In Vitro gonadotropin-releasing hormone release from hypothalamic tissues of ovariectomized estrogen-treated cynomolgus macaques

The effects of in vivo 17β-estradiol (E₂) treatment on in vitro GnRH release and serum LH levels were studied to determine the loci of E₂ feedback actions and to examine the hypothalamic mechanisms by which this steroid may regulate LH secretion in monkeys. Ovariectomized cynomolgus macaques received sc Silastic capsule implants containing E₂ and were killed 12, 36, 42, or 48 h later. At least one control (CTL) animal received a blank implant and was killed concurrently with each E₂-treated monkey. Three untreated animals were used in validation experiments. Before death, each animal was anesthetized with ketamine (15 mg/kg, im), and blood samples were drawn for subsequent LH analysis by Leydig cell bioassay. A diencephalic tissue block was obtained at autopsy and immediately immersed in Krebs-Ringer-phosphate medium (KRP). Mediobasal hypothalamic (MBH) and anterior hypothalamic/preoptic (AH/POA) fragments were quickly dissected from the block and placed in separate superfusion chambers maintained at 37 C. Tissues were superfused at 50 µl/min with KRP, and 10-min fractions were collected, acidified, and stored at -20 C for subsequent GnRH RIA. Basal immunoreactive GnRH (IR-GnRH) release was measurable from MBH (0.367 ± 0.063 pg/min) and AH/POA (0.176 ± 0.065 pg/min) fragments from CTL monkeys. In validation experiments, IR-GnRH release was increased 3- to 7-fold by superfusion with 60 mM K⁺-KRP only in the presence of Ca⁺². Superfusate IR-GnRH coeluted with synthetic GnRH from a Sephadex G-25 chromatographic column, and superfusate and tissue extract GnRH showed appropriate LH-releasing capacities, as determined by rat pituitary cell culture assay. IR-GnRH release rates from MBH or AH/POA tissues varied as a function of in vivo estrogen treatment. GnRH release from both tissues was increased in the E₂-treated group killed at 12 h when LH levels were suppressed. Thirty-six hours after E₂ treatment, in vitro GnRH release was not significantly different from CTL values. GnRH release rates from MBH and AH/POA tissues obtained 42 h after E₂ treatment were significantly greater than CTL release rates (P < 0.01). This increased in vitro GnRH release at 42 h occurred during the apparent rising phase of the LH surge. Elevated GnRH release was not sustained at 48 h, when surge levels of LH were apparent. Our data demonstrate that GnRH terminals in the MBH and AH/POA release GnRH in response to depolarizing stimuli in a Ca⁺²-dependent manner, that GnRH released from both tissues is immunologically and biologically indistinguishable from the synthetic decapeptide, and that in vivo E₂ treatment potentiates the in vitro secretory activity of GnRH terminals during initiation of the LH surge.