Anatomical correlates of driving and modulatory glutamatergic projections in the primate visual system

Abstract

Glutamatergic projections in the mammalian brain can be segregated into two major types based on their anatomy, connectivity, and physiological response properties. These two types are named drivers, which directly modify or ‘drive’ activity in their postsynaptic targets, or modulators, which produce slight fluctuations in the postsynaptic activity of their target cells. Driving and modulatory projections utilize distinct presynaptic and postsynaptic mechanisms to affect glutamatergic transmission between neurons, and recent studies have identified two presynaptic proteins that may contribute to the functional properties of these projection types. These proteins are vesicular glutamate transporters 1 and 2 (VGLUT1 and VGLUT2), which regulate the sequestration and release of glutamate from presynaptic vesicles in excitatory neurons. We examined the distribution patterns of VGLUT1 and VGLUT2 across known driving and modulatory projections in the mammalian visual system to determine whether these proteins differentially identify each type of projection. We find that VGLUT2 predominates in driving projections while VGLUT1 predominates in modulatory projections. However, some visual projections appear to use both VGLUTs for a greater range of neural responses. The expression patterns of VGLUT1 and VGLUT2 across primates and closely related nonprimates also revealed the evolution and specialization of some visual projections within the primate lineage. These findings have informed our hypotheses on the relative contributions of single glutamatergic projections to interconnected neural networks that process visual information in the primate brain.