Pulsatile exocytosis is functionally associated with GnRH gene expression in immortalized GnRH-expressing cells

Pulsatile release of GnRH is essential for proper reproductive function, but little information is available on the molecular processes underlying this intermittent activity. Recently, GnRH gene expression (GnRH-GE) episodes and exocytotic pulses have been identified separately in individual GnRH-expressing cells, raising the exciting possibility that both activities are linked functionally and are fundamental to the pulsatile process. To explore this, we monitored GnRH-GE (using a GnRH promoter-driven luciferase reporter) and exocytosis (by FM1-43 fluorescence) in the same, living GT1-7 cells. Our results revealed a strong temporal association between exocytotic pulses and GnRH-GE episodes. To determine whether a functional link existed, we blocked one process and evaluated the other. Transcriptional inhibition with actinomycin D had only a modest influence on exocytosis, suggesting that exocytotic pulse activity was not dictated acutely by episodes of gene expression. In contrast, blockage of exocytosis with anti-SNAP-25 (which obstructs secretory granule fusion) abolished GnRH-GE pulse activity, indicating that part of the exocytotic process is responsible for triggering episodes of GnRH-GE. When taken together, our findings suggest that a careful balance is maintained between release and biosynthesis in GT1-7 cells. Such a property may be important in the hypothalamus to ensure that GnRH neurons are in a constant state of readiness to respond to changes in reproductive function.