Spatial water maze learning using celestial cues by the meadow vole, Microtus pennsylvanicus

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Bchavioural Brain Research 61 (1994) 97-100

BEHAVIOURAL BRAIN RESEARCH

Research Report

Spatial water maze learning using celestial cues by the meadow vole, Microt us pennsylvanicus Martin Kavaliers*, Liisa A.M. Galea ,\"eurowience Program and Division ol' Oral Biolo~9". l'~wulo" 0/ Dentistry, University of Western Ontario, Lomlon, Ont.. Canada N6A 5 C1 (Received 2 August 1993: revised 4 December 1993; accepted 6 December 1993)

Abstract The Morris water maze is widely used to evaluate to evaluate the spatial learning ability of rodents under laboratory settings. The present study demonstrates that reproductive male meadow voles, Microtus pennsylvanicus, are able to acquire and retain a spatial water maze task using celestial cues. Voles were able to acquire a modified outdoor Morris water maze task over 4 trials per day, whereby they had to learn and remember the location of a submerged hidden platform, using the position of the sun and associated celestial cues. Their proficiency on this task was related to the availability of the celestial cues, with voles displaying significantly poorer spatial navigation on overcast than clear days and when the testing time (and position of the sun and associated celestial cues) was shifted t?om morning to afternoon. These findings with meadow voles support the ecological relevance of the water maze task.

Key words." Morris water maze: Spatial ability; Spatial learning and memory; Celestial cue; Sun orientation; Meadow vole

1. Introduction

Performance in laboratory spatial tasks has been proposed to be related to spatial abilities to solve ecologically important problems [18] and to provide a measure of the effects of sexual and evolutionary factors on spatial learning and memory [8,10,13,17]. The Morris water maze [15,16] is widely used to evaluate spatial learning ability in rodents [1,14]. The maze contains a small platform hidden under just the surface of an opaque solution of water. The task requires the animals to swim and learn to find the position of the platform using distal, extramaze spatial cues. However, to date the Morris water maze has apparently been used exclusively indoors in the laboratory. Whether or not rodents can acquire a water maze spatial task outdoors in an ecologically relevant setting using natural celestial cues is not known. The meadow vole, Microtus pennsvh'anicus, is a polygynous microtine rodent which displays excellent performance in the hidden platform water maze task in the laboratory [9,12]. Meadow voles swim well and make diurnal aquatic excursions in the wild [3,5]. There are also reports that they live in marshes where only clumps of vegetation * Corresponding author. Fax: (1) (519) 661-3875

extend above the water [11]. Meadow voles have also been shown to respond to and utilize distal visual and celestial cues as illustrated by their responses to overhead avian predators and sun-compass orientation [7]. These behavioral features indicate the meadow vole as an appropriate subject for the investigation of water maze learning with natural celestial cues. In the present study we examined the spatial performance of breeding male meadow voles in a modified outdoor Morris water maze task, whereby the animals had to acquire and retain the location of a submerged hidden platform using celestial CUES.

2. Materials and methods 2.1. An#rials

Laboratory-bred reproductive adult male meadow voles (35-55 g, 2-12 months of age) were individually housed in polyethylene cages with hardwood bedding (Beta-chip). Voles were held under a natural photoperiod (approximately 15 h light:9 h dark) in a room where the sun and sky were visible. Food (Purina Rat Chow 5015) and water were available ad libitum.

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2.2. Water maze The water maze used was black circular plastic pool (110 cm diameter and 30 cm height) filled to a depth of 14 cm with water (22 + 2 ° C). Four starting locations, designated as north, south, east and west were located at the corresponding compass points on the pool rim. A hidden platform (7 cm in diameter, a weighed glass jar with a terry-cloth cover), submerged 0.5 cm below the surface, was located 15 cm from the edge of the pool in the northeast quadrant. A film of hardwood bedding (Beta-chip) was placed on the surface of the water to render it opaque and ensure that the platform was invisible to the voles. The water was changed every day. The pool was placed on a wooden stand inside a wooden frame that was covered by heavy black plastic. The plastic and frame extended 40 cm above the edges of the pool and excluded all visible external landmarks except for celestial cues. The orientation (e.g. which side faced east) of the frame and plastic were changed each day to eliminate any possible local cues. The pool and frame were placed in a large open field. The experimenter stood at a randomly determined position during each of the trials.

2.3. Experimental procedures Individual voles (n = 8) were released into the maze from one of the four equally spaced starting points which were ordered in a random manner between blocks (days). Initial testing took place over 6 consecutive days (Days 1-6), while final testing took place over three consecutive days (Days 17-19), with the voles receiving one block of four trials per day. During each trial, the voles were given 60 s to find the platform. If a vole found the platform it was allowed to remain on it for 15 s. If a vole failed to find the platform within the 60 s limit, it was placed on the platform for 15 s. Voles were given a 45 s rest period in a dry cage between each of the four daily trials. These initial and final acquisition trials were conducted at 10.00 h on clear sunny days, in which animals had a direct view of the sky and the position of the sun. On Day 20 after the final acquisition trial a probe trial was conducted. During the probe trial the platform was removed and the animals was allowed to swim for 60 s in the maze. The time spent in the quadrant which had previously contained the platform was recorded. All animals were released from the starting point that was opposite to the quadrant which had previously contained the platform, with the location of the experimenter being randomly determined. On Days 8-16 between the initial and final acquisition tests, determinations were made of the effects of different sky conditions, and shifts in test time (and sun position

and associated celestial cues) on spatial pert\~rmance in the water maze. (i) Sky conditioo--Utilizing available meterological forecasts, spatial performance (1 block of 4 trials) was examined on 2 separate occasions on a pattern of a clear morning, followed by an overcast-misty morning, followed by a clear morning. (ii) Shffi in test t i m e - - O n two other sets of clear days 2 separate determinations were made of spatial performance on a cleat" morning (10.00 h), followed by a clear afternoon (14.00 hi. and tt subsequent clear morning. Data were analyzed with 3 seperate repeated measures analysis of variance (ANOVA) and Tukey's procedurc, with the significance level set at (!.(}5.

3. Results

3.1. Acquisition Acquisition of the water maze task by the voles over the first 6 days (Days 1-6) and last 3 days (Days 17-19) blocks of trials is shown in Fig. 1. A repeated-measures A N O V A revealed a significant main effect of day (Fs,,~5 = 390.1, P<0.001). There was a significant reduction in the latency to find the platform such that the latency to find the platform was greater on Day 1 > Day 2 > Day 3 > Day 4. There were no significant differences in the latency to find the platform between Days 4, 5, 6 and the last three days (Days 17-19). There was also a retentkm of the spatial task in the probe trial conducted on day 20, with the voles spending significantly more time (39.75 + 1.36 s, P < 0 . 0 0 1 ) in the quadrant in the vicinity of the platform than by chance or in any of the other quadrants.

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DAYS, Fig. I. Water maze task acquisition b~, adult male m e a d o w voles using celestial cues. Initial pattern of acquisition on days 1-6 as well as final performance on days 17-19 is shown. La*encies presented are the m e a n o | ' 4 trials per day (block). Vertical lines d e n o t e a s t a n d a r d error of the mean, if error bars do not a p p e a r to be present they arc e m b e d d e d in the symbol, n = 8, in all cases.

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The effect of changing the testing time on spatial performance is shown in Fig. 3 (only one of the two determinations is shown). A repeated-measures A N O V A revealed that there was main effect of testing time ( F s j s = 8 3 . 5 3 , P < 0 . 0 0 1 ) , trial (Fs,21 =88.27, P < 0 . 0 0 1 ) and a significant interaction between test time and trial (F15.1o5 = 36.28, P < 0.001). When testing time was shifted from morning to afternoon there was a significant (F3,21 = 40.35, P < 0 . 0 0 1 ) increase in the latency to reach the platform. As well, there was a significant (F~.2~ = 34.47, P < 0 . 0 0 1 ) trial effect in the afternoon, with the latency to reach the platform decreasing over trials. As well, on trials 3 and 4 voles tested in the afternoon displayed a significantly ( P < 0 . 0 5 ) shorter latency to find the platform than they did on the corresponding trials on the overcast mornings. There were no significant differences in the latencies to find the platform in the mornings.

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Fig. 2. Spatial p c r t b r m a n c c b> adult male m e a d o w voles on t w o clear d a ? s (pre a n d post) i n t e r s p e r s e d with p e r f o r m a n c e on tin o v e r c a s t day. Latencies p r e s e n t e d are the m e a n o f 4 trials per d a y (block). Vertical lines d e n o t e u s t a n d a r d e r r o r o f the m e a n , if e r r o r b a r s d o not a p p e a r to be present they are e m b e d d e d in the symbol, n - 8, in all cases.

3.2. Ski' condition

The effects of sky condition on spatial performance are shown in Fig. 2 (only one of the two determinations is shown). A repeated-measures A N O V A revealed a main effect of sky condition (F5.~5 = 168.15, P < 0 . 0 0 1 ) , trial (F~.21 - 26.92 P < 0 . 0 0 1 ) , and a significant interaction between day and trial (Fis,to5 = 16.39, P < 0 . 0 0 1 ) . Post-hoc tests revealed that on overcast days there was poorer spatial navigation and a significantly ( P < 0 . 0 0 1 ) greater latency to reach the platform than on clear days. There was also a significant ( P < 0 . 0 0 1 ) trial effect on the overcast days, with the time taken to reach the platform decreasing over trials. There were no significant differences in the latencies to find the platform on the clear days.

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Fig. 3. Spatial p e r f ' o r m a n c e by adult male m e a d o w voles on two d a y s at the n o r m a l testing time ( m o r n i n g ) i n t e r s p e r s e d by p e r f o r m a n c e at a shifted testing time ( a f t e r n o o n ) . Vertical lines d e n o t e a s t a n d a r d error o f the m e a n , if e r r o r b a r s d o not a p p e a r to be p r e s e n t they are e m b e d d e d in the symbol, n - S, m all cases.

4. D i s c u s s i o n

The results of the present study show that adult male meadow voles are able to acquire and retain a spatial water maze task using celestial cues. Voles were able to acquire a modified, outdoor Morris water maze task, whereby they had to learn and remember the location of a submerged hidden platform, using the position of the sun and associated celestial cues. The proficiency of the voles on this task was related to the availability and, or nature of the celestial cues, with voles displaying significantly poorer spatial navigation and performance on overcast than clear days. These differences in spatial navigation are consistent with the reduced water maze performance that is reported in rats exposed to obstructed distal cues [19]. This reduced performance on overcast days also supports the suggestion that the voles are directly using celestial cues (position of the sun and associated sky cues) for the acquisition of the spatial task. It also indicates that the voles are not using the randomly determined position of the observer as an important cues. It further indicates that the voles are using celestial spatial cues and not adopting particular swimming strategies to acquire the spatial task. The improvement over trials also suggests that the voles are able to process and utilize celestial information other than just the position of the sun. The present findings confirm and extend previous demonstration of spatial learning by meadow voles in a Morris water maze task using various distal visual cues in the laboratory [8,12]. The present results also agree with a variety of other studies that have shown that in the laboratory, rodents are able to locate and rember the position of a hidden shelter or food source on the basis of its

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Acknowledgements

cues, including light sources [2,4]. In this regard, a series of studies have clearly d e m o n s t r a t e d that golden hamsters are able to use a single light as u n a m b i g u o u s directional reference [6,20]. The present findings are also consistent with the earlier d e m o n s t r a t i o n of sun c o m p a s s orientation by m e a d o w voles [7]. In the present study, voles whose test times were shifted to the afternoon were, over four trials, able to acquire the water maze task, suggesting a c o m p e n s a t i o n , or correction for, the shift in the position of the sun. There is evidence suggesting that sun compass orientation may be prevalent in spatial orientation in everyday tasks and not just restricted to migration and homing. For example, scrub jays seem to use their sun c o m p a s s to directionally retrace their caches despite the ample availability of familiar l a n d m a r k s [21]. This raises the possibility of a sun c o m p a s s c o m p o n e n t being associated with the spatial navigation and acquisition displayed by m e a d o w voles. The lack of an immediate shift with altered testing times (morning to afteroon) in the present study may be due to the voles having only a limited experience with natural sun and sky cues in their holding conditions. It should, however, be noted that voles tested in the afternoon did display a significantly better acquisition than those tested on the overcast morning, further reinforcing the use of celestial cues. The relatively poorer afternoon acquisition is unlikely to be related to any state d e p e n d e n t learning, with the results of pharmacological and e n v i r o n m e n t a l m a n i p u lations having yielded minimal evidence for a state dependency in Morris water maze performance. Clock-shift experiments examining the effects of shifts in the light-dark cycle on spatial water maze performance are necessary to further examine the role of a sun compass c o m p o n e n t . Together, these studies indicate that rodents can use distal light and celestial cues for spatial navigation. However, all of the aforementioned studies have involved analysis of homing and orientation and only indirectly memory, whereas the present study is directly concerned with, and demonstrates the use of celestial cues in spatial learning and memory. Further investigations are necessary to determine the exact nature of the celestial cues being used and how they are integrated with other potential spatial cues. The present results provide further support for the suggestions that performance on the Morris water maze is related to spatial abilities to solve ecologically important problems. In view of the m o u n t i n g evidence that seasonal, sexual and ecological factors can "affect spatial performance in the laboratory [8,10,13,18], it would be of interest to determine how these differences extend to, and affect, spatial performance in an ethologically relevant setting.

This work was supported by a Natural Sciences and Engineering Research Council of C a n a d a grant to M.K.

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