Dopamine signal transduction in striatal neurons in Parkinson’s disease

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Project Summary Parkinson’s disease (PD) is characterized by motor abnormalities primarily caused by loss of midbrain dopamine (DA) cells, whose principal function is to modulate the striatal neuron activity. DA replacement with L-Dopa produces significant improvement of motor deficits, but over time the overall efficacy of L-Dopa declines. Together disease progression and chronic dopaminergic treatment induces maladaptive plasticity in striatal projection neurons (SPNs), which results in altered responses to DA inputs. DA signaling in SPNs is transduced by various molecular cascades following DA receptor activation. The first step in these cascades is the regulation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) syntheses. These cyclic nucleotides activate their dependent protein kinases PKA and PKG, which are key mediators in multiple cell functions. Thus, DA modulation of SPNs is largely transduced by the cAMP-PKA and cGMP-PKG pathways. However, the roles of these enzyme pathways for DA responses after neurons have undergone significant maladaptive plasticity changes are not known. Furthermore, there may be different functional outcomes from activation of these enzyme pathways in distinct SPN subpopulations. Based on our pharmacological studies targeting nucleotide levels that suggest a key role of the cGMP-PKG pathway, we propose to explore the impact of manipulating PKG activity selectively in SPN subpopulations. This study of DA signal transduction in SPN subpopulations is aimed at identifying key cellular mechanisms that can be targeted to improve the efficacy of dopaminergic therapies. This project has two specific aims consisting of behavioral and physiological analyses in rodent models of PD including chronic stages. In the first aim, we will manipulate the activity of PKG in selective neuronal subtypes. For this purpose, we will use transgenic rats expressing a Cre system, and then inject viral vectors to overexpress PKG in direct or indirect SPNs. A battery of behavioral tests will be used to assess the role of the PKG pathway in the motor impairment of parkinsonian models as well as its reversal by dopaminergic stimulation. In the second aim, we will analyze the neuronal activity in response to overexpression of PKG, which will allow us to establish physiological correlates of behavioral results. For these recordings, we will also use transgenic rats and apply optogenetics to identify direct and indirect SPNs. Our approach includes the use of novel animal models and technologies to address key pathophysiologic mechanisms of DA signal transduction in PD. The main objectives of these exploratory studies are (I) to reveal particular roles of the cGMP-PKG pathway depending on the cellular subtype, and (II) to generate foundation data for continuation of this important research program. The long-term goal of the program is developing new strategies to alleviate the motor impairment of patients during the long course of PD.