Search

Oxytocin (OXT) has been implicated in the regulation of social behaviors, including intermale offensive aggression. Recently, we showed that acute enhancement of brain OXT levels markedly suppressed offensive aggression and increased social exploration in resident rats confronted with an intruder in their home territory. Moreover, a different responsivity to the exogenous OXTergic manipulation was observed among individuals based on their baseline aggression. In this study we aimed at evaluating the behavioral response to chronically enhancing or attenuating central OXT levels, and at scrutinizing whether the trait-aggression moderates the treatment-induced behavioral changes. To this end, resident male wild-type Groningen rats were continuously (via osmotic minipumps) intracerebroventricularly infused with synthetic OXT or a selective OXT receptor (OXTR) antagonist for 7days. Changes in behavior were assessed performing a resident-intruder test before and at the end of the treatment period, as well as after 7days of withdrawal. Chronic infusion of OXT was found to selectively suppress aggression and enhance social exploration. Chronic blockage of OXTRs instead increased introductory aggressive behavior (i.e. lateral threat), yet without affecting the total duration of the aggression. The magnitude of the anti-aggressive changes correlated positively with the level of baseline aggression. Interestingly, OXT-induced behavioral changes persisted 7days after cessation of the treatment. In conclusion, these findings provide further evidence that enhanced functional activity of the central OXTergic system decreases social offensive aggression while it increases social explorative behavior. The data also indicate that chronically enhancing brain OXT levels may cause enduring anti-aggressive and pro-social explorative behavioral effects., (Copyright © 2014 Elsevier Inc. All rights reserved.)

The mere presence of elevated plasma levels of corticosterone is generally regarded as evidence of compromised well-being. However, environmental stimuli do not necessarily need to be of a noxious or adverse nature to elicit activation of the stress response systems. In the present study, the physiological and neuroendocrine responses to repeated social stimuli that can be regarded as emotional opposites, i.e. social defeat and sexual behavior, were compared. Similar corticosterone responses were observed in animals confronted for the first time with either a highly aggressive male intruder or a receptive female, but a decrease was noticed in defeated rats tested during a third interaction. Only if animals are being physically attacked does the corticosterone response remain similar to the one observed during sexual behavior. In addition, the number of activated cells in the parvocellular hypothalamic paraventricular nucleus, as visualized by c-Fos immunocytochemistry, shows no difference between rats 1h after the third exposure to defeat or sex. Finally, biotelemetric recordings of heart rate, body temperature and locomotor activity show a robust response to both social stimuli that is generally, however, higher in animals being confronted with a receptive female. The data clearly indicate that acute plasma corticosterone levels are not reflecting the emotional valence of a salient stimulus. The magnitude of the response seems to be a direct reflection of the behavioral activity and hence of the metabolic requirements of activated tissues. Next to its direct metabolic role, acute increases in plasma corticosterone will have neurobiological and behavioral effects that largely depend on the neural circuitry that is activated by the stimulus that triggered its release., (Copyright © 2011 Elsevier Inc. All rights reserved.)

Individual variation in behavior and physiology is a widespread and ecologically functional phenomenon in nature in virtually all vertebrate species. Due to domestication of laboratory animals, studies may suffer from a strong selection bias. This paper summarizes behavioral, neuroendocrine and neurobiological studies using the natural individual variation in rats and mice. Individual behavioral characteristics appear to be consistent over time and across situations. The individual variation has at least two dimensions in which the quality of the response to a challenging condition (coping style) is independent from the quantity of that response (stress reactivity). The neurobiology reveals important differences in the homeostatic control of the serotonergic neuron and the neuropeptides vasopressin and oxytocin in relation to coping style. It is argued that a careful exploitation of the broad natural and biologically functional individual variation in behavior and physiology may help in developing better animal models for understanding individual disease vulnerability., (Copyright 2010 Elsevier Inc. All rights reserved.)

In humans and other primates low cerebrospinal fluid (CSF) levels of the major serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA) have been correlated to high aggressiveness. This finding forms the basis of the 5-HT deficiency hypothesis of aggression. Surprisingly, this correlation has not been confirmed in rodents so far, while manipulation studies aimed to investigate the link between 5-HT and aggressive behaviour are mostly carried out in rodents. In this study the relation between aggression and CSF monoamine and metabolite concentrations was investigated in male Wildtype Groningen rats. In sharp contrast to the hypothesis and our expectation, a clear positive correlation was found between the individual level of trait-like aggressiveness and CSF concentrations of 5-HT, 5-HIAA, norepinephrine (NE), dopamine (DA), and 3,4-dihydroxyphenylacetic acid (DOPAC). Shortly after the acute display of aggressive behaviour (as a state-like phenomenon), decreased 5-HT levels and an increase in 5-HIAA/5-HT ratio and NE concentrations were found. Surprisingly, pharmacological challenges known to influence 5-HT transmission and aggressive behaviour did not affect CSF 5-HT and 5-HIAA concentrations, only the NE level was increased. Lesioning 5-HT terminals by 5,7-dihydroxytryptamine (5,7-DHT) administration caused a decrease in CSF 5-HT and 5-HIAA, but without affecting aggressive behaviour. The observed positive correlation between CSF 5-HIAA and trait aggressiveness makes it questionable whether a direct extrapolation of neurobiological mechanisms of aggression between species is justified. Interpretation of CSF metabolite levels in terms of activity of neural substrates requires a far more detailed knowledge of the dynamics and kinetics of a neurotransmitter after its release.