Neurofilament proteins are preferentially expressed in descending output neurons of the cat the superior colliculus: A study using SMI-32

Physiological studies indicate that the output neurons in the multisensory (i.e. intermediate and deep) laminae of the cat superior colliculus receive converging information from widespread regions of the neuraxis, integrate this information, and then relay the product to regions of the brainstem involved in the control of head and eye movements. Yet, an understanding of the neuroanatomy of these converging afferents has been hampered because many terminals contact distal dendrites that are difficult to label with the neurochemical markers generally used to visualize superior colliculus output neurons. Here we show that the SMI-32 antibody, directed at the non-phosphorylated epitopes of high molecular weight neurofilament proteins, is an effective marker for these superior colliculus output neurons. It is also one that can label their distal dendrites. Superior colliculus sections processed for SMI-32 revealed numerous labeled neurons with varying morphologies within the deep laminae. In contrast, few labeled neurons were observed in the superficial laminae. Neurons with large somata in the lateral aspects of the deep superior colliculus were particularly well labeled, and many of their secondary and tertiary dendrites were clearly visible. Injections of the fluorescent biotinylated dextran amine into the pontine reticular formation revealed that approximately 80% of the SMI-32 immunostained neurons also contained retrogradely transported biotinylated dextran amine, indicating that SMI-32 is a common cytoskeletal component expressed in descending output neurons. Superior colliculus output neurons also are known to express the calcium-binding protein parvalbumin, and many SMI-32 immunostained neurons also proved to be parvalbumin immunostained. These studies suggest that SMI-32 can serve as a useful immunohistochemical marker for detailing the somatic and dendritic morphology of superior colliculus output neurons and for facilitating evaluations of their input/output relationships.