Activation Of 5-HT2 Receptors Research Articles

Receptor-ligand interaction at 5-HT3 serotonin receptors: a cluster approach.

A fundamental understanding of the interaction of ligands with biological receptors is important because many drugs exert their influence via receptors. Using a cluster approach, we have studied the role of structural and electronic parameters on receptor-ligand binding by carrying out density functional theory based calculations. As model systems, we have studied substituted arylguanidines, which activate 5-HT3 receptors in a manner similar to that of serotonin. The geometries of the arylguanidine derivatives were fully optimized to obtain the lowest energy structures. Electronic properties such as binding energies, dipole moments, polarizabilities, and electron affinities, as well as geometric properties, such as molecular volume and dihedral angles were calculated, and their relationship with binding affinity was evaluated. Results obtained were compared to experimental ligand-receptor binding affinity data available. These fundamental studies show that though both electronic and geometric properties of the ligands are important for binding, the electron affinities of the substituent species play a dominant role. Potential new fundamental indices for ligand-receptor affinity are also discussed.

The broad-spectrum antiemetic effects ETI-385 result from stimulation of 5-HT1A and 5-HT1D receptors.

In the present study, we describe how a nonstoichiometric ratio of the isomers of 8-hydroxy-2-(di-n-propylamino)tetralin (DPAT) produce a broad-spectrum of antiemetic effects in cats and shrews. Determination of the receptor profile of the isomers and testing them separately in cats revealed superior antiemetic effects but severe defensive behavior with the R isomer compared with the S isomer. Differing ratios yielded the best results with the 1:8 (R-S) ratio producing a drug more potent than DPAT and with negligible defensive behavior side effects. Studies with selective 5-HT1D ligands led to the conclusion that this site contributes antiemetic efficacy but is not related to defensive behavior, which is most likely a consequence of 5-HT7 receptor activation. ETI-385 was effective in preventing emetic responses to provocative motion, drugs acting at the chemical trigger zone and cisplatin in both cats and shrews. The results support a clinical trial of this drug for antiemetic effects.

Nociceptive vocalization response in guinea pigs modulated by opioidergic, GABAergic and serotonergic neurotransmission in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN) is involved in the control of several physiological functions, including nociceptive modulation. This nucleus is one of the main sources of serotonin to the CNS and neuromodulators such as opioids and GABA may be are important for its release. This study evaluated the influence of serotonergic, GABAergic and opioidergic stimulation, as well as their interactions in the DRN, on vocalization nociceptive response during a peripheral noxious stimulus application in guinea pigs. Morphine (1.1 nmol), bicuculline (0.50 nmol) and alpha-methyl-5-HT (1.6 nmol) microinjection on the DRN produces antinociception. The antinociception produced by morphine (1.1 nmol) and alpha-methyl-5-HT (1.6 nmol) into the DRN was blocked by prior microinjection of naloxone (0.7 nmol). The alpha-methyl-5-HT effect blocked by naloxone may indicate the existence of 5-HT2A receptors on enkephalinergic interneurons within the dorsal raphe. Pretreatment with muscimol (0.26 nmol) also prevented the antinociceptive effect caused by morphine (1.1 nmol) when administered alone at the same site, indicating an interaction between GABAergic and opioidergic interneurons. The antinociception produced by bicuculline (0.5 nmol) in the DRN was blocked by prior administration of 8-OH-DPAT (0.5 nmol), a 5-HT1A agonist. This may indicate that the 5-HT autoreceptor activation by 8-OH-DPAT at DRN effector neurons can oppose the bicuculline disinhibition effect applied to the same effectors. Thus, we suggest that 5-HT2 receptor activation in the DRN promotes endorphin/enkephalin release that may disinhibit efferent serotonergic neurons of this present structure by inhibiting GABAergic interneurons, resulting in antinociception.

Sa2042 Dependence of Colonic Motor Patterns on 5-HT3 and 5-HT4 Receptor Activation in the Rat

Sa2042 Dependence of Colonic Motor Patterns on 5-HT3 and 5-HT4 Receptor Activation in the Rat

Natural compounds boldine and menthol are antagonists of human 5‐HT3 receptors: implications for treating gastrointestinal disorders

Impaired 5-HT3 receptor function is likely involved in the pathogenesis of functional gastrointestinal disorders (FGID) and 5-HT3 receptor antagonists are effective treatments for chemotherapy-induced nausea and vomiting (CINV) and irritable bowel syndrome (IBS). The monoterpene alcohol menthol and the aporphine alkaloid boldine combat symptoms of gastrointestinal diseases; both interact with other members of the Cys-loop ligand-gated ion channel family and may therefore also act on 5-HT3 receptors. The impact of boldine and menthol on human recombinant homomeric 5-HT3 A- and heteromeric 5-HT3 AB receptors in HEK293 cells was determined by radioligand binding, a luminescence-based Ca(2+) assay, and a membrane potential assay. 5-HT3 protein and mRNA expression was assessed in human colon tissue. Boldine and menthol inhibited the 5-HT-induced activation of 5-HT3 receptors in the low and middle micromolar range, respectively. Boldine was a competitive antagonist of both receptors being 6.5- to 10-fold more potent at 5-HT3 A- vs 5-HT3 AB receptors. Menthol non-competitively and stereoselectively inhibited both receptors: In contrast to (+)-menthol, (-)-menthol was significantly more potent toward 5-HT3 A- vs 5-HT3 AB receptors. We show co-expression of 5-HT3A and 5-HT3B subunits in the human gut epithelium, the lamina propria, the myenteric plexus, and the muscular cell layer. The demonstrated 5-HT3 inhibitory effects may be relevant for boldine's and menthol's alleviating properties on FGID and may encourage clinical studies with the compounds or the plant extracts for CINV and IBS treatment. The found receptor-discriminative properties make boldine and (-)-menthol to potentially useful tools for analyzing structural differences between these receptor subtypes.

The Anticonvulsant Effects of SR 57227 on Pentylenetetrazole-Induced Seizure in Mice

Recently, studies have shown that serotonin plays an important role in the control of seizure. However, the specific role of 5-HT receptor subtypes is not yet well described, in particular that of the 5-HT3 receptor. The present study was aimed to investigate the role of 5-HT3 receptor on the pentylenetetrazole (PTZ)-induced seizure in mice. Firstly, seizure latency was significantly prolonged by a 5-HT3 receptor agonist SR 57227 in a dose-dependent manner. Seizure score and mortality were also decreased by SR 57227 in PTZ-treated mice. Furthermore, these anticonvulsant effects of SR 57227 were inhibited by a 5-HT3 receptor antagonist ondansetron. However, ondansetron alone had no effect on seizure latency, seizure score or mortality at different doses. Immunohistochemical studies have also shown that c-Fos expression was significantly increased in hippocampus (dentate gyrus, CA1, CA3 and CA4) of PTZ-treated mice. Furthermore, c-Fos expression was significantly inhibited by ondansetron in mice treated with PTZ and SR 57227. An ELISA study showed that SR 57227 attenuated the PTZ-induced inhibitory effects of GABA levels in hippocampus and cortex, and the attenuated effects of SR 57227 were antagonized by ondansetron in hippocampus but not cortex. Our findings suggest that activation of 5-HT3 receptor by SR 57227, which plays an important role on the control of seizure induced by PTZ, may be related to GABA activity in hippocampus. Therefore, 5-HT3 receptor subtype is a potential target for the treatment of epilepsy.

GPR43 activation with COX inhibition induces duodenal injury via 5‐HT pathway (905.1)

The reason why NSAIDs injure the small intestinal mucosa, but spare the duodenal mucosa is unknown. The short‐chain fatty acid receptor GRP43 is expressed in 5‐HT containing enterochromaffin cells, and a selective GPR43 agonist phenylacetamide‐1 (PA1) increases duodenal HCO3‐ secretion (DBS) via 5HT4 receptor activation. We hypothesized that GPR43 activation modulates duodenal mucosal defenses via 5‐HT release in an NSAID‐induced intestinal injury model.We measured DBS with pH and CO2 electrodes in vivo through a duodenal loop, perfused with PA1 (0.1 ‐ 10 µM) while measuring released 5‐HT in the portal vein (PV). Intestinal injury was induced by indomethacin (IND, 10 mg/kg, sc) with or without PA1 (0.1 ‐ 1 mg/kg, ig), the 5HT3 antagonist ondansetron (Ond, 3 mg/kg, ip), omeprazole (OPZ, 10 mg/kg, ig), or atropine (10 mg/kg, ig).Luminal perfusion with PA1 dose‐dependently increased DBS accompanied by 5‐HT release into PV. IND induced small intestinal ulcers with duodenal sparing. PA1 + IND dose‐dependently induced duodenal lesions, whereas PA1 alone had no effect. PA1 + IND‐induced duodenal lesions were inhibited by Ond or OPZ, but not by atropine. Non‐duodenal intestinal lesions were reduced by Ond or atropine, but not by OPZ as previously reported.These results suggest that although GPR43 activation enhances mucosal defenses by increased DBS via 5HT4 activation, GPR43 activation combined with COX inhibition may increase the vulnerability of the duodenal mucosa to gastric acid via 5HT3 activation.Grant Funding Source: Supported by VA Merit Review, NIH R01 DK54221

The role of endothelium-derived hyperpolarizing factor and cyclooxygenase pathways in the inhibitory serotonergic response to the pressor effect elicited by sympathetic stimulation in chronic sarpogrelate treated rats

We have demonstrated that the antagonism of 5-HT2 receptors produces an enhancement of serotonergic sympathoinhibitory effect by 5-HT1D and 5-HT7 activation. The aim of this work was to determine mechanisms involved in the 5-hydroxytriptaminergic inhibitory action on the pressor responses elicited by sympathostimulation in pithed rats treated with a 5-HT2 receptor blocker. The blockade of 5-HT2 receptors was induced by orally sarpogrelate treatment (30mg/kg/day). Two weeks later, animals were anaesthetized and pithed. A bolus injection of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10µg/kg), a guanylyl cyclase inhibitor, or indomethacin (2mg/kg), a non-selective COX inhibitor, prior to the infusion of (2S)(+)-5-(1,3,5-trimethylpyrazol-4-yl)-2-(dimethylamino)tetralin, AS-19 (5µg/kg/min) were not able to abolish its inhibitory action. However, i.v. administration of glibenclamide (20mg/kg), a blocker of ATP-sensitive K+ channels, completely reversed AS-19 sympathoinhibitory action. The inhibitory effect of 2-[5-[3-(4-methylsulfonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indol-3-yl]ethanamine, L-694,247 (5µg/kg/min) was abolished by indomethacin, whereas pretreatment with ODQ had no effect. Nimesulide (3mg/kg), a COX-2 inhibitor, completely reversed the inhibitory action of L-694,247, whereas 1-[[4,5-bis (4-methoxyphenyl)-2-thiazolyl]carbonyl]-4-methylpiperazine hydrochloride (FR122047) (3mg/kg), a COX-1 inhibitor, partially blocked this action. The sympathoinhibition by 5-HT (20µg/kg/min) could not be elicited after i.v. treatment with indomethacin plus glibenclamide. In conclusion, these results suggest that in chronic sarpogrelate-treated rats, the inhibitory serotonergic effect of the pressor responses induced by electrical stimulation of the sympathetic outflow via 5-HT7 and 5-HT1D receptor activation is mediated by KATP channel-mediated smooth muscle hyperpolarization and the COX pathway, respectively.

5-Hydroxytryptamine–Mediated Neurotransmission Modulates Spontaneous and Vagal-Evoked Glutamate Release in the Nucleus of the Solitary Tract Effect of Uptake Blockade

The effect of blockade of either 5-hydroxytryptamine (5-HT)/serotonin transporter (SERT) with citalopram or the organic cation transporter 3 (OCT3)/plasma membrane monoamine transporter (PMAT) with decynium-22 (D-22) on spontaneous and evoked release of 5-HT in the nucleus tractus solitarius (NTS) was investigated in rat brainstem slices treated with gabazine. 5-HT release was measured indirectly by changes in the frequency and amplitude of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) [in the presence of tetrodotoxin (TTX)] and evoked EPSCs. Blockade of 5-HT3 receptors with granisetron reduced, whereas the 5-HT3 agonist phenylbiguanide increased, the frequency of mEPSCs. 5-HT decreased mEPSC frequency at low concentrations and increased frequency at high concentrations. This inhibition was blocked by the 5-HT1A antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide (WAY-100635), which was ineffective on its own, whereas the excitation was reversed by granisetron. The addition of citalopram or D-22 caused inhibition, which was prevented by 5-HT1A blockade. Thus, in the NTS, the spontaneous release of 5-HT is able to activate 5-HT3 receptors, but not 5-HT1A receptors, as the release in their vicinity is removed by uptake. The ineffectiveness of corticosterone suggests that the low-affinity, high-capacity transporter is PMAT, not OCT3. For evoked 5-HT release, only D-22 caused an increase in the amplitude of EPSCs, with a decrease in the paired pulse ratio, and increased the number of spontaneous EPSCs after 20-Hz stimulation. Thus, for the evoked release of 5-HT, the low-affinity, high-capacity transporter PMAT, but not 5-HT transporter (5-HTT)/SERT, is important in the regulation of changes in 5-HT extracellular concentration.

Adrenergic α1 receptor activation is sufficient, but not necessary for phrenic long-term facilitation

Acute intermittent hypoxia (AIH; three 5-min hypoxic episodes) causes a form of phrenic motor facilitation (pMF) known as phrenic long-term facilitation (pLTF); pLTF is initiated by spinal activation of Gq protein-coupled 5-HT2 receptors. Because α1 adrenergic receptors are expressed in the phrenic motor nucleus and are also Gq protein-coupled, we hypothesized that α1 receptors are sufficient, but not necessary for AIH-induced pLTF. In anesthetized, paralyzed, and ventilated rats, episodic spinal application of the α1 receptor agonist phenylephrine (PE) elicited dose-dependent pMF (10 and 100 μM, P < 0.05; but not 1 μM). PE-induced pMF was blocked by the α1 receptor antagonist prazosin (1 mM; -20 ± 20% at 60 min, -5 ± 21% at 90 min; n = 6). Although α1 receptor activation is sufficient to induce pMF, it was not necessary for AIH-induced pLTF because intrathecal prazosin (1 mM) did not alter AIH-induced pLTF (56 ± 9% at 60 min, 78 ± 12% at 90 min; n = 9). Intravenous (iv) prazosin (150 μg/kg) appeared to reduce pLTF (21 ± 9% at 60 min, 26 ± 8% at 90 min), but this effect was not significant. Hypoglossal long-term facilitation was unaffected by intrathecal prazosin, but was blocked by iv prazosin (-4 ± 14% at 60 min, -13 ± 18% at 90 min), suggesting different LTF mechanisms in different motor neuron pools. In conclusion, Gq protein-coupled α1 adrenergic receptors evoke pMF, but they are not necessary for AIH-induced pLTF.

5-HT4-receptors modulate induction of long-term depression but not potentiation at hippocampal output synapses in acute rat brain slices.

The subiculum is the principal target of CA1 pyramidal cells and mediates hippocampal output to various cortical and subcortical regions of the brain. The majority of subicular pyramidal cells are burst-spiking neurons. Previous studies indicated that high frequency stimulation in subicular burst-spiking cells causes presynaptic NMDA-receptor dependent long-term potentiation (LTP) whereas low frequency stimulation induces postsynaptic NMDA-receptor-dependent long-term depression (LTD). In the present study, we investigate the effect of 5-hydroxytryptamine type 4 (5-HT4) receptor activation and blockade on both forms of synaptic plasticity in burst-spiking cells. We demonstrate that neither activation nor block of 5-HT4 receptors modulate the induction or expression of LTP. In contrast, activation of 5-HT4 receptors facilitates expression of LTD, and block of the 5-HT4 receptor prevents induction of short-term depression and LTD. As 5-HT4 receptors are positively coupled to adenylate cyclase 1 (AC1), 5-HT4 receptors might modulate PKA activity through AC1. Since LTD is blocked in the presence of 5-HT4 receptor antagonists, our data are consistent with 5-HT4 receptor activation by ambient serotonin or intrinsically active 5-HT4 receptors. Our findings provide new insight into aminergic modulation of hippocampal output.

Regulation of Amyloid Precursor Protein Processing by Serotonin Signaling

Proteolytic processing of the amyloid precursor protein (APP) by the β- and γ-secretases releases the amyloid-β peptide (Aβ), which deposits in senile plaques and contributes to the etiology of Alzheimer's disease (AD). The α-secretase cleaves APP in the Aβ peptide sequence to generate soluble APPα (sAPPα). Upregulation of α-secretase activity through the 5-hydroxytryptamine 4 (5-HT4) receptor has been shown to reduce Aβ production, amyloid plaque load and to improve cognitive impairment in transgenic mouse models of AD. Consequently, activation of 5-HT4 receptors following agonist stimulation is considered to be a therapeutic strategy for AD treatment; however, the signaling cascade involved in 5-HT4 receptor-stimulated proteolysis of APP remains to be determined. Here we used chemical and siRNA inhibition to identify the proteins which mediate 5-HT4d receptor-stimulated α-secretase activity in the SH-SY5Y human neuronal cell line. We show that G protein and Src dependent activation of phospholipase C are required for α-secretase activity, while, unexpectedly, adenylyl cyclase and cAMP are not involved. Further elucidation of the signaling pathway indicates that inositol triphosphate phosphorylation and casein kinase 2 activation is also a prerequisite for α-secretase activity. Our findings provide a novel route to explore the treatment of AD through 5-HT4 receptor-induced α-secretase activation.

Sevoflurane induced amnesia inhibits hippocampal Arc expression partially through 5-hydroxytryptamine-7 receptors in the bilateral basolateral amygdala in rats

This study aimed to investigate whether the regulation of 5-hydroxytryptamine-7 (5-HT7) receptors in the bilateral basolateral amygdala (BLA) could alter the amnesic effects of sevoflurane and change the hippocampal expression of Arc and neural apoptosis. Male Sprague-Dawley rats were randomized into ten groups. First, the animals received bilateral injection of SB269970 (20, 50, or 100pmol/0.2μl) or saline (0.2μl) or AS-19 (2, 10, or 50pmol/0.2μl), followed by inhalation of 2% sevoflurane or air for 2h. Then, fear conditioning training was carried out, and the percentage of freezing was detected 24h later. Furthermore, hippocampal Arc protein level and neural apoptosis were measured. Pre-training inhalation of sevoflurane reduced the extent of freezing, and hippocampal Arc expression. The largest dose of SB269970 (100pmol) could block sevoflurane-induced amnesia and reverse the inhibitive effect of sevoflurane on Arc expression, while the maximal dose of AS-19 could exacerbate the amnesic effect, and further inhibit Arc expression. Furthermore, pre-training inhalation of 2% sevoflurane for 6h could not induce neural apoptosis in the hippocampus. The amnesic effect of sevoflurane might partly attribute to its impairment of memory formation in the hippocampus via activation of 5-HT7 receptors in the BLA.

The relationship between perioperative nausea and vomiting and serum serotonin concentrations in patients undergoing cesarean section under epidural anesthesia.

BackgroundSerotonin-also known as 5-hydroxytryptamine or 5-HT-can induce nausea and vomiting (NV) by peripheral mechanisms via the activation of 5-HT3 receptors. In this study, we observed perioperative NV, including intraoperative NV, and changes in serum 5-HT concentrations. We evaluated the relationship between perioperative NV and serum 5-HT levels in patients undergoing cesarean section under epidural anesthesia, and carried out a pilot study to determine if further studies on a larger scale were justified.MethodsTwenty-eight patients who were scheduled for cesarean section under epidural anesthesia were included in the study. Patients were assigned to 2 groups according to the occurrence of NV after induction, i.e., an NV-positive or an NV-negative group. Serum 5-HT concentrations were measured before induction, at the time that NV occurred (in the case of the NV-positive group) or 5 min after the umbilical cord clamping (in the case of the NV-negative group) during surgery, and at 2 h postoperatively.ResultsNV occurred in 10 of the 28 patients. No significant differences in serum 5-HT concentrations were found within or between the two groups.ConclusionsThis study suggests that there is no correlation between serum 5-HT concentration and the occurrence of perioperative NV in patients undergoing cesarean section under epidural anesthesia, and the findings do not seem to support further investigations regarding a possible relationship between serum 5-HT concentration and perioperative NV.

Spinal 5-HT3 receptors mediate descending facilitation and contribute to behavioral hypersensitivity via a reciprocal neuron-glial signaling cascade.

BackgroundIt has been recently recognized that the descending serotonin (5-HT) system from the rostral ventromedial medulla (RVM) in the brainstem and the 5-HT3 receptor subtype in the spinal dorsal horn are involved in enhanced descending pain facilitation after tissue and nerve injury. However, the mechanisms underlying the activation of the 5-HT3 receptor and its contribution to facilitation of pain remain unclear.ResultsIn the present study, activation of spinal 5-HT3 receptors by intrathecal injection of a selective 5-HT3 receptor agonist SR 57227 induced spinal glial hyperactivity, neuronal hyperexcitability and pain hypersensitivity in rats. We found that there was neuron-to-microglia signaling via the chemokine fractalkine, microglia to astrocyte signaling via cytokine IL-18, astrocyte to neuronal signaling by IL-1β, and enhanced activation of NMDA receptors in the spinal dorsal horn. Glial hyperactivation in spinal dorsal horn after hindpaw inflammation was also attenuated by molecular depletion of the descending 5-HT system by intra-RVM Tph-2 shRNA interference.ConclusionsThese findings offer new insights into the cellular and molecular mechanisms at the spinal level responsible for descending 5-HT-mediated pain facilitation during the development of persistent pain after tissue and nerve injury. New pain therapies should focus on prime targets of descending facilitation-induced glial involvement, and in particular the blocking of intercellular signaling transduction between neurons and glia.

Serotonin regulation of subthalamic neurons

The subthalamic nucleus (STN) is a key component of the basal ganglia. As the only basal ganglia nucleus comprised of mostly glutamatergic neurons, STN neurons provide a key driving force to their target neurons. Thus, regulation of STN neuron activity is important. One STN regulator is the serotonin (5-HT) system. The STN receives a dense 5-HT innervation. 5-HT1A, 5-HT1B, 5-HT2C, and 5-HT4 receptors are expressed in the STN. 5-HT may regulate the STN via several mechanisms. First, 5-HT may affect STN neuron excitability directly by either inhibiting a subpopulation of STN neurons via activation of 5-HT1A receptors or exciting STN neurons through activation of 5-HT2C and 5-HT4 receptors. Second, 5-HT may affect synaptic inputs to the STN. Via activation of 5-HT1B receptors on the afferent terminals, 5-HT inhibits glutamatergic input to the STN, but the inhibitory effect on GABAergic input is smaller. Third, 5-HT may regulate the STN glutamatergic output by activating presynaptic 5-HT1B receptors, thus reducing burst firing in target neurons. Last, 5-HT may affect glutamate release at the intra-STN axon collaterals and regulate the recurrent excitation. These mechanisms may work in concert to fine-tune the intensity and pattern of STN activity and reduce STN output bursts.

Visceral analgesic effect of 5-HT4 receptor agonist in rats involves the rostroventral medulla (RVM)

The 5-HT4 receptor agonist tegaserod (TEG) has been reported to modulate visceral pain. However, the underlying mechanism remains unknown. The objective of the present study was to examine the analgesic mechanism and site of action of TEG. In male rats, visceral pain was assessed by measuring visceromotor response (VMR) to colorectal distension (CRD). Inflammation was induced by intracolonic injection of tri-nitrobenzene sulfonic acid (TNBS). The effect of TEG on the VMR was tested by injecting intraperitoneal (i.p.), intrathecal (i.t.), intracerebroventricular (i.c.v) or in the rostroventral medulla (RVM). The effect of the drug was also tested on responses of CRD-sensitive pelvic nerve afferents (PNA) and lumbo-sacral (LS) spinal neurons. Systemic injection of TEG attenuated VMR in naive and TNBS-treated rats. Similarly, supraspinal, but not spinal, injection of TEG attenuated the VMR. While GR113808, (selective 5-HT4 antagonist) blocked the effect, naloxone (NLX) an opioid receptor antagonist reversed the effect of TEG. Although i.t. NLX did not block the inhibitory effect of TEG in VMR study, i.t. injection of α2-adrenergic receptor antagonist yohimbine blocked the effect of TEG when given systemically. While TEG had no effect on the responses of CRD-sensitive PNA, it inhibited the responses of CRD-sensitive LS neurons in spinal intact condition. This inhibition was blocked by GR113808, NLX and β-funaltrexamine (β-FNA) when injected into the RVM. Results indicate that TEG produces analgesia via activation of supraspinal 5-HT4 receptors which triggers the release of opioids at supraspinal site, which activates descending noradrenergic pathways to the spinal cord to produce analgesia.

Differential role of the basolateral amygdala 5-HT3 and 5-HT4 serotonin receptors upon ACPA-induced anxiolytic-like behaviors and emotional memory deficit in mice

Background and aimThe critical role of cannabinoidergic and serotonergic systems of the amygdala in modulation of anxiety-like behaviors and emotional memory has already been demonstrated. The present study aimed to investigate the possible role of the basolateral amygdala (BLA) 5-HT3 and 5-HT4 serotonergic systems upon ACPA (CB1 cannabinoid receptor agonist)-induced anxiolytic-like behaviors and emotional memory impairment using the elevated plus-maze (EPM) test-retest paradigm in male mice. Methodbilateral guide-cannulae were implanted to allow intra-BLA microinjection of serotonergic agents. Resultsthe intraperitoneal injection of ACPA could induce anxiolytic-like behaviors and reduce the emotional memory formation. Intra-BLA injection of M-Chlorophenylbiguanide (M-Chl, a 5-HT3 serotonin receptor agonist) neither altered the anxiety-like behaviors nor the emotional memory formation by itself, while the higher dose of Y-25130 (a 5-HT3 serotonin receptor antagonist) reduced the emotional memory formation and locomotor activity but not the anxiety-like behaviors. Furthermore, injection of a higher dose of RS67333 and RS23597 (as 5-HT4 serotonin receptor agonist and antagonist, respectively) did not alter the anxiety-like behaviors, while reduced the emotional memory formation. In addition, the intra-BLA injection of M-Chl but not Y-25130 and RS67333 restored the ACPA-induced anxiolytic-like behaviors and emotional memory deficit, while a higher dose of RS67333 decreased the locomotor activity. Moreover, the intra-BLA microinjection of RS23597 could restore the ACPA-induced anxiolytic-like behaviors but not the emotional memory deficit. Conclusionbased on our findings, ACPA seems to induce its anxiolytic-like behaviors and emotional memory formation deficits via activation and deactivation of the BLA 5-HT4 and 5-HT3 serotonin receptors.

Low-frequency stimulation evokes serotonin release in the nucleus accumbens and induces long-term depression via production of endocannabinoid

The nucleus accumbens (NAc), a major component of the mesolimbic system, is involved in the mediation of reinforcing and addictive properties of many dependence-producing drugs. Glutamatergic synapses within the NAc can express plasticity, including a form of endocannabinoid (eCB)-long-term depression (LTD). Recent evidences demonstrate cross talk between eCB signaling pathways and those of other receptor systems, including serotonin (5-HT); the extensive colocalization of CB1 and 5-HT receptors within the NAc suggests the potential for interplay between them. In the present study, we found that 20-min low-frequency (4 Hz) stimulation (LFS-4Hz) of glutamatergic afferences in rat brain slices induces a novel form of eCB-LTD in the NAc core, which requires 5-HT2 and CB1 receptor activation and L-type voltage-gated Ca(2+) channel opening. Moreover, we found that exogenous 5-HT application (5 μM, 20 min) induces an analogous LTD (5-HT-LTD) at the same synapses, requiring the activation of the same receptors and the opening of the same Ca(2+) channels; LFS-4Hz-LTD and 5-HT-LTD were mutually occlusive. Present results suggest that LFS-4Hz induces the release of 5-HT, which acts at 5-HT2 postsynaptic receptors, increasing Ca(2+) influx through L-type voltage-gated channels and 2-arachidonoylglycerol production and release; the eCB travels retrogradely and binds to presynaptic CB1 receptors, causing a long-lasting decrease of glutamate release, resulting in LTD. These observations might be helpful to understand the neurophysiological mechanisms underlying drug addiction, major depression, and other psychiatric disorders characterized by dysfunction of 5-HT neurotransmission in the NAc.

Rapid Anxiolytic Effects of a 5-HT4 Receptor Agonist Are Mediated by a Neurogenesis-Independent Mechanism

Selective serotonin reuptake inhibitors (SSRIs) display a delayed onset of action of several weeks. Past work in naive rats showed that 5-HT₄ receptor agonists had rapid effects on depression-related behaviors and on hippocampal neurogenesis. We decided to investigate whether 5-HT₄ receptor stimulation was necessary for the effects of SSRIs in a mouse model of anxiety/depression, and whether hippocampal neurogenesis contributed to these effects. Using the mouse corticosterone model of anxiety/depression, we assessed whether chronic treatment with a 5-HT₄ receptor agonist (RS67333, 1.5 mg/kg/day) had effects on anxiety- and depression-related behaviors, as well as on hippocampal neurogenesis in comparison with chronic fluoxetine treatment (18 mg/kg/day). Then, using our anxiety/depression model combined with ablation of hippocampal neurogenesis, we investigated whether neurogenesis was necessary for the behavioral effects of subchronic (7 days) or chronic (28 days) RS67333 treatment. We also assessed whether a 5-HT₄ receptor antagonist (GR125487, 1 mg/kg/day) could prevent the behavioral and neurogenic effects of fluoxetine. Chronic treatment with RS67333, similar to fluoxetine, induced anxiolytic/antidepressant-like activity and stimulated adult hippocampal neurogenesis, specifically facilitating maturation of newborn neurons. However, unlike fluoxetine, anxiolytic effects of RS67333 were already present after 7 days and did not require hippocampal neurogenesis. Chronic treatment with GR125487 prevented both anxiolytic/antidepressant-like and neurogenic effects of fluoxetine, indicating that 5-HT₄ receptor activation is necessary for these effects of SSRIs. 5-HT₄ receptor stimulation could represent an innovative and rapid onset therapeutic approach to treat depression with comorbid anxiety.