The recently discovered protein leptin has a molecular mass of 16 kDa, consists of 146 amino acids, and is synthesized and secreted by adipose tissue. Leptin affects feed intake, the neuroendocrine-axis, and immunological processes. The protein was first identified as the gene product that is deficient in the obese ob/ob mouse. Leptin serves as a circulating signal of nutritional status and plays a pivotal role in regulation of body weight, energy expenditure, growth, and reproduction.

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The objective of this study was to determine whether AIMAX (a dithiocarbamoylhydrazine derivative) is suitable for determining the effects of norepinephrine (NE) depletion on reproduction in domestic animals. Therefore, the effect of AIMAX (n = 6) on concentrations of biogenic amines in the medial basal hypothalamus (MBH) and anterior hypothalamic area (AHA) of ovariectomized (OVX) rats primed with ovarian steroids was compared to that of diethyldithiocarbamate (DDC; n = 5), a potent dopamine-beta-hydroxylase (DBH) inhibitor, which is chemically similar to AIMAX. Rats that received only ovarian steroids and saline injections served as controls (n = 6). Treatment with DDC resulted in sedation and reduced body temperature. In contrast, rats behaved normally after AIMAX treatment. AI-MAX reduced (P < .05) NE but increased (P < .05) dopamine (DA) concentrations in MBH and AHA compared with controls. Similar changes in NE and DA concentrations were observed in DDC-treated rats. However, elevated epinephrine (EPI) levels were measured in MBH and AHA of only DDC-treated rats. Serum LH concentrations were suppressed (P < .005) in both AIMAX- and DDC-treated rats compared with control animals. Because AIMAX, like DDC, suppressed hypothalamic NE content and LH secretion, AIMAX should be useful in studying effects of NE depletion on gonadotropin secretion in domestic animals.

This study was conducted to determine whether progesterone inhibits luteinizing hormone (LH) secretion in female pigs by a direct action on the pituitary gland. Eight ovariectomized, hypophysial stalk-transected gilts were given 1-pg pulses of gonadotropin-releasing hormone iv every 45 min from Day 0 to 12. On Days 5-12, each of four gilts received either progesterone or oil vehicle im at 12-hr intervals. Serum progesterone concentrations in steroid-treated gilts reached 70 2 6.8 ng/ml (mean 2 SE) by Day 8 and remained elevated thereafter, whereas serum progesterone concentrations in oil-treated controls were less than 1 ng/ml for the entire study. Daily serum LH concentrations were not different between gilts treated with progesterone or oil. The 1-pg pulses of gonadotropin-releasing hormone reliably evoked pulses of LH in both treatment groups. The LH pulse frequency and amplitude, assessed from samples collected every 15 min for 6 hr on Day 12, were similar for progesterone-and oil-treated gilts. These results provide evidence that progesterone does not act at the pituitary gland to alter LH secretion in pigs.gonadotropin-releasing hormone (GnRH) from the hypothalamus (5). Progesterone, therefore, may decrease LH secretion in the pig by acting directly on the pituitary gland and/or inhibiting the secretion of GnRH. In an effort to determine the site of the negative feedback of progesterone in pigs, we assessed LH secretion after the administration of progesterone to hypophysial stalktransected (HST) gilts given unvarying pulses of GnRH.