Activation of 5-HT(2C) receptors in the dorsal periaqueductal gray increases antinociception in mice exposed to the elevated plus-maze

Defensive and Emotional Behavior Modulation by Serotonin in the Periaqueductal Gray

Serotonin 5-hydroxytryptamine (5-HT) is a key neurotransmitter for the modulation and/or regulation of numerous physiological processes and psychiatric disorders (e.g., behaviors related to anxiety, pain, aggressiveness, etc.). The periaqueductal gray matter (PAG) is considered an integrating center for active and passive defensive behaviors, and electrical stimulation of this area has been shown to evoke behavioral responses of panic, fight-flight, freezing, among others. The serotonergic activity in PAG is influenced by the activation of other brain areas such as the medial hypothalamus, paraventricular nucleus of the hypothalamus, amygdala, dorsal raphe nucleus, and ventrolateral orbital cortex. In addition, activation of other receptors within PAG (i.e., CB1, Oxytocin, µ-opioid receptor (MOR), and γ-aminobutyric acid (GABA_A)) promotes serotonin release. Therefore, this review aims to document evidence suggesting that the PAG-evoked behavioral responses of anxiety, panic, fear, analgesia, and aggression are influenced by the activation of 5-HT_1A and 5-HT_2A/C receptors and their participation in the treatment of various mental disorders.

Association of plasma tryptophan concentration with periaqueductal gray matter functional connectivity in migraine patients

Altered periaqueductal gray matter (PAG) functional connectivity contributes to brain hyperexcitability in migraine. Although tryptophan modulates neurotransmission in PAG projections through its metabolic pathways, the effect of plasma tryptophan on PAG functional connectivity (PAG-FC) in migraine has not been investigated yet. In this study, using a matched case-control design PAG-FC was measured during a resting-state functional magnetic resonance imaging session in migraine without aura patients (n = 27) and healthy controls (n = 27), and its relationship with plasma tryptophan concentration (TRP) was assessed. In addition, correlations of PAG-FC with age at migraine onset, migraine frequency, trait-anxiety and depressive symptoms were tested and the effect of TRP on these correlations was explored. Our results demonstrated that migraineurs had higher TRP compared to controls. In addition, altered PAG-FC in regions responsible for fear-cascade and pain modulation correlated with TRP only in migraineurs. There was no significant correlation in controls. It suggests increased sensitivity to TRP in migraine patients compared to controls. Trait-anxiety and depressive symptoms correlated with PAG-FC in migraine patients, and these correlations were modulated by TRP in regions responsible for emotional aspects of pain processing, but TRP did not interfere with processes that contribute to migraine attack generation or attack frequency.

Involvement of Serotonergic System in Oxaliplatin-Induced Neuropathic Pain

Oxaliplatin is a chemotherapeutic agent widely used against colorectal and breast cancers; however, it can also induce peripheral neuropathy that can rapidly occur even after a single infusion in up to 80–90% of treated patients. Numerous efforts have been made to understand the underlying mechanism and find an effective therapeutic agent that could diminish pain without damaging its anti-tumor effect. However, its mechanism is not yet clearly understood. The serotonergic system, as part of the descending pain inhibitory system, has been reported to be involved in different types of pain. The malfunction of serotonin (5-hydroxytryptamine; 5-HT) or its receptors has been associated with the development and maintenance of pain. However, its role in oxaliplatin-induced neuropathy has not been clearly elucidated. In this review, 16 in vivo studies focused on the role of the serotonergic system in oxaliplatin-induced neuropathic pain were analyzed. Five studies analyzed the involvement of 5-HT, while fourteen studies observed the role of its receptors in oxaliplatin-induced allodynia. The results show that 5-HT is not involved in the development of oxaliplatin-induced allodynia, but increasing the activity of the 5-HT_1A, 5-HT_2A, and 5-HT₃ receptors and decreasing the action of 5-HT_2C and 5-HT₆ receptors may help inhibit pain.

Involvement of 5-HT2A, 5-HT2B and 5-HT2C receptors in mediating the ventrolateral orbital cortex-induced antiallodynia in a rat model of neuropathic pain

The present study examined the roles of 5-HT2A, 5-HT2B and 5-HT2C receptor subtypes in mediating the ventrolateral orbital cortex (VLO)-induced antiallodynia in a rat model of neuropathic pain induced by spared nerve injury (SNI). Change of mechanical paw withdrawal threshold (PWT) was measured using von-Frey filaments. Microinjection of preferential or selective 5-HT2A/C, 5-HT2B and 5-HT2C receptor agonists, (±)-1-(2,5-Dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI), α-methyl-5-(2-thienylmethoxy)-1H-Indole-3-ethanamine hydrochloride (BW723C86) and 1-(3-Chlorophenyl)-piperazine hydrochloride (m-CPP) into the VLO significantly depressed allodynia induced by SNI, and the inhibitory effect of DOI was blocked or attenuated by selective 5-HT2A/C receptor antagonists ketanserin (+)-tartrate salt (ketanserin) and 5-HT2A receptor antagonist R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenylethyl)]-4-piperidinemethanol (M100907); the effects of BW723C86 and m-CPP were antagonized by 5-HT2B receptor antagonists N-(1-Methyl-1H-5-indolyl)-N'-(3-methyl-5-isothiazolyl)urea (SB204741) and 5-HT2C receptor antagonist RS102221 hydrochloride hydrate (RS-102221), respectively. These results suggest that 5-HT2A, 5-HT2B, 5-HT2C receptor subtypes are involved in mediating the VLO-induced antiallodynia in the neuropathic pain state.

Chronic Fluoxetine Impairs the Effects of 5-HT1A and 5-HT2C Receptors Activation in the PAG and Amygdala on Antinociception Induced by Aversive Situation in Mice

Growing evidence suggests an important role of fluoxetine with serotonin 5-HT_1A and 5-HT_2C receptors in the modulation of emotion and nociception in brain areas such as the amygdala and periaqueductal gray (PAG). Acute fluoxetine impairs 5-HT_2C (but not 5-HT_1A) receptor activation in the amygdaloid complex. Given that fluoxetine produces its clinical therapeutic effects only when given chronically, this study investigated the effects of chronic treatment with fluoxetine on the effects produced by 5-HT_1A or 5-HT_2C receptors activation in the amygdala or PAG on fear-induced antinociception. We recorded the effects of chronic fluoxetine on serotonin and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) levels as well as serotonin turnover; 5-HT_1A and 5-HT_2C receptor protein levels in the amygdala and PAG. Also, we evaluated the effects of chronic fluoxetine combined with intra-amygdala or intra-PAG injection of MK-212 (a 5-HT_2C agonist; 0.63 nmol) or 8-OH-DPAT (a 5-HT_1A agonist; 10 nmol) on the antinociceptive response in mice confined in the open arm of the elevated plus-maze (EPM). Nociception was assessed with the writhing test induced by intraperitoneal injection of 0.6% acetic acid. Results showed that fluoxetine (20 mg/kg, s.c.) enhanced the open-arm induced antinociception (OAA) and reduced the number of writhes in mice confined in the enclosed arm, featuring an analgesic effect. In addition, fluoxetine increased the expression of 5-HT_2C receptors and 5-HT levels whereas reduced its turnover in the amygdala. While fluoxetine did not change 5-HT and 5-HIAA levels, and its turnover in the PAG, it up-regulated 5HT_1A and 5-HT_2C receptors in this midbrain area. Chronic fluoxetine (5.0 mg/Kg, an intrinsically inactive dose on nociception) antagonized the enhancement of OAA produced by intra-amygdala or intra-PAG injection of MK-212. Fluoxetine also impaired the attenuation of OAA induced by intra-amygdala injection of 8-OH-DPAT and totally prevented OAA in mice that received intra-PAG 8-OH-DPAT. These results suggest that (i) 5-HT may facilitate nociception and intensify OAA, acting at amygdala 5-HT_1A and 5-HT_2C receptors, respectively, and (ii) fluoxetine modulates the OAA through activation of 5-HT_2C receptors within the PAG. These findings indicate that chronic fluoxetine impairs the effects of 5-HT_1A and 5-HT_2C receptors activation in the amygdala and PAG on fear-induced antinociception in mice.

Valproate reverses stress-induced somatic hyperalgesia and visceral hypersensitivity by up-regulating spinal 5-HT2C receptor expression in female rats

Sodium valproate (VPA) has analgesic effects in clinical and experimental studies, but the mechanisms are still unclear. The present study examined the effects of VPA on stress-induced somatic hyperalgesia and visceral hypersensitivity and the role of 5-HT_2C receptors in the spinal cord. Repeated 3 day forced swim (FS) significantly reduced the thermal withdrawal latency and mechanical withdrawal threshold, and increased the magnitude of the visceromotor response to colorectal distention compared to the baseline values in rats. The somatic hyperalgesia and visceral hypersensitivity were accompanied by significant down-regulation of 5-HT_2C receptor expression in the L4-L5 and L6-S1 dorsal spinal cord. Intraperitoneal administration of VPA (300 mg/kg) before each FS and 1 day post FS prevented the development of somatic hyperalgesia and visceral hypersensitivity induced by FS stress, as well as down-regulation of 5-HT_2C receptors in the spinal cord. The reversal of somatic hyperalgesia and visceral hypersensitivity by VPA in FS rats was blocked by intrathecal administration of the selective 5-HT_2C receptor antagonist RS-102221 (30 μg/10 μL) 30 min after each VPA injection. The results suggest that VPA attenuates FS-induced somatic hyperalgesia and visceral hypersensitivity by restoring down-regulated function of 5-HT_2C receptors in the spinal cord.

Interplay between 5-HT2C and 5-HT1A receptors in the dorsal periaqueductal gray in the modulation of fear-induced antinociception in mice

The confinement of rodents to the open arm of the elevated-plus maze provokes antinociception (OAA). As a type of defensive reaction, the OAA has been investigated through systemic and intramesencephalic (e.g., dorsal portion of the periaqueductal gray - dPAG) injections of anxiolytic-like drugs [e.g., serotonergic (5-HT) receptor agonists or antagonists]. Here we investigated the effects of (i) intra-dPAG injections of a 5HT_2C receptor agonist (MK-212; 0.21 or 0.63 nmol) and antagonist (SB 242084; 0.01, 0.1 or 1.0 nmol); (ii) combined injections of SB 242084 and MK-212 into the dPAG; (iii) combined injections of SB 242084 with 8-OHDPAT (10 nmol) into the dPAG on the OAA in male Swiss mice. Nociception was assessed with the writhing test induced by acetic acid injection. Results showed that (i) intra-dPAG injection of MK-212 (0.63 nmol) increased the OAA; (ii) intra-dPAG SB 242084 (1.0 nmol) prevented the OAA; (iii) SB 242084 (0.1 nmol, a dose devoid of intrinsic effect on nociception) blocked the OAA enhancement provoked by MK-212 and enabled 8-OH-DPAT to prevent the OAA. These results suggest that OAA is mediated by 5-HT_2C receptors within the dPAG. Intra-dPAG SB242084 administration provoked similar results on the effects produced by MK-212 and 8-OH-DPAT on OAA. In addition, the dPAG 5-HT_1A and 5-HT_2C receptors interact each other in the modulation of OAA.

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.

Rat exposure in mice with neuropathic pain induces fear and antinociception that is not reversed by 5-HT2C receptor activation in the dorsal periaqueductal gray

Previous studies have demonstrated that serotonin 5-HT2C receptors in the dorsal periaqueductal gray (dPAG) mediate both anxiety and antinociception in mice submitted to the elevated plus maze. The present study examined the effects of intra-dPAG infusion of the serotonin 5-HT2C receptor agonist (MK-212) in the defensive reactions and antinociception in mice with neurophatic pain confronted by a predator. Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve, and predator confrontation was performed using the rat exposure test (RET). Our results demonstrated that both sham-operated and CCI mice exhibited intense defensive reactions when confronted by rats. However, rat-exposed CCI mice showed reduced pain reactivity in comparison to CCI mice exposed to a toy rat. Intra-dPAG infusion of MK-212 prior to predator exposure did not significantly alter defensive or antinociceptive responses. To our knowledge, our results represent the first evidence of RET-induced antinociception in mice. Moreover, the results of the present study suggest that 5-HT2C receptor activation in the dPAG is not critically involved in the control of predator-evoked fearful or antinociceptive responses.

Role of TRPV1 channels of the dorsal periaqueductal gray in the modulation of nociception and open elevated plus maze-induced antinociception in mice

Recent findings have identified the presence of transient receptor potential vanilloid-1 (TRPV1) channels within the dorsal portion of the periaqueductal gray (dPAG), suggesting their involvement in the control of pain and environmentally-induced antinociception. Environmentally, antinociception may be achieved through the use of an open elevated plus maze (oEPM, an EPM with 4 open arms), a highly aversive environmental situation. Here, we investigated the role of these TRPV1 channels within the dPAG in the modulation of a tonic pain and in the oEPM-induced antinociception. Male Swiss mice, under the nociceptive effect of 2.5% formalin injected into the right hind paw, received intra-dPAG injections of the TRPV1 agonist (capsaicin: 0, 0.01, 0.1 or 1.0 nmol/0.2 μL; Experiment 1) or antagonist (capsazepine: 0, 10 or 30 nmol/0.2 μL; Experiment 2) or combined injections of capsazepine (30 nmol) and capsaicin (1.0 nmol) (Experiment 3) and the time spent licking the formalin-injected paw was recorded. In Experiment 4, mice received intra-dPAG capsazepine (0 or 30 nmol) and were exposed to the oEPM or to a control situation, an enclosed EPM (eEPM; an EPM with 4 enclosed arms). Results showed that while capsaicin (1 nmol) decreased the time spent licking the formalin-injected paw, capsazepine did not change nociceptive response. Capsazepine (30 nmol) blocked pain inhibition induced by capsaicin and mildly attenuated the oEPM-induced antinociception. Our results revealed an important role of TRPV1 channels within the dPAG in the modulation of pain and in the phenomenon known as fear-induced antinociception in mice.