NeuN Expression in Spinal Neurons Projecting to the Cerebellum

We studied the distinctive features of NeuN immunolabeling in the neurons located in the four cat spinal cord structures sending projections to the cerebellum: Clarke's nucleus and border cells (L4 segment), central cervical nucleus (C3 segment), and Stilling's nucleus (S2 segment). Using morphometric and densitometric analysis, we demonstrated that all the neurons of interest share a striking feature, namely an extremely low level of cytoplasmic NeuN immunolabeling against a high level of nuclear immunolabeling of this neuronal nuclear antigen, commonly used as a neuronal marker. The neuronal soma size averaged 1000-1850 mu m(2), which is comparable to another type of large neurons detected in slices, motoneurons (1140-1660 mu m(2)). Therefore, we used motoneuronal populations in the corresponding segments to compare the magnitude of their optical density. The relative optical density of the neurons of interest was several times lower than that of the motoneurons (0.060 +/- 0.030 vs 0.330 +/- 0.127). No significant differences in optical density were revealed between the above four spinal cord structures. Given that all these cell populations are morphologically unique and similar, we believe that the features of NeuN expression can be used as a simple tool to visualize neurons sending projections to the cerebellum. This can be instrumental both in targeted morphological examination and in histological control after experimental exposures.