mr-2266 and Seizures

The present study was designed to investigate the pro- or anticonvulsant effect of tramadol using maximal electroshock (MES) test. An attempt was also made to determine the possible opioid receptor mechanism involved. MES seizures were induced through transauricular electrodes (60 mA, 0.2s) and the seizure severity was assessed by the duration of tonic hindlimb extensor phase. Intraperitoneal (i.p.) administration of tramadol resulted in a dose-dependent anticonvulsant action; the ED50 for the effect was 33 mg/kg. The anti-MES effect of tramadol was antagonized by the low doses (0.05 and 0.1 mg/kg, s.c.) of MR 2266, a selective kappa receptor antagonist and also by the high doses (1.0 and 5.0 mg/kg, i.p.) but not the low doses (0.1 and 0.25 mg/kg) of naloxone. The results suggest that the anti-MES effect of tramadol is mediated by kappa receptors, since MR 2266 and naloxone (in high doses) are known to block these receptors.

Topics: Animals; Anticonvulsants; Benzomorphans; Drug Interactions; Electroshock; Female; Male; Mice; Naloxone; Narcotic Antagonists; Receptors, Opioid, kappa; Seizures; Tramadol

Intracerebroventricular (ICV) administration of kappa-agonists (PD 117302, U-50488H and U-69593) induced convulsions in a dose-related manner in mice. The dose at which 50% of animals convulsed (CD50) was in nmol ranges for all opioids. Among the opioids used, PD 117302 was the most potent convulsant. ICV administration of either vehicle alone or U-53445E, a non-kappa-opioid (+) enantiomer of U-50488H did not induce convulsions. The convulsive response of kappa-agonists was differentially susceptible for antagonism by naloxone and/or MR 2266. Collectively, these findings support the view that convulsions induced by kappa-agonists in mice involve stereospecific opioid receptor mechanisms. Furthermore, the convulsant effect of kappa-agonists could not be modified by pretreatment with MK-801, ketamine, muscimol or baclofen. It is concluded that kappa-opioid but not NMDA or GABA receptor mechanisms are involved in convulsions induced by kappa-agonists. These results are the first experimental evidence implicating stereospecific kappa-receptor mechanisms in opioid-induced convulsions in mice.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Analgesics; Animals; Baclofen; Benzeneacetamides; Benzomorphans; Cerebral Ventricles; Convulsants; Dizocilpine Maleate; Injections, Intraventricular; Ketamine; Mice; Mice, Inbred Strains; Muscimol; Naloxone; Narcotic Antagonists; Narcotics; Pyrroles; Pyrrolidines; Receptors, Opioid; Receptors, Opioid, kappa; Seizures; Stereoisomerism; Thiophenes

The effects of mu-agonists (morphine, fentanyl) and kappa-agonists (U-50,488, U-69,593, bremazocine, nalbuphine, tifluadom) on the electroconvulsive threshold were studied in mice. The threshold could be significantly elevated by all drugs tested in a dose range that was in the order of magnitude of the antinociceptive ED50. Mice tolerant to the antielectroshock effect of morphine still reacted to U-69,593. The antagonism of the anticonvulsant effect by the mu-antagonist naloxone and the kappa-antagonist MR 2266 was receptor-specific only with fentanyl and U-50,488. The other opioid agonists were either antagonized by both drugs (morphine, U-69,593, bremazocine, nalbuphine) or even by the opposite antagonist (tifluadom). A synergistic effect of mu- and kappa-stimulation is assumed for the mediation of the antielectroshock effect of opioid drugs, but drugs with high affinity and intrinsic activity at one receptor type (fentanyl, U-50,488) are obviously able to bring about their antielectroshock effect through the one respective opioid binding site.

Topics: 3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; Animals; Benzeneacetamides; Benzodiazepines; Benzomorphans; Electroshock; Fentanyl; Male; Mice; Morphinans; Morphine; Nalbuphine; Naloxone; Pyrrolidines; Receptors, Opioid; Receptors, Opioid, kappa; Receptors, Opioid, mu; Seizures

Pentylenetetrazol (PTZ, 45 mg/kg, ip) impaired retention of a one-trial step-through inhibitory avoidance task when injected into male Swiss mice 10 min after training, as indicated by retention performance 48 h later. The amnestic effect of PTZ was prevented by naltrexone (0.01 or 0.10 mg/kg, ip) administered after training, but prior to PTZ-treatment. On the contrary, neither naltrexone methyl bromide (0.01, 0.10, or 10.0 mg/kg, ip), a quaternarium analog of naltrexone, nor MR2266 (0.01 or 0.10 mg/kg, ip), a putative kappa opiate receptor antagonist, modified the behavioral effects of PTZ. On the other hand, the body seizures produced by PTZ were unaffected by any of the three opiate receptor antagonists that were given before the convulsant. Taken together, these results suggest that the effects of PTZ on retention are mediated, at least in part, by opioid peptides of central origin, and rules out a possible participation of opioid peptides derived from prodynorphin-precursor molecule. Administration of beta-endorphin (0.01 or 0.10 microgram/kg, ip) 10 min prior to testing attenuate the retrograde amnesia caused by PTZ. The effect of beta-endorphin was prevented by the simultaneous administration of naltrexone (0.10 mg/kg, ip) prior to testing. Naltrexone has no effect of its own upon retrieval. These results suggest that the impairment of retention induced by PTZ is probably due, at least in part, to a release of opioid peptides in the brain during the post-training period. PTZ given after training do not affect consolidation or memory storage, as mice thus treated may retrieve the learned information when they are submitted to an appropriate neurohumoral and/or hormonal state in the test session, that is, beta-endorphin injection. Therefore, the action of PTZ would be primarily at the level of the mechanism that make stored information available for late retrieval.

Topics: Animals; Avoidance Learning; Benzomorphans; beta-Endorphin; Brain; Dose-Response Relationship, Drug; Male; Memory; Mice; Mice, Inbred Strains; Naltrexone; Narcotic Antagonists; Pentylenetetrazole; Quaternary Ammonium Compounds; Receptors, Opioid; Retention, Psychology; Seizures