SIZE-RELATED PROPERTIES OF VESTIBULAR AFFERENT-FIBERS IN THE FROG - DIFFERENTIAL SYNAPTIC ACTIVATION OF N-METHYL-D-ASPARTATE AND NON-N-METHYL-D-ASPARTATE RECEPTORS

Vestibular afferent fibers exhibit a specific, cell size-related uptak e of aspartate and glycine [Straka H, et al. (1995) Neuroscience 70, 6 85-696]. A similar, size-related coexistence of glycine and glutamate had been reported earlier for these fibers [Reichenberger I. and Dieri nger N. (1994) J. comp. Neurol. 349, 603-614]. Taken together, these r esults suggest a size-related co-release of both amino acids and the a ctivation of different glutamate receptors in second order vestibular neurons. To test this hypothesis we stimulated the VIIIth nerve and re corded the responses of central vestibular neurons in the isolated bra instem of frogs before and during the application of the N-methyl-D-as partate antagonists (7-chlorokynurenic acid and D-(-)-2-amino-5-phosph onovaleric acid). The presence of either one of these antagonists prov oked a dose-dependent and Mg2+-sensitive partial block of the monosyna ptic responses recorded extra- or intracellularly. This implies that a fferent-evoked responses in central vestibular neurons are composed of N-methyl-D-aspartate and non-N-methyl-D-aspartate receptor-mediated c omponents. In most of the intracellularly recorded neurons (21 out of 24) the relative amplitude of the N-methyl-D-aspartate receptor-mediat ed component decreased with an increase in stimulus intensity. Since e lectric stimulation recruits thick afferents at a lower current intens ity than thin afferent fibers, our results imply a co-activation of N- methyl-D-aspartate and non-N-methyl-D-aspartate receptors by thick ves tibular afferents. At a given stimulus intensity the amplitude of the N-methyl-D-aspartate receptor-mediated component differed between neur ons. The results of this study extend the list of known anatomical, hi stochemical and physiological properties that distinguish thick from t hinner vestibular afferent fibers. In spite of this detailed knowledge , however, the physiological role of thick vestibular afferents is so far unclear. The novel concept of a size-related co-activation of N-me thyl-D-aspartate and non-N-methyl-D-aspartate receptors by vestibular afferent fibers establishes the basis for more specific physiological hypotheses.