The projection of locus coeruleus neurons to the spinal cord in the rat determined by anterograde tracing combined with immunocytochemistry

Pontospinal noradrenergic neurons located in the A5, A7 and locus coeruleus/subcoeruleus (LC/SC) nuclei are the major source of the noradrenergic innervation of the spinal cord. However, the specific terminations of spinally-projecting noradrenergic neurons located in these nuclei have not been clearly defined. The purpose of the experiments described in this report was to more precisely define the spinal terminations of neurons located in the LC/SC using the anterograde tracer phaseolus vulgaris-leucoagglutinin in combination with dopamine-beta-hydroxylase (D beta H) immunocytochemistry. In addition, the spinal cord regions in which LC/SC neurons terminate was assessed by measuring the reduction in the density of D beta H-immunoreactive axon terminals in specific spinal cord regions after a unilateral electrolytic lesion that included LC/SC neurons. The results of these experiments indicate that the axons of LC neurons are located primarily in the ipsilateral ventral funiculus and terminate most heavily in the medial part of laminae VII and VIII, the motoneuron pool of lamina IX, and lamina X. LC neurons provide a moderately dense innervation of the ventral part of the dorsal horn, but only a very sparse innervation of the superficial dorsal horn. The SC projects ipsilaterally in the ventrolateral funiculus and terminates diffusely in the intermediate and ventral laminae of the spinal cord.