Bates College. Rats with lesions of the medial septum were more likely to begin swimming in the wrong direction, swim farther, and, therefore, require more time ...
Behavioral Neuroscience 1988, Vol. 102, No. 2. 289-293
Copyright 1988 by the American Psychological Association, Inc. 07 35-7044/88/S00.75
Medial Septal Lesions Disrupt Spatial Mapping Ability in Rats
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
John E. Kelsey and Beth A. Landry Bates College Rats with lesions of the medial septum were more likely to begin swimming in the wrong direction, swim farther, and, therefore, require more time to find a platform hidden in a Morris water tank than were control rats. Although the performance of the rats with medial septal lesions did improve over trials, their asymptotic performance also failed to equal that of the controls. Movement of the platform to a new position in the tank disrupted the performance of both groups, and, again, the rats with medial septal lesions were slower to locate the moved platform. However, this deficit was completely eliminated when a visual cue indicating the location of the moved platform was introduced. We suggest that these data indicate that damage to the septo-hippocampal cholinergic projection system produces a deficit in the formation or utilization of a spatial map (reference memory) that represents the location of a place with respect to the surrounding distal cues.
The two cholinergic pathways of the basal forebrain, one projecting from the medial septal area and diagonal band of Broca to the hippocampus (Lewis & Shute, 1967) and the other one projecting from the nucleus basalis of Meynert (nbM) to the cortex (Wainer, Levey, Mufson, & Mesulam, 1984), have both been implicated in spatial memory. For example, lesions of the neurons originating in the medial septum or the nbM (Hepler, Wenk, Cribbs, Olton, & Coyle, 1985), bilateral damage to the afferent and efferent connections of the hippocampus (Olton, Walker, & Gage, 1978), and systemic injections of cholinergic receptor blockers such as scopolamine (Watts, Stevens, & Robinson, 1981) all disrupt performance in the radial-arm maze. Rats with these manipulations were more likely than control rats to reenter arms they had previously visited and depleted of food on that trial; that is, they behaved as though they could not remember the location of the arms they had already visited. Many of these same manipulations have also been shown to produce deficits in another test of spatial memory, the Morris water task (Morris, 1981, 1984). In this task, a rat is placed in a circular tank filled with opaque water and, in order to escape, is required to swim to a small platform hidden just below the surface of the water in a fixed location in the tank. Rats with nbM or hippocampal lesions or injections of the cholinergic receptor blocker, atropine, all required more time to find the platform than did control rats (Morris, Garrud, Rawlins, & O'Keefe, 1982; Sutherland, Whishaw, & Kolb, 1983; Whishaw, O'Connor, & Dunnett, 1985). These effects did not reflect a deficit in swimming but appeared to reflect a deficit in locating the platform. Consequently, these investigators concluded that these treatments disrupted the ability to form or utilize a spatial map (reference memory)
that represents the location of the platform with respect to the distal cues surrounding the tank. Although lesions of the medial septum, which disrupt the cholinergic projection to the hippocampus, have been shown to disrupt performance in the radial-arm maze (Hepler et al., 1985), the effect of these lesions on performance in the Morris water task has not been examined. If the deficits produced by hippocampal lesions and injections of atropine in the Morris water task are due to damage to the septo-hippocampal cholinergic projection system as surmised, then lesions of the medial septum should produce similar deficits. The intent of the present experiment was to examine this hypothesis by determining if lesions of the medial septum in rats would disrupt the ability to find a platform hidden in a Morris water tank. Following initial acquisition, the effects of these lesions on spatial mapping ability were further assessed by moving the platform to a new location and, for half of the animals, indicating the location of the moved platform with a visual cue.
Method
Subjects The subjects were 24 male Sprague-Dawley rats from Sasco-King Laboratories that weighed from 324 to 500 g at the beginning of testing. They were single housed in a colony that was lighted from 0700 to 1900 hr each day, and they had ad-lib access to food and water except during testing.
Apparatus The grey stainless steel circular water tank was 137 cm in diameter and 71 cm deep. The tank was placed in a large teaching laboratory between a row of laboratory benches and a row of unshaded windows. The 5.3 x 5.3 x 33.5-cm platform was made of wire mesh spaced 0.6 cm apart and was painted white. A yellow swing-arm lamp was used as a visual cue to indicate the location of the platform for some of the rats.
This study is based on research submitted to Bates College by Beth A. Landry as partial fulfillment of the requirements for a Bachelor of Science degree. Beth A. Landry is now at the Institute for Biogerontology Research, Sun City, Arizona. Correspondence should be addressed to John E. Kelsey, Department of Psychology, Bates College, Lewiston, Maine 04240.
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Surgery
Histology
All rats were anesthetized with sodium pentobarbital ip (Ncmbutal; 50 mg/kg) and placed in a stereotax. Electrolytic lesions were made in the medial septum of 12 rats by passing 1.5 mA of anodal dc current for 20 s through an electrode, which had been implanted at a 10° angle to the following coordinates: AP = 8.4, H = 0.2, and L = 0.0 (Pellegrino & Cushman, 1967). The electrode was a No. 1 stainless steel insect pin that was insulated except for the flattened cross section of the tip. Sham lesions for which the electrode was lowered to within 1 mm of the septum and no current was passed were performed on 5 rats, and sham lesions for which holes were drilled in the skull but no electrode was lowered were performed on 3 rats. Four control rats were unoperated. Because these three procedures for producing control rats did not differentially affect performance in the Morris water task, these 12 rats formed a single control group. Testing began 14 to 50 days following surgery.
Following behavioral testing, the rats with lesions were sacrificed with an overdose of ether. Each rat was then perfused intracardially with isotonic saline followed by 10% formol-saline. The brains were frozen, and 64-pm thick sections were taken through the area of the lesion. These sections were mounted on slides, stained with cresyl violet, and examined through a dissecting microscope.
Procedure Pairs of rats from the same group (lesion or control) were run for six trials per day for 7 consecutive days. For the first 4'A days (27 trials), the platform was hidden in the same location in the pool, 30 cm from the tank wall. For the next 2'/2 days (Trials 28-42), the platform was moved to a new location 130° clockwise from the original position and 20 cm from the wall. During these latter 15 trials, the location of the moved platform was indicated for half of the rats from each group by attaching the swing-arm lamp to a lab bench and extending the lamp so that the portion holding the bulb was about 30 cm directly above the platform. The procedure each day was as follows. The platform was positioned in the pool, and the pool was filled with 20 °C water to a depth of 36 cm, which was about 2.5 cm above the top of the platform. Powdered milk (1,300 ml) was added to the water to make the white platform less visible. Following an ip injection of saline (I ml/kg), the rats were taken to the experimental room in plastic cages containing wood shavings and placed on the lab benches above the pool for 20 min. Each rat was then placed directly onto the platform in the pool for 45 s, so that the rat could determine the position of the platform with respect to the pool and the consistent surrounding distal cues such as the lab benches, windows, overhead lights, and experimenter. After 3 min in its cage, each rat was then placed into the pool facing the wall at one of three starting positions. One starting position was 43° clockwise from the platform's initial location, and the other two were spaced 155° and 210° clockwise from the first position. The starting position for each trial was randomly chosen daily, such that each position was used within each block of three trials. After the rat was released, its swimming path was drawn on a representation of the pool, and the time required to reach the platform was recorded to the nearest 0.1 s with a stopwatch. If the rat failed to find the platform within 120 s, the experimenter guided the rat to the platform using the rat's tail as a rudder. When the rat reached the platform, it was allowed to remain there for 45 s for purposes of orientation. Each rat was then returned to its cage for an intertrial interval (ITI) of approximately 3 min during which time the second rat in the pair received its trial in the pool. Trials were alternated between each rat in a pair in this fashion until each rat had received six trials per day. Following the first three trials on Day 5, the platform was moved to its new location. Rats from each of the two main groups, lesion and control, were divided into two equivalent subgroups based on their swimming latencies on Day 4. For two subgroups, one from each main group, the position of the moved platform remained unmarked. For the remaining two subgroups, the location of the moved platform was indicated by the suspended yellow lamp. Except for these changes, trials continued as before for 2 Vi days (15 trials).
Data Analysis The main data consisted of averages of the escape latencies for the first three and last three trials each day. All of the statistical tests conducted were three-way groups (2) by cue (2) by blocks of trials (2-9) analyses of variance with repeated measures on the last factor. Following attainment of a significant groups by cue interaction, simple effects tests were conducted. Following attainment of a significant triple interaction, subsequent comparisons were made by means of Newman-Keuls tests. To examine the effects of the lesions on acquisition, the latencies for the first nine blocks of three trials (Days 1-4'A) were analyzed. To examine the effects of the visual cue, the latencies for the trials following movement of the platform (the last five blocks of trials on Days 4'/2-7) were analyzed separately. Similar, but less complete, analyses were also performed on the mean distance swum and on the median initial heading error as measured after the rat had swum 25 cm. Distance swum was measured by tracing the drawing of the path swum with a map gauge, and the initial heading error was measured as the angle between the tangent to the rat's path after 25 cm of swimming and the platform.
Results histological As can be seen in Figure 1, the medial septal lesions extended from AP = 9.4 to AP = 7.0, with the maximal
Figure 1.
Reconstruction of a typical medial septal lesion.
SEPTAL LESIONS DISRUPT SPATIAL MAPPING
damage occurring at AP = 8.2 (Pellegrino & Cushman, 1967). These lesions consistently destroyed the medial septal nucleus and the dorsal tip of the diagonal band of Broca. Damage to the lateral septal nuclei was incomplete and frequently unilateral. Although many of the lesions damaged the anterior medial parolfactorial area, damage to the posterior septal nuclei was slight. The data from 2 rats, 1 from each subgroup, were eliminated, because the lesions of these rats produced only unilateral damage to the medial septal nucleus.
This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.
Behavioral Escape latency and distance swum. Analyses of the escape latencies and distance swum revealed identical patterns of effects both prior to and following movement of the platform. Consequently, only the analyses of the escape latency data are presented here. As can be seen in the left portion of Figure 2, the control rats rapidly learned to locate the hidden platform, requiring an average of only 5 s per trial by Day 4. In contrast, the rats with medial septal lesions acquired this task more slowly. These conclusions were supported by a significant main effect of blocks, F(8, 144) = 22.50, p < .001, which indicates improvement across trials, and a significant main effect of groups, F(\, 18) = 13.32, p < .01, which indicates that the rats with medial septal lesions required more time to reach the platform than did the controls. An insignificant groups by blocks interaction, .F(8, 144) = 0.88, indicated that although the rats with medial septal lesions were consistently
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slower to find the platform, they improved their swimming times across trials as rapidly as did the controls. As can be seen in the right portion of Figure 2, moving the platform to a new location after the first three trials of Day 5 disrupted the performance of all four groups. Moreover, as indicated by a significant groups by cue interaction, f\{, 18) = 6.58, p < .05, and a significant groups by cue by blocks interaction, F(4, 72) = 2.74, p < .05, providing a visual cue that indicated the location of the moved platform did not affect the rats with medial septal lesions and the control rats equally. Similar to initial acquisition, the rats with medial septal lesions that received no visual cue required more time to find the moved platform than did the control rats, F(l, 18) = 27.70, p < .001. In marked contrast, the rats with medial septal lesions that were provided with the visual cue learned to find the platform as rapidly as the control rats, F(l, 18) = 2.63, p > .10, and more rapidly than the rats with medial septal lesions that received no cue, F( 1, 18) = 21.82, p < .001. Although the presence of the visual cue also helped the control rats on the first three trials, p < .05, this facilitating effect quickly disappeared. By the second block of three trials, the control rats that received the visual cue were escaping no faster than the controls that received no cue. Initial heading error. Analysis of the initial heading errors measured after 25 cm of swimming revealed a slightly different pattern of effects. First, these data were so variable that we elected to use the median rather than the mean to represent performance for a three-trial block. An analysis of the heading
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errors on the first three trials of Day 1, first three trials of Day 5 just before moving the platform, and the last three trials of Day 7 revealed a significant groups effect, F(l, 18) = 7.55, p < .05, which indicates that the rats with medial septal lesions were less accurate in their initial headings than were the control rats. As can be seen in Figure 3, this difference occurred primarily on Days 5 and 7. Although the groups by cue by blocks interaction was not significant, F{2, 36) = 1.41, subsequent analyses indicated that whereas the control rats improved their heading errors during training from Day 1 to Day 5, p < .05, the rats with medial septal lesions did not. In contrast to the effects on latency and distance swum, however, there was no evidence that providing a visual cue after the platform was moved was helpful in correcting the initial heading errors of the rats with medial septal lesions (see Figure 3). Neither the main effect of cue or any of the double interactions with cue were close to being significant, Fs(2, 36) < 0.90. Discussion Our data demonstrated that rats with medial septal lesions were deficient at finding a platform hidden in a Morris water tank. Even after several days of training, the rats with medial septal lesions were more likely to begin swimming in the wrong direction, swim farther, and consequently take longer to find the hidden platform than were the control rats. We hypothesize that this deficit occurs because these lesions disrupt a septo-hippocampal cholinergic projection system that is important for the formation or utilization of a spatial map (reference memory) that represents the location of the platform with respect to the surrounding distal cues. Because the electrolytic lesions damaged several neurochemical systems passing through the medial septum, this
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