LAUSR

The vestibular system serves a variety of functions, i.e., postural control, spatial orientation, and gaze stabilization, during head and body movements. Thereby, the vestibuloocular reflex (VOR) provides an efficient rapidly acting three neuron arc transmitting signals about angular and linear head motions to the extraocular muscles to elicit compensatory eye movements [1]. Phylogenetically, the VOR represents one of the oldest types of eye movements, and the secondary vestibulo-ocular connections seem to be organised similarly in all vertebrates [2], i.e., secondary vestibular neurons in the vestibular nuclei receive primary canal afferents via the vestibular nerve and project to motoneurons in the abducens, trochlear, and oculomotor nuclei. The rather similar cytoarchitecture of the vestibular complex in all vertebrates allows four main nuclei composed of smallto large-sized neurons of variable shape to be distinguished [3]: the superior (SV), medial (MV), lateral (LV), and descending (DV) vestibular nuclei, though specific functions and histochemical features are not necessarily confined to these nuclear borders [4–6]. For example, contralateral excitatory vestibulo-ocular projections arise from central magnocellular regions of the medial (MVm) and superior vestibular nucleus (SV), whereas the smaller cells surrounding this more magnocellular core in MVm and SV seem to represent mainly commissural and intrinsic connections [7]. For a clinicopathological analysis of caseswithoculomotor or vestibular disorders, it is important to identify functional cell groups of the oculomotor/vestibular system to be able to correlate their damage with the observed deficits. Based on combined tract-tracing and immunohistochemical studies in monkeys, histochemicalmarkers for saccadic burst neurons or omnipause neurons have been established andwere applied to human brainstem sections to identify the homologue cell groups [8–11]. Here, it was our aim to find a histochemical marker for the secondary vestibular neurons in humans. The reanalysis of monkey brainstem sections immunostained for non-phosphorylated neurofilaments (NPNF) revealed similar neuron populations as those seen of retrogradely labelled vestibulo-ocular neurons after a tract-tracer injection into the oculomotor nucleus (nIII) (Fig. 1a, b) [12]. The combined immunofluorescence staining for the tracer (wheatgerm agglutinin, WGA; or WGA conjugated to horseradish peroxidase, WGA-HRP) and NPNF (monoclonal anti NPNF 1: 1000; SMI32, Sternbergermonoclonals) revealed that the vast majority of tracer-labelled neurons in the vestibular nuclei express NPNF, thereby confirming a previous preliminary observation (Fig. 1c, d) [12]. The large-sized NPNF-positive neurons in themagnocellularMVm and core region of the SV are considered the secondary vestibular neurons (Fig. 1b). The transfer to human tissue was carried out on three control human brainstems (age 62–75; immersion fixation in 10% formalin) obtained 24–72 h after death from bodies donated to the Anatomical Institute and from the Neurobiobank Munich approved by the ethics committee of the Ludwig Maximilians University. Series of frozen or paraffin sections were stained with Luxol Fast Blue (LFB) & A. Horn [email protected]