ROLE OF THE MEDULLARY RAPHE NUCLEI IN THE RESPIRATORY RESPONSE TO CO2

We characterized the role of neurons within the midline of the medulla oblongata on phrenic and hypoglossal nerve responses to hypercapnia d uring early development. Studies were performed on decorticate or anes thetized, vagotomized and mechanically ventilated 14-20 day old piglet s. Reversible withdrawal of midline neuronal activity was induced by m icroinjections of lidocaine (2%, 300 nl; n = 10) and lesioning was cau sed by microinjections of the neurotoxic agent, ibotenic acid (n = 12) , at the same sites. At any given end-tidal CO2, peak phrenic and hypo glossal activities after lidocaine were significantly lower than in th e control period (P < 0.01). Similarly, 1-2 h after injections of ibot enic acid, both phrenic and hypoglossal nerve responses to CO, were si gnificantly lower than in the control period (P < 0.01). The results i ndicate for the first time that the medullary midline neurons are requ ired for full expression of ventilatory responses to hypercapnia and r aise the possibility that dysfunction of these nuclei may contribute t o respiratory instability during early postnatal life. (C) 1998 Elsevi er Science B.V.