A microiontophoretic study of the role of excitatory amino acids at the afferent synapses of mammalian inner hair cells.

The results presented indicate that L-glutamate is involved in mediating excitatory afferent neuro-transmission in the mammalian cochlea. Glutamate applied directly into the subsynaptic inner hair cell region of the adult guinea pig with the aid of microiontophoretic techniques evoked neuronal firing. By using competitive antagonists for glutamate receptors, a further study was carried out in order to identify possible receptor types. Such included N-methyl-D-aspartate (NMDA), kainate and quisqualate receptors, which are defined by selective agonist action. The glutamate-induced cochlear firing rate was antagonized by D-2-amino-7-phosphonoheptanoate (AP-7), suggesting that the receptor involved is of the NMDA type. Further studies with glutamic acid diethylester (GDEE) showed that glutamate-induced activity could also be blocked by this substance. Since GDEE depresses responses to quisqualate, non-NMDA receptors also have to be taken into consideration.