Early life events and peptic ulcer susceptibility: an experimental model

On the nature and consequences of early loss

OBJECTIVE

To describe how an animal model system can be used to explore basic questions about the nature of loss and the effects of early loss on later vulnerability to disease.

METHOD

The physiological and behavioral responses of infant rats to separation from their mothers are first described and then analyzed experimentally into component mechanisms.

RESULTS

These studies have revealed an extensive layer of processes underlying the psychological constructs generally used to understand the response to loss. Hidden within the observable interactions of parent and offspring, we found a number of discrete sensorimotor, thermal, and nutrient-based events that have unexpected long-term regulatory effects on specific components of infant physiology and behavior. Release from all of these inhibitory and excitatory regulators together during maternal separation constitutes a novel mechanism by which the experience of loss can be translated into a complex patterned response. Evidence for early regulatory processes has also been found in monkey and human mother-infant interactions. Here they may well constitute the building blocks from which attachment and object representations develop. We and others have found long-term effects of loss, and of selective replacement of regulators, on behavioral development and on later vulnerability to disease.

CONCLUSIONS

The results give us a new understanding of early attachment as a developmental force and of human grief as a risk to health.

Early relationships as regulators of infant physiology and behavior

In recent years, animal research has revealed a network of simple behavioral and biological processes that underlie the psychological constructs we use to define early social relationships. Hidden within the observable interactions of parent and offspring are sensorimotor, thermal and nutrient-based events which have unexpected and widespread regulatory effects on infant behavior and physiology. The complex pattern of responses resulting from early separation in infant rats can be traced to the abrupt withdrawal of a number of discrete, independent regulatory processes which had been acting on individual components of the infant's physiology and behavior. These regulatory processes also appear to mediate long-term shaping effects exerted by early relationships, for example, on the vulnerability of the adult rat to hypertension and stress ulcer. In human development, early regulatory interactions may provide a bridge between biological and psychological processes in the development of our earliest mental representations.

The neurobiology of stress ulcers

We have reviewed the neurobiology of stress ulcers from animal models to potential pharmacotherapeutic mechanisms. The evidence strongly supports the hypothesis that certain stress-related gastric lesions are 'brain-driven' events which may be more effectively managed through central manipulations than by altering local, gastric factors. Recent advances in the use of anxiolytic and antidepressant drugs in the management of stress-related gastric mucosal injury further supports the contention that a brain-gut axis, which may have nervous, peptidergic and classic monoaminergic components, modulates the intricate and complicated pattern of communication between the brain and the stomach. Delineation of the precise pathways which make up this communication as well as their manipulation by various pharmacological agents will be the focus of future research endeavour.

Experience with younger siblings facilitates maternal responsiveness in pubertal Norway rats

The latency of maternal behavior (MB) onset induced by cohabitation with pups (sensitization) increases in female and male Long-Evans rats from 24 days of age (1-2 day latency) to 42 days of age (4-5 day latency). This decline in maternal responsiveness at Day 42 did not occur in 50% of rats ("responders") which lived with their dam and younger siblings (conceived during the postpartum estrus) for either 4 or 11-18 days after their birth. In contrast to cessation of suckling at about Day 25 postpartum in the absence of a 2nd litter, when a 2nd litter was born greater than Day 26 or less than Day 26 postpartum, suckling continued for a mean of 32 and 35.5 days, respectively (and up to Day 40). Also, time spent suckling during the 10 days following the birth of the 2nd litter was more than five times as great if the 2nd litter was born with little or no delay (less than Day 26). The offspring from the 1st litter spent about 80% of the diurnal observation time in proximity to the dam and younger siblings, and instances of nurturance (retrieving, crouching, licking) toward the young littermates were observed. However, the only dam-litter interaction significantly related to subsequent maternal responsiveness by 1st litter offspring was a positive relation between the prior incidence of aggression by mothers towards juveniles and sensitization latency.

Early weaning effects on voluntary ethanol consumption and stress responsivity in rats

The present study examined the effect of early maternal deprivation (early weaning) on voluntary ethanol consumption and responses to cold-immobilization stress in adult rats. Rats were weaned at 16 and 21 days of age and housed individually with food and water ad lib until they reached 190 +/- 5 g at which time half of the animals from each group were exposed to increasing concentrations (3 to 9%) of ethanol in a free choice with water every alternate day. Following acquisition of ethanol drinking, animals were provided with water and ethanol (9%) daily for eight days. At the end of this period, animals were divided into stressed and non-stressed groups. Stressed animals were exposed to cold immobilization stress for 3 hr. Results showed that the early-weaned animals consumed significantly more ethanol as compared to normal-weaned animals. Stomach pathology data revealed little ulcer formation in water-only groups. However, normal-weaned/ethanol-exposed animals exhibited significantly more severe ulcers when compared to all other water- or ethanol-exposed groups. We suggest that early maternal deprivation may predispose animals to increased ethanol consumption as adults. Stress ulcer susceptibility in these animals was likely influenced by interactions among the effects of early weaning, ethanol intake and handling and needs further clarification.

Effects of early experience on responsiveness to ethanol: a preliminary report

The present study examined the hypothesis that early experiences which alter an organism's response to novel or aversive stimuli and hence its ability to cope with stress might also alter sensitivity to and preference for alcohol. On day 1 of pregnancy, females were assigned to 1 of 5 groups: Handled (H), Nonhandled (NH), Malnourished (MAL), Prenatally stressed (PRE) or Control (CON). Offspring were tested in adulthood to measure their adrenocortical response to stress (ether plus cardiac puncture), adrenocortical response to IP ethanol injection and alcohol preference. Data indicate that early experience did alter responsiveness to stress in adulthood; early handling decreased later stress responsiveness and prenatal stress increased later stress responsiveness. In addition, although early experiences did not alter the corticosterone response to IP ethanol at 60 min post injection, both preweaning handling and perinatal malnutrition reduced alcohol preference in adulthood.

The ulcerogenic effect of a rest period after exposure to water-restraint stress in rats

Shock-induced gastric ulceration in rats is exacerbated by allowing animals to rest between the period of stress and sacrifice. Conflicting data have been reported for similar post-stress rest effects using restraint stress. In Experiment 1, rats restrained in water for 2 h and then allowed a 2-h rest period exhibited significantly greater gastric ulceration than rats sacrificed immediately after the 2-h restraint-in-water stress period. The strength of this phenomenon is confirmed by Experiment 2, in which rats challenged with restraint in water for 1.25 h and then allowed a 1.25-h rest period before sacrifice exhibited greater ulceration than animals maintained in the restraint-in-water stress for the full 2 1/2 h. Corticosterone levels for these two treatment groups were similar at the time of sacrifice. Post-stressor rest therefore actively contributes to the ulcerogenic processes initiated by this form of stress challenge.

Development of maternal behaviors in prepubertal rats at three ages: age-characteristic patterns of responses

Individual 18-, 24-, or 30-day-old male and female Wistar prepubertal rats (juveniles) were continuously exposed to 3-8-day old pups in order to determine developmental differences in the expression of maternal and other behaviors during the process of sensitization. The results that emerged from this study are as follows: Maternal behaviors are neither consistent nor integrated in their expression prepubertally; exposure to pups facilitates the emergence of some maternal behaviors in juveniles more easily than others, e.g., retrieving and anogenital licking more than ventral crouching or nestbuilding; there are age-related differences in the expression of different maternal behaviors, e.g., 24-day-olds retrieve more than at other ages; in our strain, few, if any, gender differences are apparent in the expression of maternal behaviors prepubertally; juvenile play behaviors (charge, pounce) are related to retrieving behavior in the context of exposure to pups. Overall, the results suggest that maternal behaviors are distinct developmentally and probably involve different mechanisms in prepubertal animals.

Some behavioral factors related to the effects of cold-restraint stress in rats: a factor analytic-multiple regression approach

Recently, some individual differences in rats have been shown to be related to stress-induced physiological responses. As yet there has been no attempt to incorporate measurement techniques from the psychometric field to this line of research. The present study was conducted to examine the utility of applying such methods to animal research. Physiological responses to cold-restraint stress in 26 male rats were investigated using a factor analytic-multiple regression procedure. Nineteen behavioral and physiological measures obtained during open-field testing, motor activity monitoring, and passive avoidance learning were subjected to a principle components factor analysis. Five factors were extracted which reflected exploratory behavior, general activity level, metabolic rate, behavioral reactivity, and autonomic reactivity. The obtained factor scores were used to predict physiological responses to four hours of supine cold-restraint using a step-wise multiple regression procedure. General activity level was the best predictor of adrenal weight and temperature loss. Autonomic reactivity was the best predictor of ulcer incidence and severity. Applications of these statistical procedures are discussed.

Early weaning predisposes rats to exacerbated activity-stress ulcer formation

Rats were weaned early (Day 15), normally (Day 21) or late (Day 27). When they reached 200 g (+/- 10) they were placed into the activity-stress paradigm. Half the rats in each weaning group were housed in activity-wheel cages and fed only 1 hr each day. The remaining rats served as food-yoked home cage controls and were fed only that amount of food eaten by their wheel-housed partners. Early weaned rats ran more than normally or late weaned animals. Early weaned rats also died at a faster rate and exhibited a significantly greater cumulative ulcer length than did normally or late weaned rats, although ulcer incidence did not differ between the groups. No ulcers occurred in home cage control rats. Early maternal deprivation may generally predispose rats to stress-induced gastrointestinal disease, whether or not such disease has an acid etiology.

The telencephalic limbic system and experimental gastric pathology: a review

The effects of lesions and stimulations of the telencephalic limbic system on experimental gastric ulcers and erosions are reviewed. It is concluded that the centromedial amygdala and the anterior cingulate gyrus are facilitatory structures, whereas the medial prefrontal cortex, posterior cingulate cortex, entorhinal cortex, hippocampus and posterolateral amygdala are inhibitory areas during stressful experiences, e.g., immobilization. Both the centromedial amygdala and the anterior cingulate gyrus may be part of an "ancillary" pain system, mediating the affective components of aversive experiences. The inhibitory structures, on the other hand, are assumed to be part of a "preventive" mechanism which is initiated by the selective nuclear binding of glucocorticoids when under stress.