Endogenous opioids and post-ictal increase in seizure threshold in Mongolian gerbils

Evoked responses of the dentate gyrus during seizures in developing gerbils with inherited epilepsy

When they are 1-2 mo old, domesticated Mongolian gerbils begin having initially mild seizures which become more severe with age. To evaluate the development of this increasing seizure severity, we obtained field potential responses of the dentate gyrus to paired-pulse stimulation of the perforant path during seizures. In 18 gerbils that were 1.5-8.0 mo old, 73 seizures were analyzed. We measured population spike amplitude, the slope of the field excitatory postsynaptic potential (fEPSP), and the population spike amplitude ratio (2nd/1st) to evaluate excitatory and inhibitory synaptic processes. In gerbils <2 mo old, exposure to a novel environment was followed by an increase in population spike amplitude and then by seizure onset, but population spike amplitude ratio and fEPSP slope remained at baseline levels, and multiple population spikes were never evoked. As previously reported for chronically epileptic gerbils, these findings provide little evidence of a disinhibitory seizure-initiating mechanism in the dentate gyrus when young gerbils begin having seizures. In young gerbils evoked responses changed little during the behaviorally mild seizures. In contrast, most seizures in older gerbils included generalized convulsions, postictal depression, and evoked responses that changed dramatically. In older gerbils, shortly after seizure onset the dentate gyrus became hyperexcitable. Population spike amplitude and fEPSP slope peaked, and multiple population spikes were evoked, suggesting that mechanisms for seizure amplification and spread are more developed in older gerbils. Next, dentate gyrus excitability decreased precipitously, and population spike amplitude and fEPSP slope diminished. This period of hypoexcitability began before the end of the seizure, suggesting it may contribute to seizure termination. After the convulsive phase of the seizure, older gerbils remained motionless during a period of postictal depression, and population spike amplitude remained suppressed until the abrupt switch to normal exploratory activity. These findings suggest that the mechanisms of postictal depression may suppress granule cell excitability. The population spike amplitude ratio peaked after the convulsive phase and then gradually returned to the baseline level an average of 12 min after seizure onset, suggesting that granule cell inhibition recovers within minutes after a spontaneous seizure. Although it is unclear whether the seizure-related changes in evoked responses are a cause or an effect of increased seizure severity in older gerbils, their analysis provides clues about developmental changes in the mechanisms of seizure spread and termination.

HPLC/RIA analysis of bioactive methionine enkephalin content in the seizure-susceptible El mouse brain

We previously reported a deficit of methionine enkephalin-like immunoreactivity (ME-LI), in the cerebral cortex, septal area, hippocampus, and striatum and the abnormal metabolism of opioid peptides in the hippocampus and striatum of seizure-susceptible El mice, which are involved in the pathogenesis of seizures. However, these findings suggest that the ME-LI does not necessarily reflect the bioactive methionine enkephalin (ME). Herein, we measured the biologically active peptide, ME excluding cross-reactive substances by using HPLC coupled with radioimmunoassay to clarify the abnormal function of enkephalinergic neurons in the El mouse brain. The ME content in 25-day-old El mice that had no seizures was significantly decreased in the hippocampus and septal area, as compared with corresponding regions in ddY mice (seizure-nonsusceptible; the mother strain of El). At the age of 50 days when El mice displayed abortive seizures, this content in both stimulated El[s] and nonstimulated El[ns] was significantly reduced in the septal area and cerebral cortex. At the age of 150 days when El mice exhibit tonic-clonic seizures, this content in both El[s] and El[ns] was significantly reduced in the septal area, cerebral cortex and striatum. These findings were generally compatible with our previous findings. This study further supports our hypothesis that a deficit of anticonvulsant endogenous ME, in the cerebral cortex, septal area, and hippocampus of seizure-susceptible El mice play an important role in the pathogenesis of seizures.

The kappa-opioid receptor agonist, U50,488H, exerts antidystonic activity in a mutant hamster model of generalized dystonia

The kappa-opioid receptor agonist, U50,488H (trans-(+-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl]- benzeneacetamide), has recently been reported to induce dystonia after s.c. administration of 5-10 mg/kg in guinea pigs. The dystonic movements observed in response to U50,488H resembled those previously reported to occur spontaneously or in response to mild environmental stimuli in a mutant hamster model of paroxysmal generalized dystonia. This prompted us to study the effects of opioid receptor antagonists and of U50,488H in the mutant hamster model. Naloxone and naltrexone, 1 and 10 mg/kg i.p., were either ineffective or tended to induce prodystonic effects in the mutant hamsters. In contrast, U50,488H markedly reduced the severity of dystonic movements at doses of 1-10 mg/kg s.c. In non-dystonic hamsters, U50,488H reduced locomotor activity but did not produce dystonic-like symptoms. The data from mutant hamsters demonstrate that stimulation of kappa-opioid receptors is a powerful means of attenuating dystonic movements in a genetic animal model of idiopathic dystonia. The antidystonic effects of U50,488H might relate to interactions between kappa-opioid receptors and dopaminergic and glutamatergic neurotransmission.

Effects of naloxone on seizure latencies of black (a/a) Mongolian gerbils (Meriones unguiculatus)

The effects of the opiate antagonist, naloxone, on seizure tendencies of the black or nonagouti line of Mongolian gerbils (Meriones unguiculatus) carrying the a/a coat color allele was investigated. The animals were tested under the following conditions: mock infection; 0.9% NaCl injection; and naloxone at doses of 1, 2, 5, and 10 mg/kg body weight. The results indicated that when naloxone was at doses of 2 mg/kg body weight and higher, there was an increase in the animals' seizure latencies relative to that manifested under the basal condition.

Endogenous opiates: 1987

This paper is the tenth installment of our annual review of the research during the past year involving the endogenous opiate system. It covers the nonanalgesia and behavioral studies of the opiate peptides published in 1987. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal activity; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.