Cross-Streams Through the Ventral Posteromedial Thalamic Nucleus to Convey Vibrissal Information

Whisker detection is crucial to adapt to the environment for some animals, but how the nervous system processes and integrates whisker information is still an open question. It is well-known that two main parallel pathways through Ventral posteromedial thalamic nucleus (VPM) ascend to the barrel cortex, and classical theory suggests that the cross-talk from trigeminal nucleus interpolaris (Sp5i) to principal nucleus (Pr5) between the main parallel pathways contributes to the multi-whisker integration in barrel columns. Moreover, some studies suggest there are other cross-streams between the parallel pathways. To confirm their existence, in this study we used a dual-viral labeling strategy and high-resolution, large-volume light imaging to get the complete morphology of individual VPM neurons and trace their projections. We found some new thalamocortical projections from the ventral lateral part of VPM (VPMvl) to barrel columns. In addition, the retrograde-viral labeling and imaging results showed there were the large trigeminothalamic projections from Sp5i to the dorsomedial section of VPM (VPMdm). Our results reveal new cross-streams between the parallel pathways through VPM, which may involve the execution of multi-whisker integration in barrel columns.

NOCICEPTIVE RESPONSES OF THE LATERAL THALAMIC NUCLEUS EVOKED BY PINCH IN ANESTHETIZED RATS

Pinch was used as a noxious stimulus and applied to the tail and hindlimbs of rats to investigate the nociceptive responsiveness of neurons in the lateral thalamus. Pinch stimuli were applied with calibrated pincher, and glass micropipettes filled with 3 M NaCl served as recording electrodes. The lateral thalamus of six rats under ketamine anesthesia was probed, including the ventrobasal (VB) complex and the posterior thalamic nucleus ( Po ), while the noxious stimulus was applied. Neuronal responses in the lateral thalamus were identified with the aid of an audio monitoring device and an oscilloscope, and several inclusion criteria had to be met. A total of 168 neurons were studied across the ventral posterolateral thalamic nucleus (VPL), ventral posteromedial thalamic nucleus (VPM), and Po of the thalamus. Of the neurons assessed in the lateral thalamus, 38 responded to the nociceptive stimuli, and most were located within the Po region of the thalamus. Very few were located in the VB. The number of responsive neurons in the anterior region of the lateral thalamus was significantly greater than the number of responsive neurons in the posterior region of the lateral thalamus. This finding indicates that the anterior region of the lateral thalamus, especially the Po area, may respond more vigorously to noxious stimuli than the posterior region.

Increases in local cerebral blood flow associated with somatosensory activation are not mediated by NO

Effects of inhibition of nitric oxide (NO) synthase by NG-nitro-L-arginine methyl ester (L-NAME) on the increases in local cerebral blood flow (LCBF) produced in the whisker-to-barrel sensory pathway by vibrissal stimulation were studied in conscious rats with the autoradiographic iodo[14C]antipyrine method. Unilateral whisker stroking increased LCBF in the ipsilateral trigeminal spinal and principal sensory nuclei, contralateral ventral posteromedial thalamic nucleus, and contralateral somatosensory barrel cortex. Intravenous L-NAME (30 mg/kg) lowered baseline LCBF without altering the percent increases due to stimulation. Intracisternal infusions of L-NAME in doses about 10 times the molar content of free arginine in brain inhibited brain NO synthesis activity by 88%, but the percent augmentations of LCBF by stimulation remained unchanged. Chronic treatment with L-NAME (50 mg/kg ip twice daily for 4 days) inhibited NO synthase activity in brain by 84% but also failed to reduce the percent increases in LCBF due to stimulation. These results indicate that NO does not mediate the increases in LCBF associated with functional activation.