SYNAPTIC INPUT-INDUCED INCREASE IN INTRANEURONAL CA2-RATS( IN THE MEDIAL VESTIBULAR NUCLEUS OF YOUNG)

In the medial vestibular nucleus (MVN), an input-dependent influx of C a2+ into neurons through N-methyl-D-aspartate (NMDA) receptor-linked c hannels and/or voltage-dependent Ca2+ channels is suggested as underly ing certain mechanisms of plasticity of the vestibular system. To see whether there is an increase in intracellular Ca2+ induced by afferent synaptic inputs to MVN neurons, we measured changes in [Ca(2+])i with microfluorometry using a Ca2+ indicator, rhod-2, following electrical stimulation of ipsilateral vestibular afferents and commissural fiber s in slice preparations of the brainstem of young rats (4-7 days postn atal). Single shock stimulation of ipsilateral afferents or commissura l fibers induced an increase in fluorescence intensity lasting for sev eral seconds. An application of 2-amino-5-phosphonovaleric acid (APV), an antagonist of NMDA receptors, almost completely blocked this stimu lus-induced rise in fluorescence intensity. Nifedipine, an L-type Ca2 channel blocker, also reduced the stimulus-induced rise in fluorescen ce intensity to 44-51% of the control value. These results suggest tha t synaptic inputs from the afferent and commissural pathways induce an influx of Ca2+ into MVN neurons due, at least in part, to the activat ion of NMDA receptors and the subsequent operation of L-type Ca2+ chan nels in young rats.