Coping style predicts the (in)sensitivity for developing hyperinsulinemia on a high fat diet in rats

Effects of personality on assessments of anxiety and cognition

Individual variation in responses to commonly used tests of anxiety and spatial memory is often reported. While this variation is frequently considered to be 'noise', evidence suggests that it is, at least partially, related to consistent individual differences in behavioral responses (i.e., personality). The same tests used to assess anxiety are often used to profile personality traits, but personality differences are rarely considered when testing treatment differences in anxiety. Focusing on the rat literature, we describe fundamental principles involved in anxiety and spatial memory tests and we discuss how personality differences and housing conditions can influence behavioral responses in these tests. We propose that an opportunity exists to increase stress resiliency in environmentally sensitive individuals by providing environmental enrichment. We conclude by discussing different approaches to incorporating personality measures into the design and analysis of future studies; given the potential that variation masks research outcomes, we suggest that a strategy which considers the individual and its housing can contribute to improving research reproducibility.

Roman high and low avoidance rats differ in their response to chronic olanzapine treatment at the level of body weight regulation, glucose homeostasis, and cortico-mesolimbic gene expression

Olanzapine, an antipsychotic agent mainly used for treating schizophrenia, is frequently associated with body weight gain and diabetes mellitus. Nonetheless, studies have shown that not every individual is equally susceptible to olanzapine's weight-gaining effect. Therefore, Roman high and low avoidance rat strains were examined on their responsiveness to olanzapine treatment. The Roman high avoidance rat shares many behavioral and physiological characteristics with human schizophrenia, such as increased central dopaminergic sensitivity, whereas the Roman low avoidance rat has been shown to be prone to diet-induced obesity and insulin resistance. The data revealed that only the Roman high avoidance rats are susceptible to olanzapine-induced weight gain and attenuated glucose tolerance. Here it is suggested that the specific olanzapine-induced weight gain in Roman high avoidance rats could be related to augmented dopaminergic sensitivity at baseline through increased expression of prefrontal cortex dopamine receptor D1 mRNA and nucleus accumbens dopamine receptor D2 mRNA expression. Regression analyses revealed that olanzapine-induced weight gain in the Roman high avoidance rat is above all related to increased prolactin levels, whereas changes in glucose homeostasis is best explained by differences in central dopaminergic receptor expressions between strains and treatment. Our data indicates that individual differences in dopaminergic receptor expression in the cortico-mesolimbic system are related to susceptibility to olanzapine-induced weight gain.

Failure to upregulate Agrp and Orexin in response to activity based anorexia in weight loss vulnerable rats characterized by passive stress coping and prenatal stress experience

We hypothesize that anorexia nervosa (AN) poses a physiological stress. Therefore, the way an individual copes with stress may affect AN vulnerability. Since prenatal stress (PNS) exposure alters stress responsivity in offspring this may increase their risk of developing AN. We tested this hypothesis using the activity based anorexia (ABA) rat model in control and PNS rats that were characterized by either proactive or passive stress-coping behavior. We found that PNS passively coping rats ate less and lost more weight during the ABA paradigm. Exposure to ABA resulted in higher baseline corticosterone and lower insulin levels in all groups. However, leptin levels were only decreased in rats with a proactive stress-coping style. Similarly, ghrelin levels were increased only in proactively coping ABA rats. Neuropeptide Y (Npy) expression was increased and proopiomelanocortin (Pomc) expression was decreased in all rats exposed to ABA. In contrast, agouti-related peptide (Agrp) and orexin (Hctr) expression were increased in all but the PNS passively coping ABA rats. Furthermore, DNA methylation of the orexin gene was increased after ABA in proactive coping rats and not in passive coping rats. Overall our study suggests that passive PNS rats have innate impairments in leptin and ghrelin in responses to starvation combined with prenatal stress associated impairments in Agrp and orexin expression in response to starvation. These impairments may underlie decreased food intake and associated heightened body weight loss during ABA in the passively coping PNS rats.

Individual differences in the effects of prenatal stress exposure in rodents

Exposure to prenatal stress alters the phenotype of the offspring in adulthood. When the prenatal and adult environments do not match, these alterations may induce pathology risk. There are, however, large individual differences in the effects of prenatal stress. While some individuals seem vulnerable, others appear to be relatively resistant to its effects. In this review we discuss potential mechanisms underlying these individual differences with a focus on animal models. Differences between rodent models selected for stress coping traits are discussed. In addition, the role of circulating factors, like glucocorticoids and cytokines, factors involved in brain development and influences of epigenetic and genetic factors in prenatal stress induced phenotype are covered.

Prenatal stress and stress coping style interact to predict metabolic risk in male rats

Both prenatal stress (PNS) exposure and a passive stress-coping style have been identified as risk factors for insulin resistance in rats. In the current study, we test the hypothesis that PNS and stress-coping style may interact in predicting susceptibility for metabolic disease. To test this hypothesis, adult male control and PNS offspring were behaviorally characterized using a defensive burying test to have either a passive or proactive stress-coping style. In adulthood, all rats were fed either a standard chow or a high-fat diet for 3 weeks. After 3 weeks of diet exposure, glucose and insulin levels were assessed during an oral glucose tolerance test. Under high-fat diet conditions, PNS rats display elevated glucose and insulin responses to the oral glucose tolerance test, indicative of glucose intolerance. Interestingly, these effects of PNS were far more pronounced in rats characterized by a passive stress-coping style. Additionally, the passively coping PNS rats also gained more weight on the high-fat diet than all other rats tested. This observation suggests that a stressful prenatal environment in combination with a passive stress-coping strategy may prime an individual to be sensitive to diet-induced obesity and type 2 diabetes.

Similarities and differences between "proactive" and "passive" stress-coping rats in responses to sucrose, NaCl, citric acid, and quinine

A stress-coping style describes a set of behavioral and physiological measures that characterize an individual's response to stressful stimuli. It would follow that different stress-coping styles are associated with differential sensitivity for taste stimuli. Animals with stress-coping characteristics better suited to an environment in which new foods are more frequently encountered may show enhanced orosensitivity to cues that signal toxins and/or nutritional value. Here, rats were categorized as "proactive" or "passive" based on results from a defensive burying test. Next, the brief-access taste procedure was used to compare unconditioned licking responses to a concentration array of compounds that humans describe as "sweet" (sucrose), "salty" (NaCl), "sour" (citric acid), and "bitter" (quinine) across the 2 groups. Both groups displayed concentration-dependent lick responses to sucrose, NaCl, citric acid, and quinine. The passive group initiated significantly fewer trials to sucrose than the proactive rats, but the groups did not significantly differ in trial initiation for the other 3 test compounds. Thus, differences in food intake, body weight, and glucose homeostasis between the stress-coping styles are not likely driven by alterations in orosensory responsivity. However, the current findings lend support to the hypothesis that the 2 groups differ in reward-related signaling mechanisms.

Blunted cortisol response to stress is associated with higher body mass index in low-income preschool-aged children

No known studies have tested the hypothesis that a blunted pattern of cortisol reactivity to stress, which is often found following exposure to chronic life stressors, is associated with a higher body mass index (BMI) in very young children. Low-income children (n=218, mean age 56.6 (range: 38.1-78.5; SD 7.0) months, 49.1% male, 56.4% white, 16.1% black, 11.5% Hispanic/Latino) participated in a series of behavioral tasks designed to elicit stress. Cortisol was sampled in saliva 5 times during the protocol, and area under the curve (AUC), representing total cortisol output during stress elicitation, was calculated. Children were weighed and height measured and body mass index (BMI) z-score was calculated. Linear regression was used to evaluate the association between cortisol AUC and BMI z-score, controlling for child age, sex, and race/ethnicity (non-Hispanic white vs. not); primary caregiver weight status (overweight, defined as BMI ≥ 25 vs. not); and family income-to-needs ratio. Mean child BMI z-score was 0.88 (SD=1.03). Mean cortisol AUC was 6.11 μg/dL/min (SD=10.44). In the fully adjusted model, for each 1-standard deviation unit decrease in cortisol AUC, the child's BMI z-score increased by 0.17 (SE 0.07) standard deviation units (p<0.02). A blunted cortisol response to stress, as is often seen following chronic stress exposure, is associated with increased BMI z-score in very young children. Further work is needed to understand how associations between stress, cortisol, and elevated body mass index may develop very early in the lifespan.

Adiposity is associated with blunted cardiovascular, neuroendocrine and cognitive responses to acute mental stress

Obesity and mental stress are potent risk factors for cardiovascular disease but their relationship with each other is unclear. Resilience to stress may differ according to adiposity. Early studies that addressed this are difficult to interpret due to conflicting findings and limited methods. Recent advances in assessment of cardiovascular stress responses and of fat distribution allow accurate assessment of associations between adiposity and stress responsiveness. We measured responses to the Montreal Imaging Stress Task in healthy men (N = 43) and women (N = 45) with a wide range of BMIs. Heart rate (HR) and blood pressure (BP) measures were used with novel magnetic resonance measures of stroke volume (SV), cardiac output (CO), total peripheral resistance (TPR) and arterial compliance to assess cardiovascular responses. Salivary cortisol and the number and speed of answers to mathematics problems in the task were used to assess neuroendocrine and cognitive responses, respectively. Visceral and subcutaneous fat was measured using T₂^*-IDEAL. Greater BMI was associated with generalised blunting of cardiovascular (HR:β = −0.50 bpm.unit⁻¹, P = 0.009; SV:β = −0.33 mL.unit⁻¹, P = 0.01; CO:β = −61 mL.min⁻¹.unit⁻¹, P = 0.002; systolic BP:β = −0.41 mmHg.unit⁻¹, P = 0.01; TPR:β = 0.11 WU.unit⁻¹, P = 0.02), cognitive (correct answers: r = −0.28, P = 0.01; time to answer: r = 0.26, P = 0.02) and endocrine responses (cortisol: r = −0.25, P = 0.04) to stress. These associations were largely determined by visceral adiposity except for those related to cognitive performance, which were determined by both visceral and subcutaneous adiposity. Our findings suggest that adiposity is associated with centrally reduced stress responsiveness. Although this may mitigate some long-term health risks of stress responsiveness, reduced performance under stress may be a more immediate negative consequence.

Forced and voluntary exercise counteract insulin resistance in rats: the role of coping style

There are large individual differences in the success rates of exercise intervention programs aimed at the prevention and treatment of obesity-related disorders. In the present study, we tested the hypothesis that differences in coping style may impact the success rates of these intervention programs. We tested insulin responses before and after voluntary wheel running in both passive (insulin resistant) Roman Low Avoidance (RLA) and proactive (insulin sensitive) Roman High Avoidance (RHA) rats using intravenous glucose tolerance tests (IVGTTs). To control for a potential difference between voluntary and forced exercise, we also included RLA and RHA rats that were subjected to forced running. We found the following: 1) when given the opportunity to run voluntarily in a running wheel, passive RLA rats run more than proactively than RHA rats; 2) voluntary exercise leads to a normalization of insulin responses during an IVGTTs in RLA rats; and 3) there were no behavioral and physiological differences in efficacy between voluntary and forced running. We conclude that exercise, both forced and voluntary, is a successful lifestyle intervention for the treatment of hyperinsulinemia, especially in individuals with a passive coping style.

Personality, a key factor in personalized medicine?

The incidence of health problems resulting from obesity is growing and obesity and its related diseases has become one of the main causes in death in industrialized societies. Environmental influences are crucial for the interactions between genetic, neurohormonal and metabolic factors that may be important in understanding individual differences in the development of obesity and metabolic diseases like type 2 diabetes. In particular the interactions between the personality of an individual and the environment play a key role in predicting the chance for successful treatment. Our experimental data clearly point out that the success of interventions designed to prevent or treat metabolic diseases could be considerably improved by adjusting the intervention to the personality of the individual. Furthermore, certain physiological and neuroendocrine characteristics of a personality are strong indicators for pathology development, both in experimental animals and humans. Future research should focus on the identification of easily measurable physiological and neuroendocrine markers indicative of the coping style or personality in humans.

The biological control of voluntary exercise, spontaneous physical activity and daily energy expenditure in relation to obesity: human and rodent perspectives

Mammals expend energy in many ways, including basic cellular maintenance and repair, digestion, thermoregulation, locomotion, growth and reproduction. These processes can vary tremendously among species and individuals, potentially leading to large variation in daily energy expenditure (DEE). Locomotor energy costs can be substantial for large-bodied species and those with high-activity lifestyles. For humans in industrialized societies, locomotion necessary for daily activities is often relatively low, so it has been presumed that activity energy expenditure and DEE are lower than in our ancestors. Whether this is true and has contributed to a rise in obesity is controversial. In humans, much attention has centered on spontaneous physical activity (SPA) or non-exercise activity thermogenesis (NEAT), the latter sometimes defined so broadly as to include all energy expended due to activity, exclusive of volitional exercise. Given that most people in Western societies engage in little voluntary exercise, increasing NEAT may be an effective way to maintain DEE and combat overweight and obesity. One way to promote NEAT is to decrease the amount of time spent on sedentary behaviours (e.g. watching television). The effects of voluntary exercise on other components of physical activity are highly variable in humans, partly as a function of age, and have rarely been studied in rodents. However, most rodent studies indicate that food consumption increases in the presence of wheels; therefore, other aspects of physical activity are not reduced enough to compensate for the energetic cost of wheel running. Most rodent studies also show negative effects of wheel access on body fat, especially in males. Sedentary behaviours per se have not been studied in rodents in relation to obesity. Several lines of evidence demonstrate the important role of dopamine, in addition to other neural signaling networks (e.g. the endocannabinoid system), in the control of voluntary exercise. A largely separate literature points to a key role for orexins in SPA and NEAT. Brain reward centers are involved in both types of physical activities and eating behaviours, likely leading to complex interactions. Moreover, voluntary exercise and, possibly, eating can be addictive. A growing body of research considers the relationships between personality traits and physical activity, appetite, obesity and other aspects of physical and mental health. Future studies should explore the neurobiology, endocrinology and genetics of physical activity and sedentary behaviour by examining key brain areas, neurotransmitters and hormones involved in motivation, reward and/or the regulation of energy balance.

Individual variation in the (patho)physiology of energy balance

There are large individual differences in the susceptibility for metabolic disorders such as obesity, the metabolic syndrome and type 2 diabetes. Unfortunately, most animal studies in this field ignore the importance of individual variation which limits the face validity of these studies for translation to the human situation. We have performed a series of studies that were particularly focused on the individual differences in the (patho)physiology of energy balance. The studies were performed with passive and proactive individuals of two different rat strains: the Roman High and Low Avoidance rats and the Wild type Groningen rat. The data reveal that passive and proactive individuals differ significantly on several parameters, i.e. body composition, Hypothalamic-Pituitary-Adrenal (HPA) axis activity, plasma levels of insulin and leptin, intestinal transit time, systolic blood pressure and meal patterns. We also found that the selection line of the Roman Low Avoidance rat may be considered as a non-obese animal model for the metabolic syndrome, since these rats display, under sedentary conditions, many of the related symptoms such as hypertension, visceral adiposity and insulin resistance during an intravenous glucose tolerance test. These symptoms disappeared when the animals were allowed to exercise voluntarily in a running wheel. We conclude that experiments with passive and proactive individuals are highly relevant for studying the (patho)physiology and behavior of energy balance and the related metabolic disorders.

Pharmacological treatment of hyperinsulineamia in rats depends on coping style

Passive and proactive coping styles are associated with marked differences in behavioral and neuroendocrine responses. Previous studies revealed that the passive individuals are more prone to hyperinsulinemia. Likewise, we hypothesize that different coping styles may require different drugs to treat this. We tested this by treating passive and proactive rats (Roman Low Avoidance and Roman High Avoidance rats respectively) with either Rosiglitazone or with RU486. After eight days of treatment we performed and intravenous glucose tolerance test (IVGTT) and we compared the insulin and glucose levels with those measured during the IVGTT at baseline. Rosiglitazone improved insulin levels during an IVGTT in both passive and proactive coping styles. RU486, however, lowered insulin levels only in rats with a passive coping style. This study suggests that insight in the neuroendocrine differences between passive and proactive coping styles may provide an extra impulse to improve treatment of insulin resistance, since it allows the application of drugs targeted at the individual.