Neurobiological Mechanisms Involved in the Establishment and Maintenance of Dominance Hierarchies and its Modulation by Stress in Rats

Stress can have a strong and long lasting effect on the establishment and maintenance of dominance hierarchies in rats. When two rats of a pair have not been stressed (non-stressed pair) before a first social encounter, they establish a dominance hierarchy, but the rank that is obtained during this first encounter is not maintained until a second encounter one week later. When one rat of the pair is stressed (stressed pair) before the first encounter, the stressed rat becomes subordinate during the first encounter and the hierarchy that is formed is long lasting and stable. In this thesis, the neurobiological mechanisms underlying the effects of stress on dominance hierarchies were studied. In the first study, we investigated the role of increasing corticosterone levels before or just after a first encounter between two rats of a pair in the establishment and the long-term maintenance of a dominance hierarchy. We show that pre-social encounter corticosterone treatment does not affect the outcome of the hierarchy during a first encounter, but induces a long-term memory for the hierarchy when the corticosterone-injected rat becomes dominant during the encounter, but not when it becomes subordinate. Post-social encounter corticosterone leads to a long-term maintenance of the hierarchy only when the subordinate rat of the pair is injected with corticosterone. This corticosterone effect implicates glucocorticoids in the consolidation of the memory for a recently established hierarchy. Next, we investigated the immediate and long-term changes in mRNA levels of receptors for oxytocin and vasopressin in medial amygdala and lateral septum. In subordinate rats of the stressed pairs, we found a downregulation in oxytocin receptor mRNA in medial amygdala at three hours after the first encounter, and a downregulation in vasopressin 1a receptor mRNA in lateral septum one week after the first encounter. We show that administration of an oxytocin antagonist in the medial amygdala of the subordinate rat immediately after a first encounter induces a long-term memory for the established hierarchy. Administration of a vasopressin 1a receptor antagonist in the lateral septum of the subordinate rat just before a second encounter mimics the effects of exposure to acute stress on the long-term establishment of a dominance hierarchy. These results suggest a role for oxytocin and vasopressin in the effects of stress on the long-term establishment of dominance hierarchies. Finally, we investigated the long-term effects of acute stress and hierarchy formation on the establishment of a second hierarchy with an unfamiliar opponent. We show that the behavior of stressed subordinate rats towards an unfamiliar opponent depends on the previous status of the opponent: when paired with a previously dominant rat they become subordinate, whereas they become dominant when paired with a previously subordinate rat. Furthermore, we show that monoamine oxidase A and androgen receptor mRNA levels are downregulated in the lateral septum of stressed subordinate rats at one week after the first encounter; i.e., at the time when we have observed their flexible social behavior against unfamiliar opponents. The monoamine oxidase A inhibitor clorgyline was injected into the lateral septum to examine whether there was a causal relationship between changes in monoamine oxidase A mRNA levels and offensive behavior towards an unfamiliar subordinate opponent. However, our results did not support this hypothesis. Taken together, we have shown that exposure to acute stress and a social encounter leads to changes in the mRNA levels of receptors for oxytocin, vasopressin and androgens and of monoamine oxidase A. These changes might underlie the effect of stress on the formation of a stable and long-lasting dominance hierarchy. Corticosterone plays a role in the long-term maintenance of the recently established hierarchy and hence it is an ideal candidate to mediate some of the observed neurobiological changes.