The Different Roles of 5-HT2 and 5-HT3 Receptors on Antinociceptive Effect of Paroxetine in Chemical Stimuli in Mice

Paroxetine increases delta opioid responsiveness in sensory neurons

There are currently no Food and Drug Administration (FDA)-approved delta opioid receptor (DOR)-selective agonists, despite having fewer side effects in rodents and non-human primates compared to traditional mu opioid receptor (MOR) therapeutics (Vanderah, 2010). Targeting peripheral receptors is an attractive strategy to reduce abuse potential. However, peripheral opioid receptors do not readily respond to agonists unless primed by inflammation, which would limit their efficacy in non-inflammatory pain patients (Stein et al., 1989). It was recently identified that G protein-coupled receptor kinase 2 (GRK2) maintains DOR incompetence in non-inflamed nociceptors (Brackley et al., 2016; Brackley et al., 2017). Here, we report that paroxetine, a selective serotonin reuptake inhibitor and potent GRK2 inhibitor (Thal et al., 2012), reduces chronic GRK2 association with membrane DOR, thereby enhancing peripheral DOR-mediated analgesic competence in the absence of inflammation. Interestingly, paroxetine's effects on GRK2 in vivo are limited to peripheral tissues in the male rat. The effects of paroxetine on DOR competence are notably antagonized by GRK2 overexpression. This is the first study to suggest that paroxetine induces peripheral DOR analgesic competence through a GRK2-dependent mechanism, improving analgesic efficacy in non-inflamed tissue. Because paroxetine targets the protein that governs peripheral opioid receptor responsiveness, and does so in the absence of inflammation, we propose that paroxetine may be suitable as a co-therapy with peripherally-restrictive doses of opioids to improve analgesic efficacy in non-inflammatory pain conditions.Significance StatementOpioids that target MOR represent the gold-standard for analgesic healthcare, despite widespread abuse potential and the ongoing opioid-epidemic. Work herein uncovers the therapeutic potential of targeting peripheral DOR for analgesic utility with an FDA-approved GRK2 inhibitor paroxetine to boost efficacy and reduce side effect profiles. Analgesic pain management targeting DOR with increased efficacy through adjuvant paroxetine treatment could reduce over-reliance on MOR agonist opioids for pain relief and usher in new options for analgesia.

The role of serotonergic and catecholaminergic systems for possible antidepressant activity of apigenin

Background and objective

Although, the anti-depressant like effects of apigenin (APG) are documented in the literature, the underlying mechanism for exerting such an effect is still not clear. In this research, an attempt was made to determine the possible role of APG for antidepressant activity through serotonergic and catecholaminergic systems using standardized animal models.

Materials and methods

The antidepressant property of APG was determine by involving tail suspension (TST) and modified forced swimming tests (MFST). The effect of APG was evaluated at 25 and 50 mg/kg. In mechanistic models, animals were pretreated with catecholaminergic and serotonergic antagonists prior to administration of APG. The results obtained were statistically analyzed to determine the level of significance.

Results

The period of immobility in both models (TST and MFST) was significantly reduced by APG (25 and 50 mg/kg). The best therapetuic dose of APG (50 mg/kg) was selected for the mechanistic study. The anti-immobility effect of APG declined to a significant extent upon pretreatment with catecholaminergic antagonists (α-methyl-para-tyrosine methyl ester; SCH 23390; sulpiride; phentolamine) and serotonergic inhibitors (p-clorophenylalanine-methyl-ester; ondansetron) in both TST and MFST models. The antidepressant benefits of apigenin were only modestly reversed when rats were given propranolol.

Conclusions

The findings suggest that APG's antidepressant effect is mediated by the α-adrenergic, dopaminergic and 5-HT3 serotonergic receptors.

Anti-inflammatory and antinociceptive activities of a phenylpropanoid-enriched fraction of Duguetia furfuracea

A previous study reported the in vivo anti-inflammatory and antinociceptive activities of essential oil of the underground stem bark of Duguetia furfuracea, termed EODf. This study aimed to obtain a phenylpropanoid-enriched fraction from the D. furfuracea (EFDf) essential oil and to investigate its anti-inflammatory and antinociceptive effects. The chemical composition of the EFDf was determined by gas chromatography-mass spectrometry (GC-MS). The in vivo anti-inflammatory activity was evaluated with a lipopolysaccharide (LPS)-induced paw oedema model. The effects of the EFDf on the polymorphonuclear leukocyte recruitment and the inducible nitric oxide synthase (iNOS) expression were evaluated in mice footpads. Moreover, the in vivo antinociceptive effect was assayed using the formalin test and the LPS-induced thermal hyperalgesia model. In the EFDf, 8 major compounds were identified, with α-asarone (36.4%) and 2,4,5-trimethoxystyrene (27.8%) the main constituents. A higher concentration of phenylpropanoid derivatives was found in the EFDf, 64.2% compared to the EODf (38%). The oral (p.o.) treatment with the EFDf at a dose of 3 mg/kg significantly attenuated the paw oedema, polymorphonuclear leukocyte migration, iNOS expression, and tumour necrosis factor alpha (TNF-α) production. The EFDf (10 and 30 mg/kg) also inhibited both phases of the formalin test and caused a significant increase in the reaction time in the LPS-induced thermal hyperalgesia model. Finally, EFDf-treated animals did not show any alteration of motor coordination. The results suggest that the enrichment of 2,4,5-trimethoxystyrene and α-asarone enhances the anti-inflammatory activity of the EFDf compared to the EODf. In contrast, the antinociception promoted by the EFDf was similar to the EODf and was mediated via activation of adenosinergic and opioidergic receptors.

Computational Methods for the Identification of Molecular Targets of Toxic Food Additives. Butylated Hydroxytoluene as a Case Study

Butylated hydroxytoluene (BHT) is one of the most commonly used synthetic antioxidants in food, cosmetic, pharmaceutical and petrochemical products. BHT is considered safe for human health; however, its widespread use together with the potential toxicological effects have increased consumers concern about the use of this synthetic food additive. In addition, the estimated daily intake of BHT has been demonstrated to exceed the recommended acceptable threshold. In the present work, using BHT as a case study, the usefulness of computational techniques, such as reverse screening and molecular docking, in identifying protein-ligand interactions of food additives at the bases of their toxicological effects has been probed. The computational methods here employed have been useful for the identification of several potential unknown targets of BHT, suggesting a possible explanation for its toxic effects. In silico analyses can be employed to identify new macromolecular targets of synthetic food additives and to explore their functional mechanisms or side effects. Noteworthy, this could be important for the cases in which there is an evident lack of experimental studies, as is the case for BHT.

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.

Antidepressant-like effect of gallic acid in mice: Dual involvement of serotonergic and catecholaminergic systems

AIMS

This study was planned to examine the antidepressant potency of gallic acid (30 and 60mg/kg), a phenolic acid widely distributed in nature, together with its possible underlying monoaminergic mechanisms.

MAIN METHODS

Antidepressant-like activity was assessed using the tail suspension (TST) and the modified forced swimming tests (MFST). Locomotor activity was evaluated in an activity cage.

KEY FINDINGS

Administration of gallic acid at 60mg/kg reduced the immobility duration of mice in both the TST and MFST without any changes in the locomotor activity. The anti-immobility effect observed in the TST was abolished with pre-treatment of p-chlorophenylalanine methyl ester (an inhibitor of serotonin synthesis; 100mg/kg i.p. administered for 4-consecutive days), ketanserin (a 5-HT2A/2C antagonist; 1mg/kg i.p.), ondansetron (a 5-HT3 antagonist; 0.3mg/kg i.p.), α-methyl-para-tyrosine methyl ester (an inhibitor of catecholamine synthesis; 100mg/kg i.p.), phentolamine (non-selective alpha-adrenoceptor antagonist; 5mg/kg i.p.), SCH 23390 (a dopamine D1 antagonist; 0.05mg/kg s.c.), and sulpiride (a dopamine D2/D3 antagonist; 50mg/kg i.p.). However, NAN 190 (a 5-HT1A antagonist; 0.5mg/kg i.p.) and propranolol (a non-selective β-adrenoceptor antagonist; 5mg/kg i.p.) pre-treatments were ineffective at reversing the antidepressant-like effects of gallic acid.

SIGNIFICANCE

The results of the present study indicate that gallic acid seems to have a dual mechanism of action by increasing not only serotonin but also catecholamine levels in synaptic clefts of the central nervous system. Further alpha adrenergic, 5-HT2A/2C and 5-HT3 serotonergic, and D1, D2, and D3 dopaminergic receptors also seem to be involved in this antidepressant-like activity.

Serotonin induces peripheral mechanical antihyperalgesic effects in mice

The role of serotonin (5-HT) in nociception will vary according to the subtypes of receptors activated. When administered peripherally, it induces pain in humans and in rats by activation of 5-HT1, 5-HT2 and 5-HT3 receptors. In addition, endogenous 5-HT produced in situ, is involved in the nociceptive response induced by formalin in rat's paw inflammation, possibly via 5-HT3 receptors. Moreover, it has been shown that 5-HT released in the dorsal horn of the spinal cord by stimulation of the periaqueductal gray causes activation of inhibitory interneurons, resulting in inhibition of spinal neurons. In the present study we evaluated the effect of serotonin and its receptors at peripheral antinociception. The mice paw pressure test was used in animals that had increased sensitivity by an intraplantar injection of PGE2 (2 µg). We used selective antagonists of serotonin receptors (isamoltan 5-HT1B, BRL 15572 5-HT1D, ketanserin 5-HT2A, ondansetron 5-HT3 and SB-269970 5-HT7). Administration of serotonin into the right hind paw (62.5, 125, 250 and 500 ng and 1 µg) produced a dose-dependent peripheral mechanical antihyperalgesic effect of serotonin in mice. Selective antagonists for 5-HT1B, 5-HT2A, 5-HT3 receptors at doses of 0.1, 1 and 10 µg, reversed the antihyperalgesic effect induced by 250 ng serotonin. In contrast, selective antagonists for 5-HT1D and 5-HT7 receptors were unable to reverse the antihyperalgesic effect induced by serotonin. These results demonstrated for the first time, the peripheral mechanical antihyperalgesic effect of serotonin, and participation of 5-HT1B, 5-HT2A and 5-HT3 receptors in this event.

Molecular interactions between general anesthetics and the 5HT2B receptor

BACKGROUND

Serotonin modulates many processes through a family of seven serotonin receptors. However, no studies have screened for interactions between general anesthetics currently in clinical use and serotonergic G-protein-coupled receptors (GPCRs). Given that both intravenous and inhalational anesthetics have been shown to target other classes of GPCRs, we hypothesized that general anesthetics might interact directly with some serotonin receptors and thus modify their function.

METHODS

Radioligand binding assays were performed to screen serotonin receptors for interactions with propofol and isoflurane as well as for affinity determinations. Docking calculations using the crystal structure of 5-HT2B were performed to computationally confirm the binding assay results and locate anesthetic binding sites.

RESULTS

The 5-HT2B class of receptors interacted significantly with both propofol and isoflurane in the primary screen. The affinities for isoflurane and propofol were determined to be 7.78 and .95 μM, respectively, which were at or below the clinical concentrations for both anesthetics. The estimated free energy derived from docking calculations for propofol (-6.70 kcal/mol) and isoflurane (-5.10 kcal/mol) correlated with affinities from the binding assay. The anesthetics were predicted to dock at a pharmacologically relevant binding site of 5HT2B.

CONCLUSIONS

The molecular interactions between propofol and isoflurane with the 5-HT2B class of receptors were discovered and characterized. This finding implicates the serotonergic GPCRs as potential anesthetic targets.

Desvenlafaxine succinate ameliorates visceral hypersensitivity but delays solid gastric emptying in rats

Desvenlafaxine succinate (DVS) is a novel serotonin and norepinephrine reuptake inhibitor. The aim of this study was to investigate the effects of DVS on visceral hypersensitivity and solid gastric emptying in a rodent model of gastric hyperalgesia. Twenty-eight gastric hyperalgesia rats and 20 control rats were used. Visceral sensitivity during gastric distention (GD) was assessed by recording of electromyogram (EMG) at pressures of 20, 40, 60, and 80 mmHg. DVS with doses of 1, 10, and 30 mg/kg were administrated by gavage, 5-HT1A antagonist (WAY-100635, 0.3 mg/kg) was given subcutaneously, and 5-HT2A antagonist (ketanserin, 1 mg/kg) was given intraperitoneally. The level of norepinephrine in plasma was measured by enzyme-linked immunosorbent assay. We found that 1) visceral hypersensitivity induced by acetic acid was validated. 2) DVS dose-dependently reduced visceral hypersensitivity in the gastric hypersensitivity rats. The EMG (% of baseline value without GD) during GD at 60 and 80 mmHg with DVS at a dose of 30 mg/kg were 119.4 ± 2.3% (vs. saline 150.9 ± 2.7%, P < 0.001) and 128.2 ± 3.2% (vs. saline 171.1 ± 2.4%, P < 0.001). Similar findings were observed at a dose of 10 mg/kg. DVS at a dose of 1 mg/kg reduced visceral hypersensitivity only during GD at 60 mmHg. 3) Neither WAY-100635 nor ketanserin blocked the effect of DVS on visceral sensitivity. 4) DVS at 30 mg/kg significantly increased plasma NE level (P = 0.012 vs. saline). 5) DVS at 30 mg/kg significantly delayed solid gastric emptying (P < 0.05 vs. saline). We conclude that DVS reduces visceral sensitivity in a rodent model of visceral hypersensitivity and delays solid gastric emptying. Caution should be made when DVS is used for treating patients.

Modification of the behavioral effects of morphine in rats by serotonin 5-HT₁A and 5-HT₂A receptor agonists: antinociception, drug discrimination, and locomotor activity

RATIONALE

Indirect-acting serotonin (5-HT) receptor agonists can enhance the antinociceptive effects of morphine; however, the specific 5-HT receptor subtype(s) mediating this enhancement is not established.

OBJECTIVE

This study examined interactions between morphine and both 5-HT(1A) and 5-HT(2A) receptor agonists in rats using measures of antinociception (radiant heat tail flick and warm water tail withdrawal), drug discrimination (3.2 mg/kg morphine versus saline), and locomotion.

METHODS

Male Sprague-Dawley rats (n = 7-8 per group) were used to examine the effects of morphine alone and in combination with DOM (5-HT(2A) agonist) and 8-OH-DPAT (5-HT(1A) agonist).

RESULTS

DOM did not modify antinociceptive or discriminative stimulus effects while modestly attenuating locomotor-stimulating effects of morphine; the effect of DOM (0.32 mg/kg) on morphine-induced locomotion was prevented by the 5-HT(2A) receptor-selective antagonist MDL 100907. In contrast, 8-OH-DPAT (0.032-0.32 mg/kg) fully attenuated the antinociceptive effects (both procedures), did not modify the discriminative stimulus effects, and enhanced (0.32 mg/kg) the locomotor-stimulating effects of morphine. These effects of 8-OH-DPAT were prevented by the 5-HT(1A) receptor-selective antagonist WAY100635.

CONCLUSION

Agonists acting at 5-HT(1A) or 5-HT(2A) receptors do not modify all effects of mu opioid receptor agonists in a similar manner. Moreover, interactions between 5-HT and opioid receptor agonists vary significantly between rats and nonhuman primates, underscoring the value of comparing drug interactions across a broad range of conditions and in multiple species.

Effects of 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) and quipazine on heroin self-administration in rhesus monkeys

RATIONALE

The serotonin (5-HT) system is involved in pain modulation, and 5-HT receptor agonists can enhance antinociceptive effects of mu opioid receptor agonists. Less is known about the actions of 5-HT receptor agonists on other effects of opioids.

OBJECTIVE

This study examined the effects of non-contingent and contingent administration of the 5-HT(2A) receptor agonists DOM and quipazine on i.v. heroin self-administration in rhesus monkeys.

RESULTS

Heroin (0.0001-0.1 mg/kg/infusion) generated an inverted U-shaped dose-response function. Non-contingent administration of DOM (0.1-0.32 mg/kg) flattened the dose-response function in three monkeys and eliminated heroin self-administration in a fourth monkey. Contingent DOM (0.0032-0.032 mg/kg/infusion) alone did not maintain responding above that maintained by saline, and, when added to self-administered heroin, monkeys responded less than for the same unit doses of heroin alone. Non-contingent (0.32-3.2 mg/kg) and contingent (0.0032-0.56 mg/kg/infusion) administration of quipazine flattened the dose-response function in two monkeys, increasing responding maintained by small unit doses of heroin and saline, but failed to enhance responding for heroin in two other monkeys.

CONCLUSION

This study shows that DOM does not enhance, and might attenuate, the positive reinforcing effects of the mu opioid receptor agonist heroin. Quipazine increased responding for saline and small doses of heroin; those effects were modest and observed in only two subjects. Taken together, these data suggest that 5-HT(2A) receptor agonists do not significantly enhance the reinforcing effectiveness of mu opioid receptor agonists and support the view that administering 5-HT drugs in combination with opioids to treat pain might not enhance abuse liability.

Anti-depressant-like effect of vitexin in BALB/c mice and evidence for the involvement of monoaminergic mechanisms

The present study was designed to investigate the putative effect of vitexin, a flavone C-glucoside present in some drugs, medicinal plants and nutraceuticals, on the central nervous system. Vitexin (10-30 mg/kg) did not show significant alterations in the behaviour of mice tested in hole-board, plus-maze or activity cage tests. However, immobility time of the mice significantly reduced by vitexin administrations in both the tail-suspension and modified forced swimming tests. The anti-immobility effect of vitexin in the tail-suspension test was reversed with α-methyl-para-tyrosine methyl ester (AMPT, an inhibitor of catecholamine synthesis, 100mg/kg, i.p.), yohimbine (an α(2)-adrenoceptor antagonist, 1mg/kg, i.p.), NAN 190 (a 5-HT(1A) antagonist, 0.5mg/kg, i.p.), SCH 23390 (a dopamine D(1) antagonist, 0.05 mg/kg, s.c.) and sulpiride (a dopamine D(2)/D(3) antagonist, 50mg/kg, i.p.). The same effect was not reversed, however, by p-chlorophenylalanine methyl ester (PCPA; an inhibitor of serotonin synthesis 100mg/kg, i.p., administered for 4 consecutive days), ketanserin (a 5-HT(2A/2C) antagonist, 1-4 mg/kg, i.p.), ondansetron (a 5-HT(3) antagonist, 0.1-0.4 mg/kg, i.p.), prazosin (an α(1)-adrenoceptor antagonist, 1-4 mg/kg, i.p.), or propranolol (a non-selective β-adrenoceptor antagonist, 5-20mg/kg, i.p.). These results suggest that the anti-depressant-like effect of vitexin is mediated through an increase in catecholamine levels in the synaptic cleft as well as through interactions with the serotonergic 5-HT(1A), noradrenergic α(2), and dopaminergic D(1), D(2), and D(3) receptors. To our knowledge, this is the first study to show findings that indicate an anti-depressant-like effect of vitexin and its underlying mechanisms.

Antinociceptive and anti-inflammatory effects of olive oil (Olea europeae L.) in mice

CONTEXT

Olive [Olea europaea L. (Oleaceae)] is a long-lived evergreen tree that is widespread in different parts of the world.

OBJECTIVE

Olive oil has been reported to relieve pain; however, there is still insufficient data in the literature on the subject. Thus, it is considered worthwhile investigating the antinociceptive and anti-inflammatory effects of olive oil in adult male Balb/C mice.

MATERIALS AND METHODS

The antinociceptive effects were studied using formalin, hot plate and writhing tests. The acute anti-inflammatory effects of olive oil in mice were studied using xylene ear edema test. Olive oil (1, 5 and 10 ml/kg body wt.) was injected intraperitoneally. Intact animals served as controls.

RESULTS

Our results showed that the olive oil only decreased the second phase of formalin-induced pain. In the hot plate test, olive oil did not raise the pain threshold over the 60 min duration of the test. Olive oil exhibited antinociceptive activity against writhing-induced pain by acetic acid. In the xylene ear edema test, olive oil showed significant anti-inflammatory activity in the mice.

DISCUSSION AND CONCLUSION

The present data indicated that olive oil has antinociceptive and anti-inflammatory effects in mice but further investigation of these effects is required to elucidate the mechanism(s) involved in analgesic and anti-inflammatory effects of Olea europaea oil.

The antinociceptive effects of intravenous tianeptine in colorectal distension-induced visceral pain in rats: the role of 5-HT₃ receptors

Tianeptine is an unusual tricyclic antidepressant drug. In this study, we aimed to investigate the antinociceptive effect of tianeptine on visceral pain in rats and to determine whether possible antinociceptive effect of tianeptine is mediated by serotonergic (5-HT(2,3)) and noradrenergic (α(1,2)) receptor subtypes. Male Sprague Dawley rats (250-300 g) were supplied with a venous catheter, for drug administrations, and enameled nichrome electrodes, for electromyography, at external oblique musculature. Colorectal distension (CRD) was employed as the noxious visceral stimulus and the visceromotor response (VMR) to CRD was quantified electromyographically before and 5, 15, 30, 60, 90 and 120 min after tianeptine administration. Antagonists were administered 10 min before tianeptine for their ability to change tianeptine antinociception. Intravenous administration of tianeptine (2.5-20 mg/kg) produced a dose-dependent reduction in VMR. Administration of 5-HT(3) receptor antagonist ondansetron (0.5, 1 and 2 mg/kg), but not 5-HT(2) receptor antagonist ketanserine (0.5, 1 and 2 mg/kg), reduced the antinociceptive effect of tianeptine (10mg/kg). In addition, administration of α(1)-adrenoceptor antagonist prazosin (1 mg/kg) or α(2)-adrenoceptor antagonist yohimbine (1 mg/kg) did not cause any significant effect on the tianeptine-induced antinociception. Our data indicate that intravenous tianeptine exerts a pronounced antinociception against CRD-induced visceral pain in rats, and suggests that the antinociceptive effect of tianeptine appears to be mediated in part by 5-HT(3) receptors, but does not involve 5-HT(2) receptors or α-adrenoceptors.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Regional cerebral blood flow in patients with orally localized somatoform pain disorder: a single photon emission computed tomography study

AIM

Somatoform pain disorder is characterized by persistent and chronic pain at one or more sites without an associated general medical condition and in which psychological factors are thought to play a role. This study aimed to investigate the pathological features of somatoform pain disorder localized to the oral region by single photon emission computed tomography (SPECT).

METHODS

Ten patients (nine females and one male; average age 55.0 ± 14.4 years) having somatoform pain disorder with oral symptoms participated. SPECT was performed using N-isopropyl-4-[(123) I] iodoamphetamine intravenous injections, and regional cerebral blood flow (rCBF) was assessed by three-dimensional stereotactic surface projections. We also selected 12 healthy individuals (seven females and five males; average age 61.8 ± 13.2 years) to act as controls.

RESULTS

Both the patient and control groups showed no atrophy or infarction on CT or magnetic resonance imaging. The patient group showed higher rCBF in the subcortical area, especially in the thalamus and cingulate gyri, than the control group. In contrast, the patient group showed lower rCBF in the bilateral frontal and occipital lobes as well as in the left temporal lobe.

CONCLUSIONS

These results suggest that the biological process involved in somatoform pain disorder of the oral region is characterized by changes in limbic and cortical functions. The finding that somatoform pain disorder with oral symptoms is associated with brain functional changes will help to develop treatment regimes for this disorder and clarify the underlying pathology.

Effect of the inhibition of CYP3A4 or CYP2D6 on the pharmacokinetics and pharmacodynamics of oxycodone

PURPOSE

The main metabolic pathways of oxycodone, a potent opioid analgetic, are N-demethylation (CYP3A4) to inactive noroxycodone and O-demethylation (CYP2D6) to active oxymorphone. We performed a three-way, placebo-controlled, double-blind cross-over study to assess the pharmacokinetic and pharmacodynamic consequences of drug interactions with oxycodone.

METHODS

The 12 participants (CYP2D6 extensive metabolizers) were pre-treated with placebo, ketoconazole or paroxetine before oral oxycodone ingestion (0.2 mg/kg).

RESULTS

Pre-treatment with ketoconazole increased the AUC for oxycodone 2- to 3-fold compared with placebo or paroxetine. In combination with placebo, oxycodone induced the expected decrease in pupil diameter. This decrease was accentuated in the presence of ketoconazole, but blunted by paroxetine. In comparison to pre-treatment with placebo, ketoconazole increased nausea, drowsiness, and pruritus associated with oxycodone. In contrast, the effect of pre-treatment with paroxetine on the above-mentioned adverse events was not different from that of placebo. Ketoconazole increased the analgetic effect of oxycodone, whereas paroxetine was not different from placebo.

CONCLUSIONS

Inhibition of CYP3A4 by ketoconazole increases the exposure and some pharmacodynamic effects of oxycodone. Paroxetine pretreatment inhibits CYP2D6 without inducing relevant changes in oxycodone exposure, and partially blunts the pharmacodynamic effects of oxycodone due to intrinsic pharmacological activities. Pharmacodynamic changes associated with CYP3A4 inhibition may be clinically important in patients treated with oxycodone.

Usefulness of antidepressants for improving the neuropathic pain-like state and pain-induced anxiety through actions at different brain sites

Clinically, it is well known that chronic pain induces depression, anxiety, and a reduced quality of life. There have been many reports on the relationship between pain and emotion. We previously reported that chronic pain induced anxiety with changes in opioidergic function in the central nervous system. In this study, we evaluated the anxiolytic-like effects of several types of antidepressants under a chronic neuropathic pain-like state and searched for the brain site of action where antidepressants show anxiolytic or antinociceptive effects. Sciatic nerve-ligated mice exhibited thermal hyperalgesia and tactile allodynia from days 7 to 28 after nerve ligation. At 4 weeks after ligation, these mice showed a significant anxiety-related behavior in the light-dark test and the elevated plus-maze test. Under these conditions, repeated administration of antidepressants, including the tricyclic antidepressant (TCA) imipramine, the serotonin noradrenaline reuptake inhibitor (SNRI) milnacipran, and the selective serotonin reuptake inhibitor (SSRI) paroxetine, significantly prevented the anxiety-related behaviors induced by chronic neuropathic pain. These antidepressants also produced a significant reduction in thermal hyperalgesia and tactile allodynia. Moreover, the microinjection of paroxetine into the basolateral amygdala or cingulate cortex reduced anxiety-related behavior, and microinjection into the primary somatosensory cortex significantly attenuated thermal hyperalgesia. These findings suggest that serotonergic antidepressants are effective for treating anxiety associated with chronic neuropathic pain and may be useful for treating neuropathic pain with emotional dysfunction such as anxiety. Furthermore, SSRIs show anxiolytic and antinociceptive effects by acting on different brain regions.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Sympathetic nervous system and neurotransmitters: their possible role in neuroimmunomodulation of multiple sclerosis and some other autoimmune diseases

Multiple sclerosis is still a disease without a cure. Although intensive research efforts have led to the development of drugs that modify the activity of the disease, most of them have various side effects and are expensive. At the same time it is becoming apparent that some remedies usually used to treat somatic and psychic disorders also have immunomodulating properties, and may help manage multiple sclerosis and other autoimmune diseases. We describe here the role of the sympathetic nervous system in the neuro-immune interaction in multiple sclerosis and other immune diseases with increased cellular immunity as well as neurochemical disturbances that take place in these disorders.

Possible involvement of opioidergic systems in the antinociceptive effect of the selective serotonin reuptake inhibitors in sciatic nerve-injured mice

The involvement of opioid receptor activation in the antinociceptive effect of either fluvoxamine, a selective serotonin reuptake inhibitor, or serotonin (5-HT) on thermal hyperalgesia and mechanical allodynia in a model of neuropathic pain in mice induced by sciatic nerve ligation was examined. The experiments were conducted 2 or 6 weeks after unilateral sciatic nerve ligation. Ipsilateral thermal hyperalgesia and mechanical allodynia were observed both 2 and 6 weeks after sciatic nerve ligation. Neither s.c. fluvoxamine nor i.t. 5-HT affected sciatic nerve ligation-induced thermal hyperalgesia or mechanical allodynia in mice 2 weeks after sciatic nerve ligation. However, the same dose of either fluvoxamine or 5-HT significantly reduced mechanical allodynia but not thermal hyperalgesia in sciatic nerve ligated mice 6 weeks after surgery. The antinociceptive effect of fluvoxamine on sciatic nerve ligation-induced mechanical allodynia in mice 6 weeks after surgery was completely abolished by pretreatment with either naloxone, a nonselective opioid receptor antagonist, or beta-funaltrexamine, a selective mu-opioid receptor antagonist. Furthermore, pretreatment with naltrindole, a selective delta-opioid receptor antagonist, partially but significantly inhibited the antinociceptive effect of fluvoxamine in sciatic nerve ligated mice at the 6th postoperative week. The antinociceptive effect induced by i.t. 5-HT was also completely antagonized by either naloxone or beta-funaltrexamine, and partially inhibited by naltrindole. However, pretreatment with nor-binaltorphimine, a selective kappa-opioid receptor antagonist, had no effect against either s.c. fluvoxamine- or i.t. 5-HT-induced antinociception. These results suggest that the antinociceptive effect of s.c. fluvoxamine or i.t. 5-HT in the chronic state of sciatic nerve ligation-induced neuropathic pain may be related to opioidergic activity, mainly through the activation of spinal mu- and delta-opioid receptors.

The Effects of Morphine on Human 5-HT3A Receptors

5-HT3 receptors are ligand-gated ion channels that are involved in the modulation of emesis and pain. In this study, we investigated whether the opioid analgesic, morphine, exerts specific effects on human 5-HT3 receptors. Whole-cell patches from HEK-293 cells stably transfected with the human 5-HT3A receptor cDNA were used to determine the effects of morphine on the 5-HT-induced currents using the patch clamp technique. At negative membrane potentials, 5-HT induced inward currents in a concentration-dependent manner. The 5-HT3 receptor antagonist, ondansetron, (0.3 nM) reversibly inhibited the 5-HT-induced signals. Morphine reversibly suppressed 5-HT-induced peak currents as a function of concentration (IC50 = 1.1 microM, Hill coefficient = 1.2). The block by morphine decreased with increasing 5-HT concentrations, suggesting a competitive effect. In addition, the activation, as well as the inactivation, kinetics of the currents were significantly slowed in the presence of morphine. The morphine antagonist, naloxone, also inhibited 5-HT-induced currents (e.g., at 3 microM by 17%). The effects of morphine and naloxone were not additive. The potency of morphine and the competitivity of the blocking effect points to a specific mechanism at a receptor site rather than an unspecific membrane effect.

Effect of gender on antinociceptive effect of paroxetine in hot plate test in mice

PURPOSE

Many studies have reported both a gender difference in the rates of depression and its treatment by using any of the widely used antidepressant drug groups. Some studies suggest that females respond more poorly to tricyclic antidepressants than males and appear to respond better to selective serotonin reuptake inhibitors (SSRI). There is no study investigating the analgesic/antinociceptive effects of antidepressant drugs on the basis of gender difference. In this study, we aimed to investigate the antinociceptive effect of paroxetine on the basis of gender difference.

METHODS

The antinociceptive effect of paroxetine was tested using hot plate test in Balb/c mice (30-40 g). The animals were divided into eight groups on the basis of gender.

FINDINGS

While paroxetine did not induce an antinociceptive effect in both sex at a dose of 1 mg kg(-1), it showed significant antinociceptive effects in both sex at a dose of 5 or 10 mg kg(-1). None of the doses of paroxetine revealed a gender difference in its antinociceptive action.

CONCLUSION

There are several studies showing positive or negative evidence on the gender difference of paroxetine's antidepressant effect, but in the literature there is no study about the gender difference of paroxetine's or any other SSRI drug's antinociceptive effect. In conclusion, our results do not show any gender difference in antinociceptive effect of paroxetine that may be important especially when it would be used as an adjuvant agent in some painful conditions.

Disturbance of Response to Acute Thermal Pain in Naturally Occurring Cholecystokinin-A Receptor Gene Knockout Otsuka Long-Evans Tokushima Fatty (OLETF) Rats

Otsuka Long-Evans Tokushima Fatty (OLETF) rats lack cholecystokinin-A receptor (CCK-AR) because of a genetic abnormality. We observed that body temperature homeostasis in response to changes in ambient temperature was deteriorated in OLETF rats, while the functions of the signal outputs from the hypothalamus to effectors were not impaired. Deteriorated homeostasis was also seen in CCK-AR deficient (-/-) mice. In the present study, we examined whether the sensory pathway involved in transmitting signals about temperature from the skin to the brain was impaired in OLETF rats. To elucidate the involvement of CCK-AR function, we conducted the same experiment in CCK-AR(-/-) mice. Responses to thermal pain were assessed using the Hargreaves' plantar test apparatus. Shortening of withdrawal latency was observed in OLETF rats compared to control rats, indicating thermal hyperalgesia. Behavioral responses following paw withdrawal were disturbed in OLETF rats. The 5-hydroxytryptamine (5-HT) and 5-hydroxyindole acetic acid contents in the hippocampus and frontal cortex of OLETF rats were significantly higher than in those of the controls. CCK-AR(-/-) mice did not show any differences from wild-type mice. In conclusion, OLETF rats showed thermal hyperalgesia and disturbed responses to thermal pain, and an alteration of 5-HT function might have a role in this disturbance.