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Objective To investigate the effect of D-AP5 (D-2-amino-5-phosphonopentanoate, a specific NMDA-antagonist) on the increase of intracellular free Ca2+ concentration (［Ca2+］i) induced by glutamate in isolated cochlear inner hair cells (IHCs), and to detect the autoreceptors of the IHC membrane. Methods When a laser scanning confocal microscope (LSCM) was used, the exogenous glutamate (Glu)-induced changes in ［Ca2+］i of isolated IHCs and OHCs of guinea pig cochlea were observed with fluo-3, a fluorescent probe for ［Ca2+］i. After D-AP5 or CNQX (6--cyano--7--nitroguinoxaline--2, 3--dione, a specific AMPA- antagonist) was administrated, the exogenous glutamate (Glu)-induced changes in ［Ca2+］i of isolated IHCs were recorded. Results In the presence of a low concentration Glu (3.85?μmol/L), there was an increase of ［Ca2+］i in IHCs, whereas there was no change in OHCs. When 50?μmol/L of D-AP5 was administrated in advance, Glu did not induce a corresponding increase in ［Ca2+］i in IHCs, and 50?μmol/L of CNQX did not completely block the increase of ［Ca2+］i in IHCs. Conclusions These results suggest that the autoreceptors existing in the IHC membrane are mainly of NMDA type, while there are relatively few AMPA receptors. Exogenous Glu is capable of increasing ［Ca2+］i in IHCs by acting on the NMDA autoreceptor of IHCs in a positive feedback manner.
