5-Hydroxytryptamine and catecholamines in developing sympathetic cells of the rat.

Appearance of 5-hydroxytryptamine (5-HT) in developing sympathetic cells of prenatal rats was studied using the indirect immunofluorescence method. In consecutive sections, tyrosine hydroxylase (TH) immunoreactivity was considered as a marker for catecholamine-synthesizing cells in general, while phenylethanolamine N-methyltransferase (PNMT) immunoreactivity was used as an indicator of adrenaline synthesis. 5-HT immunoreactivity was observed for the first time in 12.5-day-old embryos in developing sympathetic chain ganglia. On day 13.5, 5-HT-immunoreactive cells were first seen on the preaortic region and on day 14.5 in the developing adrenal gland. Comparison with consecutive sections stained for TH revealed that all TH-immunoreactive cells were also 5-HT-immunoreactive. During later development, however, 5-HT immunoreactivity was retained by some cell types in each sympathetic tissue. In the ganglia, most developing principal nerve cells gradually lost their 5-HT immunoreactivity, while all so-called small intensely fluorescent (SIF) cells remained intensely 5-HT-immunoreactive. In the adrenal medulla, all catecholamine-containing cells showed 5-HT immunoreactivity until day 16.5. The first adrenaline-synthesizing cells appeared at this stage. Occasionally on day 16.5 and constantly on day 17.5 noradrenaline cell islets were distinguished among adrenaline cells. The adrenaline cells retained intense 5-HT immunoreactivity, while the noradrenaline cells were non-reactive to it. In the main retroperitoneal paraganglion, two noradrenaline cell populations were distinguished from day 15.5, one being 5-HT-immunoreactive and the other non-reactive. A third population appeared in this tissue at the time of birth, consisting of adrenaline-synthesizing cells which were also 5-HT-immunoreactive. These results indicate that the 3 sympathetic tissues undergo similar developmental changes: 5-HT immunoreactivity occurs in conjunction with the initiation of catecholamine synthesis and appears first in all catecholamine cells. During maturation it is confined to certain subpopulations in each tissue, i.e. the SIF cells and some principal nerve cells of the ganglion, the adrenaline cells of the adrenal medulla, the adrenaline cells and some noradrenaline cells of the paraganglionic tissue.