Developmental Changes in Hyperpolarization-Activated Currents Ih and IK(IR) in Isolated Rat Intracardiac Neurons

School of Biomedical Sciences, Department of Physiology and Pharmacology, University of Queensland, Brisbane, Queensland 4072, Australia Hogg, R. C., A. A. Harper, and D. J. Adams. Developmental Changes in Hyperpolarization-Activated Currents I h and I K(IR) in Isolated Rat Intracardiac Neurons. J. Neurophysiol. 86: 312-320, 2001. The hyperpolarization-activated nonselective cation current, I h , was investigated in neonatal and adult rat intracardiac neurons. I h was observed in all neurons studied and displayed slow time-dependent rectification. I h was isolated by blockade with external Cs + (2 mM) and was inhibited irreversibly by the bradycardic agent, ZD 7288. Current density of I h was approximately twofold greater in neurons from neonatal ( 4.1 pA/pF at 130 mV) as compared with adult ( 2.3 pA/pF) rats; however, the reversal potential and activation parameters were unchanged. The reversal potential and amplitude of I h was sensitive to changes in external Na + and K + concentrations. An inwardly rectifying K + current, I K(IR) , was also present in intracardiac neurons from adult but not neonatal rats and was blocked by extracellular Ba 2+ . I K(IR) was present in approximately one-third of the adult intracardiac neurons studied, with a current density of 0.6 pA/pF at 130 mV. I K(IR) displayed rapid activation kinetics and no time-dependent rectification consistent with the rapidly activating, inward K + rectifier described in other mammalian autonomic neurons. I K(IR) was sensitive to changes in external K + , whereby raising the external K + concentration from 3 to 15 mM shifted the reversal potential by approximately +36 mV. Substitution of external Na + had no effect on the reversal potential or amplitude of I K(IR) . I K(IR) density increases as a function of postnatal development in a population of rat intracardiac neurons, which together with a concomitant decrease in I h may contribute to changes in the modulation of neuronal excitability in adult versus neonatal rat intracardiac ganglia.