Pilocarpine-Induced Seizure Susceptibility in Rats Following Prenatal Methylazoxymethanol Treatment

Abstract

Recent progress in brain imaging has revealed a high frequency of cortical malformations in childhood epilepsy. 1,2) However, very little is knownregarding the pathophysiological mechanisms of epileptogenicity in the malformed cortex, partly because of the rarity of experimental studies with animal models.3) In rats, the application of methylazoxymethanol (MAM) in utero (E15) results in the formation of dysplastic regions in the neocortex and the CA1 region of the hippocampus, as well as in the heterotopic clusters of neurons in the subcortical white matter.4,5) Some MAM-treated pups, with both neocortical and hippocampal dysgenesis, have a lower threshold for seizure activity.6) Heterotopic neurons in the MAM model lack potassium channels and exhibit burster firing properties 7,8) and heterotopia received abundant GABAergic innervations in the MAM exposed rats. 9) Heterotopic CA1 pyramidal neurons appear to have atypical electrophysiological and morphological characteristics 10) and may form abnormal connections with neocortical regions. 11) Although it is well established that these models mimic the structural aspects of human early-onset epilepsy syndrome, there were few behavioral changes regarding spontaneous and recurrent seizures in the MAM rats. When MAM animals were examined specifically for spontaneous seizures, no activity was detected, although some EEG and sleep-cycle irregularities exist. [12][13][14][15] Behavioral changes are considered to be important factors in evaluating the usefulness of epilepsy animal models. [16][17][18] Spontaneous electrographic and behavioral seizures have been observed in pilocarpine-induced status epilepticus (SE), [19][20][21] although spontaneous epileptic seizures have not yet been observed in MAM-exposed rats. This indicates that the MAM rats were resistant to the development of spontaneous and recurrent seizures. Since spontaneous and recurrent seizures are typical characteristics of epilepsy, developing a similar seizure model in the MAM rats is important in the study of pathophysiological mechanisms that contribute to epileptogenesis.In the present study, we examined the facilitated effect of MAM-treated animals on seizure induced by pilocarpine. MATERIALS AND METHODS AnimalsFemale rats (Sprague-Dawley) with known insemination times were obtained. Pregnant rats were injected with 25 mg/kg MAM dissolved in 0.9% saline. Intraperitoneal injections were made on embryonic 15 d. Experimental procedures were performed in accordance with the animal care guidelines of NIH and the Korean Academy of Medical Sciences. All animals were maintained in a 12-h light-dark cycle and were provided with food and water ad libitum.In Vivo Recording Sprague-Dawley rats were anesthetized with urethane (1.3 g/kg), according to procedures reported elsewhere.22) Briefly, the recording electrode was located in the hippocampus (AP: Ϫ3.8 mm from bregma; L: 2.5 mm). A concentric bipolar stimulating electrode was inserted into the contralateral fimbria-fornix (AP: Ϫ1.3, L: 1.0, V: 4.8 mm) to s...

Single injection of a low dose of pentylenetetrazole leads to epileptogenesis in an animal model of cortical dysplasia

“…The increased severity of acute response to this low dose of PTZ is not an unexpected or a new finding, as indicated by previously published data. Increased, as well as decreased, susceptibility to different seizure‐inducing drugs has been reported previously in various models of irradiation‐ or drug‐induced‐dysplasia (de Feo et al., 1995; Baraban & Schwartzkroin, 1996; Germano & Sperber, 1998; Holmes et al., 1999; Kabova et al., 2000; Choi et al., 2005; Setkowicz et al., 2005). These alterations in susceptibility apparently correlate with the degree of induced dysplasia and the time point during pregnancy at which fetuses are subjected to the procedure that disrupts normal nervous system development (Riggs et al., 1956; Hicks & D’Amato, 1980; Kellinghaus et al., 2004; Setkowicz et al., 2006).…”

GABAergic Synaptic Inhibition Is Reduced before Seizure Onset in a Genetic Model of Cortical Malformation

“…Our immunohistochemical evidence shows that the distribution of parvalbumin-IR interneurons is disturbed in the tish brain, such that the normotopic cortex contains fewer interneurons than control cortex. Altered interneuron distribution appears to be a common feature in brains with malformations (Ferrer et al, 1994;Mathern et al, 1995;Jacobs et al, 1996;Marco et al, 1996;Hablitz and DeFazio, 1998;Rosen et al, 1998;Spreafico et al, 1998;Roper et al, 1999;Schwarz et al, 2000;Choi et al, 2005). Consequently, the questions of how different interneuron populations find or do not find their final cortical destinations and survive or perish during early development are critical issues to explore.…”

Novel Animal Models of Pediatric Epilepsy

“…When a "double-hit" combining MAM and pilocarpine is used to induce SE in adult rats, it has been established that prenatal induced cortical development results in more severe post-SE induced epilepsy. MAM pilocarpine rats showed abnormally large cortical pyramidal neurons with neurofilament over-expression, suggesting some similarities with dysplastic neurons in humans [83]. These data show that pre-existing experimental cortical malformations enhance epileptogenesis.…”