The Journal of Neurophysiology Vol. 86 No. 1 July 2001, pp. 312-320
Copyright ©2001 by the American Physiological Society

Developmental Changes in Hyperpolarization-Activated Currents I_h and I_K(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 Ba²⁺. 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.