Propidium iodide (PI) stains Nissl bodies and may serve as a quick marker for total neuronal cell count.

Propidium iodide (PI) reacts with both DNA and RNA and is a commonly used fluorescent reagent for nucleic acid staining. The aim of the study was to compare the cellular staining patterns of PI with that of Nissl staining in rat nervous tissues and to report a modified staining method that selectively labels Nissl bodies in neurons. Cryosections and paraffin sections of different tissues of normal Sprague-Dawley rats, including trigeminal ganglia, dorsal root ganglia, spinal cord, liver, and small intestine, were stained by either PI or the hematoxylin and eosin method. Some sections were treated with RNase or DNase before the above staining, and some were double stained with PI and a Nissl stain. The sections were observed by light, fluorescence or confocal microscopy. Results showed strong PI signals detected as patterns of granules in the neuronal cytoplasm of all nervous tissues, whereas the staining of neuronal nuclei was weaker. In contrast, nuclei of neuroglial cells were strongly stained by PI, while the cytoplasm was not obviously stained. Pretreatment of the neural tissue with RNase abolished the PI signals. Furthermore, the PI positive granules in neuronal cytoplasm co-localized with Nissl bodies stained by the fluorescent Nissl stain. When the tissue was pretreated with DNase, PI only stained the cytoplasmic granules of neurons, but not that of glial cells. Our results show that PI stains Nissl bodies and may serve as an economical and convenient neuron marker for neuronal cell counting when specific neural markers such as antibodies are not readily available.