Anxiogenic effects in the rat elevated plus-maze of 5-HT(2C) agonists into ventral but not dorsal hippocampus

Anxiolytic-like effects of extremely low frequency electric field in stressed rats: involvement of 5-HT2C receptors

BACKGROUND

Possible modulatory effects of noninvasive brain stimulation have gained interest recently. In our study, the effect of low frequency electric fields (LF-EF) on stress-induced electrophysiological, behavioral changes and the possible involvement of serotonergic 5-HT2C receptors were investigated.

MATERIALS AND METHODS

A total of eight groups including the control groups were formed by applying LF-EF with or without a 5-HT2C receptor agonist to naïve or acute stress exposed rats to demonstrate the effects of LF-EF. LF-EF administration at 10 kV/m was started 30 min before acute stress application and lasted for 1 h in total. Anxiety levels and social interaction were evaluated using the elevated plus maze test and social interaction test, respectively. Auditory evoked potentials (AEP) were recorded by using ascending loudness paradigms. Loudness dependence AEP (LDAEP) was calculated by using amplitude values to analyze serotonergic transmission. Serotonin and glucocorticoid levels were measured in the frontal cortex and hippocampus.

RESULTS

It was observed that the applied LF-EF reduced the anxiety behavior, and attenuated the LDAEP responses in stress and/or 5-HT2C receptor agonist applied groups. In parallel, an increase in serotonin levels and a decrease in glucocorticoid levels were observed. However, LF-EF exposure was ineffective in impaired social interaction.

CONCLUSIONS

Our findings show that 10 kV/m LF-EF administration may modulate the neural network and physiological responses associated with mild acute stress. 5-HT2C receptor dependent functions are thought to play a role in the anxiolytic effect of LF-EF.

5-HT2C agonists and antagonists block different components of behavioral responses to potential, distal, and proximal threat in zebrafish

Serotonin (5-HT) receptors have been implicated in responses to aversive stimuli in mammals and fish, but its precise role is still unknown. Moreover, since at least seven families of 5-HT receptors exist in vertebrates, the role of specific receptors is still debated. Aversive stimuli can be classified as indicators of proximal, distal, or potential threat, initiating responses that are appropriate for each of these threat levels. Responses to potential threat usually involve cautious exploration and increased alertness, while responses to distal and proximal threat involve a fight-flight-freeze reaction. We exposed adult zebrafish to a conspecific alarm substance (CAS) and observed behavior during (distal threat) and after (potential threat) exposure, and treated with the 5-HT_2C receptor agonists MK-212 or WAY-161503 or with the antagonist RS-102221. The agonists blocked CAS-elicited defensive behavior (distal threat), but not post-exposure increases in defensive behavior (potential threat), suggesting inhibition of responses to distal threat. MK-212 blocked changes in freezing elicited by acute restraint stress, a model of proximal threat, while RS-102221 blocked changes in geotaxis elicited this stressor. We also found that RS-102221, a 5-HT_2C receptor antagonist, produced small effect on behavior during and after exposure to CAS. Preprint: https://www.biorxiv.org/content/10.1101/2020.10.04.324202; Data and scripts: https://github.com/lanec-unifesspa/5-HT-CAS/tree/master/data/5HT2C.

Theoretical, and epistemological challenges in scientific investigations of complex emotional states in animals

This review brings to light critical epistemological and theoretical considerations when studying complex emotional states in animals. We discuss anthropomorphic and Umwelt perspectives of nonhuman animals and the ways in which distinct theories of consciousness and neural processing may restrict the potential for the development of knowledge on the topic. Within the same line of argumentation, we consider influences of the debate between monism and dualism and psychology's behaviorism and cognitive theories. Finally, we contrast the affective consciousness, higher-order emotional consciousness, and constructed emotion theories to further our understanding of complex emotional states in animals.

Different roles of distinct serotonergic pathways in anxiety-like behavior, antidepressant-like, and anti-impulsive effects

Serotonergic agents have been widely used for treatment of psychiatric disorders, but the therapeutic effects are insufficient and these drugs often induce severe side effects. We need to specify the distinct serotonergic pathways underlying each mental function to overcome these problems. Preclinical studies have demonstrated that the central serotonergic system is involved in several emotional/cognitive functions including anxiety, depression, and impulse control, but it remains unclear whether each function is regulated by a different serotonergic system. We used optogenetic strategy to increase central serotonergic activity in mice and evaluated the behavioral consequences. Pharmacological and genetic tools were used to determine the subtype of 5-HT receptors responsible for the observed effects. We demonstrated that the serotonergic activation in the median raphe nucleus enhanced anxiety-like behavior, the serotonergic activation in the dorsal raphe nucleus exerted antidepressant-like effects, and the serotonergic activation in the median or dorsal raphe nucleus suppressed impulsive action. We also found that different serotonergic terminals, ventral hippocampus, ventral tegmental area/substantia nigra, and subthalamic/parasubthalamic nucleus, are involved in regulating anxiety-like behavior, antidepressant-like, and anti-impulsive effects, respectively. Furthermore, we found, using triple-transgenic mice, that the stimulation of the 5-HT_2C receptor is required to evoke anxiety-like behavior, but not to exert anti-impulsive effects. These results suggest the need for pathway-specific treatments and provide important insights that will help the development of more effective and safer therapeutics. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Emotional Roles of Mono-Aminergic Neurotransmitters in Major Depressive Disorder and Anxiety Disorders

A growing body of researches support a role for dysfunction of serotoninergic, noradrenergic, and dopaminergic systems in the neurobiological processes involved in major depression disorder (MDD) and anxiety disorders (ADs). The physiological changes underlying abnormal signaling of 5-HT, NE, and DA may be due to either reduced presynaptic release of these neurotransmitters or aberrant signal transductions, and thus contributing to the alterations in regulation or function of receptors and/or impaired intracellular signal processing. Animal models demonstrate crucial responsiveness to disturbance of 5-HT, NE, and DA neurotransmissions. Postmortem and biochemical studies have shown altered concentrations of 5-HT, NE, and DA metabolites in brain regions that contribute importantly to regulation of mood and motivation in patients with MDD or ADs. Neuroimaging studies have found abnormal 5-HT, NE, and DA receptors binding and regulation in regard to receptor numbers. Medications that act on 5-HT, NE, and DA neurons or receptors, such as SSRIs and SNRIs, show efficacy in both MDD and ADs. The overlapping treatment response presumably suggests a common mechanism underlying the interaction of these disorders. In this paper, we reviewed studies from multiple disciplines to interpret the role of altered 5-HT, NE and DA mono-amine neurotransmitter functions in both MDD and ADs.

Studies into the anxiolytic actions of agomelatine in social isolation reared rats: Role of corticosterone and sex

Anxiety disorders are severely disabling, while current pharmacological treatments are complicated by delayed onset, low remission rates and side-effects. Sex is also noted to contribute towards illness severity and treatment response. Agomelatine is a melatonin (MT₁/MT₂) agonist and serotonin (5-HT_2C) antagonist purported to be anxiolytic in clinical and some pre-clinical studies. We undertook a detailed analysis of agomelatine's anxiolytic activity in a neurodevelopmental model of anxiety, the social isolation reared rat. Rats received sub-chronic treatment with vehicle or agomelatine (40 mg/kg per day intraperitoneally at 16:00 h for 16 days), with behaviour analysed in the open field test, social interaction test and elevated plus maze. The contribution of corticosterone and sex was also studied. Social isolation rearing increased locomotor activity and reduced social interaction in the social interaction test, and was anxiogenic in the elevated plus maze in males and females. Agomelatine reversed these behaviours. Male and female social isolation reared rats developed anxiety-like behaviours to a similar degree, although response to agomelatine was superior in male rats. Social isolation rearing decreased plasma corticosterone in both sexes and tended to higher levels in females, although agomelatine did not affect corticosterone in either sex. Concluding, agomelatine is anxiolytic in SIR rats, although correcting altered corticosterone could not be implicated. Sex-related differences in the response to agomelatine are evident.

Behavioral Effects of Systemic, Infralimbic and Prelimbic Injections of a Serotonin 5-HT2A Antagonist in Carioca High- and Low-Conditioned Freezing Rats

The role of serotonin (5-hydroxytryptamine [5-HT]) and 5-HT2A receptors in anxiety has been extensively studied, mostly without considering individual differences in trait anxiety. Our laboratory developed two lines of animals that are bred for high and low freezing responses to contextual cues that are previously associated with footshock (Carioca High-conditioned Freezing [CHF] and Carioca Low-conditioned Freezing [CLF]). The present study investigated whether ketanserin, a preferential 5-HT2A receptor blocker, exerts distinct anxiety-like profiles in these two lines of animals. In the first experiment, the animals received a systemic injection of ketanserin and were exposed to the elevated plus maze (EPM). In the second experiment, these two lines of animals received microinjections of ketanserin in the infralimbic (IL) and prelimbic (PL) cortices and were exposed to either the EPM or a contextual fear conditioning paradigm. The two rat lines exhibited bidirectional effects on anxiety-like behavior in the EPM and opposite responses to ketanserin. Both systemic and intra-IL cortex injections of ketanserin exerted anxiolytic-like effects in CHF rats but anxiogenic-like effects in CLF rats. Microinjections of ketanserin in the PL cortex also exerted anxiolytic-like effects in CHF rats but had no effect in CLF rats. These results suggest that the behavioral effects of 5-HT2A receptor antagonism might depend on genetic variability associated with baseline reactions to threatening situations and 5-HT2A receptor expression in the IL and PL cortices. Highlights -CHF and CLF rats are two bidirectional lines that are based on contextual fear conditioning.-CHF rats have a more "anxious" phenotype than CLF rats in the EPM.-The 5-HT2A receptor antagonist ketanserin had opposite behavioral effects in CHF and CLF rats.-Systemic and IL injections either decreased (CHF) or increased (CLF) anxiety-like behavior.-PL injections either decreased (CHF) anxiety-like behavior or had no effect (CLF).

Marine cyanobacteria-derived serotonin receptor 2C active fraction induces psychoactive behavioral effects in mice

CONTEXT

Marine cyanobacteria offer a robust resource for natural products drug discovery due to the secondary metabolites they produce.

OBJECTIVE

To identify novel cyanobacterial compounds that exhibit CNS psychoactive effects.

MATERIALS AND METHODS

Cyanobacteria were collected from Las Perlas Archipelago, Panama and subjected to dichloromethane/methanol extraction and fractionation by column chromatography before being screened for affinity against a panel of CNS targets. A 50:50 ethyl acetate:methanol fraction of one cyanobacterial extract (2064H) was subjected to HPLC and the major peak was isolated (2064H3). At a dose of 20 μg per animal, 2064H and 2064H3 were tested in mice using behavioral assays that included the forced swim, open field and formalin tests.

RESULTS

2064H was shown to bind to the serotonin 2C (5-HT_2C) receptor, a known target for depression and pain treatment. 2064H showed 59.6% inhibition of binding of [³H]-mesulergine with an IC₅₀ value of 179 ng/mL and did not show inhibition of binding greater than 45% with any other receptors tested. Both 2064H and 2064H3 decreased immobility time in the first minute of the tail suspension test. 2064H increased time, distance and number of entries in the center region in the first half of the open field test. 2064H increased overall nocifensive behaviors in the formalin test.

DISCUSSION AND CONCLUSION

Overall, manipulating the 5-HT_2C receptor with these receptor-specific ligands derived from cyanobacteria altered pain, depression and anxiety-like behaviors, illustrating the importance of this receptor in affective behaviors. These results demonstrate the potential of cyanobacteria as a source for CNS active compounds.

Putative Epigenetic Involvement of the Endocannabinoid System in Anxiety- and Depression-Related Behaviors Caused by Nicotine as a Stressor

Like various stressors, the addictive use of nicotine (NC) is associated with emotional symptoms such as anxiety and depression, although the underlying mechanisms have not yet been fully elucidated due to the complicated involvement of target neurotransmitter systems. In the elicitation of these emotional symptoms, the fundamental involvement of epigenetic mechanisms such as histone acetylation has recently been suggested. Furthermore, among the interacting neurotransmitter systems implicated in the effects of NC and stressors, the endocannabinoid (ECB) system is considered to contribute indispensably to anxiety and depression. In the present study, the epigenetic involvement of histone acetylation induced by histone deacetylase (HDAC) inhibitors was investigated in anxiety- and depression-related behavioral alterations caused by NC and/or immobilization stress (IM). Moreover, based on the contributing roles of the ECB system, the interacting influence of ECB ligands on the effects of HDAC inhibitors was evaluated in order to examine epigenetic therapeutic interventions. Anxiety-like (elevated plus-maze test) and depression-like (forced swimming test) behaviors, which were observed in mice treated with repeated (4 days) NC (subcutaneous 0.8 mg/kg) and/or IM (10 min), were blocked by the HDAC inhibitors sodium butyrate (SB) and valproic acid (VA). The cannabinoid type 1 (CB1) agonist ACPA (arachidonylcyclopropylamide; AC) also antagonized these behaviors. Conversely, the CB1 antagonist SR 141716A (SR), which counteracted the effects of AC, attenuated the anxiolytic-like effects of the HDAC inhibitors commonly in the NC and/or IM groups. SR also attenuated the antidepressant-like effects of the HDAC inhibitors, most notably in the IM group. From these results, the combined involvement of histone acetylation and ECB system was shown in anxiety- and depression-related behaviors. In the NC treatment groups, the limited influence of SR against the HDAC inhibitor-induced antidepressant-like effects may reflect the characteristic involvement of histone acetylation within the NC-related neurotransmitter systems other than the ECB system.

Serotonin 5-HT7 Receptor in the Ventral Hippocampus Modulates the Retrieval of Fear Memory and Stress-Induced Defecation

BACKGROUND

Patients with posttraumatic stress disorder or panic disorder are often troubled by inappropriate retrieval of fear memory. Moreover, these disorders are often comorbid with irritable bowel syndrome. The main aim of the present study is to elucidate the involvement of hippocampal serotonergic systems in fear memory retrieval and stress-induced defecation.

METHODS AND RESULTS

Microinjection of serotonin7 receptor antagonist, but not other serotonin receptor antagonists (serotonin 1A, 2A, 2C, 3, 4, and 6), into the rat ventral hippocampus significantly suppressed the expression of freezing behavior, an index of fear memory retrieval, and decreased the amount of feces, an index of stress-induced defecation, in the contextual fear conditioning test. Electrophysiological data indicated that the serotonin7 receptor agonist increased the frequency of action potentials in the ventral hippocampal CA3 pyramidal neuron via the activation of the hyperpolarization-activated nonselective cation current Ih. Moreover, in situ hybridization demonstrated that Htr7 mRNA was abundantly expressed in the CA3 compared with other subregions of the hippocampus and that these Htr7 mRNA-positive cells coexpressed hyperpolarization-activated cyclic nucleotide-gated channel 2 and 4 mRNAs, which are components of the Ih channel.

CONCLUSIONS

These results indicated that the released serotonin activates the serotonin7 receptor in the CA3 ventral hippocampus subregion, enhances the sensitivity to inputs via hyperpolarization-activated cyclic nucleotide 2 and 4 channels, and thereby facilitates fear memory retrieval. The serotonin7 receptor might be a target of drug development for the treatment of mental disorders involving fear memory and gastrointestinal problems.

EFEITOS DO EXERCÍCIO FÍSICO EM COMPORTAMENTOS RELACIONADOS COM A ANSIEDADE EM RATOS

Introdução: Os efeitos do exercício físico são associados à redução da ansiedade e à manutenção desse estado por diversas horas. Contudo, poucos trabalhos têm investigado os efeitos do exercício agudo sobre a ansiedade. Objetivo: Investigar o efeito agudo da intensidade do exercício físico sobre comportamento relacionado com a ansiedade em modelos animais. Métodos: Cinquenta e seis ratos Wistar machos foram distribuídos aleatoriamente em oito grupos: controle, 0%, 5% e 50% de carga correspondente ao peso corporal dos animais. Os grupos foram submetidos a um protocolo de natação que consistiu na realização de 30 minutos de exercício para os grupos 0% e 5% e dez sessões de dez saltos intercalados por um minuto de descanso para o grupo 50%. Após a natação, os animais foram expostos aos testes de campo aberto e labirinto em cruz elevado. Os dados foram avaliados estatisticamente pela One-way ANOVA e teste post hoc de Bonferroni, adotando-se P<0,05. Resultados: Na análise dos dados no teste de campo aberto foram encontradas diferenças significativas nas medidas de cruzamento (F(3,20) = 9,60; p < 0,01), ocorrendo um aumento de cruzamentos do grupo 5% (81,2 ± 14,8) com relação ao grupo controle (45,0 ± 8,7). Foi encontrada diferença significativa no percentual de entrada nos braços abertos (F(3,28) = 5,63; p < 0,01), havendo uma diminuição no grupo 50% (25,4 ± 17,0) com relação ao grupo controle (46,8 ± 9,1). Conclusão: A prática de exercício físico na intensidade de 5% apresentou efeitos ansiolíticos, enquanto na intensidade de 50%, apresentou efeitos ansiogênicos.

Decreased serotonin2C receptor responses in male patients with schizophrenia

Serotonin (5-HT)2C receptors in brain affect psychosis, reward, substance abuse, anxiety, other behaviors, appetite, body temperature, and other physiological measures. They also have been implicated in antipsychotic drug efficacy and side effects. We previously reported that the hyperthermia following administration of MK-212, a predominantly 5-HT(2C) receptor agonist, was diminished in a small sample of patients with schizophrenia (SCH), suggesting decreased 5-HT(2C) receptor responsiveness. We have now studied the responses to oral MK-212 and placebo in a larger sample of unmedicated male SCH (n = 69) and normal controls (CON) (n = 33), and assessed the influence of comorbid substance abuse (SA) on oral body temperature, behavioral responses, etc. The placebo-adjusted oral body temperature response to MK-212 was significantly lower in SCH compared to CON and not significantly different between the SCH with or without SA. Some behavioral responses to MK-212, e.g. self-rated feelings of increased anxiety, depression and decreased calmness, or good overall feeling, were significantly lower in the SCH patients compared to CON. These results add to the evidence for diminished 5-HT(2C) receptor responsiveness in SCH patients compared to CON and are consistent with reported association of HTR(2C) polymorphisms, leading to decreased expression or function of the HTR(2C) in patients with SCH.

Controversies on the role of 5-HT(2C) receptors in the mechanisms of action of antidepressant drugs

Evidence from the various sources indicates alterations in 5-HT2C receptor functions in anxiety, depression and suicide, and other stress-related disorders treated with antidepressant drugs. Although the notion of a 5-HT2C receptor desensitization following antidepressant treatments is rather well anchored in the literature, this concept is mainly based on in vitro assays and/or behavioral assays (hypolocomotion, hyperthermia) that have poor relevance to anxio-depressive disorders. Our objective herein is to provide a comprehensive overview of the studies that have assessed the effects of antidepressant drugs on 5-HT2C receptors. Relevant molecular (second messengers, editing), neurochemical (receptor binding and mRNA levels), physiological (5-HT2C receptor-induced hyperthermia and hormone release), behavioral (5-HT2C receptor-induced changes in feeding, anxiety, defense and motor activity) data are summarized and discussed. Setting the record straight about drug-induced changes in 5-HT2C receptor function in specific brain regions should help to determine which pharmacotherapeutic strategy is best for affective and anxiety disorders.

GPR30 activation decreases anxiety in the open field test but not in the elevated plus maze test in female mice

The GPR30 is a novel estrogen receptor (ER) that is a candidate membrane ER based on its binding to 17β estradiol and its rapid signaling properties such as activation of the extracellular-regulated kinase (ERK) pathway. Its distribution in the mouse limbic system predicts a role for this receptor in the estrogenic modulation of anxiety behaviors in the mouse. A previous study showed that chronic administration of a selective agonist to the GPR30 receptor, G-1, in the female rat can improve spatial memory, suggesting that GPR30 plays a role in hippocampal-dependent cognition. In this study, we investigated the effect of a similar chronic administration of G-1 on behaviors that denote anxiety in adult ovariectomized female mice, using the elevated plus maze (EPM) and the open field test as well as the activation of the ERK pathway in the hippocampus. Although estradiol benzoate had no effect on behaviors in the EPM or the open field, G-1 had an anxiolytic effect solely in the open field that was independent of ERK signaling in either the ventral or dorsal hippocampus. Such an anxiolytic effect may underlie the ability of G-1 to increase spatial memory, by acting on the hippocampus.

Corticotropin-releasing factor-related peptides, serotonergic systems, and emotional behavior

Corticotropin-releasing factor (CRF) is a 41-amino acid neuropeptide that is involved in stress-related physiology and behavior, including control of the hypothalamic-pituitary-adrenal (HPA) axis. Members of the CRF family of neuropeptides, including urocortin 1 (UCN 1), UCN 2, and UCN 3, bind to the G protein-coupled receptors, CRF type 1 (CRF₁) and CRF₂ receptors. In addition, CRF binding protein (CRFBP) binds both CRF and UCN 1 and can modulate their activities. There are multiple mechanisms through which CRF-related peptides may influence emotional behavior, one of which is through altering the activity of brainstem neuromodulatory systems, including serotonergic systems. CRF and CRF-related peptides act within the dorsal raphe nucleus (DR), the major source for serotonin (5-HT) in the brain, to alter the neuronal activity of specific subsets of serotonergic neurons and to influence stress-related behavior. CRF-containing axonal fibers innervate the DR in a topographically organized manner, which may contribute to the ability of CRF to alter the activity of specific subsets of serotonergic neurons. CRF and CRF-related peptides can either increase or decrease serotonergic neuronal firing rates and serotonin release, depending on their concentrations and on the specific CRF receptor subtype(s) involved. This review aims to describe the interactions between CRF-related peptides and serotonergic systems, the consequences for stress-related behavior, and implications for vulnerability to anxiety and affective disorders.

Working memory- and anxiety-related behavioral effects of repeated nicotine as a stressor: the role of cannabinoid receptors

Background

Like emotional symptoms such as anxiety, modulations in working memory are among the frequently-reported but controversial psychiatric symptoms associated with nicotine (NC) administration. In the present study, repeated NC-induced modulations in working memory, along with concurrently-observed anxiety-related behavioral alterations, were investigated in mice, and compared with the effects of a typical cognition-impairing stressor, immobilization stress (IM). Furthermore, considering the structural and functional contributions of brain cannabinoid (CB) receptors in NC-induced psychiatric symptoms including emotional symptoms, the interactive effects of brain CB receptor ligands (CB ligands) and NC and/or IM on the working memory- and anxiety-related behaviors were examined.

Results

Statistically significant working memory impairment-like behavioral alterations in the Y-maze test and anxiety-like behavioral alterations in the elevated plus-maze (EPM) test were observed in the groups of mice treated with 0.8 mg/kg NC (subcutaneous (s.c.) 0.8 mg/kg treatment, 4 days) and/or IM (10 min treatment, 4 days). In the group of mice treated with NC plus IM (NC-IM group), an enhancement of the behavioral alterations was observed. Among the CB type 1 (CB1) antagonist AM 251 (AM), the non-selective CB agonist CP 55,940 (CP), and the CB1 partial agonist/antagonist virodhamine (VD), significant recovering effects were provided by AM (0.2-2.5 mg/kg) and VD (5 mg/kg) against the working memory impairment-like behaviors, whereas significant anxiolytic-like effects (recoveries from both attenuated percentage of entries into open arms and attenuated percentage of time spent on open arms) were provided by VD (1–10 mg/kg) and CP (2 mg/kg) against the anxiety-like behaviors.

Conclusions

Although working memory impairment- and anxiety-like behavioral alterations were commonly induced in the NC, IM, and NC-IM groups and the therapeutic involvement of CB receptors was shown, there were discrepancies in the types of effective CB ligands between the working memory- and anxiety-related behaviors. The differential involvements of CB receptor subtypes and indirectly activated neurotransmitter systems may contribute to these discrepancies.

5-HT2 ligands in the treatment of anxiety and depression

INTRODUCTION

One third of depressed patients do not respond adequately to conventional antidepressants including the selective serotonin reuptake inhibitors (SSRIs). Therefore, multi-target drugs or augmentation strategies have been developed for the management of SSRIs-resistant patients. In this context, the 5-HT(2) receptor subtypes represent promising targets but their precise roles have yet to be determined.

AREAS COVERED

The aim of this review is to shed some light on the preclinical evidence supporting the use of 5-HT(2A) and/or 5-HT(2C) receptor antagonists such as antipsychotics, as potential effective adjuncts in SSRIs-resistant depression. This review synthesizes the current literature about the behavioral, electrophysiological and neurochemical effects of 5-HT(2) receptors ligands on the monoaminergic systems but also on adult hippocampal neurogenesis.

EXPERT OPINION

Although studies support the hypothesis that the inactivation of 5-HT(2A) and/or 5-HT(2C) receptors might be of interest to reinforce different facets of the therapeutic activity of SSRIs, this pharmacological strategy remains debatable notably because of the lack of chronic data in relevant animal models. Conversely, emerging evidence suggests that the activation of 5-HT(2B) receptor is required for antidepressant-like activity, opening the way to new therapeutic approaches. However, the potential risks related to the enhancement of monoaminergic neurotransmissions could represent a major concern.

Envolvimento de receptores 5-HT2C do hipocampo ventral em comportamentos de defesa de ratos no labirinto em cruz elevado

A ativação farmacológica dos receptores 5-HT2C induz comportamentos de defesa em modelos animais. O estudo busca investigar se o bloqueio seletivo de receptores 5-HT2C no hipocampo ventral (HV) previne comportamentos defensivos induzidos por um agonista de receptor 5-HT2C administrado perifericamente em ratos expostos ao labirinto em cruz elevado (LCE). Quinze minutos após injeções intraperitoniais (IP, 1ml/kg) do agonista 5-HT2C WAY-161503, ratos foram microinjetados bilateralmente no HV com o antagonista seletivo de receptores 5-HT2C SB-242084 (0, 0,1, 0,5 ou 1.5μg). Dez minutos após, cada animal foi exposto ao LCE para o registro de categorias de ansiedade. Injeções sistêmicas do WAY-161503 reduziram seletivamente as explorações nos braços abertos e aumentaram padrões de avaliação de risco. Esse efeito foi atenuado de maneira dose-dependente pela microinjeção de SB-242084 no HV, confirmando a ação ansiogênica de agonistas 5-HT2C e sugerindo que esse perfil comportamental seja mediado, pelo menos em parte, por receptores 5-HT2C do HV.

Beneficial behavioural and neurogenic effects of agomelatine in a model of depression/anxiety

Agomelatine (S20098) is a novel antidepressant drug with melatonergic agonist and 5-HT2C receptor antagonist properties, displaying antidepressant/anxiolytic-like properties in animal models and in humans. In a depression/anxiety-like mouse model in which the response of the HPA axis is blunted, we investigated whether agomelatine could reverse behavioural deficits related to depression/anxiety compared to the classical selective serotonin reuptake inhibitor, fluoxetine. Adult mice were treated for 8 wk with either vehicle or corticosterone (35 μg/ml.d) via drinking water. During the final 4 wk, animals were treated with vehicle, agomelatine (10 or 40 mg/kg i.p.) or fluoxetine (18 mg/kg i.p.) and tested in several behavioural paradigms and also evaluated for home-cage activity. Our results showed that the depressive/anxiety-like phenotype induced by corticosterone treatment is reversed by either chronic agomelatine or fluoxetine treatment. Moreover, agomelatine increased the dark/light ratio of home-cage activity in vehicle-treated mice and reversed the alterations in this ratio induced by chronic corticosterone, suggesting a normalization of disturbed circadian rhythms. Finally, we investigated the effects of this new antidepressant on neurogenesis. Agomelatine reversed the decreased cell proliferation in the whole hippocampus in corticosterone-treated mice and increased maturation of newborn neurons in both vehicle- and corticosterone-treated mice. Overall, the present study suggests that agomelatine, with its distinct mechanism of action based on the synergy between the melatonergic agonist and 5-HT2C antagonist properties, provides a distinct antidepressant/anxiolytic spectrum including circadian rhythm normalization.

Serotonin-2C receptors in the basolateral nucleus of the amygdala mediate the anxiogenic effect of acute imipramine and fluoxetine administration

A growing body of evidence indicates that facilitation of serotonin-2C receptor (5-HT2CR)-mediated neurotransmission in the basolateral nucleus of the amygdala (BLA) is involved in anxiety generation. We investigated here whether BLA 5-HT2CRs exert a differential role in the regulation of defensive behaviours related to generalized anxiety (inhibitory avoidance) and panic (escape) disorders. We also evaluated whether activation of BLA 5-HT2CRs accounts for the anxiogenic effect caused by acute systemic administration of the antidepressants imipramine and fluoxetine. Male Wistar rats were tested in the elevated T-maze after intra-BLA injection of the endogenous agonist 5-HT, the 5-HT2CR agonist MK-212 or the 5-HT2CR antagonist SB-242084. This test allows the measurement of inhibitory avoidance acquisition and escape expression. We also investigated whether intra-BLA administration of SB-242084 interferes with the acute anxiogenic effect caused by imipramine and fluoxetine in the Vogel conflict test, and imipramine in the elevated T-maze. While intra-BLA administration of 5-HT and MK-212 facilitated inhibitory avoidance acquisition, suggesting an anxiogenic effect, SB-242084 had the opposite effect. None of these drugs affected escape performance. Intra-BLA injection of a sub-effective dose of SB-242084 fully blocked the anxiogenic effect caused either by the local microinjection of 5-HT or the systemic administration of imipramine and fluoxetine. Our findings indicate that 5-HT2CRs in BLA are selectively involved in the regulation of defensive behaviours associated with generalized anxiety, but not panic. The results also provide the first direct evidence that activation of BLA 5-HT2CRs accounts for the short-term aversive effect of antidepressants.

Evaluation of the emotional phenotype and serotonergic neurotransmission of fatty acid amide hydrolase-deficient mice

RATIONALE

By enhancing brain anandamide tone, inhibitors of fatty acid amide hydrolase (FAAH) induce anxiolytic-like effects in rodents and enhance brain serotonergic transmission. Mice lacking the faah gene (FAAH(-/-)) show higher anandamide levels. However, their emotional phenotype is still debated and their brain serotonergic tone remained unexplored.

OBJECTIVES AND METHODS

In this study, we tested FAAH(-/-) mice in the social interaction and the open field tests performed under different lighting conditions (dim and bright) since variations of the experimental context were proposed to explain opposite findings. Moreover, by microdialysis performed under dim light, we analyzed their serotonergic transmission in frontal cortex (FC) and ventral hippocampus (vHIPP).

RESULTS

In both light conditions, FAAH(-/-) mice showed reduced emotionality, compared to wt controls, as suggested by the increased rearing and reduced thigmotaxis displayed in the open field and by the longer time spent in social interaction. Basal serotonergic tone was higher in the FC of mutant mice as compared to control mice, while no difference was observed in the vHIPP. K(+)-induced depolarization produced similar increases of serotonin in both areas of both genotypes. An acute treatment with the CB1 antagonist rimonabant completely abolished the emotional phenotype of FAAH(-/-) mice and prevented the K(+)-stimulated release of serotonin in their FC and vHIPP, without producing any effect in wt mice.

CONCLUSIONS

Our results support the role of FAAH in the regulation of emotional reactivity and suggest that anandamide-mediated hyperactivation of CB1 is responsible for the emotional phenotype of FAAH(-/-) mice and for their enhanced serotonergic tone.

Region- and phase-dependent effects of 5-HT(1A) and 5-HT(2C) receptor activation on adult neurogenesis

Adult neurogenesis and serotoninergic transmission are associated to mood disorders and their treatments. The present study focused on the effects of chronic activation of 5-HT(1A) and 5-HT(2C) receptors on newborn cell survival in the dentate gyrus (DG) and olfactory bulb (OB), and examined whether potential neurogenic zones as the prefrontal cortex (PFC) and striatum (ST) are reactive to these treatments. Administration of 8-OH-DPAT, but not RO600,175 increases neurogenesis and survival of late differentiating cells (15-21days) in the DG. Both 8-OH-DPAT and RO600,175 increase neurogenesis in the OB, but only 8-OH-DPAT affected cell survival, inducing a parallel decrease in the number of BrdU cells in the OB and increase in the SVZ, which suggests an impaired migration. In the PFC and ST, 8-OH-DPAT and R0600,175 increase gliogenesis (NG2-labeled cells). This study provides new insights on the serotonergic regulation of critical phases of neurogenesis helpful to understand the neurogenic and gliogenic effects of antidepressant treatments in different brain regions.

Implication of the 5-HT2A and 5-HT2C (but not 5HT1A) receptors located within the periaqueductal gray in the elevated plus-maze test-retest paradigm in mice

A single exposure to the elevated plus-maze test (EPM) increases open arms avoidance and reduces or abolishes the anxiolytic-like effect of benzodiazepines assessed during a second trial, a phenomenon defined as "one-trial tolerance" (OTT). It has been emphasized that the dorsal portion of the midbrain periaqueductal gray (dPAG) plays a role on this enhanced aversion phenomenon in maze-experienced rodents. Given that intra-dPAG injections of a wide range of serotonergic 5-HT(1A), 5-HT(2A) and 5-HT(2C) receptor agonists produce anxiolytic-like effects in maze-naïve rodents, the present study examined the effects of the 5-HT(1A) receptor agonist 8-OH-DPAT (5.6 and 10.0nmol in 0.15microl) the preferential 5-HT(2A) receptor agonist DOI (2.0 and 8.0nmol in 0.1microl) and the preferential 5-HT(2C) receptor agonist MK-212 (21.2 and 63.6nmol in 0.1microl) microinjected into the dPAG prior to Trial 1 and Trial 2 on the behaviour of mice in the EPM. Test sessions were recorded and subsequently scored for anxiety-like behaviour (percentage of open arms entries and time) as well as general locomotor activity (closed arm entries). The results showed a lack of 8-OH-DPAT (5.6 and 10.0nmol) effect on the behaviour of maze-naïve and maze-experienced mice, while intra-dPAG microinfusions of DOI (8nmol) reduced anxiety-like behaviour only in maze-experienced mice that had received a similar treatment prior to Trial 1. Furthermore, intra-dPAG MK-212 (63.6nmol) showed an anxiolytic-like effect on both Trial 1 and Trial 2. Importantly, these effects were observed in the absence of any significant change in closed arm entries, the parameter considered to be a valid index of locomotor activity in the plus-maze. These results support the dPAG as a crucial structure involved in the neurobiology of the OTT phenomenon as well as accounting the role of the 5-HT(2A) and 5-HT(2C) receptors located within this midbrain structure on the emotional state induced by EPM test and retest paradigm mice.

Mechanisms contributing to the phase-dependent regulation of neurogenesis by the novel antidepressant, agomelatine, in the adult rat hippocampus

Agomelatine is a novel antidepressant acting as a melatonergic receptor agonist and serotonergic (5-HT(2C)) receptor antagonist. In adult rats, chronic agomelatine treatment enhanced cell proliferation and neurogenesis in the ventral hippocampus (VH), a region pertinent to mood disorders. This study compared the effects of agomelatine on cell proliferation, maturation, and survival and investigated the cellular mechanisms underlying these effects. Agomelatine increased the ratio of mature vs immature neurons and enhanced neurite outgrowth of granular cells, suggesting an acceleration of maturation. The influence of agomelatine on maturation and survival was accompanied by a selective increase in the levels of BDNF (brain-derived neurotrophic factor) vs those of VEGF (vascular endothelial factor) and IGF-1 (insulin-like growth factor 1), which were not affected. Agomelatine also activated several cellular signals (extracellular signal-regulated kinase1/2, protein kinase B, and glycogen synthase kinase 3beta) known to be modulated by antidepressants and implicated in the control of proliferation/survival. Furthermore, as agomelatine possesses both melatonergic agonist and serotonergic (5-HT(2C)) antagonist properties, we determined whether melatonin and 5-HT(2C) receptor antagonists similarly influence cell proliferation and survival. Only the 5-HT(2C) receptor antagonists, SB243,213 or S32006, but not melatonin, mimicked the effects of agomelatine on cell proliferation in VH. The promoting effect of agomelatine on survival was not reproduced by the 5-HT(2C) receptor antagonists or melatonin alone. However, it was blocked by a melatonin antagonist, S22153. These results show that agomelatine treatment facilitates all stages of neurogenesis and suggest that a joint effect of melatonin agonism and 5HT(2C) antagonism may be involved in promotion by agomelatine of survival in the hippocampus.

Upregulation of 5-HT2C receptors in hippocampus of pilocarpine-induced epileptic rats: antagonism by Bacopa monnieri

Emotional disturbances, depressive mood, anxiety, aggressive behavior, and memory impairment are the common psychiatric features associated with temporal lobe epilepsy (TLE). The present study was carried out to investigate the role of Bacopa monnieri extract in hippocampus of pilocarpine-induced temporal lobe epileptic rats through the 5-HT(2C) receptor in relation to depression. Our results showed upregulation of 5-HT(2C) receptors with a decreased affinity in hippocampus of pilocarpine-induced epileptic rats. Also, there was an increase in 5-HT(2C) gene expression and inositol triphosphate content in epileptic hippocampus. Carbamazepine and B. monnieri treatments reversed the alterations in 5-HT(2C) receptor binding, gene expression, and inositol triphosphate content in treated epileptic rats as compared to untreated epileptic rats. The forced swim test confirmed the depressive behavior pattern during epilepsy that was nearly completely reversed by B. monnieri treatment.

Behavioral profile and dorsal hippocampal cells in carioca high-conditioned freezing rats

Selection for contextual fear conditioning is an important behavioral paradigm for studying the role of genetic variables and their interaction with the surrounding environment in the etiology and development of anxiety disorders. Recently, a new line of animals selectively bred for high levels of freezing in response to contextual cues previously associated with footshock was developed from a Wistar population. The purpose of the present study was to evaluate the emotional and cognitive aspects of this new line of animals, which has been named Carioca High-Freezing (CHF). For the characterization of anxious behavior, CHF and control animals were tested in the elevated plus-maze (EPM) and the social interaction test. CHF animals were significantly more anxious than control rats in terms of both the number of entries into EPM open arms and the percentage of time spent in these arms. The time spent in social interaction behavior was also significantly decreased. No statistical differences were found in locomotor activity, as measured by both the number of entries into the closed arms of the EPM and the number of crossings into the social interaction test arena. No differences between CHF and control groups were found in the depression forced swimming test, suggesting that the anxiety trait selected in the CHF line did not interact with affective disorders traits such as those for depression. Cognitive aspects of the CHF rats were evaluated in the object recognition task. Results from this test indicated no difference between the two groups. The present study also encompassed histological analysis of the dorsal hippocampus from CHF and control animals. Results revealed an absence of qualitative and quantitative differences between these two groups of animals in cells located in the dentate gyrus, CA1, and CA3 areas. Therefore, future studies are required to further investigate the possible neural mechanisms involved in the origin and development of the anxious phenotype observed in this model.

Overexpression of 5-HT2C receptors in forebrain leads to elevated anxiety and hypoactivity

The 5-HT(2C) receptor has been implicated in mood and eating disorders. In general, it is accepted that 5-HT(2C) receptor agonists increase anxiety behaviours and induce hypophagia. However, pharmacological analysis of the roles of these receptors is hampered by the lack of selective ligands and the complex regulation of receptor isoforms and expression levels. Therefore, the exact role of 5-HT(2C) receptors in mood disorders remain controversial, some suggesting agonists and others suggesting antagonists may be efficacious antidepressants, while there is general agreement that antagonists are beneficial anxiolytics. In order to test the hypothesis that increased 5-HT(2C) receptor expression, and thus increased 5-HT(2C) receptor signalling, is causative in mood disorders, we have undertaken a transgenic approach, directly altering the 5-HT(2C) receptor number in the forebrain and evaluating the consequences on behaviour. Transgenic mice overexpressing 5-HT(2C) receptors under the control of the CaMKIIalpha promoter (C2CR mice) have elevated 5-HT(2C) receptor mRNA levels in cerebral cortex and limbic areas (including the hippocampus and amygdala), but normal levels in the hypothalamus, resulting in > 100% increase in the number of 5-HT(2C) ligand binding sites in the forebrain. The C2CR mice show increased anxiety-like behaviour in the elevated plus-maze, decreased wheel-running behaviour and reduced activity in a novel environment. These behaviours were observed in the C2CR mice without stimulation by exogenous ligands. Our findings support a role for 5-HT(2C) receptor signalling in anxiety disorders. The C2CR mouse model offers a novel and effective approach for studying disorders associated with 5-HT(2C) receptors.

Nicotine (NC)-induced "depressive" behavioral symptoms and effects of antidepressants including cannabinoids (CBs)

Depression is one of the frequently-observed psychiatric symptoms associated with nicotine (NC) use. In the present study, considering the unique effects of NC (e.g. antidepressant effects have also been reported), the time course of the NC-induced depressive behavioral alterations in a mouse model was compared with a typical depression-inducing stressor. Furthermore, based on the involvement of cannabinoid (CB) receptors in the behavioral effects of NC, the effects of antidepressants including CB ligands (CBs) against the NC-induced behavioral alterations were also investigated. Repeated subcutaneous NC treatments (0.3 mg/kg, 4 days), like repeated immobilization stress (IM) treatments (10 min, 4 days), caused prolonged depressive effects (increased immobility time) at both 2 hr and 1 day time points after the last treatment in the tail suspension test. However, in the NC group, depressive effects (suppressed swimming behaviors) were observed only at the 2 hr time point in the forced swimming test. The antidepressants amitriptyline, clomipramine and fluvoxamine, the endogenous mixed CB agonist/antagonist virodhamine and the anandamide-like cannabimimetic O-2093 provided antagonistic effects against the depressive behaviors in the tail suspension test. However, in the forced swimming test, NC-induced depressive behaviors were antagonized only by the CBs virodhamine and O-2093. The present results demonstrated depressive effects of NC in two typical behavioral tests, which support the risk of repeated NC use. The shortened behavioral alterations in the forced swimming test, as compared to the IM group, seemed to reflect the neuronal modifications peculiar to NC, which are antagonized by some CBs.

Effects of systemic 5-HT(1B) receptor compounds on ventral tegmental area intracranial self-stimulation thresholds in rats

Serotonin 1B (5-HT(1B)) receptors may play a role in regulating motivation and reward-related behaviours. To date, no studies have investigated the effects of the highly selective 5-HT(1B) receptor agonist CP 94253, on the reward model of ventral tegmental area intracranial self-stimulation. The current study investigated the hypothesis that 5-HT(1B) receptors play an inhibitory role in ventral tegmental area ICSS. Using Sprague-Dawley rats, the effects of the selective 5-HT(1B) receptor agonist CP 94253 (0-5.0 mg/kg) and the 5-HT(1B/1D) receptor antagonist GR 127935 (10.0 mg/kg) were investigated in rats trained to respond for ventral tegmental area ICSS; results were compared using rate-frequency threshold analysis. The highest dose of CP 94253 (5.0 mg/kg) tested in ventral tegmental area ICSS produced an increase in rate-frequency thresholds without affecting maximal response rates. This effect was attenuated by GR 127935 which did not show any effects when administered alone. These results suggest that 5-HT(1B) receptors play an inhibitory role in regulating ventral tegmental area ICSS.

S32006, a novel 5-HT2C receptor antagonist displaying broad-based antidepressant and anxiolytic properties in rodent models

RATIONALE

Serotonin (5-HT)(2C) receptors are implicated in the control of mood, and their blockade is of potential interest for the management of anxiodepressive states.

OBJECTIVES

Herein, we characterized the in vitro and in vivo pharmacological profile of the novel benzourea derivative, S32006.

MATERIALS AND METHODS

Standard cellular, electrophysiological, neurochemical, and behavioral procedures were used.

RESULTS

S32006 displayed high affinity for human (h)5-HT(2C) and h5-HT(2B) receptors (pK (i)s, 8.4 and 8.0, respectively). By contrast, it had negligible (100-fold lower) affinity for h5-HT(2A) receptors and all other sites examined. In measures of Gq-protein coupling/phospholipase C activation, S32006 displayed potent antagonist properties at h5-HT(2C) receptors (pK (B) values, 8.8/8.2) and h5-HT(2B) receptors (7.8/7.7). In vivo, S32006 dose-dependently (2.5-40.0 mg/kg, i.p. and p.o.) abolished the induction of penile erections and a discriminative stimulus by the 5-HT(2C) receptor agonist, Ro60,0175, in rats. It elevated dialysis levels of noradrenaline and dopamine in the frontal cortex of freely moving rats, and accelerated the firing rate of ventrotegmental dopaminergic and locus ceruleus adrenergic neurons. At similar doses, S32006 decreased immobility in a forced-swim test in rats, reduced the motor depression elicited by 5-HT(2C) and alpha(2)-adrenoceptor agonists, and inhibited both aggressive and marble-burying behavior in mice. Supporting antidepressant properties, chronic (2-5 weeks) administration of S32006 suppressed "anhedonia" in a chronic mild stress procedure and increased both expression of BDNF and cell proliferation in rat dentate gyrus. Finally, S32006 (0.63-40 mg/kg, i.p. and p.o) displayed anxiolytic properties in Vogel conflict and social interaction tests in rats.

CONCLUSION

S32006 is a potent 5-HT(2C) receptor antagonist, and possesses antidepressant and anxiolytic properties in diverse rodent models.

The effects of intra-cerebral drug infusions on animals' unconditioned fear reactions: a systematic review

Intra-cerebral (i.c.) microinfusion of selective receptor agonists and antagonists into behaving animals can provide both neuroanatomical and neurochemical insights into the neural mechanisms of anxiety. However, there have been no systematic reviews of the results of this experimental approach that include both a range of unconditioned anxiety reactions and a sufficiently broad theoretical context. Here we focus on amino acid, monoamine, cholinergic and peptidergic receptor ligands microinfused into neural structures previously implicated in anxiety, and subsequent behavioral effects in animal models of unconditioned anxiety or fear. GABAA receptor agonists and glutamate receptor antagonists produced the most robust anxiolytic-like behavioral effects, in the majority of neural substrates and animal models. In contrast, ligands of the other receptor systems had more selective, site-specific anti-anxiety effects. For example, 5-HT1A receptor agonists produced anxiolytic-like effects in the raphe nuclei, but inconsistent effects in the amygdala, septum, and hippocampus. Conversely, 5-HT3 receptor antagonists produced anxiolytic-like effects in the amygdala but not in the raphe nuclei. Nicotinic receptor agonists produced anxiolytic-like effects in the raphe and anxiogenic effects in the septum and hippocampus. Unexpectedly, physostigmine, a general cholinergic agonist, produced anxiolytic-like effects in the hippocampus. Neuropeptide receptors, although they are popular targets for the development of selective anxiolytic agents, had the least reliable effects across different animal models and brain structures, perhaps due in part to the fact that selective receptor ligands are relatively scarce. While some inconsistencies in the microinfusion data can easily be attributed to pharmacological variables such as dose or ligand selectivity, in other instances pharmacological explanations are more difficult to invoke: e.g., even the same dose of a known anxiolytic compound (midazolam) with a known mechanism of action (the benzodiazepine-GABAA receptor complex), can selectively affect different fear reactions depending upon the different subregions of the nucleus into which it is infused (CeA versus BLA). These particular functional dissociations are important and may depend on the ability of a GABAA receptor agonist to interact with distinct isoforms and combinations of GABAA receptor subunits (e.g., alpha1-6, beta1-3, Upsilon1-2, delta), many of which are unevenly distributed throughout the brain. Although this molecular hypothesis awaits thorough evaluation, the microinfusion data overall give some support for a model of "anxiety" that is functionally segregated along different levels of a neural hierarchy, analogous in some ways to the organization of sensorimotor systems.

Prenatal restraint stress generates two distinct behavioral and neurochemical profiles in male and female rats

Prenatal Restraint Stress (PRS) in rats is a validated model of early stress resulting in permanent behavioral and neurobiological outcomes. Although sexual dimorphism in the effects of PRS has been hypothesized for more than 30 years, few studies in this long period have directly addressed the issue. Our group has uncovered a pronounced gender difference in the effects of PRS (stress delivered to the mothers 3 times per day during the last 10 days of pregnancy) on anxiety, spatial learning, and a series of neurobiological parameters classically associated with hippocampus-dependent behaviors. Adult male rats subjected to PRS (“PRS rats”) showed increased anxiety-like behavior in the elevated plus maze (EPM), a reduction in the survival of newborn cells in the dentate gyrus, a reduction in the activity of mGlu1/5 metabotropic glutamate receptors in the ventral hippocampus, and an increase in the levels of brain-derived neurotrophic factor (BDNF) and pro-BDNF in the hippocampus. In contrast, female PRS rats displayed reduced anxiety in the EPM, improved learning in the Morris water maze, an increase in the activity of mGlu1/5 receptors in the ventral and dorsal hippocampus, and no changes in hippocampal neurogenesis or BDNF levels. The direction of the changes in neurogenesis, BDNF levels and mGlu receptor function in PRS animals was not consistent with the behavioral changes, suggesting that PRS perturbs the interdependency of these particular parameters and their relation to hippocampus-dependent behavior. Our data suggest that the epigenetic changes in hippocampal neuroplasticity induced by early environmental challenges are critically sex-dependent and that the behavioral outcome may diverge in males and females.