Parkinson's disease (PD) is the second most common neurodegenerative disorder after Alzheimer's disease and affects about 1% of the population aged 65 and older. It is often thought of as a condition that selectively targets the dopamine (DA) neuronal system for destruction, giving rise to the well-known motor deficits of tremor, bradykinesia, rigidity and postural instability. The first-line pharmacological treatment for PD is L-DOPA, which is intended to replenish brain DA levels and thereby provide significant relief from these movement problems. It is not widely appreciated but the serotonin (5HT) and norepinephrine (NE) neuronal systems are also severely degraded in PD. While it is appropriate that so much attention is focused on the motor symptomatology of PD, increased interest in non-motor manifestations of PD is called for in light of the fact that approximately 80% of PD patients suffer from co-morbid neuropsychiatric conditions such as sleep disorders, anxiety and dementia. The most prevalent affective disorder is depression. The non-motor symptoms (NMS) of PD, whether related to the disease process or induced by L-DOPA, are not trivial and contribute to worsened disability, impaired quality of life and shortened life expectancy. In fact, it has been determined that NMS of PD have a greater impact on health-related quality of life than motor symptoms. It is also clear that affective disorders in PD are not simply a consequence of psychological distress due to the development of a chronic debilitating disease. Many of the NMS of PD can be rationally linked to reductions in function of the 5HT and NE neuronal systems and this is reinforced by the strategy usually followed when treating the NMS of PD- use of drugs that increase the synaptic levels of these monoamines or that activate their receptors. These treatments (i.e. blockers of the 5HT and NE transporters, receptor agonists) have not been that effective and in some cases, they even oppose the therapeutic efficacy of L-DOPA. The rationale for studies in this application starts with the recognition that 5HT and NE deficits, in the face of extensive losses of DA neurons, likely contribute to the NMS of PD. The appearance of NMS cannot be accounted for by the singular loss of DA neurons in PD. The proposed work in this application will take advantage of the availability in our laboratory of an innovative mouse model that lacks the gene for tryptophan hydroxylase 2 (TPH2). This new model was created using a Cre-ERT2-Lox recombination approach to induce the loss of TPH2 and 5HT after tamoxifen treatment of adult mice. Mice with 5HT deficits will be treated with 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP), a neurotoxin that targets dopamine neurons for destruction, and/or with N-(2- chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a highly selective toxin that targets NE neurons. The emergence of NMS will be assessed using a battery of behavioral, neurological, neurochemical and cognitive tests. Thereafter, mice lacking the monoamines will be treated with L-DOPA, L-DOPS (Droxidopa) and/or 5- HTP to replenish brain levels of DA, NE and 5HT, respectively. It is hypothesized that the NMS of PD emerge when DA neurons are lost on a background of 5HT and NE depletion. It is further hypothesized that 5-HTP and L-DOPS will effectively relieve the NMS of PD without diminishing the therapeutic efficacy of L-DOPA in treating the motor deficits. 5-HTP and L-DOPS are amino acid precursors to 5HT and NE, respectively, and have been used to treat humans in numerous clinical trials. Therefore, their use to reduce the NMS of PD represents a safe, simple and translational approach.
The research in this project will examine the cell and molecular mechanisms that underlie the non-motor symptoms of Parkinson's disease. This project recognizes that the Parkinson's brain has undergone pathological changes that result in lowered function in serotonin and norepinephrine neurons, in addition to losses of dopamine neurons. L-DOPA provides relief from the motor symptoms of Parkinson's via its ability to increase brain dopamine. However, effective treatments for the non-motor symptoms do not exist presently. Work in this application will use a novel knockout mouse model that lacks brain serotonin to study the emergence of non-motor symptoms after the loss of dopamine and norepinephrine neurons. 5-HTP and L- DOPS will be used to restore brain serotonin and norepinephrine levels, respectively, and to provide relief from the serious non-motor symptoms that are frequently associated with Parkinson's disease. This project therefore has high relevance to the VA health care mission and to RRD research objectives.
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