Descending corticofugal neurons in layer 5 of rabbit S1: evidence for potent corticocortical, but not thalamocortical, input.

Extracellular recordings were obtained from descending corticofugal neurons of layer 5 (CF-5 neurons) in primary somatosensory cortex (S1) of awake rabbits. These cells were identified by antidromic activation via stimulation sites in ventrobasal (VB) thalamus. Recordings were also obtained from putative GABA-ergic interneurons (suspected inhibitory interneurons, SINs) located in the same microelectrode penetrations, and in close proximity (+/- 300 microns) to the CF-5 neurons. In some experiments, the above populations were recorded simultaneously with neurons in the topographically aligned VB thalamic barreloid. Each of several experimental strategies failed to reveal evidence of monosynaptic thalamic input to CF-5 neurons, but revealed a clear monosynaptic input to neighboring SINs: (1) whereas CF-5 neurons responded at very long synaptic latencies to intense electrical stimulation of VB thalamus, neighboring SINs responded at short latencies; (2) whereas cross-correlations between CF-5 neurons and topographically aligned VB neurons failed to show significant peaks indicative of monosynaptic VB input, neighboring SINs did show such peaks; and (3) whereas CF-5 neurons were unresponsive to microstimulation of topographically aligned VB thalamic barreloids, neighboring SINs were very responsive to such stimulation. Both CF-5 neurons and neighboring SINs responded to electrical stimulation of the corpus callosum with a robust, short-latency synaptic response. This finding demonstrates that CF-5 neurons are capable of vigorous, short-latency responses to excitatory synaptic input. These data suggest considerable specificity in the thalamocortical connectivity of subpopulations within layer 5, and support the notion that CF-5 neurons are dominated by corticocortical rather than thalamocortical input.