Journal of Neuroscience, Vol 13, 768-781, Copyright © 1993 by Society for Neuroscience
Organization of long-range inhibitory connections with rat visual cortex
CT McDonald and A Burkhalter
Department of Neurology, Washington University School of Medicine, St. Louis, Missouri 63110.
We have studied the laminar organization of local long-range inhibitory
connections within rat primary visual cortex (area 17) by combining
retrograde tracing of nerve cell bodies with glutamic acid decarboxylase
immunocytochemistry. While most inhibitory connections are confined to
within 0.4 mm of the injection site, a subset of neurons at the layer 5/6
border provide long-range (> 1 mm) inhibitory connections within area
17. However, other cell layers that contain similar local long-range
horizontal connections, that is, lower layer 2/3, upper layer 5, and lower
layer 6 (Burkhalter and Charles, 1990), show a much more restricted
distribution of inhibitory connections. This suggests that cells at the
layer 5/6 border play a role in the direct inhibition of neurons at a
distant point of the topographic map. Similar double labeling studies
reveal long-range inhibitory connections between visual areas. Following
injections of fluorescent tracers into area 17, in horizontal sections
inhibitory connections can be identified that are up to 8 mm long, linking
the extrastriate subdivisions 18a and 18b with striate cortex. Conversely,
injections of fluorescent tracers into the cytoarchitectonic subdivision
18a reveal local long-range inhibitory connections within 18a, long-range
inhibitory connections between 18a and the cytoarchitectonic subdivision
18b, and inhibitory forward connections from area 17 to 18a. These results
suggest that the communication between different cortical areas can be
influenced by direct inhibitory connections.
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