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 pa rabrachial nucleus. We investigated whether differences observed in the sha pe of action potential afterhyperpolarizations in lateral parabrachial nucl eus neurons, and the ability of these neurons to accommodate, correlated wi th the presence of this current. A voltage-activated inwardly rectifying cu rrent that increased in amplitude with hyperpolarization was observed in 83 % of the neurons examined. Under voltage-clamp recording conditions, this c urrent 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 mu M 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 c urrent injections. The presence of this current did not correlate with spec ific action potential characteristics or the ability of lateral parabrachia l nucleus neurons to accommodate, as the kinetics and voltage-dependent cha racteristics are such that this hyperpolarizing-activated current does not affect neuronal excitability at or near the resting membrane potential. How ever, it may act as an important depolarizing mechanism that prevents neuro ns 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 con sistent with the H-current or Q-current. This current may function as a neg ative feedback mechanism that is activated under conditions of intense hype rpolarization so as to ensure that lateral parabrachial nucleus neurons are in a more suitable state of readiness to respond appropriately to afferent input. (C) 1999 IBRO. Published by Elsevier Science Ltd.