The Brain Nucleus Locus Coeruleus: Restricted Afferent Control of a Broad Efferent Network

TLDR

The locus coeruleus receives major afferent input only from the paragigantocellularis and prepositus hypoglossi nuclei, with minor contributions possibly from the dorsal paraventricular hypothalamic nucleus and spinal intermediate gray, while other reported sources such as amygdala, nucleus tractus solitarius, and spinal dorsal horn show no consistent retrograde labeling. Dense focal injections of wheat germ agglutinin–conjugated horseradish peroxidase in the locus coeruleus labeled afferent neurons in only a few brain regions, and anterograde tracing and electrophysiology confirmed the absence of input from previously reported areas, redefining the anatomical organization of the locus coeruleus and impacting hypotheses about its functions.

Abstract

Dense, focal injections of wheat germ agglutinin conjugated-horseradish peroxidase in the locus coeruleus of rats labeled afferent neurons in unexpectedly few brain regions. Major inputs emanate from only two nuclei—the paragigantocellularis and the prepositus hypoglossi, both in the rostral medulla. The dorsal cap of the paraventricular hypothalamic nucleus and the spinal intermediate gray are possible minor afferents to locus coeruleus. Other areas reported to project to locus coeruleus (for example, amygdala, nucleus tractus solitarius, and spinal dorsal horn) did not exhibit consistent retrograde labeling. Anterograde tracing and electrophysiologic experiments confirmed the absence of input to locus coeruleus from these areas, which instead terminate in targets adjacent to locus coeruleus. These findings redefine the anatomic organization of the locus coeruleus, and have implications for hypotheses concerning the functions of this noradrenergic brain nucleus.