Noradrenergic neurons in the locus coeruleus of birds express TrkA, transport NGF, and respond to NGF.

The chicken locus coeruleus contains a population of noradrenergic neurons which express the neurotrophin receptor p75 (von Bartheld and Bothwell, 1992). To determine which neurotrophin may regulate the development of noradrenergic neurons in the chicken locus coeruleus, expression of trk receptors, retrograde transport of neurotrophins, and responses to NGF were examined. P75-expressing noradrenergic neurons were found to project to the basal forebrain. They transport radio-iodinated NGF after injections into this target. The retrograde transport of NGF is specific to the noradrenergic neuronal population as evidenced by double labeling with antibodies against dopamine-beta-hydroxylase. The same neuronal population expresses trkA receptor mRNA. The size of noradrenergic neurons in the locus coeruleus proper, but not in the nucleus subcoeruleus, is significantly increased after injections of NGF into the telencephalon, consistent with the hypothesis that target-derived NGF provides trophic support. Noradrenergic coeruleus neurons are rescued from toxic effects of 6-hydroxydopamine injected into the telencephalon when NGF is injected into the midbrain. NGF has no rescue effect when it is coinjected with 6-hydroxy-dopamine into the telencephalon. In explant or dissociated cultures, noradrenergic coeruleus neurons do not respond to elevated levels of NGF with increased neurite outgrowth. Taken together, these results suggest that NGF plays a role in the development and maintenance of noradrenergic coeruleus neurons in the chick brain. The data also support our previous conclusion that major species differences exist between birds (chicken) and mammals with regard to trophic regulation of presumptive homologous neuronal populations.