Augmentation with a 5-HT(1A), but not a 5-HT(1B) receptor antagonist critically depends on the dose of citalopram

Amygdala biomarkers of treatment response in major depressive disorder: An fMRI systematic review of SSRI antidepressants

Functional neuroimaging studies have demonstrated abnormal activity and functional connectivity (FC) of the amygdala among individuals with major depressive disorder (MDD), which may be rectified with selective serotonin reuptake inhibitor (SSRI) treatment. This systematic review aimed to identify changes in the amygdala on functional magnetic resonance imaging (fMRI) scans among individuals with MDD who received SSRIs. A search for fMRI studies examining amygdala correlates of SSRI response via fMRI was conducted through OVID (MEDLINE, PsycINFO, and Embase). The end date was April 4th, 2023. In total, 623 records were screened, and 16 studies were included in this review. While the search pertained to SSRIs broadly, the included studies were escitalopram-, citalopram-, fluoxetine-, sertraline-, and paroxetine-specific. Decreases in event-related amygdala activity were found following 6-to-12-week SSRI treatment, particularly in response to negative stimuli. Eight-week courses of SSRI pharmacotherapy were associated with increased event-related amygdala FC (i.e., with the prefrontal [PFC] and anterior cingulate cortices, insula, thalamus, caudate nucleus, and putamen) and decreased resting-state effective connectivity (i.e., amygdala-PFC). Preliminary evidence suggests that SSRIs may alter amygdala activity and FC in MDD. Additional studies are needed to corroborate findings. Future research should employ long-term follow-ups to determine whether effects persist after treatment termination.

Buspirone, a 5-HT1A agonist attenuates social isolation-induced behavior deficits in rats: a comparative study with fluoxetine

Social isolation is a potent stressor in both humans and animals that results in increased anger-like emotion, (anger in humans), aggression and suicidal ideation in humans [suicidal trait-related behavior in rats (STRB)]. The study's purpose was to compare the effects of buspirone (BUS) and fluoxetine (Flx) on social isolation-induced behavior deficits in rats. The male Wistar rats were randomized into six groups and caged individually for 14 days except for the non stress control (nSC) group. They were then divided into the following groups, stress control (SC), Flx (30), BUS (10), BUS (20) and BUS (40) and treated from day 14 to day 28. On the last day of treatment behavior parameters were recorded. Serum cortisol, blood pressure (BP) measurement, magnetic resonance imaging (MRI) of the rat's brain and brain-derived neurotrophic factor (BDNF) expression were performed. SC group showed a significant increase in anger-like emotion, aggression, irritability score, learned helplessness, increased cortisol level and reduced BDNF. These behavioral deficits were attenuated by BUS and Flx, Both were found to be equally beneficial in preventing anger-like emotions and aggression. Flx, which has been found to promote suicidal thoughts in people, did not reduce irritability in rats, showing that it did not affect it. BUS significantly improved all behavioral traits also reduced cortisol levels, significantly increased BDNF and normalized BP. Neuroimaging studies in SC brains showed a reduction in amygdala size compared to nSC, BUS treatment mitigated this reduction. Buspirone is effective in preventing social isolation induced behavioural-deficits.

Effect of selective serotonin reuptake inhibitor discontinuation on anxiety-like behaviours in mice

BACKGROUND

Abrupt cessation of therapy with a selective serotonin reuptake inhibitor (SSRI) is associated with a discontinuation syndrome, typified by numerous disabling symptoms, including anxiety. Surprisingly, little is known of the behavioural effect of SSRI discontinuation in animals.

AIM

Here, the effect of SSRI discontinuation on anxiety-like behaviour was systematically investigated in mice.

METHODS

Experiments were based on a three-arm experimental design comprising saline, continued SSRI and discontinued SSRI. Mice were assessed 2 days after SSRI discontinuation over a 5-day period using the elevated plus maze (EPM) and other anxiety tests.

RESULTS

An exploratory experiment found cessation of paroxetine (12 days) was associated with decreased open-arm exploration and reduced total distance travelled, in male but not female mice. Follow-up studies confirmed a discontinuation effect on the EPM in male mice after paroxetine (12 days) and also citalopram (12 days). Mice receiving continued paroxetine (but not citalopram) also showed decreased open-arm exploration but this was dissociable from the effects of discontinuation. The discontinuation response to paroxetine did not strengthen after 28 days of treatment but was absent after 7 days of treatment. A discontinuation response was not discernible in other anxiety and fear-learning tests applied 3-5 days after treatment cessation. Finally, discontinuation effects on the EPM were typically associated with decreased locomotion on the test. However, separate locomotor testing implicated anxiety-provoked behavioural inhibition rather than a general reduction in motor activity.

CONCLUSION

Overall, this study provides evidence for a short-lasting behavioural discontinuation response to cessation of SSRI treatment in mice.

Inter-individual differences in serotonin and glutamate co-transmission reflect differentiation in context-induced conditioned 50-kHz USVs response after morphine withdrawal

A growing body of research provides compelling evidence that in rats 50-kHz USVs are a form of expression of positive emotions. Context-induced 50-kHz USVs emission is variable among rats, indicating individual differences in contextual response bound up with pharmacological reward. The aims of this study were to: extract the most important neurotransmitters related to context-induced conditioned 50-kHz USVs response; find biological basis of existing inter-individual differences in context-induced conditioned 50-kHz USVs response; create a model of all-to-all neurotransmitters correlations. The data collected here confirms that re-exposure to the context of morphine administration after the withdrawal period increases the level of 50-kHz USVs and this contextual response is associated with elevated serotonin concentrations in amygdala, hippocampus and mPFC and with increased Glu/Gln ratio in nucleus accumbens. The concentration of serotonin increases simultaneously in amygdala, nucleus accumbens and hippocampus. Moreover, 5-HT concentration in amygdala is bound up with glutamate level in this structure as well as in hippocampus. Furthermore, Glu/Gln ratio in nucleus accumbens has strong associations with Glu/Gln ratio simultaneously in VTA, amygdala, striatum and hippocampus. All-to-all-analysis indicate that concentration of glutamate in hippocampus is proportional to glutamate in VTA and GABA concentration in the hippocampus. We have also demonstrated that Glu/GABA ratio in VTA and amygdala was elevated after post withdrawal re-exposure to the pharmacological reward paired context. Presented analysis indicates a strong correlation between serotonergic and glutamatergic systems in context-induced conditioned response. The strength of this co-transmission correlates with the number of 50-kHz USVs emitted in response to morphine-paired context.

Looking back (and in)to the future: A personal reflection on 'Serotonin autoreceptor function and antidepressant drug action' (Hjorth et al., 2000)

Our article in this journal some 15 years ago focussed on the role of serotonin (5-HT) autoreceptors in the mechanism of action of antidepressant drugs. Specifically in this regard, the results were summarised of rat microdialysis studies carried out to examine: (a) the relative importance of 5-HT_1A and 5-HT_1B autoreceptors, including (b) possible regional variation, and (c) potential changes in autoreceptor responsiveness following chronic selective serotonin reuptake inhibitor administration. In the present reflection piece, I recap some of the key findings against a brief background and provide an account of their bearing within the context of subsequent endeavours in the antidepressant drug research and development field. I conclude by shortly commenting on selected topics relevant to novel, interesting advances and avenues for future research.

Lu AA21004, a novel multimodal antidepressant, produces regionally selective increases of multiple neurotransmitters--a rat microdialysis and electrophysiology study

The monoaminergic network, including serotonin (5-HT), norepinephrine (NE), and dopamine (DA) pathways, is highly interconnected and has a well-established role in mood disorders. Preclinical research suggests that 5-HT receptor subtypes, including 5-HT1A, 5-HT1B, 5-HT3, and 5-HT7 receptors as well as the 5-HT transporter (SERT), may have important roles in treating depression. This study evaluated the neuropharmacological profile of Lu AA21004, a novel multimodal antidepressant combining 5-HT3 and 5-HT7 receptor antagonism, 5-HT1B receptor partial agonism, 5-HT1A receptor agonism, and SERT inhibition in recombinant cell lines. Extracellular 5-HT, NE and DA levels were evaluated in the ventral hippocampus (vHC), medial prefrontal cortex (mPFC) and nucleus accumbens (NAc) after acute and subchronic treatment with Lu AA21004 or escitalopram. The acute effects of LuAA21004 on NE and DA neuronal firing were also evaluated in the locus coeruleus (LC) and ventral tegmental area (VTA), respectively. Acute Lu AA21004 dose-dependently increased 5-HT in the vHC, mPFC and NAc. Maximal 5-HT levels in the vHC were higher than those in the mPFC. Furthermore, mPFC 5-HT levels were increased at low SERT occupancy levels. In the vHC and mPFC, but not the NAc, high Lu AA21004 doses increased NE and DA levels. Lu AA21004 slightly decreased LC NE neuronal firing and had no effect on VTA DA firing. Results are discussed in context of occupancy at 5-HT3, 5-HT1B and 5-HT1A receptors and SERT. In conclusion, Lu AA21004, acting via two pharmacological modalities, 5-HT receptor modulation and SERT inhibition, results in a brain region-dependent increase of multiple neurotransmitter concentrations.

Organic cation transporter 2 controls brain norepinephrine and serotonin clearance and antidepressant response

High-affinity transporters for norepinephrine (NE) and serotonin (5-HT), which ensure neurotransmitter clearance at the synapse, are the principal targets of widely used antidepressant drugs. Antidepressants targeting these high-affinity transporters, however, do not provide positive treatment outcomes for all patients. Other monoamine transport systems, with lower affinity, have been detected in the brain, but their role is largely unknown. Here we report that OCT2, a member of the polyspecific organic cation transporter (OCT) family, is expressed notably in the limbic system and implicated in anxiety and depression-related behaviors in the mouse. Genetic deletion of OCT2 in mice produced a significant reduction in brain tissue concentrations of NE and 5-HT and in ex vivo uptake of both these neurotransmitters in the presence of the dual 5-HT-NE transport blocker, venlafaxine. In vivo clearance of NE and 5-HT evaluated using microiontophoretic electrophysiology was diminished in the hippocampus of OCT2(-/-) mice in the presence of venlafaxine, thereby affecting postsynaptic neuronal activity. OCT2(-/-) mice displayed an altered sensitivity to acute treatments with NE- and/or 5-HT-selective transport blockers in the forced-swim test. Moreover, the mutant mice were insensitive to long-term venlafaxine treatment in a more realistic, corticosterone-induced, chronic depression model. Our findings identify OCT2 as an important postsynaptic determinant of aminergic tonus and mood-related behaviors and a potential pharmacological target for mood disorders therapy.

Current therapeutic strategies for premature ejaculation and future perspectives

Premature ejaculation (PE) is a common sexual disorder in men that is mediated by disturbances in the peripheral and central nervous systems. Although all pharmaceutical treatments for PE are currently used 'off-label', some novel oral agents and some newer methods of drug administration now provide important relief to PE patients. However, the aetiology of this condition has still not been unified, primarily because of the lack of a standard animal model for basic research and the absence of a widely accepted definition and assessment tool for evidence-based clinical studies in patients with PE. In this review, we focus on the current therapeutic strategies and future treatment perspectives for PE.

Chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain

BACKGROUND

Serotonin (5-HT) is a neurotransmitter with important roles in the regulation of neurobehavioral processes, particularly those regulating affect in humans. Drugs that potentiate serotonergic neurotransmission by selectively inhibiting the reuptake of serotonin (SSRIs) are widely used for the treatment of psychiatric disorders. Although the regulation of serotonin synthesis may be an factor in SSRI efficacy, the effect of chronic SSRI administration on 5-HT synthesis is not well understood. Here, we describe effects of chronic administration of the SSRI citalopram (CIT) on 5-HT synthesis and content in the mouse forebrain.

METHODOLOGY/PRINCIPAL FINDINGS

Citalopram was administered continuously to adult male C57BL/6J mice via osmotic minipump for 2 days, 14 days or 28 days. Plasma citalopram levels were found to be within the clinical range. 5-HT synthesis was assessed using the decarboxylase inhibition method. Citalopram administration caused a suppression of 5-HT synthesis at all time points. CIT treatment also caused a reduction in forebrain 5-HIAA content. Following chronic CIT treatment, forebrain 5-HT stores were more sensitive to the depleting effects of acute decarboxylase inhibition.

CONCLUSIONS/SIGNIFICANCE

Taken together, these results demonstrate that chronic citalopram administration causes a sustained suppression of serotonin synthesis in the mouse forebrain. Furthermore, our results indicate that chronic 5-HT reuptake inhibition renders 5-HT brain stores more sensitive to alterations in serotonin synthesis. These results suggest that the regulation of 5-HT synthesis warrants consideration in efforts to develop novel antidepressant strategies.

Long-term citalopram maintenance in mice: selective reduction of alcohol-heightened aggression

BACKGROUND

Selective serotonin reuptake inhibitors (SSRIs) alleviate many affective disturbances in human clinical populations and are used in animal models to study the influence of serotonin (5-HT) on aggressive behavior and impulsivity.

OBJECTIVE

We hypothesized that long-term SSRI treatment may reduce aggressive behavior escalated by alcohol consumption in mice. Therefore, aggression was tested in male CFW mice to determine whether repeated citalopram (CIT) administration reduces alcohol-heightened aggression.

MATERIALS AND METHODS

Resident male mice self-administered alcohol by performing an operant response on a panel placed in their home cage that delivered a 6% alcohol solution. Mice repeatedly confronted an intruder 15 min after self-administration of either 1 g/kg alcohol (EtOH) or water (H(2)O). Aggressive behaviors were higher in most mice when tests occurred after EtOH intake relative to H(2)O. Once baseline aggression was established, animals were injected (i.p.) twice daily with 10 mg/kg CIT or saline (SAL) for 32 days. Every 4 days throughout the CIT treatment period, aggressive encounters occurred 6 h after CIT injections, with testing conditions alternating between EtOH and H(2)O intake.

RESULTS

Aggression was only modestly affected by CIT in the first 2 weeks of treatment. However, by day 17 of CIT treatment, alcohol-heightened aggressive behavior was abolished, while baseline aggression remained stable. These data lend support for the role of the 5-HT transporter in the control of alcohol-related aggressive behavior, and the time course of effects suggests that a change in density of 5HT(1A) autoreceptors is necessary before antidepressant drugs produce beneficial outcomes.

Augmentation of SSRI effects on serotonin by 5-HT2C antagonists: mechanistic studies

The treatment of depression may be improved by using an augmentation approach involving selective serotonin reuptake inhibitors (SSRIs) in combination with compounds that focus on antagonism of inhibitory serotonin receptors. Using microdialysis coupled to HPLC, it has recently been shown that the systemic co-administration of 5-HT(2C) antagonists with SSRIs augmented the acute effect of SSRIs on extracellular 5-HT. In this paper, we have investigated the mechanism through which this augmentation occurs. The increase in extracellular 5-HT was not observed when both compounds were locally infused. However, varying the route of administration for both compounds differentially revealed that an augmentation took place when the 5-HT(2C) antagonist was locally infused into ventral hippocampus and the SSRI given systemically, but not when systemic 5-HT(2C) antagonist was co-administered with the local infusion of citalopram. This suggests that the release of extracellular serotonin in ventral hippocampus may be controlled by (an)other brain area(s). As 5-HT(2C) receptors are not considered to be autoreceptors, this would implicate that other neurotransmitter systems are involved in this process. To investigate which neurotransmitter systems were involved in the interaction, systemic citalopram was challenged with several glutamatergic, GABA-ergic, noradrenergic, and dopaminergic compounds to determine their effects on serotonin release in ventral hippocampus. It was determined that the involvement of glutamate, norepinephrine, and dopamine in the augmentation did not seem likely, whereas evidence implicated a role for the GABA-ergic system in the augmentation.

An integrated microdialysis rat model for multiple pharmacokinetic/pharmacodynamic investigations of serotonergic agents

INTRODUCTION

Integrated in vivo models applying intracerebral microdialysis in conjunction with automated serial blood sampling in conscious, freely moving rodents are an attractive approach for pharmacokinetic (PK) and simultaneous pharmacokinetic/pharmacodynamic (PK/PD) investigations of CNS active drugs within the same animal. In this work, the ability to obtain and correlate data in this manner was evaluated for the selective serotonin (5-HT) reuptake inhibitor (SSRI) escitalopram.

METHODS

An instrumented rat model equipped with an intracerebral hippocampal microdialysis probe and indwelling arterial and venous catheters was applied in the studies. Concomitant with brain microdialysis, serial blood sampling was conducted by means of an automated blood sampling device. The feasibility of the rat model for simultaneous PK/PD investigations was examined by monitoring plasma and brain extracellular concentrations of escitalopram along with SSRI-associated pharmacological activity, monitored as changes in brain 5-HT levels and plasma corticosterone levels.

RESULTS

Combining intracerebral microdialysis and automated blood sampling did not cause any detectable physiological changes with respect to basal levels of plasma corticosterone or brain 5-HT levels. Furthermore, the PK of escitalopram in hippocampus following intravenous injection was not influenced by the presence of vascular catheters. Conversion of escitalopram dialysate concentrations into absolute extracellular levels by means of in vivo retrodialysis was verified by the no-net-flux method, which gave similar recovery estimates. The PK of escitalopram could be characterized simultaneously in plasma and the hippocampus of conscious, freely moving rats. Concomitantly, the modulatory and functional effects of escitalopram could be monitored as increases in brain 5-HT and plasma corticosterone levels following drug administration.

DISCUSSION

The applicability of intracerebral microdialysis combined with arterial blood sampling was demonstrated for simultaneous PK/PD investigations of escitalopram in individual rats under non-stressful conditions. Together, these temporal relationships provide multiple PK/PD information in individual animals, hence minimizing inter-animal variation using a reduced number of animals.

5-hydroxytryptamine in premature ejaculation: opportunities for therapeutic intervention

Ejaculation, although mediated by a spinal ejaculation generator, is subject to descending supraspinal modulation from several brain regions. 5-Hydroxytryptamine (5-HT or serotonin) is involved in ejaculatory control, with its ejaculation-retarding effects likely to be attributable to activation of 5-HT1B and 5-HT2C receptors, both spinally and supraspinally. By contrast, stimulation of 5-HT1A receptors precipitates ejaculation. Selective serotonin reuptake inhibitors (SSRIs), which are used for treatment of psychiatric disorders, can delay ejaculation in humans and are widely used 'off-label' for treatment of premature ejaculation. SSRIs require 1-2 weeks' chronic dosing to be effective, similar to their use for treatment of depression. However, a new short-acting SSRI is effective 'on demand' and might represent the first of a new generation of therapies targeted to premature ejaculation.

Pharmacokinetic modelling of blood–brain barrier transport of escitalopram in rats

This study examined the pharmacokinetics and distribution of escitalopram in the brain extracellular fluid in rats by the concurrent use of intracerebral microdialysis and serial blood sampling. Following three constant intravenous infusions, drug concentrations in the hippocampus and plasma were monitored for 6 h. To estimate the integrated pharmacokinetics and intercompartmental transport parameters, including blood-brain barrier (BBB) transport over the entire dose range, unbound brain and plasma escitalopram concentration data from all doses were simultaneously analysed using compartmental modelling. The pharmacokinetic analysis revealed that systemic clearance decreased as a function of dose, which was incorporated in the integrated model. Escitalopram was rapidly and extensively transported across the BBB and distributed into the brain extracellular fluid. The modelling resulted in an estimated influx clearance into the brain of 535 microl/min/g brain, resulting in an unbound brain-to-plasma AUC ratio of 0.8 independent of escitalopram dose. The model may be applied for preclinical evaluations or predictions of escitalopram concentration-time courses in plasma as well as at the target site in the CNS for various dosing scenarios. In addition, this modelling approach may also be valuable for studying BBB transport characteristics for other psychotropic agents.

Premature Ejaculation

Premature ejaculation (PE) is a frequent male sexual complaint that is mediated mainly by disturbances of serotonergic neurotransmission and certain serotonin (5-HT) receptors and, to a lesser extent, oxytocinergic neurotransmission in the CNS. The current Diagnostic Manual of Mental Disorders (fourth edition, revised text) [DSM-IV-TR] definition of PE has a low positive predictive value and is inadequate for clinical, epidemiological and drug treatment research. Categorisation of PE into four well defined syndromes has recently been proposed for the pending DSM (fifth edition) definition of PE. Over the last decade, an increasing number of studies of drug treatment of PE have been published. A meta-analysis of those studies, conducted in accordance with current standards of evidence-based medicine, demonstrated similar efficacies for daily treatment with the serotonergic antidepressants paroxetine hemihydrate, clomipramine, sertraline and fluoxetine, with paroxetine (hydrochloride) hemihydrate exerting the strongest effect on ejaculation. On the basis of fundamental insights into serotonergic neurotransmission, it has been suggested that on-demand selective serotonin reuptake inhibitor (SSRI) treatment will not lead to similarly impressive delays in ejaculation as has been observed with daily SSRI treatment. Indeed, some on-demand studies with SSRIs and studies with the new SSRI dapoxetine have shown a weak ejaculation-delaying effect after 1-2 hours of drug intake. Apart from daily treatment with SSRIs, PE can be delayed by on-demand use of topical anaesthetics and tramadol. Treatment with phosphodiesterase type 5 inhibitors should not be prescribed to men with PE with normal erectile function, but may be used if PE is accompanied by erectile difficulties. There is no scientific support for treatment of PE with intracavernous injection of vasoactive drugs. Animal studies have shown that strong immediate ejaculation delay may be induced by administration of a combination of an SSRI with a serotonin 5-HT(1A) receptor antagonist. The combination of an SSRI and any other compound that immediately and potently raises serotonin neurotransmission and/or use of oxytocin receptor antagonists may form the basis for the development of new on-demand and/or daily drugs for the treatment of PE.

New agents in the treatment of premature ejaculation

Premature ejaculation (PE) is a common male sexual disorder. Recent normative data suggest that men with an intravaginal ejaculatory latency time (IELT) of less than 1 minute have "definite" PE, while men with IELTs between 1 and 1.5 minutes have "probable" PE. Although there is insufficient empirical evidence to identify the etiology of PE, there is limited correlational evidence to suggest that men with PE have high levels of sexual anxiety and inherited altered sensitivity of central 5-HT (serotonin) receptors. Pharmacological modulation of the ejaculatory threshold using off-label daily or on-demand selective serotonin re-uptake inhibitors (SSRIs) offers patients a high likelihood of achieving improved ejaculatory control within a few days of initiating treatment, consequential improvements in sexual desire and other sexual domains and is well tolerated. Investigational drugs such as the ejaculo-selective serotonin transport inhibitors (ESSTIs) such as dapoxetine and UK-390,957 represent a major development in sexual medicine. These drugs offer patients the convenience of on-demand dosing, significant improvements in IELT, ejaculatory control, and sexual satisfaction with minimal adverse effects.

Investigation of the SSRI augmentation properties of 5-HT2 receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT(2) receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT(2) receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT(2) receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2mg kg(-1)) and higher (4 mg kg(-1)) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT(2) receptor antagonist ketanserin (4 mg kg(-1)) augmented the effect of 4 mg kg(-1) but not 2mg kg(-1) citalopram. The effect of 4 mg kg(-1) citalopram was also augmented by pre-treatment with either the 5-HT(2C) receptor antagonist SB 242084 (0.5mg kg(-1)) or the 5-HT(2A) receptor antagonist MDL 100907 (0.5mg kg(-1)). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5mg kg(-1)) and higher (20mg kg(-1)) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg(-1)). These results confirm recent findings that 5-HT(2) receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT(2C) and possibly 5-HT(2A) receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT(2) receptor antagonist properties of this drug.

Emerging drugs for premature ejaculation

Lifelong premature ejaculation (PE) is a frequent male sexual dysfunction and is thought to be mediated in part by disturbances of serotonergic (5-hydroxytryptamine; 5-HT) neurotransmission and ejaculation-mediating 5-HT receptors in the CNS. The aetiology of the dysfunction is unclear, but probably includes neurobiological and environmental factors. Lifelong PE is a syndrome characterised by a cluster of symptoms. Rapid ejaculations become manifest around the first sexual encounters in puberty or adolescence. Intravaginal ejaculation latency time usually occurs within 30-60 s, or maximally within 2 min after vaginal penetration, is present with nearly every sexual partner, and remains similar throughout life or may aggravate during ageing. The syndrome may lead to secondary psychological, sexual and relationship problems. Daily treatment with some selective serotonin re-uptake inhibitors (SSRIs) leads to strong ejaculation delay, but may be accompanied by side effects. New treatment with SSRIs with a short half-life (if approved) for on-demand use 1-2 h prior to coitus exerts less ejaculation-delaying effects than daily SSRI strategies. Animal studies have shown that strong, immediate ejaculation delay may be induced by the combination of an SSRI with a 5-HT(1A) receptor antagonist. The combination of an SSRI and any other compound that immediately strongly raises 5-HT neurotransmission may form the basis for the development of new on-demand drugs to treat PE.

The effect of chronic selective serotonin reuptake inhibitor treatment on serotonin 1B receptor sensitivity and HPA axis activity

The authors have investigated 5-HT 1B receptor function in prefrontal cortex and dorsal hippocampus as well as the HPA axis response after subchronic (24 h) and chronic (15 days) treatment with the SSRI citalopram. All experiments were carried out in presence of citalopram to prevent rapid resensitization of the 5-HT(1B) receptors. Moreover, this more closely resembles the clinical situation. The concentration of citalopram was measured in both brain areas to ensure comparable levels in the different treatment groups. Using microdialysis, the authors found that under those conditions the effect of the 5-HT 1B receptor antagonists SB 224289 and the mixed 5-HT 1B/1D receptor antagonist GR 127935 on extracellular levels of 5-HT was unaltered by duration of treatment. Basal levels of 5-HT, however, were increased in the dorsal hippocampus following chronic treatment. In addition, plasma levels of the catecholamines adrenaline and noradrenaline and the HPA axis hormones ACTH and corticosterone were all decreased after chronic treatment.

Citalopram combined with WAY 100635 inhibits ejaculation and ejaculation-related Fos immunoreactivity

The role of 5-HT (5-hydroxytryptamine, 5-HT)(1A) receptor activation in the sexual side-effects, in particular delayed ejaculation, of selective serotonin reuptake inhibitors (SSRIs) was studied. Male Wistar rats were treated for 15 days with vehicle, the SSRI citalopram (10 mg/kg/day p.o.), the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide 3HCL (WAY 100635, 0.1 mg/kg/ day s.c.), or both drugs combined. Sexual behavior was assessed weekly. One h after the last sexual behavior test, rat brains were processed for Fos-immunohistochemistry. Acute and chronic citalopram mildly inhibited ejaculation, which was strongly augmented by co-administration of WAY 100635. WAY 100635 alone did not alter sexual behavior. Brain sites associated with ejaculation showed reduced Fos-immunoreactivity in rats treated with both citalopram and WAY 100635. Citalopram reduced Fos-immunoreactivity in the arcuate hypothalamic nucleus, an area that might link serotonergic neurotransmission to ejaculation.

Anticonvulsant action of GBR-12909 and citalopram against acute experimentally induced limbic seizures

We recently showed that intrahippocampally administered dopamine and serotonin exert concentration-dependent non-protective, protective and proconvulsant effects against limbic seizures in rats. Anticonvulsant action was mediated via, respectively, hippocampal D2 and 5-HT1A receptor stimulation, while proconvulsant effects were associated with concomitant hippocampal glutamate increases. We here examined whether increases in endogenous hippocampal dopamine and serotonin exert similar actions. Initially, dose-response experiments were performed with intrahippocampal perfusions of GBR-12909 and citalopram, respectively, selective dopamine and serotonin re-uptake blockers. Based on their effects on monoaminergic release, a potential non-protective, protective and proconvulsant concentration was selected. The predicted non-protective GBR-12909 (10 microM) and citalopram (0.5 microM) concentrations failed to prevent pilocarpine-induced seizures. The predicted protective GBR-12909 (100 microM) and citalopram (1 microM) perfusions resulted in complete anticonvulsant action, again mediated by D2 and 5-HT1A receptors. Unexpectedly, at predicted proconvulsant concentrations complete anticonvulsant action was obtained and hippocampal Glu remained unaltered. This study shows that selective monoamine re-uptake blockers have important anticonvulsant properties. Based on the previously established anticonvulsant monoamine ranges, anticonvulsant threshold concentrations can be predicted for compounds with endogenous dopamine or serotonin promoting effects. Non-selective actions curtailing glutamatergic activity may further be responsible for the unexpected anticonvulsant effects at predicted proconvulsant concentrations.

On-demand SSRI treatment of premature ejaculation: pharmacodynamic limitations for relevant ejaculation delay and consequent solutions

INTRODUCTION

Recently, the idea has emerged that on-demand use of serotonin reuptake inhibitors (SSRIs), particularly short half-life, should be equally effective in delaying ejaculation as daily SSRI treatment of premature ejaculation.

AIM

To provide evidence that SSRI-induced ejaculation delay is mainly dependent on pharmacodynamic properties of the drug and hardly on pharmacokinetic factors, and that combined SSRI administration with specific 5-hydroxytryptamine (5-HT) receptor antagonism leads acutely to stronger ejaculation delay than acute SSRI monoadministration.

METHODS

We performed a detailed analysis of serotonin neurotransmission and reviewed animal studies with 5-HT(1A) receptor antagonists. In addition, we critically reviewed existing on-demand SSRI treatments publications and the current debate on a definition of premature ejaculation.

MAIN OUTCOME MEASURES

Intravaginal ejaculation latency time (IELT).

RESULTS

Acute SSRI administration leads to only a mild or no increase of 5-HT neurotransmission and concomitant stimulation of postsynaptic 5-HT receptors. Existing on-demand SSRI treatment studies suffer from methodological insufficiencies, and the reported high-fold increases of ejaculation time contradict with neuropharmacological insights from serotonin metabolism. Animal studies show that SSRI coadministration with 5-HT(1A) receptor antagonists significantly increases the ejaculation time acutely compared to acute SSRI monoadministration.

CONCLUSION

On-demand SSRI treatment has less ejaculation-delaying effects than daily SSRI treatment. SSRIs with a short half-life are likely leading to much less ejaculation delay than current registered SSRIs. Combined use of SSRIs with 5-HT(1A) receptor antagonists increases the likelihood of clinically relevant ejaculation delay after on-demand treatment. On-demand SSRIs with short half-life that insufficiently delay ejaculation in men with IELTs less than 1 minute should be called ejaculation-delaying drugs rather than drugs against premature ejaculation.

Future antidepressants: what is in the pipeline and what is missing?

Monoamine reuptake inhibitors still reign in the treatment of major depression, but possibly not for long. While medicinal chemists have been able to reduce the side effects of these drugs, their delayed onset of action and considerable non-response rate remain problematic. Of late, serious questions have been raised regarding the efficacy of monoamine reuptake inhibitors. The present review presents an inventory of what is (and until recently was) in the antidepressant pipeline of pharmaceutical companies. Novel antidepressant compounds can be categorised into four groups depending on their target(s): (i) monoamine receptors; (ii) non-monoamine receptors; (iii) neuropeptide receptors; and (iv) hormone receptors. Other possible targets include components of post-receptor intracellular processes and elements of the immune system; to date, however, compounds specifically aimed at these targets have not been the subject of clinical trials. Development of several compounds targeted at monoamine receptors has recently been discontinued. At least five neurokinin-1 (NK(1)) receptor antagonists were until recently in phase II of clinical testing. However, the apparent interest in the NK(1) receptor should not be interpreted as representing a departure from the monoamine hypothesis since neurokinins also modulate monoaminergic systems. In the authors' view, development of future antidepressants will continue to rely on the serendipity-based monoamine hypothesis. However, an alternative approach, based on the hypothesis that chronic stress precipitates depressive symptoms, might be more productive. Unfortunately, clinical results using drugs targeted at components of the HPA axis have not been very encouraging to date. In the short run, the authors believe that augmentation strategies offer the best hope for improving the efficacy of antidepressant treatment. Several approaches to improve the efficacy of SSRIs are conceivable, such as concurrent blockade of monoamine autoreceptors and the addition of antipsychotics, neuromodulators or hormones (HPA axis and gender related). In the long-term, however, construction of a scientifically verified conceptual framework will be needed before more effective antidepressants can be developed. It can be argued that it is not depression itself that should be treated, but rather that its duration should be reduced by pharmacological means. Animal models that take this concept into consideration and identify mechanisms for acceleration of recovery from the effects of stress need to be developed.

Effects of chronic treatment with escitalopram or citalopram on extracellular 5-HT in the prefrontal cortex of rats: role of 5-HT1A receptors

1 Microdialysis was used to study the acute and chronic effects of escitalopram (S-citalopram; ESCIT) and chronic citalopram (CIT), together with the 5-HT1A receptor antagonist WAY100,635 (N-[2-[methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride) and the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), on extracellular 5-hydroxytryptamine (5-HT) levels in the rat prefrontal cortex. 2 Extracellular 5-HT rose to 234 and 298% of basal values after subcutaneous (s.c.) acute doses of 0.15 and 0.63 mg kg(-1) ESCIT. No further increase was observed at 2.5 mg kg(-1) ESCIT (290%). 3 The effect of 13-day s.c. infusion of 10 mg kg(-1) day(-1) ESCIT on extracellular 5-HT (422% of baseline) was greater than after 2 days (257% of baseline), whereas exposure to ESCIT was similar. In contrast, the increase in extracellular 5-HT induced by the infusion of CIT for 2 (306%) and 13 days (302%) was similar. However, brain and plasma levels of S-citalopram in rats infused with CIT for 13 days were lower than after 2 days. 4 Acute treatment with 2.5 mg kg(-1) ESCIT or 5 mg kg(-1) CIT raised extracellular 5-HT by 243 and 276%, respectively, in rats given chronic vehicle but had no effect in rats given ESCIT (10 mg kg(-1) day(-1)) or CIT (20 mg kg(-1) day(-1)) for 2 or 13 days, suggesting that the infused doses had maximally increased extracellular 5-HT. WAY100,635 (0.1 mg kg(-1) s.c.) increased extracellular 5-HT levels by 168, 174 and 169% of prechallenge values in rats infused with vehicle or ESCIT for 2 or 13 days, respectively. WAY100,635 enhanced extracellular 5-HT levels to 226, 153 and 164% of prechallenge values in rats infused with vehicle or CIT for 2 and 13 days, respectively. 5 8-OH-DPAT (0.025 mg kg(-1)) reduced extracellular 5-HT by 54% in control rats, but had no effect in those given ESCIT and CIT for 13 days. 6 This series of experiments led to the conclusion that chronic treatment with ESCIT desensitizes the 5-HT1A receptors, regulating the release of 5-HT in the prefrontal cortex and enhances the effect of the drug on extracellular 5-HT. They also indicate that chronic treatment with ESCIT and CIT did not prevent WAY100,635 from raising extracellular 5-HT.

Stereoselective single-dose kinetics of citalopram and its metabolites in rats

The single-dose kinetics of the enantiomers of citalopram (CIT) and its metabolites, demethylcitalopram (DCIT) and didemethylcitalopram (DDCIT), were investigated after administration of 10, 20, or 100 mg/kg (s.c.) rac-CIT to rats. Samples from serum and two brain regions were collected 1, 3, 10, or 20 h postdose for HPLC analysis. In the 100 mg/kg rats, the enantiomeric (S/R) serum concentration ratios of CIT decreased during the study period (0.93 at 1 h vs. 0.59 at 20 h; P < 0.001). In the 10 and 20 mg/kg rats, the decrease in serum S/R CIT ratios was not so evident as in the 100 mg/kg rats. In all three groups the S/R CIT ratio was almost the same in the brain as in serum, although both CIT enantiomer levels in the brain were found to be 5-10 times higher than the levels in serum. The serum and brain metabolite levels were low in the 10 and 20 mg/kg rats, whereas the levels increased during the study period in the 100 mg/kg rats. In conclusion, the CIT enantiomers were shown for the first time to be stereoselectively metabolized after single-dose administration to rats, as previously shown in steady-state dosing studies in humans and rats.

Role of extracellular serotonin levels in the effect of 5-HT1B receptor blockade

The release of serotonin (5-HT) at serotonergic nerve terminals is regulated by 5-HT(1B) autoreceptors. Several studies have reported that the effects of selective 5-HT reuptake inhibitors (SSRIs) on extracellular 5-HT are augmented by 5-HT(1B) receptor antagonists, whereas administration of these antagonists alone do not enhance 5-HT levels. It has been suggested that 5-HT(1B) receptors have low basal endogenous activity and therefore elevated endogenous 5-HT levels are needed to elicit an effect of 5-HT(1B) receptor antagonists. To test this hypothesis, different strategies were used to enhance 5-HT levels in the rat frontal cortex to assess the effects of locally applied NAS-181, a new selective 5-HT(1B) receptor antagonist. Blockade of 5-HT(1B) receptors with NAS-181 dose dependently augmented 5-HT levels when 5-HT levels were enhanced by a SSRI. No additional effect of NAS-181 on 5-HT output was found when 5-HT levels were enhanced by KCl depolarization-induced release or by preventing degradation of 5-HT with the monoamine oxidase inhibitor pargyline. In the presence of fluvoxamine, the increased 5-HT release evoked by KCl depolarization was augmented by NAS-181, supporting the idea that blockade of 5-HT transporters is necessary to measure an effect of 5-HT(1B) receptor blockade. In conclusion, the results provide circumstantial evidence that the effect of a 5-HT(1B) receptor antagonist depends on extracellular 5-HT levels, but strongly suggest that additional 5-HT reuptake inhibition is required to detect any effect of 5-HT(1B) receptor antagonist on 5-HT levels by in vivo microdialysis.

Conditional involvement of striatal serotonin3 receptors in the control of in vivo dopamine outflow in the rat striatum

Serotonin3 (5-HT3) receptors can affect motor control through an interaction with the nigrostriatal dopamine (DA) neurons, but the neurochemical basis for this interaction remains controversial. In this study, using in vivo microdialysis, we assessed the hypothesis that 5-HT3 receptor-dependent control of striatal DA release is conditioned by the degree of DA and/or 5-HT neuron activity and the means of DA release (impulse-dependent vs. impulse-independent). The different DA-releasing effects of morphine (1 and 10 mg/kg), haloperidol (0.01 mg/kg), amphetamine (1 and 2.5 mg/kg), and cocaine (10 and 20 mg/kg) were studied in the striatum of freely moving rats administered selective 5-HT3 antagonists ondansetron (0.1 mg/kg) or MDL 72222 (0.03 mg/kg). Neither of the 5-HT3 antagonists modified basal DA release by itself. Pretreatment with ondansetron or MDL 72222 reduced the increase in striatal DA release induced by 10 mg/kg morphine but not by 1 mg/kg morphine, haloperidol, amphetamine or cocaine. The effect of 10 mg/kg morphine was also prevented by intrastriatal ondansetron (1 microm) administration. Reverse dialysis with ondansetron also reduced the increase in DA release induced by the combination of haloperidol and the 5-HT reuptake inhibitor citalopram (1 mg/kg). Considering the different DA and 5-HT-releasing properties of the drugs used, our results demonstrate that striatal 5-HT3 receptors control selectively the depolarization-dependent exocytosis of DA only when central DA and 5-HT tones are increased concomitantly.

Effect of 5-HT1A receptor-mediated serotonin augmentation on Fos immunoreactivity in rat brain

The consequences of pharmacologically evoked augmented serotonin (5-hydroxytryptamine, 5-HT) release on neuronal activity in the brain, as reflected by the cellular expression of the immediate early gene c-fos, were studied. Wistar rats were treated with saline, the 5-HT reuptake inhibitor citalopram (10 micromol/kg s.c.), the 5-HT(1A) receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)-N-(2-pyridyl)cyclohexane carboxamine trihydrochloride (WAY 100635, 1 micromol/kg s.c.), or the combination of both drugs. At the given dosages, the combination of the drugs has previously been shown to enhance the cerebral release of 5-HT. Two hours and thirty minutes after administration, the brains were fixated, and Fos protein was histologically stained and quantified. The paraventricular nucleus of the hypothalamus, the central nucleus amygdala, the ventromedial hypothalamic nucleus, the dorsolateral striatum, and the nucleus accumbens shell were particularly responsive to increased 5-HT release. The results, illustrating the synergistic consequence of the combined drug treatments, are discussed in terms of activity of the limbic-hypothalamic-pituitary-adrenocortical system.

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.

Effects of chronically administered venlafaxine on 5-HT receptor activity in rat hippocampus and hypothalamus

The effects of chronic administration of the mixed serotonin [5-hydroxytryptamine (5-HT)]/norepinephrine re-uptake inhibitor venlafaxine (5 mg/kg daily by osmotic minipump for 28 days) on the sensitivity of somatodendritic 5-HT(1A) autoreceptors on serotonergic neurons innervating the hypothalamus, and on 5-HT(1B) autoreceptors in both hypothalamus and hippocampus, were determined using in vivo microdialysis in freely moving rats. Venlafaxine induced a reduction in sensitivity of 5-HT(1B) autoreceptors in hypothalamus, but did not affect the sensitivity of 5-HT(1A) autoreceptors, or of 5-HT(1B) autoreceptors in hippocampus. The corticosterone and oxytocin responses to the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.05 or 0.2 mg/kg), a measure of postsynaptic 5-HT(1A) receptor activity in the hypothalamus, were reduced in animals administered 5 or 10 mg/kg venlafaxine daily by intraperitoneal injection for 21 days. This desensitization of post-synaptic 5- HT(1A) receptors in the hypothalamus may be a consequence of increased 5-HT levels induced by desensitization of the presynaptic 5-HT(1B) receptors. These results taken together with those of previous studies suggest that the hypothalamus might be an important site of drug action, and that venlafaxine has an overall mechanism similar to that of selective serotonin re-uptake inhibitors.

Serotonergic effects and extracellular brain levels of eletriptan, zolmitriptan and sumatriptan in rat brain

In vivo microdialysis was used to assess the central serotonergic effects and extracellular brain levels of the 5-HT(1B/1D) receptor agonists eletriptan, zolmitriptan and sumatriptan in rats after intravenous and intracerebral administration, while their binding affinities and functional potencies were determined at 5-HT(1B), 5-HT(1D) and 5-HT(1A) receptors. In vitro studies showed that all three triptans are high affinity, full agonists at 5-HT(1B/1D) receptors, but that sumatriptan is functionally less potent as a 5-HT(1B/1D) agonist than zolmitriptan and eletriptan. Local intracortical perfusion with the compounds via the dialysis probe decreased cortical 5-HT (5-hydroxytryptamine, serotonin) release with ED(50) values of approximately 0.1 microM for eletriptan and zolmitriptan and 0.5 microM for sumatriptan. At 3.2 mg/kg i.v., both eletriptan and zolmitriptan decreased 5-HT levels by about 35%, while sumatriptan had no effect, despite the fact that maximal sumatriptan concentrations in cortical dialysates were higher (8.8 nM at 20 min) than those of zolmitriptan (5.9 nM at 20 min) and eletriptan (2.6 nM at 40 min). The observation that eletriptan and zolmitriptan produce almost identical central serotonergic effects, after intracerebral as well as after systemic administration, is in agreement with their comparable functional 5-HT(1B/1D) receptor agonist potencies and their free levels in cortical dialysates after 3.2 mg/kg i.v. On the other hand, the lack of central serotonergic effects of 3.2 mg/kg i.v. sumatriptan is likely due to its weaker functional 5-HT(1B/1D) receptor agonist potency than eletriptan and zolmitriptan, rather than lower brain levels, consistent with sumatriptan's fivefold lower potency after intracerebral administration.

Is the beneficial antidepressant effect of coadministration of pindolol really due to somatodendritic autoreceptor antagonism?

BACKGROUND

We investigated the combination of selective serotonin reuptake inhibitors (SSRIs) with the beta-adrenoceptor/serotonin 1A (5-HT(1A)) antagonist pindolol, based on the concept that 5-HT(1A) receptor blockade would eliminate the need for desensitization of presynaptic 5-HT(1A) receptors and therefore hasten the onset of action and improve the efficacy of SSRIs. However, since pindolol plasma levels after 2.5 mg three times a day are about 60 nmol/L, and the K(i) for the 5-HT(1A) receptor is 30 nmol/L, it is questionable whether pindolol levels in the brain would be sufficient to antagonize 5-HT(1A) receptors. Using microdialysis in the guinea pig, we correlated brain and plasma levels of pindolol with its capability of augmenting paroxetine-induced increases in brain 5-HT levels. In addition, central beta-receptor antagonism of pindolol was studied by investigating blockade of beta-agonist-induced increases in brain cyclic adenosine monophosphate (cAMP) formation.

METHODS

Using microdialysis and jugular vein catheterization, we studied the ability of systemically administered pindolol to antagonize central 5-HT(1A) and beta-adrenoceptors, while simultaneously monitoring pindolol plasma and brain concentrations.

RESULTS

Augmentation of paroxetine-induced increases in extracellular 5-HT levels in the ventral hippocampus was only observed at steady state plasma levels exceeding 7000 nmol/L (concurrent brain levels 600 nmol/L). In contrast, antagonism of beta-agonist-induced increases of brain cAMP levels was already observed at pindolol plasma levels of 70 nmol/L (concurrent brain levels < 3 nmol/L).

CONCLUSIONS

At plasma levels that are observed in patients after 2.5 mg three times a day ( approximately 60 nmol/L), pindolol produces only a partial blockade of presynaptic 5-HT(1A) autoreceptors and does not augment the SSRI-induced 5-HT increase in the guinea pig brain. It is therefore very unlikely that the favorable effects of combining pindolol with SSRIs, as reported in a number of clinical studies, are due to 5-HT(1A) antagonism. Since pindolol completely blocks central beta-adrenoreceptors at clinically relevant plasma levels, it is possible that beta-adrenoceptor antagonism is involved in mediating pindolol's beneficial effects.

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

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

Desensitisation of 5-HT autoreceptors upon pharmacokinetically monitored chronic treatment with citalopram

Rats were chronically treated with the selective serotonin re-uptake inhibitor citalopram [1-(3-dimethylaminopropyl)-1-(4-fluorophenyl)-5-phtalancarbonitril ], by means of osmotic minipumps. Using an infusion concentration of 50 mg/ml citalopram, steady-state plasma concentrations of approximately 0.3 mcM citalopram were maintained for 15 days. Citalopram plasma levels dropped below pharmacologically active concentrations 48 h after removal of the minipumps. Although chronic treatment with citalopram did induce an attenuated response by extracellular levels of 5-hydroxytryptamine (5-HT) after systemic administration of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), no effect of chronic citalopram treatment was observed when 5-HT(1B) receptor function was evaluated with a local infusion of 5-HT(1B/D) receptor agonist, sumatriptan (3-[2-dimethylamino]ethyl-N-methyl-1H-indole-5methane sulphonamide). Controversially, no augmentation of the increase of 5-HT levels was observed upon systemic administration of citalopram. It is concluded that, although chronic treatment with citalopram does induce desensitisation of 5-HT(1A) receptors, the absence of augmented effects of citalopram on 5-HT levels indicates that other mechanisms compensate for the loss of autoreceptor control.