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 ra bbits. These cells were identified by antidromic activation via stimulation sites in ventrobasal(VB) thalamus. Recordings were also obtained from puta tive GABA-ergic interneurons (suspected inhibitory interneurons, SINs) loca ted in the same microelectrode penetrations, and in close proximity (+/-300 mu m) to the CF-5 neurons. In some experiments, the above populations were recorded simultaneously with neurons in the topographically aligned VB tha lamic barreloid. Each of several experimental strategies failed to reveal e vidence of monosynaptic thalamic input to CF-5 neurons, but revealed a clea r monosynaptic input to neighboring SINs: (1) whereas CF-5 neurons responde d at very long synaptic latencies to intense electrical stimulation of VB t halamus, neighboring SINs responded at short latencies; (2) whereas cross-c orrelations between CF-5 neurons and topographically aligned VB neurons fai led to show significant peaks indicative of monosynaptic VB input, neighbor ing SINs did show such peaks; and (3) whereas CF-5 neurons were unresponsiv e to microstimulation of topographically aligned VB thalamic barreloids, ne ighboring SINs were very responsive to such stimulation. Both CF-5 neurons and neighboring SINs responded to electrical stimulation of the corpus call osum with a robust, short-latency synaptic response. This finding demonstra tes that CF-5 neurons are capable of vigorous, short-latency responses to e xcitatory synaptic input. These data suggest considerable specificity in th e thalamocortical connectivity of subpopulations within layer 5, and suppor t the notion that CF-5 neurons are dominated by corticocortical rather than thalamocortical input.