Glial cell line-derived neurotrophic factor (GDNF) is known to promote the
survival and differentiation of dopaminergic neurons of the midbrain. GDNF
also causes an enhancement of dopamine release by a mechanism which is pres
ently unclear. Using isolated dopaminergic neurons of the rat ventral tegme
ntal area in culture, we have tested the hypothesis that GDNF regulates the
establishment and functional properties of synaptic terminals. Previous st
udies have shown that single dopaminergic neurons in culture can co-release
glutamate in addition to dopamine, leading to the generation of a fast exc
itatory autaptic current via glutamate receptors, Using excitatory autaptic
currents as an assay for the activity of synapses established by identifie
d dopaminergic neurons, we found that chronically applied GDNF produced a t
hreefold increase in the amplitude of excitatory autaptic currents. This ac
tion was specific for dopaminergic neurons because GDNF had no such effect
on ventral tegmental area GABAergic neurons. The enhancement of excitatory
autaptic current amplitude caused by GDNF was accompanied by an increase in
the frequency of spontaneous miniature excitatory autaptic currents. These
observations confirmed a presynaptic locus of change. We identified synapt
ic terminals by using synapsin-l immunofluorescence. In single tyrosine hyd
roxylase-positive neurons, the number of synapsin-positive puncta which rep
resent putative synaptic terminals was found to be approximately doubled in
GDNF-treated cells at 5, 10 and 15 days in culture. The number of such mor
phologically identified terminals in isolated GASAergic neurons was unchang
ed by GDNF. These results suggest that one mechanism through which GDNF may
enhance dopamine release is through promoting the establishment of new fun
ctional synaptic terminals.