Thermal Tail Flick Research Articles

Discovery of a New Analgesic Peptide, Leptucin, from the Iranian Scorpion, Hemiscorpius lepturus.

Hemiscorpius lepturus scorpion stings do not induce considerable pain based on epidemiological surveys conducted in the southwest part of Iran. Accordingly, this study was aimed to identify the analgesic molecule in H. lepturus venom by analyzing a cDNA library of the scorpion venom gland looking for sequences having homology with known animal venom analgesic peptides. The analgesic molecule is a cysteine rich peptide of 55 amino acids. the synthetic peptide was deprotected and refolded. RP-HPLC, Ellman’s, and DLS assays confirmed the refolding accuracy. Circular dichroism (CD) showed helix and beta sheet contents. This peptide, called leptucin, demonstrated 95% analgesic activity at the dose of 0.48 mg/kg in hot plate assay. Leptucin at the doses of 0.32, 0.48, and 0.64 mg/kg showed 100% activity in thermal tail flick test. No hemolysis or cytotoxicity was observed at 8 and 16 μg. Histopathology evaluations indicated no hepatotoxicity, nephrotoxicity, and cardiotoxicity. We thus report that leptucin is the analgesic agent of H. lepturus venom. Regarding the high in vivo efficacy of leptucin and the fact it shows no observable toxicity, it could be suggested as a drug lead in a preclinical study of acute pain as well as the study of its mechanism of action.

Daily Intermittent Fasting in Mice Enhances Morphine Induced Anti‐Nociception while Mitigating Multiple Side Effects

The opioid epidemic has continued to plague the United States with high levels of abuse and poor quality of life for chronic pain patients requiring continuous use of opioids. Many potential solutions have been proposed to mitigate this epidemic including new medications to reduce the abuse and side effect profiles of opioids; however, new medications are plagued by low efficacy, abuse potential, and toxicities. These limitations suggest that non‐pharmacological interventions could be more efficacious with low side effect profiles. Intermittent fasting (IF) is a time restriction diet which has recently grown in popularity over the past decade in the prevention and mitigation of a variety of pathological states. Numerous animal and human studies have shown the benefits of IF in these disease states, but not in opioid treatment paradigms. We thus subjected male and female CD1 mice to 18‐hour fasting intervals followed by 6‐hour feed periods with standard chow for 1 week. Mice which underwent this diet displayed an enhanced anti‐nociceptive response to systemic morphine both in efficacy and duration using thermal tail flick and post‐operative paw incision pain models. While showing enhanced anti‐nociception, IF mice also demonstrated a reduction in the development of anti‐nociceptive tolerance using the thermal tail flick assay paired with daily systemic morphine treatment. We also found a reduction in the development of opioid induced constipation using both fecal mass and fecal pellet count in IF mice. Seeking a mechanism for our behavioral findings, we performed 35S‐GTPγS assays to assess the functionality of mu opioid receptors within the spinal cord and periaqueductal grey (PAG) tissues from IF mice with and without chronic morphine treatment. The resulting DAMGO concentration‐response curves demonstrated an increased efficacy of mu opioid receptors within the spinal cord in IF mice and an absence of the development of tolerance within the PAG in IF mice, suggesting site‐specific receptor mechanisms for the behavioral effects we observed. These data suggest that a daily IF diet may improve the therapeutic index of acute and chronic opioid therapies for pain patients in the clinic, both in improved analgesia and reduced tolerance and constipation.

KEA-1010, a ketamine ester analogue, retains analgesic and sedative potency but is devoid of Psychomimetic effects

BackgroundKetamine, a widely used anaesthetic and analgesic agent, is known to improve the analgesic efficacy of opioids and to attenuate central sensitisation and opioid-induced hyperalgesia. Clinical use is, however, curtailed by unwanted psychomimetic effects thought to be mediated by N-methyl-D-aspartate (NMDA) receptor antagonism. KEA-1010, a ketamine ester-analogue designed for rapid offset of hypnosis through hydrolysis mediated break-down, has been shown to result in short duration sedation yet prolonged attenuation of nociceptive responses in animal models. Here we report on behavioural effects following KEA-1010 administration to rodents.MethodsKEA-1010 was compared with racemic ketamine in its ability to produce loss of righting reflex following intravenous injection in rats. Analgesic activity was assessed in thermal tail flick latency (TFL) and paw incision models when injected acutely and when co-administered with fentanyl. Tail flick analgesic assessment was further undertaken in morphine tolerant rats. Behavioural aberration was assessed following intravenous injection in rats undergoing TFL assessment and in auditory pre-pulse inhibition models.ResultsKEA-1010 demonstrated an ED50 similar to ketamine for loss of righting reflex following bolus intravenous injection (KEA-1010 11.4 mg/kg [95% CI 10.6 to 12.3]; ketamine (racemic) 9.6 mg/kg [95% CI 8.5–10.9]). Duration of hypnosis was four-fold shorter in KEA-1010 treated animals. KEA-1010 prolonged thermal tail flick responses comparably with ketamine when administered de novo, and augmented morphine-induced prolongation of tail flick when administered acutely. The analgesic effect of KEA-1010 on thermal tail flick was preserved in opioid tolerant rats. KEA-1010 resulted in increased paw-withdrawal thresholds in a rat paw incision model, similar in magnitude yet more persistent than that seen with fentanyl injection, and additive when co-administered with fentanyl. In contrast to ketamine, behavioural aberration following KEA-1010 injection was largely absent and no pre-pulse inhibition to acoustic startle was observed following KEA-1010 administration in rats.ConclusionsKEA-1010 provides antinociceptive efficacy in acute thermal and mechanical pain models that augments standard opioid analgesia and is preserved in opioid tolerant rodents. The NMDA channel affinity and psychomimetic signature of the parent compound ketamine is largely absent for KEA-1010.

Antinociceptive and genotoxic assessments of the antagonist TRPV1 receptor SB-366791 on morphine-induced tolerance in mice.

Chronic pain is mainly treated with opioid analgesics such as morphine. However, the use of these substances can cause adverse effects, including dependence and tolerance, necessitating the discovery of a new approach to analgesic therapies. The transient receptor potential vanilloid 1 (TRPV1) is linked to thermal sensibility and has been considered as a new therapeutic option for pain treatment. This study aims to investigate the antinociceptive effect and toxicity of SB-366791, a TRPV1 antagonist. Morphine-tolerant and morphine non-tolerant Swiss mice were submitted to the hot plate and thermal tail flick tests. Toxicological evaluations of the genotoxic and mutagenic activities of SB-366791 were assessed using a comet assay and micronucleus test, and the Salmonella/microsome mutagenicity assay. In the hot plate test, intrathecal injection of SB-366791 or morphine resulted in significantly increased antinociception in non-tolerant mice. SB-366791 also led to an analgesic effect in the tail flick test. Tolerant mice that received SB-366791 demonstrated a central antinociceptive effect in both thermal tests. No genotoxic effects were observed in the comet assay and no mutagenic effects were detected in the micronucleus test or in the Salmonella/microsome assay. Behavioral results of the thermal nociception tests show that SB-366791 has antinociceptive potential in both morphine-tolerant and non-tolerant mice and does not cause genotoxic or mutagenic effects. Nevertheless, new studies should be performed to clarify the activity and participation of vanilloid channels in the antinociception of SB-366791.

Evidence for antinociceptive effects of combined administration of vitamin E and celecoxib in tail-flick and formalin test in male rats.

The aim of this study was to evaluate the effect of vitamin E co-administration with celecoxib in thermal and inflammatory pain in two model of pain assessment including thermal tail flick test of acute pain and formalin induced inflammatory model in adult male rats. Seventy two male Wistar rats were divided into a vehicle received intraperitoneally olive oil, indomethacin (20 mg/kg), vitamin E (100, 200 and 400 mg/kg), celecoxib (3, 10, 30 and 60 mg/kg) groups, and combination groups received the combination of vitamin E (100 and 200 mg/kg) and celecoxib (3, 10 and 30 mg/kg). All drugs were dissolved in olive oil. Antinociceptive effect in tail-flick was measured using Area Under Curve (AUC) of responses and Maximum Possible Effect (%MPE) and pain score was used for antinociceptive response in formalin test. Vitamin E and celecoxib changed time course of pain scores in a dose related manner in formalin test but not in tail-flick test. Vitamin E (200 mg/kg) had no effect and merely 60 mg/kg of celecoxib increased %MPE and AUC in tail-flick. The combination of vitamin E (100 or 200 mg/kg) with celecoxib (3 or 10 mg/kg) decreased pain scores compared to vehicle in both phases of formalin test, while in chronic phase (II) the pain scores of combination groups were also decreased compared to vitamin E and celecoxib. However, in tail-flick test the combination of ineffective doses of vitamin E (200 mg/kg) and celecoxib (10 and 30 mg/kg) increased %MPE and AUC compared to vehicle but not compared to celecoxib or vitamin E. Vitamin E and celecoxib showed a dose related antinociceptive effect in inflammatory but not in thermal model of acute pain. However the co-administration of vitamin E with celecoxib caused a significant increase in the antinociceptive effect which was similar to indomethacin, as a standard anti-inflammatory drug. So we suggest the concomitant use of vitamin E with celecoxib and other NSAIDs for potentiation of both anti- inflammatory and analgesic response, as well as the reduction of cardiovascular side effects of celecoxib.

Dissociation between morphine-induced spinal gliosis and analgesic tolerance by ultra-low-dose α2-adrenergic and cannabinoid CB1-receptor antagonists.

Long-term use of opioid analgesics is limited by tolerance development and undesirable adverse effects. Paradoxically, spinal administration of ultra-low-dose (ULD) G-protein-coupled receptor antagonists attenuates analgesic tolerance. Here, we determined whether systemic ULD α2-adrenergic receptor (AR) antagonists attenuate the development of morphine tolerance, whether these effects extend to the cannabinoid (CB1) receptor system, and if behavioral effects are reflected in changes in opioid-induced spinal gliosis. Male rats were treated daily with morphine (5 mg/kg) alone or in combination with ULD α2-AR (atipamezole or efaroxan; 17 ng/kg) or CB1 (rimonabant; 5 ng/kg) antagonists; control groups received ULD injections only. Thermal tail flick latencies were assessed across 7 days, before and 30 min after the injection. On day 8, spinal cords were isolated, and changes in spinal gliosis were assessed through fluorescent immunohistochemistry. Both ULD α2-AR antagonists attenuated morphine tolerance, whereas the ULD CB1 antagonist did not. In contrast, both ULD atipamezole and ULD rimonabant attenuated morphine-induced microglial reactivity and astrogliosis in deep and superficial spinal dorsal horn. So, although paradoxical effects of ULD antagonists are common to several G-protein-coupled receptor systems, these may not involve similar mechanisms. Spinal glia alone may not be the main mechanism through which tolerance is modulated.

Neurotoxicity of intrathecal injections of dexmedetomidine into the rat spinal dorsal horn.

To investigate the neurotoxicity of intrathecal injections of dexmedetomidine, Sprague-Dawley rats were intrathecally injected with dexmedetomidine at doses of 0.75, 1.50 and 3.00 μg/kg into the spinal dorsal horn. We found that c-Fos expression in the rat spinal dorsal horn peaked at 7 hours following the 3.00 μg/kg dexmedetomidine injection, while the levels of c-Fos expression following 0.75 and 1.50 μg/kg dexmedetomidine were similar to those in the spinal dorsal horn of normal rats. At 48 hours following administration, the level of c-Fos expression was similar to normal levels. In addition, the intrathecal injections of dexmedetomidine increased paw withdrawal mechanical thresholds and prolonged thermal tail flick latencies. These results indicate that dexmedetomidine has pronounced antinociceptive effects. However, dexmedetomidine appears to have neurotoxic effects in the spinal cord because it increased c-Fos expression in the spinal dorsal horn within 7 hours following administration.

Intrathecal antinociceptive interaction between the NMDA antagonist ketamine and the opioids, morphine and biphalin

Biphalin is an opioid peptide analogue that currently is under clinical development as a new type of site-directed analgesic. In rats, the intrathecal (i.t.) analgesic potency of biphalin is 1000-fold greater than morphine. Such a high activity may reflect this compound’s activation of three types of opioid receptors ( μ, δ and κ). NMDA receptors also play an important role in nociceptive processing. Therefore, we investigated in rats whether an NMDA antagonist may influence biphalin-induced antinociception. In the present study, ketamine was chosen because of the widespread safe use of this drug in clinical practice. I.t. application of ketamine alone had relatively little analgesic effect and its excitatory effects limited possible doses of the drug. Co-administration of ketamine with biphalin or morphine produced markedly greater antinociception than biphalin or morphine alone in acute, thermal tail flick testing. These results suggest that NMDA antagonists may be useful potentiators of biphalin analgesia. Thus, to obtain the same spinal antinociceptive effect, lower doses of biphalin or morphine are required when ketamine is co-administered.

Potent antinociceptive effects of TRK-820, a novel κ-opioid receptor agonist

TRK-820, a new type of 4,5-epoxymorphinan derivative, was investigated in vivo for antinociceptive activities and its selectivity on various opioid receptors in mice. Trk-820 given s.c. or p.o. Was found to be 351- and 796-fold more potent than U50,488H with acetic acid-induced abdominal constriction test. The duration of the antinociceptive effect produced by TRK-820 was longer than that produced by μ-opioid receptor agonist morphine or other κ-opioid receptor agonists. In addition, with four other antinociceptive assays, low temperature hot plate (51 °C), thermal tail flick, mechanical tail pressure and tail pinch tests, TRK-820 was also found to be 68- to 328-fold more potent than U-50488H, and 41- to 349-fold more potent than morphine in producing antinociception, as comparing the weight of the different compound. However, TRK-820 was less active in inhibiting the high temperature (55 ° C) hot plate response. The antinociceptive effects produced by TRK-820 were inhibited by nor-BNI, but not by naloxone or naltrindole (NTI) with the abdominal constriction test, indicating that the antinociception is selectively mediated by the stimulation of κ-, but not μ- or δ- opioid receptors. Co-administration of TRK-820 with morphine slightly enhanced the antinociception induced by morphine in the mouse hot plate test. On the other hand, pentazocine significantly reduced the morphine-induced antinociception. Trk-820 produced sedation at doses, which are much higher than the doses for producing antinociception. These results indicate that the potent antinociception induced by TRK-820 is mediated via the stimulation of κ-, but not μ- or δ-opiod receptors.

Surgical pain attenuates acute morphine tolerance in rats

Nociceptive stimuli, such as formalin-induced pain and adjuvant-induced arthritis, attenuate tolerance to morphine antinociception. In this study, we have explored the effect of upper and lower abdominal surgical pain on the prevention of acute tolerance to morphine antinociception in Sprague-Dawley rats. Group I received lower abdominal surgery (LAS) and i.v. morphine infusion; group II received LAS and i.v. saline infusion; group III received upper abdominal surgery (UAS) and i.v. morphine infusion; group IV received UAS and i.v. saline infusion; group V received i.v. morphine infusion; and group VI received i.v. saline infusion. The antinociceptive effects of morphine were measured by an infrared thermal tail flick test. We also measured plasma concentrations of morphine in rats receiving morphine infusions with or without surgical treatment. We found that acute tolerance to morphine antinociception developed after 2 h following i.v. infusion of morphine alone. However, both UAS and LAS significantly slowed the rate of development of acute tolerance to morphine. The area under the time-response curves (AUC) of groups I and III were mean 34,556 (SD 5607) and 32,548 (9783), respectively, which were significantly different from that of group V (18,759 (8225)) (P < 0.01). Also, there were no significant differences between groups I and III. There were no significant differences between groups for plasma morphine concentrations during the 8-h study (e.g. groups I, III and V: 179.9 (22.6), 182.7 (14.4) and 170.9 (15.8) ng ml-1 at 8 h, respectively) and we suggest that the appearance of acute morphine tolerance after morphine infusion is not pharmacokinetic in nature.

A novel bivalent opioid tetrapeptide : Effect of administration route in comparison with morphine

Biphalin is an opioid peptide analogue that currently is under clinical development as a new type of site-directed analgesic. In rats, the intrathecal (i.t.) analgesic potency of biphalin is 1000-fold greater than morphine. Such a high activity may reflect this compound’s activation of three types of opioid receptors (μ, δ and κ). NMDA receptors also play an important role in nociceptive processing. Therefore, we investigated in rats whether an NMDA antagonist may influence biphalin-induced antinociception. In the present study, ketamine was chosen because of the widespread safe use of this drug in clinical practice. I.t. application of ketamine alone had relatively little analgesic effect and its excitatory effects limited possible doses of the drug. Co-administration of ketamine with biphalin or morphine produced markedly greater antinociception than biphalin or morphine alone in acute, thermal tail flick testing. These results suggest that NMDA antagonists may be useful potentiators of biphalin analgesia. Thus, to obtain the same spinal antinociceptive effect, lower doses of biphalin or morphine are required when ketamine is co-administered.

Stimulation of spinal dopaminergic receptors: Differential effects on tail reflexes in rats

The tail-flick reflex to heat and tail-withdrawal reflex to touch were measured in spinal and intact rats given either apomorphine (0.02–1.75) μmol · kg −1) or dopamine (0.02–1.76 μmol · kg −1) in the lumbar subarachnoid space. In spinal rats both apomorphine and dopamine suppressed thermal tailflick reflex and enhanced tactile tail-withdrawal response in a dose-dependent way. The effect of apomorphine in spinal rats was counteracted by dopaminergic receptor antagonists ( cis-flupenthixol and (+)-butaclamol), but not by their stereoisomers. Phenoxybenzamine, propranolol, methysergide and naloxone failed to counteract the effects of apomorphine on tail-reflex responses in spinal rats. (+)-Butaclamol also counteracted effects of dopamine in spinal rats. Neither apomorphine nor dopamine influenced tail reflexes in intact rats, which suggests that effects of spinal dopaminergic mechanisms on these reflexes are influenced by descending supraspinal pathways.