Abstract
In this study, we sought to determine whether neurons of the chick embryo ciliary ganglia (CG), a parasympathetic cholinergic ganglia, can express catecholaminergic (CA) traits. To accomplish this, we used immunocytochemical techniques to examine the presence of the CA enzymes tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) in CGs removed from chick embryo at day 8 of development (E8). Few neurons containing TH but not PNMT were found in the E8 CG. To examine whether CG neurons express CA enzymes in vitro, CGs removed from E8 chick embryo were dissociated and kept in culture for 3 to 12 days. In 50% of the culture dishes, some neurons contain TH or PNMT after 5 days in vitro. In an equal proportion of culture plates, CG neurons did not express the enzymes. To determine whether the proportion of CG neurons expressing TH or PNMT is increased by tissue influences, ganglion cells were co-cultured with notochord. In 90% of the co-culture experiments, most neurons present in the culture dishes stained with TH or PNMT after 5 days in vitro. To test for the presence of aromatic L-amino acid decarboxylase (AADC), another CA enzyme, cultures of CGs and CGs plus notochord were incubated with levodopa and processed for the detection of CA histofluorescence. Dopamine histofluorescence was present in all neurons after 3 days in vitro irrespective of the presence of notochord, suggesting that the expressions of TH and PNMT and that of AADC are differentially regulated. This study, therefore, demonstrates that cholinergic neurons of the CG contain CA enzymes in vivo and in vitro and that the proportion of neurons expressing CA traits during development in vitro can be increased by environmental cues such as those released by the notochord.