Preferential amino acid uptake identifies Type II spiral ganglion neurons in the gerbil

The localization of 3H-labeled amino acids was compared by light and electron microscopic autoradiography in the spiral ganglion of the gerbil, following in vivo intracochlear incubations. Following incubations with taurine, a population of heavily labeled neurons could be distinguished from lightly labeled neurons. The heavily labeled population comprised 5-6% of spiral ganglion neurons, and included the least myelinated cells. Ultrastructurally, the heavily labeled neurons were characterized by a loosely coiled Schwann cell sheath covering the cell body, and the presence of abundant cytoplasmic microfilaments. The unlabeled cells showed a typical perikaryal myelin sheath and cytoplasm rich in rough endoplasmic reticulum. It is concluded that the spiral ganglion of the gerbil contains both Type I and Type II neurons, and that Type II neurons preferentially incorporate the amino acid taurine. Type II neurons are therefore biochemically as well as morphologically distinct from Type I neurons.