Parkinson's disease and Alzheimer's disease may represent two parts of a spectrum of disease characterised by a primary loss of cells of the isodendritic core. Secondary cell loss from the striatum and cerebral cortex therefore occurs as a consequence of the loss of ascending projections from the isodendritic cells. The anatomy of this system should provide a unique opportunity for therapeutic intervention. Neurotransmitter replacement treatment may be provided either by enhancing transmitter

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... lease by any remaining neurones or by direct agonists. The wide dispersal of the isodendritic projection systems affected in Parkinson's and Alzheimer's diseases and the possibility that they are tonically active create an opportunity for neurotransmitter replacement treatment. Animal studies should be able to show whether such treatment can delay secondary cell loss, and, together with human postmortem studies, whether the hypothesis that the primary lesion is a loss of isodendritic cells is correct.