DESCRIPTION: (Verbatim from the Applicant's Abstract) Attempts to develop new and effective treatments for movement disorders such as Parkinson's disease have been hampered by an insufficient knowledge of how basal ganglia receptor systems adapt to the consequences of dopamine depletion. This research focuses on determining the role of upregulated serotonin 2A receptors, which we hypothesize provide a mechanism for serotonin to exert greater control over basal ganglia transmission and locomotor function under conditions of dopamine depletion. Our preliminary studies indicate that the target of the serotonin 2A receptor mechanism is the DIRECT striatonigral pathway which utilizes tachykinin neuropeptides colocalized with GABA. New experiments of this application will test the central hypothesis that: upregulated serotonin 2A receptor signaling provides a mechanism for serotonin to enhance striatonigral transmission under conditions of dopamine depletion which influences basal ganglia function and animal behavior. In Specific Aim 1, we will determine the functional consequences of an upregulated serotonin 2A receptor system on serotonin signal transduction within the dopamine depleted striatum by measuring serotonin 2A receptor binding, its linkage to phosphoinositol hydrolysis, its modulation of striatal membrane excitability, and its ability to trans-synaptically regulate striatal tachykinin and GABA expression. In Specific Aim 2, we will determine if tachykinin striatonigral neurons react to the stimulation of upregulated serotonin 2A receptors in the dopamine depleted animal by increasing tachykinin and GABA transmission in the substantia nigra. We will also study the impact of this regulation on locomotor behavior. Finally, in Specific Aim 3, we will determine how an upregulated serotonin 2A receptor system influences the ability of the striatonigral system to regulate basal ganglia dopamine and GABA metabolism, and how these systems influence behavioral recovery of the dopamine depleted animal. Information obtained from these studies will contribute to a better understanding of basal ganglia function and may change how serotonin pathways are considered when designing new pharmacological strategies for diseases which affect dopamine transmission.