In the present preliminary investigations carried out in Xenopus laevis by means of the autoradiographic technique, we have examined the occurrence of prolactin (PRL) binding sites in some brain structures, in particular at the level of the choroid plexus cells, after intracardiac injection of ovine or rat 125I-PRL. Control animals were injected, either subsequently or simultaneously, with much higher doses of a presumptive or specific competitor (insulin or unlabeled prolactin e.g. respectively) in order to assess the specificity of the tracer. Histoautoradiographs demonstrate the occurrence of bindig radioactive PRL at the level of the capillary lumina throughout the brain, and at the levels of the median eminence, pituitary neural lobe and choroid plexus cells. The specificity of the labeling consistently evidenced by autoradiography could not be ascertained in any of the above experiments. Owing to the number of technical difficulties, we feel that, under the foregoing experimental conditions at least, the histoauto radiographic technique should be resorted only to localize, histo- and cytologically, the occurrence of PRL binding sites, while relying on the biochemical approach when actual specificity is to be ascertained.
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