- Email: [email protected]
Kainic acid-induced perirhinal cortical seizures in rats
Brain Research 800 Ž1998. 323–327
Short communication
Kainic acid-induced perirhinal cortical seizures in rats Shin-ichi Imamura
a, )
, Shigeya Tanaka c , Hideshi Tojo c , Shin-ichiro Fukumoto b , Koichi Uetsuhara c , Jun-ichi Kuratsu a , Morikuni Takigawa b
a b
Department of Neurosurgery, UniÕersity of Kagoshima, Faculty of Medicine, Sakuragaoka 8-35-1, Kagoshima 890, Japan Department of Neuropsychiatry, UniÕersity of Kagoshima, Faculty of Medicine, Sakuragaoka 8-35-1, Kagoshima 890, Japan c Department of Neurosurgery, Kagoshima City Hospital, Kagoshima, Japan Accepted 5 May 1998
Abstract Seizures induced in rats by kainic acid microinjection into the perirhinal cortex were studied electrophysiologically and behaviorally and compared with known features of seizures following kainic acid injection into the amygdala. Unlike amygdalar seizures, perirhinal cortical seizures did not generalize to become limbic seizures but rather spread to sensorimotor cortex to become manifest as motor seizures. Perirhinal cortical seizures also required larger kainic acid doses for provocation and were briefer. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Experimental epilepsy; Kainic acid; Perirhinal cortex; Sensorimotor cortex; Secondary generalization
The exact mechanism of secondary generalization of limbic seizures, originating in the limbic structures contrasted with cortical seizures generated from the neocortex, remains unclarified, especially the pathways involved. Previous studies have indicated that several structures including the substantia nigra w5,19x, claustrum w21x, and substantia innominata w8x are important in such generalization. Recently, significant involvement of the perirhinal cortex ŽPRC., which has fiber connections to such limbic structures as the hippocampus, amygdala, entorhinal cortex, pyriform cortex, and deep prepyriform cortex as well as to the neocortex w6,11,12x, has been postulated w3,4,7x. In the present study, we investigated the electrical and behavioral characteristics of seizures following the injection of the kainic acid ŽKA. into the PRC. Ten male Wistar rats Ž250 to 300 g. underwent a stereotactic operation under pentobarbital anesthesia Ž40 mgrkg i.p... A stainless steel screw was placed in contact with the dura over the left sensorimotor cortex ŽLCx. with an additional screw placed in the frontal sinus as an indifferent electrode. A stainless steel cannula Žouter diameter 0.6 mm. with an inner needle guide Ždiameter, 0.3
)
Corresponding author. Fax: q 81-99-265-4041; [email protected]
E-mail:
0006-8993r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 5 1 3 - 7
mm. was inserted into the left ŽL. PRC Žat coordinates A, 3.8; L, 7.0; and D, y2.0. in preparation for microinjection of KA. Bipolar depth electrodes were placed into the LPRC, the left basolateral nucleus of the amygdala ŽLA; coordinates A, 5.0; L, 5.0; and D, y3.0. and the left dorsal hippocampus ŽLH; coordinates A, 4.0; L, 2.0; and D, 2.5. w13x. All electrodes as well as the cannula were fixed in place with dental cement, and the electrodes were connected to a terminal plug attached to the head. The animals were allowed to recover unrestrained for 7 days after surgery with free access to food and water until the day of the experiment, when they were placed in the recording chamber for behavioral observation and electroencephalographic monitoring. The inner guide was replaced with an injection needle and 1 to 2 m g of KA ŽSigma, St. Louis, MO. dissolved in phosphate-buffered saline solution ŽPBS; 0.2 M at pH 7.4. at a concentration of 2.0 m grm l, was injected into the LPRC Ž n s 6.. The rate of injection was 1.0 m lrmin. In the control group Ž n s 4., 1.0 m l of PBS alone was injected into the LRPC. The above procedures were done under aseptic conditions. All rats were observed electrophysiologically and behaviorally over 7 days, continuously for 3 days and afterwards 6 h a day, after which the animals were perfused with 10% formalin solution under intraperitoneally administered deep pentobarbital anesthesia. After fixation and embedding in
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paraffin, 4 m m coronal sections were cut and processed for microscopic examination including staining with Cresyl violet and Luxol fast blue. In the early period Žup to 30 min. after KA injection into the LPRC, no behavioral changes such as wet-dog shaking were seen. On the electroencephalogram ŽEEG., multiple spike discharges initially appeared in the LPRC
30 to 60 min after KA injection ŽFig. 1a., and thereafter propagated to the LA and LCx ŽFig. 1b.. Interestingly, seizure propagation to the LH was slight and transient ŽFig. 1c,d.. During this period, brief motor manifestations were observed several times, such as facial twitching which sometimes included right-sided tonic facial spasm, or right or bilateral forelimb clonus. No ictal mastication
Fig. 1. Electrical and behavioral changes after KA injection into the LPRC. Ža. EEG 30 min after injection. Multiple spike discharges initially appeared in the LPRC and left amygdala ŽLA.. No behavioral changes were seen. Žb. EEG 32 min after injection. Multiple spike discharges propagated to the LA and LCx. During this period, a brief interval of motor manifestations included such features as facial twitching Žsometimes including tonic spasm of the right side of the face., and right or bilateral forelimb clonus. Mastication, salivation and wet-dog shaking were not seen. Žc. EEG 60 min after injection. Multiple spike discharges propagated to the LA, LCx and LH. However seizure propagation to the LH was slight and transient. Žd. EEG 80 min after injection. Multiple spike discharges propagated to the LCx; seizure propagation to the LA and LH was not seen. Že. EEG 120 min after injection. Seizures had subsided at this period. Aggressive behavior, commonly seen after KA-induced amygdalar seizures, was not observed.
S.-i. Imamura et al.r Brain Research 800 (1998) 323–327
or salivation were seen. Electrographic seizures subsided within 2 h ŽFig. 1e.. Afterward, the rats became normal except for infrequent interictal discharges which were seen for 1 to 2 days. Aggressive behavior, such as that usually seen after KA-induced amygdalar seizures w15,20x, was not observed. In controls, no EEG or behavioral changes were seen. Microscopic examination demonstrated focal necrosis and surrounding gliosis around the tip of the cannula in the LPRC, possibly related to the KA microinjection ŽFig. 2a.. Neuronal damage in the entorhinal and parieto-temporal cortices adjacent to the KA injection site was not seen in all of them. Similarly, in the LH ŽFig. 2b. and LA ŽFig. 2c., pyknosis of neuronal cells was not observed in five rats, and a few pyknosis was found in just one rat. And only the slight gliosis along the path of the depth electrodes was seen. In controls, merely the minimal gliosis along the tracts of the cannula and electrodes was seen. Many previous studies dealing with experimental epilepsy have focused on the amygdala and hippocampus because they are regarded as generating limbic seizures w2,15,17,18,20x. On the other hand, several other structures, such as the substantia nigra w5,19x, claustrum w21x, and substantia innominata w8x are considered to induce secondary generalization of limbic seizures by modulating the functional sites of the primary focus or the pathways of seizure propagation. The PRC, identical to Brodmann area 35 and 36 in man w9,10,12x, is regarded as a distinct relay site which contains fiber connections between several limbic structures Žhippocampus, amygdala, entorhinal cortex, pyriform cortex and deep prepyriform cortex. and the neocortex w6,11,12x. Zola-Morgan et al. w22x have suggested that the PRC might be an important region for relaying information reciprocally between the neocortex and the limbic structures, because destruction of the PRC without injury to the hippocampus causes severe memory impairment in primates. Recently, the PRC itself has been postulated to be a generator of some seizure activity w9–11,14x. In a kindling model, McIntyre et al. w10x and Sato et al. w14x have described that fully kindled PRC seizures had a very short latency from stimulation to appearance of forelimb clonus indicating that PRC kindling induces behaviorally evident motor rather than limbic seizures. To further investigate PRC seizures, we employed KA, an excito-toxic analogue of glutamate which induces distinctive seizures, when microinjected into various structures, reflecting the function of the injected sites w1,15,17,18,20x. Only 0.6–0.8 m g of KA reportedly can induce stereotyped limbic seizures such as facial twitching, mastication and salivation beginning approximately 10 min after injection into the amygdala w15,18x or the hippocampus w2,17x, with recurrence over 1 to 2 days. In the present study, 1.0 to 2.0 m g of KA injected into the PRC elicited seizures 30 to 60 min after injection. Multiple EEG spike discharges initially appeared in the LPRC, and thereafter
325
propagated to the LA and LCx. Behaviorally, only transient motor manifestations such as facial twitching and forelimb clonus were seen. These seizures completely subsided within 2 h. Interestingly, KA-induced PRC seizures first propagated to the amygdala and the sensorimotor cortex, and then transiently to the hippocampus, in contrast to KA-induced amygdalar seizures which initially remained in limbic structures and extended to the sensorimotor cortex 1 to 2 h later w15,20x. These findings suggest that the epileptic discharges in the PRC preferentially propagate to the sensorimotor cortex and amygdala rather than to the hippocampus, despite anatomic fiber connections between the PRC and hippocampus, both directly from the PRC to the CA3 region, and indirectly via a PRC–amygdala–entorhinal cortex–CA3 circuit w11,12x. Apparently, intense and continuous seizures may be required to involve these pathways. In addition, McIntyre et al. w11x recently observed that the PRC has reciprocal fibers strongly projecting to the frontal cortex rather than to the hippocampus in the rat. Therefore, it seems reasonable that the ictal discharges in the PRC propagate preferentially to the sensorimotor cortex rather than to the hippocampus. Holmes et al. w7x pointed out that infusion of N-methylD-aspartic acid ŽNMDA. antagonist into the PRC suppressed development of amygdalar kindling. Furthermore, Fukumoto et al. w3,4x reported that lesioning the PRC reduced secondary generalization in a KA-induced limbic seizure model. These results suggest that the PRC might be a relay station, and have access to a preferential pathway from the limbic system to the neocortex. Distinctive behavioral symptoms, such as right facial twitching Žsometimes with tonic spasm. and right or bilateral forelimb clonus, are most likely induced by ictal activities of the sensorimotor cortex w9,11,14,15,20x. On the other hand, lack of behavior such as wet-dog shaking, mastication, and salivation, presumably resulting from the activation of limbic structures w15,20x, suggested that electrical activities in the LH and LA were insufficient to result in these manifestations. Concerning the neuronal damage in the KA-induced amygdalar seizures, Tanaka et al. w16x determined local cerebral glucose utilization ŽLCGU. and local cerebral blood flow ŽLCBF. by autoradiography. Their findings reflected neuronal damage in several regions, especially in the CA3 region of the ipsilateral hippocampus, due to remarkable uncoupling of LCGU and LCBF in those areas. In the present study, microscopic examination demonstrated no or very slight neuronal injury in the amygdala and hippocampus, which is compatible with the EEG findings. The increase in LCGU may not have been high enough to cause neuronal injury, since propagation of the seizure discharges to these regions, especially the hippocampus, was slight and transient. In summary, KA-induced PRC seizures contrasted with amygdalar seizures in several respects: a larger dose of KA
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Fig. 2. Microscopic findings, 7 days after KA injection into the LPRC. Ža. LPRC demonstrating focal necrosis and surrounding gliosis around the tip of the cannula in the LPRC. Neuronal damage in the entorhinal and parieto-temporal cortices adjacent to the KA-injected site was not observed. ECx, entorhinal cortex; RF, rhinal fissure. Žb. LA demonstrating no neuronal damage such as pyknosis. Žc. The CA3 region of the left hippocampus demonstrates no pyknotic pyramidal cells. Cresyl violet and Luxol fast blue stain. ŽOriginal Magnification in Ža. =40; Žb. =100; Žc. =100..
S.-i. Imamura et al.r Brain Research 800 (1998) 323–327
was needed to elicit seizures; the time to onset of seizure was longer; seizure duration was markedly shorter; PRC spike discharges initially propagated to the amygdala and sensorimotor cortex; the hippocampus was less involved in seizures; the seizure subsided shortly after KA injection; and neuronal injury in the hippocampus and amygdala was minimal. From the results of the present and previous studies w3,4,6,7,9,10,14 x, it would appear that the PRC might relay the limbic seizures to the sensorimotor cortex, and also that PRC itself could generate the seizures rapidly extending to the sensorimotor cortex rather than to the limbic structures resulting in motor manifestations. Further examination will be required to test and refine this notion.
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w9x
w10x w11x
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Acknowledgements The authors would like to thank and acknowledge Kenji Kato MD ŽSecond Department of Anatomy, University of Kagoshima, Faculty of Medicine. for technical assistance.
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References w17x w1x T. Araki, T. Tanaka, S. Tanaka, Y. Yonemasu, M. Kato, I. Goto, Kainic acid-induced thalamic seizure in cats—a possible model of petit mal seizure, Brain Res. 13 Ž1992. 223–229. w2x E.A. Cavalheiro, D.A. Riche, G. Le Gal La Salle, Long-term effects of intrahippocampal kainic acid injection in rats: a method for inducing spontaneous recurrent seizures, Electroencephalogr. Clin. Neurophysiol. 53 Ž1982. 581–589. w3x S. Fukumoto, S. Tanaka, H. Tojo, K. Uetsuhara, M. Takigawa, Effect of perirhinal cortex lesion on kainic acid-induced limbic seizures, Epilepsia 37 Ž1996. 95–96, ŽSuppl... w4x S. Fukumoto, S. Tanaka, K. Akaike, H. Tojo, S. Imamura, K. Uetsuhara, M. Takigawa, The role of perirhinal cortex for secondary generalization in kainic acid-induced seizure, Epilepsia ŽSuppl.. Žin press.. w5x K. Gale, Subcortical structures and pathways involved in convulsive seizure generation, J. Clin. Neurophysiol. 9 Ž1992. 264–277. w6x T. Halonen, A. Tortorella, H. Zrebeet, K. Gale, Posterior piriform and perirhinal cortex relay seizures evoked from area tempestas: role of excitatory and inhibitory amino acid receptors, Brain Res. 652 Ž1994. 145–148. w7x K.H. Holmes, D.K. Bilkey, R. Laverty, The infusion of an NMDA
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antagonist into perirhinal cortex suppresses amygdala-kindled seizures, Brain Res. 587 Ž1992. 285–290. M. Kunimoto, S. Tanaka, T. Tanaka, Y. Yonemasu, Ibotenic acidinduced lesions of substantia innominata on focal limbic seizures in chronic cats, Jpn. J. Psychiatry Neurol. 41 Ž1987. 508–509. Y. Matsumoto, N. Yamada, K. Morimoto, D.K. Bilkey, S. Kuroda, Characterization of epileptiform field potentials recorded in the in vitro perirhinal cortex of amygdala-kindled epilepgenesis, Brain Res. 741 Ž1996. 44–51. D.C. McIntyre, M.E. Kelly, J.N. Armstrong, Kindling in the perirhinal cortex, Brain Res. 587 Ž1993. 1–6. D.C. McIntyre, M.E. Kelly, W.A. Staines, Efferent projections of the anterior perirhinal cortex in the rat, J. Comp. Neurol. 369 Ž1996. 302–318. G. Paxinos, The Rat Nervous System, Academic Press, Sydney, 1985. L.J. Pellegrino, A.S. Pellegrino, A.J. Cushman, A Stereotaxic Atlas of the Rat Brain, Plenum, New York, 1979. T. Sato, N. Yamada, Y. Kitamura, K. Otsuki, A. Orita, S. Uemura, K. Morimoto, S. Kuroda, Perirhinal cortical kindling—a behavioral and immunohistochemical study, Epilepsia 37 Ž1996. 67, ŽSuppl... S. Tanaka, S. Kondo, T. Tanaka, Y. Yonemasu, Long-term observation of rats after unilateral intra-amygdaloid injection of kainic acid, Brain Res. 463 Ž1988. 163–167. S. Tanaka, K. Sako, T. Tanaka, I. Nishihara, Y. Yonemasu, Uncoupling of local blood flow and metabolism in the hippocampal CA3 in kainic acid-induced limbic seizure status, Neuroscience 36 Ž1990. 339–348. T. Tanaka, M. Kaijima, G. Daita, S. Ohgami, Y. Yonemasu, D. Riche, Electroclinical features of kainic acid-induced status epilepticus in freely moving cats. Microinjection into the dorsal hippocampus, Electroencephalogr. Clin. Neurophysiol. 54 Ž1982. 288–300. T. Tanaka, M. Kaijima, Y. Yonemasu, C. Cepeda, Spontaneous secondarily generalized seizures induced by a single microinjection of kainic acid into unilateral amygdala in cats, Electroencephalogr. Clin. Neurophysiol. 61 Ž1985. 422–429. T. Tanaka, S. Tanaka, M. Kaijima, Y. Yonemasu, Ibotenic acid-induced nigral lesion and limbic seizure in cats, Brain Res. 498 Ž1989. 215–220. T. Tanaka, S. Tanaka, T. Fujita, K. Takano, H. Fukuda, K. Sato, Y. Yonemasu, Experimental complex partial seizures induced by a microinjection of kainic acid into limbic structures, Prog. Neurobiol. 38 Ž1992. 317–334. J.A. Wada, T. Kudo, Involvement of the claustrum in the convulsive evolution of temporal limbic seizure in feline amygdaloid kindling, Electroencephalogr. Clin. Neurophysiol. 103 Ž1997. 249–256. S. Zola-Morgan, L.R. Squire, R.P. Clower, N.L. Rempel, Damage to the perirhinal cortex exacerbates memory impairment following lesions to the hippocampal formation, J. Neurosci. 13 Ž1993. 251– 265.
Short communication
Kainic acid-induced perirhinal cortical seizures in rats Shin-ichi Imamura
a, )
, Shigeya Tanaka c , Hideshi Tojo c , Shin-ichiro Fukumoto b , Koichi Uetsuhara c , Jun-ichi Kuratsu a , Morikuni Takigawa b
a b
Department of Neurosurgery, UniÕersity of Kagoshima, Faculty of Medicine, Sakuragaoka 8-35-1, Kagoshima 890, Japan Department of Neuropsychiatry, UniÕersity of Kagoshima, Faculty of Medicine, Sakuragaoka 8-35-1, Kagoshima 890, Japan c Department of Neurosurgery, Kagoshima City Hospital, Kagoshima, Japan Accepted 5 May 1998
Abstract Seizures induced in rats by kainic acid microinjection into the perirhinal cortex were studied electrophysiologically and behaviorally and compared with known features of seizures following kainic acid injection into the amygdala. Unlike amygdalar seizures, perirhinal cortical seizures did not generalize to become limbic seizures but rather spread to sensorimotor cortex to become manifest as motor seizures. Perirhinal cortical seizures also required larger kainic acid doses for provocation and were briefer. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Experimental epilepsy; Kainic acid; Perirhinal cortex; Sensorimotor cortex; Secondary generalization
The exact mechanism of secondary generalization of limbic seizures, originating in the limbic structures contrasted with cortical seizures generated from the neocortex, remains unclarified, especially the pathways involved. Previous studies have indicated that several structures including the substantia nigra w5,19x, claustrum w21x, and substantia innominata w8x are important in such generalization. Recently, significant involvement of the perirhinal cortex ŽPRC., which has fiber connections to such limbic structures as the hippocampus, amygdala, entorhinal cortex, pyriform cortex, and deep prepyriform cortex as well as to the neocortex w6,11,12x, has been postulated w3,4,7x. In the present study, we investigated the electrical and behavioral characteristics of seizures following the injection of the kainic acid ŽKA. into the PRC. Ten male Wistar rats Ž250 to 300 g. underwent a stereotactic operation under pentobarbital anesthesia Ž40 mgrkg i.p... A stainless steel screw was placed in contact with the dura over the left sensorimotor cortex ŽLCx. with an additional screw placed in the frontal sinus as an indifferent electrode. A stainless steel cannula Žouter diameter 0.6 mm. with an inner needle guide Ždiameter, 0.3
)
Corresponding author. Fax: q 81-99-265-4041; [email protected]
E-mail:
0006-8993r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 6 - 8 9 9 3 Ž 9 8 . 0 0 5 1 3 - 7
mm. was inserted into the left ŽL. PRC Žat coordinates A, 3.8; L, 7.0; and D, y2.0. in preparation for microinjection of KA. Bipolar depth electrodes were placed into the LPRC, the left basolateral nucleus of the amygdala ŽLA; coordinates A, 5.0; L, 5.0; and D, y3.0. and the left dorsal hippocampus ŽLH; coordinates A, 4.0; L, 2.0; and D, 2.5. w13x. All electrodes as well as the cannula were fixed in place with dental cement, and the electrodes were connected to a terminal plug attached to the head. The animals were allowed to recover unrestrained for 7 days after surgery with free access to food and water until the day of the experiment, when they were placed in the recording chamber for behavioral observation and electroencephalographic monitoring. The inner guide was replaced with an injection needle and 1 to 2 m g of KA ŽSigma, St. Louis, MO. dissolved in phosphate-buffered saline solution ŽPBS; 0.2 M at pH 7.4. at a concentration of 2.0 m grm l, was injected into the LPRC Ž n s 6.. The rate of injection was 1.0 m lrmin. In the control group Ž n s 4., 1.0 m l of PBS alone was injected into the LRPC. The above procedures were done under aseptic conditions. All rats were observed electrophysiologically and behaviorally over 7 days, continuously for 3 days and afterwards 6 h a day, after which the animals were perfused with 10% formalin solution under intraperitoneally administered deep pentobarbital anesthesia. After fixation and embedding in
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paraffin, 4 m m coronal sections were cut and processed for microscopic examination including staining with Cresyl violet and Luxol fast blue. In the early period Žup to 30 min. after KA injection into the LPRC, no behavioral changes such as wet-dog shaking were seen. On the electroencephalogram ŽEEG., multiple spike discharges initially appeared in the LPRC
30 to 60 min after KA injection ŽFig. 1a., and thereafter propagated to the LA and LCx ŽFig. 1b.. Interestingly, seizure propagation to the LH was slight and transient ŽFig. 1c,d.. During this period, brief motor manifestations were observed several times, such as facial twitching which sometimes included right-sided tonic facial spasm, or right or bilateral forelimb clonus. No ictal mastication
Fig. 1. Electrical and behavioral changes after KA injection into the LPRC. Ža. EEG 30 min after injection. Multiple spike discharges initially appeared in the LPRC and left amygdala ŽLA.. No behavioral changes were seen. Žb. EEG 32 min after injection. Multiple spike discharges propagated to the LA and LCx. During this period, a brief interval of motor manifestations included such features as facial twitching Žsometimes including tonic spasm of the right side of the face., and right or bilateral forelimb clonus. Mastication, salivation and wet-dog shaking were not seen. Žc. EEG 60 min after injection. Multiple spike discharges propagated to the LA, LCx and LH. However seizure propagation to the LH was slight and transient. Žd. EEG 80 min after injection. Multiple spike discharges propagated to the LCx; seizure propagation to the LA and LH was not seen. Že. EEG 120 min after injection. Seizures had subsided at this period. Aggressive behavior, commonly seen after KA-induced amygdalar seizures, was not observed.
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or salivation were seen. Electrographic seizures subsided within 2 h ŽFig. 1e.. Afterward, the rats became normal except for infrequent interictal discharges which were seen for 1 to 2 days. Aggressive behavior, such as that usually seen after KA-induced amygdalar seizures w15,20x, was not observed. In controls, no EEG or behavioral changes were seen. Microscopic examination demonstrated focal necrosis and surrounding gliosis around the tip of the cannula in the LPRC, possibly related to the KA microinjection ŽFig. 2a.. Neuronal damage in the entorhinal and parieto-temporal cortices adjacent to the KA injection site was not seen in all of them. Similarly, in the LH ŽFig. 2b. and LA ŽFig. 2c., pyknosis of neuronal cells was not observed in five rats, and a few pyknosis was found in just one rat. And only the slight gliosis along the path of the depth electrodes was seen. In controls, merely the minimal gliosis along the tracts of the cannula and electrodes was seen. Many previous studies dealing with experimental epilepsy have focused on the amygdala and hippocampus because they are regarded as generating limbic seizures w2,15,17,18,20x. On the other hand, several other structures, such as the substantia nigra w5,19x, claustrum w21x, and substantia innominata w8x are considered to induce secondary generalization of limbic seizures by modulating the functional sites of the primary focus or the pathways of seizure propagation. The PRC, identical to Brodmann area 35 and 36 in man w9,10,12x, is regarded as a distinct relay site which contains fiber connections between several limbic structures Žhippocampus, amygdala, entorhinal cortex, pyriform cortex and deep prepyriform cortex. and the neocortex w6,11,12x. Zola-Morgan et al. w22x have suggested that the PRC might be an important region for relaying information reciprocally between the neocortex and the limbic structures, because destruction of the PRC without injury to the hippocampus causes severe memory impairment in primates. Recently, the PRC itself has been postulated to be a generator of some seizure activity w9–11,14x. In a kindling model, McIntyre et al. w10x and Sato et al. w14x have described that fully kindled PRC seizures had a very short latency from stimulation to appearance of forelimb clonus indicating that PRC kindling induces behaviorally evident motor rather than limbic seizures. To further investigate PRC seizures, we employed KA, an excito-toxic analogue of glutamate which induces distinctive seizures, when microinjected into various structures, reflecting the function of the injected sites w1,15,17,18,20x. Only 0.6–0.8 m g of KA reportedly can induce stereotyped limbic seizures such as facial twitching, mastication and salivation beginning approximately 10 min after injection into the amygdala w15,18x or the hippocampus w2,17x, with recurrence over 1 to 2 days. In the present study, 1.0 to 2.0 m g of KA injected into the PRC elicited seizures 30 to 60 min after injection. Multiple EEG spike discharges initially appeared in the LPRC, and thereafter
325
propagated to the LA and LCx. Behaviorally, only transient motor manifestations such as facial twitching and forelimb clonus were seen. These seizures completely subsided within 2 h. Interestingly, KA-induced PRC seizures first propagated to the amygdala and the sensorimotor cortex, and then transiently to the hippocampus, in contrast to KA-induced amygdalar seizures which initially remained in limbic structures and extended to the sensorimotor cortex 1 to 2 h later w15,20x. These findings suggest that the epileptic discharges in the PRC preferentially propagate to the sensorimotor cortex and amygdala rather than to the hippocampus, despite anatomic fiber connections between the PRC and hippocampus, both directly from the PRC to the CA3 region, and indirectly via a PRC–amygdala–entorhinal cortex–CA3 circuit w11,12x. Apparently, intense and continuous seizures may be required to involve these pathways. In addition, McIntyre et al. w11x recently observed that the PRC has reciprocal fibers strongly projecting to the frontal cortex rather than to the hippocampus in the rat. Therefore, it seems reasonable that the ictal discharges in the PRC propagate preferentially to the sensorimotor cortex rather than to the hippocampus. Holmes et al. w7x pointed out that infusion of N-methylD-aspartic acid ŽNMDA. antagonist into the PRC suppressed development of amygdalar kindling. Furthermore, Fukumoto et al. w3,4x reported that lesioning the PRC reduced secondary generalization in a KA-induced limbic seizure model. These results suggest that the PRC might be a relay station, and have access to a preferential pathway from the limbic system to the neocortex. Distinctive behavioral symptoms, such as right facial twitching Žsometimes with tonic spasm. and right or bilateral forelimb clonus, are most likely induced by ictal activities of the sensorimotor cortex w9,11,14,15,20x. On the other hand, lack of behavior such as wet-dog shaking, mastication, and salivation, presumably resulting from the activation of limbic structures w15,20x, suggested that electrical activities in the LH and LA were insufficient to result in these manifestations. Concerning the neuronal damage in the KA-induced amygdalar seizures, Tanaka et al. w16x determined local cerebral glucose utilization ŽLCGU. and local cerebral blood flow ŽLCBF. by autoradiography. Their findings reflected neuronal damage in several regions, especially in the CA3 region of the ipsilateral hippocampus, due to remarkable uncoupling of LCGU and LCBF in those areas. In the present study, microscopic examination demonstrated no or very slight neuronal injury in the amygdala and hippocampus, which is compatible with the EEG findings. The increase in LCGU may not have been high enough to cause neuronal injury, since propagation of the seizure discharges to these regions, especially the hippocampus, was slight and transient. In summary, KA-induced PRC seizures contrasted with amygdalar seizures in several respects: a larger dose of KA
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Fig. 2. Microscopic findings, 7 days after KA injection into the LPRC. Ža. LPRC demonstrating focal necrosis and surrounding gliosis around the tip of the cannula in the LPRC. Neuronal damage in the entorhinal and parieto-temporal cortices adjacent to the KA-injected site was not observed. ECx, entorhinal cortex; RF, rhinal fissure. Žb. LA demonstrating no neuronal damage such as pyknosis. Žc. The CA3 region of the left hippocampus demonstrates no pyknotic pyramidal cells. Cresyl violet and Luxol fast blue stain. ŽOriginal Magnification in Ža. =40; Žb. =100; Žc. =100..
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was needed to elicit seizures; the time to onset of seizure was longer; seizure duration was markedly shorter; PRC spike discharges initially propagated to the amygdala and sensorimotor cortex; the hippocampus was less involved in seizures; the seizure subsided shortly after KA injection; and neuronal injury in the hippocampus and amygdala was minimal. From the results of the present and previous studies w3,4,6,7,9,10,14 x, it would appear that the PRC might relay the limbic seizures to the sensorimotor cortex, and also that PRC itself could generate the seizures rapidly extending to the sensorimotor cortex rather than to the limbic structures resulting in motor manifestations. Further examination will be required to test and refine this notion.
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w9x
w10x w11x
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Acknowledgements The authors would like to thank and acknowledge Kenji Kato MD ŽSecond Department of Anatomy, University of Kagoshima, Faculty of Medicine. for technical assistance.
w14x
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