Respiratory pacemaker cells responsive to CO2 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 CO2, in a region of the medulla oblongata in the fe tal rat, 2 mm rostral to the obex. We now report on the CO2 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 backgrou nd and placed in the incubator with 10% CO2 for 2-3 weeks. Cells were then studied using patch-clamp techniques. Pacemaker cells with single or bursting potentials showed responsiveness to CO2 starting with pulses of 10 msec. Irregular beating or silent cells had poor or absent responsiveness to CO2. Pacemaker cells responded to norepin ephrine with increased firing potential; this action was blocked by metropo lol. PGE(2) had no effect on pacemaker-cell activity, but indomethacin incr eased the spike frequency from 336 +/- 41 to 384 +/- 65 spikes/min. Morphin e 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 act ivity of pacemaker cells from 301 +/- 35 to 452 +/- 52 spikes/min. In all o f the above, irregular beating cells responded poorly and silent cells did not respond. The findings indicate that these pacemaker cells are uniquely designed to r espond to CO2 and have some properties which allow them to respond to certa in chemical mediators in a manner similar to that of the whole respiratory system in vivo. (C) 2000 Wiley-Liss, Inc.