Neurons of origin of striatonigral axons in the cat: connectivity and Golgi markers of somatodendritic architecture.

We identified adult cat striatonigral neurons and characterized their somatodendritic architecture by single- and double-labeling with connectivity and Golgi markers. Light and electron microscopic observations demonstrated two important structural variations among striatofugal neurons. First, striatonigral axons originate from multiple types of neurons. The principal striatonigral neurons are frequent throughout the caudate nucleus and putamen, have medium spherical or ovoidal cell bodies and form spine-laden distal dendrites. The 'accessory' striatonigral neurons are sparse in limited regions of the caudate nucleus and putamen, have large fusiform cell bodies and lack spinous processes. Second, the medium spiny striatonigral neurons exhibit a range of anatomical specializations. In some cases, these cells have folded nuclear envelopes. Some of these neurons have somatic and/or proximal dendritic as well as distal dendritic spines. We conclude that adult cat striatofugal neurons are morphologically heterogeneous in terms of regional locations, somatodendritic architecture and ultrastructure. These anatomical specializations may affect the origin and processing of synaptic inputs to the neostriatum to provide functionally different routes of neurotransmission through the basal ganglia.