Differences in the distribution of responses to ATP and acetylcholine between outer hair cells of rat and guinea pig.

Adenosine 5' triphosphate (ATP) and acetylcholine (ACh) are neurotransmitters (ACh) and/or modulators (ATP) in the mammalian cochlea. In guinea pig, it appears that both neurotransmitters have a similar response distribution, with larger responses being evoked by the ligands in short hair cells compared to long hair cells (e.g., Chen et al., 1995b. Noise exposure alters the response of outer hair cells to ATP. Hear. Res. 88, 215-221.; Erostegui et al., 1994. In vitro pharmacologic characterization of a cholinergic receptor on outer hair cells. Hear. Res. 74, 135 147). The purpose of the present study was to test whether the distribution of responses to ACh and ATP in the OHCs of rat is the same as guinea pig. The ligand-induced current was monitored using the whole-cell configuration of the patch-clamp technique. Results show that in guinea pig OHCs, extracellular application of 100 microM ATP induced a current response in a majority of the same cells that responded to the application of 100 microM ACh. In contrast in rat OHCs, 100 microM ATP did not induce a current in the majority of cells that responded to the application of 100 microM ACh. N-methyl-glucamine (NMG+) substituted for K+ in the pipette solution failed to unmask an ATP-evoked inward current in rat OHCs. In addition, no response was produced in rat or guinea pig OHCs by adenosine, adenosine 5'-monophosphate (AMP) or adenosine 5'-diphosphate (ADP) at 100 microM. Results suggest that in guinea pig ACh-gated channels are present on most of the same OHCs that have ATP-gated ion channels, whereas in rat ACh-gated ion channels are present without ATP-gated channels on some OHCs.