Differential phasic modulation of short and long latency afferent sensory transmission to single neurons in the ventroposterolateral thalamus in behaving rats

Abstract

Single neurons were recorded in the forepaw area of the ventroposterolateral (VPL) thalamus of awake rats during rest and running behaviors. Locomotor step cycle dependent changes of the transmission of the short (4.0 ± 0.1–10.1 ± 0.3 ms, SEURs) and the long (10.2 ± 0.2–26.0 ± 2.1 ms, LEURs) latency somatic sensory responses were tested by generating post-stimulus time histograms of these neurons' responses to stimulation through electrodes chronically implanted under the skin of the forepaw. The magnitudes of firing during these responses were measured and normalized as percent increases over background firing. Times of footfall were determined through frame-by-frame analyses of video recordings and peri-footfall histograms were generated to differentiate a total of 40 VPL thalamic neurons into two types, footfall responsive ( n = 21) and unresponsive ( n = 19) neurons. Perifootfall gating patterns were determined for both types of cells. The SEURs and the LEURs showed significantly different phasic sensory modulation patterns across the locomotor step cycle. Major difference of the sensory modulations between footfall responsive and footfall unresponsive cells was noted during swing phase of the locomotor step cycle. In footfall responsive cells, the SEURs were suppressed most strongly just after footfall, while the LEURs were tonically suppressed during late stance and swing phases. The SEURs were disinhibited during the swing phase, while the LEURs were disinhibited during the middle stance phase. In footfall unresponsive cells, the LEURs were suppressed more strongly around footfall event than the SEURs were. These results suggest that the short and the long latency sensory responses of footfall responsive and footfall unresponsive VPL thalamic cells may be subject to temporally specific, differential gating processes during locomotion.