FOS expression in the gonadotropin-releasing hormone (GnRH) neuron does not increase during the ovarian steroid-induced GnRH surge in the rhesus monkey.

The purpose of this study was to investigate the expression of the immediate early gene, c-fos, in GnRH neurons in female rhesus monkeys as a function of generation of the LH surge. Adult monkeys were either intact (n = 6) or ovariectomized (n = 10). Intact animals received estradiol benzoate (EB; 330 micrograms in oil, sc; n = 5) or oil (n = 1). Ovariectomized animals received either EB (n = 5) or EB, followed by progesterone (P; 2.5 ml in oil, im; n = 4), or oil (n = 1). Animals were killed from 31-75 h after EB treatment. Blood samples were collected to document LH release in response to steroid treatment. A surge of LH was initiated in most animals that received EB alone or EB plus P about 30 h after steroid treatment. Animals were perfused with 4% paraformaldehyde, and brain blocks encompassing the region known to contain the majority of GnRH neurons (septum through the medial basal hypothalamus) were cut on the vibratome. Sites of FOS and GnRH immunoreactivities were demonstrated using double labels with a variety of chromogens. Regardless of the time in the surge, there were very few GnRH neurons with FOS immunoreactivity in their nuclei (0-9%). FOS-positive nuclei were seen in many other neurons in various brain regions, including the suprachiasmatic and supraoptic nuclei. There were no differences in FOS expression in GnRH neurons in intact and ovariectomized animals or in steroid- or oil-treated animals. These results suggest that FOS activation in GnRH neurons is not associated with the initiation of the secretory GnRH stimulus to the LH surge in the rhesus monkey. If confirmed, these data suggest that the GnRH nerve terminal may be the primary site for the control of the GnRH surge.