ISSN: 2167-0501 BCPC, an open access journal Psycho- and Neuropharmacology Biochem & Pharmacol Research article Open Access Shin et al., Biochem & Pharmacol 2013, S1 DOI: 10.4172/2167-0501.S1-006 Introduction Seizures results from uncontrolled, synchronous cortical activity [1], which is thought to be caused by synaptic hyperexcitability [2]. Excessive excitatory glutamate release or reduced inhibitory GABA release has been implicated in hyperexcitability and uncontrolled neuronal activity, resulting in seizures and ultimately epilepsy. Epilepsy is characterized by spontaneous, recurrent seizures. When the balance of inhibition-excitation is disrupted, epileptiform seizures develop [3,4]. GABA release plays a vital role in maintaining a sensitive balance of excitation-inhibition. GABA receptor antagonists have been found to yield epileptogenic seizures [5] and enhanced GABA receptor-mediated auto-inhibition can prevent hyperexcitability [6]. Animal models have proven to be efective in studying epilepsy in humans [7]. Te kindling model is a physiologically-relevant model of epileptogenesis [8]. Kindling is accomplished through repeated electrical stimulation to discrete structures in the brain [9], which leads to a state of persistent hyperexcitability [10]. Te frst stimulation typically evokes a local afer discharge, which is of relatively short duration and elicits minimal behavioral efects. However, with repeated stimulations, seizures progress in duration, severity and distribution [11,12], transitioning from focal, non-overt seizures to generalized motor seizures [10]. Kindling not only results in generalization of seizure activity, but also results in a permanent state of excitability due to plasticity, or complex reorganization of circuitry [11]. Kindling facilitates determinations regarding site of action [8], and allows for the unambiguous study of anticonvulsant drugs [13], as no other drugs are present in the brain to complicate interpretations regarding specifc drugs efects, as is ofen the case with chemically-induced seizures. Such is the case with stimulation of the perforant path, the primary excitatory neocortical input to the dentate gyrus of the hippocampus [14]. It is well-known that repeated stimulation of the perforant path for several days induces sub-clinical, and ultimately, full-clinical seizures. In this kindling model of epilepsy-induction, current is administered directly to the perforant path, allowing neural circuitry to behave independently of drug or other physiological elements. Te dentate gyrus is susceptible to seizures because of its intrinsic circuitry that allows oscillatory feedback excitation. Presumably, when GABA inhibition is compromised, feedback-excitation via recurrent excitatory interneurons becomes open-loop, resulting in an oscillation of synaptic and neuronal activity. Tese afer discharges have shown to elicit spreading of uncontrolled neuronal fring to other areas of the CNS with subsequent stimulations to the perforant path. Because of their inherent ability to rapidly connect and synchronize networks of neurons, gap junction (GJs) have stimulated a lot of recent interest in epilepsy research. Gap junctions are pores constructed between cells from approximately 20 members of the connexin (Cx) protein family by aggregation of six Cx proteins, forming a connexon which associates with a connexon on a neighboring cell [15]. Tese pores allow for the coupling of membrane voltages between neighboring neurons [16], thereby providing a means for intercellular *Corresponding author: Scott C Steffensen, Department of Psychology (SWKT 1050), Brigham Young University, Provo, UT 84602, USA, Tel: (801) 422-9499; Fax: (801) 422-0602; E-mail: scott_steffensen@byu.edu Received March 02, 2013; Accepted April 15, 2013; Published April 18, 2013 Citation: Shin SI, Andersen DJ, Hansen DM, Yorgason JT, Schilaty ND, et al. (2013) Connexin-36 Knock-Out Mice have Increased Threshold for Kindled Seizures: Role of GABA Inhibition. Biochem & Pharmacol S1: 006. doi:10.4172/2167-0501. S1-006 Copyright: © 2013 Shin SI, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Abstract Blockade of gap junctions (GJs) has been shown to reduce seizures in different epilepsy models. Gap junction- mediated, electrically-coupled neuronal networks have been implicated in neuronal synchronization, which is the hallmark of seizure activity. To further understand the role of GJs in seizures, in particular hippocampal seizures, we evaluated electrophysiological responses in the dentate gyrus subfeld of the hippocampus, including GABA-mediated recurrent inhibition, seizuregenic stimulation, and seizure activity in response to perforant path kindling in Cx36 knock- out (KO) mice compared to wild-type (WT) controls. In anesthetized mice, Cx36 KO mice were characterized by enhanced GABA-mediated recurrent inhibition. Stimulation of the perforant path at 10 Hz for 10 sec (i.e., 100 pulses) markedly enhanced population spike (PS) amplitudes and reduced paired-pulse GABA-mediated inhibition during the stimulation, induced an after discharge for approximately 10 sec at 7-10 sec into the stimulation, and suppressed PS amplitudes postictally for 5-10 min in both KO and WT mice. Repeated epochs of 10 Hz for 10 sec stimulation of the perforant path at 20 min intervals resulted in progressive and persistent disinhibition of paired-pulse responses in WT, but not KO mice. In freely-behaving seizure studies, stimulation of the perforant path (10 Hz for 10 sec) once/day resulted in progressive Stage I-IV seizures in both WT and KO mice. Once Stage IV was achieved, another Stage IV seizure could be elicited each day with the same behavioral response (i.e., Stage V). The threshold for kindled seizures was signifcantly greater in KO mice compared to WT controls for most stages of seizures. The Cx36 antagonist mefoquine (MFQ) and the typical anticonvulsant pentobarbital reduced Stage IV seizures in both WT and KO mice. Knock-out mice were more sensitive to the anti-epileptic effects of the typical anti-convulsant pentobarbital (20 mg/ kg). Taken together, these fndings support the emerging view that reduction in GJ-mediated GABA electrical coupling reduces seizures, perhaps through enhancement of GABAergic feedback inhibition, which results from the uncoupling of the GABA recurrent interneurons from the resistive load that is inherent in their electrical connectivity. Connexin-36 Knock-Out Mice have Increased Threshold for Kindled Seizures: Role of GABA Inhibition Samuel I Shin, Dallin J Andersen, Micah Hansen D, Jordan T Yorgason, Nathan D Schilaty, David D Busath and Scott C Steffensen* Department of Psychology and Neuroscience Center, Brigham Young University, Provo, UT 84602, USA B i o c h e m i s t r y & P h a r m a c o l o g y : O p e n A c c e s s ISSN: 2167-0501 Biochemistry & Pharmacology: Open Access