Differential Effects of Scopolamine on Working and ... - Semantic Scholar
Pharmacology Biochemistry & Behavior, Vol. 20, pp. 659--662, 1984. ©Ankho International Inc. Printed in the U.S.A.
0091-3057/84 $3.00 + .00
Differential Effects of Scopolamine on Working and Reference Memory of Rats in the Radial Maze B. A. W I R S C H I N G , * R I C H A R D J. BENINGER,* K. J H A M A N D A S , t R. J. B O E G M A N t
A N D S. R. E L - D E F R A W Y t
Departments o f * P s y c h o l o g y and tPharmacology, Queen's University Kingston Ontario, K7L 3N6, Canada R e c e i v e d 3 O c t o b e r 1983 WIRSCHING, B. A., R. J. BENINGER, K. JHAMANDAS, R. J. BOEGMAN AND S. R. EL-DEFRAWY. Differential effects of scopolamine on working and reference memory of rats in the radial maze. PHARMACOL BIOCHEM BEHAV 20(5) 659--662, 1984.--Anticholinergics have often been found to impair choice accuracy in the radial maze. Some researchers have suggested that this indicates involvement of cholinergically innervated structures in cognitive mapping while others argue that these structures mediate working memory. However, most results are open to either interpretation since the baiting method did not allow a distinction between reference and working memory errors. To further test these hypotheses this study examined the effects of systemic scopolamine on radial maze performance, using a 4-out-of-8 baiting procedure. Food-deprived Wistar rats were pretrained until working memory choice accuracy stabilized to a criterion of 87% or better. Scopolamine (0.1, 0.4 and 0.8 mg/kg, IP, 30 min before a session) significantly increased the number of working memory errors (re-entries into baited arms) whereas reference memory errors (entries into never baited arms) did not change significantly. Observed deficits appeared not to be attributable to a drug-induced disruption of motivational systems. Results confirm the behavioural similarities between the memorial effects of hippocampectomy and anticholinergics, and implicate cholinergically innervated structures in working memory. Working memory
Reference memory
Radial maze
IT has generally been accepted that the hippocampus plays a critical role in human memory. With the repeated observation that hippocampal lesions severely disrupt accuracy in the radial-arm maze this view has been extended to include animal memory [8,15]. However, in the animal literature, there is little consensus regarding the type of memory systems with which the hippocampus is involved [11, 12, 13, 14,
Scopolamine
Acetylcholine
Cholinergic neurons
trials. The baited arms, on the Other hand, formed a working memory component since the information concerning which of the baited arms had already been visited was only useful for that particular trial. It was reasoned that hippocampal rats would err on both the baited and unbaited arms if the impairment reflected a cognitive deficit. On the other hand, if the impairment was of working memory, animals would re-enter the baited arms from which food had already been eaten. Results indicated that rats with fimbria-fornix lesions performed at chance levels on the working memory component of the task while reference memory was unimpaired. These results support the hypothesis that the hippocampal system may be selectively involved in working memory. Since the hippocampal region is strongly innervated with cholinergic fibres [3,9] studies utilizing anticholinergics such as scopolamine have also attempted to evaluate the cognitive mapping versus working memory hypotheses. Although anticholinergics produce behavioural effects on radial maze performance that closely parallel those arising from hippocampal ablations [2, 5, 17, 18], it has not been possible to conclusively argue in favour of either hypothesis, since the baiting method employed did not allow a distinction between working and reference memory. The present study was undertaken to further test the two hypotheses by examining the effects of scopolamine on performance in a partially baited radial maze, with only 4 of the 8 arms baited. According to the cognitive mapping theory drugged rats should err
15]. O ' K e e f e and associates [11,12] proposed a cognitive mapping theory suggesting that the hippocampus processes and stores spatial information. This system provides the animal with a mapping strategy which enables it to locate itself in a geographic environment as well as generate place hypotheses about that environment. In support of this notion it has been found that hippocampal damage leads to a marked deficit in ability to perform maze tasks, particularly those requiring place learning [11]. In contrast to this spatial interpretation is the working memory hypothesis [13, 14, 15] suggesting that behavioural impairments observed in maze performance following hippocampal ablations stem from an inability to solve the working memory component. For instance, Olton and Papas [15] showed a differential involvement of the hippocampal system in reference versus working memory by only baiting 8 of 17 arms in a radial maze. The unbaited set formed a reference memory component of the task since the information that no food was to be found on these arms was useful for all
659
W I R S C H I N G ET AL.
660 TABLE 1 DRUG DESIGN Order 1 2 3 4 5 6
Baseline BL BL BL BL BL
Saline Saline Saline Saline Saline Saline
1 1 l 1 1 1
BL BL BL BL BL BL
0.1" 0.! 0.4 0.4 0.8 0.8
BL BE BL BL BL BL
0.4* 0.8 0.8 0.1 0.4 0.1
BL BL BL BL BL BL
0.8* 0.4 0.1 0.8 0.1 0.4
BL BL BL BL BL BL
Saline Saline Saline Saline Saline Saline
2 2 2 2 2 2
*Dose of scopolamine (mg/kg). TABLE 2 COMPARISON OF SALINE WITH PRECEDING BASELINE BL
S~
BL
S,,
Working Memory Errors Mean (_+SEM)
0.08 (+0.08)
0.33 (+0.19)
0.67 (_+0.22)
0.58 (_+0.23)
Reference Memory Errors Mean (_+SEM)
1.8 (+0.11)
2.4 (_+0.36)
0.33 (_+0.14)
0.58 (_+0.23)
n=12.
on both the unbaited and baited arms. The working memory h y p o t h e s i s , o n the o t h e r h a n d , w o u l d p r e d i c t d i s r u p t e d perf o r m a n c e o n only t h e b a i t e d a r m s . METHOD
Subjects E i g h t e e n e x p e r i m e n t a l l y n a i v e male a l b i n o rats o f the W i s t a r strain were individually h o u s e d in a climatically controlled r o o m o n a 12 h r light/dark cycle. Initial free feeding weights of 200 to 270 g w e r e d e c r e a s e d to 80% ( a d j u s t e d for g r o w t h ) b y daily feeding with m e a s u r e d rations.
Apparatus T h e radial m a z e , e l e v a t e d 50 c m a b o v e the floor, consisted o f a n o c t a g o n a l central p l a t f o r m (30 c m wide) surr o u n d e d b y 8 equally s p a c e d radial a r m s (65 c m l o n g × 10 cm wide). F o o d wells, located 1 c m f r o m the e n d o f e a c h a r m w e r e 1.0 c m d e e p a n d 1.5 c m in d i a m e t e r . T e s t i n g w a s c a r r i e d o u t in a w h i t e p a i n t e d r o o m lit by 70 W f l u o r e s c e n t t u b e s . S e v e r a l visually d i s t i n c t c u e s (e.g., door, shelf) w e r e p r e s e n t in the r o o m a n d r e m a i n e d in the s a m e p o s i t i o n with r e s p e c t to the m a z e .
Procedure D e p r i v a t i o n was b e g u n o n e w e e k p r i o r to testing. D u r i n g this p e r i o d e a c h rat was h a n d l e d daily for a p p r o x i m a t e l y o n e min. O n d a y 5 of d e p r i v a t i o n a n i m a l s were fed a small q u a n tity of F r o o t L o o p s cereal in t h e i r h o m e cage as small pieces s u b s e q u e n t l y were utilized as reinforcers. Pretraining. O n d a y 8 o f d e p r i v a t i o n a n i m a l s were p l a c e d for 10 min in pairs o n the c e n t r a l h u b o f the m a z e w i t h F r o o t L o o p pieces s c a t t e r e d o n the p l a t f o r m a n d a r m s . O n d a y 10 rats w e r e p l a c e d singly on the m a z e . Again food was scatt e r e d o n the p l a t f o r m a n d along a r a n d o m l y p r e d e t e r m i n e d
s u b s e t o f only 4 a r m s , r e f e r r e d to as the baited a r m s . T h e baiting p a t t e r n r e m a i n e d the same t h r o u g h o u t the experim e n t b u t varied from rat to rat. During d a y s 10 to 13 the 4 a r m s w e r e r e b a i t e d until the rat l e a r n e d to r u n to the e n d a n d collect the food in 10 min o r less. Type of arm e n t r y (baited or u n b a i t e d ) was r e c o r d e d . A n a r m e n t r y was defined as crossing a line 10 c m into e a c h arm. A f t e r e a c h trial the m a z e was c l e a n e d with a 2.5% c i d a r v i n e g a r solution. Formal training. E a c h rat r e c e i v e d o n e s e s s i o n p e r day, 7 d a y s a week. At t h e start o f e a c h session, the 4 p r e d e t e r m i n e d a r m s were baited at t h e i r distal e n d (note t h a t a r m s w e r e not r e b a i t e d within a session). E a c h rat w a s p l a c e d on the p l a t f o r m and left until all 4 baits were collected, 14 c h o i c e s were m a d e or 10 min had elapsed, w h i c h e v e r c a m e first. Training c o n t i n u e d until c h o i c e a c c u r a c y stabilized o v e r 4 days to a n a v e r a g e c r i t e r i o n o f 87% or b e t t e r ; thus a score o f 100% (4 out o f 4 u n r e p e a t e d baited a r m e n t r i e s within the first 4 c h o i c e s ) on at least 2 o f the 4 days w a s required. Drug testing. This p h a s e c o n s i s t e d of 5 4-day b l o c k s of drug t r e a t m e n t with o n e session p e r day. E a c h t r e a t m e n t b l o c k was followed b y n o n - d r u g b a s e l i n e s e s s i o n s that cont i n u e d until t h e 4-day c r i t e r i o n o f 87% or b e t t e r was ree s t a b l i s h e d . E a c h s e s s i o n o f the first a n d fifth t r e a t m e n t b l o c k was p r e c e d e d 30 rain b y an IP injection o f saline (1 ml/kg). S e s s i o n s o f t h e s e c o n d , third a n d f o u r t h t r e a t m e n t b l o c k s were p r e c e d e d 30 min b y a n IP injection of s c o p o l a m i n e h y d r o b r o m i d e (Sigma C h e m i c a l Co.) dissolved in distilled water, at d o s e s o f 0.1, 0.4 or 0.8 mg/kg. R a t s were r a n d o m l y a s s i g n e d to o n e of the six p o s s i b l e d o s a g e o r d e r s (see T a b l e 1). Dependent measures. Type o f e r r o r a n d r e i n f o r c e m e n t r e c e i p t s w e r e r e c o r d e d . T h e first e n t r y into a baited a r m r e g a r d l e s s o f w h e t h e r or not the bait was collected was s c o r e d as a c o r r e c t choice, while a r e - e n t r y into t h a t a r m was s c o r e d as a w o r k i n g m e m o r y error. E n t r i e s into n e v e r b a i t e d
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0091-3057/84 $3.00 + .00
Differential Effects of Scopolamine on Working and Reference Memory of Rats in the Radial Maze B. A. W I R S C H I N G , * R I C H A R D J. BENINGER,* K. J H A M A N D A S , t R. J. B O E G M A N t
A N D S. R. E L - D E F R A W Y t
Departments o f * P s y c h o l o g y and tPharmacology, Queen's University Kingston Ontario, K7L 3N6, Canada R e c e i v e d 3 O c t o b e r 1983 WIRSCHING, B. A., R. J. BENINGER, K. JHAMANDAS, R. J. BOEGMAN AND S. R. EL-DEFRAWY. Differential effects of scopolamine on working and reference memory of rats in the radial maze. PHARMACOL BIOCHEM BEHAV 20(5) 659--662, 1984.--Anticholinergics have often been found to impair choice accuracy in the radial maze. Some researchers have suggested that this indicates involvement of cholinergically innervated structures in cognitive mapping while others argue that these structures mediate working memory. However, most results are open to either interpretation since the baiting method did not allow a distinction between reference and working memory errors. To further test these hypotheses this study examined the effects of systemic scopolamine on radial maze performance, using a 4-out-of-8 baiting procedure. Food-deprived Wistar rats were pretrained until working memory choice accuracy stabilized to a criterion of 87% or better. Scopolamine (0.1, 0.4 and 0.8 mg/kg, IP, 30 min before a session) significantly increased the number of working memory errors (re-entries into baited arms) whereas reference memory errors (entries into never baited arms) did not change significantly. Observed deficits appeared not to be attributable to a drug-induced disruption of motivational systems. Results confirm the behavioural similarities between the memorial effects of hippocampectomy and anticholinergics, and implicate cholinergically innervated structures in working memory. Working memory
Reference memory
Radial maze
IT has generally been accepted that the hippocampus plays a critical role in human memory. With the repeated observation that hippocampal lesions severely disrupt accuracy in the radial-arm maze this view has been extended to include animal memory [8,15]. However, in the animal literature, there is little consensus regarding the type of memory systems with which the hippocampus is involved [11, 12, 13, 14,
Scopolamine
Acetylcholine
Cholinergic neurons
trials. The baited arms, on the Other hand, formed a working memory component since the information concerning which of the baited arms had already been visited was only useful for that particular trial. It was reasoned that hippocampal rats would err on both the baited and unbaited arms if the impairment reflected a cognitive deficit. On the other hand, if the impairment was of working memory, animals would re-enter the baited arms from which food had already been eaten. Results indicated that rats with fimbria-fornix lesions performed at chance levels on the working memory component of the task while reference memory was unimpaired. These results support the hypothesis that the hippocampal system may be selectively involved in working memory. Since the hippocampal region is strongly innervated with cholinergic fibres [3,9] studies utilizing anticholinergics such as scopolamine have also attempted to evaluate the cognitive mapping versus working memory hypotheses. Although anticholinergics produce behavioural effects on radial maze performance that closely parallel those arising from hippocampal ablations [2, 5, 17, 18], it has not been possible to conclusively argue in favour of either hypothesis, since the baiting method employed did not allow a distinction between working and reference memory. The present study was undertaken to further test the two hypotheses by examining the effects of scopolamine on performance in a partially baited radial maze, with only 4 of the 8 arms baited. According to the cognitive mapping theory drugged rats should err
15]. O ' K e e f e and associates [11,12] proposed a cognitive mapping theory suggesting that the hippocampus processes and stores spatial information. This system provides the animal with a mapping strategy which enables it to locate itself in a geographic environment as well as generate place hypotheses about that environment. In support of this notion it has been found that hippocampal damage leads to a marked deficit in ability to perform maze tasks, particularly those requiring place learning [11]. In contrast to this spatial interpretation is the working memory hypothesis [13, 14, 15] suggesting that behavioural impairments observed in maze performance following hippocampal ablations stem from an inability to solve the working memory component. For instance, Olton and Papas [15] showed a differential involvement of the hippocampal system in reference versus working memory by only baiting 8 of 17 arms in a radial maze. The unbaited set formed a reference memory component of the task since the information that no food was to be found on these arms was useful for all
659
W I R S C H I N G ET AL.
660 TABLE 1 DRUG DESIGN Order 1 2 3 4 5 6
Baseline BL BL BL BL BL
Saline Saline Saline Saline Saline Saline
1 1 l 1 1 1
BL BL BL BL BL BL
0.1" 0.! 0.4 0.4 0.8 0.8
BL BE BL BL BL BL
0.4* 0.8 0.8 0.1 0.4 0.1
BL BL BL BL BL BL
0.8* 0.4 0.1 0.8 0.1 0.4
BL BL BL BL BL BL
Saline Saline Saline Saline Saline Saline
2 2 2 2 2 2
*Dose of scopolamine (mg/kg). TABLE 2 COMPARISON OF SALINE WITH PRECEDING BASELINE BL
S~
BL
S,,
Working Memory Errors Mean (_+SEM)
0.08 (+0.08)
0.33 (+0.19)
0.67 (_+0.22)
0.58 (_+0.23)
Reference Memory Errors Mean (_+SEM)
1.8 (+0.11)
2.4 (_+0.36)
0.33 (_+0.14)
0.58 (_+0.23)
n=12.
on both the unbaited and baited arms. The working memory h y p o t h e s i s , o n the o t h e r h a n d , w o u l d p r e d i c t d i s r u p t e d perf o r m a n c e o n only t h e b a i t e d a r m s . METHOD
Subjects E i g h t e e n e x p e r i m e n t a l l y n a i v e male a l b i n o rats o f the W i s t a r strain were individually h o u s e d in a climatically controlled r o o m o n a 12 h r light/dark cycle. Initial free feeding weights of 200 to 270 g w e r e d e c r e a s e d to 80% ( a d j u s t e d for g r o w t h ) b y daily feeding with m e a s u r e d rations.
Apparatus T h e radial m a z e , e l e v a t e d 50 c m a b o v e the floor, consisted o f a n o c t a g o n a l central p l a t f o r m (30 c m wide) surr o u n d e d b y 8 equally s p a c e d radial a r m s (65 c m l o n g × 10 cm wide). F o o d wells, located 1 c m f r o m the e n d o f e a c h a r m w e r e 1.0 c m d e e p a n d 1.5 c m in d i a m e t e r . T e s t i n g w a s c a r r i e d o u t in a w h i t e p a i n t e d r o o m lit by 70 W f l u o r e s c e n t t u b e s . S e v e r a l visually d i s t i n c t c u e s (e.g., door, shelf) w e r e p r e s e n t in the r o o m a n d r e m a i n e d in the s a m e p o s i t i o n with r e s p e c t to the m a z e .
Procedure D e p r i v a t i o n was b e g u n o n e w e e k p r i o r to testing. D u r i n g this p e r i o d e a c h rat was h a n d l e d daily for a p p r o x i m a t e l y o n e min. O n d a y 5 of d e p r i v a t i o n a n i m a l s were fed a small q u a n tity of F r o o t L o o p s cereal in t h e i r h o m e cage as small pieces s u b s e q u e n t l y were utilized as reinforcers. Pretraining. O n d a y 8 o f d e p r i v a t i o n a n i m a l s were p l a c e d for 10 min in pairs o n the c e n t r a l h u b o f the m a z e w i t h F r o o t L o o p pieces s c a t t e r e d o n the p l a t f o r m a n d a r m s . O n d a y 10 rats w e r e p l a c e d singly on the m a z e . Again food was scatt e r e d o n the p l a t f o r m a n d along a r a n d o m l y p r e d e t e r m i n e d
s u b s e t o f only 4 a r m s , r e f e r r e d to as the baited a r m s . T h e baiting p a t t e r n r e m a i n e d the same t h r o u g h o u t the experim e n t b u t varied from rat to rat. During d a y s 10 to 13 the 4 a r m s w e r e r e b a i t e d until the rat l e a r n e d to r u n to the e n d a n d collect the food in 10 min o r less. Type of arm e n t r y (baited or u n b a i t e d ) was r e c o r d e d . A n a r m e n t r y was defined as crossing a line 10 c m into e a c h arm. A f t e r e a c h trial the m a z e was c l e a n e d with a 2.5% c i d a r v i n e g a r solution. Formal training. E a c h rat r e c e i v e d o n e s e s s i o n p e r day, 7 d a y s a week. At t h e start o f e a c h session, the 4 p r e d e t e r m i n e d a r m s were baited at t h e i r distal e n d (note t h a t a r m s w e r e not r e b a i t e d within a session). E a c h rat w a s p l a c e d on the p l a t f o r m and left until all 4 baits were collected, 14 c h o i c e s were m a d e or 10 min had elapsed, w h i c h e v e r c a m e first. Training c o n t i n u e d until c h o i c e a c c u r a c y stabilized o v e r 4 days to a n a v e r a g e c r i t e r i o n o f 87% or b e t t e r ; thus a score o f 100% (4 out o f 4 u n r e p e a t e d baited a r m e n t r i e s within the first 4 c h o i c e s ) on at least 2 o f the 4 days w a s required. Drug testing. This p h a s e c o n s i s t e d of 5 4-day b l o c k s of drug t r e a t m e n t with o n e session p e r day. E a c h t r e a t m e n t b l o c k was followed b y n o n - d r u g b a s e l i n e s e s s i o n s that cont i n u e d until t h e 4-day c r i t e r i o n o f 87% or b e t t e r was ree s t a b l i s h e d . E a c h s e s s i o n o f the first a n d fifth t r e a t m e n t b l o c k was p r e c e d e d 30 rain b y an IP injection o f saline (1 ml/kg). S e s s i o n s o f t h e s e c o n d , third a n d f o u r t h t r e a t m e n t b l o c k s were p r e c e d e d 30 min b y a n IP injection of s c o p o l a m i n e h y d r o b r o m i d e (Sigma C h e m i c a l Co.) dissolved in distilled water, at d o s e s o f 0.1, 0.4 or 0.8 mg/kg. R a t s were r a n d o m l y a s s i g n e d to o n e of the six p o s s i b l e d o s a g e o r d e r s (see T a b l e 1). Dependent measures. Type o f e r r o r a n d r e i n f o r c e m e n t r e c e i p t s w e r e r e c o r d e d . T h e first e n t r y into a baited a r m r e g a r d l e s s o f w h e t h e r or not the bait was collected was s c o r e d as a c o r r e c t choice, while a r e - e n t r y into t h a t a r m was s c o r e d as a w o r k i n g m e m o r y error. E n t r i e s into n e v e r b a i t e d
E F F E C T S OF S C O P O L A M I N E ON M E M O R Y (/3
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