Transneuronal tracing of neural pathways controlling activity of diaphragmmotoneurons in the ferret

Previous studies have shown that neurons in addition to those in the medull ary respiratory groups are involved in activating phrenic motoneurons durin g a number of behaviors, including vomiting and reaction to vestibular stim ulation. However, the location of premotor inspiratory neurons outside of t he main medullary respiratory groups is largely unknown, particularly in em etic species. In the present study, the transneuronal tracer pseudorabies v irus was injected into the diaphragm of the ferret, and the locations of re trogradely-labeled motoneurons and transneuronally-labeled pre-motoneurons in the brainstem and cervical and thoracic spinal cord were mapped. Injecti ons of a monosynaptic tracer, cholera toxin, were also made in order to ver ify the location of motoneurons innervating the diaphragm. Phrenic motoneur ons identified with pseudorabies virus and cholera toxin were confined larg ely to the C-5-C-7 levels of spinal cord, and often gave rise to prominent polarized dendritic arbors that extended across the midline. At post-inocul ation survival times greater than or equal to three days, transneuronally-l abeled interneurons were located in the cervical and thoracic spinal cord a nd portions of the brainstem, including the midline pontomedullary reticula r formation and the lateral medullary reticular formation. Double-labeling studies revealed that although the infected midline neurons were located in the proximity of serotonergic neurons, only a small number of the virus-co ntaining cells were positive for serotonin. These findings suggest that neurons in the midline of the medulla and pens influence the activity of phrenic motoneurons, perhaps during inspiratory b ehaviors unique to emetic animals (such as vomiting). (C) 1999 IBRO. Publis hed by Elsevier Science Ltd.