A Comparative Study of the Behavioral Patterns of RLA/Verh and RHA/Verh Rats in the Exploration Box

Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats differ in their emotionality profiles. As emotional processes are considered to play an important role in exploratory behavior, differences between the Roman sublines in responding to a novel environment can be anticipated. The present study provides a quantitative (based on the frequency of particular responses) and qualitative (based on the frequency of behavioral sequences) comparison of exploratory behavior of RHA/Verh and RLA/Verh rats. Rats of both sexes were tested individually for 30 min in an exploration box containing a few objects. Observation periods were divided, based on earlier studies, into three consecutive blocks of 5, 10, and 15 min, respectively. Analysis of quantitative data showed differences in sniffing (RHA/Verh scoring higher) and in immobility (RLA/Verh scoring higher), both persisting throughout all measurement periods. Males of both sublines were generally more active and exploratory than their respective females. The analysis of sequential data revealed that during the first 5 min RHA/Verh rats showed a less diverse but more exploratory repertoire and that during the first 15 min males of both sublines showed more behavioral sequences than females. This study indicates that the Roman rat sublines should provide a useful model to study the mechanisms of exploratory behavior.     

Neuroendocrine Correlates of Emotional Reactivity and Coping in Male Rats from the Roman High (RHA/Verh)-and Low (RLA/Verh)-Avoidance Lines

The Roman high (RHA/Verh)-and low (RLA/Verh)-avoidance rats, originally selected and bred for rapid vs. poor acquisition of a two-way active avoidance response, differ in emotional reactivity and sensitivity to stressors in various other test situations. These behavioral differences are associated with particular neuroendocrine and neurochemical characteristics. The aim of this short review is to present data currently available on the neuroendocrine profiles of RHA/Verh and RLA/Verh rats, together with more recent findings which suggest that differences in peripheral and central hormonal responses, and in hormone action on the brain, may be closely related to emotional reactivity and coping ability. Although genetic factors certainly play a major role, there is also evidence that epigenetic factors, e.g., early environmental influences, can modulate the phenotypic expression of the basic behavioral and neuroendocrine traits characterizing these lines. These psychogenetically selected lines can therefore be used as a model to investigate interactions between genes and the environment in determining each individual's sensitivity to stress and coping abilities (vulnerability model). This model may prove particularly useful for studies on the etiology and pathophysiology of anxiety and affective disorders and their neuroendocrine correlates.     

Tail pinch induced eating: psychogenetic comparison of Roman high- and low-avoidance rats

Consummatory behavior elicited by mild tail pinch in the presence of solid food was studied in male Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh) rats. RLA/Verh rats, characterized as the more emotional of the two rat lines, exhibited significantly shorter latencies to bite and eat solid food during tail pinch than the RHA/Verh rats. Treatment with 6, 12 and 24 mg/kg sodium pentobarbital prior to tail pinch failed to increase the elicited eating in either rat line.     

Selection for two-way avoidance deficit inhibits shock-induced fighting in the rat

Matched pairs of male and female, Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats were tested for shock-induced fighting, and single rats were tested for comparative sensitivity to footshocks. The main finding was that the RLA/Verh rats showed a total absence of shock-induced fighting or posturing. The females of both selected lines showed lower flinch-, shuffle- and jump-shock thresholds than did the males. Although the RHA/Verh rats had lower shuffle and jump thresholds than did the RLA/Verh rats, there was no significant difference in the initial detection of foot-shocks by either group (flinch threshold), and all 3 levels of threshold values for both were well below the 3 mA shock level used in the shock-induced fighting experiment. It was concluded that the genetically-based response suppression (freezing behavior) seen with RLA/Verh rats under these and all shock-stress conditions played a greater role in the inhibition of shock-induced fighting than did sensitivity to footshocks per se.     

Neonatal Handling and Environmental Enrichment Effects on Emotionality, Novelty/Reward Seeking, and Age-Related Cognitive and Hippocampal Impairments: Focus on the Roman Rat Lines

Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats are selected and bred for extreme divergence in two-way active avoidance acquisition. In addition, compared to RLA/Verh rats, RHA/Verh rats are (behaviorally and physiologically) less anxious or reactive to stressors, show increased novelty (sensation)-seeking behavior as well as a higher preference for rewarding substances, and are usually less efficient in learning tasks not involving shock administration. The present article reviews evidence showing that neonatal handling and/or environmental enrichment leads to enduring effects (their magnitude frequently depending upon the rat line) on those behaviors. For example, it has been found that neonatal handling reduces most of the (behavioral and physiological) signs of emotionality/anxiety in RLA/Verh rats, while environmental enrichment increases their novelty seeking (also the case with RHA/Verh rats), saccharin and ethanol intake, and sensitivity to amphetamine. Finally, initial results (currently being further elaborated upon) support a preventive action of both environmental treatments on age-related impairments in learning a spatial, water maze task as well as on hippocampal neuronal atrophy.     

Roman high- and low-avoidance rats: Present status of the swiss sublines,RHA/Verh and RLA/Verh,and effects of amphetamine on shuttle-box performance

Following a recent change in location and shuttle-box testing conditions, RHA/Verh and RLA/Verh rats were found to have maintained their characteristic patterns of avoidance (RHA/Verh) vs nonavoidance (RLA/Verh) behavior, regardless of the time of day tested. A second experiment demonstrated dose-related differences in the effects of amphetamine in both lines. In a third experiment it was seen that, whereas 2 mg/kg amphetamine was capable of increasing two-way avoidance acquisition in naive RLA/Verh rats to the levels of nondrugged RHA/Verh rats, this improvement was accompanied by substantial increases in spontaneous activity (intertrial responses). That learning was not genuinely enhanced by amphetamine was further indicated by the observation that the avoidance improvement disappeared when the drug was abruptly, or gradually, discontinued. The return toward control levels of avoidance and freezing responses was accompanied by a similar normalization of activity levels.     

Corticotropin-Releasing Hormone Modulation of a Conditioned Stress Response in the Central Amygdala of Roman High (RHA/Verh)-Avoidance and Low (RLA/Verh)-Avoidance Rats

Roman high (RHA/Verh)- and low (RLA/Verh)-avoidance rats are selected and bred for rapid versus nonacquisition of two-way, active avoidance behavior in the shuttle box. RHA/Verh rats generally show a more active coping style than do their RLA/Verh counterparts when exposed to various environmental challenges. The central nucleus of the amygdala (CeA) is known to be involved in the regulation of autonomic, neuroendocrine, and behavioral responses to stress. Its involvement in the selection of coping strategies has also been suggested. Corticotropin-releasing hormone (CRH) seems to be one of the key neurohormones in the control of CeA output. Neuroanatomical studies have revealed that the majority of CRH fibers from the CeA have direct connections with autonomic regulatory nuclei in the brain-stem, e.g. lateral parabrachial nucleus (lPB). The effects of CRH (30 ng) on modulating CeA activity were studied by infusion of CRH into the CeA during conditioned stress (inescapable foot-shocks) in RHA/Verh and RLA/Verh male rats. Heart-rate responses after CRH treatment were not changed in either line. However, distinctly different behavioral responses were seen after CRH infusion into the CeA of both rat lines. A decrease in immobility responses was seen in both RHA/Verh and RLA/Verh rats, while an increase in exploration was observed in RHA/Verh rats only in the conditioned stress situation. Rearing levels were increased in the RHA/Verh rats, whereas they were decreased in the RLA/Verh animals. As a result of CRH infusion, the number of FOS immunoreactive cells in the lPB of RLA/Verh rats was decreased, whereas an opposite response was found in RHA/Verh rats. These results indicate that the CRH system of the CeA connected with output brain-stem areas is differentially involved in the cardiovascular and behavioral responses of these rats having different coping styles.     

Diverse Sensitivity of RHA/Verh and RLA/Verh Rats to Emotional and Spatial Aspects of a Novel Environment as a Result of a Distinct Pattern of Neuronal Activation in the Fear/Anxiety Circuit

Psychogenetically selected Roman high (RHA/Verh) and Roman low (RLA/Verh) avoidance rats constitute a well-recognized model of diverse emotional reactivity. The two Swiss lines display marked behavioral and endocrine differences in reaction to a novel environment. In our study we found that these differences are accompanied by a distinct, line-specific pattern of neuronal activation within the fear/anxiety circuit. We have compared the c-Fos protein expression in the medial prefrontal cortex (mPFC), basolateral (BLA), central (CeA), medial (MeA), and cortical (CoA) nuclei of amygdala, paraventricular nucleus of the hypothalamus (PVN), and CA1, CA2, and CA3 fields of the hippocampus upon exposure to a novel situation of different stressorgeneity (open field with illuminated center, elevated plus maze, hole board test and acute restraint). Profound between-line differences in the sensitivity to emotional and spatial aspects of the behavioral challenge were observed for tests measuring spontaneous behavior. This effect seems to reflect different motivational factors driving the rat behavior, which clearly suggests that the diverse emotional reactivity of RHA/Verh and RLA/Verh rats is a result of different activation of the fear/anxiety circuit. Edited by Stephen Maxson.     

Environmental and genetic effects on food-deprivation induced stomach lesions in male rats

When Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats were individually housed in plastic cages with sawdust bedding and food-deprived (F-D) for 4–5 days, it was found that F-D RHA/Verh rats had more lesions than their unfasted controls and more lesions than F-D RLA/Verh rats. The lesions were mostly petechial in nature and located in the glandular portion of the stomach. Also, F-D RHA/Verh rats which were housed in the same room as the controls, as well as F-D RHA/Verh rats which were housed in a separate room with a strong food odor present, had more lesions than F-D RHA/Verh rats housed in the same separate room when there was no food odor, and when none of the rats present had access to food. When F-D RHA/Verh and F-D RLA/Verh rats were individually housed in metal cages with grid floors, however, a general increase in lesion scores resulted and differences between the two rat lines disappeared, as did differences among the room conditions. Also, many lesions were of an ulcerative nature and were located in the rumenal portion of the stomach. It was concluded that sensory (in this case olfactory, at least) and genetic factors are capable of playing roles in the induction of stomach lesions in rats, and that the type, extent and location of the lesions can depend upon whether or not the animals have access to sawdust bedding.     

Differences between two psychogenetically selected lines of rats in a swimming pool matching-to-place task: long-term effects of infantile stimulation

The present study explored the effects of infantile stimulation (i.e., neonatal handling or NH) on the performance of 18-month-old Roman high-avoidance (RHA/Verh) and low-avoidance (RLA/Verh) rats in a swimming pool matching-to-place (SPMP) test. This test (also called repeated acquisition and place learning-set paradigm) consists of administering pairs of consecutive trials in the Morris water maze. The difference between each odd and the consecutive even trial of a trial pair is considered to be a measure of working memory. The same rats were first tested for exploration and novelty-seeking in a hole-board test in the presence of novel objects, which showed that RHA/Verh rats were more explorative than their RLA/Verh counterparts, and that NH treatment augmented exploration in RLA/Verh rats, generally eliminating the genetically-based differences between the lines. RHA/Verh rats performed less efficiently than RLA/Verh rats in the SPMP test, and NH facilitated acquisition in the early stages of training in both rat lines, an effect that was presumably due to an improvement in the acquisition of spatial reference information. Performance during training also indicated that RHA/Verh rats showed less differentiated behavior between odd and even trials, indicating a relative working memory deficit at advanced ages in that rat line.     

Effects of Cocaine and Morphine in Rats from Two Psychogenetically Selected Lines: A Behavioral and Brain Dialysis Study

The Swiss sublines of Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh) rats differ in their reactivity to environmental and pharmacological stressors, in their sensitivity to stereotypies elicited by dopamine (DA)-mimetic agents, and in their densities of D₁ DA receptors in the terminal field of the mesoaccumbens DAergic projection, an important link in the neural networks involved in the motor effects and reinforcing properties of drugs abused by humans. The present study was therefore designed to compare the behavioral and neurochemical effects of cocaine (5 mg/kg, i.p.) and morphine (0.5 mg/kg, s.c.) in RHA/Verh and RLA/Verh rats. To this aim, we measured motor activity and DA output in the nucleus accumbens as determined by brain microdialysis. The number of counts corresponding to horizontal, vertical, and total motor activities accumulated in basal conditions during the 60-min acclimation period was significantly larger in RHA/Verh than in RLA/Verh rats. Moreover, horizontal, vertical, and total motor activities throughout the 120-min observation period that followed the administration of vehicle tended to be larger in RHA/Verh rats, although the difference between the two lines was not statistically significant. In RHA/Verh rats, locomotion, rearing, and total motor activity were significantly more intense after acute cocaine and morphine challenges than after vehicle administration, whereas no significant differences in motor activity were observed between control and cocaine- or morphine-treated RLA/Verh rats. No line-related differences were detected in the basal DA output, but the effect of cocaine on DA release was more robust in RHA/Verh rats. Likewise, the effect of morphine was more pronounced in RHA/Verh than in RLA/Verh rats. Because the mesoaccumbens DAergic pathway plays a central role in the acquisition of motivational valence by environmental and pharmacological stimuli and, therefore, in operant behavior, our results suggest that comparative behavioral and neurochemical studies in these two lines may provide useful information on the biological correlates of drug dependence.     

Biochemical Parameters of Dopaminergic and GABAergic Neurotransmission in the CNS of Roman High-Avoidance and Roman Low-Avoidance Rats

The dopaminergic (DAergic) and GABAergic pathways in the central nervous system (CNS) are involved in the control of emotions, in the reactivity to stressful stimuli, and in the positive and negative reinforcing properties of psychotropic drugs. In the present review, we summarize the differences in a range of neurochemical markers of GABA- and DA-mediated neurotransmission in the CNS of Roman high-avoidance (RHA/Verh) and Roman low-avoidance (RLA/Verh) rats, two psychogenetically selected lines that differ in what may be considered to be level of emotionality. The stimulatory effect of GABA on ³⁶Cl^– uptake was less pronounced in the cerebral cortex of RLA/Verh rats compared to RHA/Verh rats. In addition, the binding affinity of [³⁵S]TBPS, a selective ligand of the convulsant site located in the chloride channel of GABA_A receptors, was significantly lower in the hippocampus of RLA/Verh rats than in their high-avoidance counterparts. On the other hand, the density of D₁ DA receptors labeled with [³H]SCH 23390 was lower in the nucleus accumbens of RLA/Verh rats compared to RHA/Verh rats. Brain microdialysis studies demonstrated that tail-pinch stress and subconvulsant doses of the anxiogenic compound pentylenetetrazol increased the extracellular concentrations of DA in the prefrontal cortex of hypoemotive RHA/Verh rats but not in their hyperemotive RLA/Verh counterparts. These line-dependent differences in GABAergic and DAergic neurotransmission may contribute to the distinct emotionality and responsiveness to centrally active drugs of RHA/Verh and RLA/Verh rats.     

The Acoustic Startle Response in Inbred Roman High- and Low-Avoidance Rats

To investigate the emotional reactions of two rat strains selectively bred for good and poor two-way avoidance acquisition (RHA/Verh and RLA/Verh), male animals of both strains were tested in an acoustic startle response test. They received 40 acoustic stimuli followed by 10 electric foot shocks and another 30 acoustic stimuli. RLA/Verh rats showed a significantly higher startle response compared to RHA/Verh animals, indicating a stronger emotional reaction to acoustic stimuli. In addition, the former showed a stronger response to foot shocks. Combined with earlier findings, we conclude that selection for two-way avoidance learning does not result in cognitive defects in the RLA/Verh strain but, rather, in stronger emotional reactions to fearful stimuli.     

Infantile (handling) stimulation and behavior in young Roman high- and low-avoidance rats.

The effect of infantile handling stimulation on exploratory and emotional behavior of Roman high- and low-avoidance (RHA/Verh and RLA/Verh) weanling rats was investigated. Postnatally handled and nonhandled, 4-week-old males and females from both psychogenetically selected lines were exposed to a hexagonal tunnel maze, including an illuminated central arena. Postnatal handling increased exploratory behavior and decreased emotional reactivity as expressed by increased entries into the central arena and a reduction in defecations in both lines of rats. These effects were more pronounced in the RLA/Verh rats. In agreement with earlier studies using nonselected adult rats, the females of both lines (especially those from the RHA/Verh line) were more sensitive than males to the positive influences of early stimulation.     

Differential hormonal and physiological responses to stress in Roman high- and low-avoidance rats

In the present experiments, plasma concentrations of corticosterone, ACTH, prolactin, blood glucose, serum free fatty acids, core temperature and fecal boli were measured immediately following the exposure of Roman high- and low-avoidance rats (RHA/Verh, RLA/Verh) to various stressful situations. As compared to RLA/Verh rats, the RHA/Verh animals showed an attenuated emotional response in “non-self-menacing” (novel environment) situations. Differences between the 2 selected lines were not generally found in unstressed rats or in “high-stress” (ether, immobilisation, footshock) situations.     

Increased caloric intake after a high-fat preload: relation to circulating triglycerides and orexigenic peptides

To investigate mechanisms that mediate the greater food intake induced by a fat-rich diet, the present study tested an acute "preload-to-test meal" paradigm in normal-weight rats. In this paradigm, the rats were given a small high-fat (HF) compared to low-fat (LF) preload and, after an intermeal interval, allowed to consume freely on a subsequent test meal. Modified versions of this paradigm were tested to determine the robustness of the greater caloric intake induced by the HF preload while standardizing the test protocol. A HF preload of 10-15 kcals, compared to an equicaloric LF preload, significantly increased food intake by 40-50% in the subsequent test meal. This effect, a 4-6 kcal increase, was observed with HF preloads equal in energy density and palatability to the LF preloads. It was evident with preloads or test meals that were liquid or solid, preloads that were injected, test meals that had variable fat content, and natural intermeal intervals of 60-120 min. This overeating after a HF preload was invariably associated with a 2- to 3-fold increase in circulating levels of triglycerides (TG), with no change in leptin or insulin. It was also accompanied by increased expression of the orexigenic peptides, galanin in the paraventricular nucleus and orexin in the perifornical lateral hypothalamus. Moreover, if given repeatedly over several days, the HF compared to equicaloric LF preload significantly increased 24-h food intake. These results establish a protocol for studying the phenomenon of increased feeding on a HF diet under controlled conditions and suggest possible underlying mechanisms involving circulating lipids and orexigenic peptides.     

Adaptation of lipid-induced satiation is not dependent on caloric density in rats

Food intake is modulated by ingestive (gastrointestinal) and post-ingestive signals; ingested fat is potent to produce short-term satiety (satiation) but this can be modified by long-term ingestion of a high fat diet. AIM: Determine whether altered lipid-induced satiation is dependent on the fat content of the diet, rather than increased caloric density or changes in adiposity. METHODS: Initial experiments determined the differences in the microstructure of meal patterns in rats fed a high fat diet (HF: 38% fat kcal) and in rats pair-fed an isocaloric, isonitrogenous low fat diet (LF: 10% fat kcal) and changes in meal patterns measured after long-term maintenance on the HF diet. RESULTS: Rats fed the HF diet had a significant 50% increase in meal frequency compared to rats fed the LF diet; in addition, there was a significant reduction in meal size (32%) and inter meal interval (38%) consistent with induction of satiation. After 8 weeks on the HF diet, these parameters tend to approach those of rats maintained on the LF diet. There was a significant 56% decrease in the activation of neurons in the NTS in response to intragastric gavage of lipid in rats maintained for 8 weeks on the HF compared to LF diet. CONCLUSION: Dietary fat alters meal patterns consistent with induction of a short-term satiety signal. This signal is attenuated with long-term exposure to dietary lipid, in the absence of ingestion of additional calories or changes in body weight. This adaptation of short-term satiety might contribute to diet-induced obesity.     

Effect of food deprivation and maintenance diet composition on fat preference and acceptance in rats

High-fat diets typically elicit greater kcal intake and/or weight gain than low-fat diets. Palatability, caloric density, and the unique postingestive effects of fat have each been shown to contribute to high-fat diet hyperphagia. Because long-term intake reflects the sum of many individual eating episodes (meals), it is important to investigate factors that may modulate fat intake at a meal. The present studies used high-fat (hi-fat) and high-carbohydrate (hi-carb) liquid diets (both 2.3 kcal/mL) to assess the effect of hunger level (0 versus 24-h food deprivation) and fat content of the maintenance diet (12 versus 48%) on fat preference (when a choice among foods is offered in a two-bottle test), and acceptance (only one food offered) in male rats. Preference for hi-fat relative to hi-carb (two-bottle test) was enhanced by 24-h food deprivation, and by a high-fat maintenance diet. In contrast, neither deprivation nor maintenance diet composition influenced relative meal size (one-bottle test) of hi-fat and hi-carb: irrespective of test conditions, meal size of hi-fat was bigger than meal size of hi-carb.     

CCK(1) receptor is essential for normal meal patterning in mice fed high fat diet

Cholecystokinin (CCK), released by lipid in the intestine, initiates satiety by acting at cholecystokinin type 1 receptors (CCK(1)Rs) located on vagal afferent nerve terminals located in the wall of the gastrointestinal tract. In the present study, we determined the role of the CCK(1)R in the short term effects of a high fat diet on daily food intake and meal patterns using mice in which the CCK(1)R gene is deleted. CCK(1)R(-/-) and CCK(1)R(+/+) mice were fed isocaloric high fat (HF) or low fat (LF) diets ad libitum for 18 h each day and meal size, meal frequency, intermeal interval, and meal duration were determined. Daily food intake was unaltered by diet in the CCK(1)R(-/-) compared to CCK(1)R(+/+) mice. However, meal size was larger in the CCK(1)R(-/-) mice compared to CCK(1)R(+/+) mice when fed a HF diet, with a concomitant decrease in meal frequency. Meal duration was increased in mice fed HF diet regardless of phenotype. In addition, CCK(1)R(-/-) mice fed a HF diet had a 75% decrease in the time to 1st meal compared to CCK(1)R(+/+) mice following a 6 h fast. These data suggest that lack of the CCK(1)R results in diminished satiation, causing altered meal patterns including larger, less frequent meals when fed a high fat diet. These results suggest that the CCK(1)R is involved in regulating caloric intake on a meal to meal basis, but that other factors are responsible for regulation of daily food intake.     

Changes in rats' meal patterns as a function of the caloric density of the diet

Rats in a laboratory foraging paradigm were offered each of four diets which differed in caloric density, and intakes, meal frequencies, meal sizes, and eating rates were monitored. The rats maintained a constant daily caloric intake by eating more frequent, larger meals of the lower density foods. However, caloric meal size was not regulated, and significant correlations between meal size and the length of the post-meal interval were rarely found. The 24-hour pattern of calorie intake was the same regardless of diet. Higher-calorie foods were consumed at a faster rate within meals than were lower-calorie foods. The feeding patterns observed suggest that caloric intake may be regulated over a time frame of several meals rather than on a meal-to-meal basis.