Ib. Walter, TRIIODOTHYRONINE EXERTS A TROPHIC ACTION ON RAT SENSORY NEURON SURVIVAL AND NEURITE OUTGROWTH THROUGH DIFFERENT PATHWAYS, European journal of neuroscience, 8(3), 1996, pp. 455-466
Apart from several growth factors which play a crucial role in the sur
vival and development of the central and peripheral nervous systems, t
hyroid hormones can affect different processes involved in the differe
ntiation and maturation of neurons. The present study was initiated to
determine whether triiodothyronine (T-3) affects the survival and neu
rite outgrowth of primary sensory neurons in vitro. Dorsal root gangli
a (DRG) from 19-day-old embryos or newborn rats were plated in explant
or dissociated cell cultures. The effect of T-3 On neuron survival wa
s tested, either in mixed DRG cell cultures, where neurons grow with n
on-neuronal cells, or in neuron-enriched cultures where non-neuronal c
ells were eliminated at the outset. T-3, in physiological concentratio
ns, promoted the growth of neurons in mixed DRG cell cultures as well
as in neuron-enriched cultures without added nerve growth factor (NGF)
. Since neuron survival in neuron-enriched cultures cannot be promoted
by endogenous neurotrophic factors synthesized by non-neuronal cells,
the increased number of surviving neurons was due to a direct trophic
action of T-3 Another trophic effect was revealed in this study: Tg s
ustained the neurite outgrowth of sensory neurons in DRG explants. The
stimulatory effect of T-3 on nerve fibre outgrowth was considerably r
educed when non-neuronal cell proliferation was inhibited by the antim
itotic agent cytosine arabinoside, and was completely suppressed when
the great majority of non-neuronal cells were eliminated in neuron-enr
iched cultures. These results indicate that the stimulatory effect of
T-3 On neurite outgrowth is mediated through non-neuronal cells. It is
conceivable that T-3 up-regulates Schwann cell expression of a neurot
rophic factor, which in turn stimulates axon growth of sensory neurons
. Together, these results demonstrate that T-3 promotes both survival
and neurite outgrowth of primary sensory neurons in DRG cell cultures.
The trophic actions of T-3 On neuron survival and neurite outgrowth o
perate under two different pathways.