Afferent projections to the rat locus coeruleus as determined by a retrograde tracing technique

TLDR

Retrograde transport of horseradish peroxidase was used to map afferent connections to the rat locus coeruleus, a nucleus composed solely of noradrenergic neurons. A microiontophoretic HRP delivery guided by single‑cell electrophysiological landmarks ensured accurate placement within the LC. HRP labeling revealed projections from the insular cortex, amygdala, preoptic and hypothalamic areas, as well as brainstem catecholamine groups and spinal cord dorsal horns, supporting a role for the LC in arousal and autonomic regulation.

Abstract

Abstract Afferent connections to the rat locus coeruleus (LC), which contains exclusively noradrenergic neurons, have been traced using the technique of retrograde transport of horseradish peroxidase (HRP). In order to ensure accurate placement of adequate amounts of HRP in the LC, a microiontophoretic delivery technique coupled with single cell recording was employed. The use of electrophysiological “landmarks” as aids in placing the injections is described. Following HRP injections into the LC, forebrain structures containing labelled neurons included the insular cortex, the central nucleus of the amygdala, the medial, lateral and magnocellular preoptic areas, the bed nucleus of the stria terminalis, and the dorsomedial, paraventricular and lateral hypothalamic areas. In the brainstem reactive neurons were observed in the central grey substance, the reticular formation, the raphe, vestibular, solitary tract and lateral reticular nuclei. In particular, the areas of catecholamine cell groups A 1 , A 2 and A 5 appeared to contain many reactive cells. Labelled neurons were also observed in the fastigial nuclei and in the marginal zones of the dorsal horns of the spinal cord. This pattern of afferent innervation supports suggestions for a role for the LC in behavioral arousal mechanisms and autonomic regulation.