NeuN is not a reliable marker of dopamine neurons in rat substantia nigra

Quantification of neuronal cell number is a key endpoint in the characterization of neurodegenerative disease models and neuroprotective regimens. Immunohistochemistry for phenotypic markers, followed by unbiased stereology is often used to quantify the relevant neuronal population. To control for loss of phenotypic markers in the absence of cell death, or to determine if other types of neurons are lost, a general neuronal marker is often desired. Vertebrate neuron-specific nuclear protein (NeuN) is reportedly expressed in most mammalian neurons. In Parkinson's disease models, NeuN has been widely used to determine if there is actual nigral dopamine neuron loss or simply loss of tyrosine hydroxylase expression, a prominent phenotypic marker. To date, the qualitative value of NeuN expression as such a marker in the substantia nigra has not been assessed. Midbrain tissue sections from control rats were stained for NeuN and tyrosine hydroxylase and assessed by light or confocal microscopy. Here we report that NeuN expression level in the rat substantia nigra was highly variable, with many faintly stained cells that would not be meet stereological scoring criteria. Additionally, dopamine neurons with little or no NeuN expression were readily identified. Subcellular compartmentalization of NeuN expression was also variable, with many cells dorsal and ventral to the nigra exhibiting expression in both the nucleus and cytoplasm. NeuN expression also appeared to be much higher in non-dopamine neurons within the ventral midbrain. This characterization of nigral NeuN expression suggests that it is not useful as a quantitative general neuronal marker in the substantia nigra.