[3H]glycine Uptake in Rat Hippocampus: Kinetic Analysis and Autoradiographic Localization

The uptake of [3H]glycine by rat hippocampal tissue in vitro has been characterized. [3H]Glycine transport into a crude synaptosomal (P2) fraction was resolved into two components. The high affinity component (Km = 21 +/- 5.4 microM, Vmax = 490 +/- 234 pmol/3 min/mg protein) was almost completely sodium dependent whereas the low affinity component (Km = 2.214 +/- 0.958 mM, Vmax = 13.9 +/- 0.5 nmol/3 min/mg protein) was partially dependent on sodium ions. Amongst a range of amino acids, only L-serine, L-glutamate, L-proline, histidine and glycine itself inhibited [3H]glycine uptake at 1 mM. The autoradiographic localization of [3H]glycine uptake in rat hippocampal slices revealed a general pattern of labeling in dendritic regions with a sparing of pyramidal and granule neuron cell bodies. However, a laminar distribution was apparent since the amino acid was preferentially accumulated in the hilus of the dentate gyrus, in the stratum lacunosum-moleculare, in the alveus and in the molecular layer of the lower blade of the dentate gyrus. A diffuse pattern of accumulation was apparent in these areas along with dense clusters of silver grains. The clusters were associated with small cell bodies and might represent glycine uptake into astrocytes. Glycine transport mechanisms may influence the modulatory effects of this amino acid on N-methyl-D-aspartate receptor-mediated neurotransmission in the hippocampus.