Potassium currents in chick sensory neurons change with development

We used the whole-cell configuration of the giga-seal voltage-clamp to study voltage-gated potassium currents in sensory neurons dissociated from dorsal root ganglia from embryonic and hatched chicks. Neurons from 8-, 10-, 14-, and 18-day-old embryos (E8, E10, E14, E18) and 1- to 5-day-old chicks were studied under conditions which inhibited inward currents and calcium-activated currents (tetrodotoxin, no added calcium, intracellular EGTA). At all ages, potassium currents were activated by depolarizations to potentials positive to -40 mV. At a given age the amount of inactivation of outward current during 50- to 100-ms steps varied from cell to cell; some cells showed no inactivation while in others the outward current declined to about half of the peak current. On average, the amount of inactivation was fairly stable at E8, E10, E18, and in hatched chicks but showed a transient increase at E14. In contrast, currents elicited by 50-ms test steps following 2-s conditioning steps showed an age dependent change. In E8 neurons, shifting the conditioning voltage from -100 to -90 mV had little or no effect on the current at the end of the test step while earlier outward current was reduced. In cells from older embryos or hatched chicks, similar conditioning voltages caused reductions of both early and late currents during the test step. The relative amount of late current inactivated by this protocol increased as the age of the chicks increased. In addition, the amount of variation in the inactivation properties was larger in cells from older embryos and hatched birds. The changes in outward current occur during a period in which new neurons are formed and existing neurons mature and establish function.