Arcuate Nucleus Projections to Brainstem Regions Which Modulate Nociception

Anterograde tracing studies were conducted in order to identify efferents from the arcuate nucleus, which contains the hypothalamic opiocortin neuronal pool. Phaseolus vulgaris leucoagglutinin (PHA-L) was stereotaxically iontophoresed into the arcuate nucleus and the terminal fields emanating from the labelled perikarya were identified immunocytochemically. PHA-L-immunoreactive (-ir) fibers were identified in nucleus accumbens, lateral septal nucleus, bed nucleus of the stria terminalis, medial and lateral preoptic areas, anterior hypothalamus, amygdaloid complex, lateral hypothalamus, paraventricular nucleus, zona incerta, dorsal hypothalamus, periventricular gray, medial thalamus and medial habenula. In the brainstem, arcuate terminals were identified in the periaqueductal gray (PAG), dorsal raphe nucleus (DRN), nucleus raphe magnus (NRM), nucleus raphe pallidus, locus coeruleus, parabrachial nucleus, nucleus reticularis gigantocellularis pars alpha, nucleus tractus solitarius and dorsal motor nucleus of the vagus nerve. Dual immunostaining was used to identify the neurochemical content of neurons in arcuate terminal fields in the brainstem. Arcuate fiber terminals established putative contacts with serotonergic neurons in the ventrolateral PAG, DRN and NRM and with noradrenergic neurons in periventricular gray, PAG and locus coeruleus. In the PAG, arcuate fibers terminated in areas with neurons immunoreactive to substance P, neurotensin, enkephalin and cholecystokinin (CCK) and putative contacts were identified with CCK-ir cells. This study provides neuroanatomical evidence that putative opiocortin neurons in the arcuate nucleus influence a descending system which modulates nociception.