Blockade of the high-affinity noradrenaline transporter (NET) by the selective 5-HT reuptake inhibitor escitalopram: an in vivo microdialysis study in mice

Sertraline treatment influences [18F]FE-PE2I PET imaging for Parkinsonism

BACKGROUND

The dopamine transporter (DaT) PET ligand [¹⁸F]FE-PE2I is used to aid the diagnosis of Parkinson's disease. After encountering four patients with a history of daily sertraline use, who all showed atypical findings on [¹⁸F]FE-PE2I PET, we suspected that the selective serotonin reuptake inhibitor (SSRI), sertraline, might interfere with the results and lead to globally reduced striatal [¹⁸F]FE-PE2I binding due to sertraline's high affinity for DaT.

METHODS

We rescanned the four patients with [¹⁸F]FE-PE2I PET after a 5-day sertraline pause. Sertraline plasma concentration was estimated based on body weight and dose, and specific binding ratios (SBR) in caudate nucleus, known to be more preserved in Parkinson's, were used to estimate the effect on tracer binding. Comparison was made to a patient with [¹⁸F]FE-PE2I PET before and after a 7-day Modafinil pause.

RESULTS

We found a significant effect of sertraline on caudate nucleus SBR (p = 0.029). The effect showed a linear dose-dependent relationship that corresponds to a reduction in SBR by 0.32 or 0.44 for a 75 kg male or a 65 kg female, respectively, taking a daily dose of 50 mg sertraline.

CONCLUSION

Sertraline is one of the most commonly used antidepressants and in contrast to other SSRI's, sertraline show high affinity for DaT. We recommend that sertraline treatment is taken into account when patients are undergoing [¹⁸F]FE-PE2I PET especially in patients showing apparent globally reduced PE2I binding. If tolerable, pausing of the sertraline treatment should be considered, especially for doses above 50 mg/day.

Intranasal (R, S)-ketamine delivery induces sustained antidepressant effects associated with changes in cortical balance of excitatory/inhibitory synaptic activity

In 2019, an intranasal (IN) spray of esketamine SPRAVATO® was approved as a fast-acting antidepressant by drug Agencies US FDA and European EMA. At sub-anesthetic doses, (±)-ketamine, a non-competitive glutamate N-methyl-d-aspartate (NMDA) receptor antagonist, increases the overall excitability of the medial prefrontal cortex (mPFC), an effect being essential for its rapid antidepressant activity. We wondered if this effect of ketamine could come from changes in the balance between neuronal excitation and inhibition (E/I balance) in the mPFC. Here, we performed a preclinical approach to study neurochemical and behavioral responses to a single IN ketamine dose in BALB/cJ mice, a strain more sensitive to stress. By using in vivo microdialysis, we measured cortical E/I balance as the ratio between glutamate to GABA extracellular levels 24 h post-ketamine. We found, for the first time, that E/I balance was shifted in favor of excitation rather than inhibition in the mPFC but more robustly with IN KET than with a single intraperitoneal (IP) dose. Increases in plasma and brain ketamine, norketamine and HNKs levels suggest different metabolic profiles of IP and IN ketamine 30 min post-dose. A significantly larger proportion of ketamine and HNKs in the brain are derived from the IN route 30 min post-dose. It may be linked to the greater magnitude in E/I ratio following IN delivery relative to IP at t24 h. This study suggests that both IP and IN are effective brain delivery methods inducing similar sustained antidepressant efficacy of KET, but the way they induced neurotransmitter changes is slightly different.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

Role of central serotonin and noradrenaline interactions in the antidepressants' action: Electrophysiological and neurochemical evidence

The development of antidepressant drugs, in the last 6 decades, has been associated with theories based on a deficiency of serotonin (5-HT) and/or noradrenaline (NA) systems. Although the pathophysiology of major depression (MD) is not fully understood, numerous investigations have suggested that treatments with various classes of antidepressant drugs may lead to an enhanced 5-HT and/or adapted NA neurotransmissions. In this review, particular morpho-physiological aspects of these systems are first considered. Second, principal features of central 5-HT/NA interactions are examined. In this regard, the effects of the acute and sustained antidepressant administrations on these systems are discussed. Finally, future directions including novel therapeutic strategies are proposed.

Influence of citicoline on citalopram-induced antidepressant activity in depressive-like symptoms in male mice

Depression is associated with significant functional disabilities. Application of new drugs which could enhance the effectiveness of antidepressants drug and reduce side effects of their long-term use seems necessary. Citicoline is used as an effective chemical agent for improving the symptoms of some neurodegenerative diseases. Therefore, in this survey, the application of citicoline as an adjuvant drug was evaluated in mice model of depression. A total of 180 adult NMRI male albino mice were used in this study. All groups were exposed to chronic unexpected mild stress (CUMS) followed by treatment with various doses of citalopram or/and citicoline or saline for 21 days. Sucrose preference (SP), open field (OF), and forced swimming test (FST) were applied to evaluate depression symptoms in the groups. The results indicated that only citicoline at the 5 mg/kg dose had shifted its status from being noneffective to become significantly effective in the co-administered group. The means of SP, OFT, and FST of the treatment groups were significantly different in favor of co-administered group compared with the other groups as well as the control group. Based on the results, it can be concluded that administration of citicoline, as an adjuvant drug, in combination with citalopram, enhanced the effectiveness of selective serotonin reuptake inhibitors (SSRI) drugs for depression treatment.

Treatment with escitalopram modulates cardiovascular function in rats

Considering depression is three times more common in cardiac patients compared to the normal population and selective serotonin reuptake inhibitors (SSRI) as drug of choice for treating patients with cardiovascular disease and depression, our work aims to evaluate the cardiovascular effects of treatment for 21 days with escitalopram (5 mg/kg/day, ip) in rats. The treatment caused an increase in mean arterial pressure concomitant with a decrease in heart rate. Concerning heart rate variability, there was a significant reduction in the sympathetic component and an elevation of the parasympathetic component, indicating that escitalopram caused an autonomic imbalance with parasympathetic predominance. In addition, we observed a decrease in both low and very low frequency power in blood pressure variability. The cardiac autonomic blockade indicated an increase in parasympathetic modulation to the heart with escitalopram chronic treatment. However, no change was observed on baroreflex activity. On the other hand, there was a decrease in pressure response during acute restraint stress with no changes in the tachycardia response. These findings showed that despite the escitalopram be a relatively safe drug it can cause tonic effects on cardiovascular function as well as during aversive situations.

Impact of Novel Antidepressants on Cardiac 123I-Metaiodobenzylguanidine Uptake: Experimental Studies on SK-N-SH Cells and Healthy Rabbits

¹²³I-metaiodobenzylguanidine (¹²³I-MIBG) has independent prognostic value for risk stratification among heart failure patients, but the use of concomitant medication should not affect its quantitative information. We evaluated whether the 4 classes of antidepressants currently most prescribed as first-line treatment for major depressive disorder (MDD) have the potential to alter ¹²³I-MIBG imaging results. Methods: The inhibition effect of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG uptake was assessed by in vitro uptake assays using human neuroblastoma SK-N-SH cells. The half-maximal inhibitory concentration of tracer uptake was determined from dose-response curves. To evaluate the effect of intravenous pretreatment with desipramine (1.5 mg/kg) and escitalopram (2.5 or 15 mg/kg) on ¹²³I-MIBG cardiac uptake, in vivo planar ¹²³I-MIBG scanning of healthy New Zealand White rabbits was performed. Results: The half-maximal inhibitory concentrations of desipramine, escitalopram, venlafaxine, and bupropion on ¹³¹I-MIBG cellular uptake were 11.9 nM, 7.5 μM, 4.92 μM, and 12.9 μM, respectively. At the maximum serum concentration (as derived by previous clinical trials), the inhibition rates of ¹³¹I-MIBG uptake were 90.6% for desipramine, 25.5% for venlafaxine, 11.7% for bupropion, and 0.72% for escitalopram. A low inhibition rate for escitalopram in the cell uptake study triggered investigation of an in vivo rabbit model: with a dosage considerably higher than used in clinical practice, the noninhibitory effect of escitalopram was confirmed. Furthermore, pretreatment with desipramine markedly reduced cardiac ¹²³I-MIBG uptake. Conclusion: In the present in vitro binding assay and in vivo rabbit study, the selective serotonin reuptake inhibitor escitalopram had no major impact on neuronal cardiac ¹²³I-MIBG uptake within therapeutic dose ranges, whereas other types of first-line antidepressants for MDD treatment led to a significant decrease. These preliminary results warrant further confirmatory clinical trials regarding the reliability of cardiac ¹²³I-MIBG imaging, in particular, if the patient's neuropsychiatric status would not tolerate withdrawal of a potentially norepinephrine-interfering antidepressant.

Serotonin 5-HT3 receptor antagonism potentiates the antidepressant activity of citalopram

Activation of serotonin 5-HT₃ receptor (5HT3R) in the locus coeruleus (LC), the principal somatodendritic noradrenergic area, decreases LC firing activity and noradrenaline (NA) release in prefrontal cortex (PFC). Blockade of 5HT3R in coadministration with selective serotonin reuptake inhibitors (SSRIs) has been proposed as a potential strategy to accelerate the onset of action of SSRIs. Dual-probe microdialysis in rats was used to evaluate the involvement of 5HT3R in the in vivo effect exerted by the SSRI citalopram on NA release. Besides, forced swimming test (FST) was carried out in mice to evaluate the antidepressant-like effect of citalopram in combination with a 5HT3R antagonist (Y25130). Systemic administration of the 5HT3R agonist SR57227 (10 mg/kg i.p.) increased NA in LC (Emax = 200 ± 27%) and PFC (Emax = 133 ± 2%). The increase in PFC was enhanced in local presence into LC of Y25130 (50 μM) (Emax = 296 ± 41%) suggesting an inhibitory function on NA release exerted by the activation of 5HT3R located in somatodendritic areas. Citalopram administration (10 mg/kg i.p.) increased NA in LC (Emax = 185 ± 11%) and decreased it in PFC (Emax = -35 ± 7%). Intra-LC (50 μM) or systemic co-administration of Y25130 (10 mg/kg i.p.) with citalopram (10 mg/kg i.p.) switched NA release in the PFC from an inhibition to a stimulatory effect. In mice FST, systemic coadministration of citalopram (2.5 mg/kg i.p.) and Y25130 (10 mg/kg i.p.) potentiated the decrease of immobility time through the increase of both swimming and climbing behaviours. These results suggest that the addition of a 5HT3R antagonist to SSRIs could represent a feasible strategy to improve antidepressant response.

Dexmedetomidine does not interfere with meta-iodobenzylguanidine (MIBG) uptake at clinically relevant concentrations

BACKGROUND

Neuroblastoma is a pediatric malignancy, and most tumor cells express the norepinephrine transporter (NET) enabling uptake of NET ligands. Meta-iodobenzylguanidine (MIBG) is a NET-specific ligand used as a highly specific imaging agent and targeted radiotherapeutic. Patients with neuroblastoma frequently require sedation during targeted radiotherapy. Dexmedetomidine has been increasingly used to achieve efficacious sedation. There are theoretical concerns that this highly selective alpha-2 adrenergic receptor agonist may interfere with active uptake of MIBG through the NET transporter. In this study, we analyzed the impact of [125-iodine]-labeled MIBG ([¹²⁵ I]MIBG) uptake in the presence of dexmedetomidine in human neuroblastoma-derived cellular models.

PROCEDURE

Carrier-free [¹²⁵ I]MIBG was synthesized using UltraTrace® resin (Molecular Insight Pharmaceuticals, Inc., Tarrytown, NY) through radioiododestannylation from a tin precursor bound by a solid-state polymer. NET (SLC6A2) protein expression was determined in human neuroblastoma cell lines (BE2C, SKNSH and IMR5). [¹²⁵ I]MIBG internalization studies were performed using [¹²⁵ I]MIBG alone or in combination with either desipramine or dexmedetomidine. Dexmedetomidine and desipramine competitive inhibition studies were performed and concentration at 50% maximal inhibition was calculated. Finally, NET inhibitor dissociation studies were performed in which after pre-incubation with either desipramine or dexmedetomidine, cells were washed and [¹²⁵ I]MIBG was added.

RESULTS

We show dose-dependent inhibition of [¹²⁵ I]MIBG uptake by dexmedetomidine, but at several logs lower potency than the known NET inhibitor desipramine. A review of pediatric dexmedetomidine pharmacokinetic data shows that the concentrations achieved in the serum are much lower than those required to block MIBG uptake.

CONCLUSION

We conclude that dexmedetomidine will not interfere with therapeutic [¹³¹ I]MIBG efficacy.

Ethanol induced antidepressant-like effect in the mouse forced swimming test: modulation by serotonergic system

AIM

The present investigation explored the modulatory role of serotonergic transmission in the acute ethanol-induced effects on immobility time in the mouse forced swim test (FST).

METHODS AND RESULTS

Acute i.p. administration of ethanol (20% w/v, 2 or 2.5 g/kg, i.p.) decreased the immobility time in FST of mice, indicating its antidepressant-like effect while lower doses of ethanol (1, 1.5 g/kg, i.p.) were devoid of any effect in the FST. The mice pre-treated with a sub-effective dose of 5-HT_2A agonist, DOI (10 μg/mouse, i.c.v.) or 5-HT_1A receptor antagonist, WAY 100635 (0.1 μg/mouse, i.c.v.) but not with the 5-HT_2A/2C antagonist, ketanserin (1.5 μg/mouse, i.c.v.) exhibited a synergistic reduction in the immobility time induced by sub-effective dose of ethanol (1.5 g/kg, i.p.). On the other hand, ethanol (2.5 g/kg, i.p.) failed to decrease the immobility time in mice, pre-treated with 5-HT_1A agonist, 8-OH-DPAT (0.1 μg/mouse, i.c.v.) or ketanserin (1.5 μg/mouse, i.c.v.). In addition, pre-treatment with a 5-HT neuronal synthesis inhibitor, p-CPA (300 mg/kg, i.p. × 3 days) attenuated the anti-immobility effect ethanol (2.5 g/kg, i.p.) in mouse FST.

CONCLUSIONS

Thus, the results of the present study points towards the essentiality of the central 5-HT transmission at the synapse for the ethanol-induced antidepressant-like effect in the FST wherein the regulatory role of the 5-HT_1A receptor or contributory role of the 5-HT_2A/2C receptor-mediated mechanism is proposed in the anti-immobility effect of acute ethanol in mouse FST.

Chronic citalopram administration desensitizes prefrontal cortex but not somatodendritic α2-adrenoceptors in rat brain

Selective serotonin reuptake inhibitors (SSRIs) regulate brain noradrenergic neurotransmission both at somatodendritic and nerve terminal areas. Previous studies have demonstrated that noradrenaline (NA) reuptake inhibitors are able to desensitize α₂-adrenoceptor-mediated responses. The present study was undertaken to elucidate the effects of repeated treatment with the SSRI citalopram on the α₂-adrenoceptor sensitivity in locus coeruleus (LC) and prefrontal cortex (PFC), by using in vivo microdialysis and electrophysiological techniques, and in vitro stimulation of [³⁵S]GTPγS binding autoradiography. Repeated, but not acute, treatment with citalopram (5 mg/kg, i.p., 14 days) increased extracellular NA concentration selectively in PFC. The α₂-adrenoceptor agonist clonidine (0.3 mg/kg, i.p.), administered to saline-treated animals (1 ml/kg i.p., 14 days) induced NA decrease in LC (E_max = -44 ± 4%; p < 0.001) and in PFC (E_max = -61 ± 5%, p < 0.001). In citalopram chronically-treated rats, clonidine administration exerted a lower decrease of NA (E_max = -25 ± 7%; p < 0.001) in PFC whereas the effect in LC was not different to controls (E_max = -36 ± 4%). Clonidine administration (0.625-20 μg/kg, i.v.) evoked a dose-dependent decrease of the firing activity of LC noradrenergic neurons in both citalopram- (ED₅₀ = 3.2 ± 0.4 μg/kg) and saline-treated groups (ED₅₀ = 2.6 ± 0.5 μg/kg). No significant differences between groups were found in ED₅₀ values. The α₂-adrenoceptor agonist UK14304 stimulated specific [³⁵S]GTPγS binding in brain sections containing LC (144 ± 14%) and PFC (194 ± 32%) of saline-treated animals. In citalopram-treated animals, this increase did not differ from controls in LC (146 ± 22%) but was lower in PFC (141 ± 8%; p < 0.05). Taken together, long-term citalopram treatment induces a desensitization of α₂-adrenoceptors acting as axon terminal autoreceptors in PFC without changes in somatodendritic α₂-adrenoceptor sensitivity.

A Lack of Serotonin 1B Autoreceptors Results in Decreased Anxiety and Depression-Related Behaviors

The effects of serotonin (5-HT) on anxiety and depression are mediated by a number of 5-HT receptors, including autoreceptors that act to inhibit 5-HT release. While the majority of anxiety and depression-related research has focused on the 5-HT_1A receptor, the 5-HT_1B receptor has a lesser known role in modulating emotional behavior. 5-HT_1B receptors are inhibitory GPCRs located on the presynaptic terminal of both serotonin and non-serotonin neurons, where they act to inhibit neurotransmitter release. The autoreceptor population located on the axon terminals of 5-HT neurons is a difficult population to study due to their diffuse localization throughout the brain that overlaps with 5-HT_1B heteroreceptors (receptors located on non-serotonergic neurons). In order to study the contribution of 5-HT_1B autoreceptors to anxiety and depression-related behaviors, we developed a genetic mouse model that allows for selective ablation of 5-HT_1B autoreceptors. Mice lacking 5-HT_1B autoreceptors displayed the expected increases in extracellular serotonin levels in the ventral hippocampus following administration of a selective serotonin reuptake inhibitor. In behavioral studies, they displayed decreased anxiety-like behavior in the open field and antidepressant-like effects in the forced swim and sucrose preference tests. These results suggest that strategies aimed at blocking 5-HT_1B autoreceptors may be useful for the treatment of anxiety and depression.

Ketamine treatment involves medial prefrontal cortex serotonin to induce a rapid antidepressant-like activity in BALB/cJ mice

Unlike classic serotonergic antidepressant drugs, ketamine, an NMDA receptor antagonist, exhibits a rapid and persistent antidepressant (AD) activity, at sub-anaesthetic doses in treatment-resistant depressed patients and in preclinical studies in rodents. The mechanisms mediating this activity are unclear. Here, we assessed the role of the brain serotonergic system in the AD-like activity of an acute sub-anaesthetic ketamine dose. We compared ketamine and fluoxetine responses in several behavioral tests currently used to predict anxiolytic/antidepressant-like potential in rodents. We also measured their effects on extracellular serotonin levels [5-HT]_ext in the medial prefrontal cortex (mPFCx) and brainstem dorsal raphe nucleus (DRN), a serotonergic nucleus involved in emotional behavior, and on 5-HT cell firing in the DRN in highly anxious BALB/cJ mice. Ketamine (10 mg/kg i.p.) had no anxiolytic-like effect, but displayed a long lasting AD-like activity, i.e., 24 h post-administration, compared to fluoxetine (18 mg/kg i.p.). Ketamine (144%) and fluoxetine (171%) increased mPFCx [5-HT]_ext compared to vehicle. Ketamine-induced AD-like effect was abolished by a tryptophan hydroxylase inhibitor, para-chlorophenylalanine (PCPA) pointing out the role of the 5-HT system in its behavioral activity. Interestingly, increase in cortical [5-HT]_ext following intra-mPFCx ketamine bilateral injection (0.25 μg/side) was correlated with its AD-like activity as measured on swimming duration in the FST in the same mice. Furthermore, pre-treatment with a selective AMPA receptor antagonist (intra-DRN NBQX) blunted the effects of intra-mPFCx ketamine on both the swimming duration in the FST and mPFCx [5-HT]_ext suggesting that the AD-like activity of ketamine required activation of DRN AMPA receptors and recruited the prefrontal cortex/brainstem DRN neural circuit in BALB/c mice. These results confirm a key role of cortical 5-HT release in ketamine's AD-like activity following the blockade of glutamatergic NMDA receptors. Tight interactions between mPFCx glutamatergic and serotonergic systems may explain the differences in this activity between ketamine and fluoxetine in vivo. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

In vivo studies of effects of antidepressants on parotid salivary secretion in the rat

Tricyclic antidepressants (TCA) are well-known xerogenic drugs, while antidepressants such as selective serotonin reuptake inhibitors (SSRI) are considered less xerogenic. The antimuscarinic effect of the TCAs has been considered to be the principal mechanism causing a dry mouth. Although the muscarinic receptor is commonly targeted by xerogenic pharmaceuticals, the salivation reflex arc may be affected at other levels as well. We currently wondered whether or not antidepressants exert an inhibition of the salivary reflex not only at the glandular level but at a central level as well. In this study, the effects of a TCA (clomipramine), a SSRI (citalopram) and a serotonin-noradrenaline reuptake inhibitor (SNRI; venlafaxine) were examined on reflex- (0.5M citric acid applied on the tongue) and methacholine-evoked salivary secretion. While all three compounds inhibited citric acid-evoked secretion (-40 to -60% at 5mg/kg i.v. of the antidepressants), only clomipramine inhibited methacholine-evoked secretion (-30% at 5mg/kg i.v.). On the contrary, both citalopram and venlafaxine increased the methacholine-evoked secretion (+44 to 49%). This was particularly obvious for the salivary protein output (>200%). In the presence of α- and β-adrenoceptor antagonists, the citalopram- and venlafaxine-induced increases were reduced. Thus, antidepressants irrespective of type may exert xerogenic effects by inhibiting the salivary reflex in the central nervous system. However, while TCAs may also hamper the secretory response by antimuscarinic effects, the SSRI and the SNRI groups of pharmaceuticals seem to lack this additional xerogenic mechanism indicating a better therapeutic profile and better opportunities for pharmacological treatment of drug-induced xerostomia.

In vivo properties of KNT-127, a novel δ opioid receptor agonist: receptor internalization, antihyperalgesia and antidepressant effects in mice

BACKGROUND AND PURPOSE

Activation of δ opioid (DOP) receptors regulates pain and emotional responses, and also displays ligand-biased agonism. KNT-127 (1,2,3,4,4a,5,12,12a-octahydro-2-methyl-4aβ,1β-([1,2]benzenomethano)-2,6-diazanaphthacene-12aβ,17-diol) is a novel DOP receptor agonist inducing analgesia and antidepressant effects in mice. Here, we have assessed KNT-127 for (i) analgesia against chronic inflammatory pain; (ii) effects on depression, locomotion and DOP receptor internalization; and (iii) for cross-tolerance to analgesic and antidepressant effects of acute treatment by other DOP receptor agonists.

EXPERIMENTAL APPROACH

Inflammatory pain was induced by complete Freund's adjuvant injection into tail or hindpaw, and thermal and mechanical sensitivities were determined in mice. Locomotor and antidepressant-like effects were measured using actimetry and forced swim test respectively. In vivo KNT-127 selectivity and internalization were assessed using DOP receptor knockout mice and knock-in mice expressing fluorescent-tagged DOP receptors. KNT-127 was injected acutely at 0.1-10.0 mg·kg(-1) or administered chronically at 5 mg·kg(-1) daily over 5 days.

KEY RESULTS

Acute treatment with KNT-127 reversed inflammatory hyperalgesia, produced an antidepressant-like effect but induced neither hyperlocomotion nor receptor sequestration. Chronic treatment with KNT-127 induced tolerance and cross-tolerance to SNC80-induced analgesia, but no tolerance to SNC80-evoked hyperlocomotor or antidepressant-like effects.

CONCLUSIONS AND IMPLICATIONS

The DOP receptor agonist KNT-127 induced agonist-specific acute and chronic responses, at both behavioural and cellular levels. It displays activities similar to the other recently reported DOP agonists, AR-M1000390, ADL5747 and ADL5859, and differs from SNC80. SNC80 differs from the other DOP receptor agonists including KNT-127, by exhibiting ligand-biased tolerance at this receptor.

The interaction of escitalopram and R-citalopram at the human serotonin transporter investigated in the mouse

RATIONALE

Escitalopram appears to be a superior antidepressant to racemic citalopram. It has been hypothesized that binding of R-citalopram to the serotonin transporter (SERT) antagonizes escitalopram binding to and inhibition of the SERT, there by curtailing the elevation of extracellular 5-hydroxytryptamine (5-HTExt), and hence anti-depressant efficacy. Further, it has been suggested that a putative allosteric binding site is important for binding of escitalopram to the primary, orthosteric, site, and for R-citalopram's inhibition here of.

OBJECTIVES

Primary: Investigate at the human (h)SERT, at clinical relevant doses, whether R-citalopram antagonizes escitalopram-induced 5-HTExt elevation. Secondary: Investigate whether abolishing the putative allosteric site affects escitalopram-induced 5-HTExt elevation and/or modulates the effect of R-citalopram.

METHODS

Recombinant generation of hSERT transgenic mice; in vivo microdialysis; SERT binding; pharmacokinetics; 5-HT sensitive behaviors (tail suspension, marble burying).

RESULTS

We generated mice expressing either the wild-type human SERT (hSERT(WT)) or hSERT carrying amino acid substitutions (A505V, L506F, I507L, S574T and I575T) collectively abolishing the putative allosteric site (hSERT(ALI/VFL+SI/TT)). One mg/kg escitalopram yielded clinical relevant plasma levels and brain levels consistent with therapeutic SERT occupancy. The hSERT mice showed normal basal 5-HTExt levels. Escitalopram-induced 5-HTExt elevation was not decreased by R-citalopram co-treatment and was unaffected by loss of the allosteric site. The behavioral effects of the clinically relevant escitalopram dose were small and tended to be enhanced by R-citalopram co-administration.

CONCLUSIONS

We find no evidence that R-citalopram directly antagonizes escitalopram or that the putative allosteric site is important for hSERT inhibition by escitalopram.

Activators of AMPK: not just for Type II diabetes

Recent discoveries of AMPK activators point to the large number of therapeutic candidates that can be transformed to successful designs of novel drugs. AMPK is a universal energy sensor and influences almost all physiological processes in the cells. Thus, regulation of the cellular energy metabolism can be achieved in selective tissues via the artificial activation of AMPK by small molecules. Recently, special attention has been given to direct activators of AMPK that are regulated by several nonspecific upstream factors. The direct activation of AMPK, by definition, should lead to more specific biological activities and as a result minimize possible side effects.

On 'polypharmacy' and multi-target agents, complementary strategies for improving the treatment of depression: a comparative appraisal

Major depression is a heterogeneous disorder, both in terms of symptoms, ranging from anhedonia to cognitive impairment, and in terms of pathogenesis, with many interacting genetic, epigenetic, developmental and environmental causes. Accordingly, it seems unlikely that depressive states could be fully controlled by a drug possessing one discrete mechanism of action and, in the wake of disappointing results with several classes of highly selective agent, multi-modal treatment concepts are attracting attention. As concerns pharmacotherapy, there are essentially two core strategies. First, multi-target antidepressants that act via two or more complementary mechanisms and, second, polypharmacy, which refers to co-administration of two distinct drugs, usually in separate pills. Both multi-target agents and polypharmacy ideally couple a therapeutically unexploited action to a clinically established mechanism in order to enhance efficacy, moderate side-effects, accelerate onset of action and treat a broader range of symptoms. The melatonin MT1/MT2 agonist and 5-HT(2C) antagonist, agomelatine, which is effective in the short- and long-term treatment of depression, exemplifies the former approach, while evidence-based polypharmacy is illustrated by the adjunctive use of second-generation antipsychotics with serotonin reuptake inhibitors for treatment of resistant depression. Histone acetylation and methylation, ghrelin signalling, inflammatory modulators, metabotropic glutamate-7 receptors and trace amine-associated-1 receptors comprise attractive substrates for new multi-target and polypharmaceutical strategies. The present article outlines the rationale underpinning multi-modal approaches for treating depression, and critically compares and contrasts the pros and cons of established and potentially novel multi-target vs. polypharmaceutical treatments. On balance, the former appear the most promising for the elaboration, development and clinical implementation of innovative concepts for the more effective management of depression.

Voltammetric and mathematical evidence for dual transport mediation of serotonin clearance in vivo

The neurotransmitter serotonin underlies many of the brain's functions. Understanding serotonin neurochemistry is important for improving treatments for neuropsychiatric disorders such as depression. Antidepressants commonly target serotonin clearance via serotonin transporters and have variable clinical effects. Adjunctive therapies, targeting other systems including serotonin autoreceptors, also vary clinically and carry adverse consequences. Fast scan cyclic voltammetry is particularly well suited for studying antidepressant effects on serotonin clearance and autoreceptors by providing real-time chemical information on serotonin kinetics in vivo. However, the complex nature of in vivo serotonin responses makes it difficult to interpret experimental data with established kinetic models. Here, we electrically stimulated the mouse medial forebrain bundle to provoke and detect terminal serotonin in the substantia nigra reticulata. In response to medial forebrain bundle stimulation we found three dynamically distinct serotonin signals. To interpret these signals we developed a computational model that supports two independent serotonin reuptake mechanisms (high affinity, low efficiency reuptake mechanism, and low affinity, high efficiency reuptake system) and bolsters an important inhibitory role for the serotonin autoreceptors. Our data and analysis, afforded by the powerful combination of voltammetric and theoretical methods, gives new understanding of the chemical heterogeneity of serotonin dynamics in the brain. This diverse serotonergic matrix likely contributes to clinical variability of antidepressants.

Antidepressant activity: contribution of brain microdialysis in knock-out mice to the understanding of BDNF/5-HT transporter/5-HT autoreceptor interactions

Why antidepressants vary in terms of efficacy is currently unclear. Despite the leadership of selective serotonin reuptake inhibitors (SSRIs) in the treatment of depression, the precise neurobiological mechanisms involved in their therapeutic action are poorly understood. A better knowledge of molecular interactions between monoaminergic system, pre- and post-synaptic partners, brain neuronal circuits and regions involved may help to overcome limitations of current treatments and identify new therapeutic targets. Intracerebral in vivo microdialysis (ICM) already provided important information about the brain mechanism of action of antidepressants first in anesthetized rats in the early 1990s, and since then in conscious wild-type or knock-out mice. The principle of ICM is based on the balance between release of neurotransmitters (e.g., monoamines) and reuptake by selective transporters [e.g., serotonin transporter for serotonin 5-hydroxytryptamine (5-HT)]. Complementary to electrophysiology, this technique reflects pre-synaptic monoamines release and intrasynaptic events corresponding to ≈80% of whole brain tissue content. The inhibitory role of serotonergic autoreceptors infers that they limit somatodendritic and nerve terminal 5-HT release. It has been proposed that activation of 5-HT_1A and 5-HT_1B receptor sub-types limits the antidepressant-like activity of SSRIs. This hypothesis is based partially on results obtained in ICM experiments performed in naïve, non-stressed rodents. The present review will first remind the principle and methodology of ICM performed in mice. The crucial need of developing animal models that display anxiety and depression-like behaviors, neurochemical and brain morphological phenotypes reminiscent of these mood disorders in humans, will be underlined. Recently developed genetic mouse models have been generated to independently manipulate 5-HT_1A auto and heteroreceptors and ICM helped to clarify the role of the pre-synaptic component, i.e., by measuring extracellular levels of neurotransmitters in serotonergic nerve terminal regions and raphe nuclei. Finally, we will summarize main advantages of using ICM in mice through recent examples obtained in knock-outs (drug infusion through the ICM probe allows the search of a correlation between changes in extracellular neurotransmitter levels and antidepressant-like activity) or alternatives (infusion of a small-interfering RNA suppressing receptor functions in the mouse brain). We will also focus this review on post-synaptic components such as brain-derived neurotrophic factor in adult hippocampus that plays a crucial role in the neurogenic and anxiolytic/antidepressant-like activity of chronic SSRI treatment. Limitations of ICM will also be considered.

A proposal for refining the forced swim test in Swiss mice

The forced swim test (FST) is a preclinical test to the screening of antidepressants based on rats or mice behaviours, which is also sensitive to stimulants of motor activity. This work standardised and validated a method to register the active and passive behaviours of Swiss mice during the FST in order to strength the specificity of the test. Adult male Swiss mice were subjected to the FST for 6 min without any treatment or after intraperitoneal injection of saline (0.1 ml/10 g), antidepressants (imipramine, desipramine, or fluoxetine, 30 mg/kg) or stimulants (caffeine, 30 mg/kg or apomorphine, 10mg/kg). The latency, frequency and duration of behaviours (immobility, swimming, and climbing) were scored and summarised in bins of 6, 4, 2 or 1 min. Parameters were first analysed using Principal Components Analysis generating components putatively related to antidepressant (first and second) or to stimulant effects (third). Antidepressants and stimulants affected similarly the parameters grouped into all components. Effects of stimulants on climbing were better distinguished of antidepressants when analysed during the last 4 min of the FST. Surprisingly, the effects of antidepressants on immobility were better distinguished from saline when parameters were scored in the first 2 min. The method proposed here is able to distinguish antidepressants from stimulants of motor activity using Swiss mice in the FST. This refinement should reduce the number of mice used in preclinical evaluation of antidepressants.

Selective deletion of forebrain glycogen synthase kinase 3β reveals a central role in serotonin-sensitive anxiety and social behaviour

Serotonin (5-HT) neurotransmission is thought to underlie mental illnesses, such as bipolar disorder, depression, autism and schizophrenia. Independent studies have indicated that 5-HT or drugs acting on 5-HT neurotransmission regulate the serine/threonine kinase glycogen synthase kinase 3β (GSK3β). Furthermore, GSK3β inhibition rescues behavioural abnormalities in 5-HT-deficient mice with a loss-of-function mutation equivalent to the human variant (R441H) of tryptophan hydroxylase 2. In an effort to define neuroanatomical correlates of GSK3β activity in the regulation of behaviour, we generated CamKIIcre-floxGSK3β mice in which the gsk3b gene is postnatally inactivated in forebrain pyramidal neurons. Behavioural characterization showed that suppression of GSK3β in these brain areas has anxiolytic and pro-social effects. However, while a global reduction of GSK2β expression reduced responsiveness to amphetamine and increased resilience to social defeat, these behavioural effects were not found in CamKIIcre-floxGSK3β mice. These findings demonstrate a dissociation of behavioural effects related to GSK3 inhibition, with forebrain GSK3β being involved in the regulation of anxiety and sociability while social preference, resilience and responsiveness to psychostimulants would involve a function of this kinase in subcortical areas such as the hippocampus and striatum.