Loss of dopamine receptors in the olfactory bulb of patients with Alzheimer's disease.

The presence and localization of dopamine D2 receptors was studied by means of in vitro autoradiography with [3H]N-n-propylnorapomorphine in olfactory bulbs obtained postmortem from Alzheimer patients and age-matched controls. It appeared that, in 5 age-matched controls, the greatest density of dopamine D2 receptors was found in the glomerular layer of the bulb. In 6 of 7 Alzheimer patients, the labeling of the glomerular layer was decreased so that glomerular and granular layer did not differ in labeling density. Tangles, as revealed by Thioflavin S staining, were present in all different layers of the bulbus in Alzheimer patients. Observation of Nissl-stained preparations revealed that mitral cell bodies in bulbs of these patients were not present. Since mitral cells are projection neurons with targets in the entorhinal and piriform cortex, the observation of loss of these cells supports the hypothesis of early involvement of the olfactory system in Alzheimer's disease and the spread from the olfactory mucosa and bulb to the cerebral cortex and hippocampus via degeneration of interconnecting neurons. Moreover, in vivo detection of bulbar dopamine receptors might in the future provide a diagnostic tool for the early detection of senile dementia of the Alzheimer type.