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Expression of Neurotrophins and Their Receptors in Primary Astroglial Cultures: Induction by Cyclic AMP-Elevating Agents


Address correspondence and reprint requests to Dr. N. Belluardo at Institute of Human Physiology, Faculty of Medicine, University of Catania, Viale A. Doria, 6, 95125, Catania, Italy.


Abstract: By northern blot analysis and ribonuclease protection assay, we observed the presence of a high level of trkB mRNA in primary brain cultures devoid of neuronal cells and highly enriched in glial fibrillary acidic protein-positive astroglial cells prepared from newborn rat cerebral hemispheres, cerebral cortex, hippocampus, and striatum. In primary astroglial cultures, the more abundant trkB transcripts code for the truncated receptor without tyrosine kinase activity; probes specific for the full-length trkB mRNA did not detect any signal in northern blot analysis. By the sensitive ribonuclease protection assay, we could show the presence of trkC mRNA in cultured astrocytes, whereas no trkA mRNA was detected. We confirmed the presence of relatively high levels of nerve growth factor and neurotrophin-3 mRNA, and very low basal level of brain-derived neurotrophic factor mRNA. Moreover, we demonstrated that another member of the neurotrophin family, neurotrophin-4, is also expressed in cultured astroglial cells. In view of the fact that many functional receptors for conventional neurotransmitters or neuropeptides present on astroglial cells may act via the adenylate cyclase system, we studied also the effect of agents able to increase the intracellular cyclic AMP concentration. A sharp increase in the trkB mRNA level was observed after treatment of primary astroglial cultures with dibutyryl cyclic AMP, 8-bromo-cyclic AMP, or the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine. On the contrary, trkC mRNA levels were unaffected by treatment with cyclic AMP-elevating agents. All the neurotrophin mRNAs examined, except neurotrophin-4, were increased by 3-isobutyl-1-methylxanthine treatment. Therefore, in cultured astroglial cells, gene expression of neurotrophins and trkB is regulated by activation of the cyclic AMP-second messenger system. This process may take part in the neuronal-glial interactions during the normal neuronal activity or after injury events.