Developmental changes in membrane properties of chemoreceptor afferent neurons of the rat petrosal ganglia.

Carotid body chemoreceptors increase their responsiveness to hypoxia in the postnatal period, but the mechanism for this increase is unresolved. The purpose of the present study was to examine developmental changes in cellular characteristics of chemoreceptor afferent neurons in the petrosal ganglia with the underlying hypothesis that developmental changes occur and may account for the developmental increase in chemoreceptor responsiveness. Chemoreceptor complexes (carotid body, sinus nerve, glossopharyngeal nerve, and petrosal ganglia) were harvested from rats, aged 3-40 days, and intracellular recordings were obtained from petrosal ganglion neurons using sharp electrode impalement. All chemoreceptor neurons across ages were C fibers with conduction velocities <1 m/s and generated repetitive action potentials with depolarization. Resting membrane potential was -61.3 +/- 0.9 (SE) mV (n = 78) and input resistance was 108 +/- 6 MOmega and did not significantly change with age. Cell capacitance was 32.4 +/- 1.7 pF and did not change with age. Rheobase averaged 0.21 +/- 0.02 nA and slightly increased with age. Action potentials were followed by an afterhyperpolarization of 12.4 +/- 0.6 mV and time constant 6.9 +/- 0.5 ms; only the time constant decreased with age. These results, obtained in rat, demonstrate electrophysiologic characteristics which differ substantially from that previously described in cat chemoreceptor neurons. In general developmental changes in cell characteristics are small and are unlikely to account for the developmental increase in chemoreceptor responsiveness with age.