Summary:Purpose: The possible role of gap junctions in the manifestation and control of the duration of seizures was tested on the 4-aminopyridine-induced epilepsy model in rats in vivo, by using electrophysiologic, pharmacologic, and molecular biologic techniques.Methods: In electrophysiologic experiments, the functional states of the gap junctions were manipulated with a specific blocker (carbenoxolone) or opener (trimethylamine) at the already active focus of adult, anesthetized rats, 60 min after the induction of the first seizure, which was repeated spontaneously thereafter. Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) amplification was used to measure the levels of connexin (Cx) 32, 43, and 36 messenger RNAs (mRNAs) prepared from the areas of the already active primary and mirror foci.Results: After repeated seizures, the expression levels of Cx32, Cx43, and Cx36 mRNAs at the epileptic foci were increased significantly. Blockade of the gap junctions with carbenoxolone shortened the duration of seizures and decreased the amplitude of the seizure discharges, whereas their opening with trimethylamine lengthened the duration and increased the amplitude. Secondary epileptogenesis was facilitated when the gap junctions were opened.Conclusions: Our findings support the idea that, in epileptic foci, the gap junctions are involved in the expression of rhythmic ictal discharges and in the control of the duration and propagation of the individual seizures in vivo. Key Words: Connexins-Gap junctions-4-AP-induced seizureCarbenoxolone-Trimethylamine.Gap junctions are dynamic structures that can be modulated by a number of intracellular and extracellular factors (1-6). The extent of coupling in the in vitro seizure models (7-10) is periodic: it is increased by alkalinization at the start of a seizure, and decreased as acidification occurs toward the end of an ictal period.Our previous work (11) revealed noteworthy upregulations of connexin (Cx)32 and Cx43 mRNAs after repeated seizures both at the primary focus (Pf) and at the mirror focus (Mf, homotopic area contralateral to the Pf). Accordingly, we examined whether manipulation of the functional state of the gap junctions with a specific blocker (carbenoxolone) or opener (trimethylamine, TMA) influences the manifestation, duration, and propagation of seizures. We also were interested in whether repeated seizures influence the expression of the Cx36 gene, coding for a gap-junction protein existing predominantly in neuronal cells of the mature brain (13).

Summary:Purpose: The selective contribution of neuronal gap junction (GJ) communication via connexin 36 (Cx36) channels to epileptogenesis and to the maintenance and propagation of seizures was investigated in both the primary focus and the mirror focus by using pharmacologic approaches with the 4-aminopyridine in vivo epilepsy model.Methods: ECoG recording was performed on anesthetized adult rats, in which either quinine, a selective blocker of Cx36, or the broad-spectrum GJ blockers carbenoxolone and octanol were applied locally, before the induction or at already active epileptic foci.Results: The blockade of Cx36 channels by quinine before the induction of epileptiform activity slightly reduced the epileptogenesis. When quinine was applied after 25-30 repetitions of seizures, a new discharge pattern appeared with frequencies >15 Hz at the initiation of seizures. In spite of the increased number of seizures, the summated ictal activity decreased, because of the significant reduction in the duration of the seizures. The amplitudes of the seizure discharges of all the patterns decreased, with the exception of those with frequencies of 11-12 Hz. The blockade of Cx36 channels and the global blockade of the GJ channels resulted in qualitatively different modifications in ictogenesis.Conclusions: The blockade of Cx36 channels at the already active epileptic focus has an anticonvulsive effect and modifies the manifestation of the 1-to 18-Hz seizure discharges. Our findings indicate that the GJ communication via Cx36 channels is differently involved in the synchronization of the activities of the networks generating seizure discharges with different frequencies. Additionally, we conclude that both neuronal and glial GJ communication contribute to the manifestation and propagation of seizures in the adult rat neocortex. Key Words: Gap junctions-Cx36-4-AP-induced seizure-EpileptogenesisIctogenesis-In vivo-Quinine-Carbenoxolone.Epilepsy is one of the most prevalent neurologic disorders worldwide, but pharmacologic therapy remains the best remedy for its treatment. One reason for the incomplete effectiveness of the currently available anticonvulsants is that they were identified by using the same classic epilepsy models, which mainly involve the same actions, without a consideration of the variations in the pathophysiologic mechanisms that result in epilepsy. Growing evidence indicates that, besides the chemical synapses, direct coupling via gap junction (GJ) channels provides a second major pathway, contributing to normal and abnormal physiologic rhythms both during development and in the adult brain (1-3).GJ channels mediating electrical signaling are involved in the physiologic synchronizing mechanism in the brain (3-5) and contribute to pathologic hypersynchrony in various in vitro (6-9) and in vivo (10-13) epilepsy models.

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Summary: Purpose:The functional significance of gapjunction (GJ) channels in seizure susceptibility and induction and maintenance of seizures in the developing rat brain was investigated on the 4-aminopyridine (4-AP) in vivo epilepsy model. Methods:In electrophysiological experiments, GJs were manipulated with a blocker or opener before induction or at the active epileptic foci between postnatal days 9 and 28 (P9-28). Semiquantitative reverse transcriptase-polymerase chain reaction (RT-PCR) amplification was used to measure the levels of connexin (Cx) 26, 32, 36, and 43 mRNAs at the untreated cortex or epileptic foci.Results: The basic electrocorticogram (ECoG) and Cx messenger RNA (mRNA) expression patterns exhibited characteristic maturation; the 4-AP-induced epileptiform activity correlated well with these changes. Cx mRNA expressions were significantly upregulated around P16 (except for Cx26). The Cx26, 36, and 43 gene inducibility was highest around P16 and then declined significantly. In the youngest animals, the GJ opener induced rhythmic synchronous cortical activity. On maturation, the seizures became focalized and periodic; the discharges accelerated their amplitude and frequency increase. A transient decrease (P13-14) and then increase (P15-16) in seizure susceptibility were followed by a tendency to periodicity and focalization.Conclusions: The study suggests that GJ communication is involved in rhythm genesis and synchronization of cortical activity and may enhance the epileptogenicity of the developing brain.Key Words: Gap junction channelsSynchrony-Connexins-Development-Epileptogenicity-4-Aminopyridine.Clinical experience and various experimental data indicate that the developing nervous system is more sensitive than the mature one to different convulsive effects (1-5). Although the physiological factors underlying this differential epileptogenicity have not been fully clarified, the higher susceptibility of the immature brain can be explained by certain characteristic neurobiologic features. The developing brain exhibits a high metabolic rate, abundant neuronal and synaptic networks, the overexpression of receptors and enzymes, the depolarizing effect of γ -amino-acid, the hypersynchrony of neuronal circuits, and enhanced synaptic plasticity (6,7). In addition, the immature cerebral cortex and hippocampus have higher densities of excitatory amino acid receptors and gap junction (GJ) channels as compared with the adult organs (4,8).Intercellular communication via GJ channels is an important form of cell-to-cell communication in early brain development (8)(9)(10)(11)(12)(13)(14). Electrical coupling via GJ channels has been reported both between pairs of inhibitory neurons and among inhibitory and excitatory neurons during the Accepted January 30, 2006. Address correspondence and reprint requests to Dr. M. Szente at Department of Comparative Physiology, University of Szeged, Közép fasor 52, H-6726 Szeged, Hungary. E-mail: szente@bio.u-szeged.hu early postnatal days in the rat cortex (10). Moreover,...

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