Primary somatosensory cortex modulation of tactile responses in nucleus gracilis cells of rats

Abstract

Corticofugal influences from the primary somatosensory cortex to the gracilis nuclei were studied with single unit recordings performed in urethane-anaesthetized rats. Two types of neurons were identified: low firing rate (LF) neurons, which could be activated antidromically by medial lemniscus stimulation; and high firing rate (HF) neurons. The effects of electrically stimulating the contralateral primary somatosensory cortex were studied in two situations: when the stimulated cortical area and specific gracilis cells had overlapping receptive fields and when the receptive fields of the cells and primary somatosensory cortex did not overlap. Cortical stimulation facilitated cortical and tactile responses in most gracilis neurons (68% and 58% for LF and HF neurons, respectively) with overlapping receptive fields. When receptive fields were different, cortical stimulation inhibited tactile response in most LF neurons (58%) and some HF neurons (20%). Trains of cortical shocks during sensory stimulation demonstrated that the facilitatory and inhibitory effects outlasted the stimulation period by 5 min. The facilitatory effect was decreased by iontophoretic application of the N-methyl-D-aspartate (NMDA) receptor antagonist APV (50 mm). However, APV did not modify the intensity of the tactile response inhibition in cells with nonoverlapping receptive fields, although, its duration was decreased (<5 min). Iontophoretic application of the gamma-aminobutyric acid (GABA)(A) antagonist bicuculline (20 mm) blocked the cortically evoked inhibition in cells with nonoverlapping receptive fields. The results indicate that the somatosensory cortex precisely controls somatosensory transmission throughout the gracilis nucleus by means of NMDA and GABA(A) receptor activation.

Corticofugal Modulation of the Tactile Response Coherence of Projecting Neurons in the Gracilis Nucleus

“…A selective facilitating effect of the corticofugal feedback has been previously described in the somatosensory system (Canedo and Aguilar 2000;Nuñez 1998, 2004) and in other sensory pathways (see e.g., Jen et al 2002;Sillito et al 1994;Yan and Suga 1996). The facilitating effect is due to the activation of NMDA receptors in gracile neurons (Malmierca and Nuñez 2004;Nuñez and Buño 2001). This cortical selective feedback, called "egocentric selection" by Jen et al (2002), may play a pivotal role in gating the sensory information that reaches the thalamus and then the cortex.…”

“…Besides, in vitro experiments have shown that activation of corticofugal fibers regulates both incoming sensory signals through the dorsal column and corticofugal inputs (homo-and heterosynaptic facilitation; Buño 1999, 2001). N-Methyl-D-aspartate (NMDA) receptors are activated by glutamate released from corticofugal terminals and induce Ca 2ϩ inflow through voltage-gated channels at DCN neurons, triggering these facilitating processes (Malmierca and Nuñez 2004;Buño 1999, 2001). Although the synaptic mechanisms of the corticofugal modulation on DCN neurons have been previously studied, the dynamic changes of this corticofugal action have not yet been well established, mainly due to the lack of adequate tools for analysis of the temporal evolution of sensory responses.…”

Corticofugal Output from the Primary Somatosensory Cortex Selectively Modulates Innocuous and Noxious Inputs in the Rat Spinothalamic System

“…Conversely, the consistent facilitatory effects on brushing-evoked responses, S1 microinjections of a GABA A agonist, the lack of effects of a GABA A antagonist, and the mixed effects of DLH argue for the existence of a different, predominant inhibitory, corticofugal mechanism elicited by innocuous inputs. The inhibition of corticofugal output by muscimol may affect the lateral inhibition through monosynaptic glutamatergic inputs to the NRT from layer VI corticothalamic cells (Steriade, 2001;Pinault, 2004), pyramidal influences acting on the gracilis nuclei (Mariño et al, 1999;Malmierca and Nuñez, 2004), or the spinal cord. An increase in tactile responses indicates that such peripheral stimuli are normally depressed by the NRT or pyramidal influences.…”

Corticofugal Modulation of Initial Sound Processing in the Brain

“…Many studies in the visual, auditory and somatosensory systems have unveiled that sensory cortices differentially modulate subcortical neuron activities in a highly specific manner (Sillito et al, 1994;Yan and Suga, 1996;Zhang et al, 1997;Jen, 2000, 2007;Yan and Ehret, 2002;Malmierca and Nuñez, 2004;Castellanos et al, 2007;Ma and Suga, 2007). In the auditory system, focal cortical activation enhances the thalamic and midbrain processing of auditory information that is encoded by activated cortical neurons .…”