Differential projections from locus coeruleus to olfactory bulb and olfactory tubercle: an HRP study.

The microiontophoretic administration of horseradish peroxidase (HRP) to the olfactory bulb (OB) or olfactory tubercle (OT) in cats and rats yielded similar results in both species. After an OB HRP-injection ipsilateral and contralateral labelled neurons were seen in the piriform cortex, polymorphic layer of OT, magnocellular preoptic region, lateral hypothalamus, ventromedial hypothalamic nucleus and locus coeruleus (LC). In both species more labelled structures were found after an OT HRP-injection than after an OB HRP-injection. The substantia nigra in rats was more abundantly labelled after an OT injection than after an OB one. In cats the dorsal and the ventral raphe were also labelled. In either species, OT HRP-injections resulted in a higher frequency of LC labelled neurons than after OB injections. These results favor the hypothesis that the OT plays an important role as a relay station for efferent inflow from the brain stem en route to the OB.