Immunohistochemical analysis of the effects of cysteamine on somatostatin-like immunoreactivity in the rat central nervous system.

The brain and spinal cord of untreated and cysteamine-treated rats were analyzed with immunohistochemistry using antisera raised against somatostatin (SOM)-28(1-14) and SOM-28(15-28). Sections incubated with increasing dilutions of antiserum were evaluated subjectively on coded slides and with computer-assisted image analysis. For control experiments, antisera raised against methionine-enkephalin, neuropeptide Y (NPY) and dynorphin (DYN)(1-13) were used. The latter antiserum does not visualize the conventional DYN systems in the brain, but reacts with an unknown epitope, which here could be shown to be present in SOM neurons. In cysteamine-treated rats a marked decrease in SOM-28(15-28)-like immunoreactivity (1.1) could be recorded subjectively at all antibody concentrations in fibers in several brain areas, including nucleus accumbens, tuberculum olfactorium and the hypothalamic ventromedial and arcuate nuclei. In these areas SOM-LI is fairly weak in untreated rats. In SOM-rich regions such as the median eminence and the dorsal horn of the spinal cord, the depleting effect of cysteamine could be recorded subjectively only when diluted antisera were used. Image analysis confirmed the subjective analysis, and, in addition, differences between controls and cysteamine-treated rats could be shown also at high antiserum concentrations. SOM-28(15-28)-immunoreactive cell bodies could be seen in the brains of either control or drug-treated rats. No effect of cysteamine could be observed when antiserum raised to SOM-28(1-14) was used. Cysteamine did not seem to affect enkephalin-LI, NPY-LI or an epitope in SOM neurons reacting with DYN(1-13) antiserum. After preabsorption of SOM-28(15-28) antiserum with SOM-28(15-28) peptide, the staining patterns described above disappeared completely. However, if the SOM-28(15-28) peptide was pretreated with a high concentration (1 M) of cysteamine before being used for absorption with SOM antiserum, no blocking effect could be observed. The present results demonstrate with immunohistochemistry that cysteamine causes depletion of SOM-28(15-28) in fibers but apparently not in cell bodies. No effects on SOM-28(1-14)-LI were observed. This supports earlier evidence that cysteamine interacts with the disulphide bond in the SOM-28(15-28) molecule. The present results also emphasize that when analyzing drug effects on peptide neurons with immunohistochemical techniques, it is important to use dilution series of antibodies and preferably to carry out the analysis with objective image analysis methods.