Genetic Loss of VGLUT1 Alters Histogenesis of Retinal Glutamatergic Cells and Reveals Dynamic Expression of VGLUT2 in Cones

<b>Background/Objectives</b>: Glutamatergic neurotransmission is essential for the normal functioning of the retina. Photoreceptor to bipolar and bipolar to ganglion cell signaling is mediated by L-glutamate, which is stored in and released from vesicular glutamate transporter 1 (VGLUT1) containing synaptic vesicles. VGLUT1 is expressed postnatally, P2 onwards, and is required for the glutamatergic retinal wave observed between P10 and P12 in the developing mouse retina. P9-P13 postnatal age is critical for retinal development as VGLUT1 expressing ribbon synapses activate in the outer and inner plexiform layers, and rod/cone mediated visual signaling commences in that period. Although it has been hypothesized that glutamatergic extrinsic signaling drives cell cycle exit and initiates cellular differentiation in the developing retina, it is not clear whether intracellular, synaptic, or extrasynaptic vesicular glutamate release contributes to this process. Recent studies have attempted to decipher VGLUT's role in retinal development. Here, we investigate the potential effect of genetic loss of VGLUT1 on early postnatal histogenesis and development of retinal neural circuitry. <b>Methods</b>: We employed immunohistochemistry and electrophysiology to ascertain the density of glutamatergic, cholinergic, and dopaminergic cells, spontaneous retinal activity, and light responses in VGLUT1 null retina, and contrasted them with wildtype (WT) and melanopsin null retina. <b>Results</b>: We have demonstrated here that VGLUT1 null retina shows signs of age dependent retinal degeneration, similar to other transgenic mice models with dysfunctional photoreceptor to bipolar cell synapses. The loss of VGLUT1 specifically alters glutamatergic cell density and morphological maturation of retinal ganglion cells. Moreover, VGLUT2 expression is lost in the majority of VGLUT2 cones in the absence of VGLUT1 coexpression, except when VGLUT2 coexpresses transiently with VGLUT3 in these cones, or when VGLUT1 null mice are dark reared. <b>Conclusions</b>: We present the first evidence that synaptic or extrasynaptic postnatal glutamate release from VGLUT1 containing vesicles impacts histogenesis of glutamatergic cells, pruning of retinal ganglion cell dendrites and VGLUT2 expression in cones.