Evidence for a viscerotopic sensory representation in the cortex and thalamus in the rat

Abstract

The functional organization of the insular cortex was studied by recording neuronal responses to visceral sensory stimuli. Horseradish peroxidase (HRP) was then iontophoresed at the recording sites to identify afferents from the ventrobasal thalamus to specific visceroceptive sites in the insular cortex. The relationship of the ventrobasal thalamus to the insular cortex and to brainstem relay nuclei for the ascending visceral projections was then examined by using the axonal transport of HRP, wheat germ agglutinin conjugated to HRP (WGA-HRP), and fluorescent dyes. Of a total of 55 neurons that were tested for responses to visceral sensory stimuli, 33 units responded to atleast one visceral sensory modality: 6 received gastric mechanoreceptor input, 8 responded to taste inputs, 13 were activated by arterial chemoreceptors and/or showed respiratory related activity, and 6 responded to cardiovascular baroreceptor stimulation. On the basis of its cytoarchitecture and connections with the thalamus, the insular cortex was divided into a dorsal granular area, an intermediate dysgranular region, and a ventral agranular strip. Taste-responsive neurons were located anteriorly, primarily in the dysgranular region, whereas unit responses to general visceral modalities were distributed dorsally and posteriorly in the granular insular cortex. Gastric mechanoreceptor-responsive units were situated more dorsally and anteriorly in the granular insular cortex, while cardiopulmonary inputs were located more ventrally and posteriorly. Injections of HRP into the gustatory insular cortex resulted in retrograde labeling of neurons in the parvicellular part of the ventroposterior medial thalamic nucleus (VPMpc). Injections into the general visceral insular cortex retrogradely labeled neurons lateral to VPMpc in the ventroposterior lateral parvicellular thalamic nucleus (VPLpc). Injections of HRP, WGA-HRP, and fluorescent dyes into VPMpc and VPLpc verified that their projection to the insular cortex is topographically organized. In the same experiments, retrogradely labeled neurons in the parabrachial nucleus identified the likely subnuclei within this nucleus for relay of visceral sensory information to the thalamus. Injections of WGA-HRP into the parabrachial nucleus demonstrated that its projection to the ventrobasal thalamus is also topographically organized. These results demonstrate the relationship of general visceral and special visceral (taste) representations in the insular cortex. The ascending pathway for visceral sensory information appears to be viscerotopically organized at all levels of the neuraxis, including the insular cortex.