Role of Relaxin-3 in stress and anxiety mechanismus

Relaxin-3 (RLX3), a newly identified member of the relaxin peptide family that is primarily expressed in the brain, has been shown to be involved in various functions including behavioural and cognitive processes as well as stress and anxiety mechanisms. However, although neuroanatomical studies demonstrating RLX3 and its preferred receptor the relaxin-family-peptide 3 (RXFP3) receptor in brain areas known to be involved in the regulation of stress and anxiety responses, direct functional evidence for such a role is rare. Therefore, the main goal of the present project is to assess the role of the RLX3/RXFP3 system in stress and anxiety responses. In particular, we will investigate whether the exposure to stress and/or anxiety-provoking situations induces enhanced RLX3 transmission in brain regions within the stress/anxiety circuitry such as septum, hippocampus, amygdala, or hypothalamus. After the characterization of stress-induced RLX3 release patterns in distinct brain regions we will elucidate possible behavioural and physiological implications of endogenous RLX3 in stress and anxiety mechanisms. In a further series of experiments, we will investigate whether and how selective activation or inhibition of RLX3 neurons modulates stress and anxiety responses to find out how RLX3 function can be pharmacologically manipulated to therapeutic advantage. Finally, we will determine whether genetically determined behavioural differences in a psychopathological animal model of increased anxiety and depression would be reflected by changes in RLX3 transmission and whether these changes can be reversed by treatment with existing anxiolytic and antidepressant drugs. The results of this project are expected to contribute to a better understanding of the neuronal mechanisms how RLX3 affects stress and anxiety responses. This knowledge is of considerable value for the recent proposed role of RLX3 as a therapeutic target in the treatment of stress-related disorders such as depression and anxiety disorders and should help to develop more effective strategies in the fight against such psychopathologies.

Relaxin-3 (RLX3) is a highly conserved neuropeptide of the relaxin peptide family that has been shown to be involved in various brain functions. Notably, within the brain this neuropeptide has been identified in a tight cluster of neurons in a region known as the nucleus incertus (NI). Moreover, previous studies have shown that the majority of these neurons are activated by stress or intracerebral corticotropin-releasing-factor (CRF) injections as most of these neurons co-express CRF type1 receptors. Neuroanatomical studies investigating NI projections identified RLX3 immunoreactive fibers and terminals as well as its preferred receptor the relaxin-family-peptide 3 (RXFP3) receptor in brain areas known to be involved in the regulation of stress and anxiety responses such as the hypothalamic paraventricular nucleus (PVN) and lateral septum (LS). However, beside this neuroanatomical evidence direct functional evidence for a modulatory role of RLX3 in neuroendocrine stress function such as the regulation of the hypothalamic-pituitary- adrenal (HPA) axis is rare. Therefore, primary aim of the present project was to define a potential RLX3-sensitive stress circuit in the forebrain. By using a pharmacological approach we examined whether intracerebroventricular RLX3 administration modulates stress-induced neuronal activity in key brain areas of the stress circuitry, including the PVN and LS. We could show that intracerebroventricular administration of RLX3 attenuated stress-induced neural activation in the PVN indicating that RXFP3 receptor activation within the PVN attenuates neuroendocrine stress responses. Hence, an inhibitory role of endogenous RLX3 within the PVN on the HPA axis stress responses is suggested, which is consistent with our findings from the microinjection studies demonstrating a reduction of stress-induced ACTH and corticosterone secretion after central injections of RLX3 into the PVN. We further observed that RLX3 bilaterally administered into the LS increased active coping and reduced immobility during the forced swim stress exposure identifying the LS as a critical brain area for a functional significant involvement of RLX3 in the modulation of adequate behavioral stress coping during aversive stress exposure. The results of our project imply that the RLX3/RXFP3 receptor system in the LS and PVN is critically involved in the regulation of neuroendocrine and behavioral stress function. Thus, the results contribute to a better understanding of the neuronal mechanisms how RLX3 affects stress response which is of considerable value for the recent proposed role of RLX3 as a therapeutic target in the treatment of stress-related neuropsychiatric disorders.