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A new neuronal cell line was generated by transfection of rat olfactory epithelium with immortalizing recombinant oncogene E1A of adenovirus-2. The resulting 13.S.1.24 line of transformed cells expressed an antigenic phenotype of olfactory neuronal progenitors. Addition of dopamine to 13.S.1.24 cultures induced reduction of cell number within 2 days. Two hallmarks of apoptosis were detected in dopamine-treated cultures: internucleosomal DNA fragmentation and nuclear condensation. Dopamine did not alter the cell proliferation rate, as assessed by [3H]thymidine incorporation. Dopamine also stimulated differentiation of surviving 13.S.1.24 cells into bipolar olfactory marker protein-immunoreactive neurons. Time-dependency assessments over 1 week of treatment indicated that apoptosis and differentiation induced by dopamine were concomitant. Both apoptosis and differentiation triggered by dopamine were dose-dependent, half-maximal effects being obtained with approximately 10 microM dopamine. Mediation of both effects by dopaminergic D2 receptors was supported by several observations: active dopamine doses in micromolar ranges, quinpirole agonism and eticlopride antagonism, D2-characteristic rank order of potency among the three agonists tested, and specific binding of a selective D2-like radioligand to 13.S.1.24 cells. The present data altogether indicated that dopamine commits immortalized olfactory neuronal cells in vitro either to apoptosis or to olfactory-like differentiation via D2 dopaminergic receptors.

The mammalian olfactory epithelium (OE) is composed of primary olfactory sensory neurons (OSNs) that are renewed throughout adulthood by local, restricted neuronal progenitor cells. The molecular signals that control this neurogenesisin vivoare unknown. Using olfactory bulb ablation (OBX) in adult mice to trigger synchronous mitotic stimulation of neuronal progenitors in the OE, we show thein vivoinvolvement of a cytokine in the cellular events leading to the regeneration of the OE. We find that, of many potential mitogenic signals, only leukemia inhibitory factor (LIF) is induced before the onset of neuronal progenitor proliferation. The rise in LIF mRNA expression peaks at 8 hr after OBX, andin situRT-PCR and immunocytochemistry indicate that LIF is upregulated, in part, in the injured neurons themselves. This rise in LIF is necessary for injury-induced neurogenesis, as OBX in the LIF knock-out mouse fails to stimulate cell proliferation in the OE. Moreover, delivery of exogenous LIF to the intact adult OE using an adenoviral vector stimulates BrdU labeling in the apical OE. Taken together, these results suggest that injured OSNs release LIF as a stimulus to initiate their own replacement.