Acceleration of the effect of selected antidepressant drugs in major depression by 5-HT1A antagonists

Effectiveness of pindolol plus serotonin uptake inhibitors in depression: a meta-analysis of early and late outcomes from randomised controlled trials

BACKGROUND

Contradictory results on the efficacy of pindolol associated with selective serotonin reuptake inhibitors (SSRIs) in depressive illness have been published and no former review has produced an overall figure of its efficacy. This study aims to review the efficacy and tolerability of pindolol plus SSRIs in depressive illness.

METHODS

A meta-analysis of randomised controlled trials (RCTs) comparing pindolol plus SSRIs with placebo plus SSRIs.

RESULTS

Nine RCTs met inclusion criteria. Outcome favoured pindolol at 2 weeks time (N=5; OR=2.8; 95% CI 1.4-5.7), but not at four to 6 weeks (N=7; OR=1.4; 95% CI 0.8-2.7). Results for early outcome studies were robust to sensitivity analysis. Nineteen more studies, averaging null results, would be needed to change the overall probability (P=0.0001) to a non-significant figure.

CONCLUSIONS

Pindolol seems to hasten the response to SSRIs in depression with a timing window circumscribed to the first weeks of treatment.

R-citalopram functionally antagonises escitalopram in vivo and in vitro: evidence for kinetic interaction at the serotonin transporter

1. Clinical observations with the selective serotonin reuptake inhibitor (SSRI), S-citalopram, indicate that S-citalopram is more efficacious and produces earlier symptom relief than RS-citalopram. Since R-citalopram is at least 20-fold weaker than S-citalopram as inhibitor of the 5-HT transporter (SERT) in preclinical studies, the clinical data suggest an unexpected antagonistic interaction between the two enantiomers. We therefore characterised the interaction of R- and S-citalopram with the SERT in in vivo and in vitro assays. 2. In both behavioural (potentiation of 5-hydroxytryptophan (5-HTP)-induced behaviour) and electrophysiological studies (inhibition of 5-HT-elicited ion currents in Xenopus oocytes expressing the human SERT (hSERT) R-citalopram inhibited the effects of S-citalopram in a dose-dependent manner. With S-citalopram : R-citalopram ratios of 1 : 2 and 1 : 4, 5-HTP potentiation was significantly smaller than with S-citalopram alone. 3. R-citalopram did not antagonise the effects of another SSRI (fluoxetine) in either behavioural or electrophysiological studies. 4. In oocytes, inhibition of hSERT-mediated currents by R-citalopram was almost completely reversible and characterised by fast on- and off-sets of action. In contrast, the off-set for S-citalopram was 35-fold slower than for R-citalopram. 5. Kinetic analysis of the oocyte experiments suggests that S-citalopram binding to SERT induces a long-lasting, inhibited state of the transporter and that coapplication of R-citalopram partially relieves SERT of this persistent inhibition. 6. We propose that the kinetic interaction of R- and S-citalopram with SERT is a critical factor contributing to the antagonistic effects of R-citalopram on S-citalopram in vitro and in vivo.

Venlafaxine-induced depression of wind-up activity in mononeuropathic rats is potentiated by inhibition of brain 5-HT1A receptor expression in vivo

Antinociceptive effects of inhibiting 5-HT1A receptor expression by intracerebroventricular administration of an antisense oligodeoxynucleotide were studied in mononeuropathic rats. A 7-day period of treatment with the antisense produced: (i) reduction of mechanical hyperalgesia in the neuropathic hindlimb starting from day 5 of treatment, (ii) decrease of the hypothermic effect of 8-OH-DPAT challenge on day 6 of treatment, and (iii) potentiation of the inhibitory effect of velafaxine on spinal wind-up activity on day 7 of treatment. Results suggest a counteracting role of somatodendritic 5-HT1A receptors of raphe nuclei neurons in the antinociceptive efficacy of antidepressants with serotonergic spectrum in neuropathic pain.

Selectivity of (3)H-MADAM binding to 5-hydroxytryptamine transporters in vitro and in vivo in mice; correlation with behavioural effects

1. Binding of the novel radioligand (3)H-2-(2-dimethylaminomethyl-phenylsulphanyl)-5-methyl-phenylamine ((3)H-MADAM) to the serotonin transporter (SERT) was used to characterise a range of selective serotonin re-uptake inhibitors (SSRIs) in vitro and in vivo. 2. (3)H-MADAM bound with high affinity in a saturable manner to both human SERT expressed in CHO cells (K(d)=0.20 nm (pK(d)=9.74+/-0.12), B(max)=35+/-4 fmol mg(-1) protein) and mouse cerebral cortex membranes (K(d)=0.21 nm (pK(d)=9.66+/-0.10), B(max)=50+/-24 fmol mg(-1) protein). 3. Binding of (3)H-MADAM was highly selective for SERT in vitro as demonstrated by the in vitro profile of MADAM tested at 75 different receptors, ion channels and transporters. This was further substantiated by the pharmacological profile of the binding. Hence, the binding of (3)H-MADAM was potently inhibited by SSRIs but not by selective inhibitors of noradrenaline transport and dopamine transport. Likewise, a 5-HT(2A/2C) receptor antagonist did not inhibit (3)H-MADAM binding. 4. (3)H-MADAM binding in vivo was inhibited only by compounds which also inhibited the binding of (3)H-MADAM in vitro (the SSRIs, mixed SERT/noradrenaline transport inhibitors and clomipramine), confirming the selectivity of (3)H-MADAM for SERT also in vivo. Moreover, compounds effective in inhibiting (3)H-MADAM binding were the only ones found to be active in the mouse 5-HTP potentiation test confirming the model as a behavioural correlate to in vivo 5-HT uptake. 5. Finally, it was found that a SERT occupancy of 85-95% was necessary to produce 50% of the maximum behavioural response (ED(50)).

Onset and early behavioral effects of pharmacologically different antidepressants and placebo in depression

This study was aimed at resolving the time course of clinical action of antidepressants (ADs) and the type of early behavioral changes that precede recovery in treatment-responsive depressed patients. The first goal was to identify, during the first 2 weeks of treatment, the onset of clinical actions of the selective serotonin reuptake inhibitor (SSRI), paroxetine, and the selective noradrenergic reuptake inhibitor, desipramine (DMI). The second aim was to test the hypothesis that the two pharmacologic subtypes would induce different early behavioral changes in treatment-responsive patients. The design was a randomized, parallel group, placebo-controlled, double-blind study for 6 weeks of treatment following a 1-week washout period. The study utilized measures of the major behavioral components of the depressive disorder as well as overall severity. The results indicated that the onset of clinical actions of DMI ranged from 3 to 13 days, averaged 13 days for paroxetine, and was 16-42 days for placebo. Furthermore, as hypothesized, the different types of ADs initially impacted different behavioral aspects of the disorder. After 1 week of treatment, DMI produced greater reductions in motor retardation and depressed mood than did paroxetine and placebo, and this difference persisted at the second week of treatment. Early improvement in depressed mood-motor retardation differentiated patients who responded to DMI after 6 weeks of treatment from those that did not. Paroxetine initially reduced anxiety more in responders than in nonresponders, and by the second week, significantly improved depressed mood and distressed expression in responders to a greater extent. Depressed patients who responded to placebo showed no consistent early pattern of behavior improvement. Early drug-specific behavioral changes were highly predictive of ultimate clinical response to the different ADs, results that could eventually be applied directly to clinical practice.

Differential autoreceptor control of extracellular 5-HT in guinea pig and rat: species and regional differences

RATIONALE

Central 5-hydroxytryptamine (5-HT) release is regulated by inhibitory 5-HT autoreceptors, including 5-HT(1A) and 5-HT(1B) receptors.

OBJECTIVES

The purpose of this study was to use combinations of selective autoreceptor antagonists to elucidate the role of these receptors in controlling extracellular 5-HT in terminal areas. METHODS. Microdialysis was carried out in awake rats and guinea pigs to measure extracellular 5-HT in the frontal cortex and dentate gyrus. Using the selective 5-HT(1A) receptor antagonist, WAY-100635, and the selective 5-HT(1B) receptor antagonist, SB-224289, we have compared the roles of 5-HT(1A) and 5-HT(1B) autoreceptors in controlling extracellular 5-HT.

RESULTS

SB-224289 (4 mg/kg i.p.) alone produced a significant 50% increase in extracellular 5-HT in the dentate gyrus of guinea pigs, but not in the frontal cortex of the same animals. Co-administration of WAY-100635 (0.3 mg/kg s.c.), did not change the SB-224289-induced increase in dentate gyrus 5-HT but did produce a significant augmentation (60% increase) of guinea pig frontal cortex 5-HT. In contrast, neither autoreceptor antagonist, alone or in combination, affected extracellular 5-HT in the frontal cortex or dentate gyrus of rats.

CONCLUSIONS

These data indicate that there is a species difference in the autoreceptor control of 5-HT release. Furthermore, in the guinea pig there is a divergence between dorsal and median raphe innervated brain regions. On the basis that antagonism of 5-HT(1A) and 5-HT(1B) receptors produced an immediate increase in extracellular 5-HT in multiple brain regions in the guinea pig, it is suggested that this might be a novel mechanism for achieving antidepressant efficacy.

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.

Chronic antidepressant treatment alters serotonergic regulation of GABA transmission in prefrontal cortical pyramidal neurons

The serotonin system is highly involved in the pathophysiology of mood disorders such as depression and anxiety. Currently, the most widely used treatment for these illnesses is selective serotonin (5-HT)reuptake inhibitors, such as fluoxetine. Because of the multiplicity of 5-HT receptors and their different adaptive properties, the chronic effects of fluoxetine have remained unclear. In this study, we investigated the alteration of 5-HT functions by long-term antidepressant treatment in pyramidal neurons of prefrontal cortex (PFC), a brain region crucial for the control of emotion and cognition. One prominent function of serotonin in PFC is to regulate GABAergic inhibitory transmission. Application of 5-HT induced a large, desensitizing enhancement of the amplitude and frequency of spontaneous inhibitory postsynaptic currents (sIPSC), as well as a potent reduction of electrically evoked IPSC (eIPSC). Chronic fluoxetine treatment did not alter basal sIPSC, but reduced eIPSC in response to different stimulus strengths. Moreover, chronic (but not acute) fluoxetine treatment caused a much faster desensitization of the 5-HT effect on sIPSC, and significantly attenuated the 5-HT effect on eIPSC. Application of a 5-HT(2) receptor agonist produced similar effects as 5-HT on sIPSC and eIPSC, and these effects were similarly altered by long-term fluoxetine treatment. These electrophysiological results suggest that chronic antidepressant treatment resulted in a down-regulation of the synaptic function of forebrain 5-HT(2) receptors. Given the key role of GABAergic inhibitory transmission in controlling PFC functions, its altered regulation by serotonin after chronic fluoxetine treatment may provide a mechanism underlying the therapeutic action of antidepressants.

Milnacipran and pindolol: a randomized trial of reduction of antidepressant latency

BACKGROUND

New, better tolerated and faster treatments for depression are needed. Patients are understandably unhappy with having to wait 3 to 4 weeks for a response to an antidepressant, while experiencing side effects almost immediately. This frequently has an adverse effect on compliance and engagement with treatment.

AIMS

The primary objective was to assess the activity of pindolol on the onset of antidepressive response of milnacipran. The secondary objective was to assess the number of responders among the patients who received milnacipran and pindolol versus patients who received milnacipran and placebo. The tertiary objective was to evaluate the safety of milnacipran and pindolol versus milnacipran and placebo.

METHOD

Randomized, double-blind, placebo-controlled study over 42 days.

SETTING

Inner city London community mental health teams.

PARTICIPANTS

80 patients were selected and gave written consent to treatment, 78 were randomized (39 in each group) and evaluated for safety (intention-to-treat, ITT, safety data set), 77 (ITT efficacy data set), and 64 (per protocol, PP, data set) were evaluated for efficacy. The mean age was 31.9 for the pindolol group and 32.3 for the placebo.

INTERVENTION

All patients received milnacipran 50 mg twice a day plus either pindolol 2.5 mg (the 'pindolol group') or matching placebo (the 'placebo group') three times a day.

OUTCOME MEASURES

The main efficacy variable was the Montgomery-Asberg depression rating scale (MADRS) score at days 0, 4, 7, 10, 14, 21, 28, 42 on PP data set in an observation carried (OC) approach. Secondary efficacy variables were clinical global impression (global improvement) and Hamilton depression rating scale (HDRS).

RESULTS

Improvement in MADRS total score was greater in the pindolol group than in the placebo group from day 7 (p=0.03). Responder rates in the clinical global impression were 97.2% for the pindolol group and 60.6% for the placebo group. The treatment was well tolerated with the most common side effects being nausea (28.2%; 35.9%), vomiting (7.7%; 23.1%), hot flushes (15.4%; 5.1%) and sweating (12.8%; 12.8%).

CONCLUSION

The milnacipran and pindolol combination is safe, well tolerated and efficacious in major depression, and represents a rational strategy for the possible acceleration or potentiation of antidepressant action.

Regulation by 5-HT1A receptors of the in vivo release of 5-HT and DA in mouse frontal cortex

This study examines the effects of serotonin (5-HT)1A receptor ligands on the in vivo release of 5-HT and dopamine (DA) in the prefrontal cortex of mice. Oral MKC-242 and 8-OH-DPAT, selective 5-HT1A receptor agonists, decreased cortical 5-HT release at low and high doses, while the receptor agonists increased cortical DA release only at a high dose. Local application of the selective 5-HT1A receptor antagonist, WAY100635, via a dialysis probe, antagonized oral MKC-242-induced increase in cortical DA release, but did not affect the decrease in cortical 5-HT release. Local application of 8-OH-DPAT at 100 and 300 nM via a dialysis probe increased cortical DA release, but did not affect cortical 5-HT release. The effects of oral MKC-242 and 8-OH-DPAT on 5-HT release were blocked by low and high doses of WAY100635, while blocking the agonist-induced increase in DA release required a high dose of WAY100635. These results suggest that 5-HT release and DA release in the frontal cortex of mice are regulated by pre- and postsynaptic 5-HT1A receptors, respectively, and that the presynaptic 5-HT1A receptor-mediated response is more sensitive to inhibition by WAY100635 than the postsynaptic 5-HT1A receptor-mediated response in mice.

Antidepressant drug treatment induces Arc gene expression in the rat brain

The mechanism underlying the therapeutic effect of antidepressants is not known but neuroadaptive processes akin to long-term potentiation have been postulated. Arc (Activity-regulated, cytoskeletal-associated protein) is an effector immediate early gene implicated in LTP and other forms of neuroplasticity. Recent data show that Arc expression is regulated by brain 5-hydroxytryptamine neurones, a target of many antidepressants. Here in situ hybridisation and immunohistochemistry were used to examine whether Arc expression in rat brain is altered by antidepressant drug treatment. Repeated administration of the monoamine reuptake inhibitors paroxetine, venlafaxine or desipramine induced region-specific increases in Arc mRNA. These increases were greatest in regions of the cortex (frontal and parietal cortex) and hippocampus (CA1 layer) and absent in the caudate putamen. Repeated treatment with the monoamine oxidase inhibitor, tranylcypromine, increased Arc mRNA in a similar fashion to the monoamine reuptake inhibitors. The antidepressant drugs also increased the number of Arc-immunoreactive cells in the parietal cortex. Acute antidepressant injection, and repeated administration of the antipsychotic drug chlorpromazine, produced either limited or no changes in Arc mRNA. The data suggest that chronic treatment with antidepressant drugs induces Arc gene expression in specific regions across the rat forebrain. Up-regulation of Arc expression may be part of the process by which antidepressant drugs achieve long-term changes in synaptic function in the brain.

Impaired repression at a 5-hydroxytryptamine 1A receptor gene polymorphism associated with major depression and suicide

Inhibition of serotonergic raphe neurons is mediated by somatodendritic 5-HT1A autoreceptors, which may be increased in depressed patients. We report an association of the C(-1019)G 5-HT1A promoter polymorphism with major depression and suicide in separate cohorts. In depressed patients, the homozygous G(-1019) allele was enriched twofold versus controls (p = 0.0017 and 0.0006 for G/G genotype and G allele distribution, respectively), and in completed suicide cases the G(-1019) allele was enriched fourfold (p = 0.002 and 0.00008 for G/G genotype and G allele distribution, respectively). The C(-1019) allele was part of a 26 bp imperfect palindrome that bound transcription factors nuclear NUDR [nuclear deformed epidermal autoregulatory factor (DEAF-1)]/suppressin and Hairy/Enhancer-of-split-5 (Drosophila) (Hes5) to repress 5-HT1A or heterologous promoters, whereas the G(-1019) allele abolished repression by NUDR, but only partially impaired Hes5-mediated repression. Recombinant NUDR bound specifically to the 26 bp palindrome, and endogenous NUDR was present in the major protein-DNA complex from raphe nuclear extracts. Stable expression of NUDR in raphe cells reduced levels of endogenous 5-HT1A protein and binding. NUDR protein was colocalized with 5-HT1A receptors in serotonergic raphe cells, hippocampal and cortical neurons, and adult brain regions including raphe nuclei, indicating a role in regulating 5-HT1A autoreceptor expression. Our data indicate that NUDR is a repressor of the 5-HT1A receptor in raphe cells the function of which is abrogated by a promoter polymorphism. We suggest a novel transcriptional model in which the G(-1019) allele derepresses 5-HT1A autoreceptor expression to reduce serotonergic neurotransmission, predisposing to depression and suicide.

Effect of pindolol and milnacipran versus milnacipran and placebo on plasma prolactin and adrenocorticotrophic hormone in depressed subjects

OBJECTIVE

Evidence that pindolol accelerates the action of antidepressants has been contradictory, and it is not clear why. The present study analyses the relationship between plasma prolactin (PRL) and ACTH levels and changes in relation to a milnacipran and pindolol combination versus milnacipran plus placebo.

METHOD

Eighty depressed patients agreed to take part in a double-blind randomized trial of milnacipran plus pindolol or placebo. Fifty-eight of them agreed to also take measures of ACTH and PRL levels. ACTH and PRL plasma levels were estimated on days 0 and 42 of the 6-week study. Age, gender and time of blood collection were recorded for each individual. The Montgomery-Asberg depression rating scale (MADRS) was used to measure the response to treatment. The patients were grouped into those with higher versus lower basal ACTH levels using the median of the sample (25 ng/l).

RESULTS

There were statistical differences in MADRS scores between the treatment groups on day 42. There were correlations between PRL levels on days 0 and 42; age and PRL levels on day 0; time of the PRL sample and the PRL levels on day 0 and day 42; ACTH and PRL levels on day 42. Regression analysis of the 58 patients showed that on day 0, PRL levels were dependent on the ACTH plasma levels on day 0, the time of the collection of the blood sample and the age. On day 42, the PRL levels were dependent on the ACTH levels and the time of the blood collection but not on the age. Patients with lower baseline ACTH levels on day 0 displayed a better clinical outcome when taking the combination of milnacipran and pindolol as shown in the differences in MADRS on day 42. The same group of patients showed lower PRL levels on day 42.

CONCLUSIONS

ACTH plasma levels at baseline or screening may help to predict the response to antidepressant treatment.

The R-enantiomer of citalopram counteracts escitalopram-induced increase in extracellular 5-HT in the frontal cortex of freely moving rats

The selective serotonin (5-HT) reuptake inhibitor, citalopram, is a racemic mixture of an S(+)- and R(-)-enantiomer, escitalopram and R-citalopram, respectively. The present study compares the effects of escitalopram, R-citalopram and citalopram on extracellular levels of 5-HT in the frontal cortex of freely moving rats. In addition, co-injection of escitalopram and R-citalopram (ratios 1:2 and 1:4) were assessed. In some experiments escitalopram and R-citalopram were infused into the frontal cortex by reverse microdialysis. Finally, the extracellular level of escitalopram in the frontal cortex was studied after administration of escitalopram alone or in combination with R-citalopram. Escitalopram (1.0-3.9 mg/kg, s.c.) produced a greater maximal increase in extracellular 5-HT than citalopram (2.0-8.0 mg/kg, s.c.). R-citalopram (15.6 mg/kg s.c.) did not affect the 5-HT levels. When co-injected, R-citalopram counteracted the escitalopram-induced increase in extracellular 5-HT levels. Local infusion of the two enantiomers into the frontal cortex produced a similar inhibitory response. R-citalopram did not influence the extracellular levels of escitalopram and therefore does not exert its effect via a pharmacokinetic interaction with escitalopram. In conclusion, the 5-HT-reuptake inhibitory activity of citalopram resides in escitalopram, and the R-enantiomer counteracts this effect. This observation would predict an improved clinical profile of escitalopram compared to citalopram.

Pindolol and the acceleration of the antidepressant response

BACKGROUND

It has been suggested that treatment with selective serotonin reuptake inhibitors (SSRIs) in combination with pindolol, a partial agonist at the 5-HT(1A) receptor, may produce a fast antidepressant response. However, inconsistent results have been obtained in clinical studies with combination of the two drugs. Some studies, most using paroxetine, show an acceleration of the antidepressant response, whereas studies with other SSRIs find no marked latency reduction.

METHODS

The free SSRI concentration in patients either receiving the first dose, or in steady state treatment with the five different SSRIs citalopram, fluoxetine, fluvoxamine, paroxetine and sertraline was estimated using pharmacokinetic data for the individual drugs.

RESULTS

Due to differences between the drugs regarding protein binding, distribution volume and affinity for the 5-HT transporter (5-HTT), the 5-HT uptake inhibition obtained with clinically relevant doses differs markedly among the SSRIs.

CONCLUSIONS

A nearly complete blockade of the 5-HTT is obtained already after the first dose only with paroxetine, explaining why the latency reducing effect of pindolol preferentially is seen when pindolol is combined with paroxetine.

Differential neurochemical properties of central serotonergic transmission in Roman high- and low-avoidance rats

The selective breeding of Roman high- (RHA/Verh) and low-avoidance (RLA/Verh) rats for rapid versus poor acquisition of active avoidant behaviour has produced two behavioural phenotypes with different performances in a variety of animal models of anxiety, in which RLA/Verh rats are consistently more fearful than RHA/Verh rats. In addition, these two lines display different functional properties of brain neurotransmitters like serotonin (5-HT), known to be involved in the expression of anxiety- and depression-related behaviours. Therefore, we used brain microdialysis and [3H]-citalopram binding autoradiography to characterize further the neurochemical properties of 5-HTergic transmission in the two lines. No significant line-related differences were detected in the basal 5-HT output in the frontoparietal cortex (FPCx). In contrast, the increase in the cortical 5-HT output elicited by the systemic administration or the local application, via reverse dialysis, of chlorimipramine and fluoxetine was more robust in RHA/Verh than in RLA/Verh rats. Moreover, the binding signal of [3H]-citalopram to 5-HT re-uptake sites was more intense in the FPCx of RHA/Verh rats than in their RLA/Verh counterparts. These findings suggest that the functional tone of the 5-HTergic projection to the FPCx is stronger in the RHA/Verh line relative to the RLA/Verh line. It is proposed that RLA/Verh rats may be used as a model with heuristic value for studying the role of 5-HTergic transmission in anxiety and in the anxiolytic effects of monoamine re-uptake inhibitors.

Positron emission tomographic analysis of dose-dependent NAD-299 binding to 5-hydroxytryptamine-1A receptors in the human brain

RATIONALE

The serotonin 5-HT(1A) receptor has been ascribed a putative role in the pathophysiology and drug treatment of depression. NAD-299 (generic name robalzotan) is a new potential antidepressant with high affinity and selectivity for the 5-HT(1A) receptor.

OBJECTIVES

The aim of this positron emission tomography (PET) study was to examine the extent and time-course of 5-HT(1A) occupancy by NAD-299 in the human brain, in relation to plasma concentration after escalating single oral doses.

METHODS

Five healthy male subjects received one or more single oral doses of NAD-299 (0.5, 2.5 and 10 mg) in aqueous solution under fasting conditions. Total and unbound (after ultrafiltration) plasma concentrations of NAD-299 were determined by liquid chromatography-mass spectrometry (LC-MC), over a tentative dosage interval of 8 h. Regional 5-HT(1A) receptor occupancy in brain was calculated by the simplified reference tissue model using the radioligand [ carbonyl-(11)C]WAY-100635.

RESULTS

After the 10 mg dose, occupancy was high in the raphe (62-85%) and neocortical regions (68-75%) at time for C(max), but had declined considerably (17-44%) at 7 h after dose intake.

CONCLUSIONS

This study confirmed that the new selective 5-HT(1A) antagonist NAD-299 occupies 5-HT(1A) receptors in the living human brain in a dose-dependent manner following oral dosage. The curvilinear relationship between NAD-299 drug concentration and 5-HT(1A) receptor occupancy was established and can be used for dose selection in subsequent clinical patient studies.

Galanin attenuates basal and antidepressant drug-induced increase of extracellular serotonin and noradrenaline levels in the rat hippocampus

Galanin is co-localized with classical neurotransmitters, such as acetylcholine, serotonin (5-HT) and noradrenaline (NA) in neurons or in brain regions implicated in cognitive and affective behaviour. In the present study, the effects of galanin on extracellular 5-HT and NA levels in the rat hippocampus were measured by in vivo microdialysis under basal conditions and following systemic administration of antidepressant drugs. Galanin (1.5 nmol i.c.v.) reduced basal 5-HT and NA levels to 65% and 86% of controls, respectively. Galanin (0.5 and 1.5 nmol i.c.v.) dose-dependently attenuated the elevation of 5-HT concentrations induced by imipramine and citalopram (10 mg/kg i.p., each) from 350% to 312% and from 230% to 160%, respectively. Galanin at 1.5 nmol transiently attenuated the effect of desipramine-induced (10 mg/kg i.p.) increase in extracellular NA levels from a maximal increase of 389-296% of the predrug levels. It is concluded that intraventricularly administered galanin attenuated both basal 5-HT and NA release and antidepressant drug-induced accumulation of extracellular 5-HT and NA levels most likely via a predominant inhibitory action on serotonergic and noradrenergic neurons in the raphe and locus coeruleus, respectively. These results further emphasize a possible role of galanin in regulation of 5-HT and NA neurotransmission in depressive states and during the course of antidepressant therapy.

Modulation of serotonergic function in rat brain by VN2222, a serotonin reuptake inhibitor and 5-HT1A receptor agonist

VN2222 (1-(benzo[b]thiophen-3-yl)-3-[4-(2-methoxiphenyl piperazin-1-yl]propan-1-ol) is a potential antidepressant with high affinity for the serotonin transporter and 5-HT(1A) receptors. Locally applied, VN2222 enhanced the extracellular 5-hydroxytryptamine (5-HT) concentration (5-HT(ext)) in rat striatum to 780% of baseline whereas its systemic administration (1-10 mg/kg s.c.) reduced 5-HT(ext). In the presence of citalopram, 8-OH-DPAT or VN2222 applied in medial prefrontal cortex reduced 5-HT(ext). Fluoxetine, VN2222, and 8-OH-DPAT suppressed the firing rate of dorsal raphe 5-HT neurons (ED(50): 790, 14.9, and 0.8 microg/kg i.v., respectively). These effects were antagonized by WAY 100635. Administration of VN2222 for 2 weeks desensitized 5-HT(1A) receptors as assessed by microdialysis and single-unit recordings (ED(50) values for 8-OH-DPAT were 0.45 and 2.34 microg/kg i.v. for controls and rats treated with 6 mg/kg day VN2222). These results show that VN2222 is a mixed 5-HT reuptake inhibitor/5-HT(1A) agonist that markedly desensitizes 5-HT(1A) autoreceptors. These properties suggest that it may be a clinically effective dual action antidepressant drug.

5-HT1A receptor knockout mouse as a genetic model of anxiety

Low levels of the serotonin(1A) (5-HT(1A)) receptor have been repeatedly found in mood and anxiety disorders. Stress often exacerbates psychiatric disease and can also reduce 5-HT(1A) receptor levels. When receptor deficiency was produced in mice by genetic knockout, an anxiety-like phenotype was observed. Anxiety in mice is defined as a high level of avoidance of novel and unfamiliar environment and increased fear reaction. Other aspects of anxiety such as autonomic activation, increased stress responsiveness, and neuroendocrine abnormalities have also been described in receptor knockout mice. These data indicate that 5-HT(1A) receptor knockout mice represent a genetic animal model of anxiety with both construct and face validities. Although the core phenotype of anxiety can be reproduced in knockout mice in various inbred and outbred backgrounds, abnormalities in 5-HT dynamics and resistance to the anxiolitic drug diazepam have been seen in one but not on other genetic backgrounds. This indicates that while the development of anxiety is an invariable consequence of receptor deficit, other features induced by receptor loss are strongly modulated by other gene(s). Strain-dependent variability within the core phenotype does not diminish the value of 5-HT(1A) receptor knockout mice as a model of anxiety. Indeed, it is consistent with the manifestation of anxiety in genetically heterogeneous human population.

Regulation of 5-HT1A receptor function in brain following agonist or antidepressant administration

Adaptive changes in the serotonergic system are generally believed to underlie the therapeutic effectiveness of the azapirone anxiolytics and a variety of antidepressant drugs. The serotonin-1A (5-HT(1A)) receptor has been implicated in affective disorders. Thus, studies of the regulation of 5-HT(1A) receptor function may have important implications for our understanding the role of this receptor in the mechanism of action of these therapeutic agents. This review focuses on the regulation of central 5-HT(1A) receptor function following administration of 5-HT(1A) receptor agonists or antidepressant drugs expected to increase the synaptic concentration of the neurotransmitter 5-HT. The majority of evidence supports regional differences in the regulation of central 5-HT(1A) receptor function following repeated agonist or antidepressant administration, which may be due to differences in processes involved in desensitization of the receptor at the cellular level. Region-specific differences in the regulation of 5-HT(1A) receptor function may be based on compensatory changes distal to the receptor, such as regulatory changes at the level of effector (e.g. adenylyl cyclase or ion channel), or at the level of the G protein such as changes in G protein expression, or phosphorylation of the G protein. It may be that the increase in serotonin neurotransmission, due to somatodendritic autoreceptor desensitization following agonist or antidepressant treatment, to normo-sensitive 5-HT(1A) receptors in certain brain regions (e.g. hippocampus or cortex) and to sub-sensitive 5-HT(1A) receptors in other brain regions (e.g. amygdala or hypothalamus) underlies the therapeutic efficacy of these drugs.

Prior exposure to the elevated plus-maze sensitizes mice to the acute behavioral effects of fluoxetine and phenelzine

A single undrugged experience of the elevated plus-maze modifies future drug responses in the test. The present study investigated the effects of maze-experience on the acute behavioral effects of the monoamine oxidase inhibitor phenelzine and the serotonin reuptake inhibitor fluoxetine. Phenelzine (2.5-12.5 mg/kg) had no clear effect on plus-maze behavior in test-naive Swiss Webster mice, but dose-dependently increased anxiety-like behavior in maze-experienced subjects. Similarly, fluoxetine (5-20 mg/kg) produced non-significant trends for increased anxiety-like behavior in maze-naive mice, but significantly and dose-dependently increased anxiety-like behavior and suppressed locomotor activity in maze-experienced mice. The anxiogenic effects of the benzodiazepine receptor inverse agonist N-methyl-beta-carboline-3-carboxamide (FG 7142) (20 mg/kg) was abolished by prior test experience, suggesting an alteration in gamma-aminobutyric acid (GABA)/benzodiazepine receptor function with maze-experience. However, the benzodiazepine receptor antagonist flumazenil (5-20 mg/kg) produced a silent profile regardless of maze-experience. Present findings provide further evidence demonstrating that prior test history is a critical consideration in mouse studies of anxiety-related behavior.

Flattening the corticosterone rhythm attenuates 5-HT1A autoreceptor function in the rat: relevance for depression

Depression is associated with glucocorticoid abnormalities, in particular a flattening of the diurnal cortisol rhythm. Recent data suggest that an important factor in the aetiology of depression may be a deficit in the function and expression of 5-HT(1A) receptors, which has been reported in depressed patients. The present study assessed the possibility that this cortisol abnormality is causal in the 5-HT(1A) receptor deficits. First, a rat model of flattened glucocorticoid rhythm was developed. Controlled release corticosterone pellets implanted for 14 days flattened the corticosterone rhythm and maintained levels constant midway between the nadir and zenith levels observed in sham-operated rats. Secondly, using microdialysis to assess 5-HT release in the hippocampus, the inhibitory response to 8-OHDPAT was measured to determine the sensitivity of somatodendritic 5-HT(1A) autoreceptors. Corticosterone treatment was found to induce a significant attenuation in the response to 8-OHDPAT, indicating functional desensitization of somatodendritic 5-HT(1A) autoreceptors. There was no effect of corticosterone treatment on basal extracellular 5-HT levels. The data suggest that the glucocorticoid abnormalities associated with depression may impact on the functioning of 5-HT(1A) receptors in the brain. These findings suggest that resolution of cortisol abnormalities may be a valuable target for pharmacotherapy in the treatment of depression.

Effects of triiodothyronine and imipramine on basal 5-HT levels and 5-HT(1) autoreceptor activity in rat cortex

Clinical studies have shown that triiodothyronine (T3) both augments and accelerates the therapeutic response to antidepressant drugs, particularly tricyclics. There is evidence that this effect is mediated by the serotonergic system. We show here that T3 administered daily for 7 days over the range 0.02-0.5 mg/kg increases basal serotonin (5-hydroxytryptamine, 5-HT) levels, as measured by in vivo microdialysis in rat cortex, in a dose-dependent fashion. All the doses of T3 examined reduced 5-HT(1A) autoreceptor activity, as measured by the effect of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 0.05 mg/kg s.c.) to decrease 5-HT levels in frontal cortex. T3 administered daily for 14 days at 0.02 mg/kg also reduced 5-HT(1B) autoreceptor activity, as measured by the effect of locally administered 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93129, 10 microM) to decrease 5-HT levels. In animals administered imipramine (10 mg/kg/day by osmotic minipump) concurrently with T3 injections, no further changes in either 5-HT(1A) or 5-HT(1B) autoreceptor activity were seen. We suggest that the effect of T3 to accelerate the therapeutic actions of antidepressant drugs may be due to a combination of the actions of T3 at autoreceptors and the actions of the drugs at postsynaptic 5-HT(1A) receptors.

Comparison of the effects of antidepressants on norepinephrine and serotonin concentrations in the rat frontal cortex: an in-vivo microdialysis study

The present study employed in-vivo microdialysis techniques in the freely moving rat to systematically compare the neurochemical effects of various antidepressant agents on extracellular concentrations of norepinephrine (NE) and serotonin (5-HT) in the frontal cortex. We found that acute administration of the tricyclic antidepressant, desipramine (3-30 mg/kg, s.c.) and the dual serotonin/norepinephrine reuptake inhibitor, venlafaxine (3-30 mg/kg, s.c.), produced dose-dependent and robust increases in cortical NE concentrations (498% and 403%, respectively). Conversely, acute injection of the selective serotonin reuptake inhibitors, fluoxetine (30 mg/kg, s.c.) and paroxetine (1-10 mg/kg, s.c.), did not alter forebrain NE concentrations. However, paroxetine did produce a significant increase in cortical NE concentrations (164%) when administered at 30 mg/kg. These changes in NE were not paralleled by 5-HT, which showed no increase following administration of desipramine, venlafaxine, paroxetine or fluoxetine. Combination treatment with the 5-HT1A receptor antagonist, WAY-100635 (0.3 mg/kg, s.c.), significantly enhanced extracellular 5-HT concentrations following venlafaxine (10 and 30 mg/kg), fluoxetine (30 mg/kg) and paroxetine (3-30 mg/kg). Alternatively, WAY-100635 produced no augmentation of the antidepressant-induced changes in extracellular NE. Collectively, these studies show that paroxetine, at low to intermediate doses, and fluoxetine are selective for 5-HT versus NE systems, whereas venlafaxine produces similar effects on both 5-HT and NE levels at the effective doses tested.

The effects of acute and subchronic treatment with fluoxetine and citalopram on stimulus control by DOM

Previous reports from our laboratory have provided evidence that acute, i.e., concurrent, treatment with selective serotonin reuptake inhibitors (SSRIs) augments the stimulus effects of indoleamine and phenethylamine hallucinogens in the rat. In the present investigation, the acute effects of fluoxetine and citalopram on stimulus control induced by (-)-2,5-dimethoxy-4-methylamphetamine (DOM) were compared with those following subchronic, i.e., 10-day treatment with the SSRIs. Stimulus control was established using DOM (0.56 mg/kg; 75-min pretreatment time) in a group of 11 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. The effects of a range of doses of DOM when given alone were compared with those following both acute and subchronic pretreatment with fluoxetine and citalopram in combination with DOM. It was found that acute administration of fluoxetine and citalopram potentiated the stimulus effects of DOM. Furthermore, it was observed that the degree of potentiation was not diminished by treatment with either fluoxetine or citalopram for a period of 10 days. It is concluded that whatever adaptive changes may take place in response to a 10-day period of treatment with either citalopram or fluoxetine, these adaptations are independent of the mechanisms responsible for the potentiation of the stimulus effects of DOM by the SSRIs.

Dual NK(1) antagonists--serotonin reuptake inhibitors as potential antidepressants. Part 2: SAR and activity of benzyloxyphenethyl piperazine derivatives

The synthesis, structure-affinity relationship and activity of benzyloxyphenethyl piperazine derivatives combining NK(1) antagonism and serotonin reuptake inhibition is described. Compound 7u was shown to be active in animal models of 5-HT reuptake inhibition and central NK(1) receptor blockade, and was demonstrated to be orally active in an integrated model sensitive to both mechanisms. This class of compounds potentially represents a new generation of antidepressants.

Behavioral and neurochemical effects of 5-(4-[4-(5-Cyano-3-indolyl)-butyl)-butyl]-1-piperazinyl)-benzofuran-2-carboxamide (EMD 68843): a combined selective inhibitor of serotonin reuptake and 5-hydroxytryptamine(1A) receptor partial agonist

5-(4-[4-(5-Cyano-3-indolyl)-butyl)-butyl]-1-piperazinyl)-benzofuran-2-carboxamide (EMD 68843; vilazodone) is a novel compound with combined high affinity and selectivity for the 5-hydroxytryptamine (5-HT) transporter and 5-HT(1A) receptors. EMD 68843 was tested as a prototype compound, which benefits from dual pharmacological effects that could increase extracellular 5-HT to levels higher than those produced by conventional selective serotonin reuptake inhibitors (SSRIs). In Sf9 cells, EMD 68843 increased guanosine 5'-O-(3-[(35)S]thiotriphosphate) binding to 69% of the magnitude of the full 5-HT(1A) receptor agonist R-(1)-trans-8-hydroxy-2-[N-n-propyl-N-(39-iodo-29-propenyl)] aminotetralin (8-OH-PIPAT), indicating that it is a partial agonist at 5-HT(1A) receptors. Acute, systemic administration of EMD 68843 produced a larger maximal increase of extracellular 5-HT than the SSRI fluoxetine in both the ventral hippocampus (HPv) (558 versus 274%) and the frontal cortex (FC) (527 versus 165%). Regional differences in the response to the two drugs were also observed. These effects may be attributed to the differential regulation of 5-HT release in the HPv and FC by 5-HT(1A) autoreceptors. When challenged with the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT), EMD 68843-induced increases in extracellular 5-HT were greatly reduced in the HPv but to a lesser extent in the FC. In behavioral studies, EMD 68843 produced antidepressant-like effects in the forced swimming test in both rats and mice but only within a narrow dosage range. Like fluoxetine, EMD 68843 did not produce the symptoms of the 5-HT behavioral syndrome in rats but, unlike fluoxetine, pretreatment with EMD 68843 blocked expression of the 5-HT behavioral syndrome induced by 8-OH-DPAT. Taken together, the results show that EMD 68843 augments extracellular 5-HT levels in forebrain regions to a greater extent than fluoxetine. At higher doses, however, weak efficacy of EMD 68843 at postsynaptic 5-HT(1A) receptors may inhibit the expression of rodent antidepressant-like behaviors.

Origin and functional role of the extracellular serotonin in the midbrain raphe nuclei

There is considerable interest in the regulation of the extracellular compartment of the transmitter serotonin (5-hydroxytryptamine, 5-HT) in the midbrain raphe nuclei because it can control the activity of ascending serotonergic systems and the release of 5-HT in terminal areas of the forebrain. Several intrinsic and extrinsic factors of 5-HT neurons that regulate 5-HT release in the dorsal (DR) and median (MnR) raphe nucleus are reviewed in this article. Despite its high concentration in the extracellular space of the raphe nuclei, the origin of this pool of the transmitter remains to be determined. Regardless of its origin, is has been shown that the release of 5-HT in the rostral raphe nuclei is partly dependent on impulse flow and Ca(2+) ions. The release in the DR and MnR is critically dependent on the activation of 5-HT autoreceptors in these nuclei. Yet, it appears that 5-HT autoreceptors do not tonically inhibit 5-HT release in the raphe nuclei but rather play a role as sensors that respond to an excess of the endogenous transmitter. Both DR and MnR are equally responsive to the reduction of 5-HT release elicited by the local perfusion of 5-HT(1A) receptor agonists. In contrast, the effects of selective 5-HT(1B) receptor agonists are more pronounced in the MnR than in the DR. However, the cellular localization of 5-HT(1B) receptors in the raphe nuclei remains to be established. Furthermore, endogenous noradrenaline and GABA tonically regulate the extracellular concentration of 5-HT although the degree of tonicity appears to depend upon the sleep/wake cycle and the behavioral state of the animal. Glutamate exerts a phasic facilitatory control over the release of 5-HT in the raphe nuclei through ionotropic glutamate receptors. Overall, it appears that the extracellular concentration of 5-HT in the DR and the MnR is tightly controlled by intrinsic serotonergic mechanisms as well as afferent connections.

Dual serotonin and noradrenaline uptake inhibitor class of antidepressants potential for greater efficacy or just hype?

Preclinical and clinical studies support the rationale that development of single molecules, which would promote serotonergic and noradrenergic neurotransmission by inhibiting simultaneously the uptake of both monoamines, would potentially result in improved antidepressant drugs. Currently, the dual inhibitors of serotonin and noradrenaline uptake are venlafaxine, milnacipran and duloxetine. Based on the preclinical studies, the three drugs do show properties of inhibiting uptake of both monoamines in vitro and in vivo in the following order of decreasing potency: duloxetine, venlafaxine and milnacipran, and all exhibit low affinity at neuronal receptors of neurotransmitters, suggesting low side-effect potential. In double-blind, controlled studies, venlafaxine and milnacipran were repeatedly shown to be as efficacious as tricyclic antidepressant drugs in treating major depressive disorder, while one double-blind, placebo-controlled trial showed the antidepressant efficacy of duloxetine. Specifically designed comparative trials of dual uptake inhibitors against the other agents are needed to establish whether the dual uptake inhibitors show improvement in efficacy, rate of responders, antidepressive effects and/or remission.

Overexpression of 5-HT1B receptor in dorsal raphe nucleus using Herpes Simplex Virus gene transfer increases anxiety behavior after inescapable stress

5-HT(1B) autoreceptors have been implicated in animal models of stress and are regulated selectively by serotonin-selective reuptake inhibitors such as fluoxetine. These terminal autoreceptors regulate serotonin release from dorsal raphe nucleus (DRN) projections throughout rat forebrain. However, it has not been previously possible to manipulate 5-HT(1B) autoreceptor activity selectively without also changing 5-HT(1B) activity in other neurons mediating different behavioral responses. Therefore, we have developed a viral-mediated gene transfer strategy to express hemagglutinin-tagged 5-HT(1B) and manipulate these autoreceptors in DRN. Green fluorescent protein (GFP) was coexpressed from a separate transcriptional unit on the same amplicon to assist in monitoring infection and expression. We confirmed the expression and biological activity of both transgenic proteins in vitro. When injected directly into DRN using stereotaxic procedure, HA-5-HT(1B) receptors were expressed in serotonergic neurons and translocated to the forebrain. The effect of DRN expression of HA-5-HT(1B) on stress-induced behaviors was compared with control rats that received GFP-only amplicons. There was no change in immobility in the forced swim test. However, HA-5-HT(1B) expression significantly reduced entrances into the central region of an open-field arena after water-restraint stress without altering overall locomotor activity, but not in the absence of stress exposure. HA-5-HT(1B) expression also reduced entries into the open arms of the elevated plus maze after water restraint. Because these tests are sensitive to increases in anxiety-like behavior, our results suggest that overactivity of 5-HT(1B) autoreceptors in DRN neurons may be an important mediator of pathological responses to stressful events.

Behavioural and biochemical studies of citalopram and WAY 100635 in rat chronic mild stress model

Reversal of chronic mild stress (CMS)-induced decrease of sucrose consumption has been studied in rats after 2, 7, 14, and 35 days treatment with imipramine, citalopram (both 10 mg/kg per day, i.p.), WAY 100635 (0.2 mg/kg sc, b.i.d.), and citalopram plus WAY 100635. Bmax, Kd, and functional status [cyclic AMP (cAMP) generation] of beta1-adrenoceptors were assessed in cortical tissue at the same time points. Citalopram reversed CMS-induced reduction of sucrose intake at an earlier time point than imipramine. WAY 100635 was not effective and did not potentiate the effect of citalopram. CMS produced increase of Bmax. Imipramine decreased Bmax in controls (Days 2, 7, 14, and 35) and normalised Bmax in stressed animals (Day 35). Citalopram, WAY 100635, and the combination increased Bmax in stressed animals and controls (Days 14 and 35). Inconsistent changes of Kd values and of cAMP responses to noradrenaline (NA) stimulation were observed. Thus stress- and drug-induced effects on beta1-adrenoceptors do not appear to be a common biochemical marker of antidepressant-like activity in the CMS model.

Pharmacogenetics in affective disorders

Pharmacogenetics will be of substantial help in the field of affective disorders pharmacotherapy. The possible definition of a genetic liability profile for drug side-effects and efficacy will be of great help in treatments that need weeks to months to be effective. During the last few years, a number of groups have reported possible liability genes. The efficacy and time of onset of selective serotonin reuptake inhibitors have been associated with a polymorphism in the promoter region of the transporter (SERTPR) in many independent studies, while variants at the tryptophan hydroxylase gene, 5-HT2a receptor and G-protein beta3 have been associated with them in pilot studies. Lithium long-term prophylactic efficacy has been associated with SERTPR, TPH and inositol polyphosphate 1-phosphatase variants, though in unreplicated samples. A number of further candidate genes were not associated with these treatments. In conclusion, both acute and long-term treatments appear to be, at least to some extent, under genetic influence and preliminary data have identified possible liability genes.

Chronic fluoxetine treatment selectively uncouples raphe 5-HT(1A) receptors as measured by [(35)S]-GTP gamma S autoradiography

1. Selective Serotonin Reuptake Inhibitors (SSRIs) are thought to have a delay in therapeutic efficacy because of the need to overcome the inhibitory influence of raphe 5-HT(1A) autoreceptors. Prolonged SSRI administration has been reported to desensitize these autoreceptors. We have used [(35)S]-GTP gamma S autoradiography to determine whether this desensitization occurs at the level of receptor/G protein coupling. 2. Male mice were injected intraperitoneally once a day with saline or 20 mg kg(-1) fluoxetine for either 2 days or 14 days. 5-HT(1A) receptor binding and coupling to G proteins were assessed using [(3)H]-8-OH-DPAT and [(35)S]-GTP gamma S autoradiography, respectively. 3. The 5-HT receptor agonist 5-carboxamidotryptamine (5-CT) stimulated [(35)S]-GTP gamma S binding in the substantia nigra, as well as in hippocampus and dorsal raphe nucleus. The 5-HT(1A) receptor antagonist p-MPPF (4-fluoro-N-(2-[4-(2-methoxyphenyl)1-piperazinyl]ethyl)-N-(2-pyridinyl)benzamide) blocked this effect in the latter regions, whereas the 5-HT(1B/D) antagonist GR-127,935 (2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxylic acid [4-methoxy-3-(4-methyl-piperazin-l-yl)-phenyl]-amide) only decreased labelling in substantia nigra. 4. Fourteen-day fluoxetine treatment decreased 5-CT-stimulated [(35)S]-GTP gamma S binding in dorsal raphe (saline: 112 +/- 12% stimulation; fluoxetine: 66 +/- 13%), but not in substantia nigra (99 +/- 14% vs 103 +/- 7%) or hippocampus (157 +/- 3% vs 148 +/- 18%). Two-day fluoxetine treatment did not alter 5-CT-stimulated [(35)S]-GTP gamma S binding in any of the brain areas investigated. 5. Decreased [(35)S]-GTP gamma S binding was not due to receptor down-regulation, since the density of raphe [(3)H]-8-OH-DPAT binding sites was unaffected by fluoxetine treatment. 6. These results suggest that the desensitization of presynaptic 5-HT(1A) receptor function occurs at the level of receptor-G protein interaction on dorsal raphe neurons, and may underlie the therapeutic efficacy of long-term SSRI treatment.

Studies toward the discovery of the next generation of antidepressants. Part 2: incorporating a 5-HT(1A) antagonist component into a class of serotonin reuptake inhibitors

The design and synthesis of a novel series of indole derivatives (9) having dual 5-HT transporter reuptake and 5-HT(1A) antagonist activity are described.

Modification of serotonin neuron properties in mice lacking 5-HT1A receptors

Using null mutant mice for the 5-HT1A receptor (5-HT1A-/-), extracellular electrophysiological recordings were first conducted to evaluate the impact of its genetic deletion on the firing rate of dorsal raphe 5-hydroxytryptamine (5-HT) neurons. Experiments were also done using brain slices to assess whether any compensation phenomenon had taken place in key receptors known to control 5-HT and norepinephrine release. The mean firing rate of 5-HT neurons was nearly doubled in 5-HT1A-/- mice, although 65% of the neurons were firing in their normal range. In preloaded brain slices, the 5-HT1D/B receptor agonist sumatriptan equally inhibited the electrically evoked release of [3H]5-HT in mesencephalic slices (containing the dorsal and median raphe) from wildtype and 5-HT1A-/- mice. The 5-HT1B receptor agonist CP 93129 (1,4-dihydro-3-(1,2,3,6-tetrahydro-4-pyridinyl)-5H-pyrrol (3, 2-b) pyridin-5-one) and the alpha2-adrenoceptor agonist UK14,304 (5-bromo-N-(4, 5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine) produced the same inhibitory effect in both groups of mice in hippocampus and frontal cortex slices. No difference was observed on the UK14,304-mediated inhibition of [3H]norepinephrine from preloaded slices of the two latter structures between the two groups of mice. In conclusion, the loss of control of the 5-HT1A autoreceptor in 5-HT1A-/- mice lead to a significant enhancement of 5-HT neuronal firing, but it did not alter 5-HT or norepinephrine release in any of the brain structures examined. In addition, it was not associated with changes in the function of 5-HT1D and 5-HT1B autoreceptors and of alpha2-adrenergic heteroreceptors on 5-HT neurons, nor of that of alpha2-adrenoceptors on norepinephrine terminals.

First dual NK(1) antagonists-serotonin reuptake inhibitors: synthesis and SAR of a new class of potential antidepressants

Compounds combining NK(1) antagonism and serotonin reuptake inhibition are described, and potentially represent a new generation of antidepressants. Compound 24 displays good affinities for both the NK(1) receptor and the serotonin reuptake site (32 and 25 nM, respectively).

World Federation of Societies of Biological Psychiatry (WFSBP) Guidelines for Biological Treatment of Unipolar Depressive Disorders, Part 1: Acute and continuation treatment of major depressive disorder

These practice guidelines for the biological treatment of unipolar depressive disorders were developed by an international Task Force of the World Federation of Societies of Biological Psychiatry (WFSBP). The goal for developing these guidelines was to systematically review all available evidence pertaining to the treatment of unipolar depressive disorders, and to produce a series of practice recommendations that are clinically and scientifically meaningful based on the available evidence. These guidelines are intended for use by all physicians seeing and treating patients with these conditions. The data used for developing these guidelines have been extracted primarily from various national treatment guidelines and panels for depressive disorders, as well as from meta-analyses and reviews on the efficacy of antidepressant medications and other biological treatment interventions identified by a search of the MEDLINE database and Cochrane Library. The identified literature was evaluated with respect to the strength of evidence for its efficacy and was then categorized into four levels of evidence (A-D). This first part of the guidelines covers disease definition, classification, epidemiology and course of unipolar depressive disorders, as well as the management of the acute and continuation-phase treatment. These guidelines are primarily concerned with the biological treatment (including antidepressants, other psychopharmacological and hormonal medications, electroconvulsive therapy, light therapy, adjunctive and novel therapeutic strategies) of young adults and also, albeit to a lesser extent, children, adolescents and older adults.

Pharmacology of flibanserin

Flibanserin has preferential affinity for serotonin 5-HT(1A), dopamine D(4k), and serotonin 5-HT(2A) receptors. In vitro and in microiontophoresis, flibanserin behaves as a 5-HT(1A) agonist, a very weak partial agonist on dopamine D(4) receptors, and a 5-HT(2A) antagonist. In vivo flibanserin binds equally to 5-HT(1A) and 5-HT(2A) receptors. However, under higher levels of brain 5-HT (i.e., under stress), flibanserin may occupy 5-HT(2A) receptors in higher proportion than 5-HT(1A) receptors. The effects of flibanserin on adenylyl cyclase are different from those of buspirone and 8-OH-DPAT, two other purported 5-HT(1A) receptor agonists. Flibanserin reduces neuronal firing rate in cells of the dorsal raphe, hippocampus, and cortex with the CA1 region being the most sensitive in the brain. Flibanserin-induced reduction in firing rate in the cortex seems to be mediated through stimulation of postsynaptic 5-HT(1A) receptors, whereas the reduction of the number of active cells seems to be mediated through dopamine D(4) receptor stimulation. Flibanserin quickly desensitizes somatic 5-HT autoreceptors in the dorsal raphe and enhances tonic activation of postsynaptic 5-HT(1A) receptors in the CA3 region. Flibanserin preferentially reduces synthesis and extracellular levels of 5-HT in the cortex, where it enhances extracellular levels of NE and DA. Flibanserin displays antidepressant-like activity in most animal models sensitive to antidepressants. Such activity, however, seems qualitatively different from that exerted by other antidepressants. Flibanserin seems to act via direct or indirect stimulation of 5-HT(1A), DA, and opioid receptors in those animal models. Flibanserin does not display consistent effects in animal models of anxiety and seems to exert potential antipsychotic effects. Flibanserin may induce some sedation but does not induce observable toxic effects at pharmacologically relevant doses.

Serotonin induces a decrease of 5-HT(1A) immunoreactivity in organotypic hippocampal cultures

Serotonin reuptake inhibitors used for the treatment of depression act by increasing serotonin levels at the synaptic cleft and thereby activating 5-HT(1A) serotonin receptors. However, the effect of increased levels of serotonin on postsynaptic 5-HT(1A) receptor density is unknown. The purpose of this study was to investigate, in a culture model, how postsynaptic 5-HT(1A) receptors are influenced by serotonin. Different concentrations of serotonin (0.5, 1, 10, 50 and 100 microM) were added to organotypic hippocampal cultures and incubated for 1 week. The cultures were immunostained for the 5-HT(1A) receptor and the staining analyzed densitometrically. 5-HT(1A) levels decreased with increasing serotonin concentrations, being significant at 10, 50 and 100 microM. These results indicate that at increasing serotonin levels the density of postsynaptic 5-HT(1A) receptors is down-regulated.

Agonist-induced internalization of serotonin-1a receptors in the dorsal raphe nucleus (autoreceptors) but not hippocampus (heteroreceptors)

Serotonin-1A (5-HT(1A)) receptors in the CNS are a major target for psychotropic drugs. In nucleus raphe dorsalis (NRD) and hippocampus (CA3), the selective 5-HT(1A) agonist (+)-8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT) reduces the firing activity of serotoninergic (5-HT) and pyramidal neurons, respectively. When located on 5-HT (autoreceptors), but not on non-5-HT (heteroreceptors) neurons, 5-HT(1A) receptors are known to be subject to desensitization. Using quantitative electron microscopy after pre-embedding immunogold labeling with specific antibodies, we examined the subcellular distribution of these receptors after acute administration of 8-OH-DPAT (0.5 mg/kg, i.v.). Silver-intensified immunogold particles associated with the plasma membrane or the cytoplasm were counted in somata and dendrites within the NRD, 15 min, 1 hr and 24 hr after 8-OH-DPAT injection, and in hippocampal dendrites 1 hr after the same treatment. Significant decrease in the density of membrane labeling and concomitant increase of cytoplasmic labeling were demonstrated in the NRD, 15 min and 1 hr after 8-OH-DPAT administration, with a return to baseline level at 24 hr. Internalization was blocked by previous administration of the 5-HT(1A) antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane-carboxamide (WAY 100635), which, by itself, was without apparent effect. In hippocampus (CA3), there were no apparent changes in the distribution of the receptor after 8-OH-DPAT administration. These findings are in line with earlier results showing a desensitization of 5-HT(1A) autoreceptors but not heteroreceptors after treatment with 5-HT(1A) receptor agonist. They suggest that this desensitization is the result of autoreceptor internalization.

Combining genetic and genomic approaches to study mood disorders

Recent technological advances in genetic manipulations and DNA microarrays are profoundly altering the landscape of biological research, offering opportunities to investigate biological questions that were only dreamed of a few years ago. With this revolution comes the hope of being able to tackle some of the more arduous challenges that the central nervous system has presented to the research community. Specifically, a major goal in the study of neuropsychiatric disorders has been to identify underlying mechanisms of brain dysfunction with the expectation that these insights may allow a better diagnosis, prevention and effective treatments for these disorders. For the most part, treatments of these disorders have relied on serendipitous discovery of pharmacological entities with therapeutic efficacy, while the causes of the disorders have remained unknown. The serotonin system, and the serotonin(1A) (5-HT(1A)) receptor in particular, have been under intense investigation, mostly due to the fact that serotonergic drugs that directly or indirectly affect the 5-HT(1A) receptor, are effective therapeutic agents in treating patients with various neuropsychiatric disorders, including anxiety and depression. Genetic deletion of the receptor in mouse results in increased anxiety, thus supporting an active role for this receptor in mood regulation. However, the analysis of genetic deletion experiments can be confounded by hidden developmental roles of the missing receptor, by adaptive compensatory mechanisms, as well by the fact that the genes or gene products that are responsible for the cellular and molecular aspects of the phenotype may be several steps removed from the genetic intervention. Here, we present a combined methodological approach of tissue specific and conditional genetic manipulations, with large-scale search for altered gene expression, as an experimental framework to investigate the role of genes with complex functions and/or complex expression patterns. The 5-HT(1A) receptor is used as a model of gene product with complex functions and distributions, and as a prototypical system to which these new genetic approaches are currently being applied.

Serotonin(1A) receptors in mood disorders: a combined genetic and genomic approach

The serotonin1A (5-HT1A) receptor has been under intense investigation, mostly due to its putative role in both the etiology and therapeutic treatments of depression and anxiety-related behaviors. However, the exact contribution of this receptor to normal brain physiology and disease processes remains poorly understood, due to a complex expression pattern and multiple functions. Recent development in genetic and genomic approaches allows not only for more refined functional dissection, but also for probing large gene databases for unknown gene product interactions. Here, we describe an experimental approach that is based on a combination of regional and temporal genetic manipulations of the 5-HT1A receptor with large-scale gene expression profiling to attempt to untangle the distinct roles for this receptor in particular brain regions, as well as to identify molecular partners that mediate its function. In turn, new leads for understanding mechanisms of anxiety, depression and their pharmacological treatments may be generated.

5-hydroxytryptamine (5-HT)1A autoreceptor adaptive changes in substance P (neurokinin 1) receptor knock-out mice mimic antidepressant-induced desensitization

Antagonists at substance P receptors of the neurokinin 1 (NK1) type have been shown to represent a novel class of antidepressant drugs, with comparable clinical efficacy to the selective serotonin (5-HT) reuptake inhibitors (SSRIs). Because 5-HT(1A) receptors may be critically involved in the mechanisms of action of SSRIs, we examined whether these receptors could also be affected in a model of whole-life blockade of NK1 receptors, i.e. knock-out mice lacking the latter receptors (NK1-/-). 5-HT(1A) receptor labeling by the selective antagonist radioligand [(3)H]N-[2-[4-(2-methoxyphenyl)1-piperazinyl]-ethyl]-N-(2-pyridinyl)-cyclohexanecarboxamide (WAY 100635) and 5-HT(1A)-dependent [(35)S]GTP-gamma-S binding at the level of the dorsal raphe nucleus (DRN) in brain sections, as well as the concentration of 5-HT(1A) mRNA in the anterior raphe area were significantly reduced (-19 to -46%) in NK1-/- compared with NK1+/+ mice. Furthermore, a approximately 10-fold decrease in the potency of the 5-HT(1A) receptor agonist ipsapirone to inhibit the discharge of serotoninergic neurons in the dorsal raphe nucleus within brainstem slices, and reduced hypothermic response to 8-OH-DPAT, were noted in NK1-/- versus NK1+/+ mice. On the other hand, cortical 5-HT overflow caused by systemic injection of the SSRI paroxetine was four- to sixfold higher in freely moving NK1-/- mutants than in wild-type NK1+/+ mice. Accordingly, the constitutive lack of NK1 receptors appears to be associated with a downregulation/functional desensitization of 5-HT(1A) autoreceptors resembling that induced by chronic treatment with SSRI antidepressants. Double immunocytochemical labeling experiments suggest that such a heteroregulation of 5-HT(1A) autoreceptors in NK1-/- mutants does not reflect the existence of direct NK1-5-HT(1A) receptor interactions in normal mice.

Autoradiographic mapping of 5-HT(1B) and 5-HT(1D) receptors in the post mortem human brain using [(3)H]GR 125743

The distribution of 5-HT(1B) and 5-HT(1D) receptors in the human post mortem brain was examined using whole hemisphere autoradiography and the radioligand [(3)H]GR 125743. [(3)H]GR 125743 binding was highest in the substantia nigra and the globus pallidus. Lower levels were detected in the striatum, with the highest densities in the ventromedial parts. In the amygdala, the hippocampus, the septal region and the hypothalamus, lower [(3)H]GR 125743 binding was observed, reflecting low densities of 5-HT(1B/1D) receptors. In the cerebral cortex, binding was similar in most regions, although restricted parts of the medial occipital cortex were markedly more densely labeled. Binding densities were very low in the cerebellar cortex and in the thalamus. Two methods were used to distinguish between the two receptor subtypes, the first using ketanserin to block 5-HT(1D) receptors and the second using SB 224289 to inhibit 5-HT(1B) receptor binding. The autoradiograms indicated that in the human brain, the 5-HT(1B) receptor is much more abundant than the 5-HT(1D) receptor, which seemed to occur only in low amounts mainly in the ventral pallidum. Although [(3)H]GR 125743 is a suitable radioligand to examine the distribution of 5-HT(1B) receptors in the human brain in vitro, the selectivities of ketanserin and SB 224289 are not sufficiently high to give definite evidence for the occurrence of the 5-HT(1D) receptor in the human brain.

Modulation of antagonist binding to serotonin1A receptors from bovine hippocampus by metal ions

1. The serotonin1A (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to G-proteins. They appear to be involved in various behavioral and cognitive functions. Although specific 5-HT1A agonists have been discovered more than a decade back, the development of selective 5-HT1A antagonists has been achieved only recently. 2. We have examined the modulation of the specific antagonist [3H]p-MPPF binding to 5-HT1A receptors from bovine hippocampal membranes by monovalent and divalent metal ions. Our results show that the antagonist binding to 5-HT1A receptors is inhibited by both monovalent and divalent cations in a concentration-dependent manner. This is accompanied by a concomitant reduction in binding affinity. 3. Our results also show that the specific antagonist p-MPPF binds to all available receptors in the bovine hippocampal membrane irrespective of their state of G-protein coupling and other serotonergic ligands such as 5-HT and OH-DPAT effectively compete with the specific antagonist [3H]p-MPPF. 4. These results are relevant to ongoing analyses of the overall modulation of ligand binding in G-protein-coupled seven transmembrane domain receptors.