- Email: [email protected]
Vulnerability of the hippocampus to kainate excitotoxicity in the aged, mature and young adult rat
ELSEVIER
Neuroscience Letters 188 (1995) 117-120
H[UROSCI[NC[ LETTERS
Vulnerability of the hippocampus to kainate excitotoxicity in the aged, mature and young adult rat J.P. Kesslak a,c,*, D. Yuan b, S. Neeper b, C.W. C o t m a n a,b,c aDepartment of Neurology, University of California at lrvine, lrvine, CA 92717, USA bDepartment of Psychobiology, University of California at Irvine, lrvine, CA 92717, USA Clrvine Research Unit in Brain Aging, University of California at Irvine, lrvine, CA 92717, USA Received 31 October 1994; revised version received 10 February 1995; accepted 16 February 1995
Abstract
Sensitivity to excitotoxic damage was assessed in young adult, mature and aged male Sprague-Dawley rats. Kainic acid was injected into the hippocampus and the size of the hippocampal lesion rated. Intrahippocampal injection of kainic acid produced lesions in aged animals that were significantly smaller than lesions in the young rats (P < 0.05), while lesion size in mature rats was intermediate. Excitotoxic damage was localized primarily to the CA3 region of the hippocampus in the aged rats. Young adult rats had more damage to the hippocampus with involvement of CA1 pyramidal and dentate granule cells. These results suggest that increased age may reduce susceptibility to excitotoxic damage.
Keywords: Hippocampus; Aging; Kainic acid; Excitatory amino acids; Dementia
Neural loss in the central nervous system (CNS) occurs in response to aging, trauma and disease. Significant neural loss will impact adversely on behavioral abilities including learning and memory, coordination, motor activity, motivation and a variety of physiological measures. Glutamate excitotoxicity is considered a possible mechanism of neuronal damage in normal aging and several disorders that lead to cognitive impairments, including head trauma [6,15], prolonged epileptic seizure [2,14], sustained ischemia and hypoglycemia [1,4], domoate poisoning [20,21], and possibly Alzheimer's disease [1,21]. The incidence of neuropathology due to stroke and Alzheimer's disease increases dramatically with age, suggesting an interaction for increased excitotoxic vulnerability as age increases. Excess stimulation of receptors for the excitatory amino acids (EAA) glutamate and aspartate produces characteristic patterns of excitotoxic tissue damage [7,16]. Three glutamate receptor subtypes, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole
* Corresponding author, Irvine Research Unit in Brain Aging, University of California at lrvine, Irvine, CA 92717, USA. E-mail: [email protected].
propionic acid (AMPA), and kainic acid (KA) receptors, can mediate excitotoxic damage [4]. The glutamate agonist KA is a potent and specific neurotoxin for producing lesions in the hippocampus, an area highly involved in learning and memory [10,11,13]. KA toxicity can reliably produce neuron loss in hippocampus, amygdala, medial thalamus, lateral septum, and olfactory, pyriform, and entorhinal cortices [18,20,21]. In addition to the sensitivity to EAA neurotoxicity, the hippocampus and related structures have been implicated in the cellular basis of learning [12]. Damage to these vulnerable areas, particularly the hippocampus and related cortices has long been associated with impaired learning and memory function [9,22]. The present study focuses on intrahippocampal injection of KA. This approach reduces the variability that occurs with systemic administration of KA, produces discrete lesions, and may mimic the localized release of excess agonist occurring with excitotoxic events in vivo. The extent and distribution of neural damage produced by KA differs with the route of drug administration and dose. Following intracerebral KA injection, there is marked, dose-dependent damage at the injection site that varies in severity depending on its location. Pyramidal cells of hippocampal area CA3 and pyriform cortex are especially
0304-3940/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All rights reserved SSD1 0 3 0 4 - 3 9 4 0 ( 9 5 ) 1 1 4 1 5 - P
118
J.P. Kesslak et al. / Neuroscience Letters 188 (1995) 117-120
Fig. 1. (A) Cresyl violet stained coronal sections indicate that injection of PBS did not damage the hippocampal formation. (B) lntrahippocampal in jection of KA in the contralateral side of mature rats consistently produced loss of CA3 pyramidal cells. (C) Young rats exhibited more extensive dam age after KA administration with neural loss in CA3, CAI and dentate gyrus (DG).
119
J.P. Kesslak et aL / Neuroscience Letters 188 (1995) 117-120
vulnerable [13,18]. Intracerebral KA injection can also cause damage in areas distal from the injection site. Location of distal damage varies in a manner which suggests mediation through known efferents, possibly glutamatergic, from the injection site [7,14,18]. Systemic application of KA also causes lesions in cortical and limbic areas that receive substantial E A A innervation [18,20,21]. Some vulnerable brain regions can be spared from KA mediated damage by ligation of glutamatergic afferents [14,18]. High doses of KA increase extracellular concentrations of the potentially excitotoxic EAAs, glutamate and aspartate, in vivo and in vitro [8]. Previous studies indicate vulnerability to seizures [8,20] and excitotoxicity [20,21] increases in aged versus young individuals that receive systemic KA exposure. Peripheral interactions can be eliminated with intrahippocampal injections to avoid systemic involvement and localize excitotoxicity to a CNS structure sensitive to E A A related pathology. Male Sprague-Dawley rats from Charles River, 16 young adult ( 3 m o n t h s old), 7 mature adult (1420 months old) retired breeders and 13 aged (24 months old), were housed individually in a 12/12 h light/dark vivarium with access to food and water ad libitum. Rats were anesthetized with 50 mg/kg sodium pentobarbital and placed in a stereotaxic instrument. KA (Sigma), at 0.5 mg in 0.5 ml phosphate-buffered saline (PBS), or PBS alone was administered 0.5/tl of solution to the right and left hippocampus, respectively, using a 10-/~1 Hamilton microsyringe. Coordinates for the injections from bregma were - 2 . 6 mm posterior, _+3.0 mm lateral and -3.5 mm ventral [17]. Seizure activity was present in the KA treated animals for approximately 2 h immediately post surgery. Approximately 20 days after KA injection, animals were perfused with 4% paraformaldehyde in 0.1 M Sorensen's phosphate buffer, and the brains removed. Coronal sections 30ktm thick were cut on a cryostat (Hacker Instruments) and stained with cresyl violet. An estimate of lesion size for CA1, CA3 and dentate gyrus was determined from three areas along the anteriorposterior axis of the hippocampus. Sections were rated on a 0-3 point scale by an experimenter who was not aware of the group assignment of the animal. A rating of 0 indicated no apparent damage to the hippocampus; 1, mild degeneration proximal to the injection site but not extending beyond the cannula tract or involving less than onethird of the region; 2, moderate lesion size characterized by loss of 1/3-2/3 of the neurons within a defined region; and 3, severe lesion size defined as extensive neural loss with damage exceeding 2/3 of the region. Both KA and PBS injected sites were rated for the extent of damage. Lesion scores for the hippocampus were analyzed using a two-way nested factors analysis of variance (ANOVA) for age × position (anterior, middle and posterior). Three positions of the hippocampus were examined; anterior hippocampus which included the injection site, middle
3.0
0.) .N 2.5' or) t- 2.0'
E•
[i
Young Mature Aged
._o .J
1.5-
o 1.0n"
0.5 00
anterior
middle
Hippocampal
posterior
Position
Fig. 2. Rating of lesion size (mean _+SEM) on a 0-3 point scale indicated the most extensive neural loss was in the anterior hippocampus, the site of KA injections. As distance from the injection site increased the lesion size decreased. Young animals had the largest lesions in each of the hippocampal positions and differed significantly from the aged group in the anterior and middle positions (*P < 0.05). hippocampus and posterior hippocampus which was the most distal from the site of KA injection. Intrahippocampal injection of KA consistently produced neural loss in young, mature and aged rats (Fig. 1). Analysis of the mean lesion scores indicated a significant group effect for age (F[2,33] = 5.221, P = 0.0107), with hippocampal position indicating differences in size due to distance from the injection site (F[2,66]=66.178, P - - 0 . 0 0 0 1 ) , and no interaction between age and hippocampal position (F[4,66] = 1.997, P - - 0 . 1 0 5 2 ) . Post hoc analysis of between group comparisons with Scheffe's Ftest indicated that the anterior and middle hippocampus of young and aged groups differed significantly from each other (P < 0.05), but there was no difference between either group and the mature rats (Fig. 2). Lesion size did not differ significantly between any of the three age groups in the posterior hippocampus. The extent of damage to the dentate gyrus best differentiated the groups, with scores significantly higher in young compared to mature and aged animals. As anticipated, all groups consistently had neural loss in CA3, but the dentate and CA1 were more frequently involved in the younger animals. There was no significant damage noted in the PBS injected hippocampi. In this study, aged rats showed higher resistance to KA excitotoxicity than young rats, with mature rats being intermittent in sensitivity to excitotoxicity. Our results show decreased toxicity to KA with age, whereas others have observed an increased vulnerability to E A A damage in aged rats [8,20,21] or humans [20]. In the earlier studies, excitotoxic damage was induced by systemic kainate
120
J.P. Kesslak et al. / Neuroscience Letters 188 (1995) 117-120
e x p o s u r e b y s u b c u t a n e o u s i n j e c t i o n o f K A [8,21] or b y a c c i d e n t a l i n g e s t i o n o f d o m o i c acid [20], w h e r e a s , we u t i l i z e d i n t r a h i p p o c a m p a l i n j e c t i o n o f K A . D i f f e r e n c e s in a v a i l a b i l i t y o f K A to t h e b r a i n d u e to altered d r u g m e t a b o l i s m or i n c r e a s e d p e r m e a b i l i t y o f the b l o o d - b r a i n b a r r i e r w i t h a g e c o u l d a c c o u n t for t h e d i f f e r e n c e s b e t w e e n studies. S e v e r a l f a c t o r s t h a t c h a n g e w i t h a g e m a y i n f l u e n c e the b r a i n ' s s e n s i t i v i t y to e x c i t o t o x i c d a m a g e , i n c l u d i n g , receptor density, uptake, secondary release of neurotransmitters, a n d g e n e r a l c e l l u l a r m e t a b o l i c p r o c e s s e s . F o r e x a m p l e , w i t h i n c r e a s e d a g e t h e r e is a d e c l i n e in N M D A a n d A M P A r e c e p t o r d e n s i t y , a n d a smaller, s o m e t i m e s i n s i g n i f i c a n t , r e d u c t i o n in K A r e c e p t o r d e n s i t y [3,5,19]. P h y s i o l o g i c a l c h a n g e s a s s o c i a t e d w i t h i n c r e a s e d age m a y n o t i n v a r i a b l y alter s e n s i t i v i t y to e x c i t o t o x i c d a m a g e , as indicated by increased within group variability with age a n d a lack o f d i f f e r e n c e b e t w e e n the m a t u r e a n d aged g r o u p s . F u r t h e r s t u d y m a y r e v e a l risk factors a n d m e c h a n i s m s r e s p o n s i b l e for v u l n e r a b i l i t y to g l u t a m a t e excitot o x i c i t y a n d t h e i n t e r a c t i o n w i t h age. T h i s r e s e a r c h w a s s u p p o r t e d b y the M a c A r t h u r F o u n dation Study Group on Successful Aging and NIA Lead Award AG07918. [1] Beal, M.F., Mechanisms of excitotoxicity in neurologic diseases, FASEB J., 6 (1992) 3338-3344. [2] Ben-Ari, Y., Tremblay, E., Ottersen, O.P. and Meldrum, B.S., The role of epileptic activity in hippocampal and 'remote' cerebral lesions induced by kainic acid, Brain Res., 191 (1980) 79-97. [3] Carpenter, M.K., Parker, I. and Miledi, R., Messenger RNAs coding for receptors and channels in the cerebral cortex of adult and aged rats, Brain Res. Mol. Brain Res., 13 (1992) 1-5. [4] Choi, D.W., Excitotoxic cell death, J. Neurobiol., 23 (1992) 1261-1276. [5] Clark, A.S., Magnusson, K.R. and Cotman, C.W., In vitro autoradiography of hippocampal excitatory amino acid binding in aged Fischer 344 rats: relationship to performance on the Morris water maze, Behav. Neurosci., 106 (1992) 324-335. [6] Cotman, C.W. and Nieto, S.M., Progress in facilitating the recovery of function after central nervous system trauma, Ann. N. Y. Acad. Sci., 457 (1985) 83-104. [7] Coyle, J.T., Neurotoxic action of kainic acid, J. Neurochem., 41 (1983) 1-11.
[8] Dawson, R.J. and Wallace, D.R., Kainic acid-induced seizures in aged rats: neurochemical correlates, Brain Res. Bull., 29 (1992) 459-468. [9] Eichenbaum, H., Otto, T. and Cohen, N.J., The hippocampus what does it do? Behav. Neural. Biol., 57 (1992) 2-36. [10] Handelmann, G.E., Olton, D.S., O'Donohue, T.L., Beinfield, M.C., Jacobowitz, D.M. and Cummins, C.J., Effects of time and experience on hippocampal neurochemistry after damage to the CA3 subfield, Pharmacol. Biochem. Behav., 18 (1983) 551-561. [11] Kesslak, J.P. and Gage, F.H., Recovery of spatial alternation deficits following selective hippocampal destruction with kainic acid, Behav. Neurosci., 100 (1986) 280-283. [12] Massicotte, G. and Baudry, M., Triggers and substrates of hippocampal synaptic plasticity, Neurosci. Biobehav. Rev., 15 (1991) 415-423. [13] Nadler, J.V., Role of excitatory pathways in the hippocampal damage produced by kainic acid, Adv. Biochem. Psychopharmacol., 27 (1981) 395-402. [14] Nadler, J.V., Evenson, D.A. and Smith, E.M., Evidence from lesion studies for epileptogenic and non-epileptogenic neurotoxic interactions between kainic acid and excitatory innervation, Brain Res., 205 (1981)405-410. [15] Nilsson, P., Hillered, L., Ponten, U. and Ungerstedt, U., Changes in cortical extracellular levels of energy-related metabolites and amino acids following concussive brain injury in rats, J. Cereb. Blood Flow Metab., 10 (1990) 631~37. [16] Olney, J.W., Adamo, N.J. and Ratner, A., Monosodium glutamate effects, Science, 172 (1971) 294. [17] Pellegrino, L.J., Pellegrino, A.S. and Cushman, A.J., A Stereotaxic Atlas of the Rat Brain, Plenum Press: New York, (1986). [18] Schwob, J.E., Fuller, T., Price, J.L. and Olney, J.W., Widespread patterns of neuronal damage following systemic or intracerebral injections of kainic acid: a histological study, Neuroscience, 5 (1980) 991 1014. [19] Tamaru, M., Yoneda, Y., Ogita, K., Shimizu, J. and Nagata, Y., Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus, Brain Res., 542 (1991) 83-90. [20] Teitelbaum, J.S., Zatorre, R.J., Carpenter, S., Gendron, D., Evans, A.C., Gjedde, A. and Cashman, N.R., Neurologic sequelae of domoic acid intoxication due to the ingestion of contaminated mussels, N. Engl. J. Med., 322 (1990) 1781-1787. [21] Wozniak, D.F., Stewart, G.R., Miller, LP. and Olney, J.W., Agerelated sensitivity to kainate neurotoxicity, Exp. Neurol., 114 (1991) 250-253. [22] Zola-Morgan, S. and Squire, L.R., Neuroanatomy of Memory, Annu. Rev. Neurosci., 16 (1993) 547-563.
Neuroscience Letters 188 (1995) 117-120
H[UROSCI[NC[ LETTERS
Vulnerability of the hippocampus to kainate excitotoxicity in the aged, mature and young adult rat J.P. Kesslak a,c,*, D. Yuan b, S. Neeper b, C.W. C o t m a n a,b,c aDepartment of Neurology, University of California at lrvine, lrvine, CA 92717, USA bDepartment of Psychobiology, University of California at Irvine, lrvine, CA 92717, USA Clrvine Research Unit in Brain Aging, University of California at Irvine, lrvine, CA 92717, USA Received 31 October 1994; revised version received 10 February 1995; accepted 16 February 1995
Abstract
Sensitivity to excitotoxic damage was assessed in young adult, mature and aged male Sprague-Dawley rats. Kainic acid was injected into the hippocampus and the size of the hippocampal lesion rated. Intrahippocampal injection of kainic acid produced lesions in aged animals that were significantly smaller than lesions in the young rats (P < 0.05), while lesion size in mature rats was intermediate. Excitotoxic damage was localized primarily to the CA3 region of the hippocampus in the aged rats. Young adult rats had more damage to the hippocampus with involvement of CA1 pyramidal and dentate granule cells. These results suggest that increased age may reduce susceptibility to excitotoxic damage.
Keywords: Hippocampus; Aging; Kainic acid; Excitatory amino acids; Dementia
Neural loss in the central nervous system (CNS) occurs in response to aging, trauma and disease. Significant neural loss will impact adversely on behavioral abilities including learning and memory, coordination, motor activity, motivation and a variety of physiological measures. Glutamate excitotoxicity is considered a possible mechanism of neuronal damage in normal aging and several disorders that lead to cognitive impairments, including head trauma [6,15], prolonged epileptic seizure [2,14], sustained ischemia and hypoglycemia [1,4], domoate poisoning [20,21], and possibly Alzheimer's disease [1,21]. The incidence of neuropathology due to stroke and Alzheimer's disease increases dramatically with age, suggesting an interaction for increased excitotoxic vulnerability as age increases. Excess stimulation of receptors for the excitatory amino acids (EAA) glutamate and aspartate produces characteristic patterns of excitotoxic tissue damage [7,16]. Three glutamate receptor subtypes, N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole
* Corresponding author, Irvine Research Unit in Brain Aging, University of California at lrvine, Irvine, CA 92717, USA. E-mail: [email protected].
propionic acid (AMPA), and kainic acid (KA) receptors, can mediate excitotoxic damage [4]. The glutamate agonist KA is a potent and specific neurotoxin for producing lesions in the hippocampus, an area highly involved in learning and memory [10,11,13]. KA toxicity can reliably produce neuron loss in hippocampus, amygdala, medial thalamus, lateral septum, and olfactory, pyriform, and entorhinal cortices [18,20,21]. In addition to the sensitivity to EAA neurotoxicity, the hippocampus and related structures have been implicated in the cellular basis of learning [12]. Damage to these vulnerable areas, particularly the hippocampus and related cortices has long been associated with impaired learning and memory function [9,22]. The present study focuses on intrahippocampal injection of KA. This approach reduces the variability that occurs with systemic administration of KA, produces discrete lesions, and may mimic the localized release of excess agonist occurring with excitotoxic events in vivo. The extent and distribution of neural damage produced by KA differs with the route of drug administration and dose. Following intracerebral KA injection, there is marked, dose-dependent damage at the injection site that varies in severity depending on its location. Pyramidal cells of hippocampal area CA3 and pyriform cortex are especially
0304-3940/95/$09.50 © 1995 Elsevier Science Ireland Ltd. All rights reserved SSD1 0 3 0 4 - 3 9 4 0 ( 9 5 ) 1 1 4 1 5 - P
118
J.P. Kesslak et al. / Neuroscience Letters 188 (1995) 117-120
Fig. 1. (A) Cresyl violet stained coronal sections indicate that injection of PBS did not damage the hippocampal formation. (B) lntrahippocampal in jection of KA in the contralateral side of mature rats consistently produced loss of CA3 pyramidal cells. (C) Young rats exhibited more extensive dam age after KA administration with neural loss in CA3, CAI and dentate gyrus (DG).
119
J.P. Kesslak et aL / Neuroscience Letters 188 (1995) 117-120
vulnerable [13,18]. Intracerebral KA injection can also cause damage in areas distal from the injection site. Location of distal damage varies in a manner which suggests mediation through known efferents, possibly glutamatergic, from the injection site [7,14,18]. Systemic application of KA also causes lesions in cortical and limbic areas that receive substantial E A A innervation [18,20,21]. Some vulnerable brain regions can be spared from KA mediated damage by ligation of glutamatergic afferents [14,18]. High doses of KA increase extracellular concentrations of the potentially excitotoxic EAAs, glutamate and aspartate, in vivo and in vitro [8]. Previous studies indicate vulnerability to seizures [8,20] and excitotoxicity [20,21] increases in aged versus young individuals that receive systemic KA exposure. Peripheral interactions can be eliminated with intrahippocampal injections to avoid systemic involvement and localize excitotoxicity to a CNS structure sensitive to E A A related pathology. Male Sprague-Dawley rats from Charles River, 16 young adult ( 3 m o n t h s old), 7 mature adult (1420 months old) retired breeders and 13 aged (24 months old), were housed individually in a 12/12 h light/dark vivarium with access to food and water ad libitum. Rats were anesthetized with 50 mg/kg sodium pentobarbital and placed in a stereotaxic instrument. KA (Sigma), at 0.5 mg in 0.5 ml phosphate-buffered saline (PBS), or PBS alone was administered 0.5/tl of solution to the right and left hippocampus, respectively, using a 10-/~1 Hamilton microsyringe. Coordinates for the injections from bregma were - 2 . 6 mm posterior, _+3.0 mm lateral and -3.5 mm ventral [17]. Seizure activity was present in the KA treated animals for approximately 2 h immediately post surgery. Approximately 20 days after KA injection, animals were perfused with 4% paraformaldehyde in 0.1 M Sorensen's phosphate buffer, and the brains removed. Coronal sections 30ktm thick were cut on a cryostat (Hacker Instruments) and stained with cresyl violet. An estimate of lesion size for CA1, CA3 and dentate gyrus was determined from three areas along the anteriorposterior axis of the hippocampus. Sections were rated on a 0-3 point scale by an experimenter who was not aware of the group assignment of the animal. A rating of 0 indicated no apparent damage to the hippocampus; 1, mild degeneration proximal to the injection site but not extending beyond the cannula tract or involving less than onethird of the region; 2, moderate lesion size characterized by loss of 1/3-2/3 of the neurons within a defined region; and 3, severe lesion size defined as extensive neural loss with damage exceeding 2/3 of the region. Both KA and PBS injected sites were rated for the extent of damage. Lesion scores for the hippocampus were analyzed using a two-way nested factors analysis of variance (ANOVA) for age × position (anterior, middle and posterior). Three positions of the hippocampus were examined; anterior hippocampus which included the injection site, middle
3.0
0.) .N 2.5' or) t- 2.0'
E•
[i
Young Mature Aged
._o .J
1.5-
o 1.0n"
0.5 00
anterior
middle
Hippocampal
posterior
Position
Fig. 2. Rating of lesion size (mean _+SEM) on a 0-3 point scale indicated the most extensive neural loss was in the anterior hippocampus, the site of KA injections. As distance from the injection site increased the lesion size decreased. Young animals had the largest lesions in each of the hippocampal positions and differed significantly from the aged group in the anterior and middle positions (*P < 0.05). hippocampus and posterior hippocampus which was the most distal from the site of KA injection. Intrahippocampal injection of KA consistently produced neural loss in young, mature and aged rats (Fig. 1). Analysis of the mean lesion scores indicated a significant group effect for age (F[2,33] = 5.221, P = 0.0107), with hippocampal position indicating differences in size due to distance from the injection site (F[2,66]=66.178, P - - 0 . 0 0 0 1 ) , and no interaction between age and hippocampal position (F[4,66] = 1.997, P - - 0 . 1 0 5 2 ) . Post hoc analysis of between group comparisons with Scheffe's Ftest indicated that the anterior and middle hippocampus of young and aged groups differed significantly from each other (P < 0.05), but there was no difference between either group and the mature rats (Fig. 2). Lesion size did not differ significantly between any of the three age groups in the posterior hippocampus. The extent of damage to the dentate gyrus best differentiated the groups, with scores significantly higher in young compared to mature and aged animals. As anticipated, all groups consistently had neural loss in CA3, but the dentate and CA1 were more frequently involved in the younger animals. There was no significant damage noted in the PBS injected hippocampi. In this study, aged rats showed higher resistance to KA excitotoxicity than young rats, with mature rats being intermittent in sensitivity to excitotoxicity. Our results show decreased toxicity to KA with age, whereas others have observed an increased vulnerability to E A A damage in aged rats [8,20,21] or humans [20]. In the earlier studies, excitotoxic damage was induced by systemic kainate
120
J.P. Kesslak et al. / Neuroscience Letters 188 (1995) 117-120
e x p o s u r e b y s u b c u t a n e o u s i n j e c t i o n o f K A [8,21] or b y a c c i d e n t a l i n g e s t i o n o f d o m o i c acid [20], w h e r e a s , we u t i l i z e d i n t r a h i p p o c a m p a l i n j e c t i o n o f K A . D i f f e r e n c e s in a v a i l a b i l i t y o f K A to t h e b r a i n d u e to altered d r u g m e t a b o l i s m or i n c r e a s e d p e r m e a b i l i t y o f the b l o o d - b r a i n b a r r i e r w i t h a g e c o u l d a c c o u n t for t h e d i f f e r e n c e s b e t w e e n studies. S e v e r a l f a c t o r s t h a t c h a n g e w i t h a g e m a y i n f l u e n c e the b r a i n ' s s e n s i t i v i t y to e x c i t o t o x i c d a m a g e , i n c l u d i n g , receptor density, uptake, secondary release of neurotransmitters, a n d g e n e r a l c e l l u l a r m e t a b o l i c p r o c e s s e s . F o r e x a m p l e , w i t h i n c r e a s e d a g e t h e r e is a d e c l i n e in N M D A a n d A M P A r e c e p t o r d e n s i t y , a n d a smaller, s o m e t i m e s i n s i g n i f i c a n t , r e d u c t i o n in K A r e c e p t o r d e n s i t y [3,5,19]. P h y s i o l o g i c a l c h a n g e s a s s o c i a t e d w i t h i n c r e a s e d age m a y n o t i n v a r i a b l y alter s e n s i t i v i t y to e x c i t o t o x i c d a m a g e , as indicated by increased within group variability with age a n d a lack o f d i f f e r e n c e b e t w e e n the m a t u r e a n d aged g r o u p s . F u r t h e r s t u d y m a y r e v e a l risk factors a n d m e c h a n i s m s r e s p o n s i b l e for v u l n e r a b i l i t y to g l u t a m a t e excitot o x i c i t y a n d t h e i n t e r a c t i o n w i t h age. T h i s r e s e a r c h w a s s u p p o r t e d b y the M a c A r t h u r F o u n dation Study Group on Successful Aging and NIA Lead Award AG07918. [1] Beal, M.F., Mechanisms of excitotoxicity in neurologic diseases, FASEB J., 6 (1992) 3338-3344. [2] Ben-Ari, Y., Tremblay, E., Ottersen, O.P. and Meldrum, B.S., The role of epileptic activity in hippocampal and 'remote' cerebral lesions induced by kainic acid, Brain Res., 191 (1980) 79-97. [3] Carpenter, M.K., Parker, I. and Miledi, R., Messenger RNAs coding for receptors and channels in the cerebral cortex of adult and aged rats, Brain Res. Mol. Brain Res., 13 (1992) 1-5. [4] Choi, D.W., Excitotoxic cell death, J. Neurobiol., 23 (1992) 1261-1276. [5] Clark, A.S., Magnusson, K.R. and Cotman, C.W., In vitro autoradiography of hippocampal excitatory amino acid binding in aged Fischer 344 rats: relationship to performance on the Morris water maze, Behav. Neurosci., 106 (1992) 324-335. [6] Cotman, C.W. and Nieto, S.M., Progress in facilitating the recovery of function after central nervous system trauma, Ann. N. Y. Acad. Sci., 457 (1985) 83-104. [7] Coyle, J.T., Neurotoxic action of kainic acid, J. Neurochem., 41 (1983) 1-11.
[8] Dawson, R.J. and Wallace, D.R., Kainic acid-induced seizures in aged rats: neurochemical correlates, Brain Res. Bull., 29 (1992) 459-468. [9] Eichenbaum, H., Otto, T. and Cohen, N.J., The hippocampus what does it do? Behav. Neural. Biol., 57 (1992) 2-36. [10] Handelmann, G.E., Olton, D.S., O'Donohue, T.L., Beinfield, M.C., Jacobowitz, D.M. and Cummins, C.J., Effects of time and experience on hippocampal neurochemistry after damage to the CA3 subfield, Pharmacol. Biochem. Behav., 18 (1983) 551-561. [11] Kesslak, J.P. and Gage, F.H., Recovery of spatial alternation deficits following selective hippocampal destruction with kainic acid, Behav. Neurosci., 100 (1986) 280-283. [12] Massicotte, G. and Baudry, M., Triggers and substrates of hippocampal synaptic plasticity, Neurosci. Biobehav. Rev., 15 (1991) 415-423. [13] Nadler, J.V., Role of excitatory pathways in the hippocampal damage produced by kainic acid, Adv. Biochem. Psychopharmacol., 27 (1981) 395-402. [14] Nadler, J.V., Evenson, D.A. and Smith, E.M., Evidence from lesion studies for epileptogenic and non-epileptogenic neurotoxic interactions between kainic acid and excitatory innervation, Brain Res., 205 (1981)405-410. [15] Nilsson, P., Hillered, L., Ponten, U. and Ungerstedt, U., Changes in cortical extracellular levels of energy-related metabolites and amino acids following concussive brain injury in rats, J. Cereb. Blood Flow Metab., 10 (1990) 631~37. [16] Olney, J.W., Adamo, N.J. and Ratner, A., Monosodium glutamate effects, Science, 172 (1971) 294. [17] Pellegrino, L.J., Pellegrino, A.S. and Cushman, A.J., A Stereotaxic Atlas of the Rat Brain, Plenum Press: New York, (1986). [18] Schwob, J.E., Fuller, T., Price, J.L. and Olney, J.W., Widespread patterns of neuronal damage following systemic or intracerebral injections of kainic acid: a histological study, Neuroscience, 5 (1980) 991 1014. [19] Tamaru, M., Yoneda, Y., Ogita, K., Shimizu, J. and Nagata, Y., Age-related decreases of the N-methyl-D-aspartate receptor complex in the rat cerebral cortex and hippocampus, Brain Res., 542 (1991) 83-90. [20] Teitelbaum, J.S., Zatorre, R.J., Carpenter, S., Gendron, D., Evans, A.C., Gjedde, A. and Cashman, N.R., Neurologic sequelae of domoic acid intoxication due to the ingestion of contaminated mussels, N. Engl. J. Med., 322 (1990) 1781-1787. [21] Wozniak, D.F., Stewart, G.R., Miller, LP. and Olney, J.W., Agerelated sensitivity to kainate neurotoxicity, Exp. Neurol., 114 (1991) 250-253. [22] Zola-Morgan, S. and Squire, L.R., Neuroanatomy of Memory, Annu. Rev. Neurosci., 16 (1993) 547-563.