Prodynorphin: Relevance for HPA-axis activation and behavior

Since over two decades dynorphin, member of the opioid peptide family preferentially binding to kappa receptors was suggested to play a regulatory role in numerous functional pathways of the brain. In line with its localization in the mid- and forebrain these functions resemble learning and memory, emotional control and stress response, but also body weight regulation. Pathophysiological mechanisms with a suggested role of dynorphin/kappa receptors include addiction, depression, schizophrenia, epilepsy and bipolar disorders to name just some. Most of these functions were proposed from pharmacological and electrophysiological experiments using kappa receptor specific or general opioid receptor agonists and antagonists in animal models. However, so far we have little information on the functional relevance of endogenous prodynorphin derived peptides. This is mainly due to the complexity of the opioid system. Besides its actions on all three classical opioid receptors delta, mu and kappa, dynorphin was also shown to exert non-opioid effects mainly through direct effects on NMDA receptors. Thus, systemic or local drug applications hardly address the specific functions of endogenous prodynorphin derived peptides. Now we have the tools to investigate these open questions in prodynorphin-deficient mice. The main aim of this project is to investigate the neuroanatomical and -biochemical background of the functions of endogenous prodynorphin-derived peptides in the control of stress response, anxiety and depression like behaviour in prodynorphin knockout mice. These three systems have the hypothalamic-pituitary-adrenal axis as final output line in common. Thus, we will focus on brain areas and neurotransmitter systems known to be involved in the control of this axis. As stress and emotional control are often different in the two genders, we propose to investigate female and male mice on different genetic backgrounds. We propose a multidisciplinary approach combining state of the art neurochemical and neuropharmacological methods and behavioural testing to investigate alterations in the control of the hypothalamic-pituitary-adrenal axis of prodynorphin deficient mice. To overcome compensatory effects of germ-line knockout animals, we also propose to generate conditional knockout mice, which inducible overexpress prodynorphin. Through the use of two strategies, Tet-off/doxycycline and Cre- recombinase/loxP, this mouse strain will be suitable to simulate withdrawal after overexpression as well as local deletion of the prodynorphin gene. In addition, we want to investigate recently described anticonvulsant and neuroprotective properties of endogenous prodynorphin in more detail.

Within this project we investigated the functional implications of the endogenous opioid peptides derived from the prodynorphin gene on emotional control. We were able to demonstrate that dynorphins, the endogenous ligands of kappa opioid receptors (KOPr), induce anxiogenic effects. This becomes especially evident in situations of increased stress. We also were able to suggest the regulation of corticotropin releasing hormone (a hormone known to be involved in stress and anxiety) in the central nucleus of the amygdala (a brain region highly involved in emotional control) as a potential underlying mechanism. However, consolidation of this assumption needs further investigations. In a second line of experiments we provided evidence that a recently described oestrogen receptor GPER1 may be responsible for aversive effects induced by oestrogen. GPER1 is a representative of a class of receptors, which was until recently not considered important for steroid hormones and accordingly only little functional data are available. Our findings are interesting, because a group of women respond negative on hormone replacement therapy during menopause. It was hypothesized that this may depend on an imbalance of oestrogen receptors. However, so far it was not known, which receptor(s) would mediate aversive effects. We now provided evidence, that GPER1 might be at least in part responsible for the aversive effects induced by oestrogen. Off note is the fact that the aversive effects of GPER1 activation apparently depend on dynorphin. From a therapeutic point of view the data obtained within this project suggest that KOPr might represent a potential drug target in mood disorders. Thus, our data contribute to the idea of using KOPr antagonists as antidepressant drugs. Moreover, our findings suggest that the development of specific oestrogen receptor beta agonists might be not only important to reduce the risk of breast cancer during hormone replacement therapy, but might also reduce aversive side effects of such treatments.