A delta afferent fiber stimulation activates descending noradrenergic system from the locus coeruleus.

We compared the noradrenaline (NA) level in the dorsal horn following electrical stimulation of A delta afferent nerve fibers in the peripheral nervous system between rats with bilateral lesions of the locus coeruleus (LC) and non-operated control rats by using a microdialysis technique combined with high performance liquid chromatography. Prior to A delta afferent fiber stimulation, the NA content in the dialysate did not differ between the LC-lesioned and the control rats. During A delta afferent fiber stimulation, in the LC-lesioned rats, the NA level did not change significantly compared to that before A delta afferent fiber stimulation, whereas the NA level increased significantly in the control rats. There was a significant difference in the NA levels during A delta afferent fiber stimulation between the two groups of rats. The result suggests that descending noradrenergic neurons from the LC is involved in the increase of the NA level in the spinal cord dorsal horn produced by A delta afferent fiber stimulation.