Projection neurons of the vestibulo‐sympathetic reflex pathway

Abstract

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer FluoroGold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and FluoroGold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to FluoroGold injection sites in both RVLM and CVLM and the remainders were contralateral. As a group, cells projecting to RVLM were located slightly rostral to those with terminals in CVLM. Individual activated projection neurons were multipolar, globular or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

Imidazoleacetic acid-ribotide in vestibulo-sympathetic pathway neurons

“…Both the c-Fos and FluoroGold primary antibodies provided staining of the same vestibular regions and cells observed previously (Holstein et al 2012; Holstein et al 2014; Holstein et al 2016). …”

“…Seven of these animals received injections of the retrograde tracer FluoroGold (Fluorochrome, LLC, Denver, CO) in RVLM, five had FluoroGold injections in CVLM, and two received vestibular stimulation but no tracer injection. Sections from many (but not all) of these rats have been used in several of our previous studies (Holstein et al 2014; Holstein et al 2016). …”

“…As previously described (Holstein et al 2014; Holstein et al 2016), rats were anesthetized with isoflurane (4% induction, 2% maintenance), shaved and then placed in a computer-assisted stereotaxic frame (Leica Angle Two , Leica Microsystems, St. Louis, MO). Body temperature was maintained by laying the animal on a homeothermic pad regulated by feedback from a rectal thermometer.…”

“…Rats were anesthetized with isoflurane (as above) and then perfused transcardially 90 min after the sGVS stimulation, a time point of significant c-Fos protein accumulation in vestibular neurons after sGVS or tilt (Holstein et al 2012; Holstein et al 2014). The initial perfusion with 100 ml of 37°C 10 mM phosphate buffered saline (PBS) was followed by 500 ml of 4% paraformaldehyde/0.2% glutaraldehyde fixative in 0.1M PB (pH 7.4) at room temperature.…”

“…Adjustments of brightness and contrast were performed using the Photoshop Levels and Curves tools applied to all parts of each image. The details of our image processing strategy for figure preparation, albeit with earlier versions of the same software, are published in (Holstein et al 2014). …”