Abstract
Neurogenesis continues in the adult forebrain subventricular zone (SVZ) and the dentate gyrus of the hippocampal formation. Degeneration of dopaminergic projections in Parkinson's disease and animals reduces, whereas ciliary neurotrophic factor (CNTF) promotes, neurogenesis. We tested whether the dopaminergic system promotes neurogenesis through CNTF. Astrocytes of the SVZ and dentate gyrus expressed CNTF and were close to dopaminergic terminals. Dopaminergic denervation in adult mice reduced CNTF mRNA by approximately 60%, whereas systemic treatment with the D2 agonist quinpirole increased CNTF mRNA in the SVZ and hippocampal formation, and in cultured astrocytes by 1.5-5 fold. The effect of quinpirole in vitro was blocked by the D2 antagonist eticlopride and did not cause astroglial proliferation or hypertrophy. Systemic quinpirole injections increased proliferation in wild-type mice by approximately 25-75% but not in CNTF-/- littermates or in the SVZ of mice infused with CNTF antibodies. Quinpirole increased the number of neuroblasts in wild-type but not in CNTF-/- littermates. Neurogenesis was reduced by approximately 20% in CNTF-/- mice, confirming the endogenous role of CNTF. Nigrostriatal denervation did not affect SVZ proliferation in CNTF-/- mice, suggesting that the dopaminergic innervation normally regulates neurogenesis through CNTF. Quinpirole acted on postsynaptic receptors as it reversed the reduced proliferation seen after dopaminergic denervation in wild-type mice. Thus, CNTF mediates dopaminergic innervation- and D2 receptor-induced neurogenesis in the adult forebrain. Because CNTF is predominantly expressed in the nervous system, this mechanism and the ability to pharmacologically modulate it have implications for Parkinson's disease and cell-replacement therapies for other disorders.