In explanted embryonic chick sympathetic neurons, all-trans retinoic a
cid (RA) as well as nerve growth factor (NGF) were found to be require
d for neuronal survival and neurite outgrowth at early stages of devel
opment (day 6.5-7) in agreement with previous work (Rodriguez-Tebar an
d Rohrer [1991] Development 112:813-820). The dependence of neurons on
all-trans RA for survival diminished at later stages of development.
However, all-trans RA was found to be needed at all stages of developm
ent in order to maximize neurite outgrowth. Further, removal of all-tr
ans RA from the cultures led to a rapid degeneration of the formed neu
rites, demonstrating the essentiality of all-trans RA for both the dev
elopment of neurites, and for the maintenance of existing neurites in
cultured embryonic sympathetic neurons. The mechanism whereby all-tran
s RA exerts its effects on embryonic sympathetic neurons may involve a
ctivation of the nuclear retinoic acid and retinoid-X receptor (RAR an
d RXR) families. The results of Northern blot analyses and/or reverse
transcriptase-polymerase chain reaction (RT-PCR) studies show that emb
ryonic sympathetic ganglia express RAR beta, RAR gamma and RXR gamma m
RNAs. RXR gamma mRNA is expressed at highest levels in immature neuron
s that are not yet responsive to NGF (day 6.5-7) and message levels de
cline with increasing developmental age. In contrast, RAR beta transcr
ipt levels are barely detectable at day 6.5-7, and increase similar to
4-fold in ganglia from embryos at day 8.5-9 and decline thereafter, R
T-PCR studies show that RAR gamma mRNA is expressed both early (day 6.
5-7) and late (day 15) in ganglionic development. Transcripts for the
NGF receptors, p75(NGFR) and p140(trk) were also examined. The appeara
nce of a single 2.7 kb p140(trk) transcript coincides with the time wh
en RAR beta mRNA is maximally expressed, raising the possibility that
NGF receptors may be targets of retinoid action. Evidence is also pres
ented that all-trans RA may enhance neurite outgrowth by mechanisms ot
her than simply inducing NGF-responsiveness of neurons. (C) 1996 Wiley
-Liss, Inc.