Respiratory pacemaker cells responsive to CO(2) in the upper medulla: dose response and effects of mediators

We previously reported on the presence of respiratory pacemaker cells that are highly sensitive to CO(2), in a region of the medulla oblongata in the fetal rat, 2 mm rostral to the obex. We now report on the CO(2) dose responses of these cells, as well as their responsiveness to certain chemical agents known to affect breathing in the fetus. Twenty-day-old fetal Sprague Dawley rats were block-dissected, and the cells of target areas were dissociated as previously described. Neuronal cells were plated on a medullary background and placed in the incubator with 10% CO(2) for 2-3 weeks. Cells were then studied using patch-clamp techniques. Pacemaker cells with single or bursting potentials showed responsiveness to CO(2) starting with pulses of 10 msec. Irregular beating or silent cells had poor or absent responsiveness to CO(2). Pacemaker cells responded to norepinephrine with increased firing potential; this action was blocked by metropolol. PGE(2) had no effect on pacemaker-cell activity, but indomethacin increased the spike frequency from 336+/-41 to 384+/- 65 spikes/min. Morphine stimulated the pacemaker cells from 205+/-25 to 272+/-29 spikes/min; this was blocked by naloxone. Finally, a placental extract, which inhibited breathing in the unanesthetized fetal sheep preparation, increased the activity of pacemaker cells from 301+/-35 to 452+/-52 spikes/min. In all of the above, irregular beating cells responded poorly and silent cells did not respond. The findings indicate that these pacemaker cells are uniquely designed to respond to CO(2) and have some properties which allow them to respond to certain chemical mediators in a manner similar to that of the whole respiratory system in vivo.