•Translational research requires the use of relevant preclinical pain models and assessment tools.•Clinically relevant behavioral tests should focus on restoring function by reducing pathological ...pain in the absence of disruptive side effects.•Grimace, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis, CPP and CPA are described.•Analysis of the advantages and limitations of these tests will help researchers identify appropriate tests for their particular goal.
The primary objective of preclinical pain research is to improve the treatment of pain. Decades of research using pain-evoked tests has revealed much about mechanisms but failed to deliver new treatments. Evoked pain-tests are often limited because they ignore spontaneous pain and motor or disruptive side effects confound interpretation of results. New tests have been developed to focus more closely on clinical goals such as reducing pathological pain and restoring function. The objective of this review is to describe and discuss several of these tests. We focus on: Grimace Scale, Operant Behavior, Wheel Running, Burrowing, Nesting, Home Cage Monitoring, Gait Analysis and Conditioned Place Preference/ Aversion. A brief description of each method is presented along with an analysis of the advantages and limitations. The pros and cons of each test will help researchers identify the assessment tool most appropriate to meet their particular objective to assess pain in rodents. These tests provide another tool to unravel the mechanisms underlying chronic pain and help overcome the translational gap in drug development.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
1. Mitragynine is the major indole-based alkaloid of Mitragyna speciosa (kratom). Decoctions (teas) of the plant leaves have been used traditionally for cough, diarrhoea, pain, hypertension and for ...the treatment of opioid addiction. In the West, kratom has become increasingly utilized for mood elevation, pain treatment and as a means of self-treating opioid addiction.
2. Metabolic pathways of mitragynine were identified in human liver microsomes (HLM) and S9 fractions. A total of thirteen metabolites were identified, four oxidative metabolites and a metabolite formed by demethylation at the 9-methoxy group were the major metabolites of mitragynine.
3. The cytochrome P450 enzymes involved in the metabolism of mitragynine were identified using selective chemical inhibitors of HLM and recombinant cytochrome P450. The metabolism of mitragynine was predominantly carried out through the CYP3A4 with minor contributions by CYP2D6 and CYP2C9. The formation of five oxidative metabolites (Met2, Met4, Met6, Met8 and Met11) was catalyzed by the CYP3A4.
4. In summary, mitragynine was extensively metabolized in HLM primarily to O-demethylated and mono-oxidative metabolites. The CYP3A4 enzyme plays a predominant role in the metabolic clearance of mitragynine and also in the formation of 7-hydroxymitragynine (Met2), a known active minor alkaloid identified in the leaf material.
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DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Kratom alkaloids have mostly been evaluated for their opioid activity but less at other targets that could contribute to their physiological effects. Here, we investigated the in vitro and in vivo ...activity of kratom alkaloids at serotonin receptors (5-HTRs). Paynantheine and speciogynine exhibited high affinity for 5-HT1ARs and 5-HT2BRs, unlike the principal kratom alkaloid mitragynine. Both alkaloids produced antinociceptive properties in rats via an opioid receptor-independent mechanism, and neither activated 5-HT2BRs in vitro. Paynantheine, speciogynine, and mitragynine induced lower lip retraction and antinociception in rats, effects blocked by a selective 5-HT1AR antagonist. In vitro functional assays revealed that the in vivo 5-HT1AR agonistic effects may be due to the metabolites 9-O-desmethylspeciogynine and 9-O-desmethylpaynantheine and not the parent compounds. Both metabolites did not activate 5-HT2BR, suggesting low inherent risk of causing valvulopathy. The 5-HT1AR agonism by kratom alkaloids may contribute to the mood-enhancing effects associated with kratom use.
Kratom, Mitragyna speciosa Korth., is being widely consumed in the United States for pain management and the reduction of opioid withdrawal symptoms. The central nervous system (CNS) active alkaloids ...of kratom, including mitragynine, 7-hydroxymitragynine, and numerous additional compounds, are believed to derive their effects through opioid receptor activity. There is no literature describing the systemic exposure of many of these alkaloids after the consumption of kratom. Therefore, we have developed and validated a bioanalytical method for the simultaneous quantitation of 11 kratom alkaloids (mitragynine, 7-hydroxymitragynine, corynantheidine, speciogynine, speciociliatine, paynantheine, corynoxine, corynoxine-B, mitraphylline, ajmalicine, and isospeciofoline) in rat plasma. The validated method was used to analyze oral pharmacokinetic study samples of lyophilized kratom tea (LKT) and a marketed product, OPMS liquid shot, in rats. Among the 11 alkaloids, only mitragynine, 7-hydroxymitragynine, speciociliatine, and corynantheidine showed systemic exposure 8 h postdose, and the dose-normalized systemic exposure of these four alkaloids was higher (1.6–2.4-fold) following the administration of the commercial OPMS liquid. Paynantheine and speciogynine levels were quantifiable up to 1 h postdose, whereas none of the other alkaloids were detected. In summary, the method was successfully applied to quantify the exposure of individual kratom alkaloids after an oral dose of traditional or commercial products. This information will contribute to understanding the role of each alkaloid in the overall pharmacology of kratom and elucidating the pharmacokinetic differences between traditional and commercial kratom products.
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IJS, KILJ, NUK, PNG, UL, UM
Gabapentin (GBP) is a first-line therapy for neuropathic pain, but its mechanisms and sites of action remain uncertain. We investigated GBP-induced modulation of neuropathic pain following spinal ...nerve ligation (SNL) in rats. Intravenous or intrathecal GBP reversed evoked mechanical hypersensitivity and produced conditioned place preference (CPP) and dopamine (DA) release in the nucleus accumbens (NAc) selectively in SNL rats. Spinal GBP also significantly inhibited dorsal horn wide-dynamic-range neuronal responses to a range of evoked stimuli in SNL rats. By contrast, GBP microinjected bilaterally into the rostral anterior cingulate cortex (rACC), produced CPP, and elicited NAc DA release selectively in SNL rats but did not reverse tactile allodynia and had marginal effects on wide-dynamic-range neuronal activity. Moreover, blockade of endogenous opioid signaling in the rACC prevented intravenous GBP-induced CPP and NAc DA release but failed to block its inhibition of tactile allodynia. Gabapentin, therefore, can potentially act to produce its pain relieving effects by (a) inhibition of injury-induced spinal neuronal excitability, evoked hypersensitivity, and ongoing pain and (b) selective supraspinal modulation of affective qualities of pain, without alteration of reflexive behaviors. Consistent with previous findings of pain relief from nonopioid analgesics, GBP requires engagement of rACC endogenous opioid circuits and downstream activation of mesolimbic reward circuits reflected in learned pain-motivated behaviors. These findings support the partial separation of sensory and affective dimensions of pain in this experimental model and suggest that modulation of affective-motivational qualities of pain may be the preferential mechanism of GBP's analgesic effects in patients.
Kratom (Mitragyna speciosa) is a psychoactive plant popular in the United States for the self‐treatment of pain and opioid addiction. For standardization and quality control of raw and commercial ...kratom products, an ultra‐performance liquid chromatography−tandem mass spectrometry (UPLC−MS/MS) method was developed and validated for the quantification of ten key alkaloids, namely: corynantheidine, corynoxine, corynoxine B, 7‐hydroxymitragynine, isocorynantheidine, mitragynine, mitraphylline, paynantheine, speciociliatine, and speciogynine. Chromatographic separation of diastereomers, or alkaloids sharing same ion transitions, was achieved on an Acquity BEH C18 column with a gradient elution using a mobile phase containing acetonitrile and aqueous ammonium acetate buffer (10mM, pH 3.5). The developed method was linear over a concentration range of 1–200 ng/mL for each alkaloid. The total analysis time per sample was 22.5 minutes. The analytical method was validated for accuracy, precision, robustness, and stability. After successful validation, the method was applied for the quantification of kratom alkaloids in alkaloid‐rich fractions, ethanolic extracts, lyophilized teas, and commercial products. Mitragynine (0.7%–38.7% w/w), paynantheine (0.3%–12.8% w/w), speciociliatine (0.4%–12.3% w/w), and speciogynine (0.1%–5.3% w/w) were the major alkaloids in the analyzed kratom products/extracts. Minor kratom alkaloids (corynantheidine, corynoxine, corynoxine B, 7‐hydroxymitragynine, isocorynantheidine) were also quantified (0.01%–2.8% w/w) in the analyzed products; however mitraphylline was below the lower limit of quantification in all analyses.
Kratom, Mitragyna speciosa (Korth.), is self‐administered to treat pain and opioid withdrawal. A simple and sensitive UPLC‐MS/MS method was developed and validated for the simultaneous quantification of corynantheidine, corynoxine, corynoxine B, 7‐hydroxymitragynine, isocorynantheidine, mitragynine, mitraphylline, paynantheine, speciociliatine, and speciogynine in raw and commercial kratom products. Adequate chromatographic peak resolution between kratom alkaloids sharing same ion transitions was achieved. Developed method can be applied for the standardization and quality control of commercial kratom products.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Relief of pain is rewarding. Using a model of experimental postsurgical pain we show that blockade of afferent input from the injury with local anesthetic elicits conditioned place preference, ...activates ventral tegmental dopaminergic cells, and increases dopamine release in the nucleus accumbens. Importantly, place preference is associated with increased activity in midbrain dopaminergic neurons and blocked by dopamine antagonists injected into the nucleus accumbens. The data directly support the hypothesis that relief of pain produces negative reinforcement through activation of the mesolimbic reward-valuation circuitry.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Osteoarthritis (OA) pain is most commonly characterized by movement-triggered joint pain. However, in advanced disease, OA pain becomes persistent, ongoing and resistant to treatment with ...nonsteroidal anti-inflammatory drugs (NSAIDs). The mechanisms underlying ongoing pain in advanced OA are poorly understood. We recently showed that intra-articular (i.a.) injection of monosodium iodoacetate (MIA) into the rat knee joint produces concentration-dependent outcomes. Thus, a low dose of i.a. MIA produces NSAID-sensitive weight asymmetry without evidence of ongoing pain and a high i.a. MIA dose produces weight asymmetry and NSAID-resistant ongoing pain. In the present study, palpation of the ipsilateral hind limb of rats treated 14 days previously with high, but not low, doses of i.a. MIA produced expression of the early oncogene, FOS, in the spinal dorsal horn. Inactivation of descending pain facilitatory pathways using a microinjection of lidocaine within the rostral ventromedial medulla induced conditioned place preference selectively in rats treated with the high dose of MIA. Conditioned place preference to intra-articular lidocaine was blocked by pretreatment with duloxetine (30 mg/kg, intraperitoneally at -30 minutes). These observations are consistent with the likelihood of a neuropathic component of OA that elicits ongoing, NSAID-resistant pain and central sensitization that is mediated, in part, by descending modulatory mechanisms. This model provides a basis for exploration of underlying mechanisms promoting neuropathic components of OA pain and for the identification of mechanisms that might guide drug discovery for treatment of advanced OA pain without the need for joint replacement.
Difficulty in managing advanced OA pain often results in joint replacement therapy in these patients. Improved understanding of mechanisms driving NSAID-resistant ongoing OA pain might facilitate development of alternatives to joint replacement therapy. Our findings suggest that central sensitization and neuropathic features contribute to NSAID-resistant ongoing OA joint pain.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Tonic pain has been difficult to demonstrate in animals. Because relief of pain is rewarding, analgesic agents that are not rewarding in the absence of pain should become rewarding only when there is ...ongoing pain. We used conditioned place preference to concomitantly determine the presence of tonic pain in rats and the efficacy of agents that relieve it. This provides a new approach for investigating tonic pain in animals and for evaluating the analgesic effects of drugs.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Kratom is a Southeast Asian plant used in the United States for pain relief and mitigation of opioid withdrawal symptoms.•Speciociliatine in a minor indole alkaloid found in kratom with affinity at ...opioid receptors.•A preclinical in vivo pharmacokinetic study was performed in rats.•Higher systemic exposure and lower clearance values were found for speciociliatine compared to other kratom alkaloids.
Speciociliatine is a minor indole alkaloid found in kratom, a southeast Asian medicinal plant, used for centuries to increase energy, enhance mood, and mitigate pain and opioid dependence. An ultra-performance liquid chromatography tandem mass spectrometry method was developed and validated to quantify speciociliatine in rat plasma. The quantitation range was 3−600 ng/mL. The validated method was applied to a preclinical pharmacokinetic study in male Sprague-Dawley rats after 2.5 mg/kg intravenous (I.V.) and 20 mg/kg oral (P.O.) dosing. The plasma was analyzed to obtain concentration-time profiles and results were subjected to non-compartmental analysis to determine pharmacokinetic parameters including volume of distribution (6.2 ± 2.3 L/kg I.V.), clearance (0.7 ± 0.2 L/hr/kg), and absolute oral bioavailability (20.7 %). Speciociliatine had higher systemic exposure and lower clearance compared to the other kratom alkaloids mitragynine and corynantheidine. The speciociliatine pharmacokinetic parameters described here will help to better understand the overall effects reported with kratom product use.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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