Characterisation of DARPP-32 within the Human Basal Ganglia in Huntington’s and Parkinson’s Disease

Abstract

The basal ganglia are a set of subcortical nuclei that regulate motor, emotional and cognitive function. In the basal ganglia, the striatum is interconnected with the main output structures, the globus pallidus (GP) and the substantia nigra (SN). Dysfunction of the basal ganglia circuitry is associated with the neurodegenerative diseases, including Huntington’s disease (HD) and Parkinson’s disease (PD). In HD, a significant number of cells, termed medium spiny neurons (MSNs) are lost in the striatum and this causes the mood and motor changes in the individual. Parkinson’s disease pathology consists of the loss of dopamine in the striatum. Dopamine-and cAMP-regulated neuronal phosphoprotein of 32kDa (DARPP-32) is an MSN marker in the basal ganglia. Unpublished research in our laboratory has found strong DARPP-32 staining in neurochemical compartments called striosomes of the normal human striatum. This study aims to determine the outputs of the DARPP-32 neurons in humans by studying the GP and SN, and whether DARPP-32 expression is changed in HD and PD in the SN. To determine differences in DARPP-32 expression within the human SN between HD and PD, immunohistochemistry was performed on human SN tissue sections and digitally quantified to determine the differences in DARPP-32 expression in HD and PD individuals. This study found a significant decrease of DARPP-32 immunoreactivity in the HD SN by 66% when compared to controls. This finding links HD neuropathology and the loss of DARPP-32 immunoreactivity in HD, which may help explain the dysfunction of neuronal circuits in HD. There was also a trending increase in DARPP-32 immunoreactivity in the human PD SN. This finding extends on previous studies which have found that increased DARPP-32 activation is linked to Levodopa-induced dyskinesia (LID), and this study indicates a possible link of increased DARPP-32 in PD to LID in humans. To further characterise DARPP-32 in the control human GP, double-label immunofluorescence was performed to determine the expression and localisation of well-known basal ganglia neurochemicals, enkephalin and substance P (SP) with DARPP-32. DARPP-32 is observed to possess low co-labelling with SP in the internal GP (GPi), and low co-labelling with enkephalin in the external GP (GPe), implying the existence of separate neuronal populations of enkephalin and SP and DARPP-32. Thus, the discovery of the DARPP-32 striatopallidal pathway in humans has important implications for our understanding of the basal ganglia circuitry in health and disease.