FREQUENCY BANDS AND SPATIOTEMPORAL DYNAMICS OF  BURST STIMULATION INDUCED AFTERDISCHARGES IN HIPPOCAMPUS IN VIVO J. E. MIKKONEN AND M. PENTTONEN* A. I. Virtanen Institute for Molecular Sciences, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland Abstract—Temporal and spatial characteristics of hippocampal neuronal network activation are modified during epileptiform afterdischarges. We developed a  burst stimulation protocol to investigate subregional variations and substrates of rhythmic population spike discharges in vivo in urethane anesthetized Wistar rat hippocampus with a 14-electrode recording array and extracellular single electrode recordings. Our 64 pulse  burst stimulation protocol was constructed from electrical pulses de- livered at intervals corresponding to  (14 –25 Hz),  (2 Hz), and slow (0.5 Hz) frequencies. In each experiment these interleaved pulses were all repeated four times with unchanged intervals. Stimulation of either perforant path or fimbria fornix induced a prolonged afterdischarge pattern peaking at 200 Hz fast, 20 Hz , and 2 Hz  frequencies. Analysis of variance confirmed that the response pattern of the discharges remained constant re- gardless of the stimulation  frequency. Within the afterdis- charge the fast frequencies were restricted to independent hip- pocampal subfields whereas  and slow frequencies correlated across the subfields. Current source density (CSD) analysis revealed that the original signal propagation through subfields of the hippocampus was compromised during the  burst stim- ulation induced afterdischarge. In addition, the CSD profile of the epileptiform afterdischarge was consistently similar across the different experiments. Time-frequency analysis revealed that the  frequency afterdischarge was initiated and terminated at higher  (30 – 80 Hz) frequencies. However, the alterations in the CSD profile of the hippocampus coincided with the  fre- quency dominated discharges. We propose that hippocampal epileptiform activity at fast,  and  frequencies represents coupled oscillators at respectively increasing spatial scales in the hippocampal neuronal network in vivo. © 2004 IBRO. Pub- lished by Elsevier Ltd. All rights reserved. Key words: extracellular, neuronal network, synchrony, cou- pled oscillator. Investigation of the transition between normal and abnor- mal brain states provides insight into the network architec- ture and function of the brain. One way to understand neuronal networks is to examine disturbances in the elec- trical network of the brain during epilepsy or epileptiform brain states. Thus, periodic ultrasynchronous discharges of large neuronal ensembles are the hallmark of various epileptic conditions (McCormick and Contreras, 2001), and the study of the initial conditions of such discharges can provide information on normal brain function, as well as elucidate the mechanisms involved in the generation of epileptiform activity. Hippocampus is an important focus of epileptic activity in humans and in some animal models of temporal lobe epilepsy (McCormick and Contreras, 2001; Jefferys, 2003). Therefore, it seemed appropriate to study the mechanisms of the emergence of afterdischarges in the hippocampal neuronal networks. Epilepsy related func- tional reorganization of the hippocampal network can be examined via the electrical properties of the network. To date, the in vivo methodology of local field potential record- ing offers the most complete tool to investigate normal or pathological fast network interactions in the intact brain. Within normal brain states, the  and  frequencies have been associated with epilepsy prone internal brain activity, such as 14 Hz sleep spindles in the thalamus (Steriade et al., 1993), synchronized activity in prefrontal cortex (Liang et al., 2002), and hippocampal memory con- solidation during slow wave sleep (Buzsaki, 1989; Sirota et al., 2003). Bragin et al. (1997a), Amzica and Steriade (1999), and Hirai et al. (1999) have also reported  fre- quencies occurring in epileptic conditions in vivo. It has been claimed that  frequency stimulation can either in- duce epileptiform activity (Somjen et al., 1985; Lothman and Williamson, 1992) or that after the epileptiform syn- chrony has developed, one of its hallmarks is  frequency bursting (Pare et al., 1992; Bragin et al., 1997a; Amzica and Steriade, 1999; Hirai et al., 1999; Medvedev et al., 2000). However, only a few experiments have been con- ducted linking the  frequency induction of paroxysmal afterdischarges into the epileptiform  frequency outcome of the stimulation (Amzica and Steriade, 1999). Further- more, Bikson et al. (2003) have demonstrated that parox- ysmal  frequencies are at least initially accompanied with fast, 80 Hz, oscillations whereas Pare et al. (1992) and Steriade and Contreras (1998) have reported  frequen- cies co-occurring with  frequencies in already epileptiform conditions. A systematic investigation of an epileptiform afterdis- charge with time-sensitive interactions involving more than two frequency bands would provide novel information about the temporal structure and frequency division of the afterdischarge and facilitate the understanding of the mechanisms behind the development of the epileptiform afterdischarge. Here we report brief  burst stimulation induced epileptiform afterdischarges in rat hippocampus in vivo co-occurring at anatomically distinct scales and at three frequency bands. *Corresponding author. Tel: +358-17-162345; fax: +358-17-163030. E-mail address: markku.penttonen@uku.fi (M. Penttonen). Abbreviations: CA1, cornu ammonis region 1; CA3, cornu ammonis region 3; CSD, current source density; DG, dentate gyrus; SR, stratum radiatum. Neuroscience 130 (2005) 239 –247 0306-4522/05$30.00+0.00 © 2004 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2004.08.039 239