Developmental expression of serotonin-like immunoreactivity in the sympathoadrenal system of the chicken.

The avian sympathoadrenal system has been used as a model to examine the differentiation of cells expressing neuroactive substances derived from the neural crest. Previous studies have dealt with the expression of the "classical" neurotransmitters acetylcholine and catecholamines and of the neuropeptides somatostatin and vasoactive intestinal polypeptide. We have used immunocytochemistry to examine the developmental expression of the monoamine serotonin (5HT) in the chicken sympathoadrenal system. 5HT-like immunoreactivity (5HT-LI) was found to be transiently expressed by cells of the sympathetic ganglia very early in development (E-5 to E-8), disappearing almost entirely at more advanced embryonic stages (E-10 to E-19) and post-hatched chickens where only a population of cells similar to mammalian small intensely fluorescent cells express immunoreactivity to the amine. In contrast, in the adrenal gland of embryos and post-hatched chickens, most chromaffin cells also express 5HT-LI. Double labeling experiments show that in both the adrenal gland and the sympathetic ganglia catecholaminergic properties and somatostatin immunoreactivity are co-expressed with 5-HT-LI. Moreover, the cells that transiently express 5HT-LI in sympathetic ganglia also transiently express somatostatin. The catecholaminergic cells expressing serotonin and somatostatin appear to define a biochemical phenotype common to some chromaffin cells, small intensely fluorescent cells and early sympathoblasts.