Heterogeneity of glutamatergic and GABAergic release machineries in cerebral cortex

We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the proteins they express, by studying the degree of co-localization of synapsin (SYN) I and II, synaptophysin (SYP) I and II, SNAP-25 and SNAP-23 in VGLUT1-, VGLUT2- and VGAT-positive (+) puncta in the rat cerebral cortex. Co-localization studies showed that SYNI and II were expressed in ~90% of VGLUT1+, ~30% of VGLUT2+ and 30-50% of VGAT+ puncta; SYPI was expressed in ~95% of VGLUT1+, 30% of VGLUT2+, and 45% of VGAT+ puncta; SYPII in ~7% of VGLUT1+, 3% of VGLUT2+, and 20% of VGAT+ puncta; SNAP-25 in ~94% of VGLUT1+, 5% of VGLUT2+, and 1% of VGAT+ puncta, and SNAP-23 in ~3% of VGLUT1+, 86% of VGLUT2+, and 22% of VGAT+ puncta. Since SYPI, which is considered ubiquitous, was expressed in about half of GABAergic axon terminals, we studied its localization electron microscopically and in immunoisolated synaptic vesicles: these studies showed that ~30% of axon terminals forming symmetric synapses were SYPI-negative, and that immunoisolated VGAT-positive synaptic vesicles were relatively depleted of SYPI as compared to VGLUT1+ vesicles. Overall, the present investigation shows that in the cerebral cortex of rats distinct presynaptic proteins involved in neurotransmitter release are differentially expressed in GABAergic and in the two major types of glutamatergic axon terminals in the cerebral cortex of rats.