Effect of sound stimulation at several levels on concentrations of primary amines, including neurotransmitter candidates, in perilymph of the guinea pig inner ear.

Perilymph, which bathes the sensory cells of the cochlea, was collected from guinea pigs exposed to noise and analyzed via two cation-exchange HPLC procedures with fluorescence detection, resolving 51 and 81 primary-amine compounds, respectively, at a sensitivity limit of 0.1 pmol relative to leucine. During a first period, each animal was either exposed to noise at 80, 90, or 115 decibels sound-pressure level or maintained in silence (controls), and during a second period, the same animal was maintained in silence. Perilymph was collected during both periods, and perilymphatic components were compared, within animals and across animals, for several levels of sound stimulation. A gamma-aminobutyric acid-like component was elevated in the first period in proportion to stimulus intensity by the various methods of comparison, suggesting an auditory-neurotransmitter role for this component. Aspartic acid was elevated in the second period, 2-3.5 h after onset of sound stimulation, compatible with the release of aspartic acid from central auditory synapses. In addition, a methionine-enkephalin-like component, distinct from leucine-enkephalin, was detected in perilymph from control animals and was elevated in response to noise at 115 decibels. Regression coefficients, determined for the relation between sound intensity and first-period concentrations or the difference between first and second-period concentrations, indicated zero linear regression at p = 0.05 for glutamic acid, aspartic acid, glycine, taurine, and 39 other perilymphatic components, consistent with the hypothesis that these compounds are unlikely to be peripheral auditory neurotransmitters.