THE FIRST DEVELOPING MIXED SYNAPSES BETWEEN VESTIBULAR SENSORY NEURONS MEDIATE GLUTAMATE CHEMICAL TRANSMISSION

In the present study, the nature of the synaptic transmission responsi ble for a monophasic potential generated by vestibular nerve stimulati on of the principal cells in the chick tangential nucleus was establis hed. This work was performed in slice preparations at the critical emb ryonic age of 15-16 days, the time of first observation of morphologic ally mixed (chemical and electrical) synapses at the axosomatic ending s called spoon endings. The spoon endings are formed by the primary ve stibular fibers with the largest diameters, the colossal vestibular fi bers. This monophasic potential fits the criteria for chemical rather than electrical transmission due to the following responses in most ca ses: (i) the absence of collision between a direct spike initiated by depolarization in the principal cell and a vestibular-evoked action po tential; (ii) failure to follow high frequency stimulation (up to 50 H z); (iii) sensitivity to low calcium solution (0.1 mM). These tests in dicate that strong electrical coupling between spoon endings and princ ipal cells does not prevail at this stage. The recordings were obtaine d from principal cells injected intracellularly with biocytin, allowin g their identification by morphological criteria. The lack of tracer c oupling between the stained principal cells and their innervating vest ibular fibers (n = 17) is consistent with the absence of electrical co upling. Identification of the neurotransmitter involved in this vestib ular response was achieved by bath application of glutamate receptor a ntagonists, DL-2-amino-5-phosphonovaleric acid (40 mu M) and 6-cyano-7 -nitro-quinoxaline-2,3-dione (10 mu M), which blocked transmission rev ersibly. These results suggest that at the onset of formation of these ''mixed'' vestibular synapses, the gap junctions identified morpholog ically are likely not functional, and that the main response of the pr incipal cells to vestibular nerve stimulation is mediated by glutamate .