Morphological heterogeneity among corticogeniculate neurons in ferrets: quantification and comparison with a previous report in macaque monkeys.

The corticogeniculate (CG) pathway links the visual cortex with the lateral geniculate nucleus (LGN) of the thalamus and is the first feedback connection in the mammalian visual system. Whether functional connections between CG neurons and LGN relay neurons obey or ignore the separation of feedforward visual signals into parallel processing streams is not known. Accordingly, there is some debate about whether CG neurons are morphologically heterogeneous or homogenous. Here we characterized the morphology of CG neurons in the ferret, a visual carnivore with distinct feedforward parallel processing streams, and compared the morphology of ferret CG neurons with CG neuronal morphology previously described in macaque monkeys [Briggs et al. (2016) Neuron, 90, 388]. We used a G-deleted rabies virus as a retrograde tracer to label CG neurons in adult ferrets. We then reconstructed complete dendritic morphologies for a large sample of virus-labeled CG neurons. Quantification of CG morphology revealed three distinct CG neuronal subtypes with striking similarities to the CG neuronal subtypes observed in macaques. These findings suggest that CG neurons may be morphologically diverse in a variety of highly visual mammals in which feedforward visual pathways are organized into parallel processing streams. Accordingly, these results provide support for the notion that CG feedback is functionally parallel stream-specific in ferrets and macaques.