In this study we show that high frequency stimulation (HFS, 100 Hz) of afferent fibers to the medial vestibular nucleus (MVN) can induce opposite long-term modifications of synaptic responses in the type B neurons depending upon the stimulation pattern. Long burst stimulation (LBS: 2 s) and short burst stimulation (SBS: 0.55 s) were applied with different burst number (BN) and inter-burst interval (IBI). It results that both LBS and SBS can induce either N-methyl-D aspartate receptors (NMDARs)-mediated long-term potentiation (LTP) or long-term depression (LTD), depending on temporal organization of repetitive bursts. In particular, the IBI plays a relevant role in guiding the shift from LTP to LTD since by using both LBS and SBS LTP is induced by shorter IBI than LTD. By contrast, the sign of long-term effect does not depend on the mean impulse frequency evaluated within the entire stimulation period. Therefore, the patterns of repetitive vestibular activation with different ratio between periods of increased activity and periods of basal activity may lead to LTP or LTD probably causing different levels of postsynaptic Ca2+. On the whole, this study demonstrates that glutamatergic vestibular synapse in the MVN can undergo NMDAR-dependent bidirectional plasticity and puts forward a new aspect for understanding the adaptive and compensatory plasticity of the oculomotor responses.
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