Development of potassium conductances in perinatal rat phrenic motoneurons

Prior to the inception of inspiratory synaptic drive transmission from medu llary respiratory centers, rat phrenic motoneurons (PMNs) have action poten tial and repetitive firing characteristics typical of immature embryonic mo toneurons. During the period spanning from when respiratory bulbospinal and segmental afferent synaptic connections are formed at embryonic day 17 (E1 7) through to birth (gestational period is similar to 21 days), a pronounce d transformation of PMN electrophysiological properties occurs. In this stu dy, we test the hypothesis that the elaboration of action potential afterpo tentials and the resulting changes in repetitive firing properties are due in large part to developmental changes in PMN potassium conductances. Ionic conductances were measured via whole cell patch recordings using a cervica l slice-phrenic nerve preparation isolated from perinatal rats. Voltage- an d current-clamp recordings revealed that PMNs expressed outward rectifier ( I-KV) and A-type potassium currents that regulated PMN action potential and repetitive firing properties throughout the perinatal period. There was an age-dependent leftward shift in the activation voltage and a decrease in t he time-to-peak of I-KV during the period from E16 through to birth. The mo st dramatic change during the perinatal period was the increase in calcium- activated potassium currents after the inception of inspiratory drive trans mission at E17. Block of the maxi-type calcium-dependent potassium conducta nce caused a significant increase in action potential duration and a suppre ssion of the fast afterhyperpolarizing potential. Block of the small conduc tance calcium-dependent potassium channels resulted in a marked suppression of the medium afterhyperpolarizing potential and an increase in the repeti tive firing frequency. In conclusion, the increase in calcium-mediated pota ssium conductances are in large part responsible for the marked transformat ion in action potential shape and firing properties of PMNs from the time b etween the inception of fetal respiratory drive transmission and birth.