Perinatal changes of I-h in phrenic motoneurons

The hyperpolarization-activated cationic current (I-h) was characterized an d its maturation studied on phrenic motoneurons (PMNs), from reduced prepar ations of foetal (E18 and E21) and newborn (P0-P3) rats, using the whole-ce ll patch-clamp technique. In voltage-clamp mode, 2-s hyperpolarizing steps (5-mV, -50 to -110 mV) elicited a noninactivating inward current, blocked b y external application of Cs+ or ZD 7283. At -110 mV, I-h current density a veraged 0.67 +/- 0.41 pA/pF at E18, reached a transient peak at E21 (1.38 /- 0.11 pA/pF) and decreased at P0-P3 (0.77 +/- 0.22 pA/pF). V-1/2 was simi lar at E18 and E21 (-79 mV) but was significantly hyperpolarized at P0-P3 ( -90 mV). The time constant of activation was voltage-dependent, and signifi cantly faster at E21. Reversal potential was similar at all ages when estim ated by extrapolation or tail current procedures. It was positively shifted by 25 +/- 6 mV when external potassium was raised from 3 to 10 mM, suggest ing a similar sensitivity to K+ from E18 to P0-3. Cs+ or ZD 7288 applicatio ns on PMNs at rest in current-clamp mode, in a partitioned chamber, induced a 10 +/- 2 mV hyperpolarization at E18 and E21, and an 8 +/- 2 mV hyperpol arization at P0-3. The area of the central respiratory drive potential or c urrent was increased by 33 and 31%, respectively, at E21, but was not signi ficantly modified at E18 and P0-3. Our data suggest a critical period durin g the perinatal maturation of I-h during which it is transiently upregulate d and attenuates the influence of the central respiratory drive on PMNs jus t prior to birth.