Fentanyl Analogs Exert Antinociceptive Effects via Sodium Channel Blockade in Mice.

The formalin test is one approach to studying acute pain in rodents. Similar to formalin, injection with glutamate and veratrine can also produce a nociceptive response. This study investigated whether opioid-related compounds could suppress glutamate- and veratrine-induced nociceptive responses in mice at the same dose. The administration of morphine (3 mg/kg), hydromorphone (0.4 mg/kg), or fentanyl (0.03 mg/kg) suppressed glutamate-induced nociceptive response, but not veratrine-induced nociceptive response at the same doses. However, high doses of morphine (10 mg/kg), hydromorphone (2 mg/kg), or fentanyl (0.1 mg/kg) produced a significant reduction in the veratrine-induced nociceptive response. These results indicate that high doses are required when using morphine, hydromorphone, or fentanyl for sodium channel-related neuropathic pain, such as ectopic activity. As a result, concerns have arisen about overdose and abuse if the dose of opioids is steadily increased to relieve pain. In contrast, trimebutine (100 mg/kg) and fentanyl analog isobutyrylfentanyl (iBF; 0.1 mg/kg) suppressed both glutamate- and veratrine-induced nociceptive response. Furthermore, nor-isobutyrylfentanyl (nor-iBF; 1 mg/kg), which is a metabolite of iBF, suppressed veratrine-induced nociceptive response. Besides, the optimal antinociceptive dose of iBF, unlike fentanyl, only slightly increased locomotor activity and did not slow gastrointestinal transit. Cancer pain is a complex condition driven by inflammatory, neuropathic, and cancer-specific mechanisms. Thus, iBF may have the potential to be a superior analgesic than fentanyl.

Caffeine-Supplemented Diet Prevents Fatigue-Like Behavior in Tumor-Bearing Mice.

Caffeine is a widely consumed stimulant, known for its positive effects on physical and mental performance. These effects are potentially beneficial for ameliorating cancer-related fatigue, which affects the quality of life of patients with cancer. This study aimed to determine the anti-fatigue and antitumor effects of caffeine in tumor-bearing mice. BALB/c mice were intravenously injected with C26 colon carcinoma cells and fed with normal or 0.05% caffeine-supplemented diet. Fatigue-like behavior was assessed by running performance using a treadmill test. Lung, blood, liver, muscle, and epididymal adipose tissue samples were collected on day 13 and examined. The antitumor effect of caffeine was assessed using subcutaneous tumor-bearing mice fed with 0.05% caffeine-supplemented diet, and the tumor volume was measured. C26 tumor-bearing mice showed fatigue-like behavior associated with hypoglycemia, depleted liver glycogen and non-esterified fatty acid (NEFA) levels. C26 tumor-bearing mice fed with 0.05% caffeine-supplemented diet showed improved running performance associated with restored NEFA levels. However, exacerbated hypoglycemia and liver glycogen levels after caffeine consumption may be due to tumor-induced catabolic signals, as the tumor volume was not affected. Collectively, caffeine may exert anti-fatigue effects through enhanced lipolysis leading to restored NEFA levels, which can be used as an alternative energy source.

Cold-Restraint Stress-Induced Ultrasonic Vocalization as a Novel Tool to Measure Anxiety in Mice.

Ultrasonic vocalization (USVs) is a promising tool to measure behavioral anxiety in rodents as USV recording is noninvasive, behaviorally relevant, ethological, and reproducible. Studies reporting the effects of stress-induced USVs in adult mice remain limited and debated. We investigated the conditions under which mice emit aversive USVs and evaluated the effects of psychiatric drugs on stress-induced USVs. Male C57BL/6J mice were used. USVs during entire stress sessions were recorded according to their frequency. To investigate the effect of psychiatric drugs on USVs, the number of USVs under cold-restraint stress conditions before and after drug administration was compared. Immediately after stress exposure, blood samples were collected and plasma corticosterone levels were measured. The combination of cold and restraint stress conditions significantly increased the USV numbers and plasma corticosterone levels compared with each stress alone. A benzodiazepine anxiolytic (midazolam) and δ-opioid receptor agonist putative anxiolytic (KNT-127) significantly reduced the stress-induced USV number and plasma corticosterone levels; however, a monoaminergic antidepressant (duloxetine) and N-methyl-D-aspartic acid receptor antagonist antidepressant (ketamine) did not reduce the USV numbers. No changes were noted in the USV numbers after repeated exposure to cold-restraint stress on days 1 and 3. The suppressive effect of midazolam on day 3 was comparable to that on day 1. Stress-induced USV may be used as a quantitative measure of anxiety to systematically assess the effects of anxiolytics. Therefore, cold-restraint stress-induced USVs may be used as a novel tool to measure rodent anxiety and as a useful anxiolytic-screening system.

A 3-styrylchromone converted from trimebutine 3D pharmacophore possesses dual suppressive effects on RAGE and TLR4 signaling pathways.

Receptor for advanced glycation end-products (RAGE) and Toll-like receptors (TLRs) are potential therapeutic targets in the treatment of acute and chronic inflammatory diseases. We previously reported that trimebutine, a spasmolytic drug, suppresses RAGE pro-inflammatory signaling pathway in macrophages. The aim of this study was to convert trimebutine to a new small molecule using in silico 3D pharmacophore similarity search, and dissect the mechanistic anti-inflammatory basis. Of note, a unique 3-styrylchromone (3SC), 7-methoxy-3-trimethoxy-SC (7M3TMSC), converted from trimebutine 3D pharmacophore potently suppressed both high mobility group box 1-RAGE and lipopolysaccharide-TLR4 signaling pathways in macrophage-like RAW264.7 cells. More importantly, 7M3TMSC inhibited the phosphorylation of extracellular signaling-regulated kinase 1 and 2 (ERK1/2) and downregulated the production of cytokines, such as interleukin-6. Furthermore, 3D pharmacophore-activity relationship analyses revealed that the hydrogen bond acceptors of the trimethoxy groups in a 3-styryl moiety and the 7-methoxy-group in a chromone moiety in this compound are significant in the dual anti-inflammatory activity. Thus, 7M3TMSC may provide an important scaffold for the development of a new type of anti-inflammatory dual effective drugs targeting RAGE/TLR4-ERK1/2 signaling.

Antinociceptive activity of the novel RAGE inhibitor, papaverine, in a mouse model of chronic inflammatory pain.

Extracellular high-mobility group box 1 (HMGB1) is known to mediate the inflammatory response through pattern recognition receptors, including the receptor for advanced glycation end products (RAGE) or the toll-like receptors (TLRs). The aim of the present study was to investigate whether papaverine, a novel RAGE inhibitor, could suppress inflammatory pain in mice after several time points, which was induced by the injection of complete Freund's adjuvant (CFA). We also investigated the influence of redox modulation during a state of chronic inflammatory pain. Although papaverine did not suppress CFA-induced mechanical allodynia on Day 7, papaverine significantly suppressed CFA-induced mechanical allodynia on Days 14 and 28. In contrast, the radical scavenger N-tert-Butyl-α-phenylnitrone (PBN) suppressed mechanical allodynia in mice on Days 7 and 14, but not on Day 28. We demonstrated that the RAGE inhibitor improves mechanical allodynia in chronic inflammatory conditions. Moreover, we also found that high levels of reactive oxygen species (ROS) contributed to the early phase of CFA-induced mechanical allodynia. Precisely, lower ROS levels contributed to the inflammatory pain response via the all-thiol HMGB1/RAGE signaling pathway during the chronic state. These findings led us to propose that ROS levels modulate RAGE and/or TLR4-mediated inflammatory allodynia by regulating the concentrations of disulfide HMGB1 or all-thiol HMGB1.

Trimebutine attenuates high mobility group box 1-receptor for advanced glycation end-products inflammatory signaling pathways.

We previously identified papaverine as an inhibitor of receptor for advanced glycation end-products (RAGE) and showed its suppressive effect on high mobility group box 1 (HMGB1)-mediated responses to inflammation. Here, we found trimebutine to be a 3D pharmacophore mimetics of papaverine. Trimebutine was revealed to have more potent suppressive effects on HMGB1-induced production of pro-inflammatory cytokines, such as interleukin-6 and tumor necrosis factor-α in macrophage-like RAW264.7 cells and mouse bone marrow primarily differentiated macrophages than did papaverine. However, the inhibitory effect of trimebutine on the interaction of HMGB1 and RAGE was weaker than that of papaverine. Importantly, mechanism-of-action analyses revealed that trimebutine strongly inhibited the activation of RAGE downstream inflammatory signaling pathways, especially the activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2), which are mediator/effector kinases recruited to the intracellular domain of RAGE. Consequently, the activation of Jun amino terminal kinase, which is an important effector kinase for the up-regulation of pro-inflammatory cytokines, was inhibited. Taken together, these results suggest that trimebutine may exert its suppressive effect on the HMGB1-RAGE inflammatory signal pathways by strongly blocking the recruitment of ERK1/2 to the intracellular tail domain of RAGE in addition to its weak inhibition of the extracellular interaction of HMGB1 with RAGE. Thus, trimebutine may provide a unique scaffold for the development of novel dual inhibitors of RAGE for inflammatory diseases.

Benzothiazepines, diltiazem and JTV-519, exert an anxiolytic-like effect via neurosteroid biosynthesis in mice.

We investigated whether benzothiazepines could produce anxiolytic effects via allopregnanolone (ALLO) biosynthesis in mice. We compared the behavioral effects caused by benzothiazepines to those caused by carbamazepine and sodium valproate. We found that a pretreatment with finasteride (a 5 alpha-reductase inhibitor) suppressed carbamazepine-induced anxiolytic effects but not the effects of sodium valproate. Similar to carbamazepine, diltiazem and JTV-519 displayed anxiolytic effects that were suppressed by a pretreatment with finasteride. We clearly demonstrate that the benzothiazepines, diltiazem and JTV-519, exert an anxiolytic-like effect via ALLO biosynthesis in mice.

Synergistic Antinociceptive Activity of Tramadol/Acetaminophen Combination Mediated by µ-Opioid Receptors.

We investigated whether tramadol could suppress both neuropathic and inflammatory pain in mice at the same dose level. We also examined the effects of drugs metabolized by glucuronidase, such as acetaminophen (ACAP), indomethacin, probenecid, and valproate, on the antinociceptive activity of tramadol. The administration of 5.6 or 10 mg/kg tramadol suppressed cuff-induced mechanical allodynia, but 10 mg/kg tramadol did not suppress complete Freund's adjuvant (CFA)-induced mechanical allodynia. Although neither tramadol (10 mg/kg) nor ACAP (100 mg/kg) alone produced an antinociceptive effect, their combination suppressed CFA-induced mechanical allodynia. Moreover, pretreatment naloxone, an opioid receptor antagonist, significantly attenuated the antinociceptive effects induced by the combination of tramadol and ACAP and slowed gastrointestinal transit. Similar to ACAP, the combination of tramadol and probenecid or valproate, which has the potential to inhibit uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT), also suppressed the CFA-induced mechanical allodynia and slowed gastrointestinal transit. We concluded that tramadol was more beneficial for the treatment of neuropathic pain than inflammatory pain. Furthermore, the antinociceptive effects of the tramadol and ACAP combination were mediated by the µ-opioid receptor, and were thought to be related, at least in part, to the accumulation of the active metabolite, M1.

Eicosapentaenoic Acid Improves Cisplatin-Induced Muscle Atrophy Without Accompanying Body Weight Gain.

INTRODUCTION: Cisplatin (CDDP) induces loss of muscle mass by activating the nuclear factor (NF)-κB signaling pathway. In this study, we investigated the effects of eicosapentaenoic acid (EPA), which inhibits NF-κB activation, on CDDP-induced loss of muscle mass in mice. METHODS: Male C57BL/6J mice received a single dose of CDDP and olive oil, linseed oil, or EPA daily for 4 days. Body weight and food intake were recorded daily for 5 days. Forelimb grip strength was determined using a strain gauge on the fourth day. The mice were killed 24 h after the final dose of fatty acid and the wet weight of their gastrocnemius, soleus, and tibialis anterior muscles measured. RESULTS: Olive oil, linseed oil, and EPA all failed to prevent decrease in food intake and loss of body weight. However, only EPA prevented loss of muscle mass and strength. CONCLUSION: EPA prevents CDDP-related loss of muscle mass and muscle but not CDDP-related loss of body weight.

Evaluation of the antinociceptive activities of several sodium channel blockers using veratrine test in mice.

An important role of voltage-gated sodium channels (VGSCs) in many different pain states has been established in animal models and humans wherein sodium channel blockers partially ameliorate pain. However, behavioral tests for screening analgesics that exhibit pharmacologic action by acting on VGSCs are rarely reported, and there are no studies on antinociception using veratrine as a nociceptive agent. The aim of the present study was to examine the amount of nociceptive behavior evoked by subcutaneous administration of veratrine into the hind paw and investigate whether veratrine can be used as a VGSC agonist to test the pharmacological properties of candidate analgesics via sodium channel blockade. We report for the first time that intraplantar injection of veratrine produced a reproducible nociceptive response in mice. Furthermore, several sodium channel blockers, namely carbamazepine, valproate, mexiletine, and the selective Nav1.7 inhibitor PF-04856264, but not flecainide or pilsicainide, reduced veratrine-induced nociception. In contrast, calcium channel blockers gabapentin and ethosuximide did not change veratrine-induced nociception. The veratrine test in mice might be a useful tool, at least in part, to evaluate the potential analgesic effect of sodium channel blockers.

A delta opioid receptor agonist, KNT-127, in the prelimbic medial prefrontal cortex attenuates glial glutamate transporter blocker-induced anxiety-like behavior in mice.

We previously reported that systemic administration of a delta opioid receptor (DOP) agonist, KNT-127, produced a potent anxiolytic-like effect in rats. Interestingly, DOPs are highly distributed in the prelimbic medial prefrontal cortex (PL-PFC). In the present study, we investigated the effect of KNT-127 co-perfusion in the PL-PFC on anxiety-like behavior in mice, induced by a glial glutamate transporter inhibitor, (3S)-3-[[3-[[4-(Trifluoromethyl)benzoyl]amino]phenyl]methoxy]-l-aspartic acid (TFB-TBOA). Extracellular glutamate levels were measured in male C57BL/6N mice by in vivo microdialysis high-performance liquid chromatography/electrochemical detection, with behavior simultaneously assessed in the open field test. As expected, extracellular glutamate levels were significantly increased, and anxiety-like behavior was induced after local perfusion of TFB-TBOA in the PL-PFC. Uniquely, co-perfusion of KNT-127 in the PL-PFC diminished anxiety-like behavior induced by TFB-TBOA without affecting extracellular glutamate levels. Further, the effect of KNT-127 on anxiety-like behavior was antagonized by a selective DOP antagonist, naltrindole, suggesting that KNT-127 acts via DOPs. These findings do not support our preconceived hypothesis that KNT-127 in PL-PFC produces an anxiolytic-like effect via suppression of glutamatergic transmission. Hence, further studies are necessary to understand the mechanisms of DOP agonist-induced anxiolytic-like effects in the PL-PFC.

Role of allopregnanolone biosynthesis in acute stress-induced anxiety-like behaviors in mice.

The neurosteroid allopregnanolone (3α, 5α-tetra-hydroprogesterone: ALLO) elicits anxiolytic, anticonvulsant, and hypnotic anesthetic effects in vivo similar to those induced by other positive allosteric modulators of the GABAA receptor. Endogenous ALLO has been shown to be rapidly elevated in the brain by acute stress paradigms, such as immobilization, in animal models. The present study was designed to ascertain the role of neurosteroid biosynthesis in the anxiety-like behavior induced by immobilization stress. Mice were exposed to an immobilization stressor for 2 h. After 24 h, the mice that had been immobilized did not behave significantly differently in the elevated plus maze (EPM) test and in the elevated open platform (EOP) test than the mice that had not been immobilized. In contrast, finasteride-pretreated immobilization stressed mice did behave significantly differently in the EPM and EOP tests. These findings suggest that ALLO biosynthesis contributes to stress resistance. Furthermore, the ALLO mimetic drug alfaxalone appeared to antagonize the effects of finasteride by significantly changing the behavior in the EPM test or in the EOP test in finasteride (10 mg kg-1 )-pretreated immobilized mice. In addition, alfaxalone, unlike diazepam, did not affect the muscle tone of the mice, as measured by the grip strength test. These results suggest that alfaxalone is a promising anxiolytic candidate lacking benzodiazepine-like muscle-relaxant effects.

Involvement of µ- and δ-opioid receptor function in the rewarding effect of (±)-pentazocine.

Most opioid receptor agonists have abuse potential, and the rewarding effects of opioids can be reduced in the presence of pain. While each of the enantiomers of pentazocine has a differential pharmacologic profile, (±)-pentazocine has been used clinically for the treatment of pain. However, little information is available regarding which components of pentazocine are associated with its rewarding effects, and whether the (±)-pentazocine-induced rewarding effects can be suppressed under pain. Therefore, the present study was performed to investigate the effects of pain on the acquisition of the rewarding effects of (±)-pentazocine, and to examine the mechanism of the rewarding effects of (±)-pentazocine using the conditioned place preference paradigm. (±)-Pentazocine and (-)-pentazocine, but not (+)-pentazocine, produced significant rewarding effects. Even though the rewarding effects induced by (±)-pentazocine were significantly suppressed under pain induced by formalin, accompanied by increase of preprodynorphin mRNA levels in the nucleus accumbens, a high dose of (±)-pentazocine produced significant rewarding effects under pain. In the normal condition, (±)-pentazocine-induced rewarding effects were blocked by a low dose of naloxone, whereas the rewarding effects induced by high doses of pentazocine under pain were suppressed by naltrindole (a δ-opioid receptor antagonist). Interestingly, (±)-pentazocine did not significantly affect dopamine levels in the nucleus accumbens. These findings suggest that the rewarding effects of (-)-pentazocine may contribute to the abuse potential of (±)-pentazocine through µ- as well as δ-opioid receptors, without robust activation of the mesolimbic dopaminergic system. We also found that neural adaptations can reduce the abuse potential of (±)-pentazocine under pain.

Differential substitution for the discriminative stimulus effects of 3,4-methylenedioxymethamphetamine and methylphenidate in rats.

Previous studies have demonstrated that methylphenidate, MDMA (3,4-methylenedioxymethamphetamine), and other psychostimulants exert stimulant-like subjective effects in humans. Furthermore, MDMA and methylphenidate substitute for the discriminative stimulus effects of psychostimulants, such as amphetamine and cocaine, in animals, which suggests that MDMA and methylphenidate may produce similar discriminative stimulus effects in rats. However, there is no evidence regarding the similarities between the discriminative stimulus effects of MDMA and methylphenidate. To explore this issue, cross-substitution, substitution, and combination tests were conducted in rats that had been trained to discriminate between MDMA (2.5 mg/kg) or methylphenidate (5.0 mg/kg) and saline. In the cross-substitution tests, MDMA and methylphenidate did not cross-substitute for each other. In the substitution test, methamphetamine substituted for the discriminative stimulus effects of methylphenidate, but not for those of MDMA. Furthermore, ephedrine and bupropion, which activate dopaminergic and noradrenergic systems, substituted for the discriminative stimulus effects of methylphenidate. On the other hand, serotonin (5-HT) receptor agonists 5-HT1A and 5-HT2 fully substituted for the discriminative stimulus effects of MDMA. These results suggest that activation of the noradrenergic and dopaminergic systems is important for the discriminative stimulus effects of methylphenidate, whereas activation of the serotonergic system is crucial for the discriminative stimulus effects of MDMA. Even though MDMA, like psychostimulants, exerts stimulant-like effects, our findings clearly indicate that the discriminative stimulus effects of MDMA are distinctly different from those of other psychostimulants in rats.

Inhibitory effects of SA4503 on the rewarding effects of abused drugs.

Previous findings have shown that sigma-1 receptors (Sig-1Rs) are upregulated by the self-administration of methamphetamine, whereas Sig-1R antisense can attenuate the behavioral effects of psychostimulants in rodents. Sig-1R is an endoplasmic reticulum chaperone protein. However, the effects of Sig-1R agonist on the rewarding effects of abused drugs are not fully understood. Therefore, we examined the effects of selective Sig-1R agonists, such as SA4503 and (+)-pentazocine, on the rewarding effects of abused drugs such as methamphetamine, cocaine and morphine in rats, as measured by the conditioned place preference. Methamphetamine, cocaine and morphine induced a significant place preference. SA4503, but not (+)-pentazocine, significantly attenuated the abused drug-induced place preference. We recently showed that (+)-pentazocine exerts U50,488H-like discriminative stimulus effects, which are related to its psychotomimetic/aversive effects. However, SA4503 did not generalize to the discriminative stimulus effects of U50,488H. These results suggest that SA4503 inhibits the rewarding effects of abused drugs, and that psychotomimetic/aversive effects may not play a role in the attenuating effects of SA4503 on the rewarding effects of abused drugs.

Further evidence for blood-to-brain influx of unconjugated bile acids by passive diffusion: Determination of their brain-to-serum concentration ratios in rats by LC/MS/MS.

Bile acids (BAs) reside in the brain and are probably involved in some neurological disorders. The view that most of unconjugated BAs in the brain are derived across the blood-brain barrier from the periphery by passive diffusion depending on their hydrophobicity is currently dominant, but some studies have made conflicting claims. In this study, the correlation analysis between the rat brain and serum levels of unconjugated BAs with a wider range of hydrophobicity was conducted to obtain further evidence about the blood-to-brain influx of unconjugated BAs by passive diffusion. We first developed the precise, accurate and matrix effect-free LC/ESI-MS/MS methods for quantifying eight major unconjugated BAs in the rat brain and serum. Derivatization was employed for increasing the assay sensitivity and specificity. The analysis using these methods reproduced the strong positive correlations between the brain and serum levels, and significant higher concentrations in the serum than in the brain for all the unconjugated BAs. The BA with the higher logPow (hydrophobicity) had the higher brain-to-serum concentration ratio (mono- > di- > trihydroxy BAs). Furthermore, the hydrophobicity was considered as the stronger factor for the blood-to-brain influx of the BAs than the serum protein binding ratio. Thus, this study provided further evidence supporting that passive diffusion is the major mechanism for the blood-to-brain influx of the unconjugated BAs.

Mechanisms that underlie µ-opioid receptor agonist-induced constipation: differential involvement of µ-opioid receptor sites and responsible regions.

Reducing the side effects of pain treatment is one of the most important strategies for improving the quality of life of cancer patients. However, little is known about the mechanisms that underlie these side effects, especially constipation induced by opioid receptor agonists; i.e., do they involve naloxonazine-sensitive versus -insensitive sites or central-versus-peripheral µ-opioid receptors? The present study was designed to investigate the mechanisms of µ-opioid receptor agonist-induced constipation (i.e., the inhibition of gastrointestinal transit and colonic expulsion) that are antagonized by the peripherally restricted opioid receptor antagonist naloxone methiodide and naloxonazine in mice. Naloxonazine attenuated the fentanyl-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine or oxycodone. Naloxone methiodide suppressed the oxycodone-induced inhibition of gastrointestinal transit more potently than the inhibition induced by morphine, indicating that µ-opioid receptor agonists induce the inhibition of gastrointestinal transit through different mechanisms. Furthermore, we found that the route of administration (intracerebroventricular, intrathecally, and/or intraperitoneally) of naloxone methiodide differentially influenced the suppressive effect on the inhibition of colorectal transit induced by morphine, oxycodone, and fentanyl. These results suggest that morphine, oxycodone, and fentanyl induce constipation through different mechanisms (naloxonazine-sensitive versus naloxonazine-insensitive sites and central versus peripheral opioid receptors), and these findings may help us to understand the characteristics of the constipation induced by each µ-opioid receptor agonist and improve the quality of life by reducing constipation in patients being treated for pain.

Involvement of dopamine D2 receptor signal transduction in the discriminative stimulus effects of the κ-opioid receptor agonist U-50,488H in rats.

We have reported previously that the inhibition of both dopaminergic and psychotomimetic/hallucinogenic components plays a role in the discriminative stimulus effects of U-50,488H. However, the mechanisms that underlie the discriminative stimulus effects of U-50,488H, and especially the component that plays a significant role, have not yet been clarified. The present study was designed to further investigate the mechanism(s) of the discriminative stimulus effects of the κ-opioid receptor agonist U-50,488H in rats that had been trained to discriminate between 3.0 mg/kg U-50,488H and saline. The dopamine D2 receptor antagonist sulpiride, but not the D1 receptor antagonist SCH23390, generalized to the discriminative stimulus effects of U-50,488H. The mood-stabilizing agents lithium chloride and valproic acid, which have attenuating effects on the Akt/GSK3 pathway, also partially generalized to the discriminative stimulus effects of U-50,488H. In contrast, the 5-HT-related compound racemic 3,4-methylenedioxymethamphetamine, the cannabinoid receptor agonist WIN55,212-2, and the µ-opioid receptor agonist morphine failed to generalize to the discriminative stimulus effects of U-50,488H. These results suggest that the inhibition of the dopaminergic activity mediated by the postsynaptic D2 receptor, followed by suppression of the Akt/GSK3 pathway may be critical for the induction of the discriminative stimulus effects induced by U-50,488H.

Involvement of serotonin receptor mechanisms in the discriminative stimulus effects of ketamine in rats.

We have demonstrated previously that the ketamine-induced discriminative stimulus effect is likely to reflect the phencyclidine-like psychotomimetic effects. Therefore, the present study was designed to investigate the effects of the antipsychotics and 5-HT2 receptor antagonist on the discriminative stimulus effects of ketamine in rats. While sulpiride did not attenuate the discriminative stimulus effects of ketamine, both clozapine and ketanserin attenuated those of ketamine, suggesting that the discriminative stimulus effects of ketamine are mediated by multiple receptors, especially the 5-HT2 receptor, but not the D2 receptor. Furthermore, our findings imply that atypical antipsychotics could be useful for the treatment of psychotomimetic effects induced by ketamine.

Preparation and Evaluation of Hydrogel Film Containing Tramadol for Reduction of Peripheral Neuropathic Pain.

To develop and assess new dosage forms for the alternative to existing oral medication for peripheral neuropathy, a hydrogel film in the skin patch formation containing tramadol hydrochloride (TRA), a water-soluble drug used as an analgesic, was prepared and evaluated. A hydrogel film composed of 20%(w/w) hydroxypropyl methylcellulose (HPMC) irradiated with electron beams had high transparency and elasticity similar to commercially available wound dressings and soft tissues, suggesting that it is a suitable substrate for TRA. The inclusion of TRA was enabled by immersing the HPMC hydrogel film in TRA aqueous solution. The release and skin permeation of TRA from TRA-containing hydrogel films differed depending on the electron beam dose. Moreover, the analgesic effects in mice were confirmed in a dose-dependent manner. This study demonstrated the usefulness of a hydrogel film containing TRA as a new dosage form alternative to the existing oral medication for peripheral neuropathy.