Preferential release of newly synthesized [(3)H]GABA from striatal slices to preloaded [(3)H]GABA

The release of [(3)H]GABA which is newly synthesized from [(3)H]l-glutamic acid (GLU) has been examined using striatal slices obtained from the rat brain. It was found that 8-10% of [(3)H]GLU transported was converted to [(3)H]GABA during the incubation of striatal slices in the presence of nipecotic acid (5 x 10(?5) M). Nipecotic acid was added to the medium in order to prevent possible reuptake of [(3)H]GABA released during its synthesis, and it was found to have no significant effect on the formation of [(3)H]GABA from [(3)H]GLU as well as on the uptake of [(3)H]GLU. The application of high potassium (60 mM) stimulation exhibited a significant enhancement of the release of this newly synthesized [(3)H]GABA in a Ca(2+) dependent manner. Kinetic analysis revealed that the evoked release of newly synthesized [(3)H]GABA was approximately two times greater than that of previously-loaded [(3)H]GABA, whereas no significant difference was observed in the spontaneous release. An immobilization stress in water failed to affect the release of newly synthesized [(3)H]GABA from striatal slices despite the occurrence of a significant enhancement of GABA formation in this structure. These results suggest that newly synthesized GABA may be preferentially released from its nerve terminals in response to the excitation of neurons at least in the striatum as compared with previously accumulated GABA.