A question which is central to neurobiology relates to the regulation of biosynthesis, processing and secretion of peptides. We have examined these cellular processes using the LHRH prohormone as a model. Both nerve terminals (containing processed products) and cell bodies with their axons (containing proLHRH and partially processed products) were dissected separately. Materials from these tissues were separated by size using HPLC. Fractions were screened with antisera which were specific for different regions of the proLHRH molecule. The proLHRH was cleaved into 2 major products: LHRH and GAP34-56. While LHRH was not metabolized further, GAP was cleaved into 3500 molecular weight materials which may represent GAP1-32 and GAP34-56. In order to study regulation of biosynthesis and processing, we compared brain levels of all proLHRH-derived peptides from castrate and intact rats. Castration reduced levels of proLHRH and all its products relative to intacts; the pathway of processing was unaffected. In additional studies, nerve terminal fragments from brains of rats castrated for 14-days released approximately 2-fold less LHRH and GAP1-56 into the media than intact rats. Since tissue stores of both peptides in castrates were also reduced by 2-fold, the data were expressed in terms of the percent of peptide released. Similar percentages of peptides were released from intact and castrated rats under basal and (K+)-stimulated conditions. By contrast, castrates secreted a smaller percentage of both peptides in response to phorbol ester stimulation than intacts. Our results are significant because they describe the biosynthesis and metabolism of the LHRH precursor. In addition, our findings show that biosynthesis and secretion of proLHRH peptides may be regulated by testicular factors. Future studies will evaluate in greater detail the regulation of biosynthesis and processing, the nature of the proLHRH products, and the mechanism(s) responsible for the defect in the protein kinase C pathway in castrated rats.
