Corticofugal projections induce long-lasting effects on somatosensory responses in the trigeminal complex of the rat

Abstract

The sensory information flow at subcortical relay stations is controlled by the action of topographic connections from the neocortex. To determinate the functional properties of the somatosensory corticofugal projections to the principal (Pr5) and caudal spinal (Sp5C) trigeminal nuclei, we performed unitary recordings in anesthetized rats. To examine the effect of these cortical projections we used tactile stimulation of the whisker and electrical stimulation of somatosensory cortices. Corticofugal anatomical projections to Pr5 and Sp5C nuclei were detected by using retrograde fluorescent tracers. Neurons projecting exclusively to Pr5 were located in the cingulate cortex while neurons projecting to both Sp5C and Pr5 nuclei were located in the somatosensory and insular cortices (>75% of neurons). Physiological results indicated that primary somatosensory cortex produced a short-lasting facilitating or inhibiting effects (<5 min) of tactile responses in Pr5 nucleus through activation of NMDA glutamatergic or GABAA receptors since effects were blocked by iontophoretically application of APV and bicuculline, respectively. In contrast, stimulation of secondary somatosensory cortex did not affect most of the Pr5 neurons; however both cortices inhibited the nociceptive responses in the Sp5C nucleus through activation of glycinergic or GABAA receptors because effects were blocked by iontophoretically application of strychnine and bicuculline, respectively. These and anatomical results demonstrated that the somatosensory cortices projects to Pr5 nucleus to modulate tactile responses by excitatory and inhibitory actions, while projections to the Sp5C nucleus control nociceptive sensory transmission by only inhibitory effects. Thus, somatosensory cortices may modulate innocuous and noxious inputs simultaneously, contributing to the perception of specifically tactile or painful sensations.

Keywords: caudal spinal trigeminal nucleus; corticofugal projection; insular cortex; nociception; principal trigeminal nucleus; somatosensory cortex.

Figure 1

(A) Schematic drawing of Bregma −9.50 mm Paxinos and Watson Atlas. The gray area represents the summarized location in Pr5 of the FlGo injections in all cases. (B) Microphotograph of a coronal section in animal RE11 showing the injection side in Pr5 (limited by dashed line). (C) Schematic drawing of Bregma −15.7 mm showing the limits of the FB injection in Sp5C represented by gray area. (D) Microphotograph of a coronal section showing the injection site in Sp5C (limited by dashed line) in animal RE8. Calibration toolbar: B: 400 µm, D: 300 µm. Abbreviations: 11N: accessory nerve nucleus, 4V: 4th ventricle, 5M: motor trigeminal nucleus, 5Ma: motor trigeminal nucleus, masseter part , 7n: facial nerve, Amb: ambiguous nucleus, CC: central canal, Cu: cuneate nucleus, cu: cuneatus fasciculus, gr: gracile fasciculus, Gr: gracile nucleus, ml: medial lemniscus, mlf: medial longitudinal fasciculus, mcp: middle cerebellar peduncle, P5: peritrigeminal zone, PDTg: posterodorsal tegmental nucleus, Pr5V: principal sensory trigeminal nucleus, py: pyramidal tract, pyd: pyramidal decussation, rs: rubrospinal tract, RtTg: reticulotegmental nucleus of the pons, s5: sensory root of the trigeminal nerve, scp: superior cerebellar peduncle (brachium conjunctivum), Sol: nucleus of the solitary tract, sp5: spinal trigeminal tract, Su5: supratrigeminal nucleus, m5: motor root of the trigeminal nerve, MdD: medullary reticular nucleus, dorsal part, MPB: medial parabrachial nucleus, VCA: ventral cochlear nucleus, anterior part.

Figure 2

(A) Confocal microscope microphotograph of a coronal section of the brain in animal RE9 at Bregma 2.16 mm stereotaxic coordinates, showing double labeled neurons in S1 and Ins cortices. (B) Microphotograph showing the barrel zone of S1 cortex in animal RE8 where the fluorescence was combined with cytochrome oxidase technique. Asterisk corresponds to the barrel zone, white arrows points the retrograde labeled neurons. Cytochrome oxidase labeled neurons located in the white square, are magnificated in the inset. (C) Microphotograph of S1 cortex in animal RE9 showing the FlGo and FB labeled neurons. Black arrow shows the double labeled neurons. Inset: Confocal microscope detail of a single FlGo labeled neuron (up) and double labeled neuron (down). Calibration toolbar: A: 500 µm, B: 260 µm, inset 200 µm C: 300 µm, inset 50 µm. Abbreviations: Cg: cingulate cortex, Ins: insular cortex, M: motor cortex, S1: primary somatosensory cortex.

Figure 3

(A) Schematic drawings of coronal hemi-sections from rostral to caudal levels through rat brain showing the distribution FLGo (light dots) and FB (dark dots) labeled neurons in different cortical areas. (B) Graphic representation of percentages of single FlGo, single FB and double-labeled neurons in different cortical areas. Abbreviations: 3V: 3rd ventricle, AI: agranular insular area, CC: central canal, Den: dorsal endopiriform nucleus, Cg: cingulate cortex, CM: central medial thalamic nucleus, CPu: caudate putamen (striatum), ic: internal capsule, Ins: insular cortex, LV: lateral ventricle, M: motor cortex, och: optic chiasm, RS: retrosplenial cortex, S1: primary somatosensory cortex , S2: secondary somatosensory cortex, VCl: ventral part of claustrum, VMH: ventromedial hypothalamic nucleus, VPL: ventral posterolateral thalamic ucleus, VPM: ventral posteromedial thalamic nucleus.

Figure 4

Neuronal characteristics of Pr5 and Sp5C neurons. (A) Raw data of a representative Pr5 neuron. Neuron shows a low firing rate but spikes tended to occur in the positive wave of the EEG. The ACH on the right shows that spikes occurred with an interval of 2.1 s. In this case, the Pr5 neuron was antidromically identified as thalamic- projecting neuron because VPM electrical stimulation induced antidromic spikes (three traces are superimposed). (B) PSTHs of responses evoked by whisker stimulation or S1 cortex stimulation (left and right histograms, respectively) in a representative Pr5 neuron. (C) same PSTHs in a Sp5C neuron. PSTHs are the sum of 30 trials.

Figure 5

Long-lasting S1 and S2 cortical effects on Pr5 neurons in unmatching conditions. (A) histograms show PSTHs in control condition, 1 min and 5 min after a stimulation train in a representative Pr5 neuron. (B) plot of the mean tactile responses in control condition and after S1 (blue diamonds) or S2 (red squares) cortical electrical stimulation (3 trains of stimuli; 50 Hz during 500 ms repeated every 2 s) in Pr5 neurons recorded in an unmatched condition (n = 35 and n = 9, respectively). Cortical stimulation inhibited tactile responses for 3 min. In this and in the following figures * P < 0.05, ** P < 0.01 statistical significance after S1 stimulation train with respect to control values; # P < 0.05, ## P < 0.01 after S2 stimulation train.

Figure 6

Long-lasting S1 and S2 cortical effects on Pr5 neurons in matching conditions. (A) histograms show PSTHs in control condition, 1 min and 5 min after stimulation train in a representative Pr5 neuron. (B) plot of the mean tactile responses in control condition and after S1 (blue diamonds) or S2 (red squares) cortical electrical stimulation (3 trains of stimuli; 50 Hz during 500 ms repeated each 2 s) in Pr5 neurons recorded in the matched condition. Pr5 neurons increased their tactile response after a stimulation train in the S1 cortex (n = 21). A similar stimulation train in the S2 cortex did not modify tactile responses in Pr5 neurons (n = 12). * P < 0.05, ** P < 0.01 statistical significance after cortical stimulation train with respect to control values.

Figure 7

Bicuculline and strychnine increase tactile responses of Pr5 neurons. (A) PSTHs of a representative neuron in control conditions and after iontophoretic application of bicuculline in the Pr5 nucleus (10 mM; 50 nA; upper histograms). Bicuculline blocked GABAergic inhibition and the tactile responses increased. A similar effect was observed after application of the glycinergic receptor antagonist strychnine (100 mM; 100 nA; lower histograms). (B) plot of the mean tactile responses before (C) and 1–5 min after the application of an SI stimulation train. SI cortical stimulation inhibited Pr5 tactile responses after application of saline solution (n = 12; black circles), but the effect was blocked by bicuculline (n = 9; red squares). However, strychnine application did not affect the long-lasting cortical-evoked inhibition (n = 7; blue triangles). * P < 0.05, ** P < 0.01 statistical significance after cortical stimulation train with respect to control values.

Figure 8

The long-lasting facilitation evoked by S1 cortical stimulation was due to NMDA receptor activation. (A) PSTHs of tactile responses in control conditions and after iontophoretic application of APV (50 mM, 100 nA) in the Pr5 nucleus. In the presence of APV, tactile responses were reduced. (B) Plot of the mean tactile responses before (C) and 1–5 min after the application of a SI stimulation train in a matched condition. In the presence of APV (n = 14 cells; red squares) the long-lasting facilitation evoked by corticofugal stimulation was blocked in comparison to responses after the application of saline solution (n = 8 cells; black circles). ** P < 0.01 statistical significance after cortical stimulation train with respect to control values.

Figure 9

Nociceptive stimulation modified the firing pattern of Pr5 and Sp5C neurons and was partially reverted by S1 or S2 cortical stimulation. (A) Plot of the firing rate of Pr5 (black circles) and Sp5C (red squares) after application of capsaicin cream on the whisker pad. The firing rate increased in both types of neurons (left plot). Electrical stimulation (3 trains of stimuli; 50 Hz during 500 ms repeated each 2 s) of S1 or S2 cortex (vertical arrow) inhibited the capsaicin-evoked increase of the firing rate (right plot). (B) Plot of the tactile responses of Pr5 (black circles) and Sp5C (red squares) after application of capsaicin cream on the whisker pad. Tactile responses decreased in both types of neurons. Electrical stimulation (3 trains of stimuli; 50 Hz during 500 ms repeated every 2 s) of S1 or S2 cortex (vertical arrow) did not modify the capsaicin-evoked decrease in tactile responses.