Characterization of a hyperpolarizing-activated current in rat lateral parabrachial neurons

The present study examined the electrophysiological and kinetic properties of a hyperpolarizing-activated current in neurons located in the lateral parabrachial nucleus. We investigated whether differences observed in the shape of action potential afterhyperpolarizations in lateral parabrachial nucleus neurons, and the ability of these neurons to accommodate, correlated with the presence of this current. A voltage-activated inwardly rectifying current that increased in amplitude with hyperpolarization was observed in 83% of the neurons examined. Under voltage-clamp recording conditions, this current activated at about -70 mV, was half-activated at -96.5 mV and was blocked by bath application of 2 mM cesium, but not by 100 microM barium. In the current-clamp mode, activation of this current resulted in a transient increase in neuronal excitability at the termination of the more negative current injections. The presence of this current did not correlate with specific action potential characteristics or the ability of lateral parabrachial nucleus neurons to accommodate, as the kinetics and voltage-dependent characteristics are such that this hyperpolarizing-activated current does not affect neuronal excitability at or near the resting membrane potential. However, it may act as an important depolarizing mechanism that prevents neurons from becoming unresponsive when they are excessively hyperpolarized, These results provide evidence that the majority of neurons located in the lateral parabrachial nucleus exhibit a mixed cationic current, which is consistent with the H-current or Q-current. This current may function as a negative feedback mechanism that is activated under conditions of intense hyperpolarization so as to ensure that lateral parabrachial nucleus neurons are in a more suitable state of readiness to respond appropriately to afferent input.