Peptide YY(3-36)-induced inhibition of food intake in female monkeys.

Peptide YY (PYY) is produced in L cells of the intestine and is released after eating. PYY circulates in a truncated form designated PYY(3-36). PYY(3-36) is thought to be a physiologic anorexigenic peptide. The objective of the current study was to test the effect of exogenous PYY(3-36) on food intake in non-human primates exposed to different ovarian steroid milieus. The study was conducted in four ovariectomized cynomolgus monkeys replaced with estrogen alone for 2 weeks followed by estrogen in combination with progesterone for 2 weeks to mimic the menstrual cycle. The effect of PYY(3-36) on food intake was tested during each week of the simulated menstrual cycle by comparing the 2 h food intake following intracerebroventricular (icv) injection of artificial cerebrospinal fluid (aCSF) or PYY(3-36). Despite considerable variation in food intake following aCSF, PYY(3-36) consistently inhibited food consumption, except during week 2 of estrogen plus progesterone replacement. PYY(3-36) reduced food consumption by 16.2 g (95% confidence interval (CI)=4.5-27.9 g) and 26.6 g (95% CI=7.3-45.9 g) in weeks 1 and 2 respectively of estrogen only treatment and by 38.2 g (95% CI=26.1-50.2 g) in week 1 of estrogen plus progesterone treatment. In contrast, PYY(3-36) injected in week 2 of estrogen plus progesterone did not consistently inhibit food intake (13.1 g; CI=-49.5-75.7). This is the first study to report the effect of PYY(3-36) on food consumption in female monkeys. We conclude that icv administration of PYY(3-36) has a strong anorexic effect in female cynomolgus monkeys and that sensitivity to PYY(3-36) may be influenced by the ovarian steroid milieu.

Effect of leptin administration on ovulation in food-restricted rhesus monkeys.

A chronic negative energy balance due to low nutritional intake or increased energy expenditure alters several neuroendocrine axes. The reproductive and thyroid axes are inhibited while the adrenal axis is stimulated. In primates, anovulation resulting from a chronic negative energy balance is a condition often referred to as nutritional amenorrhea. The objective of the current study was to determine if hypoleptinemia induced by dietary restriction is responsible for these neuroendocrine changes, particularly anovulation. Five rhesus monkeys had their dietary intake gradually reduced to inhibit ovulation. Dietary restriction inhibited follicle-stimulating hormone (FSH) and triiodothyronine (T(3)) secretion and stimulated cortisol release. Recombinant human leptin (rhleptin) administered by continuous infusion into the lateral ventricle for 16 weeks inhibited cortisol secretion but failed to stimulate FSH, T(3) or ovulation. An immune response to rhleptin was noted after 3 weeks of leptin administration. Realimentation resulted in weight gain and reversed all endocrine responses to dietary restriction, including ovulation. These results do not support a role for reduced leptin secretion in anovulation induced by dietary restriction. The inability of rhleptin to reverse anovulation induced by a negative energy balance in monkeys is in contrast to its stimulatory effect on ovulation in women with functional hypothalamic amenorrhea. Different outcomes may be attributed to the degree of negative energy balance, the immune response generated by interspecies leptin administration, and/or other experimental variables such as dose or route of administration. Attributing opposing outcomes to species differences is unwarranted until these variables can be further examined.

Developing a model of nutritional amenorrhea in rhesus monkeys.

Nutritional amenorrhea is defined as cessation of menstrual cycles resulting from a chronic negative energy balance. Although it is agreed that nutritional amenorrhea results from reduced secretion of GnRH, the neuroendocrine mechanisms leading to GnRH inhibition are poorly defined. Because the invasiveness of many neuroendocrine experimental approaches precludes its use in the clinical setting, we set out to establish a model of nutritional amenorrhea in rhesus monkeys. Studies were conducted in four normal-weight and one obese female rhesus monkey. Dietary intake was gradually reduced with the goal of achieving a 15-20% weight reduction. Dietary restriction inhibited ovulation in all animals. The weight loss required to inhibit ovulation ranged from 2-11% in the four normal-weight animals and was achieved with a 23% reduction in dietary intake. The time of initiating reduced food intake to first missed ovulation was 62 +/- 13 d. Greater weight loss (46% reduction) over a longer period (10 months) was required to inhibit ovulation in the obese monkey. The onset of anovulation was not preceded by changes in menstrual cycle length or progesterone secretion. Realimentation initiated ovulation at a weight that approximated the animal's weight at the time of the last ovulatory cycle during dietary restriction. By contrast, caloric intake at the return of ovulation during realimentation was 28% greater. This is the first demonstration that chronic dietary restriction can inhibit ovulation in rhesus monkeys. This model will be useful for studying the neuroendocrine mechanisms involved in diet-induced anovulation in primates.

Caloric restriction inhibits steroid-induced gonadotropin surges in ovariectomized rhesus monkeys.

We recently reported that caloric restriction inhibited ovulation in rhesus monkeys. The objective of the current study was to determine if caloric restriction affected the positive feedback response to ovarian steroids in non-human primates. Studies were conducted in four long-term ovariectomized rhesus monkeys. Animals were given an estrogen/progesterone challenge while maintained on a normal diet and on a diet that reduced body weight by approx 20%. In all cases, animals were maintained at the desired weight [based on a calculation of body mass index (BMI)] for a minimum of 4 wk before initiating the steroid challenge. Caloric restriction reduced BMI from 23.3 +/- 0.3 to 18.9 +/- 0.2 kg/m2. The estrogen/progesterone challenge elicited an LH and FSH surge in each animal maintained at a normal BMI. By contrast, gonadotropin surges were significantly compromised when monkeys were challenged at a low BMI. In addition to affecting the reproductive axis, caloric restriction stimulated cortisol release and suppressed T3 secretion. These endocrine effects of caloric restriction are consistent with our findings in ovary-intact monkeys. In summary, our previous reports in ovary-intact animals confirmed an effect of caloric restriction on tonic gonadotropin secretion leading to anovulation. Our current results suggest the effects of caloric restriction on the reproductive axis extend beyond inhibition of tonic gonadotropin secretion to include a disturbance of phasic gonadotropin secretion.

Hypoglycemia does not affect gonadotroph responsiveness to gonadotropin-releasing hormone in rhesus monkeys.

Hypoglycemia inhibits gonadotropin secretion in primates by an undefined mechanism. Some evidence suggests that hypoglycemia inhibits gonadotropin secretion independent of gonadotropin-releasing hormone (GnRH) inhibition. To this end, the effect of insulininduced hypoglycemia on the luteinizing hormone (LH) and follicle-stimulating hormone (FSH) response to graded doses of GnRH (25, 75, and 250 ng/kg) administered at 120-min intervals was determined in rhesus monkeys. A crossover design was employed such that each animal received GnRH under both hypoglycemic and euglycemic conditions. Experiments were performed in the follicular phase. Gonadotroph responsiveness to GnRH was quantified by determining the change in area under the LH (DeltaAULHC) and FSH (DeltaAUFSHC) curves that occurred in the first 60 min following each GnRH pulse. There was no statistical difference in DeltaAULHC between euglycemic and hypoglycemic animals at any GnRH dose (25 ng/kg: p = 0.19; 75 ng/kg: p = 0.41; 250 ng/kg: p = 0.46). Similarly, changes in AUFSHC following GnRH administration were comparable in euglycemic and hypoglycemic animals (25 ng/kg: p = 0.59; 75 ng/kg: p = 0.90; 250 ng/kg: p = 0.33). We conclude that hypoglycemia had no effect on gonadotroph responsiveness to GnRH. These results are consistent with the conclusion that hypoglycemia inhibits gonadotropin secretion by acting primarily at the level of the hypothalamus to reduce GnRH secretion rather than affecting pituitary responsiveness to GnRH.

Effectiveness of photodynamic ablation for destruction of endometrial explants in a rat endometriosis model.

OBJECTIVE: To evaluate the potential of photodynamic therapy with aminolevulinic acid (ALA-PDT) for ablation of endometrial explants in a rat endometriosis model and to compare the effect of ALA-PDT, electrosurgery, and surgical resection on normal peritoneum. DESIGN: Prospective controlled experimental trial. SETTING: University medical center. ANIMAL(S): Mature Sprague-Dawley female rats. INTERVENTION(S): Induction of endometriosis and subsequent treatment with ALA-PDT; electrosurgery, and simple resection, and ALA-PDT of normal peritoneum. MAIN OUTCOME MEASURE(S): Histopathological assessment. RESULT(S): Systemic ALA followed by exposure to photoactivating light for 10 or 15 minutes resulted in ablation of all explants harvested 3-4 days after treatment. Permanent destruction was confirmed by absence of regrowth by week 3. Exposure of normal peritoneum to ALA-PDT resulted in initial necrosis, with complete recovery by day 16. Adhesions were present on day 16 in 50% of cases after electrosurgery and in 100% of cases after resection. No adhesions were present in ALA-PDT-treated animals. CONCLUSION(S): Systemic ALA followed by exposure to photoactivating light at relatively low power densities for periods as brief as 10 minutes resulted in ablation of endometriotic explants. Exposure of normal peritoneum to ALA-PDT resulted in complete resurfacing. Both electrosurgery and surgical resection resulted in a greater incidence of surface adhesions.

Estrogen-induced gonadotropin surge in rhesus monkeys is not inhibited by cortisol synthesis inhibition or hypoglycemia.

Acute administration of corticotrophin-releasing hormone (CRH) has been shown to inhibit gonadotropin secretion in several species including rodents, sheep, humans, and nonhuman primates. Similarly, a variety of acute stressors have been shown to inhibit tonic gonadotropin secretion and may do so through a CRH mechanism. Stress-induced inhibition of tonic gonadotropin secretion below levels required for follicular maturation would be expected to inhibit ovulation. An additional mechanism whereby acute stressors could interfere with ovulation is through inhibition of the preovulatory gonadotropin surge. In the present study, we determined the effect of acute activation of the hypothalamic-pituitary-adrenal (HPA) axis on phasic gonadotropin secretion in female rhesus monkeys. Activation of the HPA axis was achieved by either a hypoglycemic challenge or blockage of cortisol synthesis with metyrapone, 24 h after an estradiol benzoate challenge. Neither metyrapone nor insulin-induced hypoglycemia inhibited gonadotropin secretion. In fact, the initiation of the luteinizing hormone and follicle-stimulating hormone surge was advanced by 7.4 +/- 0.4 h (p < 0.001) and 4.8 +/- 1.4 h (p = 0.04) respectively, in metyrapone-treated monkeys compared with saline controls. By contrast, hypoglycemia did not affect the gonadotropin surge. The gonadotropin surge was preceded by increased progesterone secretion in metyrapone-treated but not insulin-treated monkeys. This difference in progesterone secretion likely explains the advancement of the gonadotropin surge in the metyrapone-treated animals.