Heterogeneous expression of carboxypeptidase E and proenkephalin mRNAs by cultured astrocytes

Cultured astrocytes have been found to express neuropeptides, neuropeptide processing enzymes and their mRNAs. Although astrocytes were shown to display regional variation in their expression of these mRNAs, it was unclear whether all astrocytes cultured from the same brain region express similar mRNA levels or if this expression is heterogeneous. We examined the individual heterogeneity of astrocytes cultured from several brain regions by in situ hybridization. Astrocytes derived from the frontal cortex, hypothalamus and cerebellum of neonatal rat brains were cultured for 3 weeks and then analyzed by in situ hybridization using 35S-labeled cRNA probes to carboxypeptidase E (CPE), proenkephalin (PE), and cyclophilin (1B15) mRNAs and an oligomeric probe complimentary to the first 45 bases of rat 18S rRNA. Frequency histograms generated by counting the grains produced over emulsion-coated cells demonstrated populations of astrocytes expressing high levels of mRNA for CPE and PE and a population expressing low to background levels of these mRNAs. In contrast, all cultured astrocytes expressed high levels of 1B15 mRNA and 18S rRNA. The percentage of cultured astrocytes expressing high levels of CPE mRNA was 42% for frontal cortex astrocytes and 23% for cerebellar astrocytes. While the percentages of cultured astrocytes expressing high levels of PE mRNA varied slightly between brain region (24-28%), the level of PE mRNA expression per cell showed greater variation between regions. The effect of culture density on the expression of PE mRNA was also examined. Approximately 55% of the cells in low density cultures expressed PE mRNA, while only 5-10% of the cells in high density cultures expressed this mRNA. These results indicate that cultured astrocytes display individual heterogeneity with regard to neuropeptide biosynthesis and that the expression of neuropeptides by these cells is regulated.