Abstract:
The striatum is composed of large numbers of medium spiny projection neurons (MSNs) and the majority of these are calbindin positive across many species, including humans. Recent studies investigating the loss of MSNs in the Huntington’s disease (HD) striatum have used a different MSN marker, DARPP-32. Several rat studies using DARPP-32 as a MSN marker, report that it identifies over 95% of MSNs. In this human study, a direct comparison of the numbers of DARPP-32 and calbindin cells using immunohistochemical techniques revealed that in the normal striatum calbindin positive MSNs (75%) exceeded DARPP-32 positive MSNs (25%). Further double immunofluorescence investigations showed that the great majority of DARPP-32 cells were localised in the striosomes and were not double labelled with calbindin, whereas some scattered DARPP-32 positive cells were localised in the matrix. Some of these DARPP-32 positive cells in the matrix were double labelled with calbindin. In the normal human striatum DARPP-32 specifically recognises a subpopulation of MSNs (around 10%), mainly in the striosomes and a second population of MSNs which are DARPP-32 and calbindin positive (approximately 60%). No previous study has reported this heterogeneity of DARPP-32 MSN distribution in the mature human striatum. HD is a genetic neurodegenerative disorder caused by an expanded CAG repeat in the huntingtin gene, resulting in a major loss of MSNs. The results of the present study which compare the loss of calbindin positive MSNs versus the loss of DARPP-32 positive MSNs in HD showed that in HD striatum the calbindin positive MSN population was much more susceptible to neurodegeneration than the DARPP-32 positive MSN population. This resulted in higher proportions of DARPP-32 positive neurons with increasing neuropathological grade. The DARPP-32 positive MSNs in the striosomes (which did not contain calbindin) and ventral striatum seemed especially more resistant to neurodegeneration. Further analysis of double immunofluorescent striatal sections showed that the proportions of DARPP-32 only (13-28%), calbindin only (5-12%), and double-labelled DARPP-32/calbindin positive MSNs (82-60%) in the HD striatum which varied according to HD grade (1-3). These results comprise a new neurochemical compartmentalisation of MSNs in the human striatum which requires a re-evaluation of the neuronal functional organization of the striatum in health and disease.