Dissociation of Hippocampal and Striatal Contributions to Spatial Navigation in the Water Maze

Two experiments were conducted to compare the effects of fornix/fimbria and caudate-putamen lesions in Long–Evans hooded rats (Rattus norvegicus) trained on two water maze tasks that differed in the type of spatial localization required for optimum solution. In Experiment 1, the lesioned rats and surgical controls were trained on the standard place task in the water maze (Morris, 1981) and given two postacquisition tests (a platform removal probe and platform relocation test). In Experiment 2, rats with similar lesions and control rats were trained on a modified cue navigation task. Fornix/fimbria lesions impaired a late stage of place task acquisition but did not impair acquisition of the cue task. Caudate-putamen lesions resulted in a severe place acquisition impairment and a transient cue acquisition impairment, both of which were characterized by an initial tendency to swim near the wall of the pool. Post-hoc analyses of the direction and angles of departure from the start points suggested that rats with fornix/fimbria lesions used non-allocentric spatial strategies to solve the place task. These rats also demonstrated a significantly weakened spatial bias for the former training quadrant on the platform removal probe and reduced flexibility in navigating to a novel platform location on the platform relocation test. In contrast, rats with caudate-putamen lesions showed a significant spatial bias for the former training quadrant but failed to cross the exact location within the quadrant where the platform was formerly positioned. The results suggest that the hippocampus mediates the allocentric spatial component of the water maze place task while the dorsomedial striatum may play an important role in the acquisition of the procedural aspects of both place and cue versions of the task.     

Acetylcholine release in the hippocampus and striatum during place and response training

These experiments examined the release of acetylcholine in the hippocampus and striatum when rats were trained, within single sessions, on place or response versions of food-rewarded mazes. Microdialysis samples of extra-cellular fluid were collected from the hippocampus and striatum at 5-min increments before, during, and after training. These samples were later analyzed for ACh content using HPLC methods. In Experiment 1, ACh release in both the hippocampus and striatum increased during training on both the place and response tasks. The magnitude of increase of training-related ACh release in the striatum was greater in rats trained on the response task than in rats trained on the place task, while the magnitude of ACh release in the hippocampus was comparable in the two tasks. Experiment 2 tested the possibility that the hippocampus was engaged and participated in learning the response task, as well as the place task, because of the availability of extra-maze cues. Rats were trained on a response version of a maze under either cue-rich or cue-poor conditions. The findings indicate that ACh release in the hippocampus increased similarly under both cue conditions, but declined during training on the cue-poor condition, when spatial processing by the hippocampus would not be suitable for solving the maze. In addition, high baseline levels of ACh release in the hippocampus predicted rapid learning in the cue-rich condition and slow learning in the cue-poor condition. These findings suggest that ACh release in the hippocampus augments response learning when extra-maze cues can be used to solve the maze but impairs response learning when extra-maze cues are not available for use in solving the maze.     

The influence of NMDA receptors in the dorsomedial striatum on response reversal learning

In mammals, the dorsomedial striatum is one brain area shown to be critical for the flexible shifting of response patterns. At present, the neurochemical mechanisms that underlie learning during a shift in response patterns are unknown. The present study examined the effects of NMDA competitive antagonist, DL-2-amino-5-phosphonopentanoic acid (AP-5), injected into the dorsomedial striatum on the acquisition and reversal of a response discrimination. Male Long-Evans rats were tested across two consecutive days in a modified cross-maze. Rats received an infusion of either saline or AP-5 (5 or 25 nmol) 5 min prior to each test session. In the acquisition phase rats learned to turn in one direction (right or left) to receive a cereal reinforcement. In the reversal learning phase rats learned to turn in the opposite direction as in the acquisition phase. In both phases, criterion was achieved when a rat made 10 consecutive correct trials. Infusions of AP-5 did not impair acquisition, but impaired reversal learning of a response discrimination in a dose-dependent fashion. The reversal learning deficit induced by AP-5 resulted from reversions back to the originally learned response pattern following the initial shift. These results suggest that activation of NMDA receptors in the dorsomedial striatum are critical for the flexible shifting of response patterns by enhancing the reliable execution of a new response pattern under changing task contingencies.     

An assessment of response, direction and place learning by rats in a water T-maze

Behavioral data suggest that distinguishable orientations may be necessary for place learning even when distal cues define different start points in the room and a unique goal location. We examined whether changes in orientation are also important in place learning and navigation in a water T-maze. In Experiment 1, rats were trained to locate a hidden platform and given a no-platform probe trial after 16 and 64 trials with the maze moved to a new position. Direction and response strategies were more prevalent than a place strategy. In Experiment 2, acquisition of place, response and direction strategies was assessed in a water T-maze that was moved between two locations during training. Rats were impaired on the place task when the maze was translated (moved to the L or R) but were successful when the maze was rotated across trials. These data are consistent with findings from appetitive tasks.     

Effects of dorsal-striatum lesions and fimbria-fornix lesions on the problem-solving strategies of rats in a shallow water maze

In solving a spatial problem, animals can use a place, cue, or response strategy. The present research was designed to evaluate the role of dorsal striatum (DS) in spatial problem solving and to compare it with that of fimbria fornix (FF). Rats were trained with a place + cue task in a shallow pool, then were divided into three groups (DS, FF, control), and lesions were made in the corresponding areas. After retraining, four probe tests were given: Test 1 (start position moved), Test 2 (goal and start positions moved), Test 3 (invisible goal), and Test 4 (curtain test). The test results suggest that the DS and Control groups performed the original task by using the place strategy, whereas the FF group used the cue strategy, which strongly implies that the DS group was impaired in the use of the cue strategy. This research also provides evidence supporting the usefulness of a shallow pool in evaluating animal behavior.     

Differential effects of inactivation of the orbitofrontal cortex on strategy set-shifting and reversal learning

Different subregions of the rodent prefrontal cortex (PFC) mediate dissociable types of behavioral flexibility. For example, lesions of the medial or orbitofrontal (OFC) regions of the PFC impair extradimensional shifts and reversal learning, respectively, when novel stimuli are used during different phases of the task. In the present study, we assessed the effects of inactivation of the OFC on strategy set-shifting and reversal learning, using a maze based set-shifting task mediated by the medial PFC. Long–Evans rats were trained initially on a visual-cue discrimination to obtain food. On the subsequent day, rats had to shift to using a response strategy (e.g., always turn left). On Day 3 (reversal), rats were required to reverse the direction of their turn (e.g., always turn right). Infusions of the local anesthetic bupivacaine into the OFC did not impair initial visual discrimination learning, nor did it impair performance on the set-shift. In contrast, inactivation of the OFC did impair reversal learning; yet, these rats ceased using the previously acquired response rule as readily as controls. Instead, rats receiving OFC inactivations made a disproportionate number of erroneous arm entries towards the visual-cue, suggested that these animals reverted back to using the original visual-cue based strategy. These findings, in addition to previous data, further support the notion that the OFC and medial PFC play dissociable roles in reversal learning and set-shifting. Furthermore, the lack of effect of OFC inactivations on the set-shift indicates that this type of behavioral flexibility does not require cognitive operations related to reversal learning.     

Anterograde and retrograde amnesia of place discrimination in retrosplenial cortex and hippocampal lesioned rats

Retrograde and anterograde amnesic effects of excitotoxic lesions of the rat retrosplenial cortex (RS) and hippocampus (HPC) were investigated. To test retrograde amnesia, rats were trained with two-arm place discrimination in a radial maze 4 wk and 1 d before surgery with a different arm pair, respectively. In the retention test 1 wk after surgery, both lesion groups showed temporally ungraded retrograde amnesia. To test anterograde amnesia, animals were trained after surgery to discriminate three arm pairs successively within a day, and then after interposition of 1- to 4-wk intervals, one of these pairs, respectively, was tested for retention. RS-lesioned rats could acquire these pairs of place discriminations rapidly but showed a retention interval-dependent impairment in the retention test. Conversely, HPC-lesioned rats took more sessions to acquire these pairs than did the control group, and their retention was approximately 70% of correct performance regardless of retention interval. Results suggest that RS and HPC have different roles in spatial memory and that RS is important for remote memory process.     

Spatial information processing consequences of DAMGO injections into the dorsal striatum

The purpose of the present study was to examine the role of the dorsal striatum, and more specifically, the patch region of the dorsal striatum, in mediating spatial learning and memory. To this end, male, Long Evans rats were bilaterally implanted with cannula aimed at the dorsal striatum. Rats were injected with different doses (0, 0.05, 0.5 or 5 microg/0.5 microl) of [3H]-[D-Ala2,MePhe4,Gly-ol5]-enkephalin (DAMGO) into the dorsal striatum daily (Exp. 1) before training on a hidden platform version of the water maze task or during a reversal water maze spatial task (Exp. 2). In both experiments, probe retention tests were given drug free. Results from Exp. 1 showed that intra-striatal injection of the low DAMGO dose (0.05) resulted in enhanced spatial acquisition while the high dose (5.0) produced impairments compared to controls. During the probe test, the low dose group showed better retention of the platform location than controls as well as an enhanced ability to alter their search strategy. In Exp. 2, pretraining alleviated the high dose impairment found in Exp. 1 suggesting a motoric impairment in this group. The low dose group continued to show an enhanced ability to alter their search strategy during the probe test compared to all other groups. The data suggest that the low dose of DAMGO, when injected into the dorsal striatum, eliminates competition with the hippocampus thereby leading to enhanced spatial processing. Alternatively, inhibition of patch-striatal neurons may attenuate a memory decay process. Both alternatives are discussed.     

Training method dramatically affects the acquisition of a place response in rats with neurotoxic lesions of the hippocampus.

A considerable number of studies have demonstrated that hippocampal damage impairs the acquisition of a place response in rats. In Experiment 1, using a four-arm plus-shaped maze, we replicated this finding. Experiment 2 showed, however, that hippocampally damaged rats can learn a place response just as well as control rats when, during the training, a salient intramaze landmark indicates the position of the goal (the west arm). After reaching criterion, the hippocampal and control groups performed the task with the same degree of mastery during a transfer test in which the intramaze signal used during the acquisition was removed. In Experiment 3, the intramaze cue was substituted by an egocentric cue. The results revealed that both control and lesioned subjects learned the spatial problem well. However, a transfer test showed that control rats learned the task using a place response strategy but hippocampally lesioned animals used a rigid, hyperspecific strategy. Taken together, these results suggest that special training procedures which encourage variability in response versus perseveration make it possible to overcome the acquisition deficit normally observed in hippocampal rats.     

Differential induction of c-Jun and Fos-like proteins in rat hippocampus and dorsal striatum after training in two water maze tasks

Research examining the neuroanatomical bases of memory in mammals suggests that the hippocampus and dorsal striatum are parts of independent memory systems that mediate "cognitive" and stimulus-response "habit" memory, respectively. At the molecular level, increasing evidence indicates a role for immediate early gene (IEG) expression in memory formation. The present experiment examined whether acquisition of cognitive and habit memory result in differential patterns of IEG protein product expression in these two brain structures. Adult male Long-Evans rats were trained in either a hippocampal-dependent spatial water maze task, or a dorsal striatal-dependent cued water maze task. Ninety minutes after task acquisition, brains were removed and processed for immunocytochemical procedures, and the number of cells expressing Fos-like immunoreactivity (Fos-like-IR) and c-Jun-IR in sections from the dorsal hippocampus and the dorsal striatum were counted. In the dorsal hippocampus of rats trained in the spatial task, there were significantly more c-Jun-IR pyramidal cells in the CA1 and CA3 regions, relative to rats that had acquired the cued task, yoked controls (free-swim), or na?ve (home cage) rats. Relative to rats receiving cued task training and control conditions, increases in Fos-like IR were also observed in the CA1 region of rats trained in the spatial task. In rats that had acquired the cued task, patches of c-Jun-IR were observed in the posteroventral striatum; no such patches were evident in rats trained in the spatial task, yoked-control rats, or na?ve rats. The results demonstrate that IEG protein product expression is up-regulated in a task-dependent and brain structure-specific manner shortly after acquisition of cognitive and habit memory tasks.     

Rule-based learning impairment in rats with lesions to the dorsal striatum.

The present study examined the effects of lesions to the dorsal striatum (DS) in Sprague-Dawley rats, when tested on the acquisition and successive shifts in the position of a goal arm in an eight-arm radial maze. In the procedure we used, rats had to retrieve the location of one baited arm among the eight arms of the maze after it had just been presented as a sample during a forced trial. After attainment of a fixed learning criterion, rats were submitted to five successive shifts in the goal location. Results showed that DS rats were able to learn the position of the goal arm during the acquisition phase as efficiently as sham-operated rats. In contrast, when the position of the goal arm was shifted, although DS rats were able to learn its new position, they made significantly more errors and required more sessions to reach criterion than sham-operated rats. These results suggested that both groups did not solve the task using the same behavioral strategy. The analysis of responses made suggested that sham-operated rats solved the task using the pairing rule between the forced and the free run (matching-to-sample rule), while DS rats solved the task using only visuo-spatial processing. These data therefore suggest that the dorsal striatum plays an important role in rule-learning ability.     

Damage to the retrosplenial cortex produces specific impairments in spatial working memory

Mounting evidence indicates that the retrosplenial cortex (RSP) has a critical role in spatial navigation. The goal of the present study was to characterize the specific nature of spatial memory deficits that are observed following damage to RSP. Rats with RSP lesions or sham lesions were first trained in a working memory task using an 8-arm radial arm maze. Rats were allowed 5 min to visit each arm and retrieve food pellets and a 5-s delay was imposed between arm choices. Consistent with previous research, rats with RSP damage committed more errors than controls. In particular, RSP-lesioned rats committed more errors of omission (failing to visit an arm of the maze), but there were no lesion effects on errors of commission (revisiting an arm). Neither group of rats exhibited a turn bias (i.e., always turning a certain direction when choosing an arm). At the end of the training phase of the experiment, both groups had reached asymptote and committed very few errors. In the subsequent test phase, a longer delay (30-s) was imposed during some sessions. Both control and RSP-lesioned rats continued to make few errors during sessions with the standard 5-s delay, but RSP-lesioned rats were impaired at the 30-s delay and committed more errors of commission, consistent with an increase in taxing spatial working memory.     

Differential involvement of M1-type and M4-type muscarinic cholinergic receptors in the dorsomedial striatum in task switching

Previous experiments have demonstrated that the rat dorsomedial striatum is one brain area that plays a crucial role in learning when conditions require a shift in strategies. Further evidence indicates that muscarinic cholinergic receptors in this brain area support adaptations in behavioral responses. Unknown is whether specific muscarinic receptor subtypes in the dorsomedial striatum contribute to a flexible shift in response patterns. The present experiments investigated whether blockade of M1-type and/or M4-type cholinergic receptors in the dorsomedial striatum underlie place reversal learning. Experiment 1 investigated the effects of the M1-type muscarinic cholinergic antagonist, muscarinic-toxin 7 (MT-7) infused into the dorsomedial striatum in place acquisition and reversal learning. Experiment 2 investigated the effects of the M4-type muscarinic cholinergic antagonist, muscarinic-toxin 3 (MT-3) injected into the dorsomedial striatum in place acquisition and reversal learning. All testing occurred in a modified cross-maze across two consecutive sessions. Bilateral injections of MT-7 into the dorsomedial striatum at 1 or 2 microg, but not 0.05 microg impaired place reversal learning. Analysis of the errors revealed that MT-7 at 1 and 2 microg significantly increased regressive errors, but not perseverative errors. An injection of MT-7 2 microg into the dorsomedial striatum prior to place acquisition did not affect learning. Experiment 2 revealed that dorsomedial striatal injections of MT-3 (0.05, 1 or 2 microg) did not affect place acquisition or reversal learning. The findings suggest that activation of M1-type muscarinic cholinergic receptors in the dorsomedial striatum, but not M4-type muscarinic cholinergic receptors facilitate the flexible shifting of response patterns by maintaining or learning a new choice pattern once selected.     

D-Cycloserine enhances memory consolidation of hippocampus-dependent latent extinction

Adult male Long-Evans rats were trained to run in a straight-alley maze for food reward and subsequently received hippocampus-dependent latent extinction training. Immediately following latent extinction, rats received peripheral injections of the NMDA receptor partial agonist D-cycloserine (DCS, 15 mg/kg), or saline. Twenty-four hours later, rats received four extinction "probe" trials. Relative to saline controls, latencies to reach the goal box on probe trials were significantly higher in rats that had received DCS. These findings indicate that memory consolidation underlying hippocampus-dependent latent extinction, a cognitive form of learning in which the previously rewarded response is not made during extinction training, can be enhanced by NMDA-receptor agonism.     

Pre-exposure to context affects learning strategy selection in mice

The multiple memory systems hypothesis proposes that different types of learning strategies are mediated by distinct neural systems in the brain. Male and female mice were tested on a water plus-maze task that could be solved by either a place or response strategy. One group of mice was pre-exposed to the same context as training and testing (PTC) and the other group was pre-exposed to a different context (PDC). Our results show that the PTC condition biased mice to place strategy use in males, but this bias was dependent on the presence of ovarian hormones in females.The participation of different brain areas in place and response learning strategies has been studied extensively (White and McDonald 2002; Gold 2004; Mizumori et al. 2004). Place strategy is an allocentric navigation strategy that depends on extramaze cues. Response strategy is an egocentric navigation strategy based on proprioceptive cues. Inactivation of the hippocampus biased animals to response strategy use, and inactivation of the striatum biased animals to place strategy use (Packard and McGaugh 1996; Lee et al. 2008). Furthermore, glutamate infusion into the hippocampus strengthened place strategy use and, conversely, glutamate infusion into the striatum enhanced response strategy use (Packard 1999). These studies suggest that the hippocampus system mediates place strategy, while the striatum system mediates response strategy.Various factors can modulate learning strategy use, including training intensity (Packard and McGaugh 1996; Martel et al. 2007). A recent study investigated the influence of training on strategy use on a probe trial conducted 1 h after training (Martel et al. 2007). Male mice displayed enhanced place strategy use when trained on 12 or 22 trials compared with four trials, suggesting an effect of training intensity on strategy choice (Martel et al. 2007). This study further investigated the effect of pre-exposure to the training and testing context (PTC). Pre-exposure enhanced place strategy use in male mice after only four trials relative to animals pre-exposed to a different context (PDC). These results suggest that a sufficient exposure to the training and testing context promotes place strategy use in mice.The type of strategy used by rats is affected by both biological sex and gonadal steroids. Male rats typically employ a place strategy, especially during the early phase of training, on both land and water T-mazes (Packard and McGaugh 1992, 1996; Packard and Teather 1997). However, strategy use by female rats depends on hormonal conditions (Dohanich 2002; Dohanich et al. 2009). Place strategy is preferred by intact female rats on the day of proestrus when estradiol levels are elevated, and by ovariectomized rats treated with estradiol (Korol and Kolo 2002; Korol 2004; Korol et al. 2004). In contrast, response strategy is more often displayed by intact females on diestrus, and by ovariectomized females that did not receive estradiol replacement (Korol and Kolo 2002; Korol et al. 2004). To date, the effects of biological sex and gonadal steroids on learning strategy have not been studied in mice.In this study, we developed a modified version of the dual-solution water plus-maze task to further investigate the role of PTC compared with PDC in male and female mice. We hypothesized that strategy choice in both sexes would be dependent on context pre-exposure, and ovarian hormones would influence strategy choice in females. Our results show that PTC significantly enhanced place strategy use in male mice. Although there was no significant difference between PTC and PDC female mice, ovariectomy significantly reduced place strategy use in the PTC females, suggesting that ovarian hormones play a significant role in strategy use in female mice.Sixteen male and 39 female 129/Sve strain mice were obtained at 2–3 mo of age from Charles River Laboratories (Boston, MA). Mice were housed in groups of four on a 12/12 light/dark cycle (lights on at 07:00 h) with free access to food and water. All protocols followed the guidelines from a protocol approved by the Animal Care and Use Committee of Tulane University in accordance with National Institutes of Health Guide for the Care and Use of Laboratory Animals.Mice were pre-exposed for 5 min to the dry plus-maze either in the context of the subsequent training and testing (PTC), or in a different context in a different room (PDC), 30 min prior to the first training trial. The maze consisted of four clear Plexiglas arms (40 cm in length, 10 cm in width, and 40 cm in height). During the pre-exposure, mice were able to visit three arms of the maze. The rooms had different visual cues surrounding the maze. No extramaze cues were placed directly at the end of any arm. After the pre-exposure, the animal was placed in its home cage. The maze was wiped clean with 70% ethanol between trials.After pre-exposures, the maze was filled to 1.5 cm above the Plexiglas escape platform (15 cm in height) with room-temperature water colored opaque with white nontoxic tempera paint. Mice were trained in the water plus-maze task (Fig. 1A). The training was ended when the animals made six correct choices or reached nine trials. The animals that made fewer than four correct choices during training were not included in the study. Trials were continued until the mouse reached the platform or a maximum of 1 min. Each trial was separated by an intertrial interval of 4 min. Throughout the training trials, one arm (north) was blocked off by a white Plexiglas shield, creating a T-shaped maze. Mice were placed in the start arm of the maze (south) and were allowed to swim to the escape platform, which was consistently located in one arm of the maze for each animal and alternated between animals (east or west). Entry of the entire animal into the maze arm that contained the escape platform was scored as a correct response during the training trials, and entry of the entire animal into the maze arm that did not contain the escape platform was scored as an incorrect response. Mice were allowed to remain on the escape platform for 15 sec before being returned to their cages. Mice that failed to find the escape platform within 60 sec were manually guided to the platform. The water was distributed across all arms of the maze and the maze walls were wiped down to reduce intramaze cues between training and probe trials. One hour after training, mice were tested on a probe trial (Fig. 1B) in order to determine their relative use of “place” and “response” strategy. On the probe trial, mice were placed into the start arm 180° opposite the start arm used during training (i.e., end of the north arm) and were allowed to make an entry into either the east or west maze arm. The white Plexiglas shield blocked the south arm during the probe trial. Mice were designated as using place or response strategy based on the probe trial. Place strategy was designated as entry of the entire animal into the arm with the platform, and response strategy was designated as entry of the entire animal into the opposite arm.Open in a separate windowFigure 1.The effects of pre-exposure to the training and testing context (PTC) or to a different context (PDC) on strategy selection of male mice. (A) Schematic diagram of the water plus-maze. Mice were released from the south arm during training trials and from the north arm during the probe test. (B) More male mice used place strategy than response strategy when pre-exposed to the same context prior to training and testing (PTC, n = 5) compared with male mice pre-exposed to a different context (PDC, n = 7, P < 0.05). (C) Latency curves show the actual latency to escape to the platform. Two-way ANOVA (non-repeated measures) revealed no significant difference across training trials in escape latencies between PDC and PTC mice (P > 0.5), although a significant effect of trial indicated that mice reduced their escape latencies across trials (P < 0.001). Values represent mean ± S.E.M.Sixteen male mice were randomly divided into two groups based on pre-exposure context, PTC or PDC. Four of the 16 males were not included in the study for failure to reach criterion (four correct out of nine trials) or failure to escape to the platform due to floating, which is a behavior commonly seen in this strain (Wolfer et al. 1997). On the probe trial PTC males used the place strategy significantly more often than PDC males (P < 0.05, χ² = 5.182, Fig. 1B). Four of five PTC males used place strategy, whereas only one of seven PDC males used place strategy. Pre-exposure of animals to the same or different context prior to training did not affect the latency to escape the platform during training. Latency to find the platform during training trials revealed a significant effect of trial (F_(8,89) = 3.830, P = 0.0007, non-repeated measures two-way ANOVA) but not pre-exposure condition (F_(1,89) = 0.103, P = 0.75, non-repeated measures two-way ANOVA; Fig. 1C). Moreover, the average swim speed of PDC male mice (6 ± 1.6 cm/sec, n = 7) was not significantly different than the average swim speed of PTC male mice (6 ± 2.5 cm/sec, n = 5; P = 0.34, t = 0.9 [t-test]). Together, these data suggest that the pre-exposure condition did not influence learning during the training period, but PTC did enhance place strategy use in the probe trial in male mice.Female mice at 3 mo of age were randomly divided into two groups: mice that would receive ovariectomy (Ovx), and a sham surgery group (Sh). Mice were anesthetized with a ketamine (80 mg/kg) and xylazine (8 mg/kg) mixture. The first group of mice (n = 20) received ovariectomy using a dorsolateral approach. The other group (n = 19) of female mice received sham surgery, which consisted of ovary exposure only. Animals were injected with the pain reliever, buprenorphine (5 mg/kg), immediately after the surgery. One week after the surgery, vaginal smears were collected from all females, including Ovx as handled controls, at the same time each morning by lavage to track their estrus cycles (Marcondes et al. 2002). After two regular cycles, Sh animals were trained and tested on the day of proestrus (high estradiol).Ovariectomy has been reported to affect anxiety levels (Walf et al. 2006), and anxiety levels may alter performance on water maze tasks. To assay possible anxiety differences between Sh and Ovx, female mice were tested on open field and elevated plus-maze (EPM) 2 wk after the surgery in a room different from the rooms used in water maze tasks. A single mouse was placed in the center of a white, Plexiglas chamber measuring 43 cm in length × 43 cm in width × 18 cm in height. The animal explored the novel environment for 15 min, and movements were monitored by a camera interfaced with a tracking system (US HVS Image). The area was divided into 16 virtual squares (10.75 × 10.75 cm) by the program, and the middle four squares were defined as the center area. The Plexiglas chamber was wiped clean with 70% ethanol between trials. The EPM consisted of four arms (5 cm in width × 30 cm in length) arranged perpendicularly in a plus shape and elevated 38 cm above the floor. Two arms were enclosed by 15.5-cm dark Plexiglas walls and two arms were open. Each animal was placed in the center of the EPM facing a closed arm and allowed to move freely for 5 min. Behavior was monitored by a camera interfaced with the tracking system.Animals with high anxiety levels tend to spend less time in the open arms of the EPM and in the center of the open field. The percent time spent in the open arms of the EPM by Ovx mice (37.9% ± 7.5%, n = 14) was not significantly different than the percent of time spent in the open arms by Sh mice (27.8% ± 6.2%, n = 15; P = 0.30, t = 1.1). The percent time spent in the center of the open field by Ovx mice (35.1% ± 7.1%, n = 14) was not significantly different from Sh mice (29.0% ± 7.1%, n = 15; P = 0.55, t = 0.61). These results indicate that ovarian hormones did not have a significant effect on the anxiety levels of the female mice tested in this study.Two weeks after the anxiety tests, the Ovx and Sh groups were divided randomly into two groups based on the pre-exposure context: Ovx PTC, Ovx PDC, Sh PTC, Sh PDC. Sh females with regular estrus cycles were trained and tested on the day of proestrus. Five Ovx and seven Sh animals were not included in the study because of floating, failing to reach criterion (four correct out of nine trials), or exhibiting irregular estrus cycles. Five of eight Sh PTC and only one of six Sh PDC females used place strategy; however, this difference was not significant (P > 0.05, χ² = 2.94, Fig. 2A). Therefore, the pre-exposure condition did not significantly affect strategy use in females at proestrus.Open in a separate windowFigure 2.The effects of ovarian hormone status and pre-exposure to the training and testing (PTC) or to a different context (PDC) on strategy selection of female mice. (A) When pre-exposed to the same context prior to training and testing (PTC), more gonadally intact female mice at proestrus (Sh, n = 8) used place strategy than response strategy compared with ovariectomized female mice (Ovx, n = 8, P < 0.05). When pre-exposed to a context different than the training and testing context (PDC), gonadally intact female mice at proestrus (Sh, n = 6) and ovariectomized mice (Ovx, n = 6) used response strategy rather than place strategy. (B) Latency curves show the actual latency to escape to the platform. Two-way ANOVA (non-repeated measures) revealed no significant differences across training trials in escape latencies between sham and ovariectomized PTC and PDC mice (P > 0.5), although a significant effect of trial indicated that mice reduced their escape latencies across trials (P < 0.0001). Values represent mean ± S.E.M.Interestingly, ovariectomy did significantly affect strategy use in PTC females. Five of eight Sh PTC and only one of eight Ovx PTC females used place strategy (P < 0.05, χ² = 4.267, Fig. 2A). One of six Sh PDC females and zero of the six Ovx PDC animals used place strategy (Fig. 2A). Therefore, both Sh and Ovx PDC females used response strategy, and ovarian hormones did not enhance place strategy use in PDC females (P > 0.05, χ² = 1.09, Fig. 2A). Ovarian hormones did enhance place strategy use in PTC females. Furthermore, PTC did not enhance place strategy use in Ovx animals. Similar to males, there was a significant effect of training trial on latency to find the platform in female animals (F_(8,189) = 10.32, P < 0.0001, Fig. 2B). Ovarian hormones or pre-exposure to either context also did not affect escape latency during training in PTC or PDC females (F_(3,189) = 0.33, P = 0.80, Fig. 2B). In addition, there was no significant difference in the average swim speed between groups (F_(3,14) = 0.15, P = 0.93, one-way ANOVA). The average swim speed for each group was as follows: Ovx PTC (5 ± 1.5 cm/sec, n = 5), Ovx PDC (5 ± 1.0 cm/sec, n = 4), Sh PTC (5 ± 1.8 cm/sec, n = 5), Sh PDC (6 ± 1.5 cm/sec, n = 4). The numbers of animals are lower because in some cases, speed was not measured. Together, these data suggest that ovarian hormones and pre-exposure condition did not influence learning during the training period, but ovarian hormones did enhance place strategy use in the probe trial in only PTC mice.Consistent with previous literature (Martel et al. 2007), we found that ∼80% of PTC males favored the use of place strategy. In addition, 63% of PTC females on proestrus also used place strategy. Ovx female mice used response strategy regardless of the pre-exposure condition. These results confirm that pre-exposure to the training and testing context significantly increased the use of place strategy or reduced response strategy in male mice, while female mice on proestrus were not significantly different than chance. Ovariectomy diminished the use of place strategy and enhanced response strategy use in our study, implicating ovarian hormones in strategy choice.Male rats rely initially on a hippocampus-dependent place strategy, and then switch to a striatum-based response strategy over training (Packard and McGaugh 1996; Packard 1999). This suggests that response strategy is incrementally learned with repeated exposure to the same task. However, a sufficient amount of time to explore the extramaze cues during or before training increased place strategy use in male mice (Martel et al. 2007). In addition, it has been proposed that the presence of an increased number of salient extramaze cues favors place strategy use in rats (Restle 1957). Therefore, it is possible that pre-exposing mice to the learning environment allowed them to build a cognitive map that facilitated the use of a spatial place strategy. Another possible advantage of pre-exposure for place strategy use is that it may reduce the impact of non-mnemonic factors, such as anxiety, on performance (Cain 1998). Indeed, it was shown that peripheral injection and infusion of anxiogenic drugs into the basolateral amygdala biased rats toward the use of response strategy (Packard 1999; Wingard and Packard 2008; Packard and Gabriele 2009).While PTC female mice were not significantly different than PDC female mice, ovariectomy did reduce place strategy choice in the PTC mice. An emerging theory proposes that estradiol modulates cognitive performance via shifts in learning strategy (Korol and Kolo 2002; Daniel and Lee 2004; Korol 2004; McElroy and Korol 2005; Zurkovsky et al. 2007). Shifts in strategy use occurred across the estrus cycle in rats such that the hippocampus-dependent strategy was favored when estradiol levels were high (Korol et al. 2004). Similarly, estradiol treatment in ovariectomized rats increased hippocampus-dependent place strategy and impaired response strategy use compared with nontreated ovariectomized females (Korol and Kolo 2002). Our results showing that the lack of ovarian hormones reduced place strategy and increased response strategy use in PTC mice are consistent with these studies.In summary, we present a new design to a traditional dual-solution land plus-maze. One issue with the land maze version of the task is that it requires food deprivation. The possible increase in the appetite as a result of ovariectomy (Wade 1975) or disruption in the estrus cycle in response to food deprivation (Daniel et al. 1999) could confound the results in females in tasks that present food reward. In order to avoid these confounds, we used a modified version of a water-escape plus-maze (Packard and Wingard 2004). In this design, compared with the water-escape plus-maze, the clear Plexiglas maze itself is filled with water, instead of placing the plus-maze into a water maze, allowing a better view of extramaze visual cues. However, unlike rats, mice tend to be prey animals when in the water; therefore they are highly motivated to escape the water (Francis et al. 1995; Van Dam et al. 2006). Consequently, the stressful nature of the task prevents mice from utilizing the spatial cues as efficiently (Frick et al. 2000). Therefore, we pre-exposed the mice to the maze while it was dry, allowing them to build a cognitive map before they were released in water. The water plus-maze is important not only for the design of future studies, but also for the evaluation of previous studies that investigated learning strategies using tasks dependent on food deprivation.     

Glucose injections into the dorsal hippocampus or dorsolateral striatum of rats prior to T-maze training: modulation of learning rates and strategy selection

The present experiments examined the effects of injecting glucose into the dorsal hippocampus or dorsolateral striatum on learning rates and on strategy selection in rats trained on a T-maze that can be solved by using either a hippocampus-sensitive place or striatum-sensitive response strategy. Percentage strategy selection on a probe trial (P(crit)) administered after rats achieved criterion (nine of 10 correct choices) varied by group. All groups predominately exhibited a response strategy on a probe trial administered after overtraining, i.e., after 90 trials. In experiment 1, rats that received intrahippocampal glucose injections showed enhanced acquisition of the T-maze and showed increased use of response solutions at P(crit) compared with that of unimplanted and artificial cerebral spinal fluid (aCSF)-treated groups. These findings suggest that glucose enhanced hippocampal functions to accelerate the rate of learning and the early adoption of a response strategy. In experiment 2, rats that received intrastriatal glucose injections exhibited place solutions early in training and reached criterion more slowly than did aCSF controls, with learning rates comparable to those of unoperated and operated-uninjected controls. Relative to unoperated, operated-uninjected and glucose-injected rats, rats that received intrastriatal aCSF injections showed enhanced acquisition of the T-maze and increased use of response solutions at P(crit). The unexpected enhanced acquisition seen after striatal aCSF injections suggests at least two possible interpretations: (1) aCSF impaired striatal function, thereby releasing competition with the hippocampus and ceding control over learning to the hippocampus during early training trials; and (2) aCSF enhanced striatal functioning to facilitate striatal-sensitive learning. With either interpretation, the results indicate that intrastriatal glucose injections compensated for the aCSF-induced effect. Finally, enhanced acquisition regardless of treatment was accompanied by rapid adoption of a response solution for the T-maze.     

Spontaneously hypertensive, Wistar Kyoto and Sprague-Dawley rats differ in their use of place and response strategies in the water radial arm maze

This study further characterises the use of mnemonic systems in the spontaneously hypertensive rat (SHR), which is frequently used as a rodent model of attention deficit hyperactivity disorder. The objective of this study was to assess the preference of male SHR, Wistar-Kyoto (WKY) and Sprague-Dawley (SD) rats for a place or response strategy when trained on an ambiguous T-maze task, and also to examine whether all strains acquired information about both strategies during ambiguous training, regardless of their preferred strategy. In the first experiment, SHR and WKY showed a preference for a response strategy on the ambiguous T-maze task; in contrast, SD displayed a preference for a place strategy. In the second experiment, all strains demonstrated that they learned information about both the response and place strategies during ambiguous training. However, on a conditioned place preference test SHR did not display as strong a preference for the place arm as WKY and SD. This finding supports previous research in a conditioned cue preference test, in which SHR did not display a preference for the cue associated with the platform. These observations that the strains differ with respect to behavioural strategy in a learning task suggest that they differ in the underlying neural circuitry that serves goal-directed behaviour, and are consistent with SHR having deficits associated with the nucleus accumbens.     

Acute Ethanol Administration Impairs Spatial Performance While Facilitating Nonspatial Performance in Rats

Acute ethanol administration produces learning and memory impairments similar to those found following lesions to the hippocampal system in rats. For example, both ethanol and hippocampal lesions impair performance on spatial learning and memory tasks while sparing performance on many nonspatial learning and memory tasks. Lesions to the hippocampal system can also alter the nature of the information that the animal uses to guide its behavior, from using spatial information to using individual cues. In the present experiment, rats were trained, while sober, to navigate on an eight-arm radial arm maze to a specific arm for food reward. During training, the rewarded arm was always in the same specific location and contained well-defined cues. After the rat learned the task, a memory test was conducted under different doses of ethanol (0.0 g/kg [saline control], 1.0, 1.5, or 2.0 g/kg, intraperitoneal). On the test day the maze was rotated so that the cued arm was 90 degrees to the right of its original position. During testing, intact rats showed a significant bias to approach the place where they had been previously rewarded, even though the cue was no longer located there. Acute ethanol administration dose dependently reduced approaches to the rewarded place. However, ethanol administration did not result in increases in random choices; rather, it resulted in a dose-dependent increase in approaches to the cued arm, now in a new location. These results extend previous research showing that acute ethanol administration and lesions to the hippocampal system produce similar effects on learning and memory in rats.