EXPRESSION OF NEUROTROPHINS AND THEIR RECEPTORS IN PRIMARY ASTROGLIALCULTURES - INDUCTION BY CYCLIC AMP-ELEVATING AGENTS

By northern blot analysis and ribonuclease protection assay, we observ ed the presence of a high level of trkB mRNA in primary brain cultures devoid of neuronal cells and highly enriched in glial fibrillary acid ic protein-positive astroglial cells prepared from newborn rat cerebra l hemispheres, cerebral cortex, hippocampus, and striatum. In primary astroglial cultures, the more abundant trkB transcripts code for the t runcated receptor without tyrosine kinase activity; probes specific fo r the full-length trkB mRNA did not detect any signal in northern blot analysis. By the sensitive ribonuclease protection assay, we could sh ow the presence of trkC mRNA in cultured astrocytes, whereas no trkA m RNA was detected. We confirmed the presence of relatively high levels of nerve growth factor and neurotrophin-3 mRNA, and Very low basal lev el of brain-derived neurotrophic factor mRNA. Moreover, we demonstrate d that another member of the neurotrophin family, neurotrophin-4, is a lso expressed in cultured astroglial cells. In view of the fact that m any functional receptors for conventional neurotransmitters or neurope ptides present on astroglial cells may act via the adenylate cyclase s ystem, we studied also the effect of agents able to increase the intra cellular cyclic AMP concentration. A sharp increase in the trkB mRNA l evel was observed after treatment of primary astroglial cultures with dibutyryl cyclic AMP, 8-bromo-cyclic AMP, or the phosphodiesterase inh ibitor, 3-isobutyl-1-methylxanthine. On the contrary, trkC mRNA levels were unaffected by treatment with cyclic AMP-elevating agents. All th e neurotrophin mRNAs examined, except neurotrophin-4, were increased b y 3-isobutyl-1-methylxanthine treatment. Therefore, in cultured astrog lial 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 neuron al activity or after injury events.