Functional significance of the apamin-sensitive conductance in rat locus coeruleus neurons.

The effects of apamin on rat locus coeruleus (LC) neurons were studied in a brain slice preparation with intracellular recording. Bath application of apamin (2-500 nM) reduced the amplitude of an intermediate component of the afterhyperpolarization (AHP) following single spontaneous action potentials, but did not change the size or time-course of fast and slow components of the AHP, spike amplitude or duration. Apamin blocked the early component of the post-stimulus hyperpolarization (PSH) which follows a train of action potentials. The size of the late component of PSH was sometimes augmented by apamin. Apamin increased the number of spikes evoked by a depolarizing current pulse and increased the slope of the spike frequency-current intensity relation. Accommodation of firing during long depolarizing pulses showed a biexponential time-course indicating 2 distinct components. Apamin specifically reduced the contribution of the fast component of accommodation and increased its time constant. These data indicate that the apamin-sensitive conductance is functionally important in accommodation at faster firing rates such as those seen during evoked spike trains in the present experiments, and which may occur in vivo during behavioral arousal and in anxiety or drug withdrawal syndromes.