Department of Neurology, University of Michigan, Ann Arbor.
Previous studies have revealed a loss of enkephalin immunoreactivity in the terminals of striatal neurons projecting to the external globus pallidus in patients with early grades of Huntington's disease (HD). To assess the status of the perikarya of striatal enkephalinergic neurons, we performed in situ hybridization histochemistry with a radiolabeled RNA probe complementary to preproenkephalin messenger RNA. We studied postmortem brain tissue of 6 patients with symptomatic HD, 7 control subjects, and 2 presymptomatic carriers of the HD allele. There was a significant reduction in the areal density of striatal neurons expressing preproenkephalin messenger RNA in the patients with symptomatic HD, but the level of labeling in the remaining cells was not altered compared with the control subjects. In the specimens from presymptomatic individuals, there was no reduction of areal density of preproenkephalin messenger RNA-containing neurons in the striatum, despite the fact that loss of enkephalin immunoreactivity in the external globus pallidus had been previously demonstrated in the same brains. The results correlate with the previous demonstration of depleted enkephalin immunoreactive terminals in the external globus pallidus in patients with symptomatic HD. They also suggest that the early loss of enkephalin immunoreactivity observed in the external globus pallidus of presymptomatic carriers of the HD allele is not related to a generalized death of striatal enkephalinergic neurons early in the course of the disease.