Relationship between afterhyperpolarization profiles and the regularity of spontaneous firings in rat medial vestibular nucleus neurons

Our previous in vivo and in vitro whole-cell patch-clamp recording studies demonstrated that neurons in the medial vestibular nucleus (MVN) could be characterized on the basis of three electrophysiological properties: afterhyperpolarization (AHP) profile; firing pattern; and response pattern to hyperpolarizing current pulses. In the present study, to clarify which types of the classified MVN neurons correspond to neurons with regular or irregular firing, we investigated their spike discharge patterns using whole-cell patch-clamp recording in both in vivo and in vitro preparations. The discharge regularity was related to AHP profiles, and we found that: (i) the coefficient of variation (CV) of interspike intervals during spike discharges was smaller in neurons exhibiting AHP with a slow component [AHP(s+)] than in those without a slow component [AHP(s-)], or with a slow AHP component preceded by afterdepolarization (ADP) [AHP(s+) with ADP]; (ii) the blockade of Ca ²⁺-dependent K⁺ channels by 100 nm apamin abolished the slow component and increased the CV in neurons exhibiting AHP(s+); and (iii) the modulation of firing (firing gain) in response to ramp current was larger in neurons exhibiting AHP(s-) than in the other two neuronal types. These results suggest that neurons exhibiting AHP(s+) are regularly discharging neurons with small firing gains to stimulus, neurons exhibiting AHP(s+) with ADP are irregularly discharging neurons with small firing gains, and neurons exhibiting AHP(s-) are irregularly discharging neurons with large firing gains. The regular firing of neurons exhibiting AHP(s+) is attributed to the activation of apamin-sensitive Ca²⁺-dependent K⁺ channels.