Retention of reference memory following ischemic hippocampal damage

Physiology& Behavior, Vol. 39, pp. 783-786. Copyright©Pergamon Journals Ltd., 1987. Printed in the U.S.A.

0031-9384/87 $3.00 + .00

BRIEF COMMUNICATION

Retention of Reference Memory Following Ischemic Hippocampal Damage HASKER

P. D A V I S , J U D I T H R . B A R A N O W S K I

Department of Psychology, University o f Colorado, Colorado Springs, CO 80933 WILLIAM

A. PULSINELLI

A N D B R U C E T. V O L P E

Department of Neurology, Cornell University Medical College 1300 York Avenue, N e w York, N Y 10021 R e c e i v e d 10 O c t o b e r 1986 DAVIS, H. P., J. R. BARANOWSKI, W. A. PULSINELLI AND B. T. VOLPE. Retention of reference memoryfollowing ischemic hippocampaldamage. PHYSIOL BEHAV 39(6) 783-786, 1987.--Retention of reference memory was evaluated in an animal model of cerebral ischemia. Rats were trained for 70 daily trials on an 8-arm radial maze with 5 arms baited, subjected to 30 minutes of forebrain ischemia, allowed to recover for 30 days, and then tested for an additional 50 trials. Post ischemic (PI) rats demonstrated normal retention of reference memory (p>0.05). Working memory was significantly impaired postoperatively (,o<0.05). Morphologie analysis showed that PI rats had primary loss of pyramidal neurons in the CAI region of hippocampus. Amnesia Cardiac arrest Hippocampus Reference memory Working memory

lschemia

WE have previously proposed that rats subjected to 30 minutes of forebrain ischemia by the method of 4-vessel occlusion provided a useful model of the morphologic and behavioral consequences of cardiac arrest in humans [4, 12, 15]. Some survivors of cardiac arrest that have come to postmortem examination after ante mortem behavioral testing showed marked loss of CA1 neurons [2, 11, 16]. After cardiac arrest, some patients showed a fairly circumscribed amnesia characterized in part by a deficit in learning new and variable information [2, 10, 16]. Similarly, post ischemic rats (PI rats) that model the consequences ofischemic insult after cardiac arrest have primary loss of CA1 hippocampal neurons and have impaired acquisition of new and/or variable information [3, 4, 12-15]. In contrast to their impaired acquisition of new information, patients with loss of CA1 neurons retain information learned prior to ischemic brain insult [2, 10, 11, 16]. Our previous studies of PI rats have only examined acquisition of reference and working memory [3, 4, 15]. Since rats were subjected to isehemia prior to maze training it was not possible to examine the retention capacity of PI rats. If PI rats accurately model the consequences o f iscbemic insult in humans, then retention of highly practiced premorbid learning should not be impaired. Accordingly, the present study examines this possibility by assessing retention of reference memory in rats trained for 70 trials prior to being subjected to 30 minutes of ischemia.

Learning and memory

Radial maze

METHOD

Subjects Male Wistar rats (Hilltop, Scottsdale, PA) weighing between 250-350 grams served as subjects. Animals were individually housed throughout the experiment, provided ad lib access to water, and maintained at 80% of their ad lib weight.

Surgical Procedure Twenty-six rats were subjected to 30 minutes of transient forebrain ischemia by permanent occlusion of the vertebral arteries and temporary occlusion of the carotid arteries. Fourteen animals survived the procedure. Three animals died during subsequent recovery days, and two animals were eliminated from behavioral analysis because no histopathologic damage was detected by light microscopy with H & E staining. In PI rats cerebral blood flow values are comparable to control values by 24--48 hours in parietal area, striatum, hippocampus, diencephalon, cerebellum, and brain stem [8]. Fifteen control animals were exposed to skin incision and carotid manipulation, and neck muscle separation and manipulation. All animals were allowed 30 days of postoperative recovery. The method has been described in detail previously [7].

Behavioral Apparatus and Procedure Rats were tested in a radial 8-arm maze described previ-

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784 REFERENCE ERRORS PRETRAINEO RATS POSTOPERATIVE TRIALS

2.5 !

CONTROL@

i

ISCHEMIC&

2.0 + nO OC OC W LU 0 Z W OC LU bW OE

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T

1.0

Z

<

0.5

0

. 70

0 75

~ 80

85

90

95

100

105

110

115

I20

BLOCKS OF FIVE TRIALS

FIG. 1. Mean postoperative reference errors (_*SEM) for PI (n=9) and control rats (n= 15) in blocks of 5 trials.

TABLE 1 SEVERITY A N D DISTRIBUTION O F I S C H E M I C B R A I N D A M A G E

Mean (_+SD) Neuropathologic Score Ischemic Rats N=9

Control Rats N=15

considered a correct choice, an initial entry of an unbaited arm was considered a reference memory error, and re-entry of a previously chosen arm was considered a working memory error. Postoperative behavioral training was blind. All trials were run between 7 a.m. and 11 a.m.

Neuropathologic Procedure Neocortex Anterior Posterior Hippocampus CAI CA2 CA3 Dorsolateral Caudate Thalamus

0.3 (-+0.1) 0

0 0

2.7 (+_0.8) 0.3 (+0.4) 0.2 (+_0.4) 2.1 (_+0.8) 0.4 (_+0.8)

0 0 0 0 0

After behavioral training, brains of all rats were perfusion-fixed with F A M (40% formaldehyde, glacial acetic acid, methanol, 1:1:8) as described previously [7]. Brain sections (7/~) were taken at the level of the anterior commissure and anterior hippocampus, and stained with hematoxylin and eosin. Brain sections were graded for damage without knowledge of the experimental condition. Damage was graded on a scale of 0--3 with 0=normal brain, l = a few neurons damaged, 2 = m a n y neurons damaged, and 3 =majority of neurons damaged. RESULTS

ously [4, 6, 15]. F o r maze adaptation individual rats were allowed to explore the maze for 15 minutes on three consecutive days with 94 nag food pellets scattered over the entire surface. F o r training, rats were given daily trials for 70 days with five of eight arms baited. The five baited arms for each rat remained constant relative to room cues throughout the experiment. Rats were allowed to make arm choices until either all 5 pellets were taken, 16 choices were made, or 10 minutes elapsed. After the training trials rats were subjected to forebrain ischemia and given 30 recovery days prior to returning to the maze. Fifty additional daily trials were given postoperatively. F o r statistical analysis, an initial entry of a baited arm was

Neuropathology The severity and distribution of ischemic neuronal damage at the level o f the anterior commissure (bregrna - 0 . 3 ram) to the anterior hippocampus (bregma - 3 . 3 mm) are shown in Table 1. Previously, we reported that moderate to severe neuronal damage of the CA1 region extended to mid-dorsal regions of the posterior hippocampus [4].

Behavior An analysis of variance with repeated measured using group and trials as independent variables was performed on

MEMORY AFTER TRANSIENT FOREBRAIN ISCHEMIA

785

WORKING ERRORS P R E T R A I N E O POSTOPERATIVE

RATS

TRIALS

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i

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I 75

I 80

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I I 100

I 105

~ I 110

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I 115

1 120

BLOCKS OF FIVE TRIALS

FIG. 2. Mean postoperative working errors (-+SEM) for PI (n=9) and control rats (n= 15) in blocks of 5 trials.

each type of error. For the 70 pretraining trials no significant difference between control and PI animals was detected for either reference or working performance [reference-F(1,22)=1.61, p>0.10; working--F(1,22)=l.83, p>0.10]. For the postoperative trials, the mean number of reference errors and working errors in blocks of 5 trials are shown in Fig. 1 and Fig. 2 respectively. No significant difference was detected between PI and control rats in reference performance, F(1,22)=3.07, p>0.05. PI rats and control rats demonstrated improved postoperative reference performance, F(49,1078)=3.7, p<0.01, and no interaction between group and trials was detected, F(49,1078)=1.34, p>0.05. PI rats made significantly more working errors than control rats, F(1,22)= 12.3, p<0.01. Both PI and control rats showed improved working performance, F(49,1078)=3.9, p<0.01, but a significant interaction between group and trials was detected, F(49,1078)=1.96, p<0.01. Application of the Least Significant Difference test at the 0.05 level indicated that the working performance of PI rats was not significantly different from control performance after 15 trials postoperatively. DISCUSSION

Reference memory for baited arms in the radial maze was not impaired in PI rats given extensive training prior to ischemic induced hippocampal damage. This is consistent with the idea that hippocampus is not required for behavioral expression of some types of information consolidated prior to hippocampal damage [9]. Normal rats in the pretraining condition reach asymptote performance on the reference aspect of this task after about thirty trials. Rats then received an additional 40 daffy trials. Thus, PI rats demonstrate retention of this over learned habit when returned to the maze

30 days after iscbemic insult. Similarly, humans who have ischemic hippocampal damage retain information learned months or years prior to the ischemic episode [9, 10, 16]. While an overall analysis of postoperative training indicated impaired working memory of PI rats, no significant impairment was detected with a post-hoc analysis by fifteen trials. This finding might be taken as support for the view that working memory eventually recovers in PI rats with CA1 hippocampal damage. However, several lines of evidence suggests that this is not always the case. Several studies have shown that working memory performance will vary as a function of the level of pretraining and the level of task difficulty (see [5] for review). Similarly, we have found when the difficulty level of the working memory aspect of the radial maze is increased by baiting all eight arms, PI rats demonstrate a persistent and significant working memory deficit [3]. Specifically, PI rats pretrained for 55 trials and then given an additional 140 trials with all eight maze arms baited were significantly impaired over the last forty trials as compared to control rats (p<0.05). During these last forty trials there was no trial or trial x group effect which indicated PI rats were not likely to improve their performance to control level. Further, when task difficulty was increased with a 12-arm radial maze in which seven arms were baited, and in which rats were pretrained for 35 trials, PI rats were significantly impaired on working memory as compared to controls despite having reached their maximal level of performance [13,14]. While PI rats have neuronal loss in dorsolateral caudate and the candate has been implicated in memory processes (see [1] for review), evidence suggests damage to this area cannot account for the initial working deficit observed in this study or for the persistent working memory deficit of PI rats observed in other reports [3, 13, 14]. In a preliminary report we

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s h o w e d that rats with large radiofrequency lesions o f the dorsolateral caudate p e r f o r m e d normally on the working m e m o r y aspect of a radial 12-arm maze, and had equivalent times for arm choices [14]. Working m e m o r y p e r f o r m a n c e of PI rats was significantly impaired as c o m p a r e d to controls or caudate lesioned animals. In conclusion, this study shows that P1 rats retain information learned prior to ischemic insult that most severely damages C A I neurons of hippocampus. Further, our study

provides additional support for the view that PI rats model the m e m o r y deficits induced by ischemic damage to the hipp o c a m p u s in some survivors of cardiac arrest. ACKNOWLEDGEMENTS This research was supported by the Burke Foundation, and a USPHS Grant NS03346, and a Grant-In-Aid (851098) from the American Heart Association to B.T.V.

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