Hyperpolarization-activated currents in the growth cone and soma of neonatal rat dorsal root ganglion neurons in culture

Dissociated dorsal root ganglion neuron growth cones and somata from neonatal rats were voltage and current clamped with the use of the perforated-patch whole cell configuration to study the occurrence and properties of slow hyperpolarization-activated currents (I-h) at both regions. Under voltage-clamp conditions I-h, blockable by 2 mM extracellular CsCl, was present in 33% of the growth cones tested. Its steady-state activation as a function of voltage could be fitted with a single Boltzmann function with a midpoint potential of -97 mV. The time course of current activation could be best described by a double-exponential function. The magnitude of the fully activated conductance was 3.5 nS and the reversal potential amounted to -29 mV. At the soma, I-h was found in 80% of the somata tested, which is much higher than occurrence at the growth cone. The steady-state activation curve of I-h at the soma, fitted with a single Boltzmann funct;on, had a midpoint potential of -92 mV, which was more positive than that in the growth cone. The double-exponential activation of the current was faster than in the growth cone. The fully activated conductance of 5.1 nS and the reversal potential of -27 mV were not significantly different from the values obtained at the growth cone. Membrane hyperpolarization by current-clamp pulses elicited depolarizing sags in 30% and 78% of the tested growth cones and somata, respectively, which is in agreement with our voltage-clamp findings. Termination of the hyperpolarizing current pulse evoked a transient membrane depolarization or an action potential at both sites. Application of 2 mM extracellular CsCl hyperpolarized the membrane potential reversibly by similar to 5 mV and blocked the depolarizing sags and action potentials following the current injections at these regions. Thus I-h contributes to the resting membrane potential and modulates the excitability of both the growth cone and the soma. Intracellular perfusion with the second messenger adenosine 3',5'-cyclic monophosphate (cAMP) was only possible at the soma by the use of the conventional whole cell configuration. Addition of 100 mu M cAMP to the pipette solution shifted the midpoint potential of the I-h activation curve from -108 to -78 mV. The current activation time course was also accelerated. The reversal potential and the fully activated conductance underlying I-h were not changed by cAMP. These results imply that cAMP primarily affects the gating kinetics of I-h. Our results show for the first time quantitative differences in I-h properties and occurrence at the growth cone and soma membrane. These differences may reflect differences in intracellular cAMP concentration and in the expression of I-h.