Cells in the anteroventral cochlear nucleus are insensitive to l-glutamate and l-aspartate; excitatory synaptic responses are not blocked by d-α-aminoadipate

Abstract

Auditory nerve fibers transmit signals from the cochlea to the 3 regions of the cochlear nuclear complex, the anteroventral (AVCN), posteroventral, and dorsal cochlear nucleus in the brainstem. It has been suggested that the amino acids l-aspartate and l-glutamate might serve as a neurotransmitter in auditory nerve fibers 6–10,13,17–20. The sensitivity of postsynaptic cells in the cochlear nuclei to these amino acids has been tested by iontophoretic techniques 4,9,10. One difficulty with these experiments is that responses were recorded only extracellularly. A second difficulty is that the concentrations needed to affect cells could not be determined. To avoid these difficulties a brain slice preparation was used to test the sensitivity of cells in the AVCN to bath applied l-glutamate and l-aspartate at concentrations ranging from 10 −5 to 10 −2 M. All cells that were tested in the cochlear nuclear complex were insensitive at all concentrations used; the resting potentials and the input resistances remained unchanged and the synaptic responses to electrical stimulation of the auditory nerve were not desensitized. All cells that were tested in the hippocampus, however, depolarized in the presence of 10 −4 M l-glutamate and l-aspartate. The synaptic responses to electrical stimulation of the auditory nerve were not blocked by d-α-aminoadipate, an amino acid which has been shown to block excitation of cells in the cochlear nuclei by auditory nerve fibers 10. The results are not consistent with l-glutamate and l-aspartate serving as neurotransmitters in the AVCN.