To understand what kind of trophic factors are up-regulated in dopamine (DA)-depleted striatum, we first analysed the up-regulation of mRNAs using a DNA microarray in DA-depleted striatum where DAergic inputs were denervated by 6-OHDA. We then investigated whether or not such trophic factors had an effect on cultured dopaminergic neurons. The microarray analysis revealed that pleiotrophin (PTN), glial-derived neurotopic factor (GDNF) and others were up-regulated in DA-depleted striatum. As PTN has been reported to have a wide range of trophic effects on neurons, we focused on the functional role of PTN in the present study. The increase in PTN mRNA was confirmed by Northern blotting at 1-3 weeks after the lesion, reaching a peak at 1 week. In embryonic day 15 mesencephalic neuron culture, PTN increased the number of tyrosine hydroxylase (TH) -positive neurons in a dose-dependent manner (125.2 +/- 2.0% of the control at 50 ng/mL), while a family protein, midkine (10 ng/mL) did not show any trophic effect (99.3 +/- 0.7%). In addition, the PTN effect on DAergic neurons was additive to the GDNF effect. As PTN did not increase the number of microtubule-associated protein-2 (MAP 2)-positive neurons or promote the proliferation of dopaminergic progenitors in a bromodeoxyuridine (BrdU) labelling study, the effect appeared to enhance the specific survival of dopaminergic neurons. Expression of PTN receptors (syndecan-3, PTP-zeta) was detected on the cultured mesencephalic neurons, and also up-regulated in DA-depleted striatum. The data indicate that PTN is up-regulated in DA-depleted striatum and exhibits a trophic effect specifically on the survival of cultured dopaminergic neurons.