Epilepsy Res., 6 (1990) 110-118
110
Elsevier EPIRES 00324
Recurrent spontaneous hippocampal seizures in the rat as a chronic sequela to limbic status epilepticus
Eric W. L o t h m a n a, E d w a r d H. B e r t r a m a, Jaideep K a p u r b and Janet L. Stringer a "Department of Neurology, Universityof Virginia School of Medicine, Charlottesville, VA 22908 (U.S.A.), and t'Department of Neurology, Medical College of Virginia, Richmond, VA 23298 (U.S. A.) (Received 17 November 1989; accepted 21 November 1989)
Key words: Epilepsy; Inhibition; Chronic seizures; Status epilepticus; Hippoeampus; Slices
A period of continuous hippocampal stimulation (CHS) establishes an acute condition of self-sustaining limbic status epilepticus (SSLSE) which is followed by chronic neuropathological changes reminiscent of hippocampal sclerosis encountered in epileptic patients. In the chronic (31 month) condition following CHS-induced SSLSE, extended electrographic monitoring in the hippocampus revealed spontaneous recurrent paroxysmal discharges. All 6 animals studied had persistent interictal spiking; 3 had multiple fully developed electrographic seizures. There was a marked diminution of paired pulse inhibition, demonstrated by a protocol known to refleet the potency of inhibition mediated by GABA A receptors. Hippocampal slices from animals that had previously experienced CHS-induced SSLSE demonstrated an increased excitability relative to slices from control animals as evidenced by epileptiform bursting in increased extracellular potassium ([K +]0) and decreased extracellular calcium ([Ca2+]0). These studies establish that CHS-induced SSLSE in rats provides all experimental model with recurrent spontaneous hippocampal seizures. Based oft electrophysiologi¢al data we suggest that a decrease in GABA-mediated inhibition and/or altered sensitivity to extracellular ions may play roles in the development of such seizures.
INTRODUCTION When a low intensity electrical stimulus train is repetitively given to particular limbic brain structures, the epileptiform responses elicited by such stimuli progressively intensify until stereotyped convulsions (so-called kindled motor seizure) appear m'2+'29. Once established, this enhanced responsiveness is long-lasting, perhaps even permanent. Based on these observations, it can be argued that kindled motor seizures are an experiCorrespondence to: Eric W. Lothman, M.D., Ph.D., Department of Neurology - - Box 394, University of Virginia Medical Center, Charlottesville, VA 22908, U.S.A.
mental counterpart of limbic seizures with secondary generalization which occur in humans. However, the kindled state in which seizures remain stimulus-locked, appearing only when electrical stimuli are provided, differs from naturally occurring epilepsy in humans which is characterized by recurrent spontaneous seizures. It has been observed that some kindled rats developed spontaneous seizures, but only after several months of stimulation zT. Over the past several years, we have examined the changes brought about by rapidly recurring hippocampal seizures (RRHS). We showed that kindling could be induced with RRHS over a matter of days as opposed to the weeks required with
0920-1211/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
111 traditional kindling ~9-21. Subsequently, we found that a deterioration of GABAergic inhibition consistently accompanied kindled responses engendered by RRHS 11'12'14. This deterioration developed quickly, over the same time course as rapid kindling of electrographic and behavioral responses, and persisted for at least 1 month after the last kindled seizures. Moreover, slices isolated from animals previously kindled with RRHS and maintained in vitro were shown to be hyperexcitable as. They generated epileptiform discharges with smaller perturbations of the concentrations of extracellular potassium ([K ÷]0) or of extracellular calcium ([Ca2÷]0) than was required with normal tissue. We have also examined the consequences of providing the same stimulus trains used for RRHS, but decreasing the time between stimuli so that stimulation was essentially continuous. Such 'continuous' hippocampal stimulation (CHS) led to a self-sustaining limbic status epilepticus (SSLSE) which was centered in the hippocampus 16. Additional acute electrophysiological experiments revealed that CHS resulted in a greater diminution of GABAergic inhibition than after RRHS 13. This suggested that a deterioration of GABAergic inhibition beyond a certain level might be a key step in the development of spontaneous epileptiform activity that marked the acute state of CHS-induced SSLSE. However, the question remained of how long afterwards the disturbance of inhibition persisted. In addition, we found chronic pathologic changes after CHS-induced SSLSE that resembled hippocampal sclerosis, a condition associated with limbic (temporal lobe) seizures in humans 3. The experiments described below had 2 major goals. First, we examined chronic functional alterations after CHS-induced SSLSE. In particular, we carried out electrophysiological recordings of both paired pulse inhibition in vivo and of responses in hippocampal slices maintained in vitro. The experiments were designed to provide direct comparisons with data obtained from rats kindled with RRHS (see above) 11'12. Second, we report that recurrent spontaneous seizures occur as a late consequence of CHS-induced SSLSE~ Features of the condition with spontaneous seizures make it a
useful model of chronic temporal lobe epilepsy encountered in humans. METHODS
Animal preparation Methods employed in these experiments are described in detail elsewhere 11,t2'14A6,34-36. Briefly, while animals were anesthetized with ketamine/ xylazine, a bipolar electrode was stereotaxically positioned in one ventral hippocampus (AP-3.6, ML 4.9, D V - 5 . 0 to dura; incisor bar +5.0) and attached to the skull. An electronic switch was used both to stimulate and record from the same electrode. The animals recovered from anesthesia and I week later afterdischarge thresholds were determined as described elsewhere 2°. Only animals with afterdischarge thresholds
110
Elsevier EPIRES 00324
Recurrent spontaneous hippocampal seizures in the rat as a chronic sequela to limbic status epilepticus
Eric W. L o t h m a n a, E d w a r d H. B e r t r a m a, Jaideep K a p u r b and Janet L. Stringer a "Department of Neurology, Universityof Virginia School of Medicine, Charlottesville, VA 22908 (U.S.A.), and t'Department of Neurology, Medical College of Virginia, Richmond, VA 23298 (U.S. A.) (Received 17 November 1989; accepted 21 November 1989)
Key words: Epilepsy; Inhibition; Chronic seizures; Status epilepticus; Hippoeampus; Slices
A period of continuous hippocampal stimulation (CHS) establishes an acute condition of self-sustaining limbic status epilepticus (SSLSE) which is followed by chronic neuropathological changes reminiscent of hippocampal sclerosis encountered in epileptic patients. In the chronic (31 month) condition following CHS-induced SSLSE, extended electrographic monitoring in the hippocampus revealed spontaneous recurrent paroxysmal discharges. All 6 animals studied had persistent interictal spiking; 3 had multiple fully developed electrographic seizures. There was a marked diminution of paired pulse inhibition, demonstrated by a protocol known to refleet the potency of inhibition mediated by GABA A receptors. Hippocampal slices from animals that had previously experienced CHS-induced SSLSE demonstrated an increased excitability relative to slices from control animals as evidenced by epileptiform bursting in increased extracellular potassium ([K +]0) and decreased extracellular calcium ([Ca2+]0). These studies establish that CHS-induced SSLSE in rats provides all experimental model with recurrent spontaneous hippocampal seizures. Based oft electrophysiologi¢al data we suggest that a decrease in GABA-mediated inhibition and/or altered sensitivity to extracellular ions may play roles in the development of such seizures.
INTRODUCTION When a low intensity electrical stimulus train is repetitively given to particular limbic brain structures, the epileptiform responses elicited by such stimuli progressively intensify until stereotyped convulsions (so-called kindled motor seizure) appear m'2+'29. Once established, this enhanced responsiveness is long-lasting, perhaps even permanent. Based on these observations, it can be argued that kindled motor seizures are an experiCorrespondence to: Eric W. Lothman, M.D., Ph.D., Department of Neurology - - Box 394, University of Virginia Medical Center, Charlottesville, VA 22908, U.S.A.
mental counterpart of limbic seizures with secondary generalization which occur in humans. However, the kindled state in which seizures remain stimulus-locked, appearing only when electrical stimuli are provided, differs from naturally occurring epilepsy in humans which is characterized by recurrent spontaneous seizures. It has been observed that some kindled rats developed spontaneous seizures, but only after several months of stimulation zT. Over the past several years, we have examined the changes brought about by rapidly recurring hippocampal seizures (RRHS). We showed that kindling could be induced with RRHS over a matter of days as opposed to the weeks required with
0920-1211/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)
111 traditional kindling ~9-21. Subsequently, we found that a deterioration of GABAergic inhibition consistently accompanied kindled responses engendered by RRHS 11'12'14. This deterioration developed quickly, over the same time course as rapid kindling of electrographic and behavioral responses, and persisted for at least 1 month after the last kindled seizures. Moreover, slices isolated from animals previously kindled with RRHS and maintained in vitro were shown to be hyperexcitable as. They generated epileptiform discharges with smaller perturbations of the concentrations of extracellular potassium ([K ÷]0) or of extracellular calcium ([Ca2÷]0) than was required with normal tissue. We have also examined the consequences of providing the same stimulus trains used for RRHS, but decreasing the time between stimuli so that stimulation was essentially continuous. Such 'continuous' hippocampal stimulation (CHS) led to a self-sustaining limbic status epilepticus (SSLSE) which was centered in the hippocampus 16. Additional acute electrophysiological experiments revealed that CHS resulted in a greater diminution of GABAergic inhibition than after RRHS 13. This suggested that a deterioration of GABAergic inhibition beyond a certain level might be a key step in the development of spontaneous epileptiform activity that marked the acute state of CHS-induced SSLSE. However, the question remained of how long afterwards the disturbance of inhibition persisted. In addition, we found chronic pathologic changes after CHS-induced SSLSE that resembled hippocampal sclerosis, a condition associated with limbic (temporal lobe) seizures in humans 3. The experiments described below had 2 major goals. First, we examined chronic functional alterations after CHS-induced SSLSE. In particular, we carried out electrophysiological recordings of both paired pulse inhibition in vivo and of responses in hippocampal slices maintained in vitro. The experiments were designed to provide direct comparisons with data obtained from rats kindled with RRHS (see above) 11'12. Second, we report that recurrent spontaneous seizures occur as a late consequence of CHS-induced SSLSE~ Features of the condition with spontaneous seizures make it a
useful model of chronic temporal lobe epilepsy encountered in humans. METHODS
Animal preparation Methods employed in these experiments are described in detail elsewhere 11,t2'14A6,34-36. Briefly, while animals were anesthetized with ketamine/ xylazine, a bipolar electrode was stereotaxically positioned in one ventral hippocampus (AP-3.6, ML 4.9, D V - 5 . 0 to dura; incisor bar +5.0) and attached to the skull. An electronic switch was used both to stimulate and record from the same electrode. The animals recovered from anesthesia and I week later afterdischarge thresholds were determined as described elsewhere 2°. Only animals with afterdischarge thresholds
