The Response of Membrane Potential (Vsubm/sub) and Intracellular pH (pHsubi/sub) to Hypercapnia in Neurons and Astrocytes from Rat Retrotrapezoid Nucleus (RTN)

We compared the response to hypercapnia (10%) in both neurons and astrocytes between a distinct area of the retrotrapezoid nucleus (RTN), the medio-caudal RTN (mcRTN) and more intermediate and rostral RTN areas (irRTN) in medullary brain slices from neonatal rats. Exposure to hypercapnic acidosis (HA) caused pH _o to decline from 7.45 to 7.15 and caused a maintained, intracellular acidification of 0.15±0.02 pH unit in 90% of neurons from both areas of the RTN (n=16). HA excited 44% of mcRTN (7 of 16) and 38% of irRTN neurons (6 of 16), increasing firing rate by 167±75% (chemosensitivity index, CI, of 256±72%) and 310±93% (CI of 292±50%) respectively. These responses did not vary throughout neonatal development. We investigated the role of pH _o by comparing the responses between HA (decreased pH _i and pH _o ) and isohydric hypercapnia (IH; decreased pH _i with constant pH _o ) in mcRTN neurons. Neurons excited by HA (firing rate increased by 156±46%; n=5) were excited to the same extent by IH (firing rate increased by 167±38%; n=5). Neurons insensitive to HA were also insensitive to IH. In astrocytes from both areas of the RTN, exposure to HA caused a maintained, intracellular acidification of 0.17±0.02 pH unit (n=6) and a depolarization of 5±1mV (n=12). In summary, a high percentage of neurons (42%) from the entire RTN are highly responsive (CI 248%) to HA; this may reflect both synaptically driven and intrinsic mechanisms of CO ₂ sensitivity. Changes of pH _i appear to be a more significant stimulus than changes of pH _o in chemosensory RTN neurons. Finally, the lack of astrocytic pH _i regulation in response to HA suggests that astrocytes do not enhance extracellular acidification during hypercapnia in the RTN in the neonatal period.