Results from studies on dissociated neonatal and adult rat intracardiac neurons reveal changes in expression of hyperpolarization-activated channels I_h and Iand of the K_ATP channel (Adams et al., 2002). We investigated the effects of the wide-spectrum K⁺ channel blocker Ba²⁺ and Iblocker Cs⁺ on the electrical properties of intracardiac ganglion neurons in a wholemount preparation from adult (≥ 6 weeks) and neonatal (2-5 days) humanely killed Wistar rats. Intracellular recordings were made using sharp glass microelectrodes and the preparation was superfused with a bicarbonate buffered physiological salt solution (PSS) maintained at 37°C. All neonatal neurons had afterhyperpolarisations (AHP) with a small amplitude and short duration. In contrast, neurones from adult ganglia could be differentiated into two groups on the basis of AHP duration to 50% recovery (AHP₅₀). Ba²⁺-containing PSS (1 mM) depolarized adult neurons by 8.1 ± 1.9mV (n = 4) and neonatal neurons by 5.5 ± 1.9 mV (n = 5). Ba²⁺ also reduced the slope conductance, determined from steady-state voltage responses at hyperpolarised membrane potentials, from 14.4 ± 1.5 to 8.7 ± 1.6 nS (p <0.001, paired t-test) and from 9.3 ± 1.4 to 7.3 ± 1.5 nS (p <0.05) in adult and neonatal ganglion neurons respectively. In adults Ba²⁺ had no effect on AHP and AHP₅₀,whereas in neonates it increased AHP amplitude by 5.1mV (p < 0.05, n = 5), but AHP₅₀ was unaltered. Application of Cs⁺ (2 mM) evoked a membrane potential hyperpolarization of ~2 mV, reduced the slope conductance and abolished the time-dependent rectification of the voltage response to hyperpolarizing current pulses in both neonates and adults. It had no effect on AHP characteristics.These results show that the developmental changes in ion channel expression in isolated intracardiac neurons are reflected in the active membrane properties of neurons in the intact ganglion. Table 1. Passive electrical properties and AHP properties of intracardiac ganglion neurones. Data presented as mean ± SE; (n), number of neurones; ns, not significant. Data compared using unpaired t-test.