Ionic conductances contributing to spike repolarization and after‐potentials in rat medial vestibular nucleus neurones.

Abstract

1. Intracellular recordings were made from 123 tonically active medial vestibular nucleus (MVN) neurones in a horizontal slice preparation of the dorsal brainstem of the rat. On the basis of their averaged action potential shapes, the cells were classified as either type A, having a single deep after-hyperpolarization (AHP; 40/123 cells, 33%), or type B, having an early fast AHP and a delayed slow AHP (83/123 cells, 67%). The two cell types were distributed throughout the rostrocaudal extent of the MVN. 2. In type A cells TEA reduced the single deep AHP and decreased the rate of spike repolarization. Depolarizing current pulses from a hyperpolarized membrane potential elicited spikes with short plateau potentials in TEA. These persisted in Ca2"-free medium but were abolished along with the spontaneous activity in 1'TX. Ca2+-free medium did not affect the initial rate of repolarization but reduced the deep AHP. Apamin and carbachol had little effect. 4-Aminopyridine (4-AP) slowed spike repolarization and the AHP amplitude by a small amount. Thus, in type A cells spike repolarization and AHP appear to be mediated largely by a TEA-sensitive potassium current (presumably IK) and an apamin-insensitive Ca2+-activated potassium current (presumably Ic).3. The early fast AHP in type B cells was readily abolished in TEA. In seven of ten type B cells tested, the spontaneous spikes developed plateau potentials of 100-120 ms duration in 10 mM TEA, which then became 7-9 s long in Ca2+-free medium. In the remaining three cells, the spontaneous plateaux were 1-75-2 s long in TEA, and were reduced to 30-100 ms in Ca2+-free medium. ITX abolished the spontaneous spikes and plateaux.The delayed AHP was abolished by apamin, which induced irregular firing. 4-AP slowed spike repolarization and abolished the fast AHP, but did not induce plateaux. Thus, in type B cells spike repolarization involves a TEA-sensitive current (presumably IK) as well as Ic and the 4-AP-sensitive potassium current IA, while the apaminsensitive potassium current IAHP is responsible for the delayed AHP. 4. The tonic activity in type B cells appears to be regulated mainly by interactions between a persistent Na+ current, which in most cells is large enough to generate plateaux when repolarization is impeded in TEA, and the hyperpolarization mediated by 'AHP. About 30% of type B cells have an additional inward Ca2+ current. In type A cells the persistent Na+ current is either not large enough to generate plateaux or is more effectively counteracted by TEA-insensitive outward currents. The pacemaker conductances in type A and type B cells thus appear to be distinct.Medial vestibular nucleus (MVN) neurones are the reciprocal inhibitory interactions between the vestibular primary central targets of afferents from the ipsilateral nuclei of the two sides, and by cerebellar Purkinje cell horizontal semi-circular canal, and their projections to the projections onto MVN neurones. Cervical proprioceptive oculomotor nuclei and the cervical spinal cord are amongst and eye p...

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

“…However, there was no objective reason to keep these intermediate MVNn as a separate category. In agreement with Johnston et al (1994), our analysis shows that MVNn are best categorized by defining only two distinct groups of neurons corresponding to the qualitatively defined type A and B MVNn.…”

“…There was therefore no objective reason to keep these MVNn as a separate category. Therefore as already suggested by Johnston et al (1994), MVNn are best categorized by defining only two groups of neurons corresponding to the type A and B MVNn. Because there is a continuum of neurons between these two cell types, the limit between the two groups has to be set somewhat arbitrarily.…”

“…For each neuron, the averaged spike profile obtained during spontaneous firing and its first derivative were used (as already done by Johnston et al 1994) to assess the presence of the A-like rectification and double AHP, the two main criteria used previously for the qualitative classification.…”

“…For this study, quantitative criteria were developed to characterize the intracellularly recorded MVNn. Using a method introduced by Johnston et al (1994), an averaged spike profile was obtained for each MVNn during spontaneous firing. This spike profile and its first derivative were used to assess the presence and quantify the strength of the A-like rectification and double AHP, which were the two main criteria used previously for the qualitative classification.…”

“…In vitro intracellular recordings have led to the identification of two main types of MVNn, the type A and type B neurons, according to their membrane properties (Gallagher et al 1985;Him and Dutia 2001;Johnston et al 1994;Serafin et al 1991a,b). It is generally admitted that MVNn represent a continuum of cells whose properties are distributed between those of two canonical types of neurons, the type A and B MVNn (Du Lac and Lisberger 1995a).…”