Relationship between motor function and ultrasonic vocalizations induced by maternal separation in rat pups.

When rat pups are isolated from their mothers, they emit ultrasonic vocalizations (USVs). Although previous studies have reported that USVs are related to anxiety, others have reported that they are related to simple, nonemotional factors, such as physiological reactions to coldness. In this study, we examined the influence of three maternal separations on rat pups. The number of USVs during 5 min of USV test under maternal separation, latency in the righting reflex as motor function, and body temperature were recorded twice (the first and second tests) before and after the pups were put in various environments for 10 min. The environments were no maternal separation (Control: CON), maternal separation with littermates (LMS), and single maternal separation with a heater (SMS). In the second test, the SMS pups had fewer USVs, a lower body temperature, and a more rapid righting reflex than the CON and LMS pups. In addition, there was no strong correlation between USVs and righting reflex. As a result, pups undergoing 10 min of SMS while being kept warm by the heater showed rapid righting reflex. Thus, by a single maternal separation, the number of USVs decreased but the decrease was unrelated to decrease in motor function.

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.

Social modulation of risky decision-making in rats (Rattus norvegicus) and tufted capuchin monkeys (Sapajus spp.).

Both human and non-human animals frequently deal with risky decisions in a social environment. Nevertheless, the influence of the social context on decision-making has been scarcely investigated. Here, we evaluated for the first time whether the presence of a conspecific influences risk preferences in rats and in tufted capuchin monkeys. Subjects received a series of choices between a constant, safe option and a variable, risky option, both alone (Alone condition) and when paired with a conspecific (Paired condition). The average payoff of the risky option was always lower than that of the safe option. Overall, the two species differed in their attitude towards risk: whereas rats were indifferent between options, capuchins exhibited a preference for the safe option. In both species, risk preferences changed in the Paired condition compared to the Alone condition, although in an opposite way. Whereas rats increased their risk preferences over time when paired with a conspecific, capuchins chose the risky option less in the Paired condition than in the Alone condition. Moreover, whereas anxiety-like behaviours decreased across sessions in rats, these behaviours where more represented in the Paired condition than in the Alone condition in capuchins. Thus, our findings extends to two distantly-related non-human species the evidence, so far available for human beings, that a decrease in anxiety corresponds to an increase in risk preferences, and vice versa. This suggests that the modulation of risk preferences by social influences observed in rats and capuchin monkeys may rely on a common, evolutionarily ancient, mechanism.

The effect of methamphetamine exposure during pregnancy and lactation on hippocampal doublecortin expression, learning and memory of rat offspring.

The aim of this study was to evaluate the effect of methamphetamine (MA) exposure during pregnancy and lactation on doublecortin (DCX) expression in the hippocampus of rat offspring and also on learning/memory. Thirty-five pregnant Wistar rats were randomly divided into seven groups of 5 rats each: three experimental groups, each receiving 5 mg/kg body weight (BW) intraperitoneal (i.p.) injections of MA during pregnancy or/and lactation; three sham groups, each receiving saline injections; one control group, receiving no injection. After the interventions, two male pups (1 and 22 days old) were randomly selected from each mother, sacrificed and their brains subjected to DCX immunohistochemistry. One additional male pup from each mother was randomly selected and maintained for 60 days for testing in the Morris water maze and passive avoidance tests. MA administration during pregnancy was found to have significantly decreased the number of DCX-positive cells in the CA1, CA3 and DG regions of the hippocampus in the 1-day pups (P ≤ 0.05) and to have significantly decreased the number of DCX-positive cells in only two regions of the hippocampus, the CA1 and DG regions, in 22-day old pups. In comparison, exposure to MA during lactation was only associated with a significant decrease in the number of DCX-positive cells in the DG. Exposure to MA during pregnancy had significant impact on the intensity of DCX expression in the hippocampus of 1- and 22-day pups (P ≤ 0.05). There was no significant difference in memory/learning among the study groups. Our results indicate the administration of MA during pregnancy had a greater effect that during the lactation period on DCX expression in the hippocampus of rat offspring.

Proximity as a predictor of social behavior in rats.

BACKGROUND: In studies that measure social behavior of a freely interacting pair rats social behavior of one rat is strongly influenced by the behavior of the other. This prevents evaluating social behavior of one single rat. NEW METHOD: We assessed the motivation to interact socially in a modified open-field, by measuring the time a rat attempted to interact with a co-specific separated by a grid in a birdcage outside of the apparatus. We propose time in front of the birdcage is an indicator of social behavior. RESULTS: We showed that the focal rat allocates more time in front of the birdcage, interacting with another rat through the grid. Also, that the presence of the other rat that attracts the focal rat. Habituation to the apparatus, repeated testing and illumination condition did not alter the proximity measures of rats. Finally, treatment with chlordiazepoxide (3.0mg/kg) either increased the time spent in front of the cage by males and females or (5.6mg/kg) increased the proximity measure of females. COMPARING WITH EXISTING METHOD: Our method prevents partners from influencing the target rat's social behavior; existing methods do not. Also, it is more sensitive to the effect of chlordiazepoxide than the broadly used method proposed by File and Hyde (1978). CONCLUSIONS: Proximity is an advantageous measure: it allows the assessment of only one focal animal without the interference of a partner; it is simple to take; it requires little interpretation skills or training from the experimenter, no special equipment or conditions.

Male contraceptive efficacy of poly herbal formulation, contracept-TM, composed of aqueous extracts of Terminalia chebula fruit and Musa balbisiana seed in rat.

CONTEXT: Terminalia chebula Retz (Combretaceae) and Musa balbisiana Colla (Musaceae) have a traditional reputation as a male contraceptive. OBJECTIVE: To determine the hypo-testicular activity of aqueous extracts of Terminalia chebula (fruit) and Musa balbisiana (seed) separately, and in composite manner at the ratio of 1:1 named as 'Contracept-TM' compared to cyproterone acetate (CPA), for developing a polyherbal contraceptive. MATERIALS AND METHODS: The separate extract of above said plants or 'Contracept-TM' at the dose of 40 mg/100 g body weight of rat/day or CPA at 2 mg/100 g body weight of rat/day was administered for 28 days. Spermiological, androgenic and oxidative stress sensors, LD50 and ED50/100 g body weight values were measured. RESULTS: Treatment of individual, 'Contracept-TM' or CPA resulted significant decrease in the count of spermatogonia A (36.36-49.09%), pre-leptotene spermatocyte (19.11-55.30%), mid-pachytene spermatocyte (28.65-47.28%) and step 7 spermatid (29.65-51.59%). Activities of testicular Δ5, 3ß (21.25-48.02%),17ß-hydroxysteroid dehydrogenases (29.75-55.08%), catalase (19.06-43.29%) and peroxidase (30.76-62.82%), levels of testosterone (28.15-63.44%), testicular cholesterol (19.61-49.33%), conjugated diene (29.69-84.99%) and thiobarbituric acid reactive substances (41.25-86.73%) were elevated compare to the control. The ED50 and LD50 values were 40 mg and 5.8 g (T. chebula), 48 mg and 6.3 g (M. bulbisiana), 40 mg and 6.0 g ('Contracept-TM'), respectively. DISCUSSION AND CONCLUSION: The said spermiological and androgenic sensors' levels were decreased significantly by 'Contracept-TM' than its constitutional individual plant extract and it may be comparable to standard anti-testicular drug like CPA. So, it may be concluded that above polyherbal formulation is potent for inducing hypo-testicular activity.

Stevia preferences in Wistar rats / Preferencia gustativa de las ratas Wistar por la estevia

BACKGROUND: The Stevia rebaudiana plant is likely to become a major source of high-potency sweetener for the growing natural-food market. S. rebaudiana is the source of a number of sweet diterpenoid glycosides, but the major sweet constituents are rebaudioside A and stevioside. These two constituents have similar pharmacokinetic and metabolic profiles in rats and humans, and thus, studies carried out with either steviol glycoside are relevant to both. Other studies illustrate the diversity of voluntary sweet intake in mammals. METHOD: This study was done using a series of two-bottle tests that compared a wide range of sweetener concentrations versus saccharin concentrations and versus water. RESULTS: Wistar rats displayed preferences for stevia extract and pure rebaudioside A solutions over water at a range of concentrations (0.001% to 0.3%), and their intake peak occurred at 0.1% concentration. They also preferred solutions prepared with a commercial rebaudioside A plus erythritol mixture to water, and their peak was at 2% concentration. CONCLUSIONS: The present study provides new information about the responses of Wistar rats to stevia compounds and commercial stevia products such as Truvia. These results could help with the appropriate dosage selection for focused behavioral and physiological studies on stevia

ANTECEDENTES: la planta Stevia rebaudiana se convertirá en una de las principales fuentes de edulcorantes debido al crecimiento del consumo de productos naturales en el mercado. S. rebaudiana contiene distintos glucósidos diterpenoides, pero los que proporcionan dulzor son el rebaudiosido A y el esteviosido. Estos dos compuestos tienen perfiles farmacocinéticos y metabólicos similares en ratas y humanos. Por otro lado, hay estudios que muestran la existencia de distintos patrones de ingesta voluntaria de edulcorantes en los mamíferos. MÉTODO: se realizaron series de la prueba de libre elección entre dos botellas. Comparamos la ingesta de un rango de concentraciones de edulcorantes frente al agua y frente a sacarina. RESULTADOS: las ratas Wistar prefieren el extracto de estevia y el rebaudiosido A (concentraciones desde 0,001% hasta 0,3%) frente al agua, la ingesta máxima fue a la concentración de 0,1%. También prefieren las soluciones preparadas con el producto comercial Truvia (rebaudiósido A y eritritol) frente al agua, la ingesta máxima fue a la concentración de 2%. CONCLUSIONES: nuestro trabajo proporciona nueva información sobre la preferencia gustativa de las ratas Wistar por distintos compuestos de estevia. Estos resultados ayudarán al diseño de estudios centrados en los efectos comportamentales y fisiológicos del consumo de estevia

Behavioral differences of male Wistar rats from different vendors in vulnerability and resilience to chronic mild stress are reflected in epigenetic regulation and expression of p11.

Outbred rat lines such as Wistar rats are commonly used for models of depressive disorders. Such rats arise from random mating schedules. Hence, genetic drift occurs in outbred populations which could lead to genotypic and phenotypic heterogeneity between rats from different vendors. Additionally, vendor specific rearing conditions could contribute to intrastrain variability. In the present study differences in behavioral responses to the chronic mild stress (CMS) model of depression within Wistar rat strains from different vendors are described. DNA methylation studies and mRNA expression analysis of p11 revealed that the behavioral differences between the substrains are reflected at the epigenetic and genetic level. The results suggest that there are breeder-dependent differences in vulnerability to stress in the CMS model of depression, which might bear on the validity of the model and contribute to contradictory findings and difficulties of replication between laboratories. P11 mRNA expression seems to be differently regulated depending on the quality of the stress response evoked by CMS exposure.

Decreased emotional reactivity of rats exposed to repeated phase shifts of light-dark cycle.

Disturbed light-dark (LD) cycles are associated with circadian disruption of physiological and behavioural rhythms and in turn with an increased risk of disease development. However, direct causal links and underlying mechanisms leading to negative health consequences still need to be revealed. In the present study, we exposed male Wistar rats to repeated phase shifts of LD cycle and analysed their ability to cope with mild emotional stressors. In experiment 1, rats were submitted to either a regular 12:12 LD cycle (CTRL rats) or 8-h phase delay shifts applied every 2days for 5weeks (SHIFT rats). Subsequently, the behaviour was examined in the open-field, black-white box and elevated plus maze tests. In experiment 2, changes in blood pressure (BP), heart rate (HR) as well as the activity of autonomic nervous system were measured in telemeterised rats in response to open-field and black-white box tests before and after 5-week exposure to shifted LD regime. Locomotor activity was consistently higher in SHIFT than CTRL rats in in the open-field and black-white box tests. Interestingly, in the elevated plus maze, SHIFT rats displayed increased risk assessment and decreased grooming compared to CTRL rats. Anxiety measures were affected only in the black-white box, where SHIFT rats displayed reduced anxiety-like behaviour compared to CTRL rats. Differences in behavioural reactivity between SHIFT and CTRL rats did not correspond with BP and HR changes. However, exposure to phase shifts increased the sympathovagal reactivity in the black-white box. Together, our results demonstrated that disturbed LD conditions decreased emotional reactivity of rats and affected their ability to cope with emotional stressors denoting an additional risk mechanism linking disrupted circadian organisation to adverse health effects.

Air puff-induced 22-kHz calls in F344 rats.

Air puff-induced ultrasonic vocalizations in adult rats, termed "22-kHz calls," have been applied as a useful animal model to develop psychoneurological and psychopharmacological studies focusing on human aversive affective disorders. To date, all previous studies on air puff-induced 22-kHz calls have used outbred rats. Furthermore, newly developed gene targeting technologies, which are essential for further advancement of biomedical experiments using air puff-induced 22-kHz calls, have enabled the production of genetically modified rats using inbred rat strains. Therefore, we considered it necessary to assess air puff-induced 22-kHz calls in inbred rats. In this study, we assessed differences in air puff-induced 22-kHz calls between inbred F344 rats and outbred Wistar rats. Male F344 rats displayed similar total (summed) duration of air puff-induced 22 kHz vocalizations to that of male Wistar rats, however, Wistar rats emitted fewer calls of longer duration, while F344 rats emitted higher number of vocalizations of shorter duration. Additionally, female F344 rats emitted fewer air puff-induced 22-kHz calls than did males, thus confirming the existence of a sex difference that was previously reported for outbred Wistar rats. The results of this study could confirm the reliability of air puff stimulus for induction of a similar amount of emissions of 22-kHz calls in different rat strains, enabling the use of air puff-induced 22-kHz calls in inbred F344 rats and derived genetically modified animals in future studies concerning human aversive affective disorders.

Sociosexual behaviors and reproductive success of rats (Rattus norvegicus) in a seminatural environment.

A promiscuous pattern of copulation has been reported in both wild and domestic rats, and multiple paternity is common. In the present study we determined whether male sociosexual behaviors were associated with reproductive success or not. Groups of rats (3 males and 4 cycling females) were housed in a seminatural environment for a period of 8 days. Sociosexual interactions were observed whenever one or several females were in behavioral estrus. Paternity of the offspring sired was determined by analyses of single nucleotide polymorphisms. Fertility was quantified either as the total number of offspring, or the mean proportion of offspring from all females, sired by the male. Although the total numbers of male social and sexual behaviors during the 8 day period were unrelated to reproductive success, male fertility was correlated with the number of intromissions and ejaculations. In addition intromission and ejaculation ratios (the proportion of mounts ending in intromission and the proportion of the total number of ejaculations received by a female that was contributed by a particular male, respectively) were also correlated with male fertility. Fewer mounts as well as fewer male pursuits of the female were demonstrated by males siring entire litters than by males siring multipaternity litters. Ejaculation order was unrelated to fertility. Male or female preferences (based on various sociosexual interactions) were not strongly related to fertility. Female preference only showed a limited effect. Specifically, preferred males (males who were sniffed most) sired fewer pups. Male dominance status did not affect female preferences, copulatory behavior or fertility. It appears that only behaviors directly related to sperm transfer are important for fertility. Social behaviors in both males and females play a very limited or no role in determining rats' fertility.

Maternal-infant separation impedes changes in feeding behavior during estrous cycle of rats.

Traumatic and stressful events during childhood are associated with the development of eating disorders. We conducted an animal study to test if association stress in childhood affects ingestive behavior later in life by using female rats that have an adjusted estrous cycle. First, electrical impedance of the vagina was conducted to test estrous cycle adjustment. Second, the effects of 6 h per day maternal separation from birth to weaning, which models a psychologically stressful experience in childhood, was used to test feeding behavior during an ovarian cycle in female adult rats with matched estrous cycles. Food and water intake in maternal separated and non-separated rats was measured in each estrous phase. Non-separated rats showed periodical changes, but maternal separated rats showed no significant changes in food and water intake during an estrous cycle. An opposing tendency for food and water intake was seen between maternal separated and non-separated rats. These observations suggest that electrical impedance of the vagina showed the highest value in the estrous phase of rats housed in a reversed light-dark cycle, and maternal separation was found to disturb changes in feeding behavior during the estrous cycle.

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.

There is more to the picture than meets the rat: a study on rodent geometric shape and proportion preferences.

In rodents, the novel object preference test has been used as a behavioral parameter for evaluation of neotic exploratory behavior, and also for memory consolidation tasks. Geometric patterns of this preference are poorly understood, and may vary among species. We evaluated in Wistar rats (Rattus norvergicus) a possible exploration preference considering aluminum tripartite rounded and cylindrical objects of different proportions: 1.2; 1.618; 1.8. At the first day, animals were exposed to 1.2; 1.6 and 1.8 rounded objects. At 24h after, these animals were exposed to the same objects, together with three new steel cylindrical objects (same proportions). ANOVA and T tests were used to quantify object exploration for each animal (p<0.05). Data analysis pointed to a longer exploration time of the object 1.2 at the three different protocols indicating a preference pattern on the first day exposition. On the second day the exploration was similar in both familiar and unfamiliar objects, revealing no novel object preference for cylinders. However, we found an object preference related to the 1.2 proportion (balls plus cylinders), in two of three position protocols. In addition, on a single exposition with both cylinders and rounded objects, rats revealed a rounded object preference. The 1.2 preference disclosed by rats also reflected the proportion of their body. From nine main measures of body ratios, seven were close to 1.2 ratio. The correspondence between body ratios and object preference may be explained by habituation learning and by sexual selection, and highlight innate factors regarding aesthetic preferences among species.

Impact of chronic stressors on the anxiety profile of pregnant rats.

The manifestation of anxiety during pregnancy can be caused by multiple factors and may have emotional and physical consequences for both the mother and the fetus. The prevalence of gestational anxiety has grown in recent years, making the development of studies for its comprehension essential. Thus, the aim of this investigation was to evaluate the effects of predictable and unpredictable chronic stressors on the anxiety profile of rats in three distinct stages of pregnancy (1st, 2nd and 3rd weeks). Wistar dams were divided into three groups: control, social separation and unpredictable chronic stress. Behavioral assessments were conducted in the Elevated Plus-Maze at the end of the 1st, 2nd and 3rd weeks of gestation. The results showed that there was increased anxiety in the proximity of parturition in control dams. Chronic stressors differentially affected the behavior of pregnant rats according to the gestational period where they were applied: social separation decreased anxiety at the end of the 3rd week, while unpredictable chronic stress caused increased anxiety, especially at the end of the 2nd gestational week. These results show that there is a critical time during pregnancy for the onset of anxiety in control rats, depending on the gestational stage. The exposure to different types of chronic stressors may result in distinct behaviors related to this disorder.

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.

Sociosexual behaviors of male rats (Rattus norvegicus) in a seminatural environment.

Groups of rats (3 males and 4 cycling females) were housed in a seminatural environment consisting of a burrow and a large open area for a period of 8 days. Sociosexual interactions were recorded whenever 1 or several females were in behavioral estrus. Male sociosexual behaviors occurred in bouts (periods of sexual activity followed by more than 60 min without any sexual behavior), which could be ended either by mount (mean ± 95% confidence interval was 38 ± 13% of all bouts), intromission (21 ± 15%), or ejaculation (40 ± 18%). Bouts had a median duration of 2.8 ± 0.9 h. Within a bout, the intensity of sexual behavior remained quite stable. Interestingly, there was no decline as the end of the bout approached. There was no relationship between the last event in the preceding bout and the interbout interval. Regardless of whether there was single or multiple females in estrus, the males copulated with all available females in an order that was not significantly different from random. Social behaviors like sniffing and anogenital sniffing were more frequent in the burrow whereas sexual interactions were more frequent in the open area. The seminatural environment employed here offers a considerable amount of information concerning sexual interactions that cannot be obtained in traditional pair tests. Whenever speculations about the biological function or adaptive value of sexual behaviors are to be made, it appears that the present approach is indicated.

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.

Preference pulses induced by reinforcement.

Eight rats responded on concurrent Variable-Ratio 20 Extinction schedules for food reinforcement. The assignment of variable-ratio reinforcement to a left or right lever varied randomly following each reinforcer, and was cued by illumination of a stimulus light above that lever. Postreinforcement preference levels decreased substantially and reliably over time when the lever that just delivered reinforcement was now in extinction; however, if that lever was once again associated with variable ratio, this decrease in same-lever preference tended to be small, and for some subjects, not in evidence. The changes in preference level to the extinction lever were well described by a modified version of Killeen, Hanson, and Osborne's (1978) induction model. Consistent with this model's attribution of preference change to induction, we attribute preference change in this report to a brief period of reinforcer-induced arousal that energizes responding to the lever that delivered the last reinforcer. After a few seconds, this induced responding diminishes, and the operant responding that remains comes under the control of the stimulus light cuing the lever providing variable-ratio reinforcement.

Responding by exclusion in Wistar rats in a simultaneous visual discrimination task.

Responding by exclusion is to select a correct alternative by rejecting other potential alternatives. Studies describe this ability in some mammals and birds. However, this type of performance has not been reported in rodents. The aim of this study was to verify the occurrence of responding by exclusion in Wistar rats after a baseline of simple simultaneous visual discrimination. Six male Wistar learned nose-poking tunnels displaying visual stimuli (projected geometric shapes) in an operant chamber. After establishing the simultaneous discrimination baseline, three probe sessions were conducted. In each session, there was a novelty-control probe (a new stimulus was presented together with a stimulus trained as the S(+)) and an exclusion probe (a second new stimulus was presented simultaneously with a stimulus trained as the S(-)). Only one rat responded to the new stimulus in one of the three novelty probe trials. Four rats responded to the three new stimuli and one responded to the new stimulus in two of the three exclusion probes. One subject responded to the S(-) in all the exclusion probes. Five of the six subjects were therefore able to choose the new stimulus, rejecting stimuli trained as the S(-). This is the first experimental evidence for performance by exclusion in rats.