Development and distribution of noradrenergic and cholinergic neurons and their trophic phenotypes in the avian...

We investigated the development of noradrenergic and cholinergic neurons in the ceruleus complex and mesencephalic tegmentum in embryonic and posthatch chickens and compared the distribution of transmitter phenotypes with the expression of nerve growth factor receptor (NGFR) mRNA and fibroblast growth factor receptor (FGFR) mRNA. Noradrenergic and cholinergic neurons were visualized by using antibodies against dopamine-beta-hydroxylase (DBH) and choline acetyltransferase (ChAT), respectively. Expression of receptors for trophic factors was determined by using in situ hybridization techniques. Noradrenergic neurons concentrate in caudal parts of the locus ceruleus and nucleus subceruleus. Cholinergic ceruleus neurons are abundant in the nucleus mesencephalicus profundus, pars ventralis (MPv) as well as in the nucleus subceruleus and locus ceruleus. This cholinergic population resembles the cholinergic pontomesencephalotegmental complex of mammals. Both DBH and ChAT label is evident at and after six days of incubation (E6). The distribution and numbers of immunolabeled neurons are similar in the embryonic and posthatch chick. Initially, many tegmental and ceruleus neurons express substantial levels of NGFR mRNA (E7-E9). After E9, expression of NGFR mRNA decreases in most of these neurons, except for a distinct subpopulation of neurons in caudal parts of the ceruleus complex with increased levels of NGFR transcripts. These NGFR-positive neurons coincide in number and distribution with the noradrenergic subpopulation of the ceruleus complex (800-900 neurons). Expression of FGFR mRNA was first detected in ceruleus neurons at E13. Neurons with FGFR transcripts have the same number and distribution as the neurons with the cholinergic phenotype (2,000-2,300 neurons). Transmitter heterogeneity in the ceruleus complex is reflected by a heterogeneity of receptors for trophic factors, with NGFR expressed in the noradrenergic subpopulation, and FGFR expressed in the cholinergic subpopulation. These findings provide evidence for new chemoarchitectonic subdivisions of the avian ceruleus complex. The data showing onset of ChAT expression prior to the onset of FGFR expression argue against a role of FGFR in the determination of the cholinergic transmitter phenotype. Expression of NGFR in the noradrenergic ceruleus subpopulation reveals remarkable species differences as compared to mammals.