ONTOGENY OF GLUTAMATE AND GAMMA-AMINOBUTYRIC-ACID RELEASE IN THE HIPPOCAMPUS OF THE GUINEA-PIG

The ontogeny of the L-glutamate (GLU) and gamma-aminobutyric acid (GAB A) neuronal systems in the guinea pig hippocampus was investigated wit h respect to tissue amino acid content, and spontaneous and K+-stimula ted release of GLU and GABA. Transverse hippocampal slices were prepar ed from the guinea pig fetus at day 45 (brain growth spurt), 55 and 63 of gestation (term, about 68 days), from the 5-days-old neonate, and from the young adult. GLU and GABA release was determined as efflux fr om hippocampal slices into Krebs'-bicarbonate medium using a dynamic, submerged, superfusion apparatus. Hippocampal GLU content decreased du ring development, whereas GABA content was constant for all the ages i nvestigated. The magnitude of spontaneous GLU efflux decreased during development; there was no measurable spontaneous GABA efflux. The K+ c oncentration-GLU efflux response curve was bell-shaped for the fetus a t the three selected gestational ages, and was curvilinear for the neo nate and adult. The apparent EC75 of K+-stimulated GLU efflux was high er for the neonate and adult compared with the fetus. In contrast, the K+ concentration-GABA efflux response curve was curvilinear, and the apparent EC75 of K+ was similar for all the ages investigated. K+-stim ulated efflux of GLU and GABA was Ca++ dependent, but this was not the case for spontaneous GLU efflux. These data indicate that, in the gui nea pig hippocampus, the GLU neuronal system is developing throughout gestation, whereas the GABA neuronal system appears to mature before t he brain growth spurt.