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Mapping of spinal cord and brainstem inputs to phrenic motoneurons using transneuronal transport of rabies virus N2C in the cat

Rabies virus N2C can be employed to transneuronally trace the neural pathways regulating activity of a particular muscle. Unlike many transneuronal tracers, rabies virus is not avidly transported by autonomic pathways, and results in little lysis of infected neurons. In the present study, rabies virus N2C was injected into the diaphragm of adult cats, which were euthanized following survival periods of 2.5–6 days. Animals were classified by the extent of infection observed in the CNS. In animals with minimal infection, the labeled spinal neurons were comprised of large cells in the C5–C6 ventral horn (presumed diaphragm motoneurons) as well as smaller presumed interneurons confined mainly to the ventral horn and the region near the central canal. Considerably more infected spinal interneurons were present in most cats, which were distributed throughout the dorsal and ventral horns in all cervical segments as well as the thoracic spinal cord. Brainstem labeling in animals with the least infection was confined to the regions known to contain the medullary and pontine respiratory groups, the retrotrapezoid and raphe nuclei, the medial medullary and pontine reticular formation, and the region ventral to the spinal trigeminal nucleus. In animals with intermediate levels of infection, the number of labeled neurons in the respiratory groups and medial reticular formation was considerably larger, and infected cells were also evident in the lateral vestibular nucleus, locus coeruleus, and the cerebellar fastigial nucleus. Infected cells in the medial reticular formation and vestibular and fastigial nuclei likely have roles other than mediating breathing‐related contractions, such as adjusting diaphragm activity during postural alterations or vomiting.