MEDIAL VESTIBULAR NUCLEUS IN THE GUINEA-PIG - APAMIN-INDUCED RHYTHMICBURST FIRING - AN IN-VITRO AND IN-VIVO STUDY

In a previous in vitro study, we have shown that guinea-pig medial ves tibular nucleus neurons (MVNn) can be grouped into two main cell types based on their intrinsic membrane properties. Subsequent in vivo and in vitro studies demonstrated that these neurons are endowed with N-me thyl-D-aspartate (NMDA) receptors and that NMDA induces rhythmic burst s in B MVNn. We now report the occurrence of rhythmic bursts in B MVNn (and in the subclass of B+LTS MVNn) which are induced by long-lasting perfusion of either apamin, a selective blocker of one type of Ca2+-d ependent K+ conductance (SK channels), or by a high Mg2+/low Ca2+ arti ficial cerebrospinal fluid. Apamin-induced bursts were studied in vitr o in brainstem slices, and in vivo in the alert unrestrained guinea-pi g. In vitro, intracellular recordings demonstrated that the frequency of the bursts was voltage dependent. These bursts were insensitive to D-2-amino-5-phosphopentanoic acid but could be abolished by tetrodotox in or blocked by the bath application of 20-50 muM of ouabain, a block er of the sodium pump. In the in vivo preparation, unilateral infusion of apamin into the vestibular nuclei induced oscillatory head and eye movements. Our data show that the blockade of a Ca2+-activated K+ con ductance may switch, in vitro and probably in vivo, the B MVNn firing pattern from a regular to a bursting firing pattern.