Pacemaker Cells Uniquely Responsive to CO₂ in Cultured Cells of the Upper Medulla: Dose Response and Effect of Neuromediators. • 1476

We have previously found specific pacemaker cells, highly sensitive to CO₂, in the areas of the nucleus ambiguus (NA) and nucleus tractus solitarius (NTS), 2 mm rostral to the obex, in the fetal rat (J. Neurosci. Res., 33:590-597, 1992 & 33:579-589, 1992). We now report on the dose response evaluation of these cells as well as on their responsiveness to neurotransmitters. Twenty days old fetuses of pregnant Dawley rats were block dissected and the cells of the target areas were dissociated as previously described. Neuronal cells were plated in medullary background and placed in the incubator with 10% CO₂ for 2 or 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(n=28 cells; 16 NA and 12 NTS). Irregularly beating or silent cells show either absent or poor responsiveness to CO₂ (50 cells; 23 NA, 27 NTS). Pacemaker cells responded to norepinephrine (n=11) with an increase in firing potential, in a dose- response manner; this action was blocked by prazosin. Irregular beating cells (n=5) responded to norepinephrine poorly; silent cells(n=18) did not respond. Pacemaker cells responded to carbachol (n=18) with an increase in activity. Irregular beating cells responded poorly and silent cells did not respond; atropine blocked these effects. There were no differences in response related to the NA or NTS areas of the medulla. The findings suggest: 1) there is a very specific group of pacemaker cells in the most crucial respiratory area of the upper medulla which responds uniquely to CO₂ in a dose-dependent manner. This property is not shared by non- pacemaker neurons; and 2) the responses to norepinephrine and carbachol are in agreement with respiratory responses observed in brainstem sectioned fetal lambs. We speculate that these pacemaker cells have been specifically designed for central CO₂ chemoreception and may be involved in the generation of breathing.

Supported by the Children's Hospital of Winnipeg Research Foundation.