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Electroencephalography, clinical seizures, and neuronal involvement in experimental encephalitis
/ Epilrpsy 1989;2:103-108 0 1989 Demos Publications
Electroencephalography, Clinical Seizures, and Neuronal Involvement in Experimental Encephalitis *Michael Schlitt, ‘Anita Bucher, ‘Ken Pilgreen, 3Kent Sauter, and 3Robert Chronister
EEG and clinical seizures in an animal model of herpes simplex encephalitis are presented. After implantation of AM radio transmitters, designed for EEG telemetry, baseline recordings from the rabbits showed preinoculation rhythms in the alpha and beta ranges. The animals were then given a focal encephalitis by inoculation of the olfactory bulb with herpes simplex virus, type 1. Three clinical seizure types were demonstrated and consisted of syndromes analogous to human partial simple, partial complex, and generalized (tonic) seizures. EEG tracings showed that partial complex seizure patterns, manifested clinically as stereotypic circling episodes, were associated with 15-Hz beta rhythms that progressed to a 1.5-Hz delta rhythm with spikes after several circuits, and finally to even slower, polymorphic, I-Hz delta activity that coincided with the end of the seizure. Episodes of rhythmic facial movements and bruxism were associated with different ~-HZ, high-amplitude EEG tracings. Tonic episodes (with some clonic activity occurring afterward) were characterized by 25-Hz rhythms alternating with activity in the 5-lo-Hz range. lnterictal tracings in animals noted to have clinical seizures showed nearly continuous rhythmic theta activity (3-8 Hz) not
associated with any obvious clinical manifestations. All animals had a viral encephalitis by histologic and immunohistochemical criteria. Key Words: Electroencephalography-Herpes
Despite
intense
interest
simplex
in the medical
manage-
ment of seizure disorders for over 100 years, the basic mechanisms that precipitate these disordered electrical discharges of the brain remain largely unknown. In order to investigate these seizure mechanisms, animal model systems have been created that result in clinical and EEG manifestations of seizures (1). However, little has been learned that would relate seizures to brain infections, even though such a relationship is quite common in clinical neurology.
From the Departments and 3Anatomy, University ter, Mobile, AL, U.S.A.
of ‘Neurosurgery, of South Alabama
*Neurology, Medical Cen-
Address correspondence and reprint requests to Dr. M. Schlitt, Department of Neurosurgery, 714 Mastin, 2451 Fillingim Street, Mobile, AL 36617, U.S.A.
virus-Encephalitis-Seizures.
Herpes simplex encephalitis is a brain infection that is regularly complicated by seizures (2). The poor understanding of the pathophysiology of this infection led to the simultaneous development of two animal models of brain disease caused by herpes simplex virus type 1 (HSV) (3,4). Both of these models were predicated upon infection of the olfactory system. Further work with one of these models recently defined three different clinical behaviors felt to represent seizures. In this model, seizures resulted in 100% mortality in experimental animals and were partially ameliorated by the administration of anticonvulsants (5). This study further defines the seizures resulting in rabbits who have been given an experimental focal encephalitis due to HSV. Three major seizure categories-simple partial, complex partial, and general1 EPILEPSY, VOL. 2, NO. 2, 1989
103
M SCHLIlT
ET AL.
ized (tonic) seizures-in by this modeI.
humans are recapitulated
Methods Five- to 9-pound New Zealand white rabbits of either sex were used. Anesthesia was accomplished with 50 mg xylazine, followed after 5 min with 75 mg ketamine, both given intramuscularly. Animals underwent implantation of AM radio transmitters designed for EEG telemetry (DSI Incorporated, Roseville, MN). This transmitter was connected to the epidural space via insulated stainless steel wires. Approximately 2 mm of wire were left exposed and were inserted through small craniectomies in the parietal portion of the rabbit skull. These wires were secured to the skull with cyanoacrylate glue. Strict aseptic technique was observed, and animals were allowed to recover from the anesthetic, Figure 1 shows the relative locations of the small craniectomies for electrode implantation and viral inoculation. Sample EEG tracings were then obtained from the animals. The EEG signal was received by a matched AM receiver in the rabbit cage and was processed through a Tektronix model 545B oscilloscope. Recordings were made on a Grass model 6A electro-
encephalograph. After baseline recordings had been obtained, animals were reanesthetized, and the left olfactory bulb was exposed, using a constant small artery in the rabbit pericrania as a guide (6). The olfactory bulb was then inoculated with 0.1 ml of a wild type of HSV utilized by this laboratory. This volume contained approximately 10,000 plaque-forming units of HSV. The rabbits then underwent continuous EEG monitoring and videotaping. When seizures occurred, animals were euthanized; the brains were removed and placed in 4% buffered paraformaldehyde for fixation. Sagittal sections were taken through the area of the brain ventral to the rhinal sulcus for confirmation of viral encephalitis. Sections were cut at 50 pm on a vibratome and rinsed well in PBS. The sections were then incubated in 10% goat serum and 1% BSA-RIA in PBS for 1 h. The sections were then incubated in a monoclonal antibody overnight at 4” C. The sections were transferred to goat anti-mouse (F(ab’)2 antibodies that were biotinylated (Pel-Freez Clinical Systems), and then they were incubated in acetylated avidin-biotinperoxidase solution (Pel-Freez Clinical Systems). The peroxidase was then demonstrated with standard solutions of diaminobenzidine.
Results Six animals were inoculated; satisfactory EEG and videotape records were obtained from four. Five of the six animals showed seizure activity and were euthanized. The sixth showed no early seizures and continues to be monitored for the development of seizures occurring late after infection.
Videotape Analysis Three seizure patterns were again identified (5). The first, and most common, in this set of experiments was a perioral movement resembling chewing, sometimes associated with bruxism. This could be present for long periods of time. These movements occurred at a rate of 2/s. The animal was difficult to arouse or stimulate during these chewing movements. The second most common event was a circling
Figure 1. Dorsal aspect of a rabbit skull with crmiectomies at the entrance of the EEG leads and ouer the olfactory bulb for injection of HSV-1.
104
J EPILEPSY, VOL. 2, NO. 2, 1989
Figure 2. EEG from a rabbit showingfacial simple seizure. Bar = 2 s.
seizures--a partial
EEG IN EXPERIMENTAL
Figure 3. EEGfrotn a rabbit durirrgaduersive behavior, followed by clockwise circuifs of the cage, shows a partial complex seizure. Numbers irldicate begimirrg of each circuit; bar = 2 s.
behavior. Animals would stiffen and look in one direction and then perform three to eight circuits of the cage. Each of these circuits would take from 3 to 8 s and would be in a consistent direction for the animal under consideration. The animal could not be dissuaded from completing these circuits, even with stimulation. The third seizure pattern observed was generalized tonic activity. The animal would stiffen and sit on his haunches and then fall to one side or the other. This activity was occasionally associated with twitching of perioral and/or extremity musculature after the tonic phase. The tonic phase would last up to 30 s; the clonic phase would last up to 15 s.
EEG Findings Perioral movements and bruxism are associated with high-voltage, ~-HZ rhythmic tracings. These
ENCEPHALITIS
rhythms occurred throughout the facial movements and were synchronous with them (Fig. 2). Circling behaviors began with loss of voltage and slowing of the rhythm to approximately 2 Hz; during these rhythms, the animals would demonstrate adversive activity. The beginning of the circuiting behavior was marked by high-voltage, 15-Hz activity that gradually slowed as the circuits took progressively longer to complete. Circuits would cease soon after a 1.5-Hz delta rhythm with spikes was noted, and the completion of circuiting behavior would correspond with the onset of a slower, polymorphic, l-Hz delta wave, after which the animal would return to more normal activities (Fig. 3). Tonic seizures were associated with slowing and synchronization of the EEG followed by 25-Hz, highvoltage activity, alternating toward the end of the tonic phase with 5-lo-Hz activity. The end of this seizure pattern was not marked by the same delta activity as the aforementioned circling episodes (Fig. 4). The clonic activity noted corresponded with the slower, 5-lo-Hz activity. Interictal tracings in animals with seizure disorders were marked by a nearly continuous rhythmic theta activity not associated with any clinical manifestations.
Histologic Findings All animals studied histologically were found to have round cell infiltrates and perivascular cuffing, identical to that seen in human herpes simplex encephalitis. Again, the areas most heavily involved by HSV were inferomedial to the rhinal sulcus, but by immunocytochemical techniques, discrete areas of
Figure 4. EEG during a tonic seizure-a getleralized seizure. Bar = 2 s.
1 EPILEPSY, VOL 2, NO. 2, 1989
105
M. SCHLITT ET AL.
Figure 5. Phototnicrograph ofmidsugittal section ofun infected rabbit, shozoirzg clusters ofirrfected celk in the citzgulate cortex. Irnrnutm pero.xidase uguimt HSV. Muguificutiorz X 40.
viral infection were found in other limbic structures, such as the cingulate gyrus (Fig. 5) (7). Higher-power examination of such areas shows that essentially only large neurons are involved (Fig. 6). Further study of these animals has shown that HSV can also involve the magnocellular regions of the basal forebrain, such as the nucleus basalis, scattered cells of the diagonal band of Broca, and large cells in the ventral pallidum. In addition, the large cells of the islets of Calleja [particularly those in the magnum islet, which is presumably cholinergic (S)] also stain for HSV antigen (Fig. 7).
Figure 6. High-powered view of cells in cingulute cortex shows lurger-sized neurons stained for HSV antigen. Magnification X 400.
106
J EPILEPSY, VOL. 2, NO. 2, 1989
Discussion Animal models of epilepsy have existed since early in this century and have involved trauma to the brain, repeated electrical stimulation, application of noxious chemicals, or injections of neurotransmitter analogs. However, aside from the model described here, there are no currently used models of epilepsy that are caused by a biologic agent. Although seizures have been documented by other investigators after infection of the rabbit brain by HSV, the route of inoculation utilized was unlikely to cause a focal
EEG IN EXPERIMENTAL
Figure 7. Photomict .O& tnugnum islet of Cdleja, rons dernonstrutit~g HS:; Mugrzificntion X 400.
ENCEPHALlTlS
..
‘7 : ..
encephalitis, and different types of seizures were not described in detail for animals that experienced seizures (9). It is of note that similar EEGs to those in this report were obtained in that study. This model is particularly important in that little is known regarding the pathogenesis of seizure disorders in patients with encephalitis. Much of the experimental work in epilepsy over the last 10 years has focused on smaller (-5-10 pm) y-aminobutyric acidstaining (GABAergic) neurons, in systems not caused by biologic agents (10). In this model, however, in animals experiencing seizures, it is the larger (-1530 pm) neurons-which usually stain for choline acetyltransferase and/or glutamate-that show productive infection of HSV-1 (11,12). If larger, longprojection neurons are responsible for seizures in viral brain infection, rather than GABAergic neurons, then perhaps the design of both anticonvulsant and antiviral medications should be refocused. The three different seizure types are of interest not only due to their diversity but also because all three types can be caused by a standardized model system. This simplifies the production of a seizure model, but it is admitted that this approach further complicates division of seizure disorders into their separate groups. Perhaps the fact that rabbits’brains are much more simple than the human brain accounts for some of the ease of causing multiple seizure types by one model system technique. However, the possibility that different seizure types have a common origin at the cellular level must be kept in mind.
The stepwise progression of the infection in this system was documented in its initial description, and it is of note that all animals in this experiment destined to have seizures had them occurring between the fourth and seventh days of inoculation. This regularity in producing a seizure syndrome will allow for delineation of progressive cellular change during the course of the infection. In particular, progressive synaptic change-thought to be important in the genesis of seizures-can be easily studied in the olfactory system of the rabbit. There may be correlations between the seizures described in this mode1 and human seizure types. The seizures consisting of perioral twitching and bruxism, which were associated with ~-HZ rhythmic activity, may relate to the rhythmic slowing seen in partial motor seizures. Adversive seizures can show widespread rhythmic slowing, as was seen in these animals prior to their circuiting the cage. The cell biological and physiological mechanisms underlying these seizures are being investigated in this laboratory currently. The relationship of the generalized seizures in this model compared to those seen in humans is apparent.
References 1. Walker AE. The past four decades-experimental epilepsy. Res Pub1 Assoc Res Nerv Ment Dis 1983;61:1-17. 2. Whitley RJ, Soong SJ, Linneman C, Liu C, Pazin G, Alford CA. NIAID Collaborative Antiviral Study Group. Herpes simplex encephalitis. ]AMA 1982;247:317-20. J EPILEPSY, VOL. 2, NO. 2, 1989
107
M. SCHLllT
ET AL.
3. Schlitt M, bkeman AD, Wilson ER, To A, Acoff RW, Harsh GR, Whitley RJ. A rabbit model of focal herpes simplex encephalitis. J Itrfecf Dis 1986;153;732-5. 4. Stroop WC, Schaefer DC. Production of encephalitis restricted to the temporal lobes by experimental reactivation of herpes simplex virus. J Infect Dis 1986; 153:721-31. 5. Schlitt M, Bucher Al’, Stroop WG, Pindak F, Bastian FO, Jennings RA, Lakeman AD, Whitley RJ. Neurovirulence in an experimental focal herpes encephalitis: relationship to observed seizures. Brairl Res 1988;440: 293-8. 6. Schlitt M, Bucher A?. Herpes simplex encephalitis in the rabbit. Comp Puthol Bull 1987;19:24. 7. Bucher Al’, Quindlen EA, Schlitt M. Black and white photography of horse-radish perioxidase-stained histological slides: technical note. Biotechniques 1989;6: 945-6. 8. Chronister RB, Sikes RW, Trow TW, DeFrance JF. The organization of nucleus accumbens. In: Chronister RB, DeFrance JR, eds. The tleurobiology of the nucleus accum-
108
J EPILEPSY, VOL. 2, NO. 2, 1989
9.
10.
11. 12.
beus. Brunswick: Haer Electrophysiological Institute, 1981:97-146. Griffith MD, Kibrick S, Dodge PR, Richardson El’. Experimental herpes simplex encephalitis. Electroencephalographic, clinical virologic, and pathologic observations in the rabbit. Electroellcepllalogr Clirl Neurophysiol 1967;23:263-9. Roberts E. Failure of GABAergic inhibition: a key to local and global seizures. In: Delgado-Escueta AV, Ward AA Jr, Woodbury DM, Porter RJ, eds. Basic mechanisnls of the epilepsies: molecular arld cellular approaches. New York: Raven Press, 1986:319%41. (Advances in neurology; vol 44.) Bak IJ, Markham CH, Cook ML, Stevens JG. Intraaxonal transport of herpes simplex virus in the rat central nervous system. Bruirl Res 1977;136:415-29. Anderson JR, Field HJ. The distribution of herpes simplex type 1 antigen in mouse central nervous system after different routes of inoculation. / Nrurol Sci 1983; 60:181-95.
Electroencephalography, Clinical Seizures, and Neuronal Involvement in Experimental Encephalitis *Michael Schlitt, ‘Anita Bucher, ‘Ken Pilgreen, 3Kent Sauter, and 3Robert Chronister
EEG and clinical seizures in an animal model of herpes simplex encephalitis are presented. After implantation of AM radio transmitters, designed for EEG telemetry, baseline recordings from the rabbits showed preinoculation rhythms in the alpha and beta ranges. The animals were then given a focal encephalitis by inoculation of the olfactory bulb with herpes simplex virus, type 1. Three clinical seizure types were demonstrated and consisted of syndromes analogous to human partial simple, partial complex, and generalized (tonic) seizures. EEG tracings showed that partial complex seizure patterns, manifested clinically as stereotypic circling episodes, were associated with 15-Hz beta rhythms that progressed to a 1.5-Hz delta rhythm with spikes after several circuits, and finally to even slower, polymorphic, I-Hz delta activity that coincided with the end of the seizure. Episodes of rhythmic facial movements and bruxism were associated with different ~-HZ, high-amplitude EEG tracings. Tonic episodes (with some clonic activity occurring afterward) were characterized by 25-Hz rhythms alternating with activity in the 5-lo-Hz range. lnterictal tracings in animals noted to have clinical seizures showed nearly continuous rhythmic theta activity (3-8 Hz) not
associated with any obvious clinical manifestations. All animals had a viral encephalitis by histologic and immunohistochemical criteria. Key Words: Electroencephalography-Herpes
Despite
intense
interest
simplex
in the medical
manage-
ment of seizure disorders for over 100 years, the basic mechanisms that precipitate these disordered electrical discharges of the brain remain largely unknown. In order to investigate these seizure mechanisms, animal model systems have been created that result in clinical and EEG manifestations of seizures (1). However, little has been learned that would relate seizures to brain infections, even though such a relationship is quite common in clinical neurology.
From the Departments and 3Anatomy, University ter, Mobile, AL, U.S.A.
of ‘Neurosurgery, of South Alabama
*Neurology, Medical Cen-
Address correspondence and reprint requests to Dr. M. Schlitt, Department of Neurosurgery, 714 Mastin, 2451 Fillingim Street, Mobile, AL 36617, U.S.A.
virus-Encephalitis-Seizures.
Herpes simplex encephalitis is a brain infection that is regularly complicated by seizures (2). The poor understanding of the pathophysiology of this infection led to the simultaneous development of two animal models of brain disease caused by herpes simplex virus type 1 (HSV) (3,4). Both of these models were predicated upon infection of the olfactory system. Further work with one of these models recently defined three different clinical behaviors felt to represent seizures. In this model, seizures resulted in 100% mortality in experimental animals and were partially ameliorated by the administration of anticonvulsants (5). This study further defines the seizures resulting in rabbits who have been given an experimental focal encephalitis due to HSV. Three major seizure categories-simple partial, complex partial, and general1 EPILEPSY, VOL. 2, NO. 2, 1989
103
M SCHLIlT
ET AL.
ized (tonic) seizures-in by this modeI.
humans are recapitulated
Methods Five- to 9-pound New Zealand white rabbits of either sex were used. Anesthesia was accomplished with 50 mg xylazine, followed after 5 min with 75 mg ketamine, both given intramuscularly. Animals underwent implantation of AM radio transmitters designed for EEG telemetry (DSI Incorporated, Roseville, MN). This transmitter was connected to the epidural space via insulated stainless steel wires. Approximately 2 mm of wire were left exposed and were inserted through small craniectomies in the parietal portion of the rabbit skull. These wires were secured to the skull with cyanoacrylate glue. Strict aseptic technique was observed, and animals were allowed to recover from the anesthetic, Figure 1 shows the relative locations of the small craniectomies for electrode implantation and viral inoculation. Sample EEG tracings were then obtained from the animals. The EEG signal was received by a matched AM receiver in the rabbit cage and was processed through a Tektronix model 545B oscilloscope. Recordings were made on a Grass model 6A electro-
encephalograph. After baseline recordings had been obtained, animals were reanesthetized, and the left olfactory bulb was exposed, using a constant small artery in the rabbit pericrania as a guide (6). The olfactory bulb was then inoculated with 0.1 ml of a wild type of HSV utilized by this laboratory. This volume contained approximately 10,000 plaque-forming units of HSV. The rabbits then underwent continuous EEG monitoring and videotaping. When seizures occurred, animals were euthanized; the brains were removed and placed in 4% buffered paraformaldehyde for fixation. Sagittal sections were taken through the area of the brain ventral to the rhinal sulcus for confirmation of viral encephalitis. Sections were cut at 50 pm on a vibratome and rinsed well in PBS. The sections were then incubated in 10% goat serum and 1% BSA-RIA in PBS for 1 h. The sections were then incubated in a monoclonal antibody overnight at 4” C. The sections were transferred to goat anti-mouse (F(ab’)2 antibodies that were biotinylated (Pel-Freez Clinical Systems), and then they were incubated in acetylated avidin-biotinperoxidase solution (Pel-Freez Clinical Systems). The peroxidase was then demonstrated with standard solutions of diaminobenzidine.
Results Six animals were inoculated; satisfactory EEG and videotape records were obtained from four. Five of the six animals showed seizure activity and were euthanized. The sixth showed no early seizures and continues to be monitored for the development of seizures occurring late after infection.
Videotape Analysis Three seizure patterns were again identified (5). The first, and most common, in this set of experiments was a perioral movement resembling chewing, sometimes associated with bruxism. This could be present for long periods of time. These movements occurred at a rate of 2/s. The animal was difficult to arouse or stimulate during these chewing movements. The second most common event was a circling
Figure 1. Dorsal aspect of a rabbit skull with crmiectomies at the entrance of the EEG leads and ouer the olfactory bulb for injection of HSV-1.
104
J EPILEPSY, VOL. 2, NO. 2, 1989
Figure 2. EEG from a rabbit showingfacial simple seizure. Bar = 2 s.
seizures--a partial
EEG IN EXPERIMENTAL
Figure 3. EEGfrotn a rabbit durirrgaduersive behavior, followed by clockwise circuifs of the cage, shows a partial complex seizure. Numbers irldicate begimirrg of each circuit; bar = 2 s.
behavior. Animals would stiffen and look in one direction and then perform three to eight circuits of the cage. Each of these circuits would take from 3 to 8 s and would be in a consistent direction for the animal under consideration. The animal could not be dissuaded from completing these circuits, even with stimulation. The third seizure pattern observed was generalized tonic activity. The animal would stiffen and sit on his haunches and then fall to one side or the other. This activity was occasionally associated with twitching of perioral and/or extremity musculature after the tonic phase. The tonic phase would last up to 30 s; the clonic phase would last up to 15 s.
EEG Findings Perioral movements and bruxism are associated with high-voltage, ~-HZ rhythmic tracings. These
ENCEPHALITIS
rhythms occurred throughout the facial movements and were synchronous with them (Fig. 2). Circling behaviors began with loss of voltage and slowing of the rhythm to approximately 2 Hz; during these rhythms, the animals would demonstrate adversive activity. The beginning of the circuiting behavior was marked by high-voltage, 15-Hz activity that gradually slowed as the circuits took progressively longer to complete. Circuits would cease soon after a 1.5-Hz delta rhythm with spikes was noted, and the completion of circuiting behavior would correspond with the onset of a slower, polymorphic, l-Hz delta wave, after which the animal would return to more normal activities (Fig. 3). Tonic seizures were associated with slowing and synchronization of the EEG followed by 25-Hz, highvoltage activity, alternating toward the end of the tonic phase with 5-lo-Hz activity. The end of this seizure pattern was not marked by the same delta activity as the aforementioned circling episodes (Fig. 4). The clonic activity noted corresponded with the slower, 5-lo-Hz activity. Interictal tracings in animals with seizure disorders were marked by a nearly continuous rhythmic theta activity not associated with any clinical manifestations.
Histologic Findings All animals studied histologically were found to have round cell infiltrates and perivascular cuffing, identical to that seen in human herpes simplex encephalitis. Again, the areas most heavily involved by HSV were inferomedial to the rhinal sulcus, but by immunocytochemical techniques, discrete areas of
Figure 4. EEG during a tonic seizure-a getleralized seizure. Bar = 2 s.
1 EPILEPSY, VOL 2, NO. 2, 1989
105
M. SCHLITT ET AL.
Figure 5. Phototnicrograph ofmidsugittal section ofun infected rabbit, shozoirzg clusters ofirrfected celk in the citzgulate cortex. Irnrnutm pero.xidase uguimt HSV. Muguificutiorz X 40.
viral infection were found in other limbic structures, such as the cingulate gyrus (Fig. 5) (7). Higher-power examination of such areas shows that essentially only large neurons are involved (Fig. 6). Further study of these animals has shown that HSV can also involve the magnocellular regions of the basal forebrain, such as the nucleus basalis, scattered cells of the diagonal band of Broca, and large cells in the ventral pallidum. In addition, the large cells of the islets of Calleja [particularly those in the magnum islet, which is presumably cholinergic (S)] also stain for HSV antigen (Fig. 7).
Figure 6. High-powered view of cells in cingulute cortex shows lurger-sized neurons stained for HSV antigen. Magnification X 400.
106
J EPILEPSY, VOL. 2, NO. 2, 1989
Discussion Animal models of epilepsy have existed since early in this century and have involved trauma to the brain, repeated electrical stimulation, application of noxious chemicals, or injections of neurotransmitter analogs. However, aside from the model described here, there are no currently used models of epilepsy that are caused by a biologic agent. Although seizures have been documented by other investigators after infection of the rabbit brain by HSV, the route of inoculation utilized was unlikely to cause a focal
EEG IN EXPERIMENTAL
Figure 7. Photomict .O& tnugnum islet of Cdleja, rons dernonstrutit~g HS:; Mugrzificntion X 400.
ENCEPHALlTlS
..
‘7 : ..
encephalitis, and different types of seizures were not described in detail for animals that experienced seizures (9). It is of note that similar EEGs to those in this report were obtained in that study. This model is particularly important in that little is known regarding the pathogenesis of seizure disorders in patients with encephalitis. Much of the experimental work in epilepsy over the last 10 years has focused on smaller (-5-10 pm) y-aminobutyric acidstaining (GABAergic) neurons, in systems not caused by biologic agents (10). In this model, however, in animals experiencing seizures, it is the larger (-1530 pm) neurons-which usually stain for choline acetyltransferase and/or glutamate-that show productive infection of HSV-1 (11,12). If larger, longprojection neurons are responsible for seizures in viral brain infection, rather than GABAergic neurons, then perhaps the design of both anticonvulsant and antiviral medications should be refocused. The three different seizure types are of interest not only due to their diversity but also because all three types can be caused by a standardized model system. This simplifies the production of a seizure model, but it is admitted that this approach further complicates division of seizure disorders into their separate groups. Perhaps the fact that rabbits’brains are much more simple than the human brain accounts for some of the ease of causing multiple seizure types by one model system technique. However, the possibility that different seizure types have a common origin at the cellular level must be kept in mind.
The stepwise progression of the infection in this system was documented in its initial description, and it is of note that all animals in this experiment destined to have seizures had them occurring between the fourth and seventh days of inoculation. This regularity in producing a seizure syndrome will allow for delineation of progressive cellular change during the course of the infection. In particular, progressive synaptic change-thought to be important in the genesis of seizures-can be easily studied in the olfactory system of the rabbit. There may be correlations between the seizures described in this mode1 and human seizure types. The seizures consisting of perioral twitching and bruxism, which were associated with ~-HZ rhythmic activity, may relate to the rhythmic slowing seen in partial motor seizures. Adversive seizures can show widespread rhythmic slowing, as was seen in these animals prior to their circuiting the cage. The cell biological and physiological mechanisms underlying these seizures are being investigated in this laboratory currently. The relationship of the generalized seizures in this model compared to those seen in humans is apparent.
References 1. Walker AE. The past four decades-experimental epilepsy. Res Pub1 Assoc Res Nerv Ment Dis 1983;61:1-17. 2. Whitley RJ, Soong SJ, Linneman C, Liu C, Pazin G, Alford CA. NIAID Collaborative Antiviral Study Group. Herpes simplex encephalitis. ]AMA 1982;247:317-20. J EPILEPSY, VOL. 2, NO. 2, 1989
107
M. SCHLllT
ET AL.
3. Schlitt M, bkeman AD, Wilson ER, To A, Acoff RW, Harsh GR, Whitley RJ. A rabbit model of focal herpes simplex encephalitis. J Itrfecf Dis 1986;153;732-5. 4. Stroop WC, Schaefer DC. Production of encephalitis restricted to the temporal lobes by experimental reactivation of herpes simplex virus. J Infect Dis 1986; 153:721-31. 5. Schlitt M, Bucher Al’, Stroop WG, Pindak F, Bastian FO, Jennings RA, Lakeman AD, Whitley RJ. Neurovirulence in an experimental focal herpes encephalitis: relationship to observed seizures. Brairl Res 1988;440: 293-8. 6. Schlitt M, Bucher A?. Herpes simplex encephalitis in the rabbit. Comp Puthol Bull 1987;19:24. 7. Bucher Al’, Quindlen EA, Schlitt M. Black and white photography of horse-radish perioxidase-stained histological slides: technical note. Biotechniques 1989;6: 945-6. 8. Chronister RB, Sikes RW, Trow TW, DeFrance JF. The organization of nucleus accumbens. In: Chronister RB, DeFrance JR, eds. The tleurobiology of the nucleus accum-
108
J EPILEPSY, VOL. 2, NO. 2, 1989
9.
10.
11. 12.
beus. Brunswick: Haer Electrophysiological Institute, 1981:97-146. Griffith MD, Kibrick S, Dodge PR, Richardson El’. Experimental herpes simplex encephalitis. Electroencephalographic, clinical virologic, and pathologic observations in the rabbit. Electroellcepllalogr Clirl Neurophysiol 1967;23:263-9. Roberts E. Failure of GABAergic inhibition: a key to local and global seizures. In: Delgado-Escueta AV, Ward AA Jr, Woodbury DM, Porter RJ, eds. Basic mechanisnls of the epilepsies: molecular arld cellular approaches. New York: Raven Press, 1986:319%41. (Advances in neurology; vol 44.) Bak IJ, Markham CH, Cook ML, Stevens JG. Intraaxonal transport of herpes simplex virus in the rat central nervous system. Bruirl Res 1977;136:415-29. Anderson JR, Field HJ. The distribution of herpes simplex type 1 antigen in mouse central nervous system after different routes of inoculation. / Nrurol Sci 1983; 60:181-95.