. Development of inward rectification and control of membrane excitability in mesencephalic V neurons. J Neurophysiol 89: 1288 -1298, 2003. First published November 20, 2002 10.1152/jn.00850.2002. The present study was performed to assess the postnatal development and functional roles of inward rectifying currents in rat mesencephalic trigeminal (Mes V) neurons, which are involved in the genesis and control of oral-motor activities. Whole cell voltage-clamp recordings obtained from Mes V neurons in brain stem slices identified fast (I KIR ) and slow (I h ) inward rectifying currents, which were specifically blocked by BaCl 2 (300 -500 M) or 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD 7288, 10 M), respectively. The whole cell current density for these channels increased between postnatal days 2 to 12 (P2-P12), and the time courses for I h activation and deactivation were each well described by two time constants. Application of ZD 7288 produced membrane hyperpolarization in the majority of cells and prolonged afterhyperpolarization repolarization. Additionally, in the presence of ZD 7288, spike frequency was decreased and adaptation was more pronounced. Interestingly, these neurons exhibited a voltage-dependent membrane resonance (Ͻ10 Hz) that was prominent around resting potential and more negative to rest and was blocked by ZD 7288. These results suggest that I h contributes to stabilizing resting membrane potential and controlling cell excitability. The presence of I h imparts the neuron with the unique property of lowfrequency membrane resonance; the ability to discriminate between synaptic inputs based on frequency content.

1. Intracellular recording and stimulation were made from guinea pig trigeminal motoneurons (TMNs) in brain stem slices. Electrophysiological properties were examined and the underlying currents responsible for motoneuron excitability were investigated by the use of current clamp and single electrode voltage clamp (SEVC) techniques. 2. The voltage responses to subthreshold hyperpolarizing or depolarizing current pulses showed voltage- and time-dependent inward rectification. SEVC analysis demonstrated that the hyperpolarizing inward rectification resulted from the development of a slowly occurring voltage-dependent inward current activated at hyperpolarized membrane potentials. This current persisted in solutions containing low Ca2+/Mn2+, tetraethylammonium (TEA), and Ba2+, whereas it was reduced by 1-3 mM cesium. The depolarizing inward rectification was mediated by a persistent sodium current (INa-P) that was completely abolished by bath application of tetrodotoxin (TTX). 3. Action potential characteristics were studied by intracellular stimulation with brief current pulses (< 3 ms) in combination with ionic substitutions or application of specific ionic conductance blocking agents. Bath application of TTX abolished the action potential, whereas 1-10 mM TEA or 0.5-2 mM 4-aminopyridine (4-AP) increased, significantly, the spike duration, suggesting participation of the delayed rectifier and A-current type conductances in spike repolarization. SEVC analysis revealed a TEA-sensitive sustained outward current and a fast, voltage-dependent, transient current with properties consistent with their roles in spike repolarization. 4. TMN afterhyperpolarizing potentials (AHPs) that followed a single spike consisted of fast and slow components usually separated by a depolarizing hump [afterdepolarization (ADP)]. The fast component was abolished by TEA or 4-AP but not by Mn2+, Co2+, or the bee venom apamin. In contrast, the slow AHP was readily reduced by Mn2+, Co2+, or apamin, suggesting participation of an apamin-sensitive, calcium-dependent K+ conductance in the production of the slow AHP. SEVC analysis and ionic substitutions demonstrated a slowly activating and deactivating calcium-dependent K+ current with properties that could account for the slow AHP observed in these neurons. 5. Repetitive discharge was examined with long depolarizing current pulses (1 s) and analysis of frequency-current plots. When evoked from resting potential (about -55 mV), spike onset from rheobase occurred rapidly and was maintained throughout the current pulse. At higher current intensities, early and late adaptations in spike discharge were observed. Frequency-current plots exhibited a bilinear relationship for the first interspike interval (ISI) in approximately 50% of the neurons tested and in most neurons tested during steady-state discharge (SS).(ABSTRACT TRUNCATED AT 400 WORDS)