Expression of 5-HT1A receptor mRNA in rat lumbar spinal dorsal horn neurons after peripheral inflammation

[6]-Shogaol Attenuates Oxaliplatin-Induced Allodynia through Serotonergic Receptors and GABA in the Spinal Cord in Mice

Although oxaliplatin is a well-known anti-cancer agent used for the treatment of colorectal cancer, treated patients often experience acute cold and mechanical allodynia as side effects. Unfortunately, no optimal treatment has been developed yet. In this study, [6]-shogaol (10 mg/kg, i.p.), which is one of the major bioactive components of Zingiber officinale roscoe (Z. officinale), significantly alleviated allodynia induced by oxaliplatin (6 mg/kg, i.p.) injection. Cold and mechanical allodynia were assessed by acetone drop and von Frey filament tests, respectively. The analgesic effect of [6]-shogaol was blocked by the intrathecal injection of 5-HT1A, 5-HT3, and GABAB receptor antagonists, NAN-190 (1 μg), MDL-72222 (15 μg), and CGP 55845 (10 μg), respectively. Furthermore, oxaliplatin injection lowered the GABA concentration in the superficial laminae of the spinal dorsal horn, whereas [6]-shogaol injection significantly elevated it. The GAD (glutamic acid decarboxylase) 65 concentration also increased after [6]-shogaol administration. However, pre-treatment of NAN-190 completely inhibited the increased GABA induced by [6]-shogaol in the spinal dorsal horn, whereas MDL-72222 partially blocked the effect. Altogether, these results suggest that [6]-shogaol could attenuate oxaliplatin-induced cold and mechanical allodynia through 5-HT_1A and 5-HT₃ receptor antagonists located in the GABAergic neurons in the spinal dorsal horn in mice.

The relationship between the serotonergic system and COVID-19 disease: A review

COVID-19 is an infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which led to a pandemic started in Wuhan, China, in 2019. The rapid spread of the disease in the world, unprecedented mortality rate, and lack of definitive treatment for the disease have led to a global effort to develop effective vaccines as well as new therapeutic interventions. Immune cells activation with excessive inflammation is an important pathophysiological feature of COVID-19 that may impair the various organs functions. Accordingly, these could cause dysfunction in the brain with some symptoms such as respiratory failure, headache, impaired consciousness, olfactory and taste disorders, and severe neurological disorders such as encephalitis. It was found that there is a two-way communication between the immune system and the nervous system through classical neurotransmitters, hormones, and cytokines. Among neurotransmitters, serotonin plays important roles in the immune system and in regulating inflammatory responses by central and peripheral mechanisms. This article aimed to review the two-way relationship between the immune and the nervous systems by focusing on the serotonergic system and the emerging COVID-19 disease.

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-HT1A, 5-HT2A, and 5-HT3 receptors and decreasing the action of 5-HT2C and 5-HT6 receptors may help inhibit pain.

Depression and Pain: use of antidepressant

Background:

Emotional disorders are common comorbid affectations that exacerbate the severity and persistence of chronic pain. Specifically, depressive symptoms can lead to an excessive duration and intensity of pain. Clinical and preclinical studies have been focused on the underlying mechanisms of chronic pain and depression comorbidity and the use of antidepressants to reduce pain.

Aim:

This review provides an overview of the comorbid relationship of chronic pain and depression, the clinical and pre-clinical studies performed on the neurobiological aspects of pain and depression, and the use of antidepressants as analgesics.

Methods:

A systematic search of literature databases was conducted according to pre-defined criteria. The authors independently conducted a focused analysis of the full-text articles.

Results:

Studies suggest that pain and depression are highly intertwined and may co-exacerbate physical and psychological symptoms. One important biochemical basis for pain and depression focuses on the serotonergic and norepinephrine system, which have been shown to play an important role in this comorbidity. Brain structures that codify pain are also involved in mood. It is evident that using serotonergic and norepinephrine antidepressants are strategies commonly employed to mitigate pain

Conclusion:

Literature indicates that pain and depression impact each other and play a prominent role in the development and maintenance of other chronic symptoms. Antidepressants continue to be a major therapeutic tool for managing chronic pain. Tricyclic antidepressants (TCAs) are more effective in reducing pain than Selective Serotonin Reuptake Inhibitors (SSRIs) and Serotonin-Noradrenaline Reuptake Inhibitors (SNRIs).

Influence of Experimental Autoimmune Prostatitis on Sexual Function and the Anti-inflammatory Efficacy of Celecoxib in a Rat Model

Experimental autoimmune prostatitis (EAP) is a well-established model induced by an autoimmune response to prostate antigen. The symptomatic, pathological, and immunological characteristics of EAP animals are highly consistent with human chronic prostatitis/chronic pelvic pain syndrome (CP/CPPS), which makes EAP an ideal model for this disease. Here, we investigate the influence of EAP on male rat sexual function and the efficacy of anti-inflammatory therapy with celecoxib. EAP rat models were established using male Wistar rats. Rats were randomly assigned to a normal control group, an EAP model group, or an EAP model with celecoxib treatment group (celecoxib group). Behavioral changes, sexual behavioral changes, and erectile function were estimated using an open-field test, a sucrose consumption test, mating experiments, and by intracavernous pressure/mean arterial pressure ratio (ICP/MAP). Histological changes in the prostate were observed by HE staining, and the serum inflammatory factors IL-1β and TNF-α levels were measured by enzyme-linked immunosorbent assay. In addition, serotonin (5-hydroxytryptamine, 5-HT), 5-HT_1A receptor, 5-HT_2C receptor, and serotonin transporter (SERT) expression levels in the hippocampus and spinal cord (T13–L1, L5–S2) were examined by immunohistochemistry and western blot analysis. Results showed that EAP rats exhibited characteristics of depression, decreased sexual drive, premature ejaculation, and increased threshold of penile erection. Moreover, all these changes were effectively alleviated by celecoxib. Significant increases in prostatic interstitial infiltration by inflammatory cells and in serum IL-1β and TNF-α levels were observed in EAP rats, and these were partially reduced by celecoxib. Additionally, the expression pattern of serotonin system regulators in the hippocampus and spinal cord were altered in EAP model rats, including a decrease in 5-HT levels and an increase in 5-HT_1A receptor levels. In conclusion, autoimmune prostatitis impaired rat sexual function, and this was effectively prevented by anti-inflammatory therapy with celecoxib. Moreover, a serotonin system disorder in the central nervous system was likely mediated via inflammation in EAP rats.

Inflammation induces developmentally regulated sumatriptan inhibition of spinal synaptic transmission

BACKGROUND AND PURPOSE

While triptans are used to treat migraine, there is evidence that they also reduce inflammation-induced pain at the spinal level. The cellular mechanisms underlying this spinal enhancement are unknown. We examined whether inflammation alters sumatriptan modulation of synaptic transmission in the rat spinal dorsal horn.

EXPERIMENTAL APPROACH

Three to four days following intraplantar injection of complete Freund's adjuvant (CFA) or saline, whole cell recordings of evoked glutamatergic EPSCs were made from lumbar lamina I-II dorsal horn neurons in rat spinal slices KEY RESULTS: In 2- to 3-week-old animals, sumatriptan reduced the amplitude of evoked EPSCs and this was greater in slices from CFA, compared to saline-injected rats. In CFA-injected animals, sumatriptan increased the paired pulse ratio of evoked EPSCs and reduced the rate of spontaneous miniature EPSCs. The 5-HT_1B and 5-HT_1D agonists CP9 3129 and PNU109291 both inhibited evoked EPSCs in CFA but not saline-injected rats. By contrast, the 5-HT_1A agonist R(+)-8-OH-DPAT inhibited evoked EPSCs in saline but not CFA-injected rats. In CFA-injected rats, the sumatriptan-induced inhibition of evoked EPSCs was reduced by the 5-HT_1B and 5-HT_1D antagonists NAS181 and BRL-15572. Intriguingly, the difference in sumatriptan inhibition between CFA and saline-injected animals was only observed in animals less than 4 weeks old.

CONCLUSION AND IMPLICATIONS

These findings indicate that inflammation induces a developmentally regulated 5-HT_1B/1D presynaptic inhibition of excitatory transmission into the rat superficial dorsal horn. Thus, triptans could potentially act as spinal analgesic agents for inflammatory pain in the juvenile setting.

Establishing GPCR Targets of hMAO Active Anthraquinones from Cassia obtusifolia Linn Seeds Using In Silico and In Vitro Methods

The present study examines the effect of human monoamine oxidase active anthraquinones emodin, alaternin (=7-hydroxyemodin), aloe-emodin, and questin from Cassia obtusifolia Linn seeds in modulating human dopamine (hD₁R, hD₃R, and hD₄R), serotonin (h5-HT_1AR), and vasopressin (hV_1AR) receptors that were predicted as prime targets from proteocheminformatics modeling via in vitro cell-based functional assays, and explores the possible mechanisms of action via in silico modeling. Emodin and alaternin showed a concentration-dependent agonist effect on hD₃R with EC₅₀ values of 21.85 ± 2.66 and 56.85 ± 4.59 μM, respectively. On hV_1AR, emodin and alaternin showed an antagonist effect with IC₅₀ values of 10.25 ± 1.97 and 11.51 ± 1.08 μM, respectively. Interestingly, questin and aloe-emodin did not have any observable effect on hV_1AR. Only alaternin was effective in antagonizing h5-HT_1AR (IC₅₀: 84.23 ± 4.12 μM). In silico studies revealed that a hydroxyl group at C1, C3, and C8 and a methyl group at C6 of anthraquinone structure are essential for hD₃R agonist and hV_1AR antagonist effects, as well as for the H-bond interaction of 1-OH group with Ser192 at a proximity of 2.0 Å. Thus, based on in silico and in vitro results, hV_1AR, hD₃R, and h5-HT_1AR appear to be prime targets of the tested anthraquinones.

Serotonergic Modulation of Nociceptive Circuits in Spinal Cord Dorsal Horn

Background:

Despite the extensive number of studies performed in the last 50 years, aimed at describing the role of serotonin and its receptors in pain modulation at the spinal cord level, several aspects are still not entirely understood. The interpretation of these results is often complicated by the use of different pain models and animal species, together with the lack of highly selective agonists and antagonists binding to serotonin receptors.

Method:

In this review, a search has been conducted on studies investigating the modulatory action exerted by serotonin on specific neurons and circuits in the spinal cord dorsal horn. Particular attention has been paid to studies employing electro-physiological techniques, both in vivo and in vitro.

Conclusion:

The effects of serotonin on pain transmission in dorsal horn depend on several factors, including the type of re-ceptors activated and the populations of neurons involved. Recently, studies performed by activating and/or recording from identified neurons have importantly contributed to the understanding of serotonergic modulation on dorsal horn circuits.

Targeting Serotonin1A Receptors for Treating Chronic Pain and Depression

The association of chronic pain with depression is becoming increasingly recognized. Treating both the conditions together is essential for an effective treatment outcome. In this regard, it is important to identify a shared mechanism involved in the association of chronic pain with depression. Central serotonin (5-hydroxytryptamine; 5-HT) neurotransmission has long been known to participate in the processing of signals related to pain. It also plays a key role in the pathogenesis and treatment of depression. Although functional responses to serotonin are mediated via the activation of multiple receptor types and subtypes, the 5-HT1A subtype is involved in the processing of nociception as well as the pathogenesis and treatment of depression. This receptor is located presynaptically, as an autoreceptor, on the perikaryon and dendritic spines of serotonin-containing neurons. It is also expressed as a heteroreceptor on neurons receiving input from serotonergic neurons. This arti-cle targets the 5-HT1A receptors to show that indiscriminate activation of pre and postsynaptic 5-HT1A receptors is likely to produce no therapeutic benefits; biased activation of the 5-HT heteroreceptors may be a useful strategy for treating chronic pain and depression individually as well as in a comorbid condition.

Monoamines as Drug Targets in Chronic Pain: Focusing on Neuropathic Pain

Monoamines are involved in regulating the endogenous pain system and indeed, peripheral and central monoaminergic dysfunction has been demonstrated in certain types of pain, particularly in neuropathic pain. Accordingly, drugs that modulate the monaminergic system and that were originally designed to treat depression are now considered to be first line treatments for certain types of neuropathic pain (e.g., serotonin and noradrenaline (and also dopamine) reuptake inhibitors). The analgesia induced by these drugs seems to be mediated by inhibiting the reuptake of these monoamines, thereby reinforcing the descending inhibitory pain pathways. Hence, it is of particular interest to study the monoaminergic mechanisms involved in the development and maintenance of chronic pain. Other analgesic drugs may also be used in combination with monoamines to facilitate descending pain inhibition (e.g., gabapentinoids and opioids) and such combinations are often also used to alleviate certain types of chronic pain. By contrast, while NSAIDs are thought to influence the monoaminergic system, they just produce consistent analgesia in inflammatory pain. Thus, in this review we will provide preclinical and clinical evidence of the role of monoamines in the modulation of chronic pain, reviewing how this system is implicated in the analgesic mechanism of action of antidepressants, gabapentinoids, atypical opioids, NSAIDs and histaminergic drugs.

The central effects of buspirone on abdominal pain in rats

BACKGROUND

Buspirone, a partial agonist of the 5-HT_1a receptor (5-HT_1a R), owing to potential antinociceptive properties may be useful in treatment of abdominal pain in IBS patients. The pain-related effects of buspirone are mediated via the 5-HT_1a Rs, specifically located within the ventrolateral medulla (VLM). The most animal studies of the 5-HT_1a R involvement in pain control have been carried out with somatic behavioral tests. The 5-HT_1a R contribution in visceral pain transmission within the VLM is unclear. The objective of our study was to evaluate the 5-HT_1a R contribution in abdominal pain transmission within the VLM.

METHODS

Using animal model of abdominal pain (urethane-anaesthetized rats), based on the noxious colorectal distension (CRD) as pain stimulus we studied effects of buspirone (1.0-4.0 mg kg^-1 , iv) on the CRD-induced VLM neuron and blood pressure responses as markers of abdominal pain before and after the 5-HT_1a R blockade by antagonist, WAY 100,635.

RESULTS

The CRD induced a significant increase in VLM neuron activity up to 201.5 ± 18.0% and depressor reactions up to 68 ± 1.8% of baseline. Buspirone (1.0-4.0 mg kg^-1 , iv) resulted in an inhibition of the CRD-induced neuron responses which were changed inversely with dose increase and decreased depressor reactions directly with dose increase. These effects were antagonized by intracerebroventricular WAY 100,635.

CONCLUSION

Buspirone exerts complex biphasic action on the pain-related VLM neuron activity inversely depending on dose. The final effect of buspirone depends on the functional balance between of activation the pre- and postsynaptic 5-HT_1a Rs in mediating pain control networks.

The potential role of serotonergic mechanisms in the spinal oxytocin-induced antinociception

The role of oxytocin (OXT) in pain modulation has been suggested. Indeed, hypothalamic paraventricular nuclei (PVN) electrical stimuli reduce the nociceptive neuronal activity (i.e., neuronal discharge associated with activation of Aδ- and C-fibers) of the spinal dorsal horn wide dynamic range (WDR) cells and nociceptive behavior. Furthermore, raphe magnus nuclei lesion reduces the PVN-induced antinociception, suggesting a functional interaction between the OXT and the serotoninergic system. The present study investigated in Wistar rats the potential role of spinal serotonergic mechanisms in the OXT- and PVN-induced antinociception. In long-term secondary mechanical allodynia and hyperalgesia induced by formalin or extracellular unitary recordings of the WDR cells we evaluated the role of 5-hydroxytryptamine (5-HT) effect on the OXT-induced antinociception. All drugs were given intrathecally (i.t.). OXT (1×10^-5-1×10^-4nmol) or 5-HT (1×10^-3-1×10^-1nmol) prevented the formalin-induced sensitization, an effect mimicked by PVN stimulation. Moreover, administration of OXT (1×10^-5nmol) plus 5-HT (1×10^-3nmol) at ineffective doses, produced antinociception. This effect was antagonized by: (i) d(CH₂)₅[Tyr(Me)²,Thr⁴,Tyr-NH₂⁹]OVT (oxytocin receptor antagonist; 2×10^-2nmol); or (ii) methiothepin (a non-specific 5-HT_1/2/5/6/7 receptor antagonist; 80nmol). Similar results were obtained with PVN stimulation plus 5-HT (5×10^-5nmol). In WDR cell recordings, the PVN-induced antinociception was enhanced by i.t. 5-HT and partly blocked when the spinal cord was pre-treated with methiothepin (80nmol). Taken together, these results suggest that serotonergic mechanisms at the spinal cord level are partly involved in the OXT-induced antinociception.

Blocking of urotensin receptors as new target for treatment of carrageenan induced inflammation in rats

This study investigated possible role of U-II and its receptor expression in inflammation by using UTR agonist and antagonist in carrageenan induced acute inflammation. Rats were divided into 5 groups as (1) Healthy control, (2) Carrageenan control, (3) Carrageenan +Indomethacin 20mg/kg, orally, (4) Carrageenan +AC7954 (U-II receptor agonist, intraperitoneally) 30mg/kg and (5) Carrageenan +SB657510 (UTR antagonist, intraperitoneally) 30mg/kg. 1h after drug administration, carrageenan was injected. At the 3rd hour after carrageenan injection, agonist produced no effect while antagonist 63% anti-inflammatory effect respectively. UTR and UT-II expression increased in carrageenan induced paw tissue. Antagonist administration prevented the decrease in an antioxidant system and also capable to decrease TNF-α and IL-6 mRNA expressions. This study showed the role of urotensin II receptors in the physiopathogenesis of acute inflammatory response that underlying many diseases accompanied by inflammation.

Descending control of itch transmission by the serotonergic system via 5-HT1A-facilitated GRP-GRPR signaling

Central serotonin (5-hydroxytryptophan, 5-HT) modulates somatosensory transduction, but how it achieves sensory modality-specific modulation remains unclear. Here we report that enhancing serotonergic tone via administration of 5-HT potentiates itch sensation, whereas mice lacking 5-HT or serotonergic neurons in the brainstem exhibit markedly reduced scratching behavior. Through pharmacological and behavioral screening, we identified 5-HT1A as a key receptor in facilitating gastrin-releasing peptide (GRP)-dependent scratching behavior. Coactivation of 5-HT1A and GRP receptors (GRPR) greatly potentiates subthreshold, GRP-induced Ca(2+) transients, and action potential firing of GRPR(+) neurons. Immunostaining, biochemical, and biophysical studies suggest that 5-HT1A and GRPR may function as receptor heteromeric complexes. Furthermore, 5-HT1A blockade significantly attenuates, whereas its activation contributes to, long-lasting itch transmission. Thus, our studies demonstrate that the descending 5-HT system facilitates GRP-GRPR signaling via 5-HT1A to augment itch-specific outputs, and a disruption of crosstalk between 5-HT1A and GRPR may be a useful antipruritic strategy.

VIDEO ABSTRACT

Neuronal calcium signaling in chronic pain

Acute physiological pain, the unpleasant sensory response to a noxious stimulus, is essential for animals and humans to avoid potential injury. Pathological pain that persists after the original insult or injury has subsided, however, not only results in individual suffering but also imposes a significant cost on society. Improving treatments for long-lasting pathological pain requires a comprehensive understanding of the biological mechanisms underlying pain perception and the development of pain chronicity. In this review, we aim to highlight some of the major findings related to the involvement of neuronal calcium signaling in the processes that mediate chronic pain.

Locally injected dexmedetomidine inhibits carrageenin-induced inflammatory responses in the injected region

BACKGROUND

Dexmedetomidine, a highly selective agonist of α2-adrenoceptors, is a commonly used sedative; however, a potent anti-inflammatory effect has also been found. In the present study we evaluated the inhibitory effect of locally injected dexmedetomidine on inflammatory responses in the injected region.

METHODS

Local inflammation was induced in the hindpaws of male mice (aged 6-8 weeks) by intraplantar injection of lambda-carrageenin. To offset the central effect of tested agents, different agents were blindly injected into the left and right paws in the pairs of comparison. The effect of dexmedetomidine on edema (increase in paw volume), the accumulation of leukocytes, and production of tumor necrosis factor-α (TNF-α) and cyclooxygenase-2 (COX-2) were evaluated after carrageenin injection, using water displacement plethysmometry, histological imaging, immunohistochemistry, and Western blotting analysis. Furthermore, we also evaluated the effect of yohimbine, a full antagonist of α2-adrenoceptors, and phenylephrine, an agonist of the α1-adrenoceptor, on dexmedetomidine's action on inflammatory responses.

RESULTS

Paw volume and amount of leukocytes in the injected region significantly increased after the injection of carrageenin. Similarly, TNF-α and COX-2 production was found in the subcutaneous region injected with carrageenin, 4 hours after injection. Dexmedetomidine significantly inhibited all increases in paw volume, leukocytes, and production of TNF-α and COX-2. Furthermore, yohimbine significantly antagonized the anti-inflammatory effects of dexmedetomidine, whereas phenylephrine did not significantly alter them.

CONCLUSIONS

The findings suggest that locally injected dexmedetomidine exhibits an anti-inflammatory effect against local acute inflammatory responses, mediated by α2-adrenoceptors.

An altered spinal serotonergic system contributes to increased thermal nociception in an animal model of depression

The olfactory bulbectomized (OB) rat, an animal model of chronic depression with comorbid anxiety, exhibits a profound dysregulation of the brain serotonergic signalling, a neurotransmission system involved in pain transmission and modulation. We here report an increased nociceptive response of OB rats in the tail flick test which is reverted after chronic, but not acute, administration of fluoxetine. Autoradiographic studies demonstrated down-regulation of 5-HT transporters ([(3)H]citalopram binding) and decreased functionality of 5-HT1A receptors (8-OH-DPAT-stimulated [(35)S]GTPγS binding) in the dorsal horn of the lumbar spinal cord in OB rats. Acute administration of fluoxetine (5-40 mg/kg i.p.) did not modify tail flick latencies in OB rats. However, chronic fluoxetine (10 mg/kg/day s.c., 14 days; osmotic minipumps) progressively attenuated OB-associated thermal hyperalgesia, and a total normalization of the nociceptive response was achieved at the end of the treatment with the antidepressant. In these animals, autoradiographic studies revealed further down-regulation of 5-HT transporters and normalization in the functionality of 5-HT1A receptors on the spinal cord. On the other hand, acute morphine (0.5-10 mg/kg s.c.) produced a similar analgesic effect in OB and sham and OB rats, and no changes were detected in the density ([(3)H]DAMGO binding) and functionality (DAMGO-stimulated [(35)S]GTPγS binding) of spinal μ-opioid receptors in OB rats before and after chronic fluoxetine. Our findings demonstrate the participation of the spinal serotonergic system in the increased thermal nociception exhibited by the OB rat and the antinociceptive effect of chronic fluoxetine in this animal model of depression.

5-HT(1A) receptor: an old target as a new attractive tool in drug discovery from central nervous system to cancer

The serotonin receptor subtype 5-HT(1A) was one of the first serotonin receptor subtypes pharmacologically characterized. This receptor subtype has long been object of intense research and is implicated in the pathogenesis and treatment of anxiety and depressive disorders. In recent years, new chemical entities targeting the 5-HT(1A) receptor (alone or in combination with other molecular targets) have been proposed for novel therapeutic uses in neuroprotection, cognitive impairment, Parkinson's disease, pain treatment, malignant carcinoid syndrome, and prostate cancer. This Perspective compares existing data on expression and signaling activity of the 5-HT(1A) receptor to a ligand with an intrinsic agonist or antagonist profile. Our purpose is also to make a complete overview, useful for underlining the features needed to select a specific pharmacological profile rather than another one. This aspect could be really interesting to consider and justify the 5-HT(1A) receptor as a new attractive target for drug discovery.

Pronociceptive effect of 5-HT(1A) receptor agonist on visceral pain involves spinal N-methyl-D-aspartate (NMDA) receptor

The functional role of serotonergic 5-HT(1A) receptors in the modulation of visceral pain is controversial. The objective of this study was to systematically examine the mechanism and site of action of a selective 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)-tetralin (DPAT) on visceral pain. In the behavioral model of visceral pain, systemic injection (5-250 μg/kg) of DPAT produced a significant increase in the viscero-motor response (VMR) to colorectal distension (CRD) and this effect was blocked by the selective 5-HT(1A) receptor antagonist WAY-100135 (5 mg/kg, s.c.). Similarly, intrathecal (i.t.) injection (5 μmol) of DPAT into the lumbo-sacral (L6-S1) spinal cord produced a significant increase in VMR. The administration of N-methyl D-aspartate (NMDA) receptor antagonist AP5 (50 μg/kg) prior to DPAT injection completely blocked the pronociceptive effect of DPAT. Similarly, DPAT failed to increase VMR in rats chronically treated with NR1 subunit-targeted antisense oligonucleotide (ON), whereas the drug increased VMR in rats treated with mismatched-ON. Chronic i.t. injection of allylglycine (AG), a γ-amino decarboxylase (GAD) enzyme inhibitor, produced significant increase in VMRs, suggesting that the inhibition of GABA synthesis produces pronociception. In AG-treated rats, i.t. injection of DPAT failed to further increase in VMR, suggesting that the DPAT action is linked to GABA release. Similarly, WAY-100135 failed to attenuate VMR in AG-treated rats, suggesting that unlike DPAT, AG action is not via the activation of 5-HT(1A) receptors. In electrophysiology experiments, DPAT (50 μg/kg) significantly increased the responses of spinal neurons to CRD, but did not influence the mechanotransduction property of CRD-sensitive pelvic nerve afferent fibers. The effect of DPAT on spinal neurons remained unaffected when tested in spinal-transected (C1-C2) rats. These results indicate that the 5-HT(1A) receptor agonist DPAT produces pronociceptive effects, primarily via the activation of presynaptic 5-HT(1A) receptors in GABAergic neuron to restrict GABA release and thereby disinhibits the excitatory glutamatergic neurons in the spinal cord.

Role of spinal 5-HT1A receptors in morphine analgesia and tolerance in rats

We here studied the involvement of spinally located 5-HT(1A) and opioid receptors, in the paradoxical effects that their activation can produce on nociception. Intrathecal (i.t.) injection of the 5-HT(1A) receptor agonist 8-hydroxy-2-[di-n-propylamino] tetralin (8-OH-DPAT) (1-10 microg) induced analgesic effects in the formalin model of tonic pain whereas in the paw pressure test, it decreased the vocalization threshold. In this latter test, i.t. 8-OH-DPAT also markedly reduced the analgesic effect of systemic morphine (5-10 mg/kg, s.c.). At 10 microg, 8-OH-DPAT totally abolished the effect of 5 mg/kg of morphine; this inhibitory effect was antagonized by pre-treatment with 0.63 mg/kg of the 5-HT(1A) antagonist WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]-ethyl]-N-(2-pyridinyl)-cyclohexanecarboxamide-trihydrochloride). In contrast, the i.t. injection of WAY-100635 (1-10 microg) dose-dependently potentiated the antinociceptive activity of a dose of morphine (2.5 mg/kg, s.c.). Furthermore, WAY-100635 (10 microg, i.t.) potentiated morphine analgesia in morphine-tolerant rats. These findings demonstrate that 5-HT(1A) receptor agonists can act in the spinal cord to produce both hyper- and hypo-algesic effects and play a major role in the opioid analgesia and tolerance.

Taltirelin, a thyrotropin-releasing hormone analog, alleviates mechanical allodynia through activation of descending monoaminergic neurons in persistent inflammatory pain

Thyrotropin-releasing hormone (TRH) and its analogs have been reported to modulate descending monoaminergic inhibitory neurons, resulting in antinociception. However, it remains unknown whether TRH exerts an antiallodynic effect during persistent pain. Here, we investigated the action of taltirelin, a stable TRH analog, on mechanical allodynia in mice with inflammatory persistent pain induced by an injection of complete Freund's adjuvant into the hindpaw. Systemic administration of 1.0 mg/kg taltirelin markedly reduced mechanical allodynia. This effect was abolished by the 6-hydroxydopamine (6-OHDA)-induced depletion of central noradrenaline. While intraperitoneal injection of the α₁-adrenoceptor antagonist prazosin had no effect, intraperitoneal and intrathecal administration of the α₂-adrenoceptor antagonist yohimbine prevented the antiallodynic action of taltirelin. In addition, DL-p-chlorophenylalanine (PCPA)-induced depletion of serotonin (5-HT) and intraperitoneal and intrathecal injection of the 5-HT(1A) receptor antagonist WAY-100635 blocked the effect of taltirelin on allodynia. These findings suggest that taltirelin alleviates mechanical allodynia in inflammatory persistent pain by modulating the descending noradrenergic and serotonergic neuronal pathways via indirect activation of spinal α₂-adrenergic and 5-HT(1A) receptors.

Multiple opioid receptors mediate the hypotensive response induced by central 5-HT(3) receptor stimulation

The aim of the present work was to investigate the role of brain μ, κ and δ opioid receptors in the central serotonergic mechanisms regulating blood pressure in rats. The data obtained show that: (1) pharmacological activation of central 5-HT(3) receptors yields a significant decrease in blood pressure; (2) the blockade of those receptors by a selective antagonist induces an acute hypertensive response; (3) the pharmacological blockade of central opioid receptors by three different opioid antagonists exhibiting variable degrees of selectivity to μ, κ and δ opioid receptors always suppressed the hypotensive response induced by central 5-HT(3) receptor stimulation; (4) the blockade of opioid receptors by the same opioid antagonists that impaired the hypotensive effect of central 5-HT(3) receptor stimulation failed to modify blood pressure in animals not submitted to pharmacological manipulations of central 5-HT(3) receptor function. It is shown that a 5-HT(3) receptor-dependent mechanism seems to be part of the brain serotonergic system that contributes to cardiovascular regulation since the hypertensive response observed after ondansetron administration indicates that central 5-HT(3) receptors exert a tonic inhibitory drive on blood pressure. Furthermore, the data obtained here clearly indicate that the hypotensive response observed after pharmacological stimulation of central 5-HT(3) receptors depends on the functional integrity of brain μ, κ and δ opioid receptors, suggesting that a functional interaction between serotonergic and opiatergic pathways in the brain is part of the complex, multifactorial system that regulates blood pressure in the central nervous system.

Effects of 5-hydroxytryptamine on substantia gelatinosa neurons of the trigeminal subnucleus caudalis in immature mice

Serotonin (5-hydroxytryptamine, 5-HT) is involved in the descending modulation of nociceptive transmission in the spinal dorsal horn. The trigeminal subnucleus caudalis (Vc; medullary dorsal horn) processes nociceptive input from the orofacial region, and 5-HT-containing axons are numerous in the superficial layers of the Vc. This study examined the actions of 5-HT on the substantia gelatinosa (SG) neurons of the Vc, using gramicidin-perforated patch-clamp recording in brainstem slice preparations from immature mice. In order to clarify the possible mechanisms underlying 5-HT actions in the SG of the Vc, the direct membrane effects of 5-HT and effects of 5-HT receptor subtype agonists were examined. 5-HT induced a hyperpolarization in the majority (64/115, 56%) of the SG neurons tested. Thirty nine (34%) SG neurons showed no response, and 12 (10%) neurons responded with depolarization. The hyperpolarizing response to 5-HT was concentration-dependent (0.1-30 μM; n=7), not desensitized by repeated application (n=22), and significantly attenuated by Ba(2+) (K(+) channel blocker; n=8). The 5-HT-induced hyperpolarization was maintained in the presence of TTX (Na(+) channel blocker), CNQX (non-NMDA glutamate receptor antagonist), AP5 (NMDA glutamate receptor antagonist), picrotoxin (GABA(A) receptor antagonist), and strychnine (glycine receptor antagonist), indicating direct postsynaptic action of 5-HT on SG neurons (n=7). The 5-HT-induced hyperpolarizing effects were mimicked by 8-OH-DPAT (5-HT(1A) receptor agonist) and α-methyl-5-HT (5-HT(2) receptor agonist) and blocked by WAY-100635 (5-HT(1A) receptor antagonist) and ketanserin (5-HT(2) receptor antagonist). Single-cell RT-PCR also revealed the presence of mRNA for 5-HT(1A) and 5-HT(2C) subtypes in the SG neurons. These results suggest that 5-HT acts directly on SG neurons and 5-HT-induced hyperpolarization is mediated, in part, by 5-HT(1A) receptors and 5-HT(2) receptors, as well as by the activation of K(+) channels, indicating an important role for 5-HT in the modulation of orofacial nociceptive processing at the level of the SG of the Vc in mice.

The antihyperalgesic effect of levetiracetam in an inflammatory model of pain in rats: mechanism of action

BACKGROUND AND PURPOSE

Levetiracetam, a novel antiepileptic drug, has recently been shown to have antinociceptive effects in various animal models of pain. The purpose of this study was to investigate the antihyperalgesic effect of levetiracetam and its mechanism of action, by examining the involvement of GABAergic, opioidergic, 5-hydroxytryptaminergic (5-HTergic) and adrenergic systems in its effect, in a rat model of inflammatory pain.

EXPERIMENTAL APPROACH

Rats were intraplantarly injected with the pro-inflammatory compound carrageenan. A paw pressure test was used to determine: (i) the effect of levetiracetam on carrageenan-induced hyperalgesia; and (ii) the effects of bicuculline (selective GABA(A) receptor antagonist), naloxone (non-selective opioid receptor antagonist), methysergide (non-selective 5-HT receptor antagonist) and yohimbine (selective alpha(2)-adrenoceptor antagonist) on the antihyperalgesic action of levetiracetam.

RESULTS

Levetiracetam (10-200 mg.kg(-1); p.o.) significantly reduced, in a dose-dependent manner, the inflammatory hyperalgesia induced by carrageenan. The antihyperalgesic effect of levetiracetam was significantly decreased after administration of bicuculline (0.5-2 mg.kg(-1); i.p.), naloxone (1-3 mg.kg(-1); i.p.), methysergide (0.25-1 mg.kg(-1); i.p.) and yohimbine (1-3 mg.kg(-1); i.p.).

CONCLUSIONS AND IMPLICATIONS

These results show that levetiracetam produced antihyperalgesia which is at least in part mediated by GABA(A), opioid, 5-HT and alpha(2)-adrenergic receptors, in an inflammatory model of pain. The efficacy of levetiracetam in this animal model of inflammatory pain suggests that it could be a potentially important agent for treating inflammatory pain conditions in humans.

Repinotan, a selective 5-HT1A-R-agonist, antagonizes morphine-induced ventilatory depression in anesthetized rats

BACKGROUND

Spontaneous breathing during mechanical ventilation improves arterial oxygenation and cardiovascular function, but is depressed by opioids during critical care. Opioid-induced ventilatory depression was shown to be counteracted in anesthetized rats by serotonin(1A)-receptor (5-HT(1A)-R)-agonist 8-OH-DPAT, which cannot be applied to humans. Repinotan hydrochloride is a selective 5-HT(1A)-R-agonist already investigated in humans, but the effects on ventilation and nociception are unknown. In this study, we sought to establish (a) the effects of repinotan on spontaneous breathing and nociception, and (b) the interaction with the standard opiate morphine.

METHODS

The dose-dependent effects of repinotan, given alone or in combination with morphine, on spontaneous minute ventilation (MV) and nociceptive tail-flick reflex latencies (TFLs) were measured simultaneously in spontaneously breathing anesthetized rats. An additional series with NaCl 0.9% and the 5-HT(1A)-R-antagonist WAY 100 135 served as controls.

RESULTS

(a) Repinotan dose-dependently activated spontaneous breathing (MV, mean [95% confidence interval]; 53% [29%-77%]) of pretreatment level) and suppressed nociception (TLF, 91% maximum possible effect [68%-114%]) with higher doses of repinotan (2-200 μg/kg). On the contrary, nociception was enhanced with a small dose of repinotan (0.2 μg/kg; TFL, -47% maximum possible effect [-95% to 2%]). Effects were prevented by 5-HT(1A)-antagonist WAY 100 135. (B) Morphine-induced depression of ventilation (MV, -72% [-100% to -44%]) was reversed by repinotan (20 μg/kg), which returned spontaneous ventilation to pretreatment levels (MV, 18% [-40% to 77%]). The morphine-induced complete depression of nociception was sustained throughout repinotan and NaCl 0.9% administration. Despite a mild decrease in mean arterial blood pressure, there were no serious cardiovascular side effects from repinotan.

CONCLUSIONS

The 5-HT(1A)-R-agonist repinotan activates spontaneous breathing in anesthetized rats even in morphine-induced ventilatory depression. The potency of 5-HT(1A)-R-agonists to stimulate spontaneous breathing and their antinociceptive effects should be researched further.

Subtype-specific changes in 5-HT receptor-mediated modulation of C fibre-evoked spinal field potentials are triggered by peripheral nerve injury

Neurotransmitter serotonin (5-HT) released from descending pain modulation pathways to the dorsal horn is crucial to spinal nociception processing. This study sought to gain insight into the modulatory roles of specific serotonin receptor subtypes in experimentally induced neuropathic pain. In rats subjected to spinal nerve ligation (SNL) surgery, we recorded field potentials evoked in the spinal dorsal horn by C fibre-input, during spinal superfusion with subtype-selective drugs. In neuropathic rats, subtype 5-HT1A agonist 8-OH-DPAT (100 nM) was found to potently depress evoked field potentials, as opposed to 5-HT2A or 5-HT2B subtype agonists TCB-2 (100 nM) or BW 723C86 (1 microM), respectively, which consistently enhanced evoked potentials. All three failed to alter spinal field potentials in sham operated rats. CP 94253 (1 microM), WAY 161503 (1 mM) or SR 57227 (at 1 microM in SNL rats, and 100 microM in sham rats), selective agonists for 5-HT1B, 5-HT2C and 5-HT3 receptors, respectively, significantly depressed evoked field potentials in both animal groups. The 5-HT4 agonist RS 67333 (1 microM) was depressant only in sham operated animals. Only after SNL, spinal superfusion with 5-HT1A- or 5-HT1B receptor-antagonists (S)-WAY 100135 (100 microM) or SB 224289 (100 microM), respectively, disinhibited C fibre-evoked potentials, whereas 5-HT2A or 5-HT2B receptor-antagonists 4F 4PP (100 microM) or SB 204741 (100 microM) depressed evoked potentials, suggesting tonic activity of all four subtypes as a consequence of experimental nerve injury. The present findings reveal profound subtype-specific changes in the functional modulatory activities of spinal serotonin receptors following peripheral nerve injury. In particular, spinal hyperexcitation promoted by receptors 5-HT2A and 5-HT2B is suggested as a novel pathogenic pathway contributing to neuropathic pain.

The role of peripheral 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F serotonergic receptors in the reduction of nociception in rats

This study assessed the possible antinociceptive role of peripheral 5-HT(1) receptor subtypes in the rat formalin test. Rats were injected into the dorsum of the hind paw with 50 microl of diluted formalin (1%). Nociceptive behavior was quantified as the number of flinches of the injected paw. Reduction of flinching was considered as antinociception. Ipsilateral, but not contralateral, peripheral administration of the 5-HT(1) receptor agonists R(+)-UH-301 (5-HT(1A); 0.1-3 microg/paw), CGS-12066A (5-HT(1B); 0.01-0.3 microg/paw), GR46611 (5-HT(1B/1D); 0.3-10 microg/paw), BRL54443 (5-HT(1E/1F); 3-300 microg/paw) or LY344864 (5-HT(1F); 3-300 microg/paw) significantly reduced formalin-induced flinching. The corresponding vehicle was devoid of any effect by itself. The local antinociceptive effect of R(+)-UH-301 (0.3 microg/paw) was significantly reduced by WAY-100635 (30-100 microg/paw; a 5-HT(1A) receptor antagonist). Moreover, the antagonists GR55562 (30-100 microg/paw; 5-HT(1B/D)) or SB224289 (30-100 microg/paw; 5-HT(1B)) dose-dependently reduced the antinociceptive effect of CGS-12066A (0.3 microg/paw) whereas GR55562 (30-100 microg/paw) or BRL15572 (30-100 microg/paw, 5-HT(1D)) reduced the antinociceptive effect of GR46611 (0.3 microg/paw). Interestingly, the effects of BRL54443 and LY344864 (300 microg/paw each) were partially reduced by methiothepin, but not by the highest doses of WAY-100635, SB224289 or BRL15572. The above antagonists did not produce any effect by themselves. These results suggest that peripheral activation of the 5-HT(1A,) 5-HT(1B), 5-HT(1D), 5-HT(1F) and, probably, 5-HT(1E) receptor subtypes leads to antinociception in the rat formalin test. Thus, the use of selective 5-HT(1) receptor agonists could be a therapeutic strategy to reduce inflammatory pain.

5-HT1A receptors are involved in the effects of xaliproden on G-protein activation, neurotransmitter release and nociception

BACKGROUND AND PURPOSE

Xaliproden (SR57746A) is a 5-HT(1A) receptor agonist and neurotrophic agent that reduces oxaliplatin-mediated neuropathy in clinical trials. The present study investigated its profile on in vitro transduction, neurochemical responses and acute nociceptive pain tests in rats.

EXPERIMENTAL APPROACH

Xaliproden was tested on models associated with 5-HT(1A) receptor activation including G-protein activation, extracellular dopamine and 5-HT levels measured by microdialysis and formalin-induced pain. Activation of 5-HT(1A) receptors was confirmed by antagonism with WAY100635.

KEY RESULTS

Xaliproden exhibited high affinity for rat (r) and human (h) 5-HT(1A) receptors (pK(i)= 8.84 and 9.00). In [(35)S]GTPgammaS (guanosine 5'-O-(3-[(35)S]thio)triphosphate) assays it activated both hippocampal r5-HT(1A)[pEC(50)/E(MAX) of 7.58/61% (%5-HT)] and recombinant h5-HT(1A) receptors (glioma C6-h5-HT(1A): 7.39/62%; HeLa-h5-HT(1A): 7.24/93%). In functional [(35)S]GTPgammaS autoradiography, xaliproden induced labelling in structures enriched with 5-HT(1A) receptors (hippocampus, lateral septum, prefrontal and entorhinal cortices). Xaliproden inhibited in vivo binding of [(3)H]WAY100635 to 5-HT(1A) receptors in mouse frontal cortex and hippocampus (ID(50): 3.5 and 3.3 mg x kg(-1), p.o. respectively). In rat, it increased extracellular dopamine levels in frontal cortex and reduced hippocampal 5-HT levels (ED(50): 1.2 and 0.7 mg x kg(-1), i.p. respectively). In a rat pain model, xaliproden inhibited paw licking and elevation (ED(50): 1 and 3 mg x kg(-1), i.p. respectively) following formalin injection in the paw. All effects were reversed by pretreatment with WAY100635.

CONCLUSIONS AND IMPLICATIONS

These results indicate that activation of 5-HT(1A) receptors is the principal mechanism of action of xaliproden and provide further support for the utility of 5-HT(1A) receptor activation as an anti-nociceptive strategy.

Expression patterns of 5-HT receptor subtypes 1A and 2A on GABAergic neurons within the spinal dorsal horn of GAD67-GFP knock-in mice

5-HT plays an important role in GABA transmission at the spinal cord level. The main purpose of the present article is to find out, which 5-HT receptor subtypes may be involved in the regulation of GABAergic transmission in the spinal dorsal horn, by using the combination of tissue RT-PCR, single-cell RT-PCR and double immunofluorescent staining on the GAD(67)-GFP knock-in mice. The present result showed that 5-HT(1A) receptor subtype mRNA was present on about half of GFP-positive GABAergic interneurons in the spinal dorsal horn, which suggests a role of 5-HT(1A) receptor in GABA-mediated modulation of nociception at the spinal cord level. However, it is unexpected that just a very limited sub-population of the total GABAergic interneurons express 5-HT(2A) receptor mRNA or proteins in the spinal dorsal horn, which suggests that the 5-HT(2A) receptor subtype may not be important for spinal GABAergic effects in nociceptive modulation.

Responses to 5-HT in morphologically identified neurons in the rat substantia gelatinosa in vitro

Bath application of 5-HT (1-1000 muM) induced a tetrodotoxin (TTX)-resistant outward current at the holding membrane potential (V(H)) of -50 mV in 104/162 (64.2%) of substantia gelatinosa (SG) neurons from the rat spinal cord in vitro. The 5-HT-induced outward current was suppressed by an external solution containing Ba(2+), or a pipette solution containing Cs(2)SO(4) and tetraethylammonium. It was reversed near the equilibrium potential of the K(+) channel. The response to 5-HT was abolished 30 min after patch formation with a pipette solution containing guanosine-5-O-(2-thiodiphosphate)-S. The 5-HT-induced outward current was mimicked by a 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide, and suppressed by a 5-HT(1A) antagonist, WAY100635, suggesting the 5HT(1A) receptor-mediated activation of K(+) channels in the outward current. In 11/162 (6.8%) SG neurons, 5-HT produced an inward current, which was mimicked by a 5-HT(3) agonist, 1-(m-chlorophenyl)-biguanide (mCPBG). The 5-HT-induced outward currents were observed in vertical cells (21/34) and small islet cells (11/34), while inward currents were induced in islet cells (1/5) and small islet (4/5) cells, but not in vertical cells. It is known that most vertical cells and islet cells in the SG are excitatory (glutamatergic) and inhibitory interneurons, respectively, while small islet cells consist of both excitatory and inhibitory neurons. Bath application of 5-HT or mCPBG increased the amplitude and the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs), but no neurons showed a decrease in sIPSC. Furthermore, frequency, but not amplitude, of miniature IPSCs increased with perfusion with 5-HT in the presence of TTX. These findings, taken together, suggest that 5-HT induces outward currents through 5-HT(1A) receptors in excitatory SG neurons. These findings also suggest that the inward currents are post- and presynaptically evoked through 5-HT(3) receptors, probably in inhibitory neurons.

Role of serotonin 5-HT1A and opioid receptors in the antiallodynic effect of tramadol in the chronic constriction injury model of neuropathic pain in rats

RATIONALE

Tramadol (1RS, 2RS)-2-[(dimethylamino)-methyl]-1-(3-methoxyphenyl)-cyclohexanol) is an atypical centrally acting analgesic agent with weak opioid receptor affinity that, like some antidepressants, enhances the extraneuronal concentrations of the monoamine neurotransmitters, noradrenaline and serotonin, by interfering with their re-uptake and release mechanisms.

OBJECTIVES

The present study was undertaken to evaluate the potential role of 5-HT(1A) receptors and opioids receptors in the analgesic effect of tramadol in neuropathic pain. With this aim, the effect of either a selective 5-HT(1A) receptor antagonist (WAY-100635, N-2-[4-(2-methoxyphenyl-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexane carboxamide) or a selective 5-HT(1A) receptor agonist (8-OH-DPAT, 8-hydroxy-2-(di-n-propylamine) tetralin hydrobromide) or an opioid receptor antagonist (naloxone; naloxone hydrochloride dihydrate) was investigated in combination with tramadol by means of the cold-plate test in the chronic constriction injury model in rats.

RESULTS

The results showed that WAY-100635 (0.8 mg/kg) significantly enhanced the antiallodynic effect of non-effective doses of tramadol (5-10 mg/kg). In contrast, 8-OH-DPAT (0.5 mg/kg) counteracted the antiallodynic effect of an effective dose of tramadol (22 mg/kg). Naloxone (0.5 mg/kg) partially counteracted the antiallodynic effect of tramadol (22 mg/kg).

CONCLUSIONS

These findings suggest the involvement of opioid and 5-HT(1A) receptors in the antinociceptive effect of tramadol and support the idea that the combination of tramadol with compounds having 5-HT(1A) antagonist properties could be a new strategy to improve tramadol-induced analgesia in neuropathic pain.

Inhibition of opioid release in the rat spinal cord by serotonin 5-HT(1A) receptors

Neurotransmitter receptors that inhibit the release of opioid peptides in the spinal cord may play an important role in modulating pain. Serotonin is an important neurotransmitter in bulbospinal descending pathways, and 5-HT(1) receptors have been shown to inhibit synaptic transmission. Our goal was to determine whether 5-HT(1A) receptors inhibit opioid release in the spinal cord. Opioid release was evoked from rat spinal cord slices by electrically stimulating one dorsal horn, and measured in situ through the internalization of micro-opioid receptors in dorsal horn neurons. Stimulation with 1000 square pulses at 500 Hz produced internalization in 60% of the mu-opioid receptor neurons in the stimulated dorsal horn, but not in the contralateral one. The selective 5-HT(1A) receptor agonist 8-hydroxy-2-dipropylaminotetralin (8-OH-DPAT) inhibited the evoked mu-opioid receptor internalization by about 50%, with an approximate IC(50) of 50 nM. The effect of 8-OH-DPAT was attributed to inhibition of opioid release and not of the receptor internalization process, because 8-OH-DPAT did not inhibit the internalization induced by incubating the slices with a micro-opioid receptor agonist (endomorphin-2, 100 nM). The selective 5-HT(1A) receptor antagonist WAY100135 (10 microM) blocked the inhibition produced by 1 microM 8-OH-DPAT. These results show that 5-HT(1A) receptors inhibit opioid release in the spinal dorsal horn, probably from a subpopulation of enkephalin-containing presynaptic terminals. Therefore, 5-HT(1A) receptors likely decrease the analgesia produced by endogenously released opioids.

Mechanical, thermal and formalin-induced nociception is differentially altered in 5-HT1A-/-, 5-HT1B-/-, 5-HT2A-/-, 5-HT3A-/- and 5-HTT-/- knock-out male mice

Extensive studies in rodents suggest that serotonin (5-HT) modulates nociceptive responses through the stimulation of several receptor types. However, it remains to demonstrate that these receptors participate in the control of nociception under physiological conditions. Pain behaviors of mutants which do not express 5-HT1A, 5-HT1B, 5-HT2A or 5-HT3A receptors, or lacking the 5-HT transporter, compared to paired wild-type mice of the same genetic background, were examined using validated tests based on different sensory modalities. Mechanical (von Frey filaments, tail pressure, tail clip tests), thermal (radiant heat, 46 degrees C water bath, hot-plate test) and formalin-induced nociception were determined in 2- to 3-month-old males. 5-HT1A knock-out mice differed from wild-types by higher thermal sensitivity (hot-plate test only), and 5-HT1B knock-out mice by higher thermal and formalin sensitivity. Both 5-HT2A and 5-HT3A knock-out mice differed from wild-types by a dramatic decrease in the formalin-induced nociceptive responses for phase II (16-45 min after injection/inflammatory phase). In contrast, neither mechanical, thermal nor formalin-induced nociception differed between mutants lacking the 5-HT transporter and paired wild-type mice. Although differences in spontaneous locomotor activity in 5-HT1B-/- (increase) and 5-HT3A-/- (decrease) knock-out mice versus paired wild-types might have confounded differences in nociception, acute 5-HT receptor blockade by selective antagonists was found to replicate in wild-type mice the effects on pain behavior, but not on locomotor activity, of the respective gene knock-out in mutants. These results support the conclusion that the complex control of pain mechanisms by 5-HT, acting at multiple receptors, is physiologically relevant in mice.

Intrathecal treatment with sigma1 receptor antagonists reduces formalin-induced phosphorylation of NMDA receptor subunit 1 and the second phase of formalin test in mice

Although previous reports have suggested that the sigma 1 (sigma(1)) receptor may be involved in pain sensation, its specific site of action has not been elucidated. The aim of present study was to determine the role of the spinal sigma(1) receptor in formalin-induced pain behavior, spinal cord Fos expression and phosphorylation of N-methyl-D-aspartate receptor subunit 1 (pNR1). Intrathecal (i.t.) pretreatment with the selective sigma(1) receptor antagonist, BD-1047 (N-[2-(3,4-dichlorophenyl)ethyl]-N-methyl-2-(dimethylamino)ethylamine dihydrobromide) (10-100 nmol) dose dependently reduced formalin-induced pain behaviors in second phase, but not first phase, of the formalin test. I.t. injection of BD-1047 also reduced formalin-evoked Fos expression and pNR1 at the protein kinase C-dependent site, serine-896 (Ser896) and the protein kinase A-dependent site, serine-897 (Ser897) in spinal dorsal horn.i.t. BMY-14802 ((alpha-(4-fluorophenyl)-4-(5-fluoro-2-pyrimidinyl)-1-piperazinebutanol hydrochloride) (10-100 nmol, sigma(1) receptor antagonist and 5-HT(1A) receptor agonist) dose dependently reduced formalin-induced pain behaviors in both phases. However, the 5-HT(1A) receptor might not be involved in the antinociceptive effect of BMY-14802 on the second phase, since i.t. pretreatment with the 5-HT(1A) receptor antagonist propranolol ((S)-1-isopropylamino-3-(1-naphthyloxy)-2-propanol hydrochloride) (injected 10 min prior to i.t. BMY-14802) partially blocked the effect of BMY-14802 on the first phase of the formalin test but did not affect the inhibitory effect of BMY-14802 on the second phase. In addition, i.t. BMY-14802 significantly reduced formalin-evoked Fos expression and pNR1 (Ser896 and Ser897) expression in spinal dorsal horn. The results of this study suggest that selective blockage of spinal sigma(1) receptors can reduce pain behaviors, spinal cord Fos expression and pNR1 (Ser896 and Ser897) expression associated with the second phase of the formalin test.

5-HT1A receptors in endogenous regulation of neuropathic hypersensitivity in the rat

The role of medullary and spinal 5-HT(1A) receptors in endogenous regulation of neuropathic hypersensitivity was studied. When administered in the rostroventromedial medulla or subcutaneously, WAY-100635, a 5-HT(1A) receptor antagonist, attenuated mechanical hypersensitivity in rats with a spinal nerve injury. Thermal or mechanical nociception outside of the injured area was not influenced by medial medullary or subcutaneous administration of WAY-100635. Intrathecal administration of WAY-100635 had no significant effect on pain-related behavior. Suppression of mechanical hypersensitivity induced by medial medullary administration of WAY-100635 was reversed by intrathecal administration of WAY-100635 or atipamezole, an alpha2-adrenoceptor antagonist, but not by naloxone, an opioid receptor antagonist. The results indicate that endogenous release of 5-HT, via action on medial medullary 5-HT(1A) receptors, tonically suppresses descending inhibition in neuropathic animals. Following medial medullary administration of a 5-HT(1A) receptor antagonist, descending pain regulatory pathways are disinhibited. This leads to selective attenuation of neuropathic hypersensitivity, due to action on spinal 5-HT(1A) receptors and alpha2-adrenoceptors.

Central serotonin 3 receptors play an important role in the modulation of nociceptive neural activity of trigeminal subnucleus caudalis and nocifensive orofacial behavior in rats with persistent temporomandibular joint inflammation

The role of central serotonin 3 receptors on neural activities recorded from superficial laminae of trigeminal subnucleus caudalis/upper cervical spinal cord junction region was investigated using rats with (Complete Freund's Adjuvant day 7 group) or without (non-Complete Freund's Adjuvant group) persistent temporomandibular joint inflammation evoked by Complete Freund's Adjuvant for 7 days. We identified two types of units, Deep-wide dynamic range units and Skin-wide dynamic range units from extracellular recordings. Deep-wide dynamic range units have mechanoreceptive fields in the deep craniofacial tissues including masseter muscle but do not have cutaneous mechanoreceptive fields. Deep-wide dynamic range unit discharges evoked by the formalin injection into masseter muscle were significantly enhanced in the late phase in Complete Freund's Adjuvant day 7 group. Discharges of Skin-wide dynamic range units evoked by the noxious pinch stimulation to facial skin in Complete Freund's Adjuvant day 7 group were significantly enhanced compared with those in non-Complete Freund's Adjuvant group. Topical administration of central serotonin 3 receptor antagonist, tropisetron, onto trigeminal subnucleus caudalis/upper cervical spinal cord junction region significantly reduced both formalin-evoked Deep-wide dynamic range unit and pinch-evoked Skin-wide dynamic range unit discharges in non-Complete Freund's Adjuvant and Complete Freund's Adjuvant day 7 groups significantly. The inhibitory effects of tropisetron on pinch-evoked Skin-wide dynamic range unit discharges were prolonged in Complete Freund's Adjuvant day 7 group compared with those in non-Complete Freund's Adjuvant group. The role of central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region was also tested by orofacial formalin test in Complete Freund's Adjuvant day 7 group. Intracisternal administration of tropisetron decreased the orofacial nocifensive behavior in the late phase evoked by the injection of formalin into the masseter muscle. These results suggest that central serotonin 3 receptors in trigeminal subnucleus caudalis/upper cervical spinal cord junction region are involved in mediating pronociceptive effects in both superficial and deep craniofacial tissues nociception during persistent temporomandibular joint inflammation.

Effects of the high-efficacy 5-HT1A receptor agonist, F 13640 in the formalin pain model: a c-Fos study

We studied the effects of the high-efficacy 5-hydroxytryptamine1A (5-HT1A) receptor agonist, F 13640 on both formalin-induced spinal cord c-Fos protein expression and pain behaviours in the rat. Replicating earlier data, F 13640 (0.63 mg/kg, i.p.; t(-15 min)) completely inhibited the elevation and licking of the formalin-injected paw. In the same animals, and in spite of the agent as in earlier data increasing the number of c-Fos labelled nuclei when it was administered alone, F 13640 markedly reduced the number of formalin-induced c-Fos labelled nuclei. This was found in both the superficial (I-II) and deep (V-VI) dorsal horn laminae (2 h post-injection: 72+/-2% and 92+/-1% of reduction, respectively; P<0.001 in either case), spinal areas that contain neurons responsive to nociceptive stimulation. Co-operation occurred so that after the co-administration of F 13640 and formalin, c-Fos expression was inferior to that induced when either stimulation was administered alone. The data provide initial evidence for the agent's inhibitory effects on noxiously evoked c-Fos expression. The results indicate that co-operation between 5-HT1A receptor activation and nociceptive stimulation powerfully inhibits responses to severe, tonic nociception.

Spinal 5-HT1A receptors differentially influence nociceptive processing according to the nature of the noxious stimulus in rats: effect of WAY-100635 on the antinociceptive activities of paracetamol, venlafaxine and 5-HT

The regulation of nociceptive processing by 5-HT at the spinal level is intricate since the neurotransmitter has been implicated in both pro and antinociception. The aim of our study was to investigate, according to the nature of the noxious stimulus, how the blockade of spinal 5-HT(1A) receptors could influence the antinociceptive actions of exogenous 5-HT as well as two analgesics involving endogenous 5-HT, paracetamol and venlafaxine. Rats were submitted either to the formalin test (tonic pain) or the paw pressure test (acute pain). WAY-100635 (40 microg/rat, i.t.), a selective 5-HT(1A) receptor antagonist, had no intrinsic action in either test. However, in the formalin test, it blocked the antinociceptive action of 5-HT (50 microg/rat, i.t.) and paracetamol (300 mg/kg, i.v.) in both phases of biting/licking behaviour and that of venlafaxine (2.5 mg/kg, s.c.) in the late phase only. In the paw pressure test, the combination of sub-effective doses of 5-HT (0.01 microg/rat, i.t.), paracetamol (50 mg/kg, i.v.) or venlafaxine (20 mg/kg, s.c.) with WAY-100635 led to a significant antinociceptive effect, which seems to depend on the reinforcement of the activity of inhibitory GABAergic interneurones. In conclusion, both direct stimulation of the spinal 5-HT(1A) receptors by 5-HT, and indirect stimulation using paracetamol or venlafaxine can differently influence pain transmission. We propose that the nature of the applied nociceptive stimulus would be responsible for the dual effect of the 5-HT(1A) receptors rather than the hyperalgesic state or the supraspinal integration of the pain message.

Dual, hyperalgesic, and analgesic effects of the high-efficacy 5-hydroxytryptamine 1A (5-HT1A) agonist F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt]: relationship with 5-HT1A receptor occupancy and kinetic parameters

The aim of the present study was to establish the relationship between the plasma and brain concentration-time profiles of F 13640 [(3-chloro-4-fluoro-phenyl)-[4-fluoro-4-{[(5-methyl-pyridin-2-ylmethyl)-amino]-methyl}piperidin-1-yl]methanone, fumaric acid salt] after acute administration and both its hyper- and hypoanalgesic effects in rats. The maximal plasma concentration (C(max)) of F 13640 after i.p. administration of 0.63 mg/kg was obtained at 15 min and decreased to half its maximal value after about 1 h. The amount of F 13640 collected by means of in vivo microdialysis in hippocampal dialysates could be measured reliably after 0.63 and 2.5 mg/kg, reached its maximum at about 1 h, and fell to half of its maximal value at about 3 h. 5-Hydroxytryptamine 1A (5-HT(1A)) receptor occupancy was estimated by ex vivo binding in rat brain sections. F 13640 inhibited [(3)H]8-hydroxy-2-[di-n-propylamino] tetralin binding ex vivo in rat hippocampus, entorhinal cortex, and frontal cortex (ED(50), 0.34 mg/kg i.p.). Maximal inhibition was reached at approximately 30 min after 0.63 mg/kg F 13640 and fell to half of its value after about 4 to 8 h. After injection (15 min) in the paw pressure test, F 13640 (0.63 mg/kg i.p.) induced an initial hyperalgesia that was followed 4 h later by a paradoxical analgesia that lasted until 8 h. In contrast, in the formalin test, F 13640 inhibited pain behaviors until 4 h after drug administration. F 13640 also produced elements of the 5-HT syndrome that lasted up to 4 h after administration. These results demonstrate that F 13640 induces hyperalgesia and/or analgesia with a time course that parallels the occupancy of 5-HT(1A) receptors and the presence of the compound in blood and brain.

Roles of serotonin receptor subtypes for the antinociception of 5-HT in the spinal cord of rats

The contribution of 5-HT (5-hydroxytryptamine) receptor subtypes to the antinociception produced by intrathecal 5-HT in the formalin test was investigated in rats. Intrathecal 5-HT suppressed both phases of behaviors produced by 5% formalin, and this was blocked by antagonists for 5-HT(1B) (3-[3-(Dimethylamino)propyl]-4-hy-droxy-N-[4-(4-pyridinyl)phenyl]benzamide dihydrochloride, GR 55562), 5-HT(2C) (N-ormethylclozapine/8-Chloro-11-(1-piperazinyl)-5H-dibenzo[b,e][1,4]diazepine, D-MC), 5-HT3 (1-Methyl-N-(8-methyl-8-azabicyclo[3.2.1]-oct-3-yl)-1H-indazole-3-carboxamide maleate, LY-278,584) and 5-HT4 receptors (4-Amino-5-chloro-2-metho-xy-benzoic acid 2-(diethylamino)ethyl ester hydrochloride, SDZ-205,557), but not the 5-HT(1D) receptor antagonist 3-[4-(4-Chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanol hydrochloride (BRL 15572). The 5-HT(1A) receptor antagonist N-[2-[4-(2-Methoxyphenyl)-1-piperazinyl]-N-2-pyridinyl-cyclohexanecarboxamide maleate (WAY-100635) decreased only the second phase antinociception of 5-HT. Intrathecal administration of agonists for 5-HT(1A) (3-(N,N-Dipropylaminoethyl)-1H-indole-5-carboxamide maleate, Dipropyl-5CT), 5-HT(1B) (7-Trifluoromethyl-4(4-met-hyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline maleate, CGS-12066A), 5-HT(2C) (6-Ch-loro-2-(1-piperazinyl)pyrazine hydrochloride, MK 212), 5-HT3 (N-(3-Chlorophenyl)imidodicarbonimidic diamide hydrochloride, m-CPBG) and 5-HT4 receptors (2-[1-(4-Piperonyl)piperazinyl]benzothiazole, BZTZ) suppressed both phases of the formalin response. The results of the present study indicate that spinal 5-HT(1B,) 5-HT(2C,) 5-HT3 and 5-HT4 receptors, but not the 5-HT(1D) receptor, mediate antinociception produced by 5-HT in the formalin test. The relevance of the 5-HT(1A) receptor is less clear because of the different effects of antagonist and agonist.

5-hydroxytryptamine1A receptor is involved in the bee venom induced inflammatory pain

Injection of bee venom into one hindpaw of rat can elicit acute inflammation together with spontaneous pain, heat hyperalgesia and mechanical hyperalgesia/allodynia in the injected paw. 5-hydroxytryptamine (5-HT)1A receptor is the predominant receptor subtype in the spinal dorsal horn mediating the function of 5-HT in nociception. The goal of the present study is to assess the role of 5-HT1A receptor in the pain associated with the bee venom induced inflammation. Here we showed that 1 or 4 h after a subcutaneous bee venom challenge, expression of 5-HT1A receptor mRNA in the ipsilateral lumbar spinal cord increased significantly by 80.94 or 37.86%, respectively. Antisense oligodeoxynucleotide knockdown of spinal 5-HT1A receptor attenuated spontaneous pain and reversed heat hyperalgesia in rats injected with bee venom. Thus, the present data suggest a facilitating role for 5-HT1A receptor in bee venom induced inflammatory pain.

Involvement of spinal neurokinin-1 receptors in the maintenance but not induction of carrageenan-induced thermal hyperalgesia in the rat

The study was undertaken to assess the antihyperalgesic effect of L-732,138, (N-acetyl-L-tryptophan-3,5-bistrifluoromethyl benzyl ester), a non-peptide neurokinin-1 (NK1) receptor antagonist in rats when given intrathecally. The peripheral inflammation associated with behavioral hyperalgesia to a thermal stimulus was induced by intraplantar (i.pl.) injection of carrageenan. The thermal hyperalgesia was measured by paw withdrawal latency. Intrathecal (i.t.) injection of L-732,138 (100 nmol) at 3h after carrageenan markedly attenuated the paw withdrawal latency of the inflamed paw, but not that of the non-inflamed paw. L-732,138 (100 nmol, i.t.) given 10 min prior to carrageenan injection had no effect on the carrageenan-induced decrease in paw withdrawal latency to noxious thermal stimulus. The results demonstrate that NK1 receptor is involved in the maintenance but not the induction and development of thermal hyperalgesia evoked by carrageenan.

The novel analgesic and high-efficacy 5-HT1A receptor agonist, F 13640 induces c-Fos protein expression in spinal cord dorsal horn neurons

The very-high-efficacy, selective 5-HT(1A) receptor agonist, F 13640 produces uniquely powerful analgesia in rat models of chronic pain by novel neuroadaptive mechanisms (inverse tolerance and co-operation with nociception) [Neuropharmacology 43 (2002) 945-958]. A signal transduction theory and evidence suggest that F 13640 initiates these mechanisms, paradoxically, by mimicking the central effects of nociceptive stimulation. We report that the i.p. injection of F 13640 induces c-Fos protein expression in the L3-L5 segments of the spinal cord. Some 65% of c-Fos protein immunoreactive (c-Fos-IR) nuclei occurred bilaterally in the dorsal horn laminae I-II and V-VI, spinal areas that contain neurons responsive to nociceptive stimulation. This pattern is not unlike that found earlier in arthritic rats, a model of somatotopically widespread nociception. Dose-response studies indicated that c-Fos protein expression was induced at doses (0.63 and 2.5 mg/kg, i.p.) at which previous studies had found F 13640 to produce hyperalgesia. Time-response studies found that c-Fos-IR nuclei appeared within 1-4 h after 0.63 mg/kg of F 13640, with a maximum at 2 h. This parallels literature evidence that c-Fos expression reaches peak late after, and outlasts, nociceptive stimulation. Similar to opioids counteracting noxiously induced c-Fos expression, 10 mg/kg (s.c.) of morphine reduced the number of c-Fos-IR nuclei induced by 0.63 mg/kg of F 13640 (by 45+/-5%; P<0.001). The induction by F 13640 of c-Fos protein expression may relate to the initial hyperalgesia which earlier data indicate the agent to produce early upon its administration.

Increase of nociceptive threshold induced by intrathecal injection of interleukin-1beta in normal and carrageenan inflammatory rat

The present study was to investigate the effect of intrathecal (i.t.) injection of interleukin-1 beta (IL-1 beta) on nociception in normal and inflammatory rats. Peripheral inflammation was induced by intraplantar injection (i.pl.) of carrageenan into unilateral hind paw. The nociceptive threshold to noxious thermal stimulation was measured by the paw withdrawal latency (PWL). Intrathecal injection of IL-1 beta (10 ng, 100 ng) significantly increased PWL in normal rats, the peak occurred at 5 min and the effect lasted for 30 min. Similarly, IL-1 beta (10 ng, 100 ng, i.t.) significantly increased the PWL and lasted for more than 60 min in inflammatory rats. Both in normal and inflammatory rats, the IL-1 beta-induced antinociceptive effect was completely abolished by IL-1ra (50 ng, i.t.), and apparently attenuated by naloxone (10 microg, i.t.) or mianserin (20 microg, i.t.). These results suggest that IL-1 beta produces antinociceptive effect by binding IL-1 receptor at the spinal level, and is related to the activation of opioid and 5-HT systems.