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 beha vioral expression in response to sensory stimulation. The responsivene ss of neurons in these regions to stimulation of cutaneous and viscera l organs indicates that sensory information reaches the amygdala and o rbital cortex. In the past, a large number of studies have thoroughly documented multiple neural pathways by which sensory information can r each these regions via relay nuclei in the brainstem and diencephalon. Recent studies reported that the amygdala and orbital cortex also rec eive direct input from the spinal cord. The aim of this study was to d etermine the magnitude and the origin of these projections in the rat. Injections of the retrograde tracer Fluoro-Gold (FG), restricted to t he amygdala, labeled several hundred neurons bilaterally (60% contrala teral) throughout the length of the spinal cord. More than 60% of labe led neurons were found in the lateral reticulated area of the deep dor sal horn and the gray matter surrounding the central canal. Many neuro ns 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. M ost labeled neurons were found in the lateral reticulated area. Neuron s located in the intermediate zone and the gray matter surrounding the central canal were found mainly in upper lumbar and upper cervical se gments. These findings, together with the anterograde tracing observat ions, provide evidence for direct projections of spinal cord neurons t o the amygdala and orbital cortex. Their laminar distribution in the s pinal cord and the involvement of the amygdala and orbital cortex in l imbic functions suggest that these pathways may play a role in neurona l circuits that enable somatosensory information, including pain, to a ffect autonomic, endocrine and behavioral functions. (C) 1993 Wiley-Li ss, Inc.