Reduced efficacy of fluoxetine following MDMA ("Ecstasy")-induced serotonin loss in rats

Long-term serotonin (5-HT) neuronal loss is currently a major cause of concern associated with recreational use of the substituted amphetamine 3,4 methylenedioxymethamphetamine (MDMA; "Ecstasy"). Such loss may be problematic considering that psychiatric disorders such as depression and anxiety and responses to first line treatments for these disorders are associated with 5-HT. In this study the effects of prior exposure to MDMA on behavioural and central neurochemical changes induced by the serotonin (5-HT) re-uptake inhibitor and antidepressant fluoxetine were examined in rats. Animals were administered MDMA (10 mg/kg. i.p.) four times daily for two consecutive days. One week later the animals were subjected to treatment with fluoxetine (10 mg/kg, i.p.). Fluoxetine treatment groups received either acute (saline injections for 20 days followed by 3 fluoxetine treatments over 24 h) or chronic (once daily fluoxetine for 21 days) drug administration. Prior exposure to MDMA resulted in an attenuation of fluoxetine-induced swimming behaviour in the modified forced swimming test (FST); a behavioural test of antidepressant action. In parallel MDMA treatment resulted in significant regional depletions of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) accompanied by a reduction in cortical [3H] paroxetine binding to nerve terminal 5-HT transporters. MDMA-induced 5-HT loss was enhanced in animals following chronic fluoxetine administration. Elimination of fluoxetine and its metabolite norfluoxetine from the brain abolished this interaction between MDMA and fluoxetine treatment. Fluoxetine administration reduced both 5-HIAA and the 5-HIAA:5-HT metabolism ratio, which was attenuated in animals pre-treated with MDMA. Overall the results show that MDMA induces long-term 5-HT loss in the rodent brain and consequently diminishes behaviour and reductions in 5-HT metabolism induced by the antidepressant fluoxetine. These results have potential clinical relevance, suggesting that 5-HT re-uptake inhibitors such as fluoxetine may be less effective at treating depression in chronic abusers of MDMA.     

Persistent blockade of potassium-evoked serotonin release from rat frontocortical terminals after fluoxetine administration

We examined 5-HT and 5-HIAA release from frontal cortex evoked by high potassium chloride concentrations in rats pretreated for 3 days with high doses of the 5-HT uptake blocker fluoxetine or of dexfenfluramine, which both releases 5-HT and blocks its reuptake. The standard fluoxetine dose (30 mg/kg i.p.) was about 4 times the drug's ED₅₀ in producing a serotonin-related behavioral effect, anorexia, while the dexfenfluramine dose (7.5 mg/kg i.p.) was about 6 times its ED₅₀. These high doses were chosen in order to elucidate the mechanism by which similar doses of fluoxetine and dexfenfluramine had been found to produce long-term changes in serotonin dynamics. Fluoxetine decreased the basal release of both compounds; dexfenfluramine decreased basal 5-HIAA efflux without affecting the release of 5-HT release. Potassium-evoked 5-HT release was unchanged after dexfenfluramine pretreatment but was suppressed by fluoxetine doses as low as 7.5 mg per kg per day. Basal release of 5-HT and 5-HIAA returned to normal after 7 days of fluoxetine pretreatment, but evoked relase continued to be suppressed. These data suggest that long-term changes in brain serotonin dynamics after high doses of dexfenfluramine or fluoxetine are related to the drug's mechanisms of action, specifically their blockade of 5-HT reuptake.     

Selective changes in the contents of noradrenaline, dopamine and serotonin in rat brain areas during aging

This study examines the age-associated changes in noradrenaline (NA), dopamine (DA), 3,4-dihydroxyphenyl-acetic acid (DOPAC), serotonin (5-HT) and 5-hydroxy-3-indoleacetic acid (5-HIAA) in different brain areas of rats. DA and DOPAC concentrations in striatum increased at third month of age, remaining without significant variations until 12th month of age, and decreasing in 24-month-old rats. DA concentration dropped in hippocampus, amygdala and brainstem of 24-month-old-rats, whereas DOPAC levels decreased only in hippocampus. These changes suggest an age-dependent deficit of the dopaminergic system, presumably related to a reduced number/activity of DA nigrostriatal and mesolimbic neurons. An age-induced decline in NA content was found in the pons-medulla, the area containing NA neuronal bodies. Concentrations of 5-HT were reduced with aging in frontal cortex, showing a tendency to decrease in all brain areas examined. The increased 5-HIAA/5-HT ratio found in frontal cortex, amygdala and striatum suggests an age-related decreased synthesis and an accelerated 5-HT metabolism. The 5-HIAA content decreased in brainstem of the oldest rats. These findings point to a selective impairment of nigrostriatal and mesolimbic DA in aging rats, whereas reductions in NA were restricted to cell bodies region and 5-HT showed changes of different extent in areas of terminals and neuronal cell bodies.     

Antidepressant-like effects of the mixture of honokiol and magnolol from the barks of Magnolia officinalis in stressed rodents

Honokiol and magnolol are the main constituents simultaneously identified in the barks of Magnolia officinalis, which have been used in traditional Chinese medicine to treat a variety of mental disorders including depression. In the present study, we reported on the antidepressant-like effects of oral administration of the mixture of honokiol and magnolol in well-validated models of depression in rodents: forced swimming test (FST), tail suspension test (TST) and chronic mild stress (CMS) model. The mixture of honokiol and magnolol significantly decreased immobility time in the mouse FST and TST, and reversed CMS-induced reduction in sucrose consumption to prevent anhedonia in rats. However, this mixture was unable to affect ambulatory or rearing behavior in the mouse open-field test. CMS induced alterations in 5-hydroxytryptamine (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) levels in various brain regions of rats. An increase in serum corticosterone concentrations and a reduction in platelet adenylyl cyclase (AC) activity were simultaneously found in the CMS rats. The mixture of honokiol and magnolol at 20 and 40 mg/kg significantly attenuated CMS-induced decreases of 5-HT levels in frontal cortex, hippocampus, striatum, hypothalamus and nucleus accumbens. And it markedly increased 5-HIAA levels in frontal cortex, striatum and nucleus accumbens at 40 mg/kg and in frontal cortex at 20 mg/kg in the CMS rats. A subsequent reduction in 5-HIAA/5-HT ratio was found in hippocampus and nucleus accumbens in the CMS rats receiving this mixture. Furthermore, the mixture of honokiol and magnolol reduced elevated corticosterone concentrations in serum to normalize the hypothalamic-pituitary-adrenal (HPA) hyperactivity in the CMS rats. It also reversed CMS-induced reduction in platelet AC activity, via upregulating the cyclic adenosine monophosphate (cAMP) pathway. These results suggested that the mixture of honokiol and magnolol possessed potent antidepressant-like properties in behaviors involved in normalization of biochemical abnormalities in brain 5-HT and 5-HIAA, serum corticosterone levels and platelet AC activity in the CMS rats. Our findings could provide a basis for examining directly the interaction of the serotonergic system, the HPA axis and AC-cAMP pathway underlying the link between depression and treatment with the mixture of honokiol and magnolol.     

In the rat forced swimming test,chronic but not subacute administration of dual 5-HT/NA antidepressant treatments may produce greater effects than selective drugs

The rat forced swimming test (FST) distinguishes selective serotonin (5-HT) and selective noradrenaline (NA) reuptake-inhibitors, which respectively increase swimming and climbing behaviours. However, NA-system-mediated inhibition of 5-HT-induced swimming prevents dual 5-HT/NA reuptake-inhibition to produce concurrently climbing with swimming. Since adaptative neurochemical processes occur in the treatment of depression, we examined the influence of long-term antidepressant treatment on these interactions. METHODS: (1) Selective [fluoxetine: 10 mg/kg; desipramine: 10 mg/kg] and non-selective [milnacipran: 40 mg/kg; mirtazapine: 20 mg/kg] antidepressants were administered subacutely (3inj) and chronically (17inj) over 16 days. (2) A subacute fluoxetine-desipramine combination (10-10 mg/kg) was administered in rats that were pre-treated with chronic-desipramine (10 mg/kg per day, 14 days). (3) NA-system-mediated interactions were further examined by combining the alpha(2)-receptor agonist clonidine (5, 10, 20, 200 microg/kg) with 10 mg/kg fluoxetine. RESULTS: (1) Long-term treatment with either fluoxetine or desipramine does not modify the behavioural response produced by their subacute administration. (2) In contrast, whereas subacute-milnacipran increases climbing solely, chronic-milnacipran produces greater anti-immobility effects and increases both climbing and swimming behaviours. Similarly, the fluoxetine-desipramine combination produces climbing solely, but increases both climbing and swimming behaviours in animals pre-treated with chronic-desipramine. Chronic but not subacute-mirtazapine increases swimming behaviour. (3) clonidine dose-dependently antagonizes fluoxetine-induced anti-immobility effects and swimming behaviour. CONCLUSIONS: Chronic enhancement of NA-transmission alters NA-system-mediated inhibition of 5-HT-induced behaviour in the FST, which may involve alpha(2)-receptors.     

Estradiol-sertraline synergy in ovariectomized rats

This study investigated estradiol (E(2)) modulation of the antidepressant effects of a selective serotonin (5-HT) reuptake inhibitor (SSRI; sertraline) and a tricyclic antidepressant (imipramine) as measured by the forced swim test (FST) followed by assessment of gene and protein expression for the 5-HT transporter (SERT) and multiple 5-HT receptors. Female Sprague-Dawley rats were ovariectomized (OVX) and two-thirds of the rats received E(2) implants (OVE). 4 weeks later, implants were withdrawn in half of the OVE rats (OVW) to capture a time point when E(2) levels were rapidly declining. Rats in each hormone group were treated with vehicle, sertraline (10mg/kg) or imipramine (10mg/kg), 24, 5 and 1h before the FST. Immediately after the FST, midbrain, hippocampus and prefrontal cortex tissue was removed and frozen for analysis of gene expression via quantitative real-time PCR (midbrain tissue) and protein expression via Western blot (prefrontal cortex and hippocampal tissue). In the FST, sertraline decreased immobility and increased swimming in OVE rats, as well as increased swimming in OVW rats. In contrast, no sertraline effect was observed in OVX rats. Rats treated with imipramine showed increased climbing but no changes in immobility or swimming. No changes in protein expression were detected in any treatment group. However, in vehicle-treated rats, E(2) increased midbrain SERT mRNA expression, with no effect on midbrain mRNA for the 5-HT receptors. In sertraline-treated rats, E(2) decreased 5-HT(2A) receptor mRNA, and E(2)-withdrawal increased 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor mRNA. In imipramine-treated rats, E(2) (and E(2)-withdrawal) did not affect mRNA expression for any of the target genes. Thus, E(2) synergized behaviorally and neurochemically with an SSRI but not a tricyclic antidepressant.     

Influence of stress on regional brain serotonin metabolism after progesterone treatment and upon plasma progesterone in the rat

Summary The effect of progesterone upon stress altered serotonin (5-HT) metabolism in various regions of the rat brain was investigated with regard to a possible connection with premenstrual and post-partum depression. When electric footshock was administered to ovariectomized rats pretreated with progesterone or its vehicle, there were generally higher 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) concentrations after progesterone. 5-HT levels were significantly higher in thalamus, hippocampus, raphe, and frontal cortex, 5-HIAA rose significantly in hippocampus, raphe, and frontal cortex. Whereas after electric footshock alone the septum showed highest increases of 5-HT and 5-HIAA and hippocampus ranged last, after pretreatment with progesterone increases of 5-HT and 5-HIAA were least pronounced in septum but rather high in hippocampus. Electric footshock administered to ovariectomized rats also resulted in an increase of plasma progesterone concentration.     

Effect of D-fenfluramine on serotonin release in brains of anaesthetized rats

Using in vivo microdialysis of brains of anaesthetized rats, we have examined the acute and chronic effects of D-fenfluramine on the release of serotonin (5-HT) and 5-HIAA within the frontal cortex, the lateral hypothalamus and the nucleus accumbens. A single dose of the drug (10 mg/kg) stimulated 5-HT release by 331-810% and decreased 5-hydroxyindoleacetic acid (5-HIAA) release by 30%, within all 3 brain areas. These changes were maximal 30 min after drug administration, and values returned to baseline after 120 min. Among animals receiving D-fenfluramine (3 or 10 mg/kg, i.p.) daily for 8 days and examined 24 h after the last dose, the basal release of 5-HT from frontal cortex was unaffected. However, the levels of 5-HT in this region, and its evoked release after a subsequent dose of D-fenfluramine (10 mg/kg), were significantly reduced in animals that had received the larger chronic dose. 5-HT release was restored to normal if such rats were given tryptophan (100 mg/kg, i.p.) 1 h prior to the acute D-fenfluramine dose; moreover, 5-HT release from, and levels in, frontal cortex also returned to normal without additional treatment after a 28-day washout period. These observations suggest that the chronic administration of D-fenfluramine fails to affect spontaneous 5-HT release in rat brain, and reduces the release evoked by acute D-fenfluramine only when very high doses are given. Moreover, this reduction is reversible with time or with administration of 5-HT's circulating precursor, tryptophan.     

Modafinil enhances the increase of extracellular serotonin levels induced by the antidepressant drugs fluoxetine and imipramine: a dual probe microdialysis study in awake rat

In view of a postulated role of the vigilance-promoting drug modafinil in depression, the interaction of modafinil and two classical antidepressant drugs, fluoxetine and imipramine, were studied in 5-HT levels in the dorsal raphe-cortical system using dual-probe microdialysis. Fluoxetine (1-10 mg/kg) dose-dependently increased dorsal raphe-cortical 5-HT levels. Modafinil at a very low dose (3 mg/kg), by itself ineffective, enhanced the fluoxetine (5 mg/kg)-induced increases of 5-HT levels in both brain areas. A synergistic interaction was observed in the prefrontal cortex with fluoxetine (1 mg/kg) in terms of 5-HT release, but not in the dorsal raphe. Imipramine (1.3 mg/kg) increased 5-HT levels in the dorsal raphe, but not in the prefrontal cortex, while the higher doses (10.9-21.8 mg/kg) caused substantial increases in both brain areas. Modafinil (3 mg/kg), injected before imipramine (1.3 mg/kg), which by itself was ineffective on cortical 5-HT levels, increased cortical 5-HT levels. On other hand, modafinil failed to affect the high-dose imipramine (10.9 mg/kg)-induced increase of 5-HT levels in the prefrontal cortex and the imipramine (1.3; 10.9 mg/kg)-induced increase of 5-HT levels in the dorsal raphe nucleus. These results demonstrate that modafinil in low doses enhances the acute effects of fluoxetine and imipramine on 5-HT levels in the dorsal raphe nucleus (fluoxetine only) and especially in the prefrontal cortex of the awake rat. These findings suggest a therapeutic potential of low doses of modafinil in the treatment of depression when combined with low doses of classical antidepressants, especially by increasing 5-HT transmission in cortical regions.     

Effects of fluoxetine or D-fenfluramine on serotonin release from, and levels in, rat frontal cortex.

Using in vivo microdialysis of frontal cortex in anesthetized rats, as well as analysis of frontal cortex homogenates, we examined the effects of chronic administration of fluoxetine (30 mg/kg, i.p.) or D-fenfluramine (7.5 mg/kg, i.p.), administered daily for 3 days, on serotonin and 5-HIAA levels a day later. Measurements were also taken after 3-, 7- , and 21-day recovery periods. Neither chronic fluoxetine nor D-fenfluramine changed basal serotonin release. Both treatments, however, transiently decreased the release of serotonin evoked by an acute dose of D-fenfluramine (10 mg/kg, i.p.). Release initially was completely suppressed in fluoxetine-pretreated animals but returned to normal by the 21st day of washout; following D-fenfluramine pretreatment, normal release was attained by the 7th day of washout. Both fluoxetine and D-fenfluramine transiently decreased 5-HIAA levels in the dialysates and tissues. Both drugs also caused prolonged changes in frontal cortex serotonin levels, D-fenfluramine lowering them but fluoxetine elevating them. These results suggest that, at comparable dosage levels relative to their ED50s, fluoxetine and D-fenfluramine cause comparable reversible effects on brain serotonin release. The drugs also cause prolonged but opposite changes in brain serotonin levels, probably reflecting differences in the extents to which they or their principal metabolites release serotonin and block its reuptake.     

Serotonin concentrations and turnover in brains of depressed suicides

5-Hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentrations and 5-HT turnover (5-HIAA/5-HT) were determined in 6 brain regions from 19 suicide victims in whom a retrospective diagnosis of depression was established, and 19 age- and sex-matched control subjects. Thirteen of the suicides were free of psychoactive drugs at the time of death; 5 were receiving antidepressant drugs. 5-HT, 5-HIAA and 5-HT turnover did not differ significantly between the total, drug-free and antidepressant-treated suicides and controls in frontal and temporal cortex, caudate and hippocampus. 5-HIAA concentration was significantly higher in amygdala of drug-free suicides than controls, whereas 5-HT and 5-HT turnover did not differ. 5-HT concentration was significantly lower in putamen of the total and antidepressant-treated suicides and a similar reduction was also apparent in the drug-free suicides. 5-HT turnover in putamen was significantly higher in the total and drug-free suicides compared to controls. 5-HT and 5-HIAA concentrations in putamen were significantly lower in drug-free suicides who died by non-violent means than in those who died by violent means. Differences between controls and suicides could not be attributed to age, sex or postmortem delay. These results offer no support for the view that 5-HT turnover is reduced in depressed subjects who commit suicide.     

Chronic variable stress and partial 5-HT denervation by parachloroamphetamine treatment in the rat: effects on behavior and monoamine neurochemistry

Chronic variable stress (CVS) and manipulations of 5-HT-ergic neurotransmission are increasingly used as animal models of depression. In the present study, CVS for 2 weeks and a partial lesion of 5-HT projections by a small dose of parachloroamphetamine (PCA, 2 mg/kg) were applied independently or in combination. CVS reduced significantly the gain in body weight and increased the number of defecations in the open field test. PCA reduced body weight only within the first 24 h after its administration. Consumption of sucrose solution and its preference to water in non-deprived rats were significantly higher in PCA-pretreated rats 2 weeks after CVS compared to control animals. In the forced swimming test, both PCA and CVS treatments reduced immobility on the first but not the second session. Both treatments reduced significantly the time rats spent in social interaction. CVS also elicited an increase in the weight of the right adrenal, but this effect was not present in the PCA-pretreated group. PCA reduced 5-HT and 5-HIAA levels in the frontal cortex, hippocampus, and septum by approximately 20%. CVS increased HVA levels in the frontal cortex. Applied together, PCA pretreatment and CVS increased dopamine turnover in the frontal cortex. Conclusively, this study has provided evidence that chronic variable stress, which elicited expected physiological and neurochemical changes, does not reduce sucrose intake or preference in non-deprived animals, but, instead, may increase it after partial 5-HT-ergic denervation; and that partial 5-HT-ergic denervation by a low dose PCA treatment has a long-lasting effect on forced swimming and social behavior similar to chronic stress.     

Differential enhancement of dialysate serotonin levels in distinct brain regions of the awake rat by modafinil: possible relevance for wakefulness and depression

The present in vivo microdialysis study evaluates the possible existence of a differential regulation of serotonergic transmission by the antinarcoleptic drug modafinil [(diphenyl-methyl)-sulfinyl-2-acetamide; Modiodal] among various brain regions of the awake rat. The results show that, in the cerebral cortex, the central amygdala, and the dorsal raphe nucleus, modafinil in the dose range of 10-100 mg/kg i.p. dose-dependently increases dialysate serotonin (5-HT) levels. In other brain areas, such as the medial preoptic area and the posterior hypothalamus, the modafinil-induced increase in dialysate 5-HT levels is observed only at tenfold higher doses (100 mg/kg), 10-30 mg/kg being ineffective. Together these data suggest that, in the frontal cortex, the amygdala, and the dorsal raphe, modafinil is more potent in enhancing extracellular 5-HT levels and presumably 5-HT transmission than in the medial preoptic area and the posterior hypothalamus. In view of the role of ascending 5-HT pathways in arousal and depression, it seems likely that the antinarcoleptic drug modafinil may also have an antidepressant potential in addition to its wakefulness-promoting action, both actions involving enhancement of 5-HT neurotransmission.     

1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) enhances tryptophan hydroxylase activity in mouse striatum, but not in the frontal cortex

We measured serotonin (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA) contents and tryptophan hydroxylase (TPH) activity in mouse striatum and frontal cortex after 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treatment (7 daily injections of 30 mg/kg). In the striatum, TPH activity was increased for at least 4 weeks after injection of MPTP, along with an increase in 5-HIAA. However, no significant change was observed in 5-HT, 5-HIAA or TPH activity in the frontal cortex. These results suggest that MPTP affects 5-HT through a change in TPH activity, specifically at nerve terminals in the striatum.     

Fluoxetine increases the extracellular levels of serotonin in the nucleus accumbens

The effects of an IP injection of the monoamine uptake inhibitor fluoxetine on the extracellular concentration of serotonin (5-HT), dopamine (DA), 5-hydroxyindoleacetic acid (5-HIAA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens of awake and freely moving rats were examined using a push-pull perfusion technique. Baseline values of 5-HT, 5-HIAA, DA, DOPAC and HVA in the perfusates were approximately 0.07, 13, 0.8, 49 and 12 pmol/hr, respectively. The IP administration of 5 and 10 mg/kg fluoxetine dose-dependently elevated the amounts of 5-HT 3- and 13-fold, respectively, in the push-pull perfusate, with the maximum reached within one hour after drug administration. Moreover, 10 mg/kg fluoxetine also significantly decreased the levels of 5-HIAA in the perfusate as much as 50% within 2-3 hours. On the other hand, no significant effect of 5 or 10 mg/kg fluoxetine was observed on the contents of DA, DOPAC and HVA in the push-pull perfusates. The data indicate that fluoxetine, in accord with its role as a 5-HT uptake inhibitor, increases the physiologically active pool of 5-HT in the nucleus accumbens under in vivo conditions.     

Regional serotonin metabolism in the brain of transgenic mice lacking monoamine oxidase A.

The effect of a lack of the gene encoding monoamine oxidase A (MAO A) in transgenic Tg8 mice on the activity of tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-HT) biosynthesis, and on the levels of 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) in the midbrain, hypothalamus, hippocampus, striatum, amygdala, and frontal cortex was studied. It was shown that mice with a genetic MAO A knockout differed from mice of the initial C3H/HeJ strain in having a higher level of 5-HT and a lower level of its metabolite, 5-HIAA, in all brain regions but the frontal cortex, where the changes were insignificant. Although the 5-HIAA/5-HT ratio in various brain regions differed considerably, the decrease of the 5-HT oxidative deamination index in Tg8 mice was similar in different brain regions (to 41-45% of control values), with the exception of the frontal cortex, where the decrease of the 5-HIAA/5-HT was somewhat smaller (to 54%). The presence of the remaining 45% +/- 1.9% of the control ratio value indicates rather effective oxidative deamination of 5-HT in MAO A knockout mice and explains the lack of severe behavioral and pathological consequences in MAO A genetic deficiency. An increase of TPH activity in mice lacking MAO A was found in the frontal cortex, hippocampus, and amygdala. No significant changes were found in the striatum, hypothalamus, and midbrain. The data show an effect of the MAO A gene mutation on TPH and indicate a uniform decrease of 5-HT catabolism in different brain regions except for the frontal cortex, which is somewhat more resistant to the lack of MAO A than other brain structures.     

Ketamine plus imipramine treatment induces antidepressant-like behavior and increases CREB and BDNF protein levels and PKA and PKC phosphorylation in rat brain

A growing body of evidence has pointed to the N-methyl-d-aspartate (NMDA) receptor antagonists as a potential therapeutic target for the treatment of major depression. The present study investigated the possibility of synergistic interactions between antidepressant imipramine with the uncompetitive NMDA receptor antagonist ketamine. Wistar rats were acutely treated with ketamine (5 and 10 mg/kg) and imipramine (10 and 20 mg/kg) and then subjected to forced swimming tests. The cAMP response element bindig (CREB) and brain-derived neurotrophic factor (BDNF) protein levels and protein kinase C (PKC) and protein kinase A (PKA) phosphorylation were assessed in the prefrontal cortex, hippocampus and amygdala by imunoblot. Imipramine at the dose of 10 mg/kg and ketamine at the dose of 5 mg/kg did not have effect on the immobility time; however, the effect of imipramine (10 and 20 mg/kg) was enhanced by both doses of ketamine. Ketamine and imipramine alone or in combination at all doses tested did not modify locomotor activity. Combined treatment with ketamine and imipramine produced stronger increases of CREB and BDNF protein levels in the prefrontal cortex, hippocampus and amygdala, and PKA phosphorylation in the hippocampus and amygdala and PKC phosphorylation in prefrontal cortex. The results described indicate that co-administration of antidepressant imipramine with ketamine may induce a more pronounced antidepressant activity than treatment with each antidepressant alone. This finding may be of particular importance in the case of drug-resistant patients and could suggest a method of obtaining significant antidepressant actions whilst limiting side effects.     

Effects of triiodothyronine and fluoxetine on 5-HT1A and 5-HT1B autoreceptor activity in rat brain: regional differences

The thyroid hormone triiodothyronine (T3) augments and accelerates the effects of antidepressant drugs. Although the majority of studies showing this have used tricyclics, a few studies have shown similar effects with the selective serotonin re-uptake inhibitor (SSRI) fluoxetine. In this study we investigated the effects of fluoxetine (5 mg/kg), T3 (20 μg/kg) and the combination of these drugs, each administered daily for 7 days, on serotonergic function in the rat brain, using in vivo microdialysis. Fluoxetine alone induced a trend towards desensitization of 5-HT_1A autoreceptors as shown by a reduction in the effect of 8-OH-DPAT to lower 5-HT levels in frontal cortex, and desensitized 5-HT_1B autoreceptors in frontal cortex. The combination of fluoxetine and T3 induced desensitization of 5-HT_1B autoreceptors in hypothalamus. Since there is evidence linking hypothalamic function and depression, we suggest that this effect may partly account for the therapeutic efficacy of the combination of an SSRI and T3.     

Functional meaning of tryptophan-induced increase of 5-HT metabolism as clarified by in vivo voltammetry

Differential pulse voltammetry with carbon fiber electrodes was used to study serotonin (5-HT) metabolism in freely moving rats. The electrodes implanted in the striatum recorded the extracellular 5-hydroxyindoleacetic acid (5-HIAA) oxidation peak after oral tryptophan (150 mg/kg). This 5-HT precursor did not modify the 5-HIAA peak in any rat tested, but it raised 5-HIAA levels determined in total tissue by a classical biochemical method (HPLC). The administration of 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.) induced an increase of 5-HIAA detectable both in the extracellular medium by voltammetry and in tissue samples. As previously shown, dorsal raphe electrical stimulation raises extracellular 5-HIAA in the striatum and this effect is enhanced by pretreatment with tryptophan. The results suggest that tryptophan in 'normal' conditions enhances 5-HT metabolism without affecting 5-HT release unless such release is stimulated. 5-HTP increases 5-HT metabolism and release.