Activation Of 5-HT1A Receptors Research Articles

Anxiolytic- and antidepressant-like effects of new phenylpiperazine derivative LQFM005 and its hydroxylated metabolite in mice

The current treatments available for anxiety and depression are only palliative. Full remission has remained elusive, characterizing unmet medical needs. In the scope of an academic drug discovery program, we describe here the design, synthesis, in vitro metabolism prediction and pharmacological characterization of a new piperazine compound, 1-(4-methoxyphenyl)−4-((1-phenyl-1H-pyrazol-4-yl)methyl)piperazine (LQFM005), and of its main putative metabolite, 4-(4-((4-(4-methoxyphenyl)piperazin-1-yl)methyl)− 1H-pyrazol-1-yl)phenol (LQFM235). The production of the metabolite was initially performed by in vitro biotransformation of LQFM005 using Aspergillus candidus and then by chemical synthesis. Oral administration of either 12 or 24 µmol/kg LQFM005 to mice did not affect spontaneous locomotor activity but increased the time spent in the center of the open field. Both LQFM005 and LQFM235 (24 µmol/kg) increased the time spent by the mice in the open arms of the elevated plus maze (EPM), a good indication of anxiolytic-like effect, and decreased the immobility time in the forced swimming test (FST), suggesting an antidepressant-like effect. The previous administration of WAY-100635 (a 5-HT1A antagonist) abolished the effects of LQFM005 in both EPM and FST. Binding experiments showed that LQFM005 and its metabolite bind to the 5-HT1A receptor with a moderate affinity (Ki around 5–9 µM). The two compounds are relatively safe, as indicated by cytotoxic assessment using the 3T3 fibroblast cell line and estimated LD50 around 600 mg/kg. In conclusion, oral administration of the newly synthesized phenylpiperazines produced anxiolytic- and antidepressant-like effects in behavioral tests, putatively in part through the activation of 5-HT1A receptors.

5-HT1A receptors mediate the analgesic effect of rosavin in a mouse model of oxaliplatin-induced peripheral neuropathic pain

Oxaliplatin, a third-generation platinum derivative, is the mainstay of current antineoplastic medications for advanced colorectal cancer therapy. However, peripheral neuropathic complications, especially cold allodynia, undermine the life-prolonging outcome of this anti-cancer agent. Rosavin, a phenylpropanoid derived originally from Rhodiola rosea, exhibits a wide range of therapeutic properties. The present study explored whether and how rosavin alleviates oxaliplatin-induced cold hypersensitivity in mice. In the acetone drop test, cold allodynia behavior was observed from days 3 to 5 after a single injection of oxaliplatin (6 mg/kg, i.p.). Cold allodynia was significantly attenuated following rosavin treatment (10 mg/kg, i.p.). Specific endogenous 5-HT depletion by three consecutive pretreatments with para-chlorophenylalanine (150 mg/kg/day, i.p.) abolished the analgesic action of rosavin; this effect was not observed following pretreatment with naloxone (opioid receptor antagonist, 10 mg/kg, i.p.). Furthermore, 5-HT1A receptor antagonist WAY-100635 (0.16 mg/kg, i.p.), but not 5-HT3 receptor antagonist MDL-72222 (1 mg/kg, i.p.), blocked rosavin-induced analgesia. These results suggest that rosavin may provide a novel approach to alleviate oxaliplatin-induced cold allodynia by recruiting the activity of 5-HT1A receptors.

Serotonergic-Muscarinic Interaction within the Prefrontal Cortex as a Novel Target to Reverse Schizophrenia-Related Cognitive Symptoms.

Recent studies revealed that the activation of serotonergic 5-HT1A and muscarinic M1, M4, or M5 receptors prevent MK-801-induced cognitive impairments in animal models. In the present study, the effectiveness of the simultaneous activation of 5-HT1A and muscarinic receptors at preventing MK-801-induced cognitive deficits in novel object recognition (NOR) or Y-maze tests was investigated. Activators of 5-HT1A (F15599), M1 (VU0357017), M4 (VU0152100), or M5 (VU0238429) receptors administered at top doses for seven days reversed MK-801-induced deficits in the NOR test, similar to the simultaneous administration of subeffective doses of F15599 (0.05 mg/kg) with VU0357017 (0.15 mg/kg), VU0152100 (0.05 mg/kg), or VU0238429 (1 mg/kg). The compounds did not prevent the MK-801-induced impairment when administered acutely. Their activity was less evident in the Y-maze. Pharmacokinetic studies revealed high brain penetration of F15599 (brain/plasma ratio 620%), which was detected in the frontal cortex (FC) up to 2 h after administration. Decreases in the brain penetration properties of the compounds were observed after acute administration of the combinations, which might have influenced behavioral responses. This negative effect on brain penetration was not observed when the compounds were administered repeatedly. Based on our results, prolonged administration of a 5-HT1A activator with muscarinic receptor ligands may be effective at reversing cognitive decline related to schizophrenia, and the FC may play a critical role in this interaction.

A Complex Impact of Systemically Administered 5-HT2A Receptor Ligands on Conditioned Fear.

BackgroundThough drugs binding to serotonergic 5-HT2A receptors have long been claimed to influence human anxiety, it remains unclear if this receptor subtype is best described as anxiety promoting or anxiety dampening. Whereas conditioned fear expressed as freezing in rats is modified by application of 5-HT2A–acting drugs locally into different brain regions, reports on the effect of systemic administration of 5-HT2A receptor agonists and 5-HT2A antagonists or inverse agonists on this behavior remain sparse.MethodsWe assessed the possible impact of systemic administration of 5-HT2A receptor agonists, 5-HT2A receptor inverse agonists, and a selective serotonin reuptake inhibitor (SSRI)—per se or in combination—on the freezing displayed by male rats when re-exposed to a conditioning chamber in which they received foot shocks 7 days earlier.ResultsThe 5-HT2A receptor agonists psilocybin and 25CN-NBOH induced a reduction in conditioned fear that was countered by pretreatment with 5-HT2A receptor inverse agonist MDL 100907. While both MDL 100907 and another 5-HT2A receptor inverse agonist, pimavanserin, failed to impact freezing per se, both compounds unmasked a robust fear-reducing effect of an SSRI, escitalopram, which by itself exerted no such effect.ConclusionsThe results indicate that 5-HT2A receptor activation is not a prerequisite for normal conditioned freezing in rats but that this receptor subtype, when selectively over-activated prior to expression, exerts a marked fear-reducing influence. However, in the presence of an SSRI, the 5-HT2A receptor, on the contrary, appears to counter an anti-freezing effect of the enhanced extracellular serotonin levels following reuptake inhibition.

Potential Link between T102C Polymorphism in the Serotonin Receptors (5-HT2A) Gene and Treatment Response of Risperidone on Schizophrenia

BACKGROUND: Schizophrenia affects 1% of population and its molecular etiology remains enigmatic despite enormous study. Approximately one-third of patients failed to respond with treatment. The 5-HT2A receptor appears to be one of important site of action of atypical antipsychotic drugs. It also has been suggested that the T102C gene polymorphism alters promoter activity and expression of 5-HT2A receptors and might be responsible for the associations with the efficacy of typical antipsychotics. AIM: In this study, we aimed to evaluate the potential link between T102C polymorphism in the Serotonin Receptors (5-HT2A) and their response to risperidone. METHODS: We studied 100 schizophrenia patients and 100 healthy volunteers as a comparison of 5-HT2A receptors gene polymorphism distribution, which were all Indonesian. The peripheral blood samples were obtained from all participants. The patients assessed by Positive and Negative Scale (PANSS) and Clinical Global Impression Scale (CGI) when admitted to the hospital. Clinical improvements then assessed 5 times in 4 weeks (when transferred to sub-acute ward, at the 1st, the 2nd, the 3rd, and the 4th week). To analyze the T102C polymorphism of 5-HT2A receptor gene, we used an allele specific polymerase chain reaction based restriction fragment length polymorphism (PCR-RFLP) method. RESULTS: In this study, there were three various polymorphisms of T102C in the serotonin receptor (5-HT2A) gene: T/T, C/C, and T/C. The patients who had homozygous for T/T genotypes were found to give better improvement to risperidone than the patients who were C/C and T/C genotypes (P:0.001). Moreover, the patients with C allele genotype T/C and C/C had minimal response (P:0.001). We found no significant difference distribution of genotypes between schizophrenia patients and healthy volunteers (P:0.498). CONCLUSION: Our result supported that there was a potential link between T102C polymorphism in the serotonin receptor (5-HT2A) gene and treatment response of risperidone. It suggested the importance of genetic screening such as examination of the Serotonin Receptor 5-HT2A (T102C) gene polymorphism to be performed to optimize therapeutic strategies with antipsychotic.

Dorsal raphe nucleus to anterior cingulate cortex 5-HTergic neural circuit modulates consolation and sociability.

Consolation is a common response to the distress of others in humans and some social animals, but the neural mechanisms underlying this behavior are not well characterized. By using socially monogamous mandarin voles, we found that optogenetic or chemogenetic inhibition of 5-HTergic neurons in the dorsal raphe nucleus (DR) or optogenetic inhibition of serotonin (5-HT) terminals in the anterior cingulate cortex (ACC) significantly decreased allogrooming time in the consolation test and reduced sociability in the three-chamber test. The release of 5-HT within the ACC and the activity of DR neurons were significantly increased during allogrooming, sniffing, and social approaching. Finally, we found that the activation of 5-HT1A receptors in the ACC was sufficient to reverse consolation and sociability deficits induced by the chemogenetic inhibition of 5-HTergic neurons in the DR. Our study provided the first direct evidence that DR-ACC 5-HTergic neural circuit is implicated in consolation-like behaviors and sociability.

Novel and atypical pathways for serotonin signaling.

Serotonin (5-HT) appeared billions of years before 5-HT receptors and synapses. It is thus not surprising that 5-HT can control biological processes independently of its receptors. One example is serotonylation, which consists of covalent binding of 5-HT to the primary amine of glutamine. Over the past 20 years, serotonylation has been involved in the regulation of many signaling mechanisms. One of the most striking examples is the recent evidence that serotonylation of histone H3 constitutes an epigenetic mark. However, the pathophysiological role of histone H3 serotonylation remains to be discovered. All but one of the 5-HT receptors are G-protein-coupled receptors (GPCRs). The signaling pathways they control are finely tuned, and new, unexpected regulatory mechanisms are being uncovered continuously. Some 5-HT receptors (5-HT2C, 5-HT4, 5-HT6, and 5-HT7) signal through mechanisms that require neither G-proteins nor β-arrestins, the two classical and almost universal GPCR signal transducers. 5-HT6 receptors are constitutively activated via their association with intracellular GPCR-interacting proteins (GIPs), including neurofibromin 1, cyclin-dependent kinase 5 (Cdk5), and G-protein-regulated inducer of neurite outgrowth 1 (GPRIN1). Interactions of 5-HT6 receptor with Cdk5 and GPRIN1 are not concomitant but occur sequentially and play a key role in dendritic tree morphogenesis. Furthermore, 5-HT6 receptor-mediated G-protein signaling in neurons is different in the cell body and primary cilium, where it is modulated by smoothened receptor activation. Finally, 5-HT2A receptors form heteromers with mGlu2 metabotropic glutamate receptors. This heteromerization results in a specific phosphorylation of mGlu2 receptor on a serine residue (Ser843) upon agonist stimulation of 5-HT2A or mGlu2 receptor. mGlu2 receptor phosphorylation on Ser843 is an essential step in engagement of Gi/o signaling not only upon mGlu2 receptor activation but also following 5-HT2A receptor activation, and thus represents a key molecular event underlying functional crosstalk between both receptors.

Eupnea and gasping in vivo are facilitated by the activation of 5-HT2A receptors.

Eupnea and gasping in infancy depend on central nervous system (CNS) serotonin (5-hydroxytryptamine; 5-HT). Although previous in vitro preparations have provided some evidence that 5-HT acts through type 2 A receptors (5-HT2A) to facilitate eupnea and gasping, here the hypothesis addressed is that 5-HT2A receptor activation is necessary for eupnea and the proper generation of gasping in vivo. To test this, we administered 2,5-dimethoxy-4-iodoamphetamine (DOI; 0.25 mg/kg i.p.), a 5-HT2A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.25 mg/kg i.p.), a 5-HT1A agonist, or vehicle (saline) to 7-9-day-old tryptophan hydroxylase 2 knockout (TPH2-/-) mice. A second experiment assessed the effect of MDL-11,939 (MDL; 10 mg/kg i.p.), the specific 5-HT2A antagonist, or vehicle (DMSO) on the gasping of wild-type (TPH2+/+) animals. Drugs were given 15 min prior to five episodes of severe hypoxia that elicited gasping. TPH2-/- breathed more slowly but had the same V̇e and V̇e/V̇o2 compared with TPH2+/+. As previously reported, the gasping of TPH2-/- was significantly delayed (P < 0.001) and occurred at a significantly lower frequency compared with TPH2+/+ (P = 0.04). For both genotypes, DOI hastened eupneic frequency but had no effect on V̇e or V̇e/V̇o2. The gasping of TPH2-/-, although unaffected by 8-OH-DPAT, was indistinguishable from the gasping of TPH2+/+ following DOI. In TPH2+/+, application of MDL led to hypoventilation (P = 0.01), a delay in the appearance of gasping (P = 0.005), and reduced gasp frequency (P = 0.05). These data show that, in vivo, 5-HT2A receptors facilitate both eupnea and gasping. As has been shown in vitro, 5-HT2A probably promotes gasping by exciting hypoxia-resistant pacemaker neurons.NEW & NOTEWORTHY Previous in vitro studies suggest that 5-HT2A receptors contribute to eupnea and are necessary for fictive gasping. The current study shows that the impaired gasping displayed by neonatal TPH2-/- mice, deficient in CNS serotonin, is restored by 5-HT2A receptor activation. Following 5-HT2A blockade, wild-type mice hypoventilated and their gasping resembled that of TPH2-/- mice. This study shows that both eupnea and gasping in vivo rely on the activation of 5-HT2A receptors.

Cerebellar 5HT-2A receptor mediates stress-induced onset of dystonia.

Stress is a key risk factor for dystonia, a debilitating motor disorder characterized by cocontractions of muscles leading to abnormal body posture. While the serotonin (5HT) system is known to control emotional responses to stress, its role in dystonia remains unclear. Here, we reveal that 5HT neurons in the dorsal raphe nuclei (DRN) send projections to the fastigial deep cerebellar nuclei (fDCN) and that photostimulation of 5HT-fDCN induces dystonia in wild-type mice. Moreover, we report that photoinhibition of 5HT-fDCN reduces dystonia in a1A tot/tot mice, a genetic model of stress-induced dystonia, and administration of a 5HT-2A receptor inverse agonist (MDL100907; 0.1 to 1 mg/kg) or shRNA-mediated knockdown of the ht2ar gene in fDCN can notably reduce the onset of dystonia in a1A tot/tot mice. These results support the serotonin theory of dystonia and suggest strategies for alleviating symptoms in human patients by blocking 5HT-2A receptors.

Injection of galanin into the dorsal hippocampus impairs emotional memory independent of 5-HT1A receptor activation

There is evidence that interaction between the neuropeptide galanin and the 5-HT1A receptor represents an integrative mechanism in the regulation of serotonergic neurotransmission. Thus, in rats intracerebroventricular (i.c.v.) galanin did not impair retention in the passive avoidance (PA) test 24 h after training, but attenuated the retention deficit caused by subcutaneous (s.c.) administration of the 5-HT1A receptor agonist 8-OH-DPAT. This impairment has been linked to postsynaptic 5-HT1A receptor activation. To confirm these results in mice, galanin was infused i.c.v. (1 nmol/mouse) in C57BL/6/Bkl mice 30 min prior to training followed by s.c. injection (0.3 mg/kg) of 8-OH-DPAT or saline 15 min before PA training. In line with previous results, i.c.v. galanin significantly attenuated the PA impairment caused by 5-HT1A receptor activation in mice. To study if the galanin 5-HT1A receptor interaction involved the dorsal hippocampus, galanin (1 nmol/mouse) was directly infused into this brain region alone or in combination with s.c. 8-OH-DPAT. However, unlike i.c.v. galanin, galanin infusion into the dorsal hippocampus alone impaired PA retention and failed to attenuate the 8-OH-DPAT-mediated PA impairment. These results indicate that the ability of i.c.v. galanin to modify 5-HT1A receptor activation is not directly mediated via receptor interactions in the dorsal hippocampus. Instead, the galanin-mediated PA impairment suggests an important inhibitory role of galanin receptors in the dorsal hippocampus for acquisition (encoding) and/or consolidation of emotional memory. In addition, the interaction between galanin and 5-HT1A receptors probably involves a wide serotonergic network that is important for the integration of emotional and cognitive behaviors.

Behavioral arrest and a characteristic slow waveform are hallmark responses to selective 5-HT2A receptor activation

Perception, emotion, and mood are powerfully modulated by serotonin receptor (5-HTR) agonists including hallucinogens. The 5-HT2AR subtype has been shown to be central to hallucinogen action, yet the precise mechanisms mediating the response to 5-HT2AR activation remain unclear. Hallucinogens induce the head twitch response (HTR) in rodents, which is the most commonly used behavioral readout of hallucinogen pharmacology. While the HTR provides a key behavioral signature, less is known about the meso level changes that are induced by 5-HT2AR activation. In response to administration of the potent and highly selective 5-HT2AR agonist 25I-NBOH in mice, we observe a disorganization of behavior which includes frequent episodes of behavioral arrest that consistently precede the HTR by a precise interval. By combining behavioral analysis with electroencephalogram (EEG) recordings we describe a characteristic pattern composed of two distinctive EEG waveforms, Phase 1 and Phase 2, that map onto behavioral arrest and the HTR respectively, with the same temporal separation. Phase 1, which underlies behavioral arrest, is a 3.5–4.5 Hz waveform, while Phase 2 is slower at 2.5–3.2 Hz. Nicotine pretreatment, considered an integral component of ritualistic hallucinogen practices, attenuates 25I-NBOH induced HTR and Phase 2 waveforms, yet increases behavioral arrest and Phase 1 waveforms. Our results suggest that in addition to the HTR, behavioral arrest and characteristic meso level slow waveforms are key hallmarks of the response to 5-HT2AR activation. Increased understanding of the response to serotonergic hallucinogens may provide mechanistic insights into perception and hallucinations, as well as regulation of mood.

Cortical influences of serotonin and glutamate on layer V pyramidal neurons.

Layer V pyramidal neurons constitute principle output neurons of the medial prefrontal cortex (mPFC)/neocortex to subcortical regions including the intralaminar/midline thalamic nuclei, amygdala, basal ganglia, brainstem nuclei and the spinal cord. The effects of 5-hydroxytryptamine (5-HT) on layer V pyramidal cells primarily reflect a range of excitatory influences through 5-HT2A receptors and inhibitory influences through non-5-HT2A receptors, including 5-HT1A receptors. While the 5-HT2A receptor is primarily a postsynaptic receptor on throughout the apical dendritic field of 5-HT2A receptors, activation of a minority of 5-HT2A receptors also appears to increase spontaneous excitatory postsynaptic currents/potentials (EPSCs/EPSPs) via a presynaptic effect on thalamocortical terminals arising from the midline and intralaminar thalamic nuclei. Activation of 5-HT2A receptors by the phenethylamine hallucinogen also appears to increase asynchronous release of glutamate upon the layer V pyramidal dendritic field, an effect that is suppressed by 5-HT itself through non-5-HT2A receptors. Serotonergic hallucinogens acting on 5-HT2A receptors also appears to increase gene expression of immediate early genes (iEG) and other receptors appearing to induce an iEG-like response like BDNF. Psychedelic hallucinogens acting on 5-HT2A receptors also induce head twitches in rodents that appear related to induction of glutamate release. These electrophysiological, biochemical and behavioral effects of serotonergic hallucinogens appear to be related to modulating glutamatergic thalamocortical neurotransmission and/or shifting the balance toward 5-HT2A receptor activation and away from non-5-HT2A receptor activation. These 5-HT2A receptor induced responses are modulated by feedback homeostatic mechanisms through mGlu2, mGlu4, and mGlu8 presynaptic receptors on thalamocortical terminals. These 5-HT2A receptor and glutamatergic interactions also appear to play a role on higher cortical functions of the mPFC such as motoric impulsivity and antidepressant-like behavioral responses on the differential-reinforcement-of low rate 72-s (DRL 72-s schedule). These mutually opposing effects between 5-HT2A receptor and mGlu autoreceptor activation (e.g., blocking 5-HT2A receptors and enhancing activity at mGlu2 receptors) may play a clinical role with respect to currently prescribed or novel antidepressant drugs. Thus, there is an important balance between 5-HT2A receptor activation and activation of mGlu autoreceptors on prefrontal cortical layer V pyramidal cells with respect to the electrophysiological, biochemical and behavioral effects serotonergic hallucinogenic drugs.

Analgesic characteristics of a newly developed α2δ ligand, mirogabalin, on inflammatory pain.

Mirogabalin is a novel α2δ ligand approved in Japan for the treatment of peripheral neuropathic pain. However, the sites of action of α2δ ligands to produce analgesic effects on inflammatory pain remain unclear. In this study, we investigated the analgesic effect and site of action of mirogabalin using the rat formalin test, an acute inflammatory pain model. Mirogabalin was administered orally, intrathecally, and intracerebroventricularly. Open field tests were performed to evaluate the effect of oral-, intrathecally, and intracerebroventricularly administered mirogabalin on locomotor activity and orientation ability. Oral mirogabalin produced an analgesic effect when the formalin test was performed 4 h, but not 1 or 2 h, after oral administration. Intrathecal, but not intracerebroventricular, administration of mirogabalin produced analgesic effects when mirogabalin was administered 10 min before formalin injection. These analgesic effects were not antagonized by idazoxan, an α2 adrenergic antagonist; WAY100135, a 5-HT1A antagonist; or naloxone, an opioid receptor antagonist. Mirogabalin attenuated moving distances 1 and 2 h after oral administration and 10 min after intracerebroventricular administration, but not 10 min after intrathecal administration. In the oral administration group, the time course of the analgesic effect was different from that of moving distance. In the intracerebroventricular group, mirogabalin attenuated moving distances but did not produce an analgesic effect. In the intrathecal group, mirogabalin produced an analgesic effect but did not affect moving distances. These findings suggest that the analgesic effect of mirogabalin on the rat formalin test is mediated by spinal action and not by the activation of α2, 5-HT1A, or opioid receptors, and that the inhibitory effect of mirogabalin on moving distances is mediated by the supraspinal brain.

Evaluation of stress resilience using automatic hole-board test

The automatic hole-board test is an experimental method that can serve as a useful tool for objectively evaluating the changes in various emotional states of mice. Previously, we found that pretreatment of mice with 5-HT1A receptor agonists 24 hr before testing resulted in the development of stress resistance, i.e. the decrease in head-dipping behaviors in the hole-board test induced by exposure to acute restraint stress is suppressed. In addition, we also created stress-adaptive and -maladaptive models in mice based on the diversity of changes in head-dipping behavior in the hole-board test induced by repeated exposure to different levels of restraint stress. Biochemical studies investigating the role of the brain 5-HT nervous system in the development of stress adaptation found an increase in the tryptophan hydroxylase, an enzyme of 5-HT synthesis, in stress-adaptive mice. Furthermore, pharmacological activation of 5-HT1A receptors by selective agonist promoted the development of stress adaptation in stress-maladapted mice. These findings suggest that the brain 5-HT nervous system could play an important role in the stress resilience. In this symposium, we would like to introduce and discuss the recent findings from studies using these animal models, which were created on the basis of the hole-board test.

The action of aripiprazole and brexpiprazole at the receptor level in singultus.

The hiccup (Latin singultus) is an involuntary periodic contraction of the diaphragm followed by glottic closure, which can be a rare side effect of aripiprazole. In contrast to the structurally closely related aripiprazole, brexpiprazole was not associated with this particular adverse drug reaction. Having two very similar drugs that differ in their ability to induce hiccups represents a unique opportunity to gain insight into the receptors involved in the pathophysiology of the symptom and differences in clinical effects between aripiprazole and brexpiprazole. The overlap between maneuvers used to terminate paroxysmal supraventricular tachycardia and those employed to terminate bouts of hiccups suggests that activation of efferent vagal fibers can be therapeutic in both instances. Recent work seems to support a pivotal role for serotonin receptors in such vagal activation. It is unlikely that a unique receptor-drug interaction could explain the different effects of the examined drugs on hiccup. The different effect is most likely the consequence of several smaller effects at more than one receptor. Brexpiprazole is a highly affine (potent) α2C antagonist and, therefore, also an indirect 5-HT1A agonist. In contrast, aripiprazole is a partial 5-HT1A agonist (weak antagonist) and an HT3 antagonist. Activation of 5-HT1A receptors enhances vagal activity while HT3 blockade reduces it. Vagus nerve activation is therapeutic for hiccups. A definitive answer continues to be elusive.

The protective effect of flesinoxan on myelin damage caused by maladaptation to stress in mice

Our previous study demonstrated that the maintenance of hippocampal myelination contributes to the development of stress adaptation. Further, we showed that 5-HT1A receptor in the hippocampus may be involved in the protection of major myelin proteins loss induced by unadaptable excessive stress. However, the mechanism by which activation of 5-HT1A receptor promote myelination remains unknown. In the present study, we investigated the protective effects of 5-HT1A receptor agonist flesinoxan on unadaptable stress-induced myelin damage and its molecular mechanisms in mice. Western blot analysis revealed that administration of flesinoxan significantly increased the expression level of BDNF, p-ERK, p-CREB, and Olig2 in the hippocampus of stress-maladaptive mice. Furthermore, in the immunohistochemical analysis, a significant increase in the number of Olig2+/BrdU+ positive cells were observed in the dentate gyrus of the hippocampus of flesinoxan-treated, but not vehicle-treated, stress-maladaptive mice. The present findings indicate that 5-HT1A receptor-mediated enhancement of oligogenesis via activation of ERK/CREB/BDNF signaling pathways may be involved in the protection of myelin loss induced by unadaptable excessive stress.

Investigating the role of 5-HT2A and 5-HT2C receptor activation in the effects of psilocybin, DOI, and citalopram on marble burying in mice

Psychedelic drugs acting as 5-hydroxyptryptamine 2A receptor (5-HT2AR) agonists have shown promise as viable treatments of psychiatric disorders, including obsessive-compulsive disorder. The marble burying test is a test of compulsive-like behavior in mice, and psychedelics acting as 5-HT2AR agonists can reduce digging in this test. We assessed the 5-HT2R contribution to the mechanisms of two 5-HT2A agonists on digging behavior in female NMRI mice, using citalopram as a reference compound. While the 5-HT2AR antagonist M100907 blocked the effect of DOI and the 5-HT2CR antagonist SB242084 blocked the effect of citalopram, neither antagonist blocked the effect of psilocybin. This study confirms 5-HT2AR agonism as a mechanism for reduced compulsive-like digging in the MB test and suggests that 5-HT2A and 5-HT2CRs can work in parallel on this type of behavior. Our results with psilocybin suggest that a 5-HT2R-independent mechanism also contributes to the effect of psilocybin on repetitive digging behavior.

NLX-101, a highly selective 5-HT1A receptor biased agonist, mediates antidepressant-like activity in rats via prefrontal cortex 5-HT1A receptors

NLX-101 (also known as F15599) exhibits nanomolar affinity, exceptional selectivity and biased agonist activation of serotonin 5-HT1A receptors. Given systemically, it displays antidepressant-like activity in the rat forced swim test (FST), and preferentially activates 5-HT1A post-synaptic heteroreceptors in the prefrontal cortex (PFC), a brain region involved in the control of mood. Here, we assessed the ability of NLX-101 to produce antidepressant-like activity in the FST following in-situ PFC unilateral microinjection. (+)8−OH-DPAT and F13714, two 5-HT1A receptor agonists that do not display cortical biased agonism, were tested as comparators. NLX-101 decreased time spent in immobility in a bi-modal manner, with a first MED of 0.25 μg (immobility reduced from 160 to 80 s) but immobility returned to control levels at the next dose (1 μg). At higher doses, immobility decreased monotonically, with a second MED of 16 μg and a maximal effect (36 s) at 32 μg. (+)8−OH-DPAT and F13714 also diminished immobility but, unlike NLX-101, they did so in a unimodal manner, with MEDs of 1 and 4 μg, and maximal responses of 31 and 4 s, for (+)8−OH-DPAT and F13714, respectively. The effects of (+)8−OH-DPAT (16 μg) and of both active doses of NLX-101 (0.25 and 16 μg) were prevented by the 5-HT1A receptor antagonist WAY-100,635 (0.63 mg/kg s.c.). In conclusion, activation of 5-HT1A receptors in the PFC by NLX-101 produces robust antidepressant-like effects in the rat FST, with a distinctive bimodal dose-response pattern. These data suggest that NLX-101 may target specific 5-HT1A receptor subpopulations in PFC, likely located on GABAergic and/or glutamatergic neurons.

Cannabidiol Acts at 5-HT1A Receptors in the Human Brain: Relevance for Treating Temporal Lobe Epilepsy.

Experimental evidence indicates that cannabidiol (CBD) induces anxiolytic and antiepileptic effects through the activation of 5-HT1A receptors. These receptors are coupled to Gi/o proteins and induce inhibitory effects. At present, the interaction of CBD with 5-HT1A receptors in the human brain is unknown. The aim of this study focused on evaluating the interaction between CBD and 5-HT1A receptors in cell membranes obtained from the hippocampus and temporal neocortex of autopsies and patients with drug-resistant mesial temporal lobe epilepsy (DR-MTLE). Cell membranes were isolated from the hippocampus and temporal neocortex of a group of patients with DR-MTLE who were submitted to epilepsy surgery (n = 11) and from a group of autopsies (n = 11). The [3H]-8-OH-DPAT binding assay was used to determine the pharmacological interaction of CBD with 5-HT1A receptors. The [35S]-GTPγS assay was used to investigate the CBD-induced activation of Gi/o proteins through its action on 5-HT1A receptors.The CBD affinity (pKi) for 5-HT1A receptors was similar for autopsies and patients with DR-MTLE (hippocampus: 4.29 and 4.47, respectively; temporal neocortex: 4.67 and 4.74, respectively). Concerning the [35S]-GTPγS assay, no statistically significant changes were observed for both hippocampal and neocortical tissue (p > 0.05) at low CBD concentrations (1 pM to 10 μM). In contrast, at high concentrations (100 μM), CBD reduced the constitutive activity of Gi/o proteins of autopsies and DR-MTLE patients (hippocampus: 39.2% and 39.6%, respectively; temporal neocortex: 35.2% and 24.4%, respectively). These changes were partially reversed in the presence of WAY-100635, an antagonist of 5-HT1A receptors, in the autopsy group (hippocampus, 59.8%, p < 0.0001; temporal neocortex, 71.5%, p < 0.0001) and the group of patients with DR-MTLE (hippocampus, 53.7%, p < 0.0001; temporal neocortex, 68.5%, p < 0.001). Our results show that CBD interacts with human 5-HT1A receptors of the hippocampus and temporal neocortex. At low concentrations, the effect of CBD upon Gi/o protein activation is limited. However, at high concentrations, CBD acts as an inverse agonist of 5-HT1A receptors. This effect could modify neuronal excitation and epileptic seizures in patients with DR-MTLE.

Taurine-mediated aggression is abolished via 5-HT1A antagonism and serotonin depletion in zebrafish

Taurine is one of the most abundant amino acids in vertebrates involved in important physiological functions, including osmoregulation, membrane stability, and neuronal activity. The pleiotropic effects of taurine support the existence of different mechanisms of action (e.g., modulation of GABAA, strychnine-sensitive glycine, and NMDA receptors), which can play a role in aggressive-related responses. However, the mechanisms underlying the effects of taurine on aggression are still poorly understood. Because aggression has been associated with diverse central mechanisms, especially serotonergic activity, we aimed to investigate the involvement of this system in taurine-induced aggression in zebrafish. We treated adult zebrafish with ρ-chlorophenylalanine (ρCPA), an inhibitor of the serotonin synthesis, as well as 5-HT1A receptor antagonist and agonist (WAY100135 and buspirone, respectively). Taurine effects were tested individually at three concentrations (42, 150, and 400 mg/L) for 60 min. We further analyzed the effects on aggression and locomotion using the mirror-induced aggression test. Taurine concentration that changed behavioral responses was selected to the succeeding pharmacological experiments using ρCPA, WAY100135, and buspirone. We found that buspirone did not alter the aggression. Yet, 42 mg/L taurine increased aggression, which was abolished by ρCPA and WAY100135, indicating the involvement of 5-HT1A receptors in taurine-mediated aggression. These set of data support an indirect mechanism mediating taurine-induced aggression via serotonin release and activation of 5-HT1A receptors in zebrafish. While the exact mechanisms underlying aggression are still unclear, our novel findings reveal a key role of the serotonergic system in the effects of taurine, supporting the use of zebrafish models to understand the neural basis of aggression in vertebrates.