Effect of inhibition of estrogen synthesis during the luteal phase on function of the corpus luteum in rhesus monkeys

It has been hypothesized that estrogen synthesized by the corpus luteum initiates luteal regression during the nonfertile menstrual cycle in primates. To study the role of endogenous estrogens in functional regression of the monkey corpus luteum, we administered the aromatase inhibitor 1,4,6-androstatriene-3,17-dione (ATD) to rhesus monkeys during the luteal phase of the menstrual cycle. Twice-daily oral administration of ATD suppressed systemic and intraluteal estrogen levels by 80-90%. The midluteal phase rise in estradiol concentrations that occurs in rhesus monkeys was completely abolished by ATD treatment. Despite suppression of estrogen synthesis during the luteal phase, mean menstrual cycle length and length of the luteal phase were not different than in control monkeys treated with vehicle only. Progesterone levels were lower in the ATD-treated group on the second and third day of treatment, but did not differ from control levels during the remainder of the cycle. These data suggest that elevated estrogen synthesis during the luteal phase of the menstrual cycle is not a prerequisite for spontaneous luteolysis in rhesus monkeys.     

Effects of fenofibrate on lipid parameters in obese rhesus monkeys

Fenofibrate is a member of the fibrate class of hypolipidemic agents used clinically to treat hypertriglyceridemia and mixed hyperlipidemia. The fibrates were developed primarily on the basis of their cholesterol and triglyceride lowering in rodents. Fibrates have historically been ineffective at lowering triglycerides in experimentally-induced dyslipidemia in nonhuman primate models. The spontaneously obese rhesus monkey is a well-recognized animal model for the study of human obesity and type 2 diabetes, and many of these monkeys exhibit naturally occurring lipid abnormalities, including elevated triglycerides and low HDL cholesterol (HDL-C), similar to patients with type 2 diabetes. To explore whether the obese rhesus model was predictive of the lipid lowering effects of fibrates, we evaluated fenofibrate in six hypertriglyceridemic, hyperinsulinemic, nondiabetic animals in a 20-week, dose-escalating study. The study consisted of a 4-week baseline period, two treatment periods of 10 mg/kg twice daily (b.i.d) for 4 weeks and 30 mg/kg b.i.d. for 8 weeks, and a 4-week washout period. Fenofibrate (30 mg/kg b.i.d) decreased serum triglycerides 55% and LDL-C 27%, whereas HDL-C increased 35%. Apolipoproteins B-100 and C-III levels were also reduced 70% and 29%, respectively. Food intake, body weight, and plasma glucose were not affected throughout the study. Interestingly, plasma insulin levels decreased 40% during the 30 mg/kg treatment period, suggesting improvement in insulin sensitivity. These results support the use of obese rhesus monkey as an excellent animal model for studying the effects of novel hypolipidemic agents, particularly agents that impact serum triglycerides and HDL-C.     

Injection of soluble vascular endothelial growth factor receptor 1 into the preovulatory follicle disrupts ovulation and subsequent luteal function in rhesus monkeys

Remarkable changes in vascular permeability and neovascularization occur within the ovulatory, luteinizing follicle. To evaluate the importance of vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) in periovulatory events, sequential experiments were designed in which vehicle (PBS/0.1% BSA; controls, n = 13) or a low dose (1.5 micro g; n = 4) or a high dose (7.5 micro g; n = 4) of a VEGF antagonist, soluble VEGF receptor 1 (sVEGFR1) chimera, was injected directly into the preovulatory follicle of rhesus monkeys the day before (Day -1) or the day of (Day 0) the midcycle LH surge during spontaneous menstrual cycles. After vehicle injection, animals typically exhibited patterns and levels of serum progesterone (P(4)) that were comparable to those of untreated animals in our colony. Following low-dose sVEGFR1 injection, serum P(4) levels were diminished in two of four animals from the early to midluteal phase, but were similar to vehicle controls thereafter. In contrast, high-dose sVEGFR1 injection decreased serum P(4) levels throughout the luteal phase compared with levels in controls (P < 0.05), but it did not cause premature menstruation. Control follicles displayed indices of rupture (protruding stigmata) and luteinization. However, sVEGFR1-injected follicles exhibited signs of distension (torn surface epithelium/tunica albuginea) and luteinization, but not necessarily timely ovulation. Histological evaluation of serial sections from ovaries removed on Day 3 after treatment revealed that all (n = 3) vehicle-injected follicles ovulated, whereas half (n = 3 of 6) the sVEGFR1-injected follicles failed to ovulate and still contained an oocyte in the antrum. No appreciable differences were apparent between treatment groups in numbers of cells in luteal tissue (Day 3 or 6 after treatment) that stained positive for immunochemical or histochemical markers of proliferative (Ki67), endothelial (platelet endothelial cell adhesion molecule 1), and steroidogenic (3beta-hydroxysteroid dehydrogenase) cells. However, there was a dose-dependent increase (P < 0.05) in extracellular space in the corpus luteum by midluteal phase in sVEGFR1-treated animals. The data suggest that acute exposure to a VEGF antagonist can impair ovulation, and the subsequent development and functional capacity of the primate corpus luteum. The results are consistent with a critical role for VEGF in normal ovarian function during the periovulatory interval in primates.     

Assessment of motor function of the hand in aged rhesus monkeys

In the elderly, intact motor functions of the upper extremity are critical for the completion of activities of daily living. Many studies have provided insight into age-related changes in motor function. However, the precise nature and extent of motor impairments of the upper extremity remains unclear. In the current study we have modified two tasks to assess hand/digit function in both young and aged rhesus monkeys. We tested monkeys from 9 to 26 years of age on these tasks to determine the level of fine motor performance across the adult age range. Compared to young monkeys (9–12 years of age), aged monkeys (15–26 years of age) were mildly impaired on fine motor control of the digits. These findings are consistent with previous studies that have found age-related impairment in fine motor function. However, the magnitude and extent of impairment in the current study does differ from previous findings and is likely due to methodological differences in the degree of task complexity.     

Effect of an LHRH agonist on pituitary and testicular function in rhesus monkeys

Male rhesus monkeys were given 100 micrograms [(imBzl)-D-His6,Pro9-NEt]-LHRH (LHRH-A), a potent LHRH agonist, s.c. daily for 40 weeks. The first dose of LHRH-A caused acute increases (2-4 h after injection) in serum LH (50-fold), FSH (2 X 5-fold) and testosterone (15-fold) concentrations. Chronic treatment led to a 95% decrease in LH and FSH responses. In spite of a marked decrease in LH response the effect on testosterone response was less evident. Administration of 50 i.u. hCG to control and LHRH-A-treated animals showed that the testicular steroidogenic response was unimpaired by the chronic treatment. Evaluation of the electroejaculated semen at regular intervals showed that there was no consistent reduction in the sperm count of LHRH-A-treated monkeys. Testicular biopsies showed that normal spermatogenesis was occurring in all treated animals, but testicular volume was significantly decreased. These results suggest that, in rhesus monkeys, the pituitary is more susceptible to desensitization by chronic LHRH agonist treatment than are the testes, and that LHRH agonists do not have direct antitesticular effect in rhesus monkeys.     

Effects of artificial feeding on aggressive behaviour of rhesus monkeys in India

Artificial feeding of rhesus monkeys (Macaca mulatta), which provides the major food source for most rhesus groups in rural and urban habitats, is common practice in India. Such feeding results in significant increases in aggressive competition within the monkey groups: during feeding periods the frequencies of aggressive threats, chases, and attacks increased two to six times above those of non-feeding periods in each of eight different groups. Fighting behaviour did not increase in most groups studied, but did show a rise in one large urban group in Calcutta. The implications of these results for the management of primate breeding colonies are discussed.     

Endocardiosis in rhesus monkeys

Endocardiosis was diagnosed as an incidental finding in two rhesus monkeys. The gross and histologic appearance of the lesions was described, and the similarity of this lesion to lesions of endocardiosis as found in dogs and man was discussed.The animals used in this study were handled in accordance with the Guide for Laboratory Animal Facilities and Care established by the National Academy of Science, National Research Council.     

Effects of puzzle feeders on pathological behavior in individually housed rhesus monkeys

Self-injurious behavior (SIB) occurs in about 10% of individually housed monkeys. Monkeys with SIB bite their own bodies frequently, occasionally inflicting wounds as a result. At present, there is no standard treatment for this phenomenon. We examined the effectiveness of puzzle feeders in alleviating SIB in monkeys with a veterinary record of self-inflicted wounding. Two groups of monkeys (SIB and controls) were exposed to puzzle feeders for a 6 week period. Three levels of maze difficulty were examined. All monkeys used the feeders, but manipulation was confined to a brief period immediately after the feeders were loaded each day (1000 h) and was infrequent during the later sampling periods (1100 and 1400 h). The most difficult maze yielded a slight increase in usage at 1100 h. During the puzzle feeder phase, whole body stereotypies, including pacing and rocking, were reduced substantially in all monkeys at 1000 h when feeder manipulation was at its highest. However, self-biting in the SIB group was unchanged. Some monkeys actually bit themselves while manipulating the feeder. Long-term effects on abnormal behavior were assessed by comparing behavior during the feeder phase to baseline periods and to a phase in which the monkeys were provisioned with treats placed directly into their food box. Whole body stereotypies, including pacing, were reduced during both treatment phases; however, the reduction was associated only with the 1000 h observation. Puzzle feeders were more effective than treats alone in alleviating whole body stereotypies. Self-biting was unchanged through all phases. Puzzle feeders are beneficial from the perspective of eliciting manipulation. They also yield transient reductions in whole body stereotypy, an effect that does not extend beyond the direct manipulation of the feeder. Puzzle feeders are ineffective in alleviating self-injurious behavior. Am. J. Primatol. 46:213–227, 1998. © 1998 Wiley-Liss, Inc.     

Self-agency in rhesus monkeys

Rhesus monkeys (Macaca mulatta) have shown the ability to monitor their own mental states, but fail the mirror self-recognition test. In humans, the sense of self-agency is closely related to self-awareness, and results from monitoring the relationship between intentional, sensorimotor and perceptual information. Humans and rhesus monkeys were trained to move a computer icon with a joystick while a distractor icon partially matched their movements. Both humans and monkeys were able to monitor and identify the icon they were controlling, suggesting they have some understanding of self-agency.     

Effects of atropine on operant test battery performance in rhesus monkeys.

The acute behavioral effects of atropine sulfate were assessed using a battery of complex food-reinforced operant tasks that included: temporal response differentiation (TRD, n = 7); delayed matching-to-sample (DMTS, n = 6), progressive ratio (PR, n = 8), incremental repeated acquisition (IRA, n = 8), and conditioned position responding (CPR, n = 8). Performance in these tasks is thought to depend primarily upon specific brain functions such as time perception, short-term memory and attention, motivation, learning, and color and position discrimination, respectively. Atropine sulfate (0.01-0.56 mg/kg iv), given 15-min pretesting, produced significant dose-dependent decreases in the number of reinforcers obtained in all tasks. Response rates decreased significantly at greater than or equal to 0.03 mg/kg for the learning and discrimination tasks, at greater than or equal to 0.10 mg/kg for the motivation and short-term memory and attention tasks, and at greater than or equal to 0.30 mg/kg for the time perception task. Response accuracies were significantly decreased at doses greater than or equal to 0.10 mg/kg for the learning, discrimination, and short-term memory and attention tasks, and at greater than or equal to 0.30 mg/kg for the time perception task. Thus, the order of task sensitivity to any disruption by atropine is learning = color and position discrimination greater than time perception = short-term memory and attention = motivation (IRA = CPR greater than TRD = DMTS = PR). Thus in monkeys, the rates of responding in operant tasks designed to model learning and color and position discrimination were the most sensitive measures to atropine's behavioral effects. Accuracy in these same task was also disrupted but at higher doses. These data support the hypothesis that cholinergic systems play a greater role in the speed (but not accuracy) of performance of our learning and discrimination tasks compared to all other tasks. Accuracy of responding in these and the short-term memory task, all of which involve the use of lights as visual stimuli, was more sensitive to disruption by atropine than those tasks which did not utilize such strong visual stimuli.     

Effects of fetal decapitation on the structure and function of Leydig cells in rhesus monkeys (Macaca mulatta).

Fetal decapitation in utero has enabled us to study the role of fetal pituitary hormones in the development of the fetal testis. Testes from males decapitated near 80 days of gestational life and later delivered at 150 days were smaller than normal and about one-tenth the normal weight. The size of the seminiferous tubules was similar in both groups; however, the number of Leydig cells seemed reduced. In addition, the Leydig cells of the experimental group contained smaller mitochondria with reduced tubular cristae, fewer lipid droplets, and reduced agranular endoplasmic reticulum. Androgen production was inhibited. Measured by radioimmunoassay, the testosterone level in the umbilical vein was 329 +/- 82 pg/ml in six decapitates fetuses, 412 +/- 62 pg/ml in ten normal fetuses. The level in the umbilical artery was 328 +/- 56 pg/ml in five decapitated fetuses, 658 +/- 140 pg/ml in normal fetuses. These studies suggest that chronic deprivation of fetal pituitary hormones inhibits the growth and development of the testis in general and of the Leydig cells in particular.     

Effects of chair restraint on the strength of the tibia in rhesus monkeys

To determine the effects of the relative inactivity and unloading on the strength of the tibias of monkeys, Macaca mulatta, we used a non-invasive test to measure bending stiffness, or EI (Nm2), a mechanical property. The technique was validated by comparisons of in vivo measurements with standard measures of EI in the same bones post-mortem (r2 = 0.95, P < 0.0001). Inter-test precision was 4.28+/-1.4%. Normative data in 24 monkeys, 3.0+/-0.7 years and 3.6+/-0.6 kg, revealed EI to be 16% higher in the right than left tibia (4.4+/-1.6 vs. 3.7+/-1.6 Nm2, P < 0.05). Five monkeys, restrained in chairs for 14 days, showed decreases in EI. There were no changes in EI in two chaired monkeys that lost weight during a 2-week space flight. The factors that account for both the decreases in bone mechanical properties after chair restraint at 1 g and lack of change after microgravity remain to be identified. Metabolic factors associated with body weight changes are suggested by our results.     

Circadian and circannual changes in the testicular function of adult rhesus monkeys

The endocrine and gametogenic status of the testes were studied in 9 healthy adult rhesus monkeys of proven fertility throughout a one-year period. Testosterone levels were estimated by radioimmunoassay in blood samples collected at 4 h intervals over a 24 h period once a month. Semen samples and testicular biopsies were also examined once a month. A well-defined circandian rhythm was evident in serum levels of testosterone. The rhythmicity was less pronounced in February and September. The 24 h mean levels of serum testosterone were high between the months of August to March and low in the months of May to July. All animals did not uniformly respond to electro-ejaculation in April and May. Semen volume and total number of spermatozoa were maximal between September and March and least from April to August. Testicular biopsies indicated that all stages of spermatogenesis were evident between September and March and the spermatogenic activity was less evident between April and August. The contents of Sertoli cells showed a seasonal cyclicity; they were laden with lipid droplets during April to August when spermatogenesis was quiescent and vacuolated during September to March when spermatogenesis was active. These studies indicate that the testing of contraceptive drugs needs to be restricted to months of September to March in male rhesus monkeys otherwise, it is possible that the naturally occurring reproductive quiscence may be attributed to the effect of the drug being tested. The data accrued from the present studies also provide quantitative information on circulating levels of testosterone which could be used as a reference background while evaluating the contraceptive drug-effects in male rhesus monkeys.     

Effects of estradiol and aging on fine manual performance in female rhesus monkeys

Aging is characterized by a progressive deterioration of motor function related to dysfunctions of the nigrostriatal system. Because estrogen has been reported to protect dopaminergic neurons and to improve the motor deficits associated with Parkinson's disease, we hypothesized that it would partially reverse the age-related decline of motor function in normal aging. We tested the effects of estrogen treatment and withdrawal on fine motor performance in five aged (21-24 years old) and five young (6-9 years old) ovariectomized female rhesus monkeys. The tests required the monkeys to use each hand to retrieve a Life Saver candy from metal rods bent in shapes of different complexity. Monkeys were tested twice a week for 8 consecutive weeks, during treatment with placebo or ethinyl estradiol (EE(2)) in alternating 14-day blocks. Each behavioral test was videotaped and subsequently scored for the duration and the success of the first trial on each shape. Both groups of monkeys improved rapidly with practice in speed and success of retrieval. The older monkeys were slower but as successful as the young monkeys in retrieving the candy. The left hand was faster than the right hand for both the aged and young females. We failed to detect any effect of EE(2) treatment on speed or success of retrieval in either group. These results confirm the slowing of fine motor performance with aging in female rhesus monkeys. They also indicate that estradiol, at least as administered in this study, does not benefit fine manual performance.