Specific glycine uptake by identified neurons of Aplysia californica. I. Autoradiography

The identified giant neurons R3-R14 in the Aplysia parietovisceral ganglion (PVG) have a rapid, Na⁺-dependent and Hg²⁺-sensitive uptake system for glycine not found in neighboring neurons. In autoradiographs of PVG incubated in [³H]glycine (glutaraldehyde fix), the cytoplasms of R3-R14 have 3-4 times more silver grains (No./100 sq.μm) than other neurons. The glycine uptake system in R3-R14 is selective (alanine, serine, leucine, and proline are taken up equally by all neurons) and is unaffected by reserpine and anisomycin. Neurons R3-R14 contain 2 times less label when ganglia are fixed in formaldehyde than when glutaraldehyde is used as a fixative. Because formaldehyde fixes free amino acids poorly, much of the glycine taken up by R3-R14 is, therefore, not incorporated into protein. In autoradiographs of PVG incubated in [³H]glycine, silver grains are distributed randomly throughout the cytoplasm and nucleus of R3-R14; no association of the grains with the dense core granules characteristic of these neurons or other cellular components was found. In contrast, grains in the neurosecretory 'bag cells' of the PVG were clustered in numerous discrete areas of the cytoplasm (Golgi complex areas) and the nucleus was only sparsely labeled. The existence of a rapid and selective glycine uptake system in R3-R14, together with their high endogenous glycine concentrations, suggests that glycine may be a neurotransmitter in these neurons.