Tail-pinch Test Research Articles

The effect of Amantadine on recovery, postoperative cognitive dysfunction and pain after propofol anesthesia in mice

IntroductionPostoperative cognitive dysfunction (POCD) encompasses a spectrum of cognitive impairments following surgery, attributed to disruptions in brain homeostasis. The pathogenesis involves glutamate toxicity, GABA receptor dysfunction, and alterations in NMDA and AMPA receptors. This study aimed to investigate the impact of pre-anesthetic amantadine administration on postoperative recovery time, POCD, and stress-related pain levels when combined with propofol anesthesia. MethodsTwenty-four adult male BALB/C mice were divided into four groups: Control, Propofol, Amantadine, and Amantadine+Propofol. Amantadine and propofol doses were administered intraperitoneally based on previous literature. Recovery time, pain levels (assessed via tail pinch and hot plate tests), cognitive functions (evaluated through Morris Water Maze), and locomotor activity (measured via Open Field Test) were recorded. ResultsAmantadine administration significantly reduced recovery time from propofol anesthesia, prolonged pain perception, and preserved cognitive functions compared to propofol alone. The time spent in the target quadrant in the Morris Water Maze was significantly longer in groups receiving amantadine. Additionally, the distance covered until finding the platform was significantly shorter in the propofol-only group. DiscussionAmantadine's neuroprotective effects, attributed to its antagonistic action on glutamate and NMDA receptors, mitigate the detrimental effects of propofol on cognitive function and pain perception. This study highlights the potential of combining amantadine with propofol to enhance postoperative outcomes. ConclusionAmantadine administration before propofol anesthesia positively influenced postoperative recovery, cognitive function preservation, and stress-related pain perception in mice. These findings suggest a potential therapeutic strategy to mitigate POCD and pain associated with surgery.

Gallic acid attenuates the Chronic construction injury of sciatic nerve induced neuropathic pain in mice

Gallic acid (GA) is one of the natural flavonoids and it is known to protect the neuronal system from various endogenous neurodegenerative processes. The present study is focused on evaluating the role of GA in chronic construction injury (CCI) of sciatic nerve-induced neuropathic pain. The test compound i.e., gallic acid (20 and 40mg/kg), and reference compound i.e., pregabalin (PrG: 5mg/kg) were administered intravenously for 10 consecutive days. The sciatic nerve injury-induced neuralgic sensations were assessed with multiple test methods like acetone drop, pinprick, plantar, tail-flick, and tail pinch tests at different time points i.e., 0, 4th, 8th, 12th, and 16th days. The biochemical level changes i.e., thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH), superoxide anion, calcium, myeloperoxidase (MPO), and tumor necrosis factor–α (TNF-α) were estimated in sciatic nerve samples. The administration of GA and PreG was shown a significant ameliorative effect against the CCI-induced pain response in a dose-dependent manner. Moreover, it also attenuated the tissue biochemical changes. The observed effects of GA were similar to reference drug PreG treated groups. Hence, GA has neuroprotective and pain relief effects against the CCI-induced neuralgia. It may be due to its natural free radical scavenging; regulation of inflammatory proteins; expression of antioxidant proteins, reduction of intracellular free calcium, and prevention of neuroinflammatory actions.

Behavioral and Pharmacokinetics Studies of N-Methyl-2-Aminoindane (NM2AI) in Mice: An Aminoindane Briefly Used in the Illicit Drug Market.

Drug forums are considered as the main platform sources that have contributed to the increase in NPS popularity, especially for those not yet known to law enforcement and therefore not yet illegal. An example is the new synthetic stimulant NM2AI, which has a very short history of human use and abuse. Little is known regarding this compound, but some information from internet forums and the scientific literature indicates NM2AI as a structural derivate of MDAI, which is known for its entactogenic activity. Indeed, the purpose of this study is to evaluate, for the first time, the in vivo acute effect induced by the intraperitoneal injection of NM2AI (1-10-30-100 mg/kg) in mice. We demonstrate the sensory (by visual placing and object tests) and physiological (core temperature measurement) function variations, nociceptor (by tail pinch test) and strength (grip test) alterations, and sensorimotor (time on rod and mobility) decrease. Moreover, we verify the mild hallucinogenic effect of NM2AI (by startle/prepulse inhibition test). Lastly, we perform a pharmacokinetic study on mice blood samples, highlighting that the main active metabolite of NM2AI is 2-aminoindane (2AI). Taken together, our data confirm the suspected entactogenic activity of NM2AI; however, these in vivo effects appear atypical and less intense with respect to those induced by the classic stimulants, in surprising analogy with what is reported by networked users.

Dual orexin receptor antagonist drug suvorexant can help in amelioration of predictable chronic mild stress-induced hyperalgesia

AimsThis study aimed to evaluate the involvement of the orexin system in predictable chronic mild stress (PCMS) and the effects of suvorexant, a dual orexin receptor antagonist, on nociceptive behavior in PCMS. Materials and methodsMale C57BL/6 J mice were separated into various PCMS groups: a control group with sawdust on the floor of the rearing cage (C), a group with mesh wire on the floor (M), and a group with water just below the mesh wire (W). Activation of lateral hypothalamic orexin neurons was assessed using immunofluorescence. In another experiment, half of the mice in each group were administered an intraperitoneal injection of suvorexant (10 mg/kg), and the remaining mice were injected with the same amount of vehicle (normal saline). Thermal hyperalgesia was examined using tail immersion and hot plate tests, while mechanical hyperalgesia was investigated using the tail pinch test after 21 days of PCMS. Key findingsAnimals subjected to PCMS showed an increased percentage of activated orexin neurons in the lateral hypothalamic region after 21 days. Mice raised in the PCMS environment showed increased pain sensitivity in several pain tests; however, the symptoms were significantly reduced by suvorexant administration. SignificanceThe findings revealed that PCMS activates hypothalamic orexin neuronal activity, and the use of suvorexant can help attenuate PCMS-induced thermal and mechanical hyperalgesia.

Ameliorative effect of gallic acid in paclitaxel-induced neuropathic pain in mice.

The present study has been investigated the role of gallic acid (GA) in paclitaxel-induced neuropathic pain. The neuropathic pain was developed with paclitaxel (PT: 2 mg/kg, i.p.) administration in mice. GA (20 and 40 mg/kg) and pregabalin (PreG: 5 mg/kg) were administered intravenously for 10 consecutive days. The neuralgic sensations were investigated by assessing various pain tests like acetone drop, pinprick, plantar, tail flick, and tail pinch test. Mice pain behaviors were evaluated on 0, 4th, 8th, 12th and 16th days. The levels of sciatic nerve thiobarbituric acid reactive substances (TBARS), reduced glutathione (GSH), superoxide anion, calcium, myeloperoxidase (MPO), and TNF-α were estimated. Treatment of GA and PreG attenuate PT induced thermal &mechanical hyperalgesia and allodynia symptoms along with the reduction of TBARS, total calcium, TNF-α, superoxide anion, and MPO activity levels; and decreased GSH level. Therefore, it has been concluded that GA has potential neuroprotective actions against PT induced neuropathic pain due to it's anti-oxidant, anti-inflammation and regulation of intracellular calcium ion concentration.

The effects of anandamide signaling enhanced by the FAAH inhibitor URB597 on coping styles in rats

Coping styles are fundamental characteristics of behavior that affect susceptibility to, and resilience during, mental and physical illness. Shifts from passive to active coping are considered therapeutic goals in many stress-related disorders, but the neural control of coping is poorly understood. Based on earlier findings, we hypothesized that coping styles are influenced by endocannabinoids. Here, we tested whether FAAH inhibition by URB597 affects behaviors aimed at controlling a critical situation and the degree to which environmental stimuli influence behavior i.e., we studied the impact of URB597 on the two main attributes of coping styles. Rats were tested in the tail-pinch test of coping and in the elevated plus-maze test that was performed under highly divergent conditions. Under the effects of URB597, rats focused their behavior more on the discomfort-inducing clamp in the tail-pinch test, i.e., they coped with the challenge more actively. In the elevated plus-maze, URB597-treated rats demonstrated an autonomous behavioral control by reducing both "wariness" induced by aversive conditions and "carelessness" resulting from favorable conditions. URB597 treatment-induced behavioral changes indicated a shift towards active coping with challenges. This behavioral change appears compatible with the previously suggested role of endocannabinoids in emotional homeostasis. Albeit further studies are required to characterize the role of endocannabinoids in coping, these findings suggest that the enhancement of endocannabinoid signaling may become a therapeutic option in emotional disorders characterized by passive coping (e.g., anxiety and depression) and in physical diseases where active coping is therapeutically desirable.

Alleviation of Morphine Withdrawal Signs but Not Tolerance by the Essential Oil ofKelussia odoratissimaMozaff.

The aim of the present study was to assess the effects of chronic and acute treatment of the essential oil (EO) of Kelussia odoratissima Mozaff. on the development of morphine tolerance and dependence in mice. Mice were rendered tolerant to and dependent on morphine by subcutaneous injection of morphine over a period of 5 days. Tolerance was assessed using the tail-pinch test and withdrawal signs of morphine were precipitated by injecting naloxone 2 h after the final morphine injection. Repeated injection of the EO of K. odoratissima (5 and 10 mg/kg) for 4 days significantly suppressed morphine-withdrawal jumps, a sign of the development of dependence to opiate as assessed by naloxone precipitation withdrawal on day 5 of testing. A single injection (25, 50, 100 mg/kg) of the EO on day 5, 1 h prior to morphine failed to produce any significant change in morphine withdrawal signs. Neither the acute nor the chronic administration of EO of the K. odoratissima did significantly influence the development of tolerance to the analgesic effect of morphine. Alleviation in morphine signs of withdrawal after chronic injection with K. odoratissima is indicative of reversal of neuronal adaptation that takes place during morphine presence in the brain.

Inhibition of morphine tolerance is mediated by painful stimuli via central mechanisms

Tolerance to morphine analgesia following repeated administration disturbs the continuation of opioid therapy for severe pain. Emerging evidence suggests that the development of morphine tolerance may be antagonized by painful stimuli. To clarify the detailed mechanisms of these phenomena, we examined the effects of several pain stimuli on morphine-induced tolerance. Subcutaneous (s.c.) injection of morphine (10 mg/kg) produced an analgesic effect, which was evaluated by tail-pinch test. Morphine-induced analgesia was diminished by repeated administration of morphine (10 mg/kg, s.c.) once a day for 5 days, demonstrating the development of tolerance. Morphine analgesic tolerance was suppressed by nerve injury-induced neuropathic pain and formalin- or carrageenan-induced inflammatory pain. Tolerance to serum corticosterone elevation by morphine (10 mg/kg), which was evaluated by fluorometric assay, was also suppressed by formalin-induced inflammatory pain. Moreover, morphine analgesia induced by intracerebroventricular (10 nmol) or intrathecal (5 nmol) injection was diminished by repeated administration of morphine s.c., and this was also suppressed by carrageenan-induced inflammatory pain. These results suggest that morphine tolerance is inhibited by several pain stimuli, including neuropathic and inflammatory pain, through central mechanisms.

Rapakinin, Arg–Ile–Tyr, derived from rapeseed napin, shows anti-opioid activity via the prostaglandin IP receptor followed by the cholecystokinin CCK 2 receptor in mice

Rapakinin, Arg–Ile–Tyr, is a vasorelaxing, anti-hypertensive and anorexigenic peptide derived from rapeseed napin. In this study, we found that rapakinin intracerebroventricularly administered to mice inhibited the analgesic effect of morphine, evaluated by the tail-pinch test. The anti-opioid activity of rapakinin was blocked by LY225910, an antagonist of the cholecystokinin (CCK) CCK 2 receptor, but not by lorglumide, an antagonist of the CCK 1 receptor. The anti-opioid activity of rapakinin was also blocked by CAY10441, an antagonist of the prostaglandin (PG) IP receptor. These results suggest that the anti-opioid activity of rapakinin is mediated by the CCK 2 and IP receptors. The anti-opioid activity induced by ciprostene, an IP receptor agonist, was blocked by LY225910, while that of CCK-8 was not blocked by CAY10441. Thus, it is demonstrated that the CCK–CCK 2 system was activated downstream of the PGI 2–IP receptor system. Taken together, rapakinin shows anti-opioid activity via the activation of the PGI 2–IP receptor system followed by the CCK–CCK 2 receptor system.

Increment of Activated Serine/Threonine Protein Phosphatase in Brain Membrane Fraction Synchronized with Antinociceptive Effect of Morphine in Mice

We have examined the involvement of serine/threonine protein phosphatase (PP) sensitive to okadaic acid (OA) in the antinociceptive effect of morphine in mice. The present study was performed to elucidate subcellular distribution and activity of OA-sensitive PPs in the brain when mice exposed to morphine. Subcutaneous administration of morphine (5 mg/kg) produced the antinociceptive effect with the maximum 30 min after its administration, evaluated by tail-pinch test. The antinociception was accompanied by an increment of activity in OA-sensitive PPs in the membrane fraction prepared from the whole brain of mice treated with morphine: The temporal profile of the morphine-induced increment of OA-sensitive PP activity was consistent with that of antinociceptive effects of morphine. The morphine-induced increase in OA-sensitive PP activity was dependent on the dose and attenuated by the concurrent administration of naloxone (1 mg/kg). To identify the subtype of OA-sensitive PPs in morphine-enhanced activity, we examined the level of PP2A and PP5, OA-sensitive PPs, in the subcellular fraction prepared from the whole brain of mice receiving morphine. Western blot revealed that morphine elicited the significant increase in the level of PP5, but not PP2A, in the membrane fraction, with the same peak time for the increment of PP5 as the antinociception. No significant change was observed in the level of OA-sensitive PPs in the cytosolic fraction at any examined time after morphine. These results suggest that the translocation of PP5 to the membrane fraction is, at least in part, involved in the antinociceptive effect of morphine in mice.

Effects of chronic administration of calcium-magnesium soft gels on morphine tolerance and dependence in mice

The aim of present study was to assess the effects of chronic administration of calcium-magnesium soft gels on the development of morphine tolerance and dependence in mice. Tolerance was assessed using the tail-pinch test and withdrawal signs of morphine were precipitated by injecting naloxone 2 h after the final morphine injection. CalMag capsules were given in final doses of 50/25, 25/12.5 and 12.5/6.25 mg/kg based on calcium/magnesium ratio. Similar doses of Ca and/or Mg were prepared, separately. CalMag at 50/25, 25/12.5 mg/kg and the mixture of calcium and magnesium (Ca + Mg) at 50/25, 25/12.5, 12.5/6.25 mg/kg and calcium at 50, 25 mg/kg significantly reduced the number of jumps. The number of standings was only reduced after the administration of CalMag at 50/25 mg/kg and Ca + Mg at 25/12.5 mg/kg. The development of morphine tolerance was prevented in all drug-treated groups, except the one which received 6.25 mg/kg Mg. The data suggested that combination of calcium and magnesium at 50/25 and 25/12.5 mg/kg prevented the development of tolerance and dependence. It seems that other ingredients of CalMag capsules do not have an important effect on preventing tolerance and withdrawal signs. Compared to the acute effects, chronic administration of Cal-Mag allowed the effective dose to be reduced. Unlike the acute treatment, chronic administration of calcium alone was effective in reducing morphine tolerance and dependence, and magnesium had no significant effect on withdrawal signs, suggestive of some pharmacological adaptations.

Angiotensin AT 2 receptor agonists act as anti-opioids via EP 3 receptor in mice

Novokinin (Arg-Pro-Leu-Lys-Pro-Trp) is a vasorelaxing and hypotensive peptide acting through the angiotensin AT 2 receptor. Centrally administrated novokinin (30 nmol/mouse) inhibited the antinociceptive effect of μ agonist morphine in mice, as evaluated by the tail-pinch test. The anti-opioid effect of novokinin was blocked by PD123319, an antagonist of the AT 2 receptor. Angiotensin II (0.01 nmol/mouse, i.c.v.) and [ p-aminophenylalanine 6]-angiotensin II [ p-NH 2Phe 6]-Ang II (0.1 nmol/mouse, i.c.v.), a highly selective AT 2 receptor agonist, also inhibited the antinociceptive effect of morphine, and the effects were also blocked by PD123319. Angiotensin II did not suppress the antinociceptive effect induced by κ or δ agonists. Novokinin, angiotensin II and [ p-NH 2Phe 6]-Ang did not have affinity for the μ receptor. The anti-opioid effects induced by these peptides were blocked by ONO-AE3-240, an antagonist of the EP 3 receptor. These results suggest that the anti-opioid effects of AT 2 agonists are mediated by the PGE 2-EP 3 receptor system downstream of the AT 2 receptor.

Analgesic activity of dipeptide Tyr-Pro

The effect of dipeptide Tyr-Pro, present in representatives of most families of opioid peptides, and two its analogs, Tyr-Pro-NH2 and Tyr-Pro-OMe, on analgesic activity was studied in different tests (tail-flick test, tail pinch (Haffner's) test, formalin test, and acetic acid writing test) describing different organization levels of pain sensitivity. Intraperitoneal administration of the dipeptide decreased the pain threshold in all above-mentioned tests. Coadministration of the dipeptide and naloxone or naloxone methiodide insignificantly decreased the dipeptide analgesic effect in the tail-flick and acetic acid writing tests. Its analogs Tyr-Pro-NH2 and Tyr-Pro-OMe demonstrated a similar analgesic activity in the tail-flick test and a higher activity in the acetic acid writing test. Administration of individual amino acids (Tyr or Pro) or their mixture had no effect on the pain threshold.

Modification of antiallodynic and antinociceptive effects of morphine by peripheral and central action of fluoxetine in a neuropathic mice model

We have previously reported that serotonin concentration was reduced in the brain of mice with neuropathic pain and that it may be related to reduction of morphine analgesic effects. To further prove this pharmacological action, we applied fluoxetine, a selective serotonin reuptake inhibitor, to determine whether it suppressed neuropathic pain and examined how its different administration routes would affect antinociceptive and antiallodynic effects of morphine in diabetic (DM) and sciatic nerve ligation (SL) mice, as models of neuropathic pain. Antiallodynia and antinociceptive effect of drugs were measured by using von Frey filament and tail pinch tests, respectively. Fluoxetine given alone, intracerebroventicularly (i.c.v., 15 microg/mouse) or intraperitoneally (i.p., 5 and 10 mg/kg) did not produce any effect in either model. However, fluoxetine given i.p. enhanced both antiallodynic and antinociceptive effects of morphine. Administration of fluoxetine i.c.v., slightly enhanced only the antiallodynic effect of morphine in SL mice. Ketanserine, a serotonin 2A receptor antagonist (i.p., 1 mg/kg) and naloxone, an opioid receptor antagonist (i.p., 3 mg/kg), blocked the combined antinociceptive effect of fluoxetine and morphine. Our data show that fluoxetine itself lacks antinociceptive properties in the two neuropathy models, but it enhances the analgesic effect of morphine in the periphery and suggests that co-administration of morphine with fluoxetine may have therapeutic potential in treatment of neuropathic pain.

Involvement of spinal Met–enkephalin in nicotine-induced antinociception in mice

Nicotine is neuronal stimulating drug in the central nervous system and elicits various effects through nicotinic acetylcholine receptors. As previously reported, nicotine has an antinociceptive effect through activation of endogenous opioid neurons. However, detailed mechanisms of nicotine-induced antinociception are uncertain. In this study, we focused on spinal cord and investigated the involvement of endogenous opioidergic neurons in nicotine-induced antinociception in mice. In the tail-pinch test, subcutaneously administered nicotine (5 mg/kg) produced maximal antinociception 0.5 h after nicotine administration; this was attenuated by mecamylamine (MEC, 3 mg/kg, s.c.) or naloxone (NLX, 1 mg/kg, s.c.) administration. Intrathecal nicotine (10 μg) produced maximal antinociception at 2 min and this was also attenuated by MEC (3 mg/kg, s.c.) or NLX (1 mg/kg, s.c.) administration. The preproenkephalin (ppENK) mRNA level in spinal cord, but not dorsal root ganglion, was significantly increased 2 h following nicotine administration and recovered to control level 4 h after nicotine (5 mg/kg, s.c.) administration. This increase in ppENK mRNA level was inhibited by MEC (3 mg/kg, s.c.). The mRNA levels of preprodynorphin and preproopiomelanocortin were not increased by nicotine (5 mg/kg, s.c.). In the dorsal horn of the lumbar spinal cord, methionine–enkephalin (Met–ENK)-like IR was remarkably reduced at 0.5 h following nicotine administration and recovered to control levels by 2 h after nicotine (3 mg/kg, s.c.) administration. These results suggest that nicotine has an antinociceptive effect by promoting the release of Met–ENK, but not dynorphins and endorphins, from activated opioidergic neurons in spinal cord.

CD26 modulates nociception in mice via its dipeptidyl-peptidase IV activity

Background CD26 is a multifunctional cell surface glycoprotein expressed by T and B cells. It exhibits a dipeptidyl-peptidase activity (DPP-IV) that cleaves the penultimate proline from the N-terminus of polypeptides, thereby regulating their activity and concentration. Methods Using CD26−/− mice resulting from targeted inactivation of the gene, we examined the consequences of a DPP-IV defect on behavioural response to nociceptive stimuli and concentration of the pain modulator peptides substance P (SP) and endomorphin 2, two DPP-IV substrates. Results CD26 inactivation induced a three-fold decrease in circulating endopeptidase activity while that found in brain extracts was normal, albeit very weak. CD26−/− mice had high SP concentrations in plasma (3.4 ± 1 pg/ml versus 1.5 ± 0.3 pg/ml, P < 10 −3) but not in brain extracts (35 ± 12 pg/ml versus 32 ± 9 pg/ml, P > 0.05). Endomorphin-2 levels in the two groups were in the same range for plasma and brain extracts. CD26−/− mice displayed short latencies to nociceptive stimuli (hot plate test: 6.6 ± 1.2 s versus 8.6 ± 1.5 s, P < 10 −4; tail pinch test: 3.1 ± 0.6 s versus 4.2 ± 0.8 s, P < 10 −3). Administration of an SP (NK1) receptor antagonist or DPP-IV to CD26−/− mice normalised latencies. DPP-IV inhibitors decreased latencies only in CD26+/+ mice. Conclusions Our observations represent the first fundamental evidence showing that DPP-IV influences pain perception via modulation of the peripheral SP concentration. Our work also highlights the role of peripheral NK1 receptors in nociception.

Morphine has an antinociceptive effect through activation of the okadaic-acid-sensitive Ser/Thr protein phosphatases PP2A and PP5 estimated by tail-pinch test in mice

Although the serine/threonine protein kinases involved in the pharmacological action of morphine are well recognized, the critical contribution of serine/threonine protein phosphatase (PP) has been appreciated on to a slight degree. We examined the involvement of subtypes of serine/threonine protein phosphatase (PP) in the antinociceptive effect of morphine in mice. The antinociceptive effect of subcutaneously administered morphine was attenuated by simultaneously intracerebroventricular (i.c.v.) or intrathecal (i.t.) injection of okadaic acid (OA), a PP inhibitor. To reveal which subtypes of PPs participated in the antinociceptive effect of morphine, mice received i.c.v. or i.t. injections of antisense oligodeoxynucleotide (AS-ODN) directed against either the PP2A or PP5 subtypes of PPs before assessment of morphine-induced antinociception. Pretreatment with AS-ODN against PP2A or PP5 via each route weakened the antinociceptive effect of morphine, accompanied by reduction of expression levels of PP in the periaqueductal gray (PAG) and the spinal cord. Subcutaneously administered morphine increased activity of OA-sensitive PPs in the PAG and the spinal cord in a dose-dependent manner; this was prevented by concurrent administration of naloxone. These results suggest that PP2A and PP5 are involved in the antinociceptive effect of morphine in mice.

Fangchinoline inhibited the antinociceptive effect of morphine in mice

Fangchinoline (FAN), a non-specific calcium antagonist, is a major alkaloidal component of the creeper Stephania tetrandra S. Moore (or fenfangji). It has been shown to possess antagonistic activity on morphine-induced antinociception in mice. This study was undertaken to assess the antagonistic mechanism. The results demonstrated that FAN (IP) attenuated morphine (SC)-induced antinociception in a dose-dependent manner with significant effect at doses of 30 and 60mg/kg body wt. (IP) in the tail-flick test but not the tail-pinch tests, carried out in mice. This antagonism was abolished by pretreatment with a serotonin precursor, 5-hydroxytryptophan (5-HTP, IP), but not by pretreatment with a noradrenaline precursor, L-dihydroxyphenylalanine (L-DOPA, IP) in the tail-flick test. On the other hand, the development of morphine-induced analgesic tolerance was not prevented by FAN. These results suggest that the serotonergic pathway may be involved in the antagonism of morphine-induced antinociception by FAN and, in agreement with other reports, also indicates the possible dissociation of the morphine analgesic effect from its tolerance-development mechanism.

Small conductance calcium-activated K+ channels, SkCa, but not voltage-gated K+ (Kv) channels, are implicated in the antinociception induced by CGS21680, a A2A adenosine receptor agonist

It has been shown that A2A adenosine receptors are implicated in pain modulation. The precise mechanism by which activation of A2A receptors produces analgesic effects, however, remains unclear. The aim of this study was to investigate the possible involvement of apamin-sensitive calcium-activated potassium channels (SKCa) and voltage-gated potassium (Kv) channels in A2A receptor activation-induced analgesic effects. Using mice, we evaluated the influence of apamin, a non specific blocker of SKCa channels, Lei-Dab7 (an analog of scorpion Leiurotoxin), a selective blocker of SKCa2 channels, and kaliotoxin (KTX) a Kv channel blocker, on the CGS 21680 (A2A adenosine receptor agonist)-induced increases in hot plate and tail pinch latencies. All drugs were injected in mice via the intracerebroventricular route. We found that apamin and Lei-Dab7, but not KTX, reduced antinociception produced by CGS21680 on the hot plate and tail pinch tests in a dose dependent manner. Lei-Dab 7 was more potent than apamin in this regard. We conclude that SKCa but not Kv channels are implicated in CGS 21680-induced antinociception.

PKC and PKA inhibitors reverse tolerance to morphine-induced hypothermia and supraspinal analgesia in mice

Morphine antinociceptive tolerance in the tail-flick test is completely reversed by inhibitors of protein kinase C (PKC) or cAMP-dependent protein kinase (PKA). The effects of these inhibitors on tolerance to supraspinally mediated antinociception, such as the hot-plate test was unknown, as well as their effects in tests of mechanical nociception. The PKC inhibitors bisinolylmaleimide I ((2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide) and Gö-7874 {2[1[(3-Dimethylaminopropyl)-5-methozyindol-3-yl]-3-(1H-indol-3-yl) hydrochloride} completely reversed the tolerance to morphine in both the hot-plate and tail-pinch tests. Similarly, the PKA inhibitor KT-5720 (8R, 9S, 11S)-(−)-9-hydroxy-9-hexoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinden-1-one also reversed tolerance in both tests. The role of PKC and PKA in mediating tolerance to morphine-induced hypothermia was also investigated. Bisinolylmaleimide I, Gö-7874 and KT-5720 only partly reversed the 32-fold level of tolerance induced by the morphine pellets. However, co-administration of bisinolylmaleimide I with KT-5720 or Gö-7874 with KT-5720 completely reversed the tolerance. This demonstrates that tolerance in a non-behavioral system involves the actions of PKC and PKA.