This is interesting. Too many dopaminergic cells in the olfactory bulb cause loss of smell....


Movement Disorders

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Volume 19, Issue 6 , Pages 687 - 692

Published Online: 8 Jan 2004

Brief Report
A 100% increase of dopaminergic cells in the olfactory bulb may explain hyposmia in Parkinson's disease
Evelien Huisman, BSc 1 *, Harry B.M. Uylings, PhD 1 2, Piet V. Hoogland, MD, PhD 1 3

Abstract
Hyposmia is one of the most prevalent symptoms of Parkinson's disease. It may occur even before the motor symptoms start. To determine whether the olfactory dysfunctions, like the motor symptoms, are associated with a loss of dopamine, the number of dopaminergic cells in the olfactory bulb of Parkinson's disease patients was studied using tyrosine hydroxylase immunohistochemistry.

The quantitative analysis reveals that the total number of tyrosine hydroxylase-immunoreactive neurons in the olfactory bulb is twice as high in Parkinson patients compared to age and gender-matched controls. Because dopamine is known to inhibit olfactory transmission in the olfactory glomeruli, we suggest that the increase of dopaminergic neurons in the olfactory bulb is responsible for the hyposmia in Parkinson patients.

The increase of dopamine in the olfactory bulb explains why olfaction does not improve with levodopa therapy.

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