Blunt olfaction in sexually sluggish male rats.

Among the intact male rats, a subpopulation has been found to show little or no sexual behavior, even after experiencing several mating sessions. This study investigated whether sexually sluggish (SS) males show behavioral differences from normal copulatory (NC) males, other than those concerning sexual behavior. The olfactory preference of males was measured through the time spent displaying nose-poking behavior directed at sexually active males and estrous females for odor exploration in a three-chamber apparatus. Both the NC and SS males showed a significant preference for the odor of estrous females compared with that of male odors. However, SS males spent significantly less time nose-poking estrous females than NC males. The food-finding test was performed after overnight fasting. Our findings showed that all the NC males found the buried pellet within 5 min, whereas over 60% of the SS males failed to find it. The males were also tested for their ability to find a buried bag containing soiled bedding from estrous female cages. The bag was found by 80% of NC males, but only by 20% of SS males. Our results suggest that SS and NC male rats differ not only in sexual behavior but also in other functions such as olfaction.

Ultrasonic vocalization in female rats: A comparison among three outbred stocks from pups to adults.

Long Evans (LE), Sprague-Dawley (SD) and Wistar (WU) are outbred rat stocks, which differ in terms of brain, physiology, pharmacological reactivity and behavior. Extending our previous work with males from these stocks, we here report the analysis of ultrasonic vocalizations (USV) in females. Identical to our previous studies, we tested them as pups for 40-kHz calls during short-term isolation, as juveniles for appetitive 50-kHz calls during a cage test or when being tickled, and finally as adults for 22-kHz calls in a fear conditioning paradigm. Stock differences were obtained in all four tests, albeit with different patterns: As pups, WU rats emitted more calls and spent more time calling than SD or LE rats. Furthermore, LE rats emitted calls with shorter durations, whereas SD emitted calls with lower peak frequencies and less frequency modulation. Furthermore, stock differences in call sub-types were detected. In the cage test, 50-kHz calls were most frequent in WU and rather few in LE rats. Call durations were longer in WU rats. When being tickled, SD females emitted calls with shorter durations and lower peak frequencies. Also, frequency modulation and call amplitude was higher in LE. Finally, the fear-conditioning test led to partly unexpected results, since many females, especially WU, did not emit 22-kHz calls even during the conditioning phase, but all stocks showed the expected behavioral immobility and responded with audible calls to the aversive shocks. These results are discussed with respect to factors of testing, development, gender, and stock.

A Barnes maze for juvenile rats delineates the emergence of spatial navigation ability.

The neural bases of cognition may be greatly informed by relating temporally defined developmental changes in behavior with concurrent alterations in neural function. A robust improvement in performance in spatial learning and memory tasks occurs at 3 wk of age in rodents. We reported that the developmental increase of spontaneous alternation in a Y-maze was related to changes in temporal dynamics of fast glutamatergic synaptic transmission in the hippocampus. We also showed that, during allothetic behaviors in the Y-maze, network oscillation power increased at frequency bands known to support spatial learning and memory in adults. However, there are no discrete learning and memory phases during free exploration in the Y-maze. Thus, we adapted the Barnes maze for use with juvenile rats. Following a single platform exposure in dim light on the day before training (to encourage exploration), animals were trained on the subsequent 2 d in bright light to find a hidden escape box and then underwent a memory test 24 h later. During escape training, the older animals learned the task in 1 d, while the younger animals required 2 d and did not reach the performance of older animals. Long-term memory performance was also superior in the older animals. Thus, we have validated the use of the Barnes maze for this developmental period and established a timeline for the ontogeny of spatial navigation ability in this maze around 3 wk of age. Subsequent work will pair in vivo recording of hippocampal oscillations and single units with this task to help identify how hippocampal maturation might relate to performance improvements.

Ontogeny of object-in-context recognition in the rat.

The object-in-context recognition (OiC) task [19] is a spontaneous exploration task that serves as an index of incidental contextual learning and memory. During the test phase, rats prefer to explore the object mismatched to the testing context based on previous object-context pairings experienced during training. The mechanisms of OiC memory have been explored in adult rats [12,35]; however, little is known about its determinants during development. Thus, the present study examined the ontogeny of the OiC task in preweanling through adolescent rats. We demonstrate that postnatal day (PD) 17, 21, 26, and 31 rats can perform the OiC task (Experiment 1) and that preference for the novel target is eliminated when rats are tested in an alternate context not encountered during training (Experiment 2). Lastly, we show that PD26 but not PD17 rats can perform the OiC task when the training contexts only differed by distal spatial cues (Experiment 3). These data demonstrate for the first time that PD17 rats can acquire and retain short-term OiC memory, which involves associative learning of object and context information. However, we also provide evidence that preweanling rats' ability to utilize certain aspects of a context (i.e., distal spatial cues) in the OiC task is not equivalent to that of their older counterparts. Implications for the development of contextual memory and its related neural substrates are discussed.

Part III: Principal component analysis: bridging the gap between strain, sex and drug effects.

Previous work has identified the adolescent period as particularly sensitive to the short- and long-term effects of marijuana and its main psychoactive component Δ9-tetrahydrocannabinol (THC). However, other studies have identified certain backgrounds as more sensitive than others, including the sex of the individual or the strain of the rat used. Further, the effects of THC may be specific to certain behavioural tasks (e.g. measures of anxiety), and the consequences of THC are not seen equally across all behavioural measures. Here, data obtained from adolescent male and female Long-Evans and Wistar rats exposed to THC and tested as adults, which, using standard ANOVA testing, showed strain- and sex-specific effects of THC, was analyzed using principal component analysis (PCA). PCA allowed for the examination of the relative contribution of our variables of interest to the variance in the data obtained from multiple behavioural tasks, including the skilled reaching task, the Morris water task, the discriminative fear-conditioning to context task, the elevated plus maze task and the conditioned place preference task to a low dose of amphetamine, as well as volumetric estimates of brain volumes and cfos activation. We observed that early life experience accounted for a large proportion of variance across data sets, although its relative contribution varied across tasks. Additionally, THC accounted for a very small proportion of the variance across all behavioural tasks. We demonstrate here that by using PCA, we were able to describe the main variables of interest and demonstrate that THC exposure had a negligible effect on the variance in the data set.

Part II: Strain- and sex-specific effects of adolescent exposure to THC on adult brain and behaviour: Variants of learning, anxiety and volumetric estimates.

Marijuana is one of the most highly used psychoactive substances in the world, and its use typically begins during adolescence, a period of substantial brain development. Females across species appear to be more susceptible to the long-term consequences of marijuana use. Despite the identification of inherent differences between rat strains including measures of anatomy, genetics and behaviour, no studies to our knowledge have examined the long-term consequences of adolescent exposure to marijuana or its main psychoactive component, Δ(9)-tetrahydrocannabinol (THC), in males and females of two widely used rat strains: Long-Evans hooded (LER) and Wistar (WR) rats. THC was administered for 14 consecutive days following puberty onset, and once they reached adulthood, changes in behaviour and in the volume of associated brain areas were quantified. Rats were assessed in behavioural tests of motor, spatial and contextual learning, and anxiety. Some tasks showed effects of injection, since handled and vehicle groups were included as controls. Performance on all tasks, except motor learning, and the volume of associated brain areas were altered with injection or THC administration, although these effects varied by strain and sex group. Finally, analysis revealed treatment-specific correlations between performance and brain volumes. This study is the first of its kind to directly compare males and females of two rat strains for the long-term consequences of adolescent THC exposure. It highlights the importance of considering strain and identifies certain rat strains as susceptible or resilient to the effects of THC.

Strain and sex differences in brain and behaviour of adult rats: Learning and memory, anxiety and volumetric estimates.

Alterations in behaviour can arise through a number of factors, including strain and sex. Here, we explored strain and sex differences between Long-Evans (LER) and Wistar (WR) male and female rats that had been trained in a myriad of behavioural tasks. Tests included those assessing motor learning (skilled reaching task), spatial learning and memory (Morris water task), contextual learning (discriminative fear-conditioning to context) and anxiety behaviour (elevated plus maze). Following behavioural assessment, associated brain areas were examined for volumetric differences, including the hippocampus and its subregions, prefrontal cortex areas and the amygdala. LER and WR differed in their rates of performance in the skilled reaching task throughout the training period. Overall, LER outperformed WR in tasks related to contextual and spatial learning, although this was not accompanied by larger volumes of associated brain areas. Males outperformed females in spatial learning, and females outperformed males in the contextual fear-conditioning task and had an associated larger amygdalar volume, although these sexual dimorphisms were only observed within the LER strain. Overall, this study highlights differences between these two rat strains as well as highlights that larger volumetric estimates of brain areas do not always confer improved function of associated behaviours.

Working-memory training: effects on delay discounting in male Long Evans rats.

Delay discounting describes the devaluation of a reward as the delay to the receipt of the reward increases. Because steep delay discounting is robustly correlated with a number of behavioral problems (e.g., substance dependence, gambling) and some evidence suggests steep discounting precedes and predicts drug-taking in humans and rats, this study sought to experimentally reduce rats' delay discounting. Human stimulant-dependent participants given working-memory training reportedly decreased their rates of discounting relative to a sham-training group (Bickel, Yi, Landes, Hill, & Baxter, 2011). To evaluate the cross-species generality of this effect, 38 male Long-Evans rats, matched on pretraining delay-discounting rates, were randomly assigned to receive 140 sessions of working-memory training or sham training (which required no memory of the sample stimulus). Large between-group differences in working memory were observed after training; however, posttraining delay-discounting rates were undifferentiated across groups. Potential explanations for these findings are discussed.

Using rats for vision research.

A wide variety of species are used for the study of visual neuroscience. This is beneficial because fundamental mechanisms and theoretical principles of vision are likely to be highly conserved, while different species exhibit different visual capacities and present different technical advantages for experiments. Eight years ago my laboratory adopted the hooded rat as our primary preparation for vision research. To some this may be surprising, as nocturnal rodents have often been presumed to have poor vision and weak visual behavior. This commentary will provide my personal perspective on how I came to work with rats; discuss an example research project for which rats have been advantageous; and comment on the opportunities and challenges of the preparation.

Predictability and heritability of individual differences in fear learning.

Our objective was to characterize individual differences in fear conditioning and extinction in an outbred rat strain, to test behavioral predictors of these individual differences, and to assess their heritability. We fear-conditioned 100 Long-Evans rats, attempted to extinguish fear the next day, and tested extinction recall on the third day. The distribution of freezing scores after fear conditioning was skewed, with most rats showing substantial freezing; after fear extinction, the distribution was bimodal with most rats showing minimal freezing, but a substantial portion showing maximal freezing. Longer rearing episodes measured prior to conditioning predicted less freezing at the beginning of extinction, but differences in extinction learning were not predicted by any baseline exploratory behaviors. We tested the heritability of extinction differences by breeding rats from the top and bottom 20% of freezing scores during extinction recall. We then ran the offspring through the same conditioning/extinction procedure, with the addition of recording ultrasonic vocalizations throughout training and testing. Only a minority of rats emitted distress vocalizations during fear acquisition, but the incidence was less frequent in the offspring of good extinguishers than in poor extinguishers or randomly bred controls. The occurrence of distress vocalizations during acquisition predicted higher levels of freezing during fear recall regardless of breeding line, but the relationship between vocalization and freezing was no longer evident following extinction training, at which point freezing levels were influenced only by breeding and not by vocalization. The heritability (h(2)) of extinction recall was estimated at 0.36, consistent with human estimates.

How domestication modulates play behavior: a comparative analysis between wild rats and a laboratory strain of Rattus norvegicus.

Laboratory rats have been widely used to study the development and neural underpinnings of play behavior. However, it is not known whether domestic rats play in the same way and at the same frequency as their wild counterparts. In this study, the play of juvenile rats from a colony of wild rats maintained in captivity was compared to that of a strain of domesticated rats (e.g., Long Evans hooded). Three predictions were tested. First, it was predicted that wild rats would incorporate more agonistic behavior in their play. This was not found, as in all cases, both the wild and the laboratory rats attacked and defended the nape during play, a nonagonistic body target. Second, because play is typically more frequent in domesticated animals than their wild progenitors, it was predicted that the wild rats should play less than the laboratory rats. This was found to be the case. Third, because wild animals tend to be less tolerant of proximity by conspecifics and tend to be more agile in their movements, it was predicted that there would be less contact between wild pair mates. This was found to be the case; data show that the play of laboratory rats involves the same target (i.e., the nape of the neck) and tactics of defense as those used by wild rats. However, the laboratory rats initiated playful attacks more frequently, and were more likely to use tactics that promoted bodily contact. These similarities and differences need to be considered when using laboratory animals as models for play in general.

Rats distinguish between absence of events and lack of evidence in contingency learning.

The goal of three experiments was to study whether rats are aware of the difference between absence of events and lack of evidence. We used a Pavlovian extinction paradigm in which lights consistently signaling sucrose were suddenly paired with the absence of sucrose. The crucial manipulation involved the absent outcomes in the extinction phase. Whereas in the Cover conditions, access to the drinking receptacle was blocked by a metal plate, in the No Cover conditions, the drinking receptacle was accessible. The Test phase showed that in the Cover conditions, the measured expectancies of sucrose were clearly at a higher level than in the No Cover conditions. We compare two competing theories potentially explaining the findings. A cognitive theory interprets the observed effect as evidence that the rats were able to understand that the cover blocked informational access to the outcome information, and therefore the changed learning input did not necessarily signify a change of the underlying contingency in the world. An alternative associationist account, renewal theory, might instead explain the relative sparing of extinction in the Cover condition as a consequence of context change. We discuss the merits of both theories as accounts of our data and conclude that the cognitive explanation is in this case preferred.

Changing within-trial array location and target object position enhances rats' (Rattus norvegicus) missing object recognition accuracy.

Six rats were trained to find a previously missing target or 'jackpot' object in a square array of four identical or different objects (the test segment of a trial) after first visiting and collecting sunflower seeds from under the other three objects (the study segment of a trial). During training, objects' local positions within the array and their global positions within the larger foraging array were varied over trials but were not changed between segments within a trial. Following this training, rats were tested on their accuracy for finding the target object when a trial's test array was sometimes moved to a different location in the foraging arena or when the position of the target object within the test array had been changed. Either of these manipulations initially slightly reduced rats' accuracy for finding the missing object but then enhanced it. Relocating test arrays of identical objects enhanced rats' performance only after 10-min inter-segment intervals (ISIs). Relocating test arrays of different objects enhanced rats' performance only after 2-min ISIs. Rats also improved their performance when they encountered the target object in a new position in test arrays of different objects. This enhancement effect occurred after either 2- or 30-min ISIs. These findings suggest that rats separately retrieved a missing (target) object's spatial and non-spatial information when they were relevant but not when they were irrelevant in a trial. The enhancement effects provide evidence for rats' limited retrieval capacity in their visuo-spatial working memory.

Evidence for individual differences in regulatory focus in rats, Rattus norvegicus.

Regulatory focus (Higgins, 1997) builds on the classic approach-avoidance distinction by identifying two important approach orientations: the promotion focus (approaching gains and attainment) and the prevention focus (approaching nonlosses and safety). Though individual differences in regulatory focus have been widely studied in human psychology, it is unknown if such differences exist in other species. To explore this possibility, we designed a series of tests for laboratory rats, paralleling human regulatory focus research on risk taking. In home-cage tests, rats (N = 23) were given an opportunity to prevent a loss by burying a noxious novel object. In solitary tests in a novel enclosure, the same rats had the opportunity to pursue gains (food rewards) and/or safety (darkness). Rats demonstrated stable individual differences on both tests (p's < .001). Complementing the human research, duration of time spent with the noxious novel object was predicted by an individual's tendency to pursue safety (p < .01) and not by the tendency to pursue gains (p > .8). Some aspects of these results were compatible with alternative approaches, such as the bold-shy axis and "if-then" personality profiles (Mischel & Shoda, 1995). Regulatory focus theory, however, was uniquely able to predict the overall pattern, which may be an indication that it could contribute to future research in animal personality, motivation, and welfare.

Rats build and update topological representations through exploration.

Although rats are able to build complex spatial representations of their surroundings during exploration, the nature of the encoded information is still a matter for debate. In particular, it is not well established if rats can process the topological structure of the environment in such a way that they are aware of the connections existing between remote places. Here, rats were first exposed for four 5-min trials to a complex environment divided into several sectors that were separated by doors allowing either unrestricted or restricted access to other sectors. In the fifth test trial, we measured the behavior of the animals while they explored the same environment in which, however, they faced changes that either altered or did not alter the topological structure of the environment. In experiment 1, closing previously opened doors prevented the rat from having direct access between corresponding sectors. In experiment 2, opening previously closed doors allowed direct access between sectors that had not been directly accessible. In each experiment, control doors allowed us to discard the mere influence of door manipulation. We compared the rats' exploratory behavior in response to door manipulations that either strongly altered or did not alter the ability to commute between sectors and found evidence that the animals displayed differential reactions to the two types of door manipulations. This implies that during exploration rats build a precise map of the connectivity of space that can be flexibly updated and used for efficient navigation.

Route selection by rats and humans in a navigational traveling salesman problem.

Spatial cognition is typically examined in non-human animals from the perspective of learning and memory. For this reason, spatial tasks are often constrained by the time necessary for training or the capacity of the animal's short-term memory. A spatial task with limited learning and memory demands could allow for more efficient study of some aspects of spatial cognition. The traveling salesman problem (TSP), used to study human visuospatial problem solving, is a simple task with modifiable learning and memory requirements. In the current study, humans and rats were characterized in a navigational version of the TSP. Subjects visited each of 10 baited targets in any sequence from a set starting location. Unlike similar experiments, the roles of learning and memory were purposely minimized; all targets were perceptually available, no distracters were used, and each configuration was tested only once. The task yielded a variety of behavioral measures, including target revisits and omissions, route length, and frequency of transitions between each pair of targets. Both humans and rats consistently chose routes that were more efficient than chance, but less efficient than optimal, and generally less efficient than routes produced by the nearest-neighbor strategy. We conclude that the TSP is a useful and flexible task for the study of spatial cognition in human and non-human animals.

Acquired equivalence of cues by presentation in a common context in rats.

It is well established that humans and other animals may treat two perceptually different cues alike, if the cues have been individually paired with a common antecedent or a common consequence. Recently, Molet et al. (Psychon Bull Rev 18:618-623, 2011) reported evidence for a new form of acquired equivalence in human conditional discrimination, namely context-mediated equivalence. In the present research, using a flavor conditioning procedure, we asked whether rats would show similar context-mediated equivalence to demonstrate that this new form of acquired equivalence is a general phenomenon. Rats experienced two flavor cues A and B each presented either in the same context, X, or each in its own distinctive context, X or Y. Subsequently, the rats experienced B with sucrose in a third context, Z, and then the generalization of conditioning to A was assessed. When tested in Context Z, consumption of A was more marked when A and B had both been presented in the same context than when they had been presented in two different contexts. Thus, importantly, in the absence of the training context, cues that shared a common context at different times came to be treated as equivalent. This represents the first evidence of context-mediated equivalence in a nonhuman species.

Rats are sensitive to ambiguity.

In the present study, we investigated response decisions made under conditions of incomplete information in rats. In Experiment 1, rats were trained on either a positive patterning (PP; A-, B-, AB+) or a negative patterning (NP; A+, B+, AB-) instrumental lever-press discrimination. Subjects that had learned an NP discrimination responded less to Cue A when Cue B was covered at test. The cover did not, however, affect test responses to Cue A in the PP condition. In Experiment 2, rats received concurrent training on both PP and NP discriminations. After concurrent training, responses to Cue A were different with B covered versus uncovered for both NP and PP discriminations. We discuss possible accounts for why exposure to a nonlinearly soluble discrimination (NP) may have affected sensitivity to cue ambiguity produced by the cover. These results have interesting implications for representational processes engaged in problem solving.

Validation of a rodent model of episodic memory.

Episodic memory consists of representations of specific episodes that happened in the past. Modeling episodic memory in animals requires careful examination of alternative explanations of performance. Putative evidence of episodic-like memory may be based on encoding failure or expectations derived from well-learned semantic rules. In Experiment 1, rats were tested in a radial maze with study and test phases separated by a retention interval. The replenishment of chocolate (at its study-phase location) depended on two factors: time of day (morning vs. afternoon) and the presence or absence of chocolate pellets at the start of the test phase. Because replenishment could not be decoded until the test phase, rats were required to encode the study episode. Success in this task rules out encoding failure. In Experiment 2, two identical mazes in different rooms were used. Chocolate replenishment was trained in one room, and then they were asked to report about a recent event in a different room, where they had no expectation that the memory assessment would occur. Rats successfully answered the unexpected question, ruling out use of expectations derived from well-learned semantic rules. Our behavioral methods for modeling episodic memory may have broad application for assessments of genetic, neuroanatomical, neurochemical, and neurophysiological bases of both episodic memory and memory disorders such as those that occur in Alzheimer's disease.

Relative potency and effectiveness of flunitrazepam, ethanol, and beta-CCE for disrupting the acquisition and retention of response sequences in rats.

Despite the knowledge that gamma-aminobutyric acid(A) modulators can affect learning and memory, their capacity for disrupting each of these complex processes is rarely compared, and often mistakenly assumed to occur with identical potency. For these reasons, the effects of flunitrazepam (0.056-3.2 mg/kg), ethanol (0.25-1.5 g/kg), and ethyl-beta-carboline-3-carboxylate (beta-CCE; 1-17.8 mg/kg) were compared in groups of rats responding under baselines that assessed learning and memory separately. The first baseline was a multiple schedule of repeated acquisition and performance of tandem response sequences, whereas the second baseline was a retention or memory procedure where a tandem response sequence was acquired and then retested after a 30-min delay. Under both procedures, responding was maintained under a second-order fixed-ratio-2 schedule of food reinforcement, and incorrect responding (errors) produced a 5-s timeout. With regard to the effects of the three drugs on sequence acquisition (learning), all three drugs dose dependently decreased the overall response rate and increased the percentage of errors. Both flunitrazepam and beta-CCE affected accuracy more potently than response rate, whereas ethanol was equipotent in affecting these two dependent measures. With regard to the effects of these drugs on sequence retention (memory), both flunitrazepam and ethanol dose dependently decreased retention at doses that had little or no effect on sequence acquisition under the multiple schedule, whereas beta-CCE decreased retention and sequence acquisition similarly at the doses tested. Together, these data show that drugs with differing capacities for altering the function of gamma-aminobutyric acid(A) receptors differ in their capacity for disrupting the acquisition and retention of response sequences and that positive modulation of this receptor complex may be more predictive of disruptions in memory than disruptions in learning.