Models and Pharmacological targets in Post-traumatic Stress Disorder and Parkinson’s Disease

Post-traumatic stress disorder (PTSD) is a chronic and multifactorial psychiatric disorder that occurs after a traumatic event and is associated with neural circuitry and structural changes. The imbalance in the hypothalamic-pituitary-adrenal (HPA) axis and the noradrenergic (NA) system induces dysregulation in many neurotransmitters of the central nervous system (CNS). Dysfunctions in neuronal circuits and neurotransmitters lead to behavioural changes, memory impairments, enhanced fear retention and impairment in fear extinction, which typically occur in PTSD. In addition, PTSD is considered a risk factor for the development of other psychiatric and neurological disorders, particularly Parkinson´s disease (PD). Serotonin reuptake inhibitors (SSRIs) are the most effective treatment, although only between 20 and 30% of patients with PTSD experience complete remission. There is therefore a pressing need to identify new pharmacological targets that can induce complete remission. The first part of the thesis aimed to examine changes in mGluR5 signalling in the hippocampus and prefrontal cortex in relation to spatial learning and memory extinction. After behavioural tests, mice were systemically treated with LiCl, followed by the mGluR5-PAM VU0360172, and endogenous InsP levels were determined by ELISA. Spatial learning and memory extinction are associated with increased mGluR5 signalling not only in the dorsal hippocampus but also in the ventral hippocampus and prefrontal cortex, two regions involved in the emotional component of learning. PD is the second most common chronic neurodegenerative disorder after Alzheimer’s disease (AD). PD is characterised by a progressive neurodegeneration of dopamine (DA)-producing neurons in the substantia nigra pars compacta (SNpc). The depletion of DA in the striatal causes the typical motor symptoms of PD. The main neuropathological hallmark of PD is the abnormal accumulation and aggregation of the protein alpha-synuclein (α-Syn) in the form of Lewy bodies and Lewy neurites. In turn, α-Syn aggregates are thought to disrupt cellular homeostasis by triggering oxidative and proteostatic stress, also activating neuroinflammation. To date, L-DOPA is the most effective treatment for the symptoms of PD, although it leads to debilitating long-term complications (L-DOPA-induced dyskinesia). There is an urgent need for therapeutic approaches that can slow the progression of PD. The second part of the thesis focuses on the evaluation of a potential disease-modifying treatment stimulating the Sigma-1 receptor (Sig-1R), which is an intracellular chaperone protein involved in cell repair and neuroinflammatory processes. In the first project, mice sustained a lesion of nigrostriatal DA neurons using the toxin 6-OHDA and were treated daily with fluvoxamine, an SSRI also acting as a Sig-1R agonist. Fluvoxamine treatment was found to bring about a gradual and marked motor recovery, accompanied by an increased number of DA neurons and a decreased number of CD68-positive microglial cells in the substantia nigra. In the second project, the nigrostriatal DA pathway was damaged by intrastriatal inoculation of preformed fibrils of mouse α-Syn (mPFFs). Although this procedure did not result in significant behavioral deficits, mice treated daily with fluvoxamine showed an increased density of dopaminergic fibers in striatal regions most affected by the mPFF. Another major aim of this thesis was to develop and characterise a mouse model of human α-Syn-driven pathology by combining two different approaches, that is, adeno-associated viral vectors (hAAV) overexpressing human alpha-synuclein and human preformed fibrils (hPFF). Mice were injected in the nigrostriatal pathway with hAAV alone, hPFF alone, or the combination hAAV and hPFF. The combined injection of hAAV in substantia nigra and hPFF in striatum was the most effective lesion approach, leading to significant motor impairment at 12 weeks. We conclude that combination of hAAV and hPFF injection can induce functionally significant changes in nigrostriatal DA transmission. In summary, the first part of the thesis reveals that spatial learning and memory extinction are associated with a change in mGluR5 signalling. The second part shows that pharmacological stimulation of Sig-1R promotes recovery after a nigrostriatal dopaminergic lesion. Finally, results from the third study show that hAAV and hPFF injections act synergistically to induce degenerative changes in the nigrostriatal pathway mimicking those found in early-stage PD-like pathology.