Impact of PACAP/PAC1 signaling in stress and anxiety regulation

Pituitary Adenylate Cyclase-Activating Polypeptide (PACAP) is a neuropeptide that was first isolated from ovine hypothalamic extracts. It is a member of the vasointestinal polypeptide (VIP)/secretin/glucagon family of neuropeptides and expressed in the brain, where it may act as a neurotransmitter and/or neuromodulator contributing to different functions including behavioral and cognitive processes as well as stress and anxiety mechanisms. However, although neuroanatomical studies demonstrating PACAP and its preferred binding sites the PAC1 receptor in brain areas known to be implicated 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 PACAP/PAC1 receptor system in stress and anxiety responses. In particular, we will investigate whether the exposure to stress and/or anxiety-provoking situations induces enhanced PACAP transmission in brain regions within the stress/anxiety circuitry such as septum, hippocampus, amygdala, or hypothalamus. After the characterization of stress-induced PACAP release patterns in distinct brain regions we will elucidate possible behavioral and physiological implications of endogenous PACAP in stress and anxiety mechanisms. In a further series of experiments, we will investigate whether and how selective activation or inhibition of PACAP neurons modulates stress and anxiety responses to find out how PACAP function can be pharmacologically manipulated to therapeutic advantage. Finally, we will determine whether genetically determined behavioral differences in a psychopathological animal model of increased anxiety and depression would be reflected by changes in PACAP/PAC1 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 PACAP affects stress and anxiety responses. This knowledge is of considerable value for the recent proposed role of the PACAP/PAC1 receptor system as a therapeutic target in the treatment of stress-related psychiatric disorders such as depression and anxiety disorders and should help to develop more effective strategies in the fight against such psychopathologies.

The neuropeptide PACAP (Pituitary Adenylate Cyclase Activating Polypeptide) is a peptide from the secretin-glucagon family, which was first extracted in 1989 from ovine hypothalamic tissue. This peptide exists in two forms (PACAP-27 and PACAP-38) and has an important function as a neurotransmitter and/or neuromodulator especially in neural tissues such as the brain. Although neuroanatomical studies have shown that both PACAP and its preferred receptor, the PAC1 receptor, are localized in brain areas that are crucial for the regulation of stress and anxiety reactions, direct functional evidence for such a role is rare. Therefore, the main goal of the present project was to assess the role of the PACAP/PAC1 receptor system in stress and anxiety responses. Notably, we could show that the exposure to emotional stressors lead to an increased expression of PACAP and its receptors (including PAC1, VPAC1 and VPAC2) in distinct forebrain areas, such as the hypothalamic nucleus paraventricularis (PVN), lateral septum (LS), bed nucleus of stria terminalis (BNST) and amygdala. Moreover, we could demonstrate that the intracerebral administration of PACAP in selected brain areas, such as PVN and LS, induced changes in the neuroendocrine and behavioral stress response. Our immunohistochemical studies, where we investigated the localization of the PAC1 receptors in stress-related areas, revealed that in the PVN these receptors are expressed on stress-activated CRF neurons. These results indicate that the stress regulatory properties of PACAP in the PVN are mediated directly by modulating stress-sensitive CRF neurons. In additional experiments, we investigated whether a selective activation or inhibition of PAC1 receptors in the LS might change the emotional and motivational behavior of rats. Accordingly, rats were administered PACAP locally into the LS and then tested in established behavioral tests to evaluate anxiety-related behavior and the motivational deficits, which are associated with symptoms of anhedonia. We observed that PACAP-treated animals show less spontaneous grooming behavior than control animals, which suggests a reduced motivational and self-care behavior. Moreover, PACAP-treated animals show increased anxiety-related behavior. Thus, the results of the present project contribute to a better understanding of how PACAP modulates stress and anxiety responses. These studies further identified distinct brain areas that seem critical for a functional significant involvement of PACAP transmission in stress-induced dysfunctions. This information is of considerable value for the recently proposed role of PACAP as a potential therapeutic target in the treatment of stress-related neuropsychiatric disorders such as depression and anxiety disorders.