Development of action potentials and apamin-sensitive after-potentials in mouse vestibular nucleus neurones

Abstract

The postnatal maturation of medial vestibular nucleus (MVN) neurones was examined in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. Using spike-shape averaging to analyse the intracellularly recorded action potentials and after-hyperpolarizations (AHPs) in each cell, all the MVN neurones recorded in the young adult (postnatal day 30; P30) mouse were shown to have either a single deep AHP (type A cells), or an early fast and a delayed slow AHP (type B cells). The relative proportions of the two subtypes were similar to those in the young adult rat. At P5, all the MVN cells recorded showed immature forms of either the type A or the type B action potential shape. Immature type A cells had broad spontaneous spikes, and the characteristic single AHP was small in amplitude. Immature type B cells had somewhat narrower spontaneous spikes that were followed by a delayed, apamin-sensitive AHP. The delayed AHP was separated from the repolarisation phase of the spike by a period of isopotentiality. Over the period P10-P15, the mean resting potentials of the MVN cells became more negative, their action potential fall-times became shorter, the single AHP in type A cells became deeper, and the early fast AHP appeared in type B cells. Until P15 cells of varying degrees of electrophysiological maturity were found in the MVN but by P30 all MVN cells recorded were typical adult type A or type B cells. Exposure to the selective blocker of SK-type Ca-activated K channels, apamin (0.3 microM), induced depolarising plateaux and burst firing in immature type B cells at rest. The duration of the apamin-induced bursts and the spike frequency during the bursts were reduced but not abolished after blockade of Ca channels in Ca-free artificial cerebrospinal fluid containing Cd2+. By contrast, in mature type B cells at rest apamin selectively abolished the delayed slow AHP but did not induce bursting activity. Apamin had no effect on the action potential shape of immature type A cells. These data show that the apamin-sensitive I(AHP) is one of the first ionic conductances to appear in type B cells, and that it plays an important role in regulating the intrinsic rhythmicity and excitability of these cells.

Long-Term Plasticity of Ipsilesional Medial Vestibular Nucleus Neurons After Unilateral Labyrinthectomy

“…Compared with the data obtained in the rat and mouse (Dutia and Johnston 1998;Him and Dutia 2001;Johnston et al 1994), in the guinea pig, the proportion of type A MVNn is higher (ϳ50% instead of 20 -25%) and the proportion of type B MVNn is lower. All type B MVNn display a double AHP during spontaneous firing in the rat and mouse (because this is the criterion used by these authors to define type B MVNn).…”

Transgenic Mouse Lines Subdivide Medial Vestibular Nucleus Neurons into Discrete, Neurochemically Distinct Populations

“…Initial attempts to classify neurons in vitro subdivided the population into two types, A and B, largely on the basis of the presence of second component to the AHP (Serafin et al, 1991;Johnston et al, 1994). Although action potentials characteristic of type A and type B neurons have been observed in various species (Straka et al, 2005), classification of neurons based on a continuous AHP profile requires semi-arbitrary subdivisions (Beraneck et al, 2003) and is not robust to changes in intrinsic physiological properties that occur with development (Dutia and Johnston, 1998;Murphy and du Lac, 2001), cellular plasticity (Nelson et al, 2003), or altered vestibular experience (Ris et al, 2002;Beraneck et al, 2003Beraneck et al, , 2004.…”

Functional Properties of Motoneurons Derived from Mouse Embryonic Stem Cells

“…Apamin had no effect on frequency-current relationships (n ϭ 6; data not shown). Whether the lack of apaminsensitive K (Ca) channels reflects a normal developmental process (Dutia and Johnston, 1998;Klocker et al, 2001) or results from our culture conditions is not clear.…”