Role of serotonin 5-HT1A receptors in the antidepressant-like effect and the antinociceptive effect of venlafaxine in mice

The alteration of gut microbiota in venlafaxine-ameliorated chronic unpredictable mild stress-induced depression in mice

Depression is associated with intestinal dysbiosis. Venlafaxine is a commonly used antidepressant in clinical practice as a serotonin and noradrenaline reuptake inhibitor. However, its effects on gut bacteria in depression remain unclear. Here, we established a mouse model of depression induced by chronic unpredictable mild stress (CUMS), and investigated the alterations of venlafaxine on the gut microbiota and potential key bacteria. Our data show that venlafaxine exerts antidepressant effects by restoring the serotonin (5-HT) system and glutamate (Glu) levels in CUMS mice. Moreover, we revealed that venlafaxine altered the diversity of gut bacteria in CUMS mice, and at genus level, Blautia, Oscillibacter, Tyzzerella, Butyricicoccus, and Enterorhabdus are the key bacteria responsible for venlafaxine-ameliorated depression in mice. Among these potential key bacteria, Blautia, Oscillibacter, and Butyricicoccus are correlated significantly with the 5-HT and 5-hydroxyindoleacetic acid levels; while Tyzzerella is correlated markedly with Glu levels. We further show that venlafaxine affected multiple functional metabolic pathways of gut bacteria in mice with CUMS-induced depression. Our results suggest that venlafaxine possibly ameliorates depression via modulating gut bacteria, and found the potential targets of its antidepressant effects.

Hybrid molecules combining GABA-A and serotonin 5-HT6 receptors activity designed to tackle neuroinflammation associated with depression

There is clear evidence that the presence of inflammatory factors and impaired GABA-ergic neurotransmission in depressed patients is associated with poor clinical outcome. We designed hybrid molecules, bearing the GABA molecule assembled with chemical fragments that interact with the serotonin 5-HT₆ receptor. Such a combination aimed to curb neuroinflammation, remodel GABA-ergic signaling, and provide antidepressant-like activity. The most promising hybrid 3B exerted nanomolar affinity for 5-HT₆ receptors and exerted agonistic properties on GABA-A receptors. Developability studies conferred that 3B exerted favorable drug-like properties and optimal brain penetration. In in vivo studies, 3B exerted robust antidepressant-like activity and proved to be highly effective in reducing levels of oxidative stress markers and the pro-inflammatory cytokine IL-6. The inetersting pharmacological profile of 3B makes it a promising candidate for further development for depression associated with neuroinflammation.

Role of 5-HT1A-mediated upregulation of brain indoleamine 2,3 dioxygenase 1 in the reduced antidepressant and antihyperalgesic effects of fluoxetine during maintenance treatment

The reduced antidepressant and antihyperalgesic effects of selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine during maintenance treatment has been reported, but little is known about the molecular mechanism of this phenomenon. In three comorbid pain and depression animal models (genetic predisposition, chronic social stress, arthritis), we showed that the fluoxetine's antidepressant and antihyperalgesic effects were reduced during the maintenance treatment. Fluoxetine exposure induced upregulation of the 5-hydroxytryptamine 1A (5-HT1A) auto-receptor and indoleamine 2,3 dioxygenase 1 (IDO1, a rate-limiting enzyme of tryptophan metabolism) in the brainstem dorsal raphe nucleus (DRN), which shifted the tryptophan metabolism away from the 5-HT biosynthesis. Mechanistically, IDO1 upregulation was downstream to fluoxetine-induced 5-HT1A receptor expression because 1) antagonism of the 5-HT1A receptor with WAY100635 or 5-HT1A receptor knockout blocked the IDO1 upregulation, and 2) inhibition of IDO1 activity did not block the 5-HT1A receptor upregulation following fluoxetine exposure. Importantly, inhibition of either the 5-HT1A receptor or IDO1 activity sustained the fluoxetine's antidepressant and antihyperalgesic effects, indicating that 5-HT1A-mediated IDO1 upregulation in the brainstem DRN contributed to the reduced antidepressant and antihyperalgesic effects of fluoxetine. These results suggest a new strategy to improving the therapeutic efficacy of SSRI during maintenance treatment.

The Absence of Caspase-8 in the Dopaminergic System Leads to Mild Autism-like Behavior

In the last decade, new non-apoptotic roles have been ascribed to apoptotic caspases. This family of proteins plays an important role in the sculpting of the brain in the early stages of development by eliminating excessive and nonfunctional synapses and extra cells. Consequently, impairments in this process can underlie many neurological and mental illnesses. This view is particularly relevant to dopamine because it plays a pleiotropic role in motor control, motivation, and reward processing. In this study, we analyze the effects of the elimination of caspase-8 (CASP8) on the development of catecholaminergic neurons using neurochemical, ultrastructural, and behavioral tests. To do this, we selectively delete the CASP8 gene in cells that express tyrosine hydroxylase with the help of recombination through the Cre-loxP system. Our results show that the number of dopaminergic neurons increases in the substantia nigra. In the striatum, the basal extracellular level of dopamine and potassium-evoked dopamine release decreased significantly in mice lacking CASP8, clearly showing the low dopamine functioning in tissues innervated by this neurotransmitter. This view is supported by electron microscopy analysis of striatal synapses. Interestingly, behavioral analysis demonstrates that mice lacking CASP8 show changes reminiscent of autism spectrum disorders (ASD). Our research reactivates the possible role of dopamine transmission in the pathogenesis of ASD and provides a mild model of autism.

Monoaminergic and Opioidergic Modulation of Brainstem Circuits: New Insights Into the Clinical Challenges of Pain Treatment?

The treatment of neuropathic pain remains a clinical challenge. Analgesic drugs and antidepressants are frequently ineffective, and opioids may induce side effects, including hyperalgesia. Recent results on brainstem pain modulatory circuits may explain those clinical challenges. The dual action of noradrenergic (NA) modulation was demonstrated in animal models of neuropathic pain. Besides the well-established antinociception due to spinal effects, the NA system may induce pronociception by directly acting on brainstem pain modulatory circuits, namely, at the locus coeruleus (LC) and medullary dorsal reticular nucleus (DRt). The serotoninergic system also has a dual action depending on the targeted spinal receptor, with an exacerbated activity of the excitatory 5-hydroxytryptamine 3 (5-HT3) receptors in neuropathic pain models. Opioids are involved in the modulation of descending modulatory circuits. During neuropathic pain, the opioidergic modulation of brainstem pain control areas is altered, with the release of enhanced local opioids along with reduced expression and desensitization of μ-opioid receptors (MOR). In the DRt, the installation of neuropathic pain increases the levels of enkephalins (ENKs) and induces desensitization of MOR, which may enhance descending facilitation (DF) from the DRt and impact the efficacy of exogenous opioids. On the whole, the data discussed in this review indicate the high plasticity of brainstem pain control circuits involving monoaminergic and opioidergic control. The data from studies of these neurochemical systems in neuropathic models indicate the importance of designing drugs that target multiple neurochemical systems, namely, maximizing the antinociceptive effects of antidepressants that inhibit the reuptake of serotonin and noradrenaline and preventing desensitization and tolerance of MOR at the brainstem.

Cardamonin Modulates Neuropathic Pain through the Possible Involvement of Serotonergic 5-HT1A Receptor Pathway in CCI-Induced Neuropathic Pain Mice Model

Cardamonin, a naturally occurring chalcone isolated from Alpinia species has shown to possess strong anti-inflammatory and anti-nociceptive activities. Previous studies have demonstrated that cardamonin exerts antihyperalgesic and antiallodynic properties in chronic constriction injury (CCI)-induced neuropathic pain animal model. However, the mechanisms underlying cardamonin's effect have yet to be fully understood. The present study aims to investigate the involvement of the serotonergic system in cardamonin induced antihyperalgesic and antiallodynic effects in CCI-induced neuropathic pain mice model. The neuropathic pain symptoms in the CCI mice model were assessed using Hargreaves Plantar test and von-Frey filament test on day 14 post-surgery. Central depletion of serotonin along the descending serotonergic pathway was done using ρ-chlorophenylalanine (PCPA, 100 mg/kg, i.p.), an inhibitor of serotonin synthesis for four consecutive days before cardamonin treatment, and was found to reverse the antihyperalgesic and antiallodynic effect produced by cardamonin. Pretreatment of the mice with several 5-HT receptor subtypes antagonists: methiothepin (5-HT1/6/77 receptor antagonist, 0.1 mg/kg), WAY 100635 (5-HT1A receptor antagonist, 1 mg/kg), isamoltane (5-HT1B receptor antagonist, 2.5 mg/kg), ketanserin (5-HT2A receptor antagonist, 0.3 mg/kg), and ondansetron (5-HT3 receptor antagonist, 0.5 mg/kg) were shown to abolish the effect of cardamonin induced antihyperalgesic and antiallodynic effects. Further evaluation of the 5-HT1A receptor subtype protein expressions reveals that cardamonin significantly upregulated its expression in the brainstem and spinal cord. Our results suggest that the serotonergic pathway is essential for cardamonin to exert its antineuropathic effect in CCI mice through the involvement of the 5-HT1A receptor subtype in the central nervous system.

5-HT1A Serotonergic, α-Adrenergic and Opioidergic Receptors Mediate the Analgesic Efficacy of Vortioxetine in Mice

Vortioxetine is a multimodal antidepressant drug that affects several brain neurochemicals and has the potential to induce various pharmacological effects on the central nervous system. Therefore, we investigated the centrally mediated analgesic efficacy of this drug and the mechanisms underlying this effect. Analgesic activity of vortioxetine (5, 10 and 20 mg/kg, p.o.) was examined by tail-clip, tail-immersion and hot-plate tests. Motor performance of animals was evaluated using Rota-rod device. Time course measurements (30-180 min) showed that vortioxetine (10 and 20 mg/kg) administrations significantly increased the response latency, percent maximum possible effect and area under the curve values in all of the nociceptive tests. These data pointed out the analgesic effect of vortioxetine on central pathways carrying acute thermal and mechanical nociceptive stimuli. Vortioxetine did not alter the motor coordination of mice indicating that the analgesic activity of this drug was specific. In mechanistic studies, pre-treatments with p-chlorophenylalanine (serotonin-synthesis inhibitor), NAN-190 (serotonin 5-HT_1A receptor antagonist), α-methyl-para-tyrosine (catecholamine-synthesis inhibitor), phentolamine (non-selective α-adrenoceptor blocker), and naloxone (non-selective opioid receptor blocker) antagonised the vortioxetine-induced analgesia. Obtained findings indicated that vortioxetine-induced analgesia is mediated by 5-HT_1A serotonergic, α-adrenergic and opioidergic receptors, and contributions of central serotonergic and catecholaminergic neurotransmissions are critical for this effect.

Selective deletion of Caspase-3 gene in the dopaminergic system exhibits autistic-like behaviour

Apoptotic caspases are thought to play critical roles in elimination of excessive and non-functional synapses and removal of extra cells during early developmental stages. Hence, an impairment of this process may thus constitute a basis for numerous neurological and psychiatric diseases. This view is especially relevant for dopamine due to its pleiotropic roles in motor control, motivation and reward processing. Here, we have analysed the effect of caspase-3 depletion on the development of catecholaminergic neurons and performed a wide array of neurochemical, ultrastructural and behavioural assays. To achieve this, we performed selective deletion of the Casp3 gene in tyrosine hydroxylase (TH)-expressing cells using Cre-loxP-mediated recombination. Histological evaluation of most relevant catecholaminergic nuclei revealed the ventral mesencephalon as the most affected region. Stereological analysis demonstrated an increase in the number of TH-positive neurons in both the substantia nigra and ventral tegmental area along with enlarged volume of the ventral midbrain. Analysis of main innervating tissues revealed a rather contrasting profile. In striatum, basal extracellular levels and potassium-evoked DA release were significantly reduced in mice lacking Casp3, a clear indication of dopaminergic hypofunction in dopaminergic innervating tissues. This view was sustained by analysis of TH-labelled dopaminergic terminals by confocal and electron microscopy. Remarkably, at a behavioural level, Casp3-deficient mice exhibited impaired social interaction, restrictive interests and repetitive stereotypies, which are considered the core symptoms of autism spectrum disorder (ASD). Our study revitalizes the potential involvement of dopaminergic transmission in ASD and provides an excellent model to get further insights in ASD pathogenesis.

Dentate gyrus activin signaling mediates the antidepressant response

Antidepressants that target monoaminergic systems, such as selective serotonin reuptake inhibitors (SSRIs), are widely used to treat neuropsychiatric disorders including major depressive disorder, several anxiety disorders, and obsessive-compulsive disorder. However, these treatments are not ideal because only a subset of patients achieve remission. The reasons why some individuals remit to antidepressant treatments while others do not are unknown. Here, we developed a paradigm to assess antidepressant treatment resistance in mice. Exposure of male C57BL/6J mice to either chronic corticosterone administration or chronic social defeat stress induces maladaptive affective behaviors. Subsequent chronic treatment with the SSRI fluoxetine reverses these maladaptive affective behavioral changes in some, but not all, of the mice, permitting stratification into persistent responders and non-responders to fluoxetine. We found several differences in expression of Activin signaling-related genes between responders and non-responders in the dentate gyrus (DG), a region that is critical for the beneficial behavioral effects of fluoxetine. Enhancement of Activin signaling in the DG converted behavioral non-responders into responders to fluoxetine treatment more effectively than commonly used second-line antidepressant treatments, while inhibition of Activin signaling in the DG converted responders into non-responders. Taken together, these results demonstrate that the behavioral response to fluoxetine can be bidirectionally modified via targeted manipulations of the DG and suggest that molecular- and neural circuit-based modulations of DG may provide a new therapeutic avenue for more effective antidepressant treatments.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Serotonergically dependent antihyperalgesic and antiallodynic effects of isoliquiritin in a mouse model of neuropathic pain

Chronic neuropathic pain poses a significant health problem worldwide, for which effective treatment is lacking. The current work aimed to investigate the potential analgesic effect of isoliquiritin, a flavonoid from Glycyrrhiza uralensis, against neuropathic pain and elucidate mechanisms. Male C57BL/6J mice were subjected to chronic constriction injury (CCI) by loose ligation of their sciatic nerves. Following CCI surgery, the neuropathic mice developed pain-like behaviors, as shown by thermal (heat) hyperalgesia in the Hargreaves test and tactile allodynia in the von Frey test. Repetitive treatment of CCI mice with isoliquiritin (p.o., twice per day for two weeks) ameliorated behavioral hyperalgesia to thermal (heat) stimuli and allodynia to tactile stimuli in a dose-dependent fashion (5, 15 and 45 mg/kg). The isoliquiritin-triggered analgesia seems serotonergically dependent, since its antihyperalgesic and antiallodynic actions were totally abolished by chemical depletion of spinal serotonin by p-chlorophenylalanine, whereas potentiated by 5-HTP (a precursor of 5-HT). Consistently, isoliquiritin-treated neuropathic mice showed escalated levels of spinal monoamines especially 5-HT, with depressed monoamine oxidase activity. Moreover, isoliquiritin-evoked antihyperalgesia and antiallodynia were preferentially counteracted by the 5-HT_1A receptor antagonist WAY-100635 delivered systematically or spinally. Of notable benefit, isoliquiritin was able to correct co-morbid behavioral symptoms of depression and anxiety evoked by neuropathic pain. Collectively, these findings demonstrate, for the first time, the therapeutic efficacy of isoliquiritin on neuropathic hypersensitivity, and this effect is dependent on the spinal serotonergic system and 5-HT_1A receptors.

Venlafaxine pharmacogenetics: a comprehensive review

Antidepressant response could be from 42 to 50% genetically determined. Venlafaxine (VEN) was the sixth most-prescribed antidepressant in the USA in 2017. Therefore, we reviewed studies which focused on the pharmacogenetics of VEN and found that there is a lack of guidelines for pharmacogenetic testing for VEN. Within investigated genetic polymorphisms, few of them can be indicated as potential predictors of VEN efficacy and tolerance. However, additional pharmacogenetic studies of VEN should be performed to reproduce already obtained results or explain contradictory ones. The individualization of pharmacotherapy is a key issue in providing patients with the highest possible quality of treatment, therefore pharmacogenetic studies should be one of the components of therapy optimization.

Differences Between the Prenatal Effects of Fluoxetine or Buspirone Alone or in Combination on Pain and Affective Behaviors in Prenatally Stressed Male and Female Rats

The selective serotonin reuptake inhibitor fluoxetine and the 5-HT1A receptor agonist buspirone are used to treat depression and anxiety. Previously we demonstrated that chronic stress during pregnancy (prenatal stress) in rats, used as a model of maternal depression risk, increased inflammatory pain and depressive-like behavior in the offspring; buspirone injected to pregnant dams was protective. Clinically, the addition of buspirone to fluoxetine increases the latter’s efficacy in treating depression in patients. Here, we investigated the influence of repeated prenatal injections of fluoxetine, buspirone or their combination on pain- and depressive-like behaviors in prenatally stressed young male and female rats. Prenatal stress augmented depressive-like behavior and both thermal and inflammatory pain (formalin test), replicating our prior findings, and increased basal levels of corticosterone in the blood plasma. Both drugs and their combination reduced the effects of prenatal stress on thermal pain and depressive-like behavior independently of sex. The combination of fluoxetine and buspirone, compared with fluoxetine, was more antinociceptive in the hot plate test in both sexes, and when compared with buspirone, was more antinociceptive only in males. A detailed study of the time-course of formalin-induced pain showed a nuanced effect of these drugs that was sex-dependent. The combination of the two drugs was less effective in females than males during the initial acute phase of nociceptive behavior in flexing + shaking behaviors, whereas that combination was more effective than fluoxetine alone in the first acute phase of licking behavior in females. The antinociceptive effect of buspirone dominated that of the drug combination and of fluoxetine alone, especially during the interphase of the formalin test in both sexes for both flexing + shaking and licking, suggesting a more effective prenatal action of buspirone on the development of a descending serotonergic inhibitory system modulating pain in the spinal cord dorsal horn neurons. Our results indicate that inflammatory pain-like responses integrated at the spinal level in males were more vulnerable to prenatal stress than females. In licking, the antinociceptive effect of fluoxetine and drug combination in the interphase was more in males than females. The data underscore the importance of considering sexual dimorphism when using drug therapy.

Dose related effects of buspirone on pain, learning / memory and food intake

The present study concerned extending the therapeutic use of buspirone for treating pain and improving cognition. Effects of single and repeated administration of buspirone were therefore monitored on pain threshold in the hot plate test and on spatial memory in the water maze test in rats. Effects on cumulative food intake were also monitored. The drug was administered intraperitoneally in doses of 0.1, 0.3, 1.0 and 2.0 mg/kg. We found that single and repeated administration of buspirone in doses of 0.1 mg/kg decreased pain threshold in the hot plate test, while doses of 1.0 and 2.0 mg/kg increased it. Effects of single and repeated administration were not different. A dose of 0.3 mg/kg had no effect. Food intake increased following single as well as repeated administration of 0.1 mg/kg buspirone; higher doses had no effect. Low doses (0.1 and 0.3 mg/kg) improved acquisition and retention of memory in the water maze test, while memory extinction was reduced. Higher doses had either no effect (1.0 mg/kg) or impaired (2.0 mg/kg) performance in this test. The results suggest potential therapeutic use of selected doses of buspirone as an analgesic and nootropic drug.

The influence of aripiprazole and venlafaxine on the antidepressant-like effect observed in prenatally stressed rats (animal model of depression)

Depression is a nosological entity which may appear alone or concomitantly (e.g. in schizophrenia). Analysis of data from both clinical and experimental studies allows a conclusion that atypical antipsychotics, such as aripiprazole (ARI), may also be effective in treating depression in addition to antidepressants. The aim of the studies was to determine antidepressant efficacy of ARI, venlafaxine (VEN) and combined therapy using both drugs, in prenatally stressed rats (animal depression model) and control group. In addition, this article was aimed at determining the effect of these drugs on locomotor activity of these animals. The effect of chronic stress used in pregnant rats and the use of drugs such as ARI (1.5 mg/kg) and VEN (20 mg/kg) were studied in forced swimming test (FST; antidepressant effect) and locomotor activity test. Performed tests confirmed the antidepressant effect of ARI, VEN and efficacy of combined drugs in FST in both prenatally stressed rats (effect present upon single administration and after 7, 14 and 21 days of testing) and control group rats (effect present upon single administration and 7 days of testing). Moreover, upon single administration of the used drugs to prenatally stressed rats, it was found sedative effect - reduced animals' locomotor activity. Study results have proven antidepressant and sedative efficacy of ARI, VEN and combined administration of these drugs. Due to the small amount of data on the above preparations, in particular in the context of animal depression models, further studies in this respect are recommended.

Mirtazapine exerts an anxiolytic-like effect through activation of the median raphe nucleus-dorsal hippocampal 5-HT pathway in contextual fear conditioning in rats

The functional role of serotonergic projections from the median raphe nucleus (MRN) to the dorsal hippocampus (DH) in anxiety remains understood poorly. The purpose of the present research was to examine the functional role of this pathway, using the contextual fear conditioning (CFC) model of anxiety. We show that intra-MRN microinjection of mirtazapine, a noradrenergic and specific serotonergic antidepressant, reduced freezing in CFC without affecting general motor activity dose-dependently, suggesting an anxiolytic-like effect. In addition, intra-MRN microinjection of mirtazapine dose-dependently increased extracellular concentrations of serotonin (5-HT) but not dopamine in the DH. Importantly, intra-DH pre-microinjection of WAY-100635, a 5-HT1A antagonist, significantly attenuated the effect of mirtazapine on freezing. These results, for the first time, suggest that activation of the MRN-DH 5-HT1A pathway exerts an anxiolytic-like effect in CFC. This is consistent with the literature that the hippocampus is essential for retrieval of contextual memory and that 5-HT1A receptor activation in the hippocampus primarily exerts an inhibitory effect on the neuronal activity.

Psychiatric agents and implications for perioperative analgesia

The use of antidepressants, anxiolytics, mood stabilizers, anticonvulsants, and major tranquilizers introduces neurochemical, behavioral, cognitive, and emotional factors that increase the complexity of medical and surgical tasks. Increasingly, various classes of psychotropic medications are being prescribed in the perioperative setting for their analgesic properties in patients with or without a psychiatric diagnosis. In many cases, the precise mechanisms of action and dose-response relationships by which these agents mediate analgesia are largely unclear. An appreciation of the side effects and adverse-effect profiles of such medications and familiarity with the clinically relevant drug interactions that may occur in the perioperative setting are imperative to ensure the best possible outcome in dealing with patients on these medications. This review focuses on various classes of psychotropic agents, which are addressed individually, with particular focus on their analgesic properties. The latest published research is summarized, deficiencies in our current collective knowledge are discussed, and evidence-based recommendations are made for clinical practice.

Chronic curcumin treatment normalizes depression-like behaviors in mice with mononeuropathy: involvement of supraspinal serotonergic system and GABAA receptor

RATIONALE

Comorbid depression is commonly observed in individuals who suffer from neuropathic pain, which necessitates improved treatment. Curcumin, a phenolic compound derived from Curcuma longa, possesses both antinociceptive and antidepressant-like activities in animal studies, suggesting its possible usefulness in treating this comorbidity.

OBJECTIVE

We investigated the effect of curcumin on depressive-like behaviors in mice with mononeuropathy, and explored the mechanism(s).

METHODS

Chronic constriction injury (CCI) was produced by loosely ligating the sciatic nerves in mice. The nociceptive behaviors were examined using Hargreaves test, and the depressive-like behaviors were determined by forced swim test (FST) and tail suspension test (TST).

RESULTS

After CCI injury, the neuropathic mice developed nociceptive and depressive-like behaviors, as shown by thermal hyperalgesia in Hargreaves test and protracted immobility time in FST and TST. Chronic treatment of neuropathic mice with curcumin (45 mg/kg, p.o., twice per day for 3 weeks) corrected their exacerbated nociceptive and depressive-like behaviors, which was abolished by chemical depletion of brain serotonin rather than noradrenaline. The paralleled antinociceptive and antidepressant-like actions of curcumin seem to be pharmacologically segregated, since intrathecal and intracerebroventricular injection of methysergide, a nonselective 5-HT receptor antagonist, separately counteracted the two actions of curcumin. Further, this antidepression was abrogated by repeated co-treatment with 5-HT1A receptor antagonist WAY-100635 and greatly attenuated by acute co-treatment with GABAA receptor antagonist bicuculline.

CONCLUSION

Curcumin can normalize the depressive-like behaviors of neuropathic mice, which may be independent of the concurrent analgesic action and possibly mediated via the supraspinal serotonergic system and downstream GABAA receptor.

Anti-nociceptive effect of vitexin mediated by the opioid system in mice

In the present study, we determined the potential anti-nociceptive activity of vitexin, a C-glycosylated flavone, by conducting some acute nociceptive tests in mice. Centrally mediated anti-nociceptive effect was evaluated by hot-plate and tail-clip tests, whereas peripherally mediated anti-nociception was assessed by acetic acid-induced writhing tests. Rota-rod test was performed to evaluate the probable effect of vitexin on the motor coordination of mice. Vitexin administered orally at doses of 10, 20, and 30 mg/kg significantly increased the reaction times of animals in the hot-plate and tail-clip tests and reduced the number of acetic acid-induced writhes and stretches in writhing tests, which clearly indicated the presence of the anti-nociceptive effect. This effect disappeared by pretreatment with naloxone (a non-selective opioid receptor antagonist, 5.48 mg/kg, i.p.), which indicated the involvement of opioid mechanisms in anti-nociception. We evaluated the contribution of mu, delta, and kappa subtypes of opioid receptors to the anti-nociceptive activity by using naloxonazine (7 mg/kg, s.c.), naltrindole (0.99 mg/kg, i.p.), and nor-binaltorphimine (1.03 mg/kg, i.p.), respectively. Pretreatment using these antagonists reversed the anti-nociceptive effect of vitexin in all the nociceptive tests, which indicated that mu, delta, and kappa opioid receptors contributed to the anti-nociceptive effect of this flavonoid. Falling latencies of mice in the Rota-rod test did not change upon the administration of vitexin, which indicated that vitexin showed specific anti-nociceptive effect. To the best of our knowledge, this is the first study on centrally and peripherally mediated anti-nociceptive effect of vitexin via opioid-related mechanisms.

Sensitivity of hippocampal 5-HT1A receptors to mild stress in BDNF-deficient mice

Serotonin 1A (5-HT(1A)) receptors in brain play an important role in cognitive and integrative functions, as well as emotional states. Decreased brain-derived neurotrophic factor (BDNF) expression and/or function, particularly in hippocampus, are implicated in the pathophysiology of stress-related disorders such as major depression. BDNF(+/-) mice are more vulnerable to stress than wild-type mice, exhibiting behavioural despair after mild handling stress. We examined the effect of mild handling stress on 5-HT(1A) receptor function, as measured by 8-OH-DPAT stimulated [(35)S]GTPγS binding, in BDNF(+/-) mice and mice with a forebrain-specific reduction in BDNF (embryonic BDNF inducible knockout mice). Our data show a remarkable sensitivity of hippocampal 5-HT1A receptors to mild stress and a deficiency in BDNF. Other 5-HT(1A) receptor populations, specifically in frontal cortex and dorsal raphe, were resistant to the combined detrimental effects of mild stress and reductions in BDNF expression. Decreases in hippocampal 5-HT(1A) receptor function induced by mild stress in BDNF-deficient mice were prevented by administration of the selective serotonin reuptake inhibitor fluoxetine, which increased activation of TrkB, the high affinity receptor for BDNF, in wild-type and BDNF(+/-) mice. In hippocampal cultures, BDNF increased the capacity of 5-HT(1A) receptors to activate G proteins, an effect eliminated by the knockout of TrkB, confirming TrkB activation increases 5-HT(1A) receptor function. The mechanisms underlying the sensitivity of hippocampal 5-HT(1A) receptors to mild stress and decreased BDNF expression remain to be elucidated and may have important implications for the emotional and cognitive impairments associated with stress-related mental illness.

Levomilnacipran (F2695), a norepinephrine-preferring SNRI: profile in vitro and in models of depression and anxiety

Levomilnacipran (LVM; F2695) is the more active enantiomer of the serotonin/norepinephrine (5-HT/NE) reuptake inhibitor (SNRI) milnacipran and is currently under development for the treatment of major depressive disorder. LVM was benchmarked against two other SNRIs, duloxetine and venlafaxine, in biochemical, neurochemical and pharmacological assays. LVM exhibited high affinity for human NE (Ki = 92.2 nM) and 5-HT (11.2 nM) transporters, and potently inhibited NE (IC50 = 10.5 nM) and 5-HT (19.0 nM) reuptake (human transporter) in vitro. LVM had 2-fold greater potency for norepinephrine relative to serotonin reuptake inhibition (i.e. NE/5-HT potency ratio: 0.6) and 17 and 27 times higher selectivity for NE reuptake inhibition compared with venlafaxine and duloxetine, respectively. LVM did not exhibit affinity for 23 off-target receptors. LVM (i.p.) increased cortical extracellular levels of 5-HT, and NE (minimal effective doses: MEDs = 20 and 10 mg/kg, respectively). In anti-depressive/anti-stress models, i.p. LVM diminished immobility time in the mouse forced swim (MED = 20 mg/kg) and tail suspension (MED = 2.5 mg/kg) tests, and reduced shock-induced ultrasonic vocalizations in rats (MED = 5 mg/kg). Duloxetine and venlafaxine were less potent (MEDs ≥ 10 mg/kg). At doses active in these three therapeutically-relevant models, LVM (i.p.) did not significantly affect spontaneous locomotor activity. In summary, LVM is a potent, selective inhibitor of NE and 5-HT transporters with preferential activity at the former. It is efficacious in models of anti-depressive/anti-stress activity, with minimal potential for locomotor side effects.

Active behaviours produced by antidepressants and opioids in the mouse tail suspension test

Most classical preclinical tests to predict antidepressant activity were initially developed to detect compounds that influenced noradrenergic and/or serotonergic activity, in accordance with the monoaminergic hypothesis of depression. However, central opioid systems are also known to influence the pathophysiology of depression. While the tail suspension test (TST) is very sensitive to several types of antidepressant, the traditional form of scoring the TST does not distinguish between different modes of action. The present study was designed to compare the behavioural effects of classical noradrenergic and/or serotonergic antidepressants in the TST with those of opioids. We developed a sampling technique to differentiate between behaviours in the TST, namely, curling, swinging and immobility. Antidepressants that inhibit noradrenaline and/or serotonin re-uptake (imipramine, venlafaxine, duloxetine, desipramine and citalopram) decreased the immobility of mice, increasing their swinging but with no effect on their curling behaviour. No differences were observed between antidepressants that act on noradrenergic or serotoninergic transmission. While opioid compounds also decreased the immobility of the mice [morphine, codeine, levorphanol, (-)-methadone, (±)-tramadol and (+)-tramadol], they selectively increased curling behaviour. Blocking opioid receptors with naloxone prevented the antidepressant-like effect of codeine, and μ-opioid receptor knockout decreased normal curling behaviour and blocked (±)-tramadol-induced curling, further demonstrating the reliability and validity of this approach. These results show that at least two behaviourally distinct processes occur in the TST, highlighting the antidepressant-like effects of opioids evident in this test. Furthermore, our data suggest that swinging and curling behaviours are mediated by enhanced monoamine and opioid neurotransmission, respectively.

The effects of serotonin/norepinephrine reuptake inhibitors and serotonin receptor agonist on morphine analgesia and tolerance in rats

Several studies have demonstrated that serotonergic and noradrenergic systems have important roles in morphine analgesia and tolerance. However, the exact mechanism underlying the development of morphine tolerance is not fully understood. The aim of this study was to investigate the possible role of serotonin/norepinephrine reuptake inhibitors (amitriptyline, venlafaxine) and serotonin receptor (5-HT(1A) and 5-HT(1B/1D)) agonist (dihydroergotamine) in morphine analgesia and tolerance in rats. To constitute morphine tolerance, animals received morphine (50 mg/kg; s.c.) once daily for 3 days. After the last dose of morphine was injected on day 4, morphine tolerance was evaluated. The analgesic effects of amitriptyline (20 mg/kg; i.p.), venlafaxine (20 mg/kg; s.c.), dihydroergotamine (100 μg/kg; i.v.) and morphine (5 mg/kg) were considered at 15- to 30-min intervals (0, 15, 30, 60, 90, and 120 min) by tail-flick and hot-plate analgesia tests. In this study, the data obtained suggested that amitriptyline and venlafaxine significantly increased the analgesic effect of morphine and attenuated the expression of morphine tolerance. However, dihydroergotamine significantly increased the analgesic effect of morphine but did not reduce the expression of morphine tolerance. In conclusion, we determined that co-administration of morphine with amitriptyline and venlafaxine increased the analgesic effects of morphine and attenuated the morphine analgesic tolerance.

Altered response to the selective serotonin reuptake inhibitor escitalopram in mice heterozygous for the serotonin transporter: an electrophysiological and neurochemical study

A serotonin (5-HT) transporter (5-HTT; SERT) polymorphism has been associated with depressive states and poor responses to selective serotonin reuptake inhibitors (SSRIs). Given the similar attenuation of SERT activity in SERT+/- mice and in humans with short allele(s) of SERT in its promoter region, it is conceivable that SERT+/- mice offer an adequate model to mimic the human subpopulation with respect to their altered response to SSRIs. This study investigated the effects of the most selective SSRI escitalopram, in heterozygous SERT+/- mice using a combined electrophysiological and neurochemical approach. Results indicated that administration of escitalopram for 2 d resulted in a 72% and 63% decrease in dorsal raphe 5-HT neuronal firing rate in SERT+/+ and SERT+/- mice, respectively. In contrast, administration of escitalopram for 21 d produced a gradual recovery of 5-HT neuronal firing rate to basal level in SERT+/+, but not in SERT+/- mice. In the hippocampus, microdialysis revealed that sustained administration of escitalopram produced a greater increase in extracellular 5-HT ([5-HT]ext) outflow in SERT+/- than in the wild-types with or without a washout of the SSRI. Nevertheless, the ability of microiontophoretically applied 5-HT to inhibit the firing rate of CA3 pyramidal neurons was not different between SERT+/+ and SERT+/- mice given escitalopram for 21 d. The data indicate that the poor response to SSRIs of depressive patients with short allele(s) of SERT is not attributable to a lesser increase in 5-HT transmission in the hippocampus.

Role for monoaminergic systems in the antidepressant-like effect of ethanol extracts from Hemerocallis citrina

ETHNOPHARMACOLOGICAL RELEVANCE

Hemerocallis citrina, a traditional herbal medicine, has been used for the improvement of emotions in Eastern-Asia countries.

AIM OF THE STUDY

Herein, we explored the antidepressant-like effect and its monoaminergic mechanism of the ethanol extracts from Hemerocallis citrina (HCE).

MATERIALS AND METHODS

Effect of HCE (90, 180 and 360 mg/kg, p.o.) on the immobility time was assessed in the mouse forced swim test (FST) and tail suspension test (TST), and locomotor activity was evaluated in the open-field test (OFT). Additionally, the monoamine neurotransmitters serotonin (5-HT), noradrenaline (NA) and dopamine (DA) levels involved in the antidepressant-like effect of HCE were also measured in the mice brain regions of frontal cortex and hippocampus.

RESULTS

HCE (90, 180 and 360 mg/kg, p.o.) administration significantly reduced the immobility time in both the FST and TST without accompanying changes in locomotor activity in the OFT. The pretreatment of mice with WAY 100635 (0.1 mg/kg, s.c., a 5-HT(1A) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) receptor antagonist), prazosin (62.5 μg/kg, i.p., an α(1)-adrenoceptor antagonist), yohimbine (1 mg/kg, i.p., an α(2)-adrenoceptor antagonist), propranolol (5 mg/kg, i.p., a β-adrenoceptor antagonist) or sulpiride (50 mg/kg, i.p., a dopamine D(2) receptor antagonist), but not SCH23390 (0.05 mg/kg, s.c., a dopamine D(1) receptor antagonist) prevented the antidepressant-like effect of HCE (360 mg/kg, p.o.) in the TST. In addition, HCE enhanced 5-HT and NA levels in the frontal cortex and hippocampus as well as elevated DA levels in the frontal cortex.

CONCLUSION

The results indicate that the antidepressant-like effect of HCE is dependent on the serotonergic (5-HT(1A) and 5-HT(2) receptors), noradrenergic (α(1)-, α(2)- and β-adrenoceptors) and dopaminergic (D(2) receptor) systems as well as the elevation of 5-HT, NA and DA levels in the mouse brain.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Increased thermal and mechanical nociceptive thresholds in rats with depressive-like behaviors

Clinical observations suggest that depressed patients were less sensitive to experimental pain than healthy subjects. However, few animal studies are reported concerning the association of depression and pain. The purpose of this study was to investigate the effects of unpredictable chronic mild stress (UCMS) induced depression on the perceived intensity of painful stimulation in rats. We measured the thermal and mechanical paw withdrawal thresholds (PWT) of normal and spinal nerve ligated (SNL) rats using hot plate test and von Frey test, respectively. The results showed that rats exposed to UCMS exhibited significantly higher thermal and mechanical pain thresholds in comparison to the non-depressed controls. In particular, the PWT of the SNL group was restored to nearly normal level after three weeks of UCMS, and even comparable to that of the control group. These results strongly suggest that the depressed subjects have decreased sensitivity to externally applied noxious stimulation, which is consistent with our previous findings.

Effect of buspirone on thermal sensory and pain thresholds in human volunteers

Background

Buspirone is a partial 5-HT_1A receptor agonist. Animal studies have shown that modulation of serotoninergic transmission at the 5-HT_1A receptor can induce analgesia in acute pain models. However, no studies have been published so far on the effects of serotonin receptor agonists on pain perception in humans.

Methods

The effects of buspirone (30 mg p.o.) on thermal sensory and pain thresholds were investigated in twelve female volunteers (26 ± 2 yrs) in a prospective, randomized, double-blind, double-dummy, placebo-controlled study with morphine (10 mg i.v.) as positive control.

Results

Morphine significantly increased the heat pain detection threshold (ΔT: placebo 1.0°C and 1.3°C, p < 0.05) at 60 minutes. Buspirone caused mild sedation in six participants at 60 minutes, but was without effect on any of the measured parameters.

Conclusion

Buspirone in the maximal recommended dose was without significant effect on thermal pain. However, as it is only a partial agonist at the 5-HT_1A receptor and also acts on other receptor types, the negative results of the present study do not rule out a possible analgesic effect of more specific 5-HT_1A receptor agonists.

5-HT(1A) receptor function in major depressive disorder

Dysfunction of the serotonin 1A receptor (5-HT(1A)) may play a role in the genesis of major depressive disorder (MDD). Here we review the pharmacological, post-mortem, positron emission tomography (PET), and genetic evidence in support of this statement. We also touch briefly on two MDD-associated phenotypes, cognitive impairment and somatic pain. The results of pharmacological challenge studies with 5-HT(1A) receptor agonists are indicative of blunted endocrine responses in depressed patients. Lithium, valproate, selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants (TCAs), and other treatment, such as electroconvulsive shock therapy (ECT), all increase post-synaptic 5-HT(1A) receptor signaling through either direct or indirect effects. Reduced somatodendritic and postsynaptic 5-HT(1A) receptor numbers or affinity have been reported in some post-mortem studies of suicide victims, a result consistent with well-replicated PET analyses demonstrating reduced 5-HT(1A) receptor binding potential in diverse regions such as the dorsal raphe, medial prefrontal cortex (mPFC), amygdala and hippocampus. 5-HT(1A) receptor knockout (KO) mice display increased anxiety-related behavior, which, unlike in their wild-type counterparts, cannot be rescued with antidepressant drug (AD) treatment. In humans, the G allele of a single nucleotide polymorphism (SNP) in the 5-HT(1A) receptor gene (HTR1A; rs6295), which abrogates a transcription factor binding site for deformed epidermal autoregulatory factor-1 (Deaf-1) and Hes5, has been reported to be over-represented in MDD cases. Conversely, the C allele has been associated with better response to AD drugs. We raise the possibility that 5-HT(1A) receptor dysfunction represents one potential mechanism underpinning MDD and other stress-related disorders.

Signal transduction and functional selectivity of F15599, a preferential post-synaptic 5-HT1A receptor agonist

BACKGROUND AND PURPOSE

Activation of post-synaptic 5-HT(1A) receptors may provide enhanced therapy against depression. We describe the signal transduction profile of F15599, a novel 5-HT(1A) receptor agonist.

EXPERIMENTAL APPROACH

F15599 was compared with a chemical congener, F13714, and with (+)8-OH-DPAT in models of signal transduction in vitro and ex vivo.

KEY RESULTS

F15599 was highly selective for 5-HT(1A) receptors in binding experiments and in [(35)S]-GTPgammaS autoradiography of rat brain, where F15599 increased labelling in regions expressing 5-HT(1A) receptors. In cell lines expressing h5-HT(1A) receptors, F15599 more potently stimulated extracellular signal-regulated kinase (ERK1/2) phosphorylation, compared with G-protein activation, internalization of h5-HT(1A) receptors or inhibition of cAMP accumulation. F13714, (+)8-OH-DPAT and 5-HT displayed a different rank order of potency for these responses. F15599 stimulated [(35)S]-GTPgammaS binding more potently in frontal cortex than raphe. F15599, unlike 5-HT, more potently and efficaciously stimulated G(alphai) than G(alphao) activation. In rat prefrontal cortex (a region expressing post-synaptic 5-HT(1A) receptors), F15599 potently activated ERK1/2 phosphorylation and strongly induced c-fos mRNA expression. In contrast, in raphe regions (expressing pre-synaptic 5-HT(1A) receptors) F15599 only weakly or did not induce c-fos mRNA expression. Finally, despite its more modest affinity in vitro, F15599 bound to 5-HT(1A) receptors in vivo almost as potently as F13714.

CONCLUSIONS AND IMPLICATIONS

F15599 showed a distinctive activation profiles for 5-HT(1A) receptor-mediated signalling pathways, unlike those of reference agonists and consistent with functional selectivity at 5-HT(1A) receptors. In rat, F15599 potently activated signalling in prefrontal cortex, a feature likely to underlie its beneficial effects in models of depression and cognition.