Bidirectional axonal transport of free glycine in identified neurons R3–R14 of Aplysia

Abstract

The axonal transport of ³H‐amino acids was studied in the axons of identified neurons R3–R14 in the parietovisceral ganglion (PVG) of the mollusc Aplysia. The PVG was incubated (3–24 hr) in media containing physiological concentrations of single ³H‐amino acids while the isolated nerve was superfused with plain or chemically altered media. The nerve was then sliced into sequential segments for biochemical analyses or fixed for autoradiography. ³H‐glycine was transported at 70 mm/day in 6X greater quantities than other amino acids which were transported at 80% of the label transported into the nerve remained as free glycine, comigrating with glycine in thin‐layer chromatographs. In autoradiographs of sections 4 mm from the ganglion‐nerve barrier, >50% of the silver grains were over R3–R14 axons which occupy 2+, by vinblastine and Nocodazole, and by low Ca²⁺ media. Autoradiographs of vinblastine‐treated nerves showed a drastic reduction in label over R3–R14 and other axons. Label was also transported retrogradely; this transport rate was similar to the orthograde rate, but 5–10 times less label moved retrogradely. Autoradiographs showed that the retrograde label was localized to R3–R14 axons. This report clearly demonstrates the rapid, selective, and bidirectional transport of a free amino acid and provides further evidence that glycine may be used as a neurochemical messenger by neurons R3–R14.