Pattern of Driver-Like Input onto Neurons of the Mouse Ventral Lateral Geniculate Nucleus

The ventral lateral geniculate nucleus (vLGN) is a retinorecipient region of thalamus that contributes to a num-ber of complex visual behaviors. Retinal axons that target vLGN terminate exclusively in the external subdivi-sion (vLGNe), which is also transcriptionally and cytoarchitectonically distinct from the internal subdivision (vLGNi). While recent studies shed light on the cell types and efferent projections of vLGNe and vLGNi, we have a crude understanding of the source and nature of the excitatory inputs driving postsynaptic activity in these regions. Here, we address this by conducting in vitro whole-cell recordings in acutely prepared thalamic slices and using electrical and optical stimulation techniques to examine the postsynaptic excitatory activity evoked by the activation of retinal or cortical layer V input onto neurons in vLGNe and vLGNi. Activation of ret-inal afferents by electrical stimulation of optic tract or optical stimulation of retinal terminals resulted in robust driver-like excitatory activity in vLGNe. Optical activation of corticothalamic terminals from layer V resulted in similar driver-like activity in both vLGNe and vLGNi. Using a dual-color optogenetic approach, we found that many vLGNe neurons received convergent input from these two sources. Both individual pathways displayed similar driver-like properties, with corticothalamic stimulation leading to a stronger form of synaptic depression than retinogeniculate stimulation. We found no evidence of convergence in vLGNi, with neurons only respond-ing to corticothalamic stimulation. These data provide insight into the influence of excitatory inputs to vLGN and reveal that only neurons in vLGNe receive convergent input from both sources.Significance StatementThe ventral lateral geniculate nucleus is traditionally thought of as a thalamic visual recipient structure. However, recent studies reveal its divergent output to a variety of nonvisual subcortical structures helps control an array of light-mediated defensive and mood-related behaviors. Despite this knowledge, we still lack an understanding of where and how inputs to this nucleus drive activity. Here we show that vLGN re-ceives strong, driver-like excitatory input from two sources: the retina and cortical layer V. The external sub-division receives convergent input from both sources, whereas the internal division receives input only from layer V. Such an arrangement has important implications for understanding the functional organization of vLGN and its role in integrating vision with internal behavioral states.