Amygdaloid signature of peripheral immune activation by bacterial lipopolysaccharide or staphylococcal enterotoxin B.

Activated immune cells produce soluble mediators that not only coordinate local and systemic immune responses but also act on the brain to initiate behavioral, neuroendocrine and metabolic adaptations. Earlier studies have shown that the amygdala, a group of nuclei located in the medial temporal lobe, is engaged in the central processing of afferent signals from the peripheral immune system. Here, we compared amygdaloid responses to lipopolysaccharide (LPS) and staphylococcal enterotoxin B (SEB), two prototypic bacterial products that elicit distinct immune responses. Intraperitoneal administration of LPS (0.1 mg/kg) or SEB (1 mg/kg) in adult rats induced substantial increases in amygdaloid neuronal activity as measured by intracerebral electroencephalography and c-fos gene expression. Amygdaloid neuronal activation was accompanied by an increase in anxiety-related behavior in the elevated plus-maze test. However, only treatment with LPS, but not SEB, enhanced amygdaloid IL-1ß and TNF-α mRNA expression. This supports the view of the immune system as a sensory organ that recognizes invading pathogens and rapidly relays this information to the brain, independent of the nature of the immune response induced. The observation that neuronal and behavioral responses to peripheral immune challenges are not necessarily accompanied by increased brain cytokine expression suggests that cytokines are not the only factors driving sickness-related responses in the CNS.

Repeated recall of learned immunosuppression: evidence from rats and men.

Akin to other physiological responses, the immune system can be modified, via Pavlovian or behavioral conditioning. It is unknown, however, whether and to what extent learned immune responses can be repeatedly recalled over time. Here we demonstrate in both rats and humans that repeated contingent pairing of a novel taste (conditioned stimulus, CS) together with the immunosuppressive drug cyclosporine A as unconditioned stimulus (US) leads to the acquisition of a learned immunosuppression. Sole presentation of the CS caused a significant inhibition of interleukin (IL)-2 and interferon (IFN)-γ production by rat splenic T cells and human peripheral T lymphocytes, closely mimicking the effect of the drug. More importantly, a comparable suppression of cytokine production was also observed after a second, unreinforced exposure to the CS that was separated from the first evocation by an interval of 6 (rats) or 11 (humans)days, respectively. Together, our findings demonstrate that a learned immunosuppression can be repeatedly recalled in both animals and humans, which is an important prerequisite for the implementation of conditioning paradigms as supportive therapy.

Acute amygdaloid response to systemic inflammation.

The amygdala, a group of nuclei located in the medial temporal lobe, is a key limbic structure involved in mood regulation, associative learning, and modulation of cognitive functions. Functional neuroanatomical studies suggest that this brain region plays also an important role in the central integration of afferent signals from the peripheral immune system. In the present study, intracerebral electroencephalography and microdialysis were employed to investigate the electrophysiological and neurochemical consequences of systemic immune activation in the amygdala of freely moving rats. Intraperitoneal administration of bacterial lipopolysaccharide (100 µg/kg) induced with a latency of about 2 h a significant increase in amygdaloid neuronal activity and a substantial rise in extracellular noradrenaline levels. Activated neurons in the amygdaloid complex, identified by c-Fos immunohistochemistry, were mainly located in the central nucleus and, to a lesser extent, in the basolateral nucleus of the amygdala. Gene expression analysis in micropunches of the amygdala revealed that endotoxin administration induced a strong time-dependent increase in IL-1ß, IL-6, and TNF-α mRNA levels indicating that these cytokines are de novo synthesized in the amygdala in response to peripheral immune activation. The changes in amygdaloid activity were timely related to an increase in anxiety-like behavior and decreased locomotor activity and exploration in the open-field. Taken together, these data give novel insights into different features of the acute amygdaloid response during experimental inflammation and provides further evidence that the amygdala integrates immune-derived information to coordinate behavioral and autonomic responses.

Electrical activity in rat cortico-limbic structures after single or repeated administration of lipopolysaccharide or staphylococcal enterotoxin B.

Immune-to-brain communication is essential for an individual to aptly respond to challenging internal and external environments. However, the specificity by which the central nervous system detects or 'senses' peripheral immune challenges is still poorly understood. In contrast to post-mortem c-Fos mapping, we recorded neural activity in vivo in two specific cortico-limbic regions relevant for processing visceral inputs and associating it with other sensory signalling, the amygdala (Am) and the insular cortex (IC). Adult rats were implanted with deep-brain monopolar electrodes and electrical activity was monitored unilaterally before and after administration of two different immunogens, the T-cell-independent antigen lipopolysaccharide (LPS) or the T-cell-dependent antigen staphylococcal enterotoxin B (SEB). In addition, the neural activity of the same individuals was analysed after single as well as repeated antigen administration, the latter inducing attenuation of the immune response. Body temperature and circulating cytokine levels confirmed the biological activity of the antigens and the success of immunization and desensitization protocols. More importantly, the present data demonstrate that neural activity of the Am and IC is not only specific for the type of immune challenge (LPS versus SEB) but seems to be also sensitive to the different immune state (naive versus desensitization). This indicates that the forebrain expresses specific patterns of electrical activity related to the type of peripheral immune activation as well as to the intensity of the stimulation, substantiating associative learning paradigms employing antigens as unconditioned stimuli. Overall, our data support the view of an intensive immune-to-brain communication, which may have evolved to achieve the complex energetic balance necessary for mounting effective immunity and improved individual adaptability by cognitive functions.

Family matters: maternal and litter-size effects on immune parameters in young laboratory rats.

A functional immune system is important for the survival of mammalian young, particularly at weaning when they lose the immunological support provided by the mother's colostrum and milk. In altricial mammals, litter size and maternal characteristics are important components of an animal's early environment, which affect postnatal growth and development. In a study of unculled litters of Long-Evans laboratory rats (Rattus norvegicus), we asked whether such parameters are also associated with the immune status of the young shortly before weaning. On postnatal day 17, we assessed numbers of several leukocyte and lymphocyte subsets, the activity of the complement system, and immunoglobulin G (IgG) concentrations in the serum. Averaging the values of all pups per litter, we found negative correlations between litter size and lymphocyte counts, complement system activity and IgG concentration. Maternal effects were seen in the positive correlation between maternal postpartum body mass and granulocyte and monocyte counts. In addition, lymphocyte and monocyte counts as well as complement activity were lower for the young of multiparous than of primiparous mothers. This suggests a trend towards a better developed immune system in such offspring, which may be relevant for their immediate and long-term survival. The effects described here have potential implications for the design and interpretation of biomedical studies of immune parameters in laboratory rats.

Litter size is negatively correlated with corticosterone levels in weanling and juvenile laboratory rats.

An animal's environment during early life can strongly affect its physiological development. For example, litter size, i.e. the number of litter siblings, has been previously shown to strongly affect early growth in many small mammal species including laboratory rats. In the present study we tested whether natural, unmanipulated litter size is also associated with differences in stress hormone levels in young Long-Evans laboratory rats. We found a negative correlation between serum corticosterone (CORT) concentrations and litter size during two different stages of juvenile life. On postnatal day 17, shortly before weaning, this relationship was apparent with respect to basal CORT values. On day 33, however, two weeks after weaning, we found this relationship only when animals were challenged by a 10-min test on an elevated plus maze, but not in control animals (basal values). Although the physiological basis of these differences is not clear, we discuss two main, not mutually exclusive possibilities: (a) delayed maturation of the HPA axis in typically lower body mass pups of large litters, and (b) that such pups, encountering greater competition for maternal resources, adjust to this presumably more stressful developmental environment by down-regulating responsiveness of the HPA axis. In conclusion, our study provides evidence that a naturally varying feature of the developmental environment of many altricial mammals - number of littermates - may contribute to individual differences in stress-related physiology. Furthermore, it suggests the need to consider litter-size effects when investigating differences in animals' stress responses.

Development of behavior in the litter huddle in rat pups: within- and between-litter differences.

Early postnatal growth in mammals can be considerably influenced by litter size and often differs among littermates in relation to birth mass. In a study of Long Evans laboratory rats we asked whether within- and between-litter differences in body mass and growth are related to behavioral development during early postnatal life. For this, we analyzed the amount of general motor activity and the display of directed, seemingly goal-oriented interactions within the litter huddle in previsual pups. During the study period from postnatal days 2 to 11, we found significant changes in pup behavior, showing a nonlinear, quadratic shape. General motor activity and, more specifically, the display of behaviors apparently directed to reaching central positions in the litter huddle increased during the first postnatal days and then decreased again. However, pups from small litters that grow more rapidly than pups from large litters, showed a faster increase in both behaviors, whereas the young from large litters reached a higher maximum. We also found striking within-litter differences in the amount of directed behavior performed by light and heavy pups, with higher levels in the former group, most probably because light pups that have a less favorable body mass-to-volume ratio and more often occupy peripheral positions in the litter huddle, make a greater effort to reach thermally favorable central positions. In conclusion, our study shows there to be consistent between-litter as well as within-litter differences in behavioral patterns during early life. These differences might have important implications for an individual's long-term behavioral and physiological performance.

Separating maternal and litter-size effects on early postnatal growth in two species of altricial small mammals.

Growth during an animal's early ontogeny can have important consequences for its health, reproduction and survival during later life. We studied pre-weaning growth rates of two species of small altricial mammals, and assessed the impact and compared the importance of two main aspects of the animals' early environment: maternal characteristics and the number of litter siblings (i.e. litter size). Study animals were wild-type European rabbits living in a field enclosure and laboratory rats kept under standardized housing conditions. Growth of rabbit pups was best explained (to 47%) by the combined effects of litter size, maternal age and whether females had given birth to another litter shortly before. Similarly, growth in rat pups was best explained (to 75%) by the combined effects of litter size, maternal body mass and whether mothers had previously given birth. In both species, litter size correlated negatively with pup growth. In rabbits, growth was greatest in litters of middle-aged females. In rats, pup growth correlated positively with maternal mass. Pups of both species showed higher growth rates when the mother had not given birth to a previous litter. Despite major differences in maternal behaviour and study conditions between the two species, the findings point to a similar ranking in the importance of the different parameters tested: Litter size, i.e. sibling number, was the most important factor, followed by maternal mass or age, and then by mothers' history of previous reproductive activity. We therefore think it likely that these findings will be valid for other mammalian species giving birth to large litters of altricial young.