GDNF enhances the synaptic efficacy of dopaminergic neurons in culture

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.