due to non-selective blockade of norepinephrine uptake. Prefrontal cortex extracellular concentrations of fluoxetine at the dose that increased extracellular monoamines were 242 nM, a concentration sufficient to block 5-HT(2C) receptors which is a potential mechanism for the fluoxetine-induced increase in catecholamines. CONCLUSION: Amongst the SSRIs examined, only fluoxetine acutely increases extracellular concentrations of norepinephrine and dopamine as well as serotonin in prefrontal cortex, suggesting that fluoxetine is an atypical SSRI.     

LY393558, a 5-hydroxytryptamine reuptake inhibitor and 5-HT(1B/1D) receptor antagonist: effects on extracellular levels of 5-hydroxytryptamine in the guinea pig and rat.

The stimulation of terminal 5-HT(1B/1D) autoreceptors limits the effects of selective serotonin reuptake inhibitors on extracellular levels of 5-hydroxytryptamine (5-HT, serotonin) in vivo. Microdialysis studies show that acute oral administration of LY393558-a 5-HT reuptake inhibitor and antagonist at both the human 5-HT(1B) and 5-HT(1D) receptor-in the dose range 1-20 mg/kg, increases extracellular levels of 5-HT in both the guinea pig hypothalamus and rat frontal cortex. In both species, the levels of 5-HT that were attained were higher than following an acute, maximally effective dose of fluoxetine (20 mg/kg orally), reaching approximately 1500% in the guinea pig hypothalamus and 700% in the rat frontal cortex. In both species, the response to LY393558 (10 mg/kg p.o.) was impulse dependent, being absent in the presence of tetrodotoxin delivered at 1 microM via the microdialysis probe. The sensitivity to tetrodotoxin contrasted with the effects seen with DL-fenfluramine. Studies in rats showed that the microdialysate 5-HT concentration achieved in the frontal cortex after an acute challenge with LY393558 (5 mg/kg p.o.) was significantly greater than following a chronic regime of fluoxetine treatment (10 mg/kg/day orally for 21 days). Moreover, in rats chronically treated with LY393558 (5 mg/kg/day orally for 21 days), the mean basal concentration, 24 h after the final pretreatment dose, was of the same magnitude as that following chronic fluoxetine. However, in contrast to the response seen in fluoxetine-pretreated animals, a challenge dose of LY393558 still elicited a further increase in extracellular 5-HT in LY393558-pretreated animals. LY393558 is a potent 5-HT reuptake inhibitor and 5-HT(1B/1D) receptor antagonist. Microdialysis studies show that acute oral administration increases extracellular levels of 5-HT, by an impulse-dependent mechanism, above those produced by a maximally effective dose of fluoxetine, and in rats to levels only achieved following chronic fluoxetine treatment. Its neurochemical profile in vivo suggests that it may be a more effective antidepressant with the potential for producing an earlier onset of clinical activity than selective serotonin reuptake inhibitors.     

Reboxetine attenuates forced swim test-induced behavioural and neurochemical alterations in the rat.

The forced swim test is a behavioural paradigm that is predicative of antidepressant activity in rodents. Until recently, research has focused on the ability of antidepressant drugs to decrease immobility in the forced swim test paradigm, but the neurochemical sequelae induced by swim stress, or the neurochemical basis of antidepressant-induced behavioural changes have received little attention. In this regard, we have recently demonstrated that forced swim test exposure increases serotonergic activity in the amygdala, frontal cortex and hippocampus and dopamine turnover in the striatum. In addition, forced swim test-exposure activates the hypothalamic pituitary adrenal axis. The purpose of the present study was to examine the effect of treatment with the selective noradrenaline reuptake inhibitor reboxetine (3, 10 and 30 mg/kg; i.p.) on immobility and defaecation scores in the forced swim test, and on forced swim test-induced neurochemical and hypothalamic pituitary adrenal axis changes in the rat. Reboxetine treatment (10 and 30 mg/kg) significantly decreased immobility and defaecation in the forced swim test in dose dependent manner. Furthermore, reboxetine produced a dose dependent attenuation of forced swim test-induced increases in serotonin turnover in the amygdala and frontal cortex and dopamine turnover in the striatum. Reboxetine (30 mg/kg) produced a modest, but non-significant, attenuation of forced swim test-induced increases in serum corticosterone concentrations. These data demonstrate that, in addition to the behavioural activity of reboxetine in the rat forced swim test paradigm, a dose-dependent attenuation of swim stress-induced increases in serotonergic and dopaminergic activity occurred in a region specific manner. These are the first data to demonstrate that treatment with the selective noradrenaline reuptake inhibitor, reboxetine can impact on the activity of other neurotransmitter systems in response to stress. Moreover, these data further demonstrate that this paradigm is a valuable tool in studying the effect of antidepressants, on both behaviour and swim stress-related alterations in central neurotransmitter function and hypothalamic pituitary adrenal axis activity in the rat.     

Tianeptine increases the extracellular concentrations of dopamine in the nucleus accumbens by a serotonin-independent mechanism.

The effect of various doses of tianeptine on the extracellular concentrations of dopamine was studied in the striatum and nucleus accumbens of the rat. At 5 (but not 2.5) mg/kg intraperitoneally, tianeptine increased the extracellular dopamine only in the nucleus accumbens. At 10 mg/kg, the effect was also seen in the striatum but it was less marked and shorter-lasting. At 10 mg/kg (i.p.), tianeptine significantly raised the extracellular concentrations of dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in both regions. The effect of 10 mg/kg tianeptine on dopamine and its metabolites was not significantly changed in animals which had received this dose twice daily for 15 days. Intracerebroventricular administration of 150 micrograms/20 microliters 5,7-dihydroxytryptamine, which markedly depleted serotonin in the brain, did not modify the effect of 10 mg/kg tianeptine on the extracellular concentrations of dopamine and HVA in the nucleus accumbens but reduced the effect on DOPAC. Various doses of tianeptine (1, 3 and 10 mg/kg i.p.) did not change the synthesis of serotonin and dopamine in the striatum and nucleus accumbens. The results show that tianeptine increased the extracellular concentrations of dopamine more in the nucleus accumbens than in striatum. The effect on the output of DA in the nucleus accumbens could be involved in the antidepressant activity of tianeptine.     

Chronic treatment with reboxetine by osmotic pumps facilitates its effect on extracellular noradrenaline and may desensitize alpha(2)-adrenoceptors in the prefrontal cortex

1. This study investigated the effect of acute (2 days) and chronic (14 days) treatment with a selective inhibitor of noradrenaline uptake, reboxetine (10 mg kg(-1) day(-1)) by osmotic pumps, on extracellular noradrenaline and the sensitivity of alpha(2)-adrenoceptors in the prefrontal cortex of rats. 2. The effect of continuous infusion of reboxetine for 14 days on cortical extracellular noradrenaline was significantly higher (599% of vehicle levels) than after 2 days (263% of vehicle levels). 3. Brain concentrations of reboxetine after 2 and 14 days of infusion were 37.9+/-17.8 and 37.1+/-7.7 ng g(-1), respectively. 4. Reboxetine infused for 2 and 14 days significantly increased extracellular dopamine in the prefrontal cortex, to a similar extent (257 and 342% of vehicle levels, respectively), whereas extracellular 5-HT was not modified by either treatment. 5. Clonidine (10 and 30 microg kg(-1) i.p.) reduced cortical extracellular noradrenaline similarly in animals treated with reboxetine or vehicle for 2 days whereas the effects in rats infused with reboxetine for 14 days were markedly less than in vehicle-treated animals. 6. Clonidine (0.05 and 0.2 microM), infused through the dialysis probe into the prefrontal cortex, reduced cortical extracellular noradrenaline much less in rats treated with reboxetine for 14 days than in vehicle-treated animals. 7. Reboxetine's effect on extracellular noradrenaline in the prefrontal cortex was greater after chronic treatment and could be associated with desensitization of terminal alpha(2)-adrenoceptors that normally serve to inhibit noradrenaline release.     

Effects of selective serotonin and serotonin/noradrenaline reuptake inhibitors on extracellular serotonin in rat diencephalon and frontal cortex

Some clinical reports suggest that tricyclic antidepressants which block both noradrenaline and serotonin (5-HT) reuptake (SNRIs) are more effective than selective 5-HT reuptake inhibitors (SSRIs) in treating severe depression. Moreover, one neurochemical study reported larger increases in extracellular 5-HT in rat frontal cortex in response to the tricyclic antidepressant imipramine compared to the SSRI fluoxetine. However, imipramine, which blocks both 5-HT and noradrenaline reuptake, also binds with relatively high affinity to receptors for noradrenaline, histamine and acetylcholine. Thus, to test the hypothesis that compounds that inhibit both 5-HT and noradrenaline reuptake produce larger increases in 5-HT efflux, we compared the effects of acute systemic administration of several SNRIs and SSRIs. Extracellular 5-HT was measured using microdialysis probes implanted in the diencephalon and frontal cortex of unanesthetized rats. We tested the SSRIs paroxetine (0.3-10 mg/kg), citalopram (10-20 mg/kg) and fluoxetine (10 mg/kg), the nonselective tricyclic antidepressant imipramine (20 mg/kg) and the more selective SNRIs duloxetine (3-30 mg/kg) and venlafaxine (30-50 mg/kg). During the lights-off period, paroxetine and duloxetine increased 5-HT in the diencephalon approximately 300 and approximately 200%, respectively. During the lights-on period, paroxetine and duloxetine each increased 5-HT approximately 400% in the diencephalon. In the frontal cortex, both paroxetine and duloxetine increased 5-HT approximately 200%. Citalopram and venlafaxine each increased 5-HT in the diencephalon approximately 300%. Fluoxetine and imipramine increased 5-HT in the diencephalon by approximately 125 and approximately 80%, respectively. Thus, these results do not support the hypothesis that compared to SSRIs, compounds which inhibit both 5-HT and noradrenaline reuptake have a larger acute effect on extracellular 5-HT.     

Reboxetine Enhances the Olanzapine-Induced Antipsychotic-Like Effect, Cortical Dopamine Outflow and NMDA Receptor-Mediated Transmission

Preclinical data have shown that addition of the selective norepinephrine transporter (NET) inhibitor reboxetine increases the antipsychotic-like effect of the D_2/3 antagonist raclopride and, in parallel, enhances cortical dopamine output. Subsequent clinical results suggested that adding reboxetine to stable treatments with various antipsychotic drugs (APDs) may improve positive, negative and depressive symptoms in schizophrenia. In this study, we investigated in rats the effects of adding reboxetine to the second-generation APD olanzapine on: (i) antipsychotic efficacy, using the conditioned avoidance response (CAR) test, (ii) extrapyramidal side effect (EPS) liability, using a catalepsy test, (iii) dopamine efflux in the medial prefrontal cortex and the nucleus accumbens, using in vivo microdialysis in freely moving animals and (iv) cortical N-methyl--aspartate (NMDA) receptor-mediated transmission, using intracellular electrophysiological recording in vitro. Reboxetine (6 mg/kg) enhanced the suppression of CAR induced by a suboptimal dose (1.25 mg/kg), but not an optimal (2.5 mg/kg) dose of olanzapine without any concomitant catalepsy. Addition of reboxetine to the low dose of olanzapine also markedly increased cortical dopamine outflow and facilitated prefrontal NMDA receptor-mediated transmission. Our data suggest that adjunctive treatment with a NET inhibitor may enhance the therapeutic effect of low-dose olanzapine in schizophrenia without increasing EPS liability and add an antidepressant action, thus in principle allowing for a dose reduction of olanzapine with a concomitant reduction of dose-related side effects, such as EPS and weight gain.     

Neurochemical responses to antidepressants in the prefrontal cortex of mice and their efficacy in preclinical models of anxiety-like and depression-like behavior: a comparative and correlational study

Rational Marble burying and forced swimming behavior are widely used and sensitive tests for identifying clinically effective antidepressant drugs, although the underlying neurobiology of these behaviors is not fully elucidated. Objectives The objective of this study was to determine the relationship between the behavioral effects of antidepressant drugs and their ability to modulate extracellular neurotransmitter levels in the prefrontal cortex. Materials and methods The effects of fluoxetine, fluvoxamine, citalopram, imipramine, and desipramine (0 to 60 mg/kg by oral gavage, except fluoxetine at 0 to 40 mg/kg) were studied independently in CD-1 mice in the marble-burying task, forced swim task and on extracellular concentrations of serotonin, norepinephrine, and dopamine in the prefrontal cortex by freely moving microdialysis. Results Fluvoxamine, fluoxetine, and citalopram all suppressed marble-burying behavior, but produced no change in immobility time in the forced swim test. In contrast, imipramine and desipramine suppressed both marble-burying behavior and increased swimming time in the forced swim test, although desipramine mildly suppressed locomotor activity at the maximal dose. Fluvoxamine, fluoxetine, and citalopram all increased extracellular levels of cortical serotonin. Desipramine and imipramine increased extracellular dopamine levels. Fluoxetine, desipramine, and imipramine increased extracellular norepinephrine levels. Correlational analysis revealed a positive correlation between efficacy of drugs in the forced swim test and cortical extracellular dopamine levels, whereas a positive correlation was found between efficacy in the marble-burying test and extracellular serotonin levels. Conclusions Although marble burying and forced swimming behavior have strong predictive validity in tests of antidepressant action, each assay appears to be underpinned by entirely different neurochemical systems.     

Overflow of noradrenaline and dopamine in frontal cortex after [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] (DSP-4) treatment: in vivo microdialysis study in anaesthetized rats

The changes in the extracellular concentration of endogenous noradrenaline and dopamine in the frontal cortex following pretreatment with noradrenergic neurotoxin DSP-4 [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] were studied by in vivo microdialysis in rats anaesthetized with chloral hydrate. Noradrenaline and dopamine levels in frontal cortex were detected only when the uptake inhibitor, nomifensine (10μM) was present in dialysis fluid. Under those conditions, the Na⁺ channel agonist veratridine and a depolarising concentration of potassium chloride (60mM), applied locally through the microdialysis probe, increased the overflow of noradrenaline. Tetrodotoxin had an opposite effect. These results indicate that most of the noradrenaline probably arose from exocytotic release. Noradrenaline efflux in the frontal cortex of DSP-4 pretreated rats (52±6.1fmol/sample) did not differ significantly from that of the control animals (69±4.9fmol/sample). Dopamine efflux was not changed either (64±9.6 and 62±3.9fmol/sample, respectively). The α₂-adrenoceptor antagonist, atipamezole (3mg/kg i.p.), increased the overflow of noradrenaline in the frontal cortex of saline-treated rats by 100%, whereas in DSP-4 treated rats the increase was only around 30%. The overflow of dopamine was not changed under the conditions described. The effect of atipamezole in DSP-4 treated rats may be of smaller magnitude due to the diminished pool of releasable noradrenaline or due to a downregulation of presynaptic α₂-adrenoceptors in the frontal cortex. The perfusion of 60mM KCl at the end of the experiment unexpectedly produced equivalent increases in noradrenaline and dopamine content in dialysates of both vehicle and DSP-4 treated rats. We conclude that the uptake inhibitor, nomifensine, and atipamezole, which had a stronger effect in vehicle-treated animals, reduced the effect of KCl-stimulation and masked the true difference in changes of noradrenaline efflux. Post-mortem tissue concentrations of noradrenaline 7 days after DSP-4 administration (50mg/kg) were significantly reduced in the frontal cortex (54%), hippocampus (62.5%) and to lesser extent in the hypothalamus (27%) as compared to vehicle-treated rats. Dopamine and 3,4-dihydroxyphenylacetic acid concentrations were not changed confirming the efficacy and selectivity of the DSP-4 lesion. These results demonstrate that one week after DSP-4 treatment the extracellular levels of noradrenaline and dopamine as assessed by in vivo microdialysis are not changed in the frontal cortex, but atipamezole-stimulated release of noradrenaline is decreased in DSP-4 treated rats. Received: 12 June 1996 / Accepted: 30 October 1996     

Contrasting Fos expression induced by acute reboxetine and fluoxetine in the rat forebrain: neuroanatomical substrates for the antidepressant effect

Rationale Antidepressants preferentially facilitating serotonin seem to be particularly effective for treating the anxiety and aggressive component of the depressive syndrome, whereas those with a noradrenergic profile seem to be more effective in reducing psychomotor retardation, although their overall antidepressant effects are about the same. However, the mechanism of this difference remains unknown.Objectives To investigate the neural substrate for the different therapeutic efficacies of fluoxetine and reboxetine, we examined the regional Fos immunoreactivity (Fos-ir) induced by the two agents.Methods Male Wistar rats (290–330 g) were given a subcutaneous injection of fluoxetine (5 or 10 mg/kg), reboxetine (5 or 10 mg/kg) or saline. Two hours later, rats were perfused through the ascending aorta and their brains were processed for Fos immunohistochemistry. Fos-ir was quantified by counting the number of Fos-ir-positive nuclei in six areas of the forebrain.Results The shell of the nucleus accumbens was the only region in which both fluoxetine and reboxetine equally increased Fos-ir expression. Fluoxetine particularly induced Fos-ir in the central nucleus of the amygdala. In contrast, reboxetine induced Fos-ir in the cingulate cortex area 3 and the lateral orbital cortex.Conclusions These results suggest that the shell region may be one possible target for the antidepressant effects of fluoxetine and reboxetine. Furthermore, the difference in their clinical effects may depend on their different target sites of action.     

The effect of risperidone on the mirtazapine-induced changes in extracellular monoamines in the rat frontal cortex

BackgroundThe aim of our study was to understand the mechanism of clinical efficacy of the combination of an antidepressant and risperidone in drug-resistant depression.MethodsWe studied the effect of an antidepressant (mirtazapine) and risperidone (atypical antipsychotic), given separately or jointly on extracellular levels of dopamine (DA), serotonin (5-HT) and noradrenaline (NA) in the rat frontal cortex. The animals were given a single intraperitoneal injection of risperidone (1 mg/kg) and mirtazapine (10 and 20 mg/kg). The release of monoamines in the rat frontal cortex was investigated using a microdialysis in freely moving animals, and monoamine levels were assayed by HPLC with coulochemical detection.ResultsRisperidone increased the cortical extracellular levels of DA, 5-HT and NA. Similarly, mirtazapine dose-dependently increased the cortical extracellular levels of the monoamines studied. A combination of mirtazapine either at the higher dose (20 mg/kg) or at both doses (10 and 20 mg/kg) with risperidone produced a significant effect on DA and NA release, respectively compared to the effect of any drug given alone. The increase in the DA (but not NA) release induced by mirtazapine plus risperidone was partly blocked by the selective 5-HT_1A antagonist WAY 100635 (0.2 mg/kg).ConclusionsOur data indicate that the increase of cortical extracellular levels of DA and NA by combined administration of mirtazapine and risperidone may be of crucial importance to the pharmacotherapy of drug resistant depression, and that, among other mechanisms, 5-HT_1A, 5-HT_2A, α₂-adrenergic and histamine H₁ receptors may play some role in this effect.     

Effect of cyclophosphamide on gene expression of cytochromes p450 and beta-actin in the HL-60 cell line

The novel antidepressant reboxetine is a selective norepinephrine reuptake inhibitor. In this study, the antidepressant-like effects of reboxetine were characterized in a modified rat forced swim test. Further, in order to investigate the role of the locus coeruleus and lateral tegmental noradrenergic systems in the mediation of reboxetine's effects, the impact of different chemical lesions of these two pathways was examined on the behavioral responses induced by reboxetine in the forced swim test. Reboxetine (5-20 mg/kg, s.c.) dose-dependently decreased immobility and swimming behavior in the forced swim test while it simultaneously increased climbing behavior. These effects were similar to those previously demonstrated with tricyclic antidepressants and are indicative of reboxetine's effects on the noradrenergic system. Discrete local injections of the neurotoxin 6-hydroxydopamine were employed to lesion the ventral noradrenergic bundle arising from cells located in the lateral tegmentum. This resulting lesion completely prevented reboxetine (10 mg/kg, s.c.)-induced decreases in immobility and increases in climbing behavior, demonstrating that an intact ventral noradrenergic bundle is required for the manifestation of reboxetine-induced antidepressant-like behavior in the test. In contrast, lesions of the dorsal noradrenergic bundle which consists of neurons arising from the nucleus locus coereleus, were achieved by systemic pretreatment with the selective noradrenergic neurotoxin N-(2-chloroethyl)-N-2-bromobenzylamine (DSP-4; 50 mg/kg, i.p.). The ability of reboxetine (10 mg/kg, s.c.) to increase climbing and decrease immobility was augmented by DSP-4 pretreatment. Furthermore, neither lesions of the dorsal noradrenergic bundle nor the ventral noradrenergic bundle altered baseline immobility scores in the forced swim test. Taken together, these data suggest that forebrain regions innervated by these two distinct noradrenergic pathways exert opposing influences on the behavioral response to reboxetine in the rat forced swim test.