Effects of putative transmitters on afferent cochlear transmission

Putative transmitters and related substances were perfused through the guinea pig scala tympani while monitoring the compound action potential of the cochlear nerve (AP) and the cochlear microphonic potential. Various substances were then ranked according to their ability to reduce the AP. The more active compounds ranked: methysergide (1 mM) greater than ATP (10 mM) = tyramine (10 mM) greater than salicylate (10 mM) greater than bicuculline (10 mM) greater than asparate (10 mM) greater than glutamate (10 mM) greater than citrate (10 mM) greater than dextrose (100 mM) greater than glycine (100 mM) greater than GABA (100 mM) greater than prostaglandin E2 (1 mM) greater than serotonin (10 mM). The activity of substances at 100 mM indicates a physical, osmotic change in the cochlear structure. Activity at 10 mM and 1 mM indicated afferent transmitter-like activity for the putative transmitters and interference with the endogenous transmitter for related substances. It is concluded that several substances can be eliminated as afferent transmitter candidates, while others warrant further examination.