Lack of the Burst Firing of Thalamocortical Relay Neurons and Resistance to Absence Seizures in Mice Lacking α1G T-Type Ca2+ Channels

al., 1996; Huguenard and Prince, 1994a). The hyperpolarization of membrane potentials induced by the activation of GABAB receptors evokes rebound burst discharges in TC neurons (Crunelli and Leresche, 1991; McCormick and Bal, 1994). This characteristic firing patDaesoo Kim,1,4 Inseon Song,1,4 Sehoon Keum,1 Taehoon Lee,1 Myung-Jin Jeong,1 Sung-Sook Kim,1 Maureen W. McEnery,2 and Hee-Sup Shin1,3,4 1 National Creative Research Initiative Center for Calcium and Learning tern of TC neurons is evoked by low-threshold Ca Department of Life Science potentials (LTCPS) (Deschènes et al., 1984; Jahnsen and Pohang University of Science and Technology Llinàs, 1984). It has therefore been proposed that lowPohang 790-784 threshold T-type Ca channels are involved in the geneKorea sis of absence seizures in the thalamocortical network 2 Department of Physiology and Biophysics (Coulter et al., 1989; Crunelli and Leresche, 1991). This Case Western Reserve University School of Medicine idea has been supported by a belief that drugs effective Cleveland, Ohio 44106 in the treatment of absence seizures, such as ethosuximide, exert their anti-absence actions through reducing T-type Ca currents (IT) in thalamic neurons (Coulter et Summary al., 1989; Kostyuk et al., 1992). In addition, T-type Ca channels were moderately increased in the thalamic T-type Ca2 currents have been proposed to be inneurons of the genetic absence epilepsy rat from Strasvolved in the genesis of spike-and-wave discharges, bourg (GAERS), a model of spontaneous absence epia sign of absence seizures, but direct evidence in vivo lepsy (Talley et al., 2000; Tsakiridou et al., 1995). to support this hypothesis has been lacking. To adResults from recent studies, however, have led to dress this question, we generated a null mutation of controversy about the role of IT in the genesis of absence the 1G subunit of T-type Ca2 channels. The thalamoseizures. For example, it was shown that ethosuximide cortical relay neurons of the 1G-deficient mice lacked failed to suppress IT, but instead affected other channels the burst mode firing of action potentials, whereas such as noninactivating Na channels and Ca -actithey showed the normal pattern of tonic mode firing. vated channels in TC neurons (Leresche et al., 1998). The 1G-deficient thalamus was specifically resistant Another controversy arose from the observation that in to the generation of spike-and-wave discharges in rethe intracellular recording of TC neurons in vivo, the sponse to GABAB receptor activation. Thus, the modumajority of neurons underwent rhythmic sequences of lation of the intrinsic firing pattern mediated by 1G IPSPs and steady hyperpolarization instead of LTCPs T-type Ca2 channels plays a critical role in the genesis during SWDs (Steriade and Contreras, 1995; Pinault et of absence seizures in the thalamocortical pathway. al., 1998). Therefore it is not clear whether T-type Ca channels in TC neurons are involved in the generation Introduction of SWDs. To address this question, we have generated a null mutation of the 1G gene that encodes the poreAbsence seizures are characterized by a brief loss of forming subunit of T-type Ca channels. This gene was consciousness associated with an electroencephalochosen because it is dominantly expressed in TC neugraphic (EEG) recording of 3 Hz bilaterally synchronous rons but not, significantly, in thalamic reticular (nRT) spike-and-wave discharges (SWDs) (Niedermeyer, neurons (Talley et al., 1999; Talley et al., 2000). We ana1996; Williams, 1950). Although earlier studies indicated lyzed the mutant mice with respect to their propensity to that thalamic neurons were involved in the genesis of generate SWDs in well-characterized pharmacological SWDs (Avoli and Gloor, 1981; Pellegrini et al., 1979), models. The results provide conclusive evidence for a more recent experiments demonstrated that the neocorcritical role of 1G in the generation of absence seizures tex is the minimal substrate for the generation of SWDs in the thalamocortical network. because such seizures occur in the cerebral cortex even after complete ipsilateral thalamectomy (Steriade and Results Contreras, 1998). A series of pharmacological studies suggested that Generation of Knockout Mice for the 1G Subunit GABAB receptors play a critical role in the genesis of of the T-Type Ca2 Channel SWDs (Crunelli and Leresche, 1991). It is known that As shown in Figure 1A, the targeting vector was deGABAB receptor agonists exacerbate seizures, whereas signed to delete most of the exon encoding amino acid GABAB receptor antagonists suppress them (Hosford et residues 82–118 that comprise the N terminus of the 1G al., 1992; Smith and Fisher, 1996; Snead, 1992). The antiprotein (Perez-Reyes et al., 1998). Because the length absence drug clonazepam is thought to act by diminishof the exon is not a multiple of three, possible exon ing GABAB-mediated inhibitory postsynaptic potentials skipping by alternative splicing does not result in func(IPSPs) in thalamocortical relay (TC) neurons (Gibbs et tional proteins. Southern analysis revealed that 30% of neo /TK embryonic stem (ES) cell clones were correctly targeted and two of these were used for making 3 Correspondence: [email protected] germline chimeras. Two germline chimeras were mated 4 Present address: Korea Institute of Science and Technology (KIST),