Temporal Alterations in Luteinizing Hormone-Releasing Hormone Concentrations in Several Discrete Brain Regions: Effects of Estrogen-Progesterone and Norepinephrine Synthesis Inhibition*

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Abstract
Studies were undertaken to identify the brain regions which display temporal changes in LHRH concentrations after progesterone (P) administration (on day 2 at 1000 h) to ovariectomized, estrogen-primed (OVX-EP) rats (day 0). Groups of OVX-EP rats were decapitated at 1000, 1200, 1400, 1600, and 1800 h and the following regions were isolated by microdissection for subsequent analyses of LHRH by RIA: nucleus (N.) preopticus medialis, N. hypothalamicus anterior, N. suprachiasmatic region (NSC), area retrochiasmatic (ARC), N. arcuate (NA) region, and median eminence (ME). In these rats, ME and NA LHRH concentrations first increased 2- and 3-fold, respectively, by 1400 h when serum gonadotropin levels were basal, and later at 1600 h LHRH levels decreased precipitously concomitant with peak serum LH and FSH levels. In the NSC and ARC regions, however, LHRH concentrations declined slowly reaching significantly lower values by 1600–1800 h. P treatment did not influence LHRH concentrations in the N. preopticus medialis or N. hypothalamicus anterior through 1000–1800 h. Pretreatment of OVX-EP rats with a norepinephrine synthesis inhibitor, diethyldithiocarbamate (550 mg/kg; 0900 h), abolished the effects of P on serum gonadotropins and the LHRH concentration in the ME, NA, NSC, and ARC regions; instead, there was a progressive decrease in LHRH concentrations, reaching significantly lower values in the NA region at 1400 h and in the ME at 1800 h. These studies show that 1) P induces rapid fluctuations in the LHRH concentration in regions within the medial basal hypothalamus; 2) on the basis of quantitative and temporal assessment of LHRH changes, P feedback action may predominantly be on LHRH neurons in the NA-ME region; and 3) dynamic changes in LHRH concentrations in response to P require an intact central noradrenergic component.