ESTIMATION OF THE NUMBER OF SOMATOSTATIN NEURONS IN THE STRIATUM - ANIN-SITU HYBRIDIZATION STUDY USING THE OPTICAL FRACTIONATOR METHOD

Somatostatin-containing neurons of the striatum constitute fewer than 5% of the total neuronal population. Their involvement in the feedforw ard inhibition of the spiny projection neurons, the modulation of othe r interneurons, and the regulation of regional blood flow indicates th at this small population of neurons plays an important role in the pro cessing of information in the striatum. As a first step in developing a quantitative structural framework within which a more rigorous analy sis can be made of the functional circuitry of the striatum, we used m odern unbiased stereological techniques to make estimates of the total number of neurons expressing mRNA for somatostatin in the striatum of rats. The strategy developed involved the application of the optical fractionator technique to relatively thick tissue sections that were h ybridized in situ with a relatively short oligonucleotide:probe conjug ated to a nonradioactive reporter molecule. The approach is generally applicable to other subpopulations of in situ hybridized cells in othe r parts of the brain and can provide a link between molecular neurobio logy and stereology. The mean total number of neurons on one side of t he striatum was estimated to be 21,300. An analysis of the sampling sc heme indicated that counting no more than 200 neurons in a systematic sample of not more than 15 sections per individual results in an estim ate with a precision that is more than sufficient for comparative and experimental studies. The issues that must be considered when analyzin g in situ hybridized tissue with modern stereological methods, the int erpretive caveats inherent in the resulting data, and the unique persp ectives provided by data like that presented here for striatal somatos tatin neurons are discussed. (C) 1996 Wiley-Liss, Inc.