The PhD research described in this thesis has focused on the role of OXT in offensive aggression in male rats. Offensive aggression is initiated by the offender and is characterized by a series of preliminary behaviors such as threatening displays prior to overt aggression, such as biting. Aggression was both qualitatively and quantitatively measured using a territorial aggression test. In this test, a territorial male rat (resident) in its own cage is confronted with an unfamiliar male conspecific (intruder). This leads to a rich repertoire of offensive behaviors undertaken by the resident towards the intruder, in which the intensity and the duration of the offense may depend on the individual. To overcome the extremely low aggression levels typically expressed by the most commonly used laboratory animals, it has been used a strain of rodents originated from wild ancestors, so-called wild-type Groningen rat. In this strain of rats, the duration of the offensive aggression
displayed by adult male individuals can vary from 0 % to 80 % of the total observation time. Besides a larger population mean in baseline aggression, this strain has a much richer repertoire of social behaviors and a wider within-population individual variability as compared to the highly domesticated laboratory rats. These phenotypic properties allow studying the link between the individual variation in aggression and the variation in the endogenous OXTergic system. This is of particular importance for drawing conclusions about the possible serenic profile of substances that affect the OXTergic system.
The content of this thesis is divided in six chapters. A brief introduction (chapter 1) presents the core aspects related to the link between OXT and aggression, and discusses the main findings of this research in the context of the current literature. The following five chapters describe experiments focused on the research questions mentioned below:
1. Does acute administration of synthetic OXT in the brain affect offensive aggression in male rats? What is the dose-dependency, the behavior- and receptor-specificity, and the variation between individuals?
2. What is the effect of chronic OXT treatment on aggression and does it lead to long-lasting changes in aggressiveness?
3. Which brain regions are involved in the OXTergic modulation of offensive aggression?
4. Is the individual variation in aggression associated with individual differences in endogenous structural/functional properties of the brain OXTergic system?
5. Does intranasal administration of OXT have similar effects on aggression as intra-cerebral administered OXT, and if so via which mechanism?
The first set of experiments (chapter 2) has shown that acute administration of OXT into the cerebral ventricles leads to a dose-dependent decrease of intermale aggression, without delaying the onset of the first attack. In addition, OXT increases social exploratory behavior of the resident towards the intruder. These behavioral effects were prevented when receptor binding of the synthetic OXT was blocked by the co-administration of a selective OXT receptor antagonist. The OXT-induced decrease of aggression was greater in animals that exhibited high basal level of aggression. Pharmacological blockade of OXT receptors has led to an increase in aggression in animals with a low basal level of aggression. These first results lead to the conclusion that elevation of brain OXT levels has serenic properties on intermale offensive aggression.
The consistent results seen after acute OXTergic manipulation prompted to test the behavioral effects of a chronic elevation (with a synthetic OXT) or attenuation (with an OXT receptor antagonist) of the central OXTergic activity (chapter 3). Chronic manipulation of the central OXT levels was performed by implanting subcutaneously osmotic mini-pumps, which were connected to a cannula in the lateral ventricle of the brain. This methodology allowed a continuous and constant cerebroventricular drug infusion for seven days. The behavioral tests confirmed the robust anti-aggressive and pro-social efficacy of sustained enhancement of central OXT levels. A remarkable finding was that these behavioral
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changes persisted for at least seven days after OXT treatment was stopped. This indicates a more lasting or permanent change in aggressive behavior induced by chronic OXT treatment. Although central infusion is a method not directly applicable to humans, the evidence of enduring effects and no rebound phenomena invites to further investigate potential long-lasting behavioral changes in a clinical therapeutic situation as well.
The unequivocal behavioral evidence for serenic effects of both acute and chronic OXT administration gave rise to the question of where exactly in the brain OXT exerts its effects on aggression (chapter 4). The experiments focused on local injections of OXT into the central nucleus of the amygdala within the limbic system, and the dorsal raphe in the brainstem. Both brain areas are densely populated with OXT receptors and the literature suggests that these brain regions play an important role in the regulation of behavioral and emotional responsivity to social stimuli. Administration of OXT in the dorsal raphe did not reveal clear behavioral effects. This is in contrast to the central amygdala where OXT micro-injection induced potent anti-aggressive and pro-social changes. These results are in support of clinical studies showing that intranasal administration of OXT reduces behavioral aversion to negative or threatening social stimuli through the dampening of amygdala reactivity.
In chapter 5, it was tested the hypothesis that the individual variation in basal aggression and the responsivity to OXT treatment are related to individual differences in the functional and/or structural properties of the endogenous OXTergic system. This hypothesis was based not only on our previous pharmacological results, but also on the idea that violent behavior in humans has been associated with low brain OXT levels and/or signaling activity. To better model human aggression and violence, the resident rats underwent repeated fighting experiences. Under these conditions, some animals develop excessive levels and abnormal/
pathological forms of aggressive behavior. Interestingly, the animals that exhibited this pathological aggression were characterized with a low level of OXT mRNA transcription in hypothalamic regions. In addition, these animals had a higher OXT receptor binding in the limbic system (central amygdala and bed nucleus of the stria terminalis). These findings are relevant for understanding the causal link between the functioning of the OXTergic system in the brain and the development of pathological forms of social behavior.
The growing attention for the behavioral effects induced in humans by nasal OXT administration, and the lack of understanding of the underlying mechanisms were the basis for the experiments described in chapter 6. Intranasal OXT administration in male resident rats had a strong anti-aggressive and pro-social action during the confrontation with the unknown male intruder. In addition, it promoted pair bonding in a test in which the male resident was given to choose between its own female partner and a novel female rat. Two experiments were focused on investigating the possible mechanisms by which intranasal OXT delivery could affect social behavior. By employing radio-biotelemetry, heart rate and blood pressure responses were measured after intranasal OXT application. The lack of autonomic physiological changes excluded the possibility that enhanced peripheral circulation of OXT, possibly occurring after intranasal OXT delivery and absorption into the
nasal blood vessels, is provoking peripheral effects that in turn may affect behavior. In a second experiment, it was tested whether intranasal OXT could activate the endogenous OXTergic system. Although the underlying mechanism still needs to be elucidated, intranasal OXT administration was indeed shown to increase the activity of OXT neurons in hypothalamic regions. This study is one of the few pre-clinical studies that aimed at a mechanistic explanation for the putative direct OXT nose-to-brain transport.