beta-blocker binding to human 5-HT(1A) receptors in vivo and in vitro: implications for antidepressant therapy

MDMA-assisted psychotherapy for PTSD: Growing evidence for memory effects mediating treatment efficacy

The application of MDMA in conjunction with psychotherapy has in recent years seen a resurgence of clinical, scientific, and public interest in the treatment of posttraumatic stress disorder (PTSD). Clinical trials have shown promising safety and efficacy, but the mechanisms underlying this treatment form remain largely unestablished. This article explores recent preclinical and clinical evidence suggesting that the treatment's efficacy may be influenced by the mnemonic effects of MDMA. We review data on the effects of MDMA on fear extinction and fear reconsolidation and the utility of these processes for PTSD treatment. We corroborate our findings by incorporating research from cognitive psychology and psychopharmacology and offer recommendations for future research.

The Serotonin 1A (5-HT1A) Receptor as a Pharmacological Target in Depression

Clinical depression is a common, debilitating and heterogenous disorder. Existing treatments for depression are inadequate for a significant minority of patients and new approaches are urgently needed. A wealth of evidence implicates the serotonin 1A (5-HT1A) receptor in the pathophysiology of depression. Stimulation of the 5-HT1A receptor is an existing therapeutic target for treating depression and anxiety, using drugs such as buspirone and tandospirone. However, activation of 5-HT1A raphe autoreceptors has also been suggested to be responsible for the delay in the therapeutic action of conventional antidepressants such as selective serotonin reuptake inhibitors (SSRIs). This narrative review provides a brief overview of the 5-HT1A receptor, the evidence implicating it in depression and in the effects of conventional antidepressant treatment. We highlight that pre- and post-synaptic 5-HT1A receptors may have divergent roles in the pathophysiology and treatment of depression. To date, developing this understanding to progress therapeutic discovery has been limited, partly due to a paucity of specific pharmacological probes suitable for use in humans. The development of 5-HT1A 'biased agonism', using compounds such as NLX-101, offers the opportunity to further elucidate the roles of pre- and post-synaptic 5-HT1A receptors. We describe how experimental medicine approaches can be helpful in profiling the effects of 5-HT1A receptor modulation on the different clinical domains of depression, and outline some potential neurocognitive models that could be used to test the effects of 5-HT1A biased agonists.

Role of hippocampal 5-HT1A receptors in the antidepressant-like phenotype of mice expressing RGS-insensitive Gαi2 protein

A single base mutation in the Gα_i2 protein (G184S) renders this Gα subunit insensitive to the negative modulatory effects of Regulator of G-protein Signaling (RGS) proteins. Mice expressing this RGS insensitive (RGSi) variant of Gα_i2 (RGSi Gα_i2) display a spontaneous antidepressant-like phenotype that is reversible by treatment with the 5-HT1A receptor (5-HT1AR) antagonist WAY100635. Here we test the hypothesis that increased activity of 5-HT1ARs in the hippocampus of RGSi Gα_i2 knock-in mice is responsible for the expression of the observed antidepressant-like behavior. We administered the 5-HT1AR antagonist WAY100635 or the agonist 8-OH-DPAT via bilateral intra-hippocampal infusion cannulae and evaluated antidepressant-like behavior using the tail suspension test (TST). WAY100635 reversed the antidepressant-like phenotype of the RGSi Gα_i2 knock-in mice and 8-OH-DPAT produced an antidepressant-like response in wild type mice that was blocked by systemic WAY100635. Furthermore, intra-hippocampal infusion of the RGS19/4 inhibitor CCG-203769 produced an antidepressant-like effect in female mice. Ex-vivo slice recording confirmed the 5-HT1AR-mediated decrease in hippocampal CA1 pyramidal neuron excitability was enhanced in the RGSi Gα_i2 knock-in mice. There was no change in hippocampal 5-HT1AR expression as measured by ligand binding but there was a compensatory reduction in Gαi proteins. The findings demonstrate that RGS protein control of hippocampal 5-HT1AR signaling is necessary and sufficient to account for the antidepressant-like phenotype in the RGSi Gα_i2 knock-in mice and that RGS proteins highly expressed in the hippocampus should be investigated as targets for novel antidepressant therapies.

Adjunctive 5-Hydroxytryptophan Slow-Release for Treatment-Resistant Depression: Clinical and Preclinical Rationale

Serotonin transporter (SERT) inhibitors treat depression by elevating brain extracellular 5-hydroxytryptamine (5-HT_Ext). However, only one-third of patients respond adequately. Treatment-resistant depression (TRD) is a major unmet need. Interestingly, elevating 5-HT_Ext beyond what is achieved by a SERT inhibitor appears to treat TRD. Adjunctive administration of 5-hydroxytryptophan (5-HTP) safely elevates 5-HT_Ext beyond the SERT inhibitor effect in humans; however, 5-HTP cannot be a clinically viable drug because of its poor pharmacokinetics. A slow-release (SR) delivery mode would be predicted to overcome the pharmacokinetic limitations of 5-HTP, substantially enhancing the pharmacological action and transforming 5-HTP into a clinically viable drug. Animal studies bear out this prediction. Thus, adjunct 5-HTP SR could be an important new treatment for TRD. Here, we review the clinical and preclinical evidence for this treatment.

Is pindolol augmentation effective in depressed patients resistant to selective serotonin reuptake inhibitors? A systematic review and meta-analysis

OBJECTIVE

This systematic review and meta-analysis was conducted to assess the use of pindolol augmentation in depressed patients resistant to selective serotonin reuptake inhibitor (SSRI) therapy.

METHODS

A comprehensive search of PubMed, Cochrane, Embase, Web of Science, and PsychINFO databases from 1970 through December 2013 was conducted. Only randomized controlled trials (RCTs) studied on unipolar SSRI-resistant depressed adults were included. The primary outcome was mean change scores of depressive symptom on the depression rating scales, assessed with standardized mean differences.

RESULTS

Five RCTs consisting of 154 patients met all inclusion and exclusion criteria. The overall pooled effect size in the primary and secondary efficacy analysis showed no significant effects of pindolol plus SSRI therapy (standardized mean difference = -0.43, p = 0.24; OR = 1.92, p = 0.39, respectively). In terms of acceptability, there was no statistical difference in either tolerability or safety between the two groups (OR = 0.46, p = 0.40; OR = 0.90, p = 0.94, respectively). These estimates remained robust through several sensitivity and subgroup analyses, except 7.5 mg-qd pindolol augmentation did show a significant benefit over 2.5-mg tid pindolol augmentation.

CONCLUSIONS

Pindolol augmentation may not be suitable for treatment-resistant depression patients with SSRI-resistant depression. However, once-daily high-dose pindolol (7.5 mg qd) appears to show a promising benefit in these patients.

No evidence that MDMA-induced enhancement of emotional empathy is related to peripheral oxytocin levels or 5-HT1a receptor activation

The present study aimed at investigating the effect of MDMA on measures of empathy and social interaction, and the roles of oxytocin and the 5-HT_1A receptor in these effects. The design was placebo-controlled within-subject with 4 treatment conditions: MDMA (75 mg), with or without pindolol (20 mg), oxytocin nasal spray (40 IU+16 IU) or placebo. Participants were 20 healthy poly-drug MDMA users, aged between 18–26 years. Cognitive and emotional empathy were assessed by means of the Reading the Mind in the Eyes Test and the Multifaceted Empathy Test. Social interaction, defined as trust and reciprocity, was assessed by means of a Trust Game and a Social Ball Tossing Game. Results showed that MDMA selectively affected emotional empathy and left cognitive empathy, trust and reciprocity unaffected. When combined with pindolol, these effects remained unchanged. Oxytocin did not affect measures of empathy and social interaction. Changes in emotional empathy were not related to oxytocin plasma levels. It was concluded that MDMA (75 mg) selectively enhances emotional empathy in humans. While the underlying neurobiological mechanism is still unknown, it is suggested that peripheral oxytocin does not seem to be the main actor in this; potential candidates are the serotonin 2A and the vasopressin 1A receptors.

Trial Registration

MDMA & PSB NTR 2636

Characterising the plasma-target occupancy relationship of the neurokinin antagonist GSK1144814 with PET

GSK1144814 is a potent, insurmountable antagonist at human NK₁ and NK₃ receptors. Understanding the relationship between plasma pharmacokinetics and receptor occupancy in the human brain, was crucial for dose selection in future clinical studies. GSK1144814 occupancy data were acquired in parallel with the first-time-in-human safety and tolerability study. [¹¹C]GR-205171 a selective NK₁ receptor PET ligand was used to estimate NK₁ occupancy at several time-points following single dose administration of GSK1144814. The time-plasma concentration-occupancy relationship post-single dose administration was assessed, and used to predict the plasma concentration-occupancy relationship following repeat dose administration. Repeat dose predictions were tested in a subsequent cohort of subjects examined following approximately 7 and 14 days dosing with GSK1144814. GSK1144814 was shown to demonstrate a dose-dependent occupancy of the NK₁ receptor with an estimated in vivo EC₅₀~0.9 ng/mL in the human brain. A direct relationship was seen between the GSK1144814 plasma concentration and its occupancy of the brain NK₁ receptor, indicating that in future clinical trials the occupancy of brain receptors can be accurately inferred from the measured plasma concentration. Our data provided support for the further progression of this compound and have optimised the likely therapeutic dose range.

Molecular and cellular mechanisms of antidepressant action

A long-standing theory is that brain monoamine signalling is critically involved in the mechanisms of antidepressant drug treatment. Theories on the nature of these mechanisms commenced with ideas developed in the 1960s that the drugs act simply by increasing monoamine availability in the synapse. However, this thinking has advanced remarkably in the last decade to concepts which position that antidepressant drug action on monoamine signalling is just the starting point for a complex sequence of neuroadaptive molecular and cellular changes that bring about the therapeutic effect. These changes include activation of one or more programmes of gene expression that leads to the strengthening of synaptic efficacy and connectivity, and even switching neural networks into a more immature developmental state. It is thought that through this increase in plasticity, key neural circuits within the limbic system are more easily remodelled by incoming emotionally relevant stimuli. This article attempts to bring together previous and current knowledge of antidepressant drug action on monoamine signalling at molecular and cellular levels, and introduces current thinking that these changes interact with neuropsychological processes ultimately to elevate mood.

Presynaptic selectivity of a ligand for serotonin 1A receptors revealed by in vivo PET assays of rat brain

A novel investigational antidepressant with high affinity for the serotonin transporter and the serotonin 1A (5-HT(1A)) receptor, called Wf-516 (structural formula: (2S)-1-[4-(3,4-dichlorophenyl)piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2-yl)benzo[b]furan-4-yloxy]propan-2-ol monohydrochloride), has been found to exert a rapid therapeutic effect, although the mechanistic basis for this potential advantage remains undetermined. We comparatively investigated the pharmacokinetics and pharmacodynamics of Wf-516 and pindolol by positron emission tomographic (PET) and autoradiographic assays of rat brains in order to elucidate their molecular interactions with presynaptic and postsynaptic 5-HT(1A) receptors. In contrast to the full receptor occupancy by pindolol in PET measurements, the binding of Wf-516 to 5-HT(1A) receptors displayed limited capacity, with relatively high receptor occupancy being achieved in regions predominantly containing presynaptic receptors. This selectivity was further proven by PET scans of neurotoxicant-treated rats deficient in presynaptic 5-HT(1A) receptors. In addition, [(35)S]guanosine 5'-O-[γ-thio]triphosphate autoradiography indicated a partial agonistic ability of Wf-516 for 5-HT(1A) receptors. This finding has lent support to reports that diverse partial agonists for 5-HT(1A) receptors exert high sensitivity for presynaptic components. Thus, the present PET data suggest a relatively high capacity of presynaptic binding sites for partial agonists. Since our in vitro and ex vivo autoradiographies failed to illustrate these distinct features of Wf-516, in vivo PET imaging is considered to be, thus far, the sole method capable of pharmacokinetically demonstrating the unique actions of Wf-516 and similar new-generation antidepressants.

The development, past achievements, and future directions of brain PET

The early developments of brain positron emission tomography (PET), including the methodological advances that have driven progress, are outlined. The considerable past achievements of brain PET have been summarized in collaboration with contributing experts in specific clinical applications including cerebrovascular disease, movement disorders, dementia, epilepsy, schizophrenia, addiction, depression and anxiety, brain tumors, drug development, and the normal healthy brain. Despite a history of improving methodology and considerable achievements, brain PET research activity is not growing and appears to have diminished. Assessments of the reasons for decline are presented and strategies proposed for reinvigorating brain PET research. Central to this is widening the access to advanced PET procedures through the introduction of lower cost cyclotron and radiochemistry technologies. The support and expertize of the existing major PET centers, and the recruitment of new biologists, bio-mathematicians and chemists to the field would be important for such a revival. New future applications need to be identified, the scope of targets imaged broadened, and the developed expertize exploited in other areas of medical research. Such reinvigoration of the field would enable PET to continue making significant contributions to advance the understanding of the normal and diseased brain and support the development of advanced treatments.

β-blockers and the risk of incident depression in the elderly

The hypothesis that β-blockers cause depression has been both confirmed and refuted in previous studies. However, in hardly any of these studies, depression was systematically and adequately assessed. The aim of this cohort study was to examine whether β-blockers, in general, highly lipid-soluble, nonselective, or serotonergic receptor-binding β-blockers, are associated with incident depression. Between 1993 and 2005, 5104 elderly persons were followed for incident depressions. Depressions were identified by regular interview and continuous monitoring of medical records. Cases were categorized as clinically relevant depressive symptoms or as depressive syndromes, the latter including Diagnostic and Statistical Manual of Mental Disorders-IV-defined depressive disorders. Pharmacies provided information on filled β-blockers. We used Cox regression with drug use as a time-dependent variable to analyze the data, adjusted for potential demographic covariates, activity of daily living, and (contra)indications for β-blockers. We found that use of β-blockers in general did not convey an increased risk of depressive symptoms (hazard ratio [HR], 0.76; 95% confidence interval [CI], 0.37-1.59) or depressive syndromes (HR, 0.99; 95% CI, 0.53-1.84). Highly lipid-soluble β-blockers, mostly propranolol in our study, were associated with depressive symptoms during the first 3 months of use (HR, 3.31; 95% CI, 1.03-10.6), but not with depressive syndromes. Nonselective or serotonergic receptor affinity was not associated with an increased risk of depressive symptoms or syndromes independent of high lipid solubility. We conclude that β-blockers in general do not convey an increased risk of depression. Lipophilic β-blockers are associated with an increased risk of depressive symptoms.

Investigation of serotonin-1A receptor function in the human psychopharmacology of MDMA

Serotonin (5-HT) release is the primary pharmacological mechanism of 3,4-methylenedioxymethamphetamine (MDMA, 'ecstasy') action in the primate brain. Dopamine release and direct stimulation of dopamine D2 and serotonin 5-HT2A receptors also contributes to the overall action of MDMA. The role of 5-HT1A receptors in the human psychopharmacology of MDMA, however, has not yet been elucidated. In order to reveal the consequences of manipulation at the 5-HT1A receptor system on cognitive and subjective effects of MDMA, a receptor blocking study using the mixed beta-adrenoreceptor blocker/5-HT1A antagonist pindolol was performed. Using a double-blind, placebo-controlled within-subject design, 15 healthy male subjects were examined under placebo (PL), 20 mg pindolol (PIN), MDMA (1.6 mg/kg b.wt.), MDMA following pre-treatment with pindolol (PIN-MDMA). Tasks from the Cambridge Neuropsychological Test Automated Battery were used for the assessment of cognitive performance. Psychometric questionnaires were applied to measure effects of treatment on core dimensions of Altered States of Consciousness, mood and state anxiety. Compared with PL, MDMA significantly impaired sustained attention and visual-spatial memory, but did not affect executive functions. Pre-treatment with PIN did not significantly alter MDMA-induced impairment of cognitive performance and only exerted a minor modulating effect on two psychometric scales affected by MDMA treatment ('positive derealization' and 'dreaminess'). Our findings suggest that MDMA differentially affects higher cognitive functions, but does not support the hypothesis from animal studies, that some of the MDMA effects are causally mediated through action at the 5-HT1A receptor system.

Novel 5-HTTLPR allele associates with higher serotonin transporter binding in putamen: a [(11)C] DASB positron emission tomography study

BACKGROUND

The serotonin transporter (5-HTT)-linked polymorphic region (5-HTTLPR) has two frequent alleles, designated long (L), and short (S). The S allele is associated with lower levels of 5-HTT mRNA and lower 5-HTT expression in human cell lines. A functional single nucleotide variant was detected within L, designated L(A) and L(G). Only L(A) is associated with high levels of in vitro 5-HTT expression, whereas L(G) is low expressing and more similar to S. We examined the possible influence of the long (A/G) variant on 5-HTT density in the living human brain using 3-(11)C-amino-4-(2-dimethylaminomethylphenyl-sulfanyl) benzonitrile ([(11)C]DASB) positron emission tomography.

METHODS

The 5-HTT binding potential (5-HTT BP), an index of 5-HTT density, was found in 43 healthy subjects genotyped for 5-HTTLPR long (A/G), and in an ethnically homogenous subsample of 30 Caucasian-Canadians.

RESULTS

The L(A)/L(A) was associated with higher 5-HTT BP in putamen (p = .026, not corrected). This association became stronger in the Caucasian subsample (p = .004) and was significant even after correcting for multiple comparisons.

CONCLUSIONS

The 5-HTTLPR long (A/G) polymorphism influences 5-HTT density leading to higher putamen 5-HTT BP in healthy L(A)/L(A) carriers of Caucasian ancestry. This finding extends the role of this polymorphism from in vitro reports of higher 5-HTT expression with the L(A)/L(A) genotype into in vivo brains of healthy human subjects.

Brain serotonin transporter binding in non-depressed patients with Parkinson's disease

Early post-mortem data suggest that damage to brain serotonin neurones might play a role in some features (e.g., depression) of Parkinson's disease (PD). However, it is not known whether such damage is a typical characteristic of living patients with PD or whether the changes are regionally widespread. To address this question we measured, by positron emission tomography imaging, levels of the brain serotonin transporter (SERT), a marker for serotonin neurones, as inferred from binding of [11C]-3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile (DASB), a second generation SERT radioligand, in subcortical and cerebral cortical brain areas of clinically advanced non-depressed (confirmed by structured psychiatric interview) patients with PD. SERT binding levels in PD were lower than those in controls in all examined brain areas, with the changes statistically significant in orbitofrontal cortex (-22%), caudate (-30%), putamen (-26%), and midbrain (-29%). However, only a slight non-significant reduction (-7%) was observed in dorsolateral pre-frontal cortex, an area implicated in major depression. Our imaging data suggests that a modest, regionally widespread loss of brain serotonergic innervation might be a common feature of advanced PD. Further investigation will be required to establish whether SERT binding is more or less decreased in those patients with PD who also have major depressive disorder.

Neuropharmacology of 5-hydroxytryptamine

This review outlines the history of our knowledge of the neuropharmacology of 5-hydroxytryptamine (5-HT; serotonin), focusing primarily on the work of U.K. scientists. The existence of a vasoconstrictive substance in the blood has been known for over 135 years. The substance was named serotonin and finally identified as 5-HT in 1949. The presence of 5-HT in the brain was reported by Gaddum in 1954 and it was Gaddum who also demonstrated that the action of 5-HT (in the gut) was antagonised by the potent hallucinogen lysergic acid diethylamide. This provoked the notion that 5-HT played a pivotal role in the control of mood and subsequent investigations have generally confirmed this hypothesis. Over the last 50 years a good understanding has been gained of the mechanisms involved in control of the storage, synthesis and degradation of 5-HT in the brain. Knowledge has also been gained on control of the functional activity of this monoamine, often by the use of behavioural models. A considerable literature also now exists on the mechanisms by which many of the drugs used to treat psychiatric illness alter the functional activity of 5-HT, particularly the drugs used to treat depression. Over the last 20 years the number of identified 5-HT receptor subtypes has increased from 2 to 14, or possibly more. A major challenge now is to utilise this knowledge to develop receptor-specific drugs and use the information gained to better treat central nervous system disorders.

Synthesis and SAR of highly potent dual 5-HT1A and 5-HT1B antagonists as potential antidepressant drugs

Novel 5-HT(1) autoreceptor ligands based on the N-4-aryl-piperazinyl-N'-ethyl-5,6,7,8-tetrahydropyrido[4', 3':4,5]thieno[2,3-d]pyrimidin-4(3H)-one core are described. Aiming at antidepressants with a novel mode of action our objective was to identify potent antagonists showing balanced affinities and high selectivity for the 5-HT(1A) and 5-HT(1B) receptors. Strategies for the development of dual 5-HT(1A) and 5-HT(1B) antagonists based on 1 and 2 as leads and the corresponding results are discussed. Isoquinoline analogue 33 displayed high affinity and an antagonistic mode of action for the 5-HT(1A) and the 5-HT(1B) receptors and was characterized further with respect to selectivity, electrically stimulated [(3)H]5-HT release and in vivo efficacy.

Effects of tryptophan depletion on the serotonin transporter in healthy humans

BACKGROUND

Lowering of brain serotonin by acute tryptophan depletion (TD) frequently leads to transient symptoms of depression in vulnerable individuals but not in euthymic healthy subjects with a negative family history of depression. The effects of TD on regional serotonin transporter binding potential (5-HTT BP), an index of 5-HTT density and affinity, were studied in healthy individuals using 3-(11)C-amino-4-(2-dimethylaminomethylphenylsulfanyl)benzonitrile ([11C]DASB) positron emission tomography (PET). Adaptive decreases in 5-HTT density and/or affinity during TD would be a possible compensatory mechanism to maintain sufficient extracellular serotonin levels during TD, thereby preventing a depressive relapse.

METHODS

Regional noninvasive 5-HTT BP was found in 25 healthy subjects using [11C]DASB PET. Fourteen subjects were scanned twice, once after TD and once after sham depletion, and 11 other healthy subjects were scanned twice to measure test-retest reliability of the method.

RESULTS

None of the healthy subjects experienced depressive symptoms during TD and there was no difference in regional 5-HTT BP during TD as compared with sham depletion.

CONCLUSIONS

Acute changes in 5-HTT density or affinity are unlikely to play a role in protecting healthy subjects against mood symptoms during TD. Other mechanisms that may be associated with greater resilience against acute lowering of extracellular serotonin should be explored to gain further insight into the neurochemical basis of different vulnerabilities to short-term depressive relapse.

Pindolol augmentation of selective serotonin reuptake inhibitors: accounting for the variability of results of placebo-controlled double-blind studies in patients with major depression

Co-administration of pindolol with SSRIs in patients with depression has been suggested as a way to both hasten and augment antidepressant response. Clinical trials have examined the efficacy of this treatment regime and conflicting results have been reported. The present review briefly presents the results of placebo-controlled double-blind trials of pindolol augmentation of SSRIs in patients with major depression, and focuses on factors that may account for the variability of findings. Additionally, a profile of the subset of patients who may most benefit from pindolol augmentation is outlined. Methodological factors such as qualitative differences in definitions of antidepressant response, the timing of pindolol administration and heterogeneous clinical characteristics of patient samples may contribute to the variability in the results of clinical trials to date. Similarly, individual differences in neuropathology, neurophysiology and genotype may also account for some of the inconsistencies in the findings. Finally, the results of recent neuroimaging studies suggest that the 2.5 mg t.i.d. dose of pindolol that has been used in all but one of these investigations may be suboptimal for achieving reliable and significant occupancy of 5-HT1A autoreceptors and may explain the contradictory nature of the results of investigations of pindolol augmentation.

5-HT1A receptors, gene repression, and depression: guilt by association

The serotonin system is implicated in major depression and suicide and is negatively regulated by somatodendritic 5-HT1A autoreceptors. Desensitization of 5-HT1A autoreceptors is implicated in the 2- to 3-week latency for antidepressant treatments. Alterations in 5-HT1A receptor levels are reported in depression and suicide, and gene knockout of the 5-HT1A receptor results in an anxiety phenotype, suggesting that abnormal transcriptional regulation of this receptor gene may underlie these disorders. The 5-HT1A receptor gene is negatively regulated in neurons by repressors including REST/NRSF, Freud-1, NUDR/Deaf-1, and Hes5. The association with major depression, suicide, and panic disorder of a new functional 5-HT1A polymorphism at C(-1019)G that selectively blocks repression of the 5-HT1A autoreceptor by NUDR further suggests a causative role for altered regulation of this receptor in predisposition to mental illness. The authors review evidence that altered transcription of the 5-HT1A receptor can affect the serotonin system and limbic and cortical areas, leading to predisposition to depression.

A positron emission tomography study of the 5-HT1A receptor in schizophrenia and during clozapine treatment

Several post-mortem studies have identified increases of 5-HT1A receptor density in frontal cortical areas in schizophrenic patients, and one has found increases in the cerebellar vermis. Clozapine has moderate affinity at the 5-HT1A receptor, and this may be of therapeutic importance. This positron emission tomography (PET) study attempted to replicate the post-mortem findings in vivo and sought an occupancy effect of clozapine at the 5-HT1A receptor. We recruited healthy controls, and patients with schizophrenia who were divided into those receiving clozapine and those receiving neuroleptics lacking 5-HT1A receptor affinity. Each volunteer received a PET scan, using the 5-HT1A receptor radioligand [carbonyl-11C]WAY-100635, and a magnetic resonance imaging scan. The cerebellar vermis was examined by comparing time-activity data between groups. For other brain regions (the raphe and subdivisions of the cerebral cortex), binding potential images were generated to reflect receptor density, then analysed using 'region of interest' and voxel-by-voxel methods. No significant changes of 5-HT1A receptor density were found in schizophrenic patients compared to controls. Two other PET studies, containing drug naïve rather than medicated schizophrenic patients, have also reported no increase in 5-HT1A receptor density in the frontal cortex. The results obtained in vivo bring into question the importance of the receptor in the pathophysiology of the illness. Clozapine did not occupy the 5-HT1A receptor at clinical doses. This is consistent with recent related PET results: 5-HT1A agonists do not appear to measurably block the binding of antagonist radiotracers in man at doses that are pharmacologically active but which are limited by tolerability.

Preferential 5-HT1A autoreceptor occupancy by pindolol is attenuated in depressed patients: effect of treatment or an endophenotype of depression?

Using positron emission tomography and the selective 5-HT1A receptor radioligand [11C]WAY100635, we previously demonstrated a preferential occupancy of 5-HT1A autoreceptors, compared to postsynaptic receptors by pindolol in healthy volunteers. We have speculated that preferential occupancy may be clinically important for the purported actions of pindolol in accelerating the antidepressant effects of selective serotonin re-uptake inhibitors (SSRIs). In this study, we have examined the preferential occupancy by pindolol of 5-HT1A autoreceptors, following three different pindolol regimes (10 mg single dose, 2.5 mg t.i.d., and 5 mg t.i.d., in 15 depressed patients on SSRIs. In addition, seven healthy volunteers were examined following a single 10 mg dose of pindolol. We found a preferential occupancy of 22.6+/-7.7% following a single dose of 10 mg of pindolol, in the healthy volunteers, which was attenuated in depressed patients on the same dose of pindolol to 2.9+/-10.8% (Student's t=3.94, df=12, p=0.002). In addition, we found a significant negative correlation between the degree of preferential occupancy and the severity of depression as assessed by the Hamilton depression rating score (HAM-D), Spearman's rho=-0.728, N=14, p=0.003, in the depressed sample. A possible mechanism underlying preferential occupancy and the attenuation of this phenomenon in depressed patients on SSRIs may include changes in the proportion of high affinity 5-HT1A sites in the autoreceptor region of the midbrain raphe. Speculatively, the degree of preferential occupancy may serve as a surrogate marker for depression, or the pharmacological effects of antidepressants.

Occupancy of agonist drugs at the 5-HT1A receptor

Drugs acting on the 5-HT1A receptor are used in the treatment of depression, generalized anxiety disorder, and schizophrenia. This study investigated 5-HT1A receptor occupancy by the 5-HT1A agonist drugs flesinoxan (a highly selective probe for the 5-HT1A receptor) and ziprasidone (a novel atypical antipsychotic drug). Using a within-subject design, 14 healthy volunteers each received two positron emission tomography scans using the selective 5-HT1A antagonist radiotracer [11C]WAY-100635. One scan constituted a baseline, while the other followed either 1 mg flesinoxan or 40 mg ziprasidone orally. In addition, rats were pretreated with intravenous flesinoxan at doses ranging from 0.001 to 5 mg/kg then [11C]WAY-100635 binding measured ex vivo. Cerebral cortical and hippocampal regions of interest, and cerebellar reference regions were sampled to estimate 5-HT1A receptor occupancy (inferred from reductions in specific radioligand binding). In man, occupancy was not significant despite volunteers experiencing side effects consistent with central serotonergic activity. The mean cerebral cortex occupancy (+/- 1 SD) for flesinoxan was 8.7% (+/- 13%), and for ziprasidone 4.6% (+/- 17%). However, in rats, flesinoxan achieved significant and dose-related occupancy (17-57%) at 0.25 mg/kg and above. We conclude that 5-HT1A receptor agonists produce detectable occupancy only at higher doses that would produce unacceptable levels of side effects in man, although lower doses are sufficient to produce pharmacological effects. The development of agonist radiotracers may increase the sensitivity of detecting agonist binding, as 5-HT1A antagonists bind equally to low- and high-affinity receptor states, while agonists bind preferentially to the high-affinity state.

Persistent reduction in brain serotonin1A receptor binding in recovered depressed men measured by positron emission tomography with [11C]WAY-100635

Positron emission tomography (PET) studies with the selective 5-HT(1A) receptor ligand, [(11)C]WAY-100635, have indicated that the binding potential (BP) of brain 5-HT(1A) receptors is lowered in unmedicated subjects with acute major depression. However, it is unclear if these changes persist after recovery from depression. To resolve this issue, we used [(11)C]WAY-100635 in conjunction with PET imaging to compare 5-HT(1A) BP in 18 healthy controls and 14 male subjects with recurrent major depression who were clinically recovered and free of antidepressant medication. BP values, derived from a reference tissue model, were analysed by region of interest and statistical parametric mapping. Both analyses showed a widespread and substantial (17%) decrease in 5-HT(1A) receptor BP in cortical areas in the recovered depressed subjects. In contrast, 5-HT(1A) BP in the raphe nuclei did not distinguish depressed subjects from controls. Our results suggest a persistent dysfunction in cortical 5-HT(1A) BP as measured by [(11)C]WAY-100635 in recovered depressed men. Lowered 5-HT(1A) receptor binding availability could represent a trait abnormality that confers vulnerability to recurrent major depression.

Once-daily high-dose pindolol for SSRI-refractory depression

Selective serotonin reuptake inhibitor (SSRI) augmentation with the 5-HT1A antagonist pindolol has met with mixed results. Recent studies using positron emission tomography (PET) suggest that pindolol doses used in these studies were too low to effect 5-HT1A autoreceptor blockade. To test the hypothesis that a single higher dose of pindolol would effectively augment antidepressant responses in SSRI-refractory patients, nine subjects with major depression unresponsive to paroxetine 40 mg/day given for 2 months or more were randomized to AM pindolol 7.5 mg (n=4) or placebo (n=5). Subjects were administered the Hamilton Depression Scale (HAM-D), the Hamilton Anxiety Scale (HAM-A), the Bech-Rafaelsen Melancholia Scale, and the Zung Depression Inventory at baseline and weeks 1, 2, 3, and 4. Subjects receiving pindolol exhibited significant improvements in all ratings beginning at week 2 which continued through week 4. Aside from transient dizziness and a five-point decrease in systolic/diastolic blood pressure associated with pindolol, no adverse effects were reported. Although results must be verified in a larger sample, these findings support previous studies indicating that pindolol can accelerate antidepressant responses during SSRI therapy. In addition, results reported here suggest that a single high dose of pindolol (7.5 mg) is a more effective augmentation strategy in SSRI-refractory patients compared with the same total dose given at 2.5 mg tid.

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.

Lack of effect of a single dose of hydrocortisone on serotonin(1A) receptors in recovered depressed patients measured by positron emission tomography with [11C]WAY-100635

BACKGROUND

Elevated cortisol levels might account for the reduction in central serotonin 1A (5-hydroxytryptamine [5-HT](1A)) receptor binding and function observed in patients with major depression. We tested this hypothesis by studying the effect of acute administration of hydrocortisone on 5-HT(1A) receptor binding potential (BP) in subjects recovered from depression.

METHODS

We studied 14 subjects (8 male, 6 female) who had recovered from at least two episodes of major depression and had been euthymic and drug free for at least 6 months. Serotonin 1A receptor BP was measured by [(11)C]WAY-100635 in conjunction with positron emission tomography. Subjects were tested on two occasions in a double-blind, random-order, crossover design after administration of either hydrocortisone (100 mg orally) or placebo 12 hours previously. Positron emission tomography scans were analyzed with a region of interest analysis.

RESULTS

Hydrocortisone treatment did not decrease 5-HT(1A) receptor BP either in the hippocampus, which was our a priori hypothesis, or in other cortical 5-HT(1A) regions; however, female subjects had a higher 5-HT(1A) receptor BP in certain brain areas compared with male subjects.

CONCLUSIONS

These data are consistent with an earlier study in healthy volunteers and do not support the proposal that decreased 5-HT(1A) receptor BP in patients with acute major depression is a consequence of cortisol hypersecretion.

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.

Effect of different 5-HT1A receptor antagonists in combination with paroxetine on expression of the immediate-early gene Arc in rat brain

Selective 5-HT(1A) receptor antagonists enhance the effect of selective serotonin reuptake inhibitors (SSRIs) on presynaptic 5-HT function, and have potential as antidepressant augmentation therapies. The present study tested the effect of different selective 5-HT(1A) receptor antagonists (WAY 100635, NAD-299, p-MPPI and LY 426965) in combination with a SSRI (paroxetine), on postsynaptic 5-HT function measured by increased expression of the immediate early gene, Arc. Paroxetine (5 mg/kg s.c.) combined with WAY 100635 (0.3 mg/kg s.c.) increased Arc mRNA in frontal, parietal and piriform cortices, and caudate putamen. Paroxetine (5 mg/kg s.c.) plus NAD-299 (1 or 5 mg/kg s.c.) had a similar effect. None of these drugs increased Arc mRNA when administered alone. Paroxetine (5 mg/kg s.c.) plus p-MPPI (8.5 mg/kg s.c.) also increased Arc mRNA but p-MPPI itself elevated Arc mRNA in many regions. Whilst LY 426965 (3 or 10 mg/kg s.c.) had no effect alone, when combined with paroxetine (5 mg/kg s.c.), the drug increased Arc mRNA in caudate putamen but not cortical regions.In conclusion, this study demonstrates that four 5-HT(1A) receptor antagonists augment the effect of an SSRI on Arc mRNA expression, which is suggestive of increased postsynaptic 5-HT function. However, the data reveal certain differences in the 5-HT(1A) receptor antagonists not recognised in models of presynaptic 5-HT function.

Responses to exercise in the heat related to measures of hypothalamic serotonergic and dopaminergic function

We have studied 12 recreationally active men to measure their responses to exercise in the heat and relate these to measures of hypothalamic function explored with a buspirone [5-hydroxytryptamine 1A (5-HT(1A)) agonist, dopaminergic D(2) antagonist] neuroendocrine challenge, with and without pretreatment with pindolol (5-HT(1A) antagonist). Pindolol treatment allowed the serotonergic and non-serotonergic components of prolactin release to be distinguished. Subjects exercised at 73 (5)% maximal rate of oxygen uptake (VO(2max)) until volitional fatigue at 35 degrees C (relative humidity, 30%). On another two occasions they underwent a buspirone challenge [0.5 mg (kg body mass)(-1)], once with, and once without, pindolol [0.5 mg (kg body mass)(-1)] pretreatment and the circulating plasma concentrations of prolactin were measured for the next 2.5 h. Rectal temperature increased throughout exercise, whilst mean skin temperature remained constant. There was a wide inter-subject variation in prolactin response to the neuroendocrine challenges. The proportion of the prolactin response to buspirone attributable to a non-serotonergic component (most likely dopaminergic) correlated both with exercise duration (r=0.657, P=0.028), rectal temperature at fatigue (r=0.623, P=0.041) and the rate of temperature rise (r=-0.669, P=0.024). Our results suggest that high activity of the dopaminergic pathways in the hypothalamus is a predictor of exercise tolerance in the heat.

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.

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.

The pharmacology of putative early-onset antidepressant strategies

Depression is a serious and burdensome illness. Although selective serotonin reuptake inhibitors (SSRIs) have improved safety and tolerability of antidepressant treatment efficacy, the delay in the onset of action have not been improved. There is evidence to suggest that the delay in onset of therapeutic activity is a function of the drugs, rather than the disease. This suggests that research into the biological characteristics of depression and its treatments may yield faster-acting antidepressants. Emerging evidence from clinical studies with mirtazapine, venlafaxine and SSRI augmentation with pindolol suggests that these treatments may relieve antidepressant symptoms more rapidly than SSRIs. The putative mechanism of action of faster-acting antidepressant strategies presented here purports that conventional antidepressants acutely increase the availability of serotonin (5-hydroxytryptamine, 5-HT) or noradrenaline (NA), preferentially at their cell body level, which triggers negative feedback mechanisms. After continued stimulation, these feedback mechanisms become desensitised and the enhanced 5-HT availability is able to enhance 5-HT and/or NA neurotransmission. Putative fast-onset antidepressants, on the other hand, may uncouple such feedback control mechanisms and enhance 5-HT and/or NA neurotransmission more rapidly. Further studies are required to characterise in detail the interactions between NA and 5-HT systems and to definitively establish the early onset of candidate antidepressants such as mirtazapine, venlafaxine and pindolol augmentation.

Is there a role for 5-HT1A agonists in the treatment of depression?

The role of serotonin (5-hydroxytryptamine; 5-HT) in the treatment of depressive and anxiety disorders is underscored by the therapeutic action of selective 5-HT reuptake inhibitors acting to enhance the degree of activation of various 5-HT receptor subtypes. The 5-HT1A receptors are particularly relevant to the antidepressant and anxiolytic responses in human beings. They are located presynaptically in the raphe nuclei, where they act as cell body autoreceptors to inhibit the firing rate of 5-HT neurons, and are located postsynaptically in limbic and cortical regions, where they also attenuate firing activity. The azapirones are full agonists at 5-HT1A autoreceptors and are generally, but not exclusively, partial agonists at postsynaptic 5-HT1A receptors. Some of these drugs, including gepirone and other 5-HT1A agonists such as buspirone, have been reported to exert anxiolytic and antidepressive activity in double-blind, placebo-controlled, and comparative trials. Their delayed therapeutic activity is believed to result from increased activation of postsynaptic 5-HT1A receptors occurring only after 5-HT neurons regain their normal firing activity. The recovery of this parameter, which is attributable to 5-HT1A autoreceptor desensitization, also restores 5-HT release. At this point, the summed effects of a normalized level of synaptic 5-HT and the exogenous 5-HT1A agonist can be exerted on postsynaptic 5-HT1A receptors. The widespread recognition of the clinical efficacy of such agents has largely been hampered by their undesirable pharmacokinetic properties. Most 5-HT1A agonists are indeed readily absorbed but are also rapidly eliminated, thereby often producing either suboptimal therapeutic responses at low doses, or cumbersome adverse effects at higher doses. Extended-release formulations allow once-daily dosing regimens, thus avoiding sharp peak plasma concentrations. This improves compliance and permits the use of higher dosages, which may be associated with enhanced efficacy and better tolerability relative to the immediate-release formulations. In sum, 5-HT1A receptor agonism represents a valuable and efficacious therapeutic approach to major depression.

Evaluation of EMD 128 130 occupancy of the 5-HT1A and the D2 receptor: a human PET study with [11C]WAY-100635 and [11C]raclopride

The use of so-called, atypical antipsychotic medication is becoming more widespread in the treatment of psychotic disorders. EMD 128 130 is a novel compound acting as an agonist at the 5-HT1A receptor, and as an antagonist at the dopamine-2 (D2) receptor. This dual action may confer additional benefits over selective D2 antagonists in the treatment of psychotic disorders. In this study, we investigated the occupancy of EMD 128 130 in vivo at the human D2 and 5-HT1A receptors with positron emission tomography using the radiotracers [11C]raclopride and [11C]WAY-100635. Seven healthy volunteers were examined before and after 5 days of treatment with EMD 128 130, administered in an incremental dose building up to 50 mg, b.d. A significant occupancy was demonstrated at the human D2 receptor (40% following a dose of 50 mg, b.d.) while there was no consistent effect observed at the 5-HT1A receptor, despite a similar affinity of EMD 128 130 for cloned human D2 and 5-HT1A receptors, and the presence of typical, central 5-HT1A agonist side-effects. The differential effects of EMD 128 130 at the D2 and the 5-HT1A receptor (antagonist at D2 receptor, agonist at the 5-HTIA receptor) may explain the differences in occupancy observed.

5-Hydroxytryptamine1A receptor occupancy by novel full antagonist 2-[4-[4-(7-chloro-2,3-dihydro-1,4-benzdioxyn-5-yl)-1-piperazinyl]butyl]-1,2-benzisothiazol-3-(2H)-one-1,1-dioxide: a[11C][O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY-100635) positron emission tomography study in humans

5-Hydroxytryptamine(1A) (5-HT(1A)) receptors have been implicated in the pathophysiology and treatment of anxiety and depression, and are a target for novel drug development. This is the first study examining the human brain in vivo occupancy by a novel, selective, silent 5-HT(1A) antagonist. 2-[4-[4-(7-Chloro-2,3-dihydro-1,4-benzdioxyn-5-yl)-1-piperazinyl]butyl]-1,2-benzisothiazol-3-(2H)-one-1,1-dioxide (DU 125530), a compound in clinical development, has potential applications in the treatment of anxiety and mood disorders. Positron emission tomography (PET) and [(11)C][O-methyl-3H]-N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride (WAY-100635), were used to assess 5-HT(1A) autoreceptor and postsynaptic receptor occupancy in 12 healthy male volunteers. Over a 10- to 40-mg daily dose range, DU 125530 was well tolerated, and exhibited a dose-dependent occupancy from 0 to 72% at 2 h post the last dose. Occupancy correlated significantly with plasma levels of DU 125530, and a fitting of the data to a standard single-site binding model gave a maximum occupancy of approximately 80%, and a half-saturation concentration (ED(50)) of approximately 7 ng/ml. At 24 h after the last dose 5-HT(1A) occupancy was approximately 50% of that achieved at 2 h. This study demonstrates that high occupancy of the human brain 5-HT(1A) receptor can be achieved at doses producing minimal acute side effects.

Therapeutic potential of serotonin antagonists in depressive disorders

Although the precise neurochemical imbalances in depression are still unknown, a role for the neurotransmitter 5-hydroxytryptamine (serotonin) has been implicated since the identification of the first effective antidepressants, imipramine and iproniazid. This led to the development of the selective serotonin re-uptake inhibitors which are widely used in the treatment of depression and depressive disorders, including generalised anxiety disorder, social phobia, obsessive compulsive disorder etc. Studies involving chronic administration in rats led to the hypothesis that the different classes of antidepressant treatment produce a common neuroadaptive change, namely an enhancement of serotonin neurotransmission, albeit via different pre and postsynaptic mechanisms. From this, it was suggested that serotonin antagonists should induce similar neuroadaptive changes, either directly or through a potentiation of other antidepressant agents. Here, the profiles of novel serotonin antagonists currently in preclinical development are reviewed and their therapeutic potential is assessed.

A database of [(11)C]WAY-100635 binding to 5-HT(1A) receptors in normal male volunteers: normative data and relationship to methodological, demographic, physiological, and behavioral variables

PET studies of [(11)C]WAY-100635 binding are proving to be a useful tool to evaluate 5-HT(1A) receptor function in vivo in humans. We describe the pattern of [(11)C]WAY-100635 binding in 61 healthy male brains and examine its variability. For all PET scans, binding potential (BP) values for [(11)C]WAY-100635 in different regions were calculated using a simplified reference tissue model, with the cerebellum as reference region. Specifically we describe (1) region of interest and SPM databases of PET [(11)C]WAY-100635 binding, including test-retest variability; (2) the sensitivity of [(11)C]WAY-100635 binding to manipulations of endogenous 5-HT; and (3) correlations between [(11)C]WAY-100635 binding and radiochemical, demographic, physiological, and behavioral variables. The regional distribution of [(11)C]WAY-100635 binding in healthy human brain was similar to that reported in vitro. The test-retest variability was approximately 12% (range 9-16%) and was similar for all methods of regional sampling. The binding of [(11)C]WAY-100635 was insensitive to changes in brain 5-HT induced by tryptophan infusion and depletion. Although BP values varied greatly across subjects (range 2.9-6.8), there were no significant correlations of regional and global BP with common radiochemical, demographic, physiological, and personality variables. Specifically, in contrast with two recent small studies, we found no decline of [(11)C]WAY-100635 binding with age in our large cohort over the age range of 24 to 53 years. Assessment of 5-HT(1A) receptors in vivo using PET and [(11)C]WAY-100635 gives reliable measures of 5-HT(1A) binding. The large between-subject variability observed could not be explained by common methodological, physiological, or behavioral factors and hence the biological basis of this variability remains to be clarified.

Toward optimal treatments for major depression

The treatment of major depression remains problematic for several reasons. In particular, the therapeutic response to medications usually does not manifest itself until a week after administration has begun, and more than half the patients will not experience a full recovery with the first antidepressant drug administration. There are, however, some pharmacologic strategies that can accelerate antidepressant response. When facing a treatment-resistant depression, combination therapy offers a more time-efficient approach to achieve remission than drug substitution. These interventions have been devised on a better understanding of the basis for the therapeutic response obtained with the first- and second-generation antidepressants, and evidence derived from controlled clinical trials of their superior effectiveness is growing. The rationale for such approaches will be described in this article, as well as their advantages and potential inconveniences. Ongoing research in this field continues to fuel the development of novel, better-tolerated, and more effective pharmacotherapies for depression.

The 'dalhousie serotonin cocktail' for treatment-resistant major depressive disorder

We describe the successful treatment of five patients with treatment-resistant major depressive disorder (TR-MDD) with a combination pharmacotherapy of pindolol, tryptophan and nefazodone. Five TR-MDD outpatients who had previously not responded to at least four different antidepressant medication trials were initiated on 300 mg/day of nefazodone, 7.5 mg/day of pindolol and 1 g/day of tryptophan. Pindolol doses remained the same throughout the 20 weeks, while tryptophan and nefazodone dosages were gradually increased to 8 g/day and 450 mg/day, respectively. The Hamilton Depression Rating Scale (HAM-D) was used to evaluate outcome. By week 4, all cases demonstrated at least 50% decrease in HAM-D scores. At the end of the trial, the group mean HAM-D score had significantly decreased from 26.8 (+/- 1.9) to 1.8 (+/- 0.8) (p < 0.001). No significant adverse effects were reported. These results suggest that if serotonin availability and release is further enhanced by tryptophan in the presence of nefazodone and pindolol, an antidepressant effect may be produced in patients who are otherwise treatment-resistant. Due to limited sample size, an open design and an 'unusually' high successful efficacy rate of this preliminary study, controlled studies are required to confirm the efficacy of this treatment strategy.

How does pindolol improve antidepressant action?

Since 1994, the beta-adrenoceptor and 5-HT(1A/1B) receptor ligand pindolol has been used to accelerate or enhance the clinical effects of antidepressant drugs, such as the selective 5-HT reuptake inhibitors (SSRIs), that act primarily on 5-HT-containing neurones. Pindolol was initially thought to act by preventing the inhibition of 5-HT release, elicited by SSRIs and other 5-HT-acting drugs, as a result of its ability to antagonize the action of 5-HT at midbrain raphe 5-HT(1A) autoreceptors that control the activity of ascending 5-HT-mediated pathways. However, the partial agonist properties of pindolol at 5-HT(1A) receptors and beta-adrenoceptors suggest that other explanations for its action are also possible. In this article, recent controversial data on the mechanism of action of pindolol, which are crucial for the development of more rapid and efficient antidepressant therapies, will be discussed.

Pharmacology of antidepressants

Presently in the United States, 21 compounds have been approved by the Food and Drug Administration as antidepressants. Two additional drugs marketed outside the United States as antidepressants have been approved for obsessive-compulsive disorder. Nearly one half of all these compounds became available within the past 12 years, whereas the first antidepressant was available more than 40 years ago. After the clinical aspects of depression are introduced in this article, the pharmacology of the newer generation drugs is reviewed in relationship to the older compounds. The information in this review will help clinicians treat acute depression with pharmacological agents.

Current perspectives on the development of non-biogenic amine-based antidepressants

Compounds that inhibit the re-uptake and/or metabolism of biogenic amines (i.e. serotonin, norepinephrine, and dopamine) have been used to treat depression for more than 40 years. Selective re-uptake inhibitors, currently the most widely prescribed class of biogenic amine-based agents, are certainly safe and relatively easy to use, but do not exhibit either a faster onset of action or greater efficacy than their predecessors. An approach to overcome the limitations that may be inherent to these 'conventional' therapies is to circumvent the monoaminergic synapse. In this review, two potential antidepressant strategies are discussed that may converge with intracellular pathways impacted by chronic treatment with biogenic amine-based agents. Drugs emerging from these strategies may offer significant advantages over currently used antidepressants.

Augmentation of fluoxetine's antidepressant action by pindolol: analysis of clinical, pharmacokinetic, and methodologic factors

In a controlled trial, the beta-adrenoceptor/5-hydroxytryptamine-1A (5-HT1A) receptor antagonist pindolol accelerated and enhanced the antidepressant effect of fluoxetine. The median times to sustained response (> or = 50% reduction of baseline severity maintained until endpoint) were 19 days for fluoxetine plus pindolol (N = 55) and 29 days for fluoxetine plus placebo (N = 56) (p = 0.01). The response rate at endpoint was 16% greater in patients treated with the combination. The plasma concentration of pindolol remained stable between 3 days (first blood sampling) and 6 weeks. Mean values were approximately 26 nM, a concentration higher than the Ki of (-)pindolol for human 5-HT1A autoreceptors (11 nM). Plasma fluoxetine and norfluoxetine concentrations increased steadily until the fourth week of treatment. Fluoxetine concentrations were lower in patients receiving the combination (p = 0.043), but there was no significant relationship to the clinical response in either group. A reanalysis of the data using a survival analysis revealed that significant differences in the time to sustained response between both groups would have also been detected (1) in a 2-week trial, (2) without a placebo lead-in phase, and (3) with less frequent visits. However, the use of "response" instead of "sustained response" as measure of clinically relevant change would have greatly diminished the difference between treatment arms (p = 0.08 instead of p = 0.01). This emphasizes the need of using stringent outcome criteria in antidepressant drug trials. A comparison of the data of all sustained responders (N = 27) in the fluoxetine-plus-placebo group with the first 27 responders in the fluoxetine-plus-pindolol group (of a total of 38) revealed a highly significant difference in the time to sustained response (18 and 10 days, respectively; p = 0.0002). This indicates that the faster response in the fluoxetine-plus-pindolol group is not a result of the greater proportion of responders.

Imaging serotonergic neurotransmission in depression: hippocampal pathophysiology may mirror global brain alterations

The recent development of [carbonyl-(11)C]WAY-100635 for serotonin (5-HT)(1A) and [(18)F]setoperone and [(18)F]altanserin for 5-HT(2A) positron emission tomography receptor imaging has allowed studies of 5-HT neurotransmission in depressive disorders. The hippocampus is likely to be an important brain structure in the pathophysiology of depression because it may mediate both cognitive deficits and hypercortisolemia found in this disorder. Decreased 5-HT(1A) binding was reported in the medial temporal cortex, which receives dense 5-HT innervation, and also throughout neocortical regions. Because the 5-HT(1A) antagonist pindolol may hasten antidepressant effects of selective serotonin reuptake inhibitor medications, its receptor occupancy has been measured in both presynaptic and postsynaptic sites. The results are controversial but suggest that pindolol has preferential occupancy of somatodendritic autoreceptors in the raphe. The results of 5-HT(2A) receptors are mixed, with one showing a significant decrease in the right orbitoinsular cortex and three not detecting a significant change. The disparate findings in patients with depression almost certainly reflect the heterogeneity of the disorder, and we highlight the utility of the hippocampus as a useful target region not only to compare depressed subjects with healthy subjects but also to correlate findings with cognitive function and activity of the limbic-hypothalamic-pituitary axis system.

Pindolol augmentation of antidepressant treatment: recent contributions from brain imaging studies

Preclinical studies suggest that augmentation of selective serotonin (5-HT) reuptake inhibitors by the 5-HT(1A) receptor agent pindolol might reduce the delay between initiation of treatment and antidepressant response, an effect largely mediated by blockade of 5-HT(1A) autoreceptors in the dorsal raphe nuclei. Although some controlled clinical trials suggest that pindolol might reduce latency to selective serotonin reuptake inhibitor response in acute depressive episodes, the effect is moderate and highly variable. Recent positron emission tomography studies investigating the occupancy of 5-HT(1A) receptors in humans by pindolol have shown that at the dose used most often in clinical trials the occupancy is low and variable, which might explain the inconsistent clinical results. Positron emission tomography studies also suggest that pindolol might be more potent at blocking 5-HT(1A) autoreceptors than at blocking postsynaptic receptors, a property that may be useful in this pharmacologic strategy. Thus, the positron emission tomography data support the potential of pindolol to augment the antidepressant response of selective serotonin reuptake inhibitors, but also imply that this potential has not been fully evaluated. Here we review the clinical trials, the positron emission tomography studies, and the possible mechanisms of pindolol augmentation. It is also suggested that positron emission tomography may be used to define therapeutic dosing early on in the process of clinical evaluation of new treatment strategies.