Processing of luteinizing hormone-releasing hormone precursor in rat neurons

Results of previous immunocytochemical studies indicate that in the rat brain proteolytic cleaving of LHRH precursors to generate the physiologically active decapeptide takes place within neuronal fibers and terminals and not within perikarya. A 69-amino acid (aa) LHRH precursor comprised of the decapeptide, a 3-aa cleavage and amidation site, and a 56-aa C-terminal extension has recently been characterized. Two antisera generated to specific aa sequences of the C-terminal extension (RM 8/5, anti aa 14-26; PS 39A, anti aa 40-53) and two directed to specific regions of the LHRH decapeptide (RM 1076, anti aa 4-8; A 422 generated to the N-terminal pGlu and C-terminal amidated Gly) were used to further examine intraneuronal sites of precursor processing. Patterns of immunoreactivity revealed with antisera directed to non-LHRH sequences of LHRH precursor paralleled those observed with antisera to the decapeptide. Immunopositive perikarya, processes, and neurovascular terminals were observed with PS 39A. Antiserum PS 39A binds to an internal aa sequence of the C-terminal extension and would, therefore, be expected to detect intact precursor LHRH as well as products of proteolytic cleavage. In contrast, only immunopositive processes and neurovascular terminals were observed with RM 8/5, an antiserum directed to an initial aa sequence of the C-terminal extension. The pattern of immunoreactivity revealed with RM 8/5 resembled that observed with an antiserum that binds the fully processed decapeptide (A 422), indicating that proteolytic cleavage of the decapeptide from the C-terminal extension is required for binding by this antiserum. Furthermore, the restricted distribution of reaction product observed with RM 8/5 relative to A 422 suggests that additional processing of the C-terminal extension may be required for binding. Such additional processing appears to occur in neurovascular terminals of the median eminence.