Blockade of morphine-induced hindlimb myoclonic seizures in mice by ketamine

Low-Dose Ketamine Pretreatment Reduces the Incidence and Severity of Myoclonus Induced by Etomidate: A Randomized, Double-Blinded, Controlled Clinical Trial

Myoclonic movement induced by etomidate is a common but undesirable problem during general anesthesia induction. To investigate the influence of pretreatment with low-dose ketamine on the incidence and severity of myoclonus induced by etomidate, 104 patients were randomized allocated to 1 of 2 equally sized groups (n = 52) to receive either intravenous low-dose ketamine 0.5 mg/kg (group K) or an equal volume of normal saline (group S) 1 minute before induction of anesthesia with 0.3-mg/kg etomidate. The incidence and severity of myoclonus were assessed for 2 minutes after administration of etomidate. Here, we found that the incidence and intensity of myoclonus were both significantly reduced in low-dose ketamine-treated group compared with saline-treated group. The incidence of adverse effects was low and similar between groups. These results demonstrate that intravenous infusion of low-dose ketamine 0.5 mg/kg 1 minute prior to etomidate administration is effective in relieving etomidate-induced myoclonic movements during general anesthesia induction.

The NMDA receptor complex as a therapeutic target in epilepsy: a review

A substantial amount of research has shown that N-methyl-D-aspartate receptors (NMDARs) may play a key role in the pathophysiology of several neurological diseases, including epilepsy. Animal models of epilepsy and clinical studies demonstrate that NMDAR activity and expression can be altered in association with epilepsy and particularly in some specific seizure types. NMDAR antagonists have been shown to have antiepileptic effects in both clinical and preclinical studies. There is some evidence that conventional antiepileptic drugs may also affect NMDAR function. In this review, we describe the evidence for the involvement of NMDARs in the pathophysiology of epilepsy and provide an overview of NMDAR antagonists that have been investigated in clinical trials and animal models of epilepsy.

Myoclonus associated with high-dose parenteral methadone

Methadone is generally believed to be devoid of neuroexcitatory properties, and its use is increasing. This paper reports two cases of myoclonus with high-dose parenteral methadone in patients with cancer under hospice care. This side effect may be dose related and/or due to the parenteral route of administration. Reduction of the dose and change of route was sufficient to eliminate the myoclonus while maintaining an adequate pain control. Possible mechanisms for methadone causing myoclonus include a redistribution of receptor saturation in the N-methyl-D-aspartate (NMDA) and delta receptors. Ketamine may be an option for patients with intractable pain who develop methadone-induced myoclonus.

Drug-induced myoclonus: frequency, mechanisms and management

Myoclonus is a sudden, abrupt, brief, 'shock-like' involuntary movement caused by muscular contractions ('positive myoclonus') or a sudden brief lapse of muscle contraction in active postural muscles ('negative myoclonus' or 'asterixis'). Various disorders can cause myoclonus including neurodegenerative and systemic metabolic disorders and CNS infections. In addition, myoclonus has been described as an adverse effect of some drugs. Level II evidence is available to indicate that levodopa, cyclic antidepressants and bismuth salts can cause myoclonus, while there is less robust evidence to associate numerous other drugs with the induction of myoclonus. The pharmacological mechanisms responsible for this adverse effect are not well established, although increased serotonergic transmission may be involved in the induction of myoclonus by several drugs. Drug-induced myoclonus usually resolves after withdrawal of the offending drug, but in some cases specific treatments are needed.

The role of alpha2-adrenoceptors in the modulatory effects of morphine on seizure susceptibility in mice

PURPOSE

To evaluate the effect of the alpha2-adrenoceptor agonist clonidine and the antagonist yohimbine on the dual modulation of seizure susceptibility induced by morphine and the anticonvulsant effect of acute stress in mice.

METHODS

The thresholds for the clonic seizures induced after intravenous administration of pentylenetetrazole (PTZ) or bicuculline were assessed in mice weighing 23-30 g. Acute stress was induced by restraining mice for 2 h in a restrainer.

RESULTS

Morphine at lower doses (0.5, 1, and 3 mg/kg) increased and, at higher doses (15, 30, and 75 mg/kg), decreased the seizure threshold. Pretreatment with clonidine (0.001-0.1 mg/kg) inhibited the anticonvulsant effect of morphine, while potentiating its proconvulsant effect. Conversely, yohimbine (0.5-2 mg/kg) potentiated the anticonvulsant effect of morphine but inhibited its proconvulsant effects. Acute stress induced an anticonvulsant effect that was reversible by naloxone (1 mg/kg) or clonidine (0.05-0.1 mg/kg) or a combination of their lower doses (0.3 and 0.01 mg/kg, respectively), while being potentiated by yohimbine (1 mg/kg).

CONCLUSIONS

alpha2-Adrenoceptors play a dual role in the anticonvulsant effects of morphine. The activation of these receptors also can decrease the anticonvulsant effect of acute restraint stress in mice.

The role of nitric oxide in anticonvulsant and proconvulsant effects of morphine in mice

Acute subcutaneous administration of lower doses of morphine (0.5, 1 and 3 mg/kg) increase the threshold of seizures induced by pentylenetetrazole (PTZ) in mice, whereas higher doses of morphine (15, 30 and 60 mg/kg) have proconvulsant effects. The effect of systemic administration of nitric oxide synthase (NOS) inhibitors N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-nitro-L-arginine (L-NNA) and nitric oxide synthase (NOS) L-arginine on biphasic effect of morphine was investigated. Acute administration of both L-NAME (1, 3 and 10 mg/kg) and L-NNA (1 and 10 mg/kg) as well as chronic pretreatment with L-NAME (1 and 10 mg/kg, 4 days) dose-dependently inhibited both the anticonvulsant and proconvulsant effects of morphine (1 and 30 mg/kg, respectively). The inhibition was complete for anticonvulsant effect while partial for proconvulsant effect. L-arginine at doses that did not affect seizure threshold per se (acute, 30 and 60 mg/kg; chronic, 60 mg/kg) potentiated both anticonvulsant and proconvulsant properties of less potent doses of morphine (0.5 and 15 mg/kg, respectively). The L-arginine induced potentiation of both phases of morphine effect was blocked by L-NAME (0.5-30 mg/kg). Moreover, low and per se non-effective doses of naloxone (0.1 mg/kg) and L-NAME (0.3, 0.5 or 1 mg/kg) showed additive effects in inhibiting both phases of morphine effects. These results support the involvement of L-arginine/nitric oxide pathway in the modulation of seizure threshold by morphine.

Acetylcodeine, an impurity of illicitly manufactured heroin, elicits convulsions, antinociception, and locomotor stimulation in mice

Acetylcodeine is one of the major impurities present in illicitly manufactured heroin (diacetylmorphine). Data on its pharmacology and toxicology are limited and its ability to alter the toxic effects of diacetylmorphine is not known. The first objective of the present study was to compare the acute pharmacological and toxicological effects of acetylcodeine to those of codeine and diacetylmorphine in mice by assessing nociception in the tail-flick test, locomotor stimulation, and convulsive behavior. The second goal of this study was to determine whether acetylcodeine would alter the convulsant effects of diacetylmorphine. The antinociceptive potencies of acetylcodeine and codeine were similar, as reflected by their ED50 (95% confidence limits) values of 35 (29-44) and 51 (40-65) micromol/kg, respectively. Acetylcodeine was somewhat less potent than codeine in stimulating locomotor behavior, with ED50 values of 28 (22-37) and 12 (6-24) micromol/kg, respectively. Diacetylmorphine was considerably more potent than the other two drugs, producing antinociception and locomotor stimulation at ED50 values of 2.4 (1.4-4.1) and 0.65 (0.36-1.2) micromol/kg, respectively. On the other hand, the convulsant effects of acetylcodeine (ED50=138 (121-157) micromol/kg) and diacetylmorphine (ED50=115 (81-163) micromol/kg) were similar in potency and both were more potent than codeine (ED50=231 (188-283) micromol/kg). Finally, a subthreshold dose of acetylcodeine (72 micromol/kg) decreased the convulsant ED50 dose of diacetylmorphine to 40 (32-49). These findings suggest that the convulsant effects of acetylcodeine are more potent than predicted by its effects on locomotor activity and antinociception. The observation that acetylcodeine potentiated the convulsant effects of diacetylmorphine suggests a mechanism for some of the heroin-related deaths reported in human addicts.

Prenatal morphine exposure alters N-methyl-D-aspartate- and kainate-induced seizures in adult male rats

The purpose of the present study was to investigate whether prenatal exposure to morphine has effects on excitatory amino acid-induced seizures. Adult male rats, exposed on embryonic days 11-18 to saline or morphine, were injected with N-methyl-D-aspartate (NMDA) (150, 175, 200, 225, and 250 mg/kg) or kainic acid (KA) (15 or 20 mg/kg) in adulthood to assess the occurrence and latency to onset of stereotypy and seizures. The latency to onset of stereotypy was significantly increased after 175 mg/kg, and decreased after 200 mg/kg of NMDA in morphine-exposed animals. The lowest dose of NMDA (150 mg/kg) induced seizures in prenatally saline-treated control male rats but not in the morphine-exposed male rats. In the KA-injected group, prenatally morphine-exposed males had shorter latency to onset of wet-dog shakes, but there were no effects on the latency to onset of clonic seizures. The data suggest that prenatal morphine exposure has long-term effects on seizure susceptibility and the onset of stereotypy in the excitatory amino acid-induced seizure models.

NMDA and opioid receptors: their interactions in antinociception, tolerance and neuroplasticity

Over the last several years, significant progress has been made in our understanding of interactions between the N-methyl-D-aspartate (NMDA) and opioid receptors. Such interactions have been demonstrated at two distinct sites: (1) modulation of NMDA receptor-mediated electrophysiological events by opioids; and (2) intracellular events involving interactions between NMDA and opioid receptors. Furthermore, a considerable number of studies have shown the involvement of such interactions in neural mechanisms of nociceptive transmission, antinociception in acute and chronic pain states, opioid tolerance/dependence, and neuroplasticity. Importantly, emerging evidence indicates that activation of NMDA receptors may differentially modulate functions mediated by distinct opioid receptor subtypes, namely mu, delta, and kappa receptors. These studies have greatly enriched our knowledge regarding both NMDA and opioid receptor systems and have shed light on neurobiology of both acute and chronic pain. The advancement of such knowledge also promotes new strategies for better clinical management of pain patients.

Endogenous opiates: 1997

This paper is the twentieth installment of our annual review of research concerning the opiate system. It summarizes papers published during 1997 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.