Application of in situ hybridization to studies of messenger RNA in the nervous system: functional correlations and their quantitation

Abstract

With a tissue as heterogeneous and complex in its organization as the nervous system, in situ hybridization offers an approach of considerable potential. The complexity of this tissue means that a technique with high resolving power is needed to identify changes in gene expression that occur in a small group of cells. The CNS is extremely heterogeneous, it contains a dozen classical transmitters and more than 40 neuropeptides all in discrete cell clusters. This gives rise to a diversity of functional cell types each with its own strict localization and interaction with other cells. Activation of specific pathways may then result in only highly localized changes in metabolism which would be difficult to detect with tissue homogenates where the transmitter being studied may only be present in 1% of cells or less. The technique of in situ hybridization however allows the detection of highly localized changes in mRNA levels in specific cell types. The applications are numerous, from tracing pathways, localization of function to particular cell groups and localization of mRNAs of unknown function. Further, abnormal gene expression in neurological disorders may give some insight into the pathogenesis of disease and examples of these different applications will be given in the text. To satisfy the requirements for in situ hybridization it is essential to preserve the tissue during preparative treatments in order to be able to accurately define the cell types containing the mRNA being studied. Many of the mRNA species of particular interest are often far from abundant e.g. receptors and neuropeptides. Thus in studies of the nervous system, resolution, structural integrity and quantitation are key issues and these aspects will be discussed in detail together with the particular applications for which these approaches have been used.