Asymmetrically lesioned mesencephalon in healthy rodents: call for caution

Stereological counting of tyrosine-hydroxylase immunoreactive (TH-IR) neurons in the mesencephalon is a pivotal parameter in assessing the extent of lesioning in animal models of Parkinson's disease. We here show that the number of TH-IR neurons often appears abnormally decreased in healthy-commercially available-mice and rats, although both the number of Nissl-stained cells and the striatal dopaminergic innervation are unaffected. This potential bias in assessing extent of neurotoxin-induced lesion and subsequent protection by pharmacological manipulation prompts us to call for caution in setting up experimental designs. (C) 2004 Elsevier B.V. All rights reserved.