Whole-cell currents in two subpopulations of cultured rat petrosal neurons with different tetrodotoxin sensitivities

In this study we use whole-cell recording to characterize at least two distinct populations of cultured neurons from perinatal rat petrosal or petrosal/jugular ganglia based on differential sensitivity of the transient inward Na+ current to tetrodotoxin. These ganglia supply chemoreceptor and baroreceptor afferents which mediate several cardiovascular reflexes. Approximately 50% of the neurons sampled had Na+ currents that were virtually unaffected by bath addition of tetrodotoxin (0.5-2.0 microM) but were abolished by choline substitution for external Na+. The majority of the remaining neurons had Na+ currents that were rapidly and reversibly blocked by 500 nM tetrodotoxin. A few cells had both tetrodotoxin-resistant and tetrodotoxin-sensitive Na+ currents. All neurons had similar voltage-activated Ca2+ and K+ currents. The inward Ca2+ current had no obvious fast transient or T-type component and appeared to be due mainly to the presence of long-lasting L-type Ca2+ channels. The outward currents consisted largely of a delayed rectifying K+ current (IKdr) and a Ca(2+)-activated K+ current (IKca), but no obvious fast transient K+ current (IA) was observed. Exposure to a chemosensory stimulus, hypoxia (PO2 approximately 20 Torr), had no effect on these neurons, in contrast to the pronounced decrease in K+ current it produces in cultured glomus cells, the presumed chemoreceptors and normal targets for a subset of petrosal neurons in vivo. Current-clamp recordings indicated that some neurons gave single spikes while others gave multiple spikes in response to long-depolarizing stimuli. No correlation between spiking behaviour and tetrodotoxin-sensitivity was observed. Thus, cultures enriched in petrosal neurons contain subpopulations with differential sensitivities to tetrodotoxin. Since many of these neurons innervate a single chemosensory target organ, the carotid body, it is of interest to know whether one or both subtypes can form functional synapses with glomus cells of the carotid body and mediate a chemoreceptor reflex.