Selective reduction by isolation rearing of 5-HT1A receptor-mediated dopamine release in vivo in the frontal cortex of mice

Serotonin (5-HT)1A receptors modulate in vivo release of brain monoaminergic neurotransmitters which may be involved in isolation-induced aggressive behavior. The present study examined the effect of isolation rearing on the 5-HT1A receptor-mediated modulation of dopamine (DA), 5-HT and noradrenaline (NA) release in the frontal cortex of mice. The selective 5-HT1A receptor agonist (S)-5-[-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242) increased the release of DA and NA and decreased the release of 5-HT in the frontal cortex of mice. The effect of MKC-242 on DA release was significantly less in isolation-reared mice than in group-reared mice, while effects of the drug on NA and 5-HT release did not differ between both groups. The effect of the other 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin on cortical DA release was also less in isolation-reared mice than in group-reared mice, and that of the drug on cortical 5-HT release did not differ between both groups. In contrast to MKC-242-induced DA release, amphetamine-induced increase in cortical DA release in vivo was greater in isolation-reared mice. The present findings suggest that isolation rearing enhances the activity of cortical dopaminergic neurons and reduces selectively the 5-HT1A receptor-mediated release of DA in the cortex.     

Combined Administration of a 5-Hydroxytryptamine (5-HT)1D Antagonist and a 5-HT Reuptake Inhibitor Synergistically Increases 5-HT Release in Guinea Pig Hypothalamus In Vivo

Abstract: In vivo microdialysis in guinea pig hypothalamus was used to study the effect of serotonin [5-hydroxytryptamine (5-HT)] subtype 1D autoreceptor blockade on the increase in extracellular 5-HT levels produced by a selective 5-HT reuptake inhibitor (SSRI). Administration of the selective 5-HT_1D antagonist GR127935 at 0.3 mg/kg had no effect, but 5 mg/kg significantly increased extracellular levels of 5-HT and 5-hydroxyindoleacetic acid to 135% of basal values. Moreover, at these doses GR127935 significantly attenuated the decrease in extracellular 5-HT levels following local perfusion with the selective 5-HT_1D agonist CP-135,807. The SSRI sertraline at 2 mg/kg increased 5-HT levels to 130% of basal levels. The combination of this low dose of sertraline with either dose of GR127935 resulted in a pronounced, long-lasting increase in 5-HT levels to 230% of basal values. These results indicate that the effects of an SSRI on terminal 5-HT are significantly enhanced by coadministration of a 5-HT_1D antagonist and confirm that in addition to somatodendritic 5-HT_1A autoreceptors, terminal 5-HT_1D autoreceptors mitigate the effect of SSRIs on terminal 5-HT. As such, antagonists of the 5-HT_1D autoreceptor could be useful as rapidly acting antidepressants and may shorten the onset of antidepressant action when combined with SSRIs.     

Acute effects of combining citalopram and pindolol on regional brain serotonin synthesis in sham operated and olfactory bulbectomized rats

The olfactory bulbectomized (OBX) rat is considered to be a good model of the pathology of human depression and also of the functional actions of antidepressant drug therapy. It has been proposed that antidepressant effects of selective serotonin reuptake inhibitors (SSRIs) can be accelerated by blocking 5-HT_1A/B autoreceptors with pindolol. The underlying mechanism is thought to involve acute unrestricting of 5-HT release and, consequently, relatively enhanced 5-HT turnover throughout the forebrain serotonergic networks. The effect of this combination on 5-HT turnover in sham operated or OBX rats can be assessed at the level of 5-HT synthesis, a very important presynaptic step in serotonergic neurotransmission, using the α-[¹⁴C]methyl-l-tryptophan autoradiography method. In sham rats, acute citalopram (20 mg/kg) treatment increased synthesis at almost all serotonergic terminal regions but slightly decreased synthesis at serotonergic cell body regions (i.e. dorsal and median (not significant) raphe; ～16%). Combining pindolol (10 mg/kg) with citalopram further increased synthesis at many regions in sham rats (relative to treatment with only citalopram). In OBX rats, citalopram decreased synthesis at a few terminal regions and greatly decreased synthesis at the dorsal and median raphe (～45%; relative to OBX rats treated with saline). Combining pindolol with citalopram greatly increased synthesis at almost all regions in OBX rats (relative to treatment with only citalopram). These results suggest that acute citalopram effects result in elevated terminal 5-HT synthesis, but these effects are restrained by 5-HT_1A/B autoreceptor feedback to different degrees in sham and OBX rats. Moreover, 5-HT_1A/B autoreceptor feedback is stronger in OBX rats and may underlie the delay of SSRI effects in OBX rats and, correspondingly, in human depression. Pindolol acceleration and augmentation of SSRI antidepressant therapy for human depression may be mediated by attenuation of 5-HT_1A/B autoreceptor feedback, permitting unhindered SSRI effects on serotonergic terminals.     

Activation of 5-HT(1A) but not 5-HT(1B) receptors attenuates an increase in extracellular dopamine derived from exogenously administered L-DOPA in the striatum with nigrostriatal denervation

In order to determine whether L-DOPA-derived extracellular dopamine (DA) in the striatum with dopaminergic denervation is affected by activation of serotonin autoreceptors (5-HT(1A) and 5-HT(1B) receptors), we applied in vivo brain microdialysis technique to 6-hydroxydopamine-lesioned rats and examined the effects of the selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and the selective 5-HT(1B) receptor agonist CGS-12066 A on L-DOPA-derived extracellular DA levels. Single L-DOPA injection (50 mg/kg i.p.) caused a rapid increase and a following decrease of extracellular DA, with a peak value at 100 min after L-DOPA injection. Pretreatment with both 0.3 mg/kg and 1 mg/kg 8-OH-DPAT (i.p.) significantly attenuated an increase in L-DOPA-derived extracellular DA and the times of peak DA levels were prolonged to 150 min and 225 min after L-DOPA injection, respectively. These 8-OH-DPAT-induced changes in L-DOPA-derived extracellular DA were antagonized by further pretreatment with WAY-100635, a selective 5-HT(1A) antagonist. In contrast, intrastriatal perfusion with the 5-HT(1B) agonist CGS-12066 A (10 nM and 100 nM) did not induce any changes in L-DOPA-derived extracellular DA. Thus, stimulation of 5-HT(1A) but not 5-HT(1B) receptors attenuated an increase in extracellular DA derived from exogenous L-DOPA. These results support the hypothesis that serotonergic neurons are primarily responsible for the storage and release of DA derived from exogenous L-DOPA in the absence of dopaminergic neurons.     

Acute and chronic effects of citalopram on postsynaptic 5-hydroxytryptamine(1A) receptor-mediated feedback: a microdialysis study in the amygdala

Microdialysis was used to assess the involvement of postsynaptic 5-hydroxytryptamine(1A) (5-HT(1A)) receptors in the regulation of extracellular 5-HT in the amygdala. Local infusion of the 5-HT(1A) receptor agonist flesinoxan (0.3, 1, 3 microM) for 30 min into the amygdala maximally decreased 5-HT to 50% of basal level. Systemic administration of citalopram (10 micromol/kg) increased 5-HT to 175% of basal level. Local infusion of 1 microM of the 5-HT(1A) receptor antagonist WAY 100.635 into the amygdala augmented the effect of citalopram to more than 500% of basal 5-HT level. 5-HT(1A) receptor responsiveness after chronic citalopram treatment was determined in two ways. First, by local infusion of 1 microM flesinoxan for 30 min into the amygdala, which showed a significant 63% reduction in response (area under the concentration-time curve; AUC) for the citalopram group compared to the saline group. Second, by systemic administration of citalopram (10 micromol/kg), which increased 5-HT to 350% of basal level. The effect was larger than in untreated animals, but more important, local infusion of 1 microM WAY 100.635 into the amygdala now failed to augment the effect of citalopram. Both the flesinoxan and WAY 100.635 data suggest an involvement of postsynaptic 5-HT(1A) receptor-mediated feedback in the amygdala, which diminishes following chronic citalopram treatment.     

Blockade of substance P (neurokinin 1) receptors enhances extracellular serotonin when combined with a selective serotonin reuptake inhibitor: an in vivo microdialysis study in mice

Abstract Substance P antagonists of the neurokinin-1 receptor type (NK1) are gaining growing interest as new antidepressant therapies. It has been postulated that these drugs exert this putative therapeutic effect without direct interactions with serotonin (5-HT) neurones. Our recent microdialysis experiment performed in NK1 receptor knockout mice suggested evidence of changes in 5-HT neuronal function (Froger et al. 2001). The aim of the present study was to evaluate the effects of coadministration of the selective 5-HT reuptake inhibitor (SSRI) paroxetine with a NK1 receptor antagonist (GR205171 or L733060), given either intraperitoneally (i.p.) or locally into the dorsal raphe nucleus, on extracellular levels of 5-HT ([5-HT]ext) in the frontal cortex and the dorsal raphe nucleus using in vivo microdialysis in awake, freely moving mice. The systemic or intraraphe administration of a NK1 receptor antagonist did not change basal cortical [5-HT]ext in mice. A single systemic dose of paroxetine (4 mg/kg; i.p.) resulted in a statistically significant increase in [5-HT]ext with a larger extent in the dorsal raphe nucleus (+ 138% over basal AUC values), than in the frontal cortex (+ 52% over basal AUC values). Co-administration of paroxetine (4 mg/kg; i.p.) with the NK1 receptor antagonists, GR205171 (30 mg/kg; i.p.) or L733060 (40 mg/kg; i.p.), potentiated the effects of paroxetine on cortical [5-HT]ext in wild-type mice, whereas GR205171 (30 mg/kg; i.p.) had no effect on paroxetine-induced increase in cortical [5-HT]ext in NK1 receptor knock-out mice. When GR205171 (300 micro mol/L) was perfused by 'reverse microdialysis' into the dorsal raphe nucleus, it potentiated the effects of paroxetine on cortical [5-HT]ext, and inhibited paroxetine-induced increase in [5-HT]ext in the dorsal raphe nucleus. Finally, in mice whose 5-HT transporters were first blocked by a local perfusion of 1 micro mol/L of citalopram into the frontal cortex, a single dose of paroxetine (4 mg/kg i.p.) decreased cortical 5-HT release, and GR205171 (30 mg/kg i.p.) reversed this effect. The present findings suggest that NK1 receptor antagonists, when combined with a SSRI, augment 5-HT release by modulating substance P/5-HT interactions in the dorsal raphe nucleus.     

Selective serotonin reuptake blockade increases extracellular dopamine in noradrenaline-rich isocortical but not prefrontal areas: dependence on serotonin-1A receptors and independence from noradrenergic innervation

The present study investigated the effects of two serotonin (5-HT) uptake inhibitors, citalopram and paroxetine, and of a non-selective noradrenaline (NA) and 5-HT uptake blocker, imipramine, on extracellular NA and dopamine (DA) in the prefrontal cortex (PfCX), parietal cortex (ParCX) and occipital cortex (OccCX). Citalopram, the most selective 5-HT uptake blocker, increased dialysate DA in the OccCX and ParCX but not in the PfCX and this effect was prevented in the OccCX by WAY-100635, an antagonist of serotonin-1A (5-HT(1A)) receptors, but not by dorsal noradrenergic bundle (DNAB) lesions that reduced to unmeasurable levels basal dialysate NA but did not affect dialysate DA. Paroxetine, a less selective 5-HT uptake inhibitor than citalopram, at the dose of 5 mg/kg, increased DA in the OccCX but not in the PfCX; however, at doses of 10 mg/kg, which increase PfCX NA, paroxetine increased DA also in this area. Imipramine increased dialysate DA and NA both in the PfCX and in the OccCX and this effect was abolished by DNAB lesions and was reduced but not abolished by WAY-100635. Administration of doses of reboxetine and citalopram that do not increase DA release in the OccCX if given separately, markedly increased DA when combined. These results indicate that endogenous 5-HT, raised by selective blockade of the 5-HT carrier, can increase extracellular DA in the OccCX and in the ParCX by stimulating 5-HT(1A) receptors independently from the presence of NA terminals, although blockade of 5-HT and NA carrier can strongly interact to raise extracellular DA in this area. These observations are consistent with the existence of DA neurons separate from the NA ones contributing to extracellular DA even in NA-rich/DA poor isocortical areas.     

Tandospirone potentiates the fluoxetine-induced increases in extracellular dopamine via 5-HT(1A) receptors in the rat medial frontal cortex.

Recent clinical studies suggest that 5-HT(1A) receptor agonists, including buspirone, may have an antidepressant effect and potentiate the efficacy of selective serotonin reuptake inhibitors (SSRI) in major depressive disorders. In the present study, we investigated the effect of tandospirone, a highly potent and selective 5-HT(1A) receptor agonist, on dopamine release and potentiation of fluoxetine-induced dopamine outflow in the medial frontal cortex using microdialysis in freely moving rats. Intraperitoneal injection of tandospirone (5 mg/kg) increased dopamine release to about 190% of basal levels. Pretreatment with the selective 5-HT(1A) receptor antagonist, WAY 100635 (1mg/kg), blocked the effect of tandospirone. Local application of WAY 100635 (10 microM) via microdialysis probe antagonized the increase in dopamine release in the medial frontal cortex induced by systemic injection of tandospirone. Fluoxetine (10 mg/kg) also increased dopamine release in the medial frontal cortex, to 200% of basal levels, and the simultaneous administration of tandospirone and fluoxetine increased the release to 380%. These results indicate that tandospirone potentiates the fluoxetine-induced increase in dopamine release via 5-HT(1A) receptors in the rat medial frontal cortex, and suggest that tandospirone may have therapeutic potential for the treatment of depression.     

Sustained pharmacological blockade of NK1 substance P receptors causes functional desensitization of dorsal raphe 5-HT 1A autoreceptors in mice

Antagonists at NK1 substance P receptors have demonstrated similar antidepressant properties in both animal paradigms and in human as selective serotonin reuptake inhibitors (SSRIs) that induce desensitization of 5-HT 1A autoreceptors within the dorsal raphe nucleus (DRN). We investigated whether this receptor adaptation also occurs upon NK1 receptor blockade. C57B/L6J mice were treated for 21 days with the selective NK1 receptor antagonist GR 205171 (10 mg/kg daily) through subcutaneously implanted osmotic mini pumps, and DRN 5-HT 1A autoreceptor functioning was assessed using various approaches. Recording of DRN serotonergic neurons in brainstem slices showed that GR 205171 treatment reduced (by approximately 1.5 fold) the potency of the 5-HT 1A receptor agonist, ipsapirone, to inhibit cell firing. In parallel, the 5-HT 1A autoreceptor-mediated [35S]GTP-gamma-S binding induced by 5-carboxamidotryptamine onto the DRN in brainstem sections was significantly decreased in GR 205171-treated mice. In vivo microdialysis showed that the cortical 5-HT overflow caused by acute injection of the SSRI paroxetine (1 mg/kg) was twice as high in GR 205171-treated as in vehicle-treated controls. In the DRN, basal 5-HT outflow was significantly enhanced by GR 205171 treatment. These data supported the hypothesis that chronic NK1 receptor blockade induces a functional desensitization of 5-HT 1A autoreceptors similar to that observed with SSRIs.     

Serotonin-2C antagonism augments the effect of citalopram on serotonin and dopamine levels in the ventral tegmental area and nucleus accumbens

Many patients with major depression do not respond to selective serotonin reuptake inhibitors (SSRIs). Lack of response could be due to inhibition of dopamine (DA) release by serotonin (5-HT) through 5-HT_2C receptors. Combining an SSRI with a 5-HT_2C antagonist may result in improved efficacy by causing simultaneous increases of 5-HT and DA. In order to test this augmentation strategy, male Wistar rats were treated (s.c.) with an acute dose of the SSRI citalopram (Cit, 5 mg/kg), the 5-HT_2C antagonist SB 242084 (SB, 2 mg/kg), or Cit + SB, and the effect on 5-HT and DA release in the nucleus accumbens (NAcc) was assessed by microdialysis. In a separate experiment, animals were treated with vehicle, Cit (20 mg/kg/d), SB (2 mg/kg/d) or Cit + SB for a period of 2 days (s.c.), and the impact on the release of 5-HT and DA in the ventral tegmental area (VTA) and NAcc was studied. On the day of microdialysis, 5-HT_2C receptor sensitivity was assessed with an SB challenge. Acutely administered Cit + SB increased 5-HT release in the NAcc more than Cit alone. SB alone increased DA release in the NAcc (not in the VTA), but when administered together with Cit, this effect was abolished. A 2-day treatment with Cit or Cit + SB increased 5-HT release in both VTA and NAcc. Combining Cit with SB augmented the effect of Cit in the VTA. DA release in VTA and NAcc was only significantly increased after 2-days of treatment with Cit + SB. In conclusion, Cit + SB had synergistic effects on 5-HT and DA release after 2-days of treatment, probably related to a decreased tonic inhibition of DA release via 5-HT_2C receptors. Regional differences occur and future studies should elucidate if this augmentation strategy is beneficial at the behavioral level.     

Selective stimulation of serotonin2c receptors blocks the enhancement of striatal and accumbal dopamine release induced by nicotine administration

The effects of acute and repeated nicotine administration on the extracellular levels of dopamine (DA) in the corpus striatum and the nucleus accumbens were studied in conscious, freely moving rats by in vivo microdialysis. Acute intraperitoneal (i.p.) injection of nicotine (1 mg/kg) increased DA outflow both in the corpus striatum and the nucleus accumbens. Repeated daily injection of nicotine (1 mg/kg, i.p.) for 10 consecutive days caused a significant increase in basal DA outflow both in the corpus striatum and the nucleus accumbens. Acute challenge with nicotine (1 mg/kg, i.p.) in animals treated repeatedly with this drug enhanced DA extracellular levels in both brain areas. However, the effect of nicotine was potentiated in the nucleus accumbens, but not in the corpus striatum. To test the hypothesis that stimulation of 5-HT (5-hydroxytryptamine, serotonin)(2C) receptors could affect nicotine-induced DA release, the selective 5-HT(2C) receptor agonist RO 60-0175 was used. Pretreatment with RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently prevented the enhancement in DA release elicited by acute nicotine in the corpus striatum, but was devoid of any significant effect in the nucleus accumbens. RO 60-0175 (1 and 3 mg/kg, i.p.) dose-dependently reduced the stimulatory effect on striatal and accumbal DA release induced by an acute challenge with nicotine (1 mg/kg, i.p.) in rats treated repeatedly with this alkaloid. However, only the effect of 3 mg/kg RO 60-0175 reached statistical significance. The inhibitory effect of RO 60-0175 on DA release induced by nicotine in the corpus striatum and the nucleus accumbens was completely prevented by SB 242084 (0.5 mg/kg, i.p.) and SB 243213 (0.5 mg/kg, i.p.), two selective antagonists of 5-HT(2C) receptors. It is concluded that selective activation of 5-HT(2C) receptors can block the stimulatory action of nicotine on central DA function, an effect that might be relevant for the reported antiaddictive properties of RO 60-0175.     

Combined chronic treatment with citalopram and lithium does not modify the regional neurochemistry of nitric oxide in rat brain.

A substantial number of patients do not respond sufficiently to antidepressant drugs and are therefore often co-medicated with lithium as an augmentation strategy. Also inhibitors of nitric oxide synthase (NOS) have been used as an augmentation strategy, while inhibitors of NOS exhibit antidepressant-like properties in various animal models. Therefore, we hypothesized that modulation of NOS may be involved in the long-term effects of antidepressants and lithium, and studied the influence of acute and chronic administration of citalopram, alone or in combination with lithium, on NOS activity in hippocampus, cerebellum, and frontal cortex, by determination of L-citrulline being formed. We found that administration of acute or chronic citalopram (5 mg/kg and 20 mg/kg/24h, respectively) alone or in combination with subchronic lithium (60 mmol/kg chow pellet) did not influence the activity of NOS ex vivo in all regions compared to control. In contrast, high doses of lithium caused a significant decrease in NOS activity in vitro. We conclude that basal conditions are unsuitable for the study of antidepressant effects on NOS, and that the neurochemistry of nitric oxide remains unaltered following chronic citalopram or subchronic lithium under normal physiological conditions.     

An index of 5-HT synthesis changes during early antidepressant treatment: alpha-[11C]methyl-L-tryptophan PET study

The antidepressant selective serotonin transporter inhibitors (SSRIs) are clinically active after a delay of several weeks. Indeed, the rapid increase of serotonin (5-HT) caused by SSRIs, stimulates the 5-HT(1A) autoreceptors, which exert a negative feedback on the 5-HT neurotransmission. Only when autoreceptors are desensitized, can SSRIs exert their therapeutic activity. The 5-HT(1A) receptor antagonist pindolol has been used to accelerate the clinical effects of antidepressant by preventing the negative feedback. Using the alpha-[(11)C]methyl-L-tryptophan/positron emission tomography (PET), the goal of the present double-blind, randomized study was to compare the changes in alpha-[(11)C]methyl-L-tryptophan trapping, an index of serotonin synthesis, in patients suffering from unipolar depression treated with the SSRI citalopram (20 mg/day) plus placebo versus patients treated with citalopram plus pindol (7.5 mg/day). PET and Hamilton depression rating scale (HDRS-17) were performed at baseline, and after 10 and 24 days of antidepressant treatment. Results show that the combination citalopram plus pindol, compared to citalopram alone shows a more rapid and greater increase of an index of 5-HT synthesis in prefrontal cortex (BA 9). This research is the first human PET study demonstrating that, after 24 days, the combination SSRIs plus pindolol produces a greater increase of the metabolism of serotonin in the prefrontal cortex, an area associated to depressive symptoms.     

Role of serotonin receptors in the delta-opioid immunosuppression

The present study indicates involvement of serotoninergic (5-HTergic) mechanisms in immunosuppression by DSLET (100 mkg/kg), a selective agonist of the delta2-opioid receptors, in CBA mice. 8-OH-DPAT (0.1 mg/kg), a selective agonist of the 5-HT(1A)-autoreceptors, WAY-100635 (1, 3 mg/kg) and ketanserin (1, 3 mg/kg), a selective antagonists of the postsynaptic 5-HT(1A)- and 5-HT(2A)-receptors, respectively, prevented immunosuppressive effect of DSLET. A possible differential role for 5-HT-receptors in delta-opioid immunosuppression is suggested.     

The serotonergic antidepressant nefazodone inhibits the serotonin transporter: In vivo and ex vivo studies

Nefazodone HCl (Serzone^®) is a new antidepressant with a chemical structure unrelated to selective serotonin reuptake inhibitors (SSRIs), tricyclics, tetracyclics, or monoamine oxidase inhibitors (MAOIs). Nefazodone is active in a number of preclinical tests for antidepressant activity and shows clinical efficacy in the treatment of depression with a more favorable side-effect profile than the structurally similar antidepressant trazodone. Previous studies have shown that nefazodone is a potent antagonist of 5-HT_2A receptors and binds to the serotonin transporter in vitro and in vivo. Nefazodone also binds to the norepinephrine transporter in vitro and in acute ex vivo studies. To further investigate the ability of nefazodone to modify serotonergic transmission, the ability of systemically administered nefazodone to inhibit the serotonin transporter was assessed by investigating the ability of nefazodone to prevent p-chloroamphetamine- (PCA) induced depletions of cortical 5-HT concentrations. In addition, the ability of acute and subchronic nefazodone administration to inhibit ex vivo [³H]-5-HT uptake was assessed. Acute administration of nefazodone (30, 100, and 150 mg/kg) antagonized PCA-induced depletion of cortical 5-HT concentrations in a dose-dependent manner at 1,2, and 3 hours post-treatment. This effect was directly correlated with serum nefazodone concentrations. Both 100 mg/kg and 150 mg/kg of nefazodone were equipotent with fluoxetine (10 mg/kg) over the course of the experiment with respect to sparing of 5-HT depletion. Acute administration of nefazodone (100 and 150 mg/kg s.c.) significantly increased the K_m, for [³H]-5-HT uptake in rat cortical synaptosomes from 60 nmol/L in controls to 230 and 242 nmol/L in nefazodone-treated rats, respectively. Subchronic administration of nefazodone (100 and 150 mg/kg, s.c., b.i.d. × 5.5 days) reduced [³H]-5-HT uptake by 24% and 29%, respectively. Sub-chronic dosing with fluoxetine (5 mg/kg, s.c., b.i.d. × 5.5 days) reduced [³H]-5-HT uptake by 65% These experiments confirm and extend previous reports concerning the ability of nefazodone to inhibit the 5-HT transporter in vivo.     

Effects of chronic paroxetine treatment on dialysate serotonin in 5-HT1B receptor knockout mice

The role of serotonin (5-HT)1B receptors in the mechanism of action of selective serotonin re-uptake inhibitors (SSRI) was studied by using intracerebral in vivo microdialysis in conscious, freely moving wild-type and 5-HT1B receptor knockout (KO 5-HT1B) mice in order to compare the effects of chronic administration of paroxetine via osmotic minipumps (1 mg per kg per day for 14 days) on extracellular 5-HT levels ([5-HT]ext) in the medial prefrontal cortex and ventral hippocampus. Basal [5-HT]ext values in the medial prefrontal cortex and ventral hippocampus, approximately 20 h after removing the minipump, were not altered by chronic paroxetine treatment in both genotypes. On day 15, in the ventral hippocampus, an acute paroxetine challenge (1 mg/kg i.p.) induced a larger increase in [5-HT]ext in saline-pretreated mutant than in wild-type mice. This difference between the two genotypes in the effect of the paroxetine challenge persisted following chronic paroxetine treatment. Conversely, in the medial prefrontal cortex, the paroxetine challenge increased [5-HT]ext similarly in saline-pretreated mice of both genotypes. Such a challenge produced a further increase in cortical [5-HT]ext compared with that in saline-pretreated groups of both genotypes, but no differences were found between genotypes following chronic treatment. To avoid the interaction with raphe 5-HT1A autoreceptors, 1 micro m paroxetine was perfused locally through the dialysis probe implanted in the ventral hippocampus; similar increases in hippocampal [5-HT]ext were found in acutely or chronically treated wild-type mice. Systemic administration of the mixed 5-HT1B/1D receptor antagonist GR 127935 (4 mg/kg) in chronically treated wild-type mice potentiated the effect of a paroxetine challenge dose on [5-HT]ext in the ventral hippocampus, whereas systemic administration of the selective 5-HT1A receptor antagonist WAY 100635 did not. By using the zero net flux method of quantitative microdialysis in the medial prefrontal cortex and ventral hippocampus of wild-type and KO 5-HT1B mice, we found that basal [5-HT]ext and the extraction fraction of 5-HT were similar in the medial prefrontal cortex and ventral hippocampus of both genotypes, suggesting that no compensatory response to the constitutive deletion of the 5-HT1B receptor involving changes in 5-HT uptake capacity occurred in vivo. As steady-state brain concentrations of paroxetine at day 14 were similar in both genotypes, it is unlikely that differences in the effects of a paroxetine challenge on hippocampal [5-HT]ext are due to alterations of the drug's pharmacokinetic properties in mutants. These data suggest that there are differences between the ventral hippocampus and medial prefrontal cortex in activation of terminal 5-HT1B autoreceptors and their role in regulating dialysate 5-HT levels. These presynaptic receptors retain their capacity to limit 5-HT release mainly in the ventral hippocampus following chronic paroxetine treatment in mice.     

Repeated Immobilization Stress Increases the Binding of c-Fos-Like Proteins to a Rat Dopamine β-Hydroxylase Promoter Enhancer Sequence

Abstract: The present study addressed the possibility that disinhibition of serotonin (5-HT) autoreceptor-mediated negative feedback might potentiate the elevation of nerve terminal 5-HT output induced by selective 5-HT reuptake blockade. To this end, rats were given citalopram and the 5-HT autoreceptor-blocking agents ( S )-UH-301 (5-HT_1A) and (−)-penbutolol (5-HT_1A/1B), and the effect on extracellular 5-HT in the ventral hippocampus was monitored by means of in vivo microdialysis. Citalopram (5 mg/kg, s.c.) approximately doubled the 5-HT output, a response that was markedly augmented by ( S )-UH-301 (3 mg/kg, s.c.) and (−)-penbutolol (8 mg/kg, s.c.) and by combined treatment with ( S )-UH-301 (3 mg/kg, s.c.) plus (−)-penbutolol (1 μ M ; via the dialysis perfusion medium), but not by (−)-penbutolol (1 μ M ) alone. These findings provide evidence that 5-HT, in particular 5-HT_1A, autoreceptor-mediated negative feedback mechanisms are pivotal in determining the nerve terminal 5-HT output level after 5-HT reuptake inhibition. These findings have important implications for the interplay between different processes controlling 5-HT transmission in vivo and might possibly offer a lead toward novel, therapeutically exploitable principles.     

Effect of pharmacologically selective antidepressants on serotonin uptake in rat platelets

We tested a hypothesis that a long-term administration of antidepressants acting through different primary biochemical mechanisms is associated with changes in the platelet serotonin (5-hydroxytryptamine, 5-HT) transport. Laboratory rats were administered norepinephrine reuptake inhibitors (desipramine, maprotiline), selective 5-HT reuptake inhibitor (citalopram), reversible monoamine oxidase inhibitor (moclobemide), and lithium (inositol monophosphatase inhibitor among others) during a 4-week period. Apparent kinetic parameters of platelet 5-HT transport were analyzed. Significant decrease in apparent Michaelis constant (K(M)) was found after the administration of all tested antidepressants except for desipramine. There was certain increase in maximal velocity (V(max)) values following the administration of desipramine, maprotiline, and citalopram; however, the all V(max) changes were not significant. V(max)/K(M) ratio representing limiting permeability at low extracellular concentrations of 5-HT was systematically increased in all the tested drugs, but significant changes were occurred only in maprotiline- and citalopram-treated rats. Adaptive changes in platelet 5-HT transport induced by citalopram were opposite to the acute inhibitory effect of this drug on 5-HT transporter activity. An increase in limiting membrane permeability for 5-HT could be included in the common adaptive effect of the long-term administration of antidepressants that differ in pharmacologic selectivity.     

Selective Serotonin Reuptake Inhibitors Potentiate the Rapid Antidepressant-Like Effects of Serotonin4 Receptor Agonists in the Rat

Background

We have recently reported that serotonin₄ (5-HT₄) receptor agonists have a promising potential as fast-acting antidepressants. Here, we assess the extent to which this property may be optimized by the concomitant use of conventional antidepressants.

Methodology/Principal Findings

We found that, in acute conditions, the 5-HT₄ agonist prucalopride was able to counteract the inhibitory effect of the selective serotonin reuptake inhibitors (SSRI) fluvoxamine and citalopram on 5-HT neuron impulse flow, in Dorsal Raphé Nucleus (DRN) cells selected for their high (>1.8 Hz) basal discharge. The co-administration of both prucalopride and RS 67333 with citalopram for 3 days elicited an enhancement of DRN 5-HT neuron average firing rate, very similar to what was observed with either 5-HT₄ agonist alone. At the postsynaptic level, this translated into the manifestation of a tonus on hippocampal postsynaptic 5-HT_1A receptors, that was two to three times stronger when the 5-HT₄ agonist was combined with citalopram. Similarly, co-administration of citalopram synergistically potentiated the enhancing effect of RS 67333 on CREB protein phosphorylation within the hippocampus. Finally, in the Forced Swimming Test, the combination of RS 67333 with various SSRIs (fluvoxamine, citalopram and fluoxetine) was more effective to reduce time of immobility than the separate administration of each compound.

Conclusions/Significance

These findings strongly suggest that the adjunction of an SSRI to a 5-HT₄ agonist may help to optimize the fast-acting antidepressant efficacy of the latter.     

Serotonergic Facilitation of Acetylcholine Release In Vivo from Rat Dorsal Hippocampus via Serotonin 5-HT3 Receptors

Abstract: The serotonin (5-HT) releaser d -fenfluramine and its active metabolite d -norfenfluramine, or the 5-HT-uptake inhibitor citalopram, by increasing synaptic 5-HT availability, facilitated in vivo release of acetylcholine (ACh) from dorsal hippocampi of freely moving rats as determined by the microdialysis technique. The effects of d -norfenfluramine (7.5 mg/kg i.p.) and citalopram (10 μ M , applied by reverse dialysis) were prevented by a 14-day chemical lesion of the raphe nuclei, suggesting mediation by the 5-HT system in the cholinergic action of the drugs. The increase in extracellular ACh content induced by d -norfenfluramine (5 mg/kg i.p.) was antagonized by the 5-HT₃ receptor antagonists tropisetron (0.5 mg/kg i.p.) and DAU 6215 (60 μg/kg i.p.), but not by the mixed 5-HT₁ and 5-HT₂ receptor antagonist metergoline (2 mg/kg s.c.). In accordance with an involvement of the 5-HT₃ receptor in the ACh facilitation induced by d-norfenfluramine is the finding that the selective 5-HT₃ receptor agonist 2-methyl-serotonin (250 μg i.c.v., or 10 μ M applied by reverse dialysis) raised ACh release. The effect of the intracerebroventricular drug was prevented by the 5-HT₃ antagonists DAU 6215 (60 μg/kg i.p.) and ondansetron (60 μg/kg s.c.). These antagonists by themselves did not modify the basal ACh release, indicating that 5-HT does not tonically activate the 5-HT₃ receptors involved. In conclusion, the overall regulatory control exerted by 5-HT in vivo is to facilitate hippocampal ACh release. This is mediated by 5-HT₃ receptors probably located in the dorsal hippocampi.