Putrescine as an important source of GABA in the postnatal rat subventricular zone

The subventricular zone (SVZ) is a neurogenic region that continually gives rise to olfactory bulb (OB) GABAergic interneurons in mammals. The newly generated neuroblasts already express GABA while migrating to this structure along the rostral migratory stream (RMS). Here, we investigate in early postnatal rat if SVZ/RMS cells undertake the same synthetic pathway by which GABA is produced in differentiated neurons, i.e. the decarboxylation of glutamate by the glutamic acid decarboxylase (GAD), or, if an alternative pathway, the conversion of putrescine into GABA, also contributes to GABA synthesis. We show here that GAD immunoreactivity is not significantly detectable within the SVZ/RMS. However, strong immunolabeling is found within the OB. Nevertheless, low GAD enzymatic activity (as compared with OB) is detected in the SVZ/RMS. SVZ/RMS explants convert approximately 30% of all captured radiolabeled putrescine into GABA in vitro, showing that this pathway is important for GABA synthesis in the SVZ. We also show that SVZ/RMS, OB and choroid plexus explants are able to synthesize putrescine, as analyzed by ornithine decarboxylase (ODC) activity, providing neuroblasts with different sources of putrescine for GABA production. During early stages of neuroblast differentiation, in which neurotransmitter choice may still be undefined, an alternative pathway for GABA synthesis guarantees the production of GABA, necessary for neuroblast proliferation and migration in the SVZ/RMS.