Intracerebroventricular administration of kappa-agonists induces convulsions in mice

Salvinorin A Does Not Affect Seizure Threshold in Mice

The κ-opioid receptor has recently gained attention as a new molecular target in the treatment of many psychiatric and neurological disorders including epilepsy. Salvinorin A is a potent plant-derived hallucinogen that acts as a highly selective κ-opioid receptor agonist. It has unique structure and pharmacological properties, but its influence on seizure susceptibility has not been studied so far. Therefore, the aim of the present study was to investigate the effect of salvinorin A on seizure thresholds in three acute seizure tests in mice. We also examined its effect on muscular strength and motor coordination. The obtained results showed that salvinorin A (0.1-10 mg/kg, i.p.) did not significantly affect the thresholds for the first myoclonic twitch, generalized clonic seizure, or forelimb tonus in the intravenous pentylenetetrazole seizure threshold test in mice. Likewise, it failed to affect the thresholds for tonic hindlimb extension and psychomotor seizures in the maximal electroshock- and 6 Hz-induced seizure threshold tests, respectively. Moreover, no changes in motor coordination (assessed in the chimney test) or muscular strength (assessed in the grip-strength test) were observed. This is a preliminary report only, and further studies are warranted to better characterize the effects of salvinorin A on seizure and epilepsy.

Recurrent seizure activity after epidural morphine in a post-partum woman

PURPOSE

We report on a primiparous woman who suffered recurrent seizure activity after repeated small doses of epidural morphine to highlight the neuroexcitation potential of neuraxial opioids in the epileptic patient.

CLINICAL FEATURES

Seizure activities as a complication of opioid administration have been reported in laboratory animals and humans. We report the case of a 30-yr-old primiparous woman with a history of epilepsy under carbamazepine treatment, who had epidural anesthesia for elective Cesarean section at 38 weeks gestation. Postoperatively, 1.5 mg of morphine were administered epidurally for pain control. Three hours later the patient suffered from clonic movements of the right arm without loss of consciousness. One day later, she again received 1 mg of epidural morphine twice at a 12-hr interval and similar seizure episodes recurred eight hours after each dose. A relation between the administration of morphine and seizure activity was suspected and the use of opioids for pain control was stopped. The patient was discharged on the fifth postoperative day and, more than one year after the last episode, she remains free of any seizure activity.

CONCLUSION

Our report indicates that even a remote history of epilepsy carries a pro-convulsant potential in the peripartum period, even following the administration of small doses of epidural morphine.

Nonopioid actions of U50,488 enantiomers contribute to their peripheral cutaneous antinociceptive effects

The ability of arylacetamide kappa-opioid receptor agonists (kappa-ORAs) to block sodium channels by a nonopioid mechanism has been previously documented. The present experiments were undertaken to test whether two enantiomers of the arylacetamide kappa-ORA (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488), (+)-(1R,2R)-U50,488 and (-)-(1S,2S)-U50,488, are antinociceptive in the formalin model by a peripheral, nonopioid receptor-mediated mechanism. Although both enantiomers have been previously shown to block sodium channels with comparable potencies, only (-)-(1S,2S)-U50,488 has activity at the kappa-opioid receptor (KOR). In the formalin test, intrapaw administration of U50,488 enantiomers as well as lidocaine exhibited significant dose-related attenuation of formalin-induced flinching behavior. The rank order of potency of the drugs tested was (-)-(1S,2S)-U50,488 > (+)-(1R,2R)-U50,488 > lidocaine. The antinociception produced by lower doses of (-)-(1S,2S)-U50,488 was blocked by intrapaw nor-binaltorphimine as well as by antisense knockdown of the KOR. Such pretreatments, however, did not block the antinociception produced by (+)-(1R,2R) U50,488, lidocaine, or higher doses of (-)-(1S,2S)-U50,488. These data suggest that the sodium channel blocking effects of U50,488 and similar kappa-ORAs can contribute to their peripheral antinociceptive actions.

Sodium channel blocking actions of the kappa-opioid receptor agonist U50,488 contribute to its visceral antinociceptive effects

The goal of the present study was to determine whether the kappa-opioid receptor agonist (ORA) U50,488 attenuates behavioral and primary afferent nerve responses to noxious colorectal distension (CRD) by sodium channel blockade. We tested the analgesic kappa-ORA (+/-)-trans U50,488, its enantiomers (-)-trans (1S,2S)-U50,488 and non kappa-ORA (+)-trans (1R,2R)-U50,488, and/or its diastereomer (-)-cis (1S,2R)-U50,488 for their ability to attenuate visceromotor and pelvic nerve afferent fiber responses to noxious CRD in vivo and voltage-activated sodium current in colon sensory neurons in vitro. In unanesthetized rats, subcutaneous administration of U50,488, (1S,2S)-U50,488, and (1R,2R)-U50,488 attenuated the behavioral visceromotor response to noxious CRD; the rank order of potency was: (1S,2S)-U50,488 > U50,488 angle quotation mark, right (1R,2R)-U50,488. U50,488 and its stereoisomers also inhibited responses of decentralized pelvic nerve afferent fibers to noxious CRD in a dose-dependent manner. Cumulative doses of 16 mg/kg of (1S,2S)-U50,488, (1S,2R)-U50,488, and (1R,2R)-U50,488 reduced responses to a mean 29, 30, and 47% of control, respectively. The mean inhibitory doses of these drugs were not different (range: 6.6-10.8 mg/kg). Sodium channel blockers mexiletine and carbamazepine mimicked the effect of U50,488. In contrast, the kappa-ORAs dynorphin (1-13) and ICI 204,488 were ineffective in attenuating pelvic nerve activity. Perfusion of (1S,2S)-U50,488, (1S,2R)-U50,488, or (1R,2R)-U50,488 on colon sensory neurons in vitro decreased voltage-activated sodium currents. This inhibition by U50,488 and its stereoisomers was not opioid receptor-mediated because it could not be reversed by the opioid receptor antagonist naloxone and was also not a G protein-mediated effect. The results reported here suggest that the visceral antinociceptive effects of U50,488 and its stereoisomers are contributed to by their peripheral sodium channel blocking actions.

Elevated prodynorphin expression associated with ethanol withdrawal convulsions

The hypothesis that kappa-opioid system activity may in part mediate convulsions exhibited during ethanol withdrawal was tested by exposing Withdrawal Seizure-Prone (WSP) and Withdrawal Seizure-Resistant (WSR) mice to chronic ethanol. Whole brain was harvested for RNA isolation and prodynorphin mRNA steady-state levels in whole brain were examined using Northern blot analysis. The data revealed significantly increased levels of prodynorphin mRNA expression in mice susceptible to ethanol withdrawal convulsions after withdrawal, with no corresponding increase in prodynorphin steady-state levels in mice resistant to ethanol withdrawal convulsions. These findings were not due to basal differences in prodynorphin expression between the WSP and WSR mice. To verify that the differences observed were not due to an ethanol-induced global alteration in gene transcription, mRNA levels of the housekeeping gene glyceraldehyde-3-phosphate dehydrogenase were measured. Glyceraldehyde-3-phosphate dehydrogenase expression was unchanged following both chronic exposure to ethanol and chronic exposure followed by withdrawal. These results extend our understanding of prodynorphin's role in generalized seizure activity to include ethanol withdrawal-induced convulsions. Our findings suggest that prodynorphin expression is modulated during ethanol withdrawal convulsions, or alternatively, prodynorphin may mediate the severity of ethanol withdrawal convulsions.

Synthesis and biological activity of histogranin and related peptides

Histogranin (HN) was first isolated from bovine adrenal medulla and shown to be a pentadecapeptide displaying N-methyl-D-aspartate (NMDA) receptor antagonist activity. To determine the active pharmacophore of HN, fragments of the peptide were synthesized and their structure-activity relationships studied by measuring their ability to displace the binding of [125I][Ser1]HN to rat brain membrane preparations and to block NMDA-induced convulsions in mice. In the binding assay, only the full length peptide HN and HN(1-10) displayed a high affinity (Ki of 72 and 162 nM, respectively). All other tested fragments with deletions at the N- and (or) C-terminals of the molecule showed large (16- to 2500-fold) decreases in potency. The least active peptide fragment tested was HN(6-10) (Ki of 164 microM). In vivo, HN and HN(2-15) (100 nmol; i.c.v.) produced 94 and 40% protection against NMDA-induced convulsions in mice, respectively. None of the other peptide fragments displayed significant anticonvulsant activity. The protective activity of HN (60 and 100 nmol) was markedly antagonized by coadministration of HN(1-10) (100 nmol). The results indicate that the in vivo anti-NMDA and in vitro binding activities of HN and related peptides, with the exception of HN(1-10), depend upon the integrity of the molecule. On the other hand, the high affinity of HN(1-10) for HN binding sites correlates well with its antagonist effects towards the activity of the parent peptide.

N-methyl-D-aspartate receptor antagonist activity and phencyclidine-like behavioral effects of the pentadecapeptide, [Ser1]histogranin

The behavioral and pharmacologic profiles of [Ser1]histogranin ([Ser1]HN) were assessed by monitoring its ability to displace the binding of the specific N-methyl-D-aspartate (NMDA) receptor ligand, [3H]CGP 39653, to block the convulsant effects of NMDA and other excitatory agents in mice, and to produce phencyclidine (PCP)-like behavioral effects in rats. The peptide potently inhibited [3H]CGP 39653 binding to membrane preparations of rat brain with an IC50 of 198 nM and a maximal inhibition of 34% of the specific binding activity. Saturation binding experiments with [3H]CGP 39653 in the absence and presence of [Ser1]HN (2 microM) indicated that the inhibitory effect of the peptide was noncompetititive, producing a decrease in the maximal number of binding sites (Bmax of 62.5 fmol/mg protein as compared with 91.3 fmol/mg protein in control), but no significant change in the affinity (Kd of 4.5 nM as compared with 5.1 nM in control). Intracerebroventricular (ICV) injection of [Ser1]HN (10-100 nmol) in mice evoked a dose-dependent and selective blockade of NMDA-induced convulsions. In rats, [Ser1]HN (2.5-100 nmol, ICV) produced dose-dependent stereotypy, ataxia, and locomotion similar to those observed with PCP, at doses ranging between 50 and 400 nmol. The data indicate that [Ser1]HN noncompetitively interacts with the NMDA receptor, an action that goes along with its in vivo NMDA receptor antagonist activity and PCP-like behavioral effects.

Enkephalinase inhibition and hippocampal excitatory effects of exogenous and endogenous opioids

1. The relationships between the in vivo and in vitro epileptogenic effects of opioids or enkephalins and the electrophysiological activity of inhibitors of endogenous enkephalinase were analyzed. 2. The functional effects of the inhibition of the endogenous enkephalinase has been compared with the role of the endogenous opioid peptidergic system in the control of neuronal excitability.

Methodological variables during analysis of in vivo cerebellar cyclic GMP, an indirect marker of nitric oxide release

In light of the recent recognition of the physiological significance of nitric oxide, there is considerable interest in the methodological variables that can confound the results of the cerebellar cGMP analysis from in vivo experiments. In this study, using male Swiss Webster mice, the effect of such methodological variables as 1) weight of the animals; 2) tissue extraction procedures used in radioimmunoassay for cGMP; and 3) the commercial source of the assay kit on, harmaline-, pentylenetetrazole- or SNAP-induced increase in cerebellar cGMP in vivo were evaluated. Results indicate that mice in the 15- to 19-g weight range are most sensitive and best suited for in vivo drug effects on cerebellar cGMP. Furthermore, for the extraction of cerebellar cGMP, use of ice-cold 0.5N hydrochloric acid and subsequent dilution of the sample in assay buffer is the simplest and fastest method. Present data also indicate that the source of the radioimmunoassay kit has a significant effect on the cerebellar cGMP results. Based on the present results, the protocol developed and the guidelines drawn are timely and of high practical significance for research in the area of pharmacology of nitric oxide.

Chronic administration of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine, and drug-induced increase in cerebellar cyclic GMP in vivo

N omega-nitro-L-arginine (NG-nitro-L-arginine) is a potent nitric oxide synthase inhibitor which crosses the blood brain barrier and does not undergo extensive metabolism in vivo. In this study, effect of chronic pretreatment of N omega-nitro-L-arginine (75 mg/kg, i.p., twice daily for 7 days) on the harmaline- (100 mg/kg, s.c.), picrotoxin- (4 mg/kg, s.c.), pentylenetetrazole- (50 mg/kg, i.p.), and L-glutamic acid- (400 micrograms/10 microliters/mouse, i.c.v.) induced increase in cerebellar cGMP was assessed. All the four drugs produced significant increase in cerebellar cGMP in vehicle pretreated control animals. Cerebellar cGMP increased induced by harmaline, picrotoxin, and L-glutamic acid was attenuated in N omega-nitro-L-arginine pretreated animals. These results indicate that in vivo cerebellar cGMP levels are increased by the prototype excitatory amino acid receptor agonist, L-glutamic acid and also by the drugs which augment the excitatory amino acid transmission. Furthermore, parenteral chronic administration of N omega-nitro-L-arginine blocks NO synthase in the brain and hence cerebellar cGMP response in chronic N omega-nitro-L-arginine treated animals could be used as a tool to assess the physiological functions of nitric oxide in vivo.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.