Role of phosphodiesterase 10A in striatal medium spiny neuron activity during L-DOPA-induced dyskinesia

Abstract

Parkinson's disease (PD) is a progressive neurodegenerative disorder that affects 1% of people over the age of 60. The primary motor symptoms of PD include resting tremor, slowness of movements, rigidity and postural instability. These symptoms result from the degeneration of the dopaminergic cells in the substantia nigra pars compacta (SNc). L-DOPA remains the gold standard treatment for PD. However, with repeated administration, L-DOPA can cause abnormal involuntary movements (e.g. L-DOPA-induced dyskinesia; LID) in 75-80% of PD patients. There is a clear need for novel preclinical and clinical research to identify an effective antidyskinetic intervention that provides PD patients with a better quality of life during the on-state of L-DOPA treatment. The cellular, synaptic, and circuit mechanisms responsible for LID are unknown, but several key findings suggest that abnormal activity in the input nucleus of the basal ganglia, the striatum, is responsible. The striatum receives massive cortical excitatory inputs and is densely innervated by dopaminergic projections. The glutamatergic and dopaminergic information is integrated within the striatal medium spiny neurons (MSNs) and transmitted to the output nuclei of the basal ganglia, the internal portion of the globus pallidus and the substantia nigra pars reticulata (SNr). MSNs can project directly (dMSNs) or indirectly (iMSNs) to the output nuclei of basal ganglia. dMSNs express preferentially D1 dopaminergic receptors as well as the neuropeptides dynorphin and substance P. The iMSNs express mainly D2 dopaminergic receptors and the neuropeptide enkephalin. Models of PD propose that chronic L-DOPA exposure generates a hyperdopaminergia state that contributes to imbalance in the activity of striatal MSNs, generating hyperactivity of dMSNs and hypoactivity of iMSNs. This imbalance between MSN activity during the on-state of L-DOPA treatment would be responsible for the appearance of LID. Unfortunately, direct electrophysiological evidence is lacking. Using in vivo electrophysiological recordings of spontaneous and cortically evoked activity in the 6-hydroxydopamine(OHDA)-lesioned rat model of PD, we aim to characterize the physiology of dMSNs and iMSNs during the on-state of L-DOPA treatment in dyskinetic rats. Dissecting these two groups of neurons will be the first step in developing improved PD therapeutics. Once this part of the study is completed, we will test how selective targeting of phosphodiesterase 10A (PDE10A) signaling impacts on spontaneous and cortically-evoked activity of MSNs in the dyskinetic striatum. By the end of the project, we expect to demonstrate clear preclinical evidence for the long-term antidyskinetic properties achieved with PDE10A inhibition. The clinical implications of this discovery are expected to advance the treatment options for patients with PD. (AU)