Definition of neuronal circuitry controlling the activity of phrenic and abdominal motoneurons in the ferret using recombinant strains of pseudorabies virus

During a number of behaviors, including vomiting and some postural adjustme nts, activity of both the diaphragm and abdominal muscles increases. Previo us transneuronal tracing studies using injection of pseudorabies virus (PRV ) into either the diaphragm or rectus abdominis (RA) of the ferret demonstr ated that motoneurons innervating these muscles receive inputs from neurons in circumscribed regions of the spinal cord and brainstem, some of which h ave an overlapping distribution in the magnocellular part of the medullary reticular formation (MRF). This observation raises two possibilities: that two populations of MRF neurons provide independent inputs to inspiratory an d expiratory motoneurons or that single MRF neurons have collateralized pro jections to both groups of motoneurons. The present study sought to disting uish between these prospects. For this purpose, recombinant isogenic strain s of PRV were injected into these respiratory muscles in nine ferrets; the strain injected into the diaphragm expressed beta-galactosidase, whereas th at injected into RA expressed green fluorescent protein. Immunofluorescence localization of the unique reporters of each virus revealed three populati ons of infected premotor neurons, two of which expressed only one virus and a third group that contained both viruses. Dual-infected neurons were pred ominantly located in the magnocellular part of the MRF, but were absent fro m both the dorsal and ventral respiratory cell groups. These data suggest t hat coactivation of inspiratory and expiratory muscles during behaviors suc h as emesis and some postural adjustments can be elicited through collatera lized projections from a single group of brainstem neurons located in the M RF.