Kazumi Yoshizawa

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.

A putative sigma1 receptor antagonist NE‐100 attenuates the discriminative stimulus effects of ketamine in rats

Ketamine, one of the dissociative anaesthetic agents, has been shown to produce psychotomimetic effects. It has been well documented that activation of sigma receptors is responsible for the pathogenesis of some psychiatric disorders. In the present study, the effects of NE‐100, a putative sigma 1 receptor antagonist, was investigated in rats trained to discriminate between ketamine (5 mg/kg, i.p.) from saline under a fixed‐ratio 10 food‐reinforced procedure. Here we report for the first time that NE‐100 (1 mg/kg) produced a shift to the right in the dose–response curve for ketamines discriminative stimulus effects. These results suggest that the sigma 1 receptor is, at least in part, involved in the discriminative stimulus effects of ketamine.

Usefulness of olanzapine as an adjunct to opioid treatment and for the treatment of neuropathic pain.

Background: The use of opioids for pain management is often associated with nausea and vomiting. Although conventional antipsychotics are often used to counter emesis, they can be associated with extrapyramidal symptoms. However, chronic pain can induce sleep disturbance. The authors investigated the effects of the atypical antipsychotic olanzapine on morphine-induced emesis and the sleep dysregulation associated with chronic pain. Methods: A receptor binding assay was performed using mouse whole brain tissue. The emetic response in ferrets was evaluated by counting retching and vomiting behaviors. Catalepsy in mice was evaluated by placing both of their forepaws over a horizontal bar. Released dopamine was measured by an in vivo microdialysis study. Sleep disturbance in mice in a neuropathic pain-like state was assayed by electroencephalogram and electromyogram recordings. Results: Olanzapine showed high affinity for muscarinic M1 receptor in brain tissue. Olanzapine decreased morphine-induced nausea and vomiting in a dose-dependent manner. However, olanzapine at a dose that had an antiemetic effect (0.03 mg/kg) did not induce catalepsy or hyperglycemia. In addition, olanzapine at this dose had no effect on the morphine-induced release of dopamine or inhibition of gastrointestinal transit. Finally, olanzapine inhibited thermal hyperalgesia and completely alleviated the sleep disturbance induced by sciatic nerve ligation. Conclusion: These findings suggest that olanzapine may be useful for the treatment of morphine-induced emesis and as an adjunct for the treatment of neuropathic pain associated with sleep disturbance.

Role of the NMDA receptor subunit in the expression of the discriminative stimulus effect induced by ketamine

Ketamine, which is a non-competitive NMDA receptor antagonist, has been used as a dissociative anesthetic agent. However, chronic use of ketamine produces psychotomimetic effects, such as nightmares, hallucination and delusion. Therefore, the present study was designed to ascertain the role of the NMDA receptor and sigma receptor in the discriminative stimulus effect induced by ketamine. Fischer 344 rats were trained to discriminate between ketamine (5 mg/kg, i.p.) and saline under a fixed-ratio 10 food-reinforced procedure. Non-competitive antagonists for both NR2A- and NR2B-containing NMDA receptors, such as phencyclidine (0.1--1 mg/kg, i.p.) and dizocilpine (3--30 microg/kg, i.p.), and the NR2A-containing NMDA receptor-preferred antagonist dextromethorphan (3--56 mg/kg, i.p.) fully substituted for the ketamine cue in a dose-dependent manner. By contrast, the NR2B-containing NMDA receptor antagonist ifenprodil (5--20 mg/kg, i.p.) exhibited no generalization. Additionally, the competitive NMDA antagonist 3-[(+/-)-2-carboxypiperazine-4-yl] propyl-1-phosphonic acid ((+/-)-CPP; 0.3--5.6 mg/kg, i.p.) and a sigma receptor ligand DTG (0.3--3 mg/kg, s.c.) displayed no generalization to the ketamine cue. These results suggest that NR1/NR2A subunit containing NMDA antagonism may be critical for the production of the ketamine cue.

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.

Sigma‐1 receptor function is critical for both the discriminative stimulus and aversive effects of the kappa‐opioid receptor agonist U‐50488H

The present study was undertaken to identify possible similarities between the effects of kappa‐opioid receptor agonist, N‐methyl‐D‐aspartate‐receptor antagonist, and sigma receptor agonist on the discriminative stimulus effects of U‐50488H, and the possible involvement of sigma receptors in the discriminative stimulus and aversive effects of U‐50488H. The kappa‐opioid receptor agonist U‐50488H produced significant place aversion as measured by the conditioned place preference procedure, and this effect was completely abolished by treatment with the putative sigma‐1 receptor antagonist NE‐100. In addition, phencyclidine (+)‐SKF‐10047 and (+)‐pentazocine, which are sigma receptor agonists, generalized to the discriminative stimulus effects of U‐50488H in rats that had been trained to discriminate between U‐50488H (3.0 mg/kg) and saline. Furthermore, NE‐100 significantly attenuated the discriminative stimulus effects of U‐50488H and the U‐50488H‐like discriminative stimulus effects of phencyclidine. These results suggest that the sigma‐1 receptor is responsible for both the discriminative stimulus effects and aversive effects of U‐50488H.

Activation of extracellular signal-regulated kinase is critical for the discriminative stimulus effects induced by U-50,488H

We previously demonstrated that the discriminative stimulus effects of the κ‐opioid receptor agonist U‐50,488H were associated with its aversive effects in rats. However, its molecular signaling mechanisms are not fully understood. The aim of this study was to investigate the intracellular signaling that plays a role in mediating the discriminative stimulus effect induced by U‐50,488H. To better understand the involvement of molecular signaling mechanisms in the discriminative stimulus effects of U‐50,488H, rats were subjected to a drug discrimination paradigm, and levels of immunoreactivity and mRNA expression were determined in these rats. Although U‐50,488H‐trained rats did not show changes in the mRNA expression of typical dopamine (DA) receptors, NMDA receptor subunits, or transcriptional activators, there were remarkable changes in the levels of immunoreactivity of several phosphorylated protein kinases. The levels of immunoreactivity of phosphorylated p38 MAPK and phosphorylated calcium/calmodulin‐dependent protein kinase II (CaMKII) were significantly increased in the nucleus accumbens and amygdala in lever‐press, yoked and discrimination groups compared to a naive group. Furthermore, the level of phosphorylated cAMP response element‐binding protein was also increased in both the discrimination and yoked groups. In contrast, the immunoreactivity of phosphorylated extracellular signaling‐regulated kinase (ERK 1/2) was specifically increased in the discrimination group. These results suggest that the ERK signaling pathway in the nucleus accumbens and amygdala may be critical for the expression of the discriminative stimulus effects of U‐50,488H. Synapse, 2011.

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.

Role of dopamine D2 and D3 receptors in mediating the U-50,488H discriminative cue: comparison with methamphetamine and cocaine

Substitutions of the dopamine D2 or D3 receptor agonists for the discriminative stimulus effect induced by U‐50,488H, methamphetamine (METH) and cocaine in rats were examined. The D2 receptor agonist R‐propylnorapomorphine [(−)‐NPA] failed to substitute for U‐50,488H cue, while the D3 receptor‐preferred agonist (+/–)‐7‐hydroxy‐dipropylaminotetralin hydrobromide (7‐OH‐DPAT) produced dose‐related increases in drug‐appropriate responding up to 0.03 mg/kg, which fully substituted. At doses greater than 0.03 mg/kg of 7‐OH‐DPAT, there was a dose‐dependent decrease in the percentage of responses on the U‐50,488H‐appropriate lever. Furthermore (−)‐NPA and 7‐OH‐DPAT at high doses substituted for the discriminative stimulus effect induced by both METH and cocaine, indicating that 7‐OH‐DPAT at high doses may interact with D2 receptors. These results suggest that the stimulation of D2 receptor may be critical for the production of the discriminative stimulus effect induced by METH and cocaine, whereas the stimulation of D3 receptor may contribute to the production of the U‐50,488H cue.

Unconjugated bile acids in rat brain: Analytical method based on LC/ESI-MS/MS with chemical derivatization and estimation of their origin by comparison to serum levels

Although some studies have revealed the implication of bile acids (BAs) and neurological diseases, the levels and origin of the BAs in the brain are not fully understood. In this study, we first developed and validated a sensitive and specific method for the determination of three unconjugated BAs [cholic acid (CA), chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA)] in the rat brain by liquid chromatography/electrospray ionization-tandem mass spectrometry combined with chemical derivatization. The measured brain concentrations (mean±standard deviation, n=10) of normal rats were 58.7±48.8, 14.2±11.7 and 13.2±8.7ng/g tissue for CA, CDCA and DCA, respectively. For their origin, we developed the hypothesis that they might be mostly derived from the periphery. To test this hypothesis, the brain BA levels were compared with the serum levels. The brain levels had high correlations with the serum levels, and were always lower than the serum levels for the three unconjugated BAs. Furthermore, the higher brain-to-serum concentration ratios were found for the BAs with higher logD values (higher lipophilicity). Moreover, the brains of the rats intraperitoneally administered with deuterium-labeled CA and CDCA were also analyzed; the deuterium-labeled BAs were detected in the brain of the rats administered with these compounds. Based on all the results, we concluded that the BAs found in the brain are mostly derived from the periphery and the major mechanism for the transportation of the unconjugated BAs to the brain is by passive diffusion.