OVERLAPPING AND DISTINCT ACTIONS OF THE NEUROTROPHINS BDNF, NT-3, ANDNT-4 5 ON CULTURED DOPAMINERGIC AND GABAERGIC NEURONS OF THE VENTRAL MESENCEPHALON/

The neurotrophins brain-derived neurotrophic factor (BDNF), neurotroph in-3 (NT-3), and neurotrophin-4/5 (NT-4/5) were compared for their eff ects in promoting the survival and/or regulation of expression of phen otypic markers of dopaminergic and GABAergic neurons in cultures deriv ed from embryonic rat ventral mesencephalon. Dopaminergic neuron numbe r and phenotypic expression were monitored by tyrosine hydroxylase (TH ) immunocytochemistry, and measurement of high-affinity dopamine uptak e activity and dopamine content, respectively. High-affinity GABA upta ke, glutamic acid decarboxylase (GAD) activity, and endogenous GABA co ntent were used to detect GABAergic neurons. Seven days of treatment w ith either BDNF or NT-3 resulted in dose-dependent increases in the nu mber of TH-positive neurons, with maximal responses of 3-fold and 2.3- fold, respectively. Dopamine uptake activity and dopamine content were similarly increased. The effects of BDNF and NT-3 on dopamine uptake activity showed no additivity. NT-4/5 treatment elicited the greatest increase (7-fold) in the number of TH-positive neurons, as well as a 2 .6-fold increase in dopamine content. In marked contrast to BDNF or NT -3, NT-4/5 had no effect on dopamine uptake capacity. BDNF, NT-3, or N T-4/5 also produced dose-dependent elevations of 2-3-fold in GABA upta ke activity. These effects were not additive. GAD activity was increas ed by BDNF (1.8-fold) and NT-3 (threefold) treatment, but not by NT-4/ 5, whereas GABA content was increased to a similar extent by all three neurotrophins. NGF had no effect on any of the parameters measured in this study. Northern analyses indicated that the mRNAs encoding TrkB and TrkC, the functional high-affinity receptors for BDNF and NT-4/5, and NT-3, respectively, are expressed in the substantia nigra of adult rat brain, as well as in cultures of developing ventral mesencephalon . Taken together, our results indicate that BDNF and NT-3 have broadly similar effects in promoting the survival and differentiated phenotyp e of both dopaminergic and GABAergic neurons of the developing substan tia nigra. Although BDNF and NT-4/5 are thought to act through the sam e high-affinity receptor, TrkB, it is evident that these two neurotrop hins have distinct as well as overlapping actions toward mesencephalic dopaminergic or GABAergic neurons.