Retrograde labeling of neurons in the spinal cord that project directly to the amygdala or the orbital cortex in the rat

The amygdala and orbital cortex are thought to play an important role in the regulation of autonomic functions, hormonal secretion, and behavioral expression in response to sensory stimulation. The responsiveness of neurons in these regions to stimulation of cutaneous and visceral organs indicates that sensory information reaches the amygdala and orbital cortex. In the past, a large number of studies have thoroughly documented multiple neural pathways by which sensory information can reach these regions via relay nuclei in the brainstem and diencephalon. Recent studies reported that the amygdala and orbital cortex also receive direct input from the spinal cord. The aim of this study was to determine the magnitude and the origin of these projections in the rat. Injections of the retrograde tracer Fluoro-Gold (FG), restricted to the amygdala, labeled several hundred neurons bilaterally (60% contralateral) throughout the length of the spinal cord. More than 60% of labeled neurons were found in the lateral reticulated area of the deep dorsal horn and the gray matter surrounding the central canal. Many neurons were also found in the lateral spinal nucleus. Labeled neurons were concentrated in upper lumbar and upper cervical segments. Injections of Fluoro-Gold that were centered in the orbital cortex labeled only a small number of neurons (73% contralateral) within the spinal cord. Most labeled neurons were found in the lateral reticulated area. Neurons located in the intermediate zone and the gray matter surrounding the central canal were found mainly in upper lumbar and upper cervical segments. These findings, together with the anterograde tracing observations, provide evidence for direct projections of spinal cord neurons to the amygdala and orbital cortex. Their laminar distribution in the spinal cord and the involvement of the amygdala and orbital cortex in limbic functions suggest that these pathways may play a role in neuronal circuits that enable somatosensory information, including pain, to affect autonomic, endocrine and behavioral functions.