Quantitative In Situ Hybridization Using Radioactive Probes to Study Gene Expression in Heterocellular Systems

For several years, quantitative in situ hybridization has emerged as a particularly powerful technique to study gene expression within complex heterocellular systems, including the central nervous system, by measuring mRNA levels and their variations under experimental or physiological conditions. In comparison with other quantitative techniques such as Northern blot or dot blot, this approach makes possible the analysis of gene expression, not only at the regional level but also at the cellular level, which appears particularly useful for the study of heterogeneous cellular populations (1 -3 ). Quantitative in situ hybridization has been greatly improved during the past 10 years, especially by the development of various image analyzer systems. Moreover, the technique has reached a very high level of sensitivity by using appropriate standardization and methods of measurement (1 -9 ). Two levels of analysis yield different and complementary information: (1) macroscopic study using densitometry on X-ray films allows the measurement of variations in mRNA at the anatomical level and (2) microscopic study allows quantitation of mRNA at the cellular level (see Note 1 ).