Perspectives On Medical Research

Volume 5, 1995

Aping Science

A Critical Analysis of Research at the Yerkes
Regional Primate Research Center

D. Reproductive Physiology

Reproductive physiological investigations have historically been a mainstay of the primate center research, constituting roughly 15 % of its activities. While this critique does not include all reproductive physiological investigations at Yerkes, it does address those that Yerkes officials have highlighted as relevant to human health problems. Specifically, since the 1980s, Yerkes researchers have focused on experiments in the areas of hormonal influences on reproductive system development and function, electroejaculation for semen collection, and means of fertility modification.

GnRH Experiments

GnRH (gonadotropin releasing hormone) is a hypothalamic hormone, one variety of which, LHRH (luteinizing-releasing hormone), stimulates the pituitary gland to produce LH (luteinizing hormone, one of the gonadotropic hormones). In females, LH induces ovarian follicular growth and corpus luteum formation, which increases estrogen levels, and in males it stimulates testicular interstitial cells to produce testosterone. GnRH agonists are synthetic copies of LHRH, and these experimental drugs alter hormonal signals for reproductive function and development in both male and female primates. GnRH agonists overstimulate the pituitary gland, altering serum LH levels and resulting in a wide range of hormonal disturbances.

Through a variety of complex mechanisms, which vary between species, GnRH influences the immune system, the reproductive system, and perhaps other systems as well. There are at least seven different GnRH forms in nature, including at least two in vertebrates.¹ In different species, these hormones act at different receptors of different cell types, and mediate their effects by different mechanisms.^2-5 Furthermore, in different species, different modulators exert different effects on GnRH levels.³ As P. Michael Conn and colleagues have observed, "A view is now emerging that the choice of model (specifically the animal, its age and endocrine status, and the culture conditions) substantially influences the results obtained and, consequently, the putative roles for specific effector molecules in the model."⁴ It would appear, then, any application of animal model data to humans would be at best very tenuous.

Since 1984, Yerkes researchers have utilized GnRH agonists to explore the effects of changing sex-hormone function in both sexes of rhesus monkeys and other nonhuman primates. Recent publications have addressed treatment for endometriosis, antifertility treatment for men, reproductive system development, bone loss accompanying oophorectomy or menopause, and treatment for endometriosis.


Paper 1: Endometriosis⁶

Prior to this report, GnRH agonists had already been tested in human clinical trials for effectiveness against endometriosis.7'° David Mann and co-workers produced experimental endometriosis by surgically excising uterine tissue, mincing it, and planting it back into several areas of the monkeys' abdominal cavities. Two months later, they inspected the sites for growth, and then they obtained biopsies during a third surgical procedure at six months.

This process of "disease" induction differs considerably from that of women with spontaneous endometriosis. There is no reason to expect that this unnatural, mechanistic production of 'tendometriosis" approximates the actual disease. It is like sewing milk-secreting glandular fatty tissue onto men's breasts to study mammary gland function. Furthermore, these monkeys were no doubt physically and psychologically stressed and altered by the multiple major surgical interventions, factors not reflective of experiences of women with endometriosis.

Mann and co-workers found some benefits from GnRH and certain other hormonal treatments. Similarly, several hormonal regimens had previously been tested in women with endometriosis, with varying efficacy and side-effects. The argument used to support animal studies, that they are required as screening procedures prior to human experimentation, does not apply here, as human studies occurred first. The argument that animal studies can clarify human studies is fallacious, for only human trials yield information that can be applied directly and reliably to humans, and they remain the only way to discover optimal therapy for humans.

Data from monkeys is, at best, tenuous for humans, because the monkeys' anatomy and physiology differs from that of humans, the pathogenesis of the monkey "disease" is totally unlike that of the human, and the stress inherent in the laboratory environment does not correspond to human life.


Papers 2 and 3: Inhibition of Sperm Production^11,12

Regarding Mann and colleagues' report on GnRH as a male contraceptive, Yerkes officials have written:

Yerkes scientists tested a hormone agent [GnRH agonist] for its safety and effectiveness as a male contraceptive in humans.... Although the agent halted sperm production, it produced undesirable side effects that, it is hoped, will be eliminated in present research. The side effects included decreased libido and testis size.¹³

Prior to Mann et al.'s work, a GnRH agonist had already been tested by clinical researchers as a human male contraceptive, but it had tended to decrease male libido.¹⁴ To remedy this, these researchers tried to maintain libido with testosterone, and they succeeded in reducing sperm counts considerably while maintaining libido in humans.¹⁵

Relative to humans, rhesus monkeys are resistant to the antifertility effects of GnRH,¹⁶ and testosterone offsets the GnRH agonist's effect on sperm counts more in rhesus monkeys than in humans.¹⁷ Yerkes researchers reported that, using a continuous infusion GnRH agonist delivery system, GnRH agonist plus testosterone can substantially suppress sperm production in male rhesus monkeys. Therefore, by considerably manipulating their experimental conditions, they have been able to reproduce in nonhuman primates a drug affect already clinically demonstrated in humans. In this study, they did not assess libido in the monkeys. Although, in theory, they could have recorded sexual behavior, this is different from libido, which involves sexual desire and therefore cannot be ascertained in nonhumans.


Paper 4: Early Male Sexual Maturation¹⁸

Yerkes officials have maintained that GnRH experiments have also elucidated primate sexual maturation:

The results of Yerkes research by Drs, David Mann and Kim Wallen suggest that the sexual and behavioral maturation of young male primates depends on the brief period of testicular activity that occurs in humans and rhesus macaque males immediately following birth. When testicular activity was pharmacologically suppressed in rhesus males, the puberty was delayed at least one year. These studies may serve as a model of human cryptorchidism, in which the failure of the testes to descend into the scrotum can suppress the post-birth testicular activation typical of human males.¹³

Using a GnRH agonist, Mann's group disrupted the pituitary-testicular hormonal balance to determine whether normal hormonal changes shortly after birth, i.e., a sudden increase in testosterone, is necessary for normal sexual development of male rhesus monkeys.¹⁸ While it has long been known that boys with subnormal blood-levels of LH are cryptorchid (testicles undescended) and may have decreased sperm production as adults, the precise role of testosterone and other hormones remains unclear.

The Yerkes researchers derived their hypothesis about the importance of neonatal testosterone and other hormones from clinical observations of cryptorchid boys¹⁹ and children with other hormonal deficiencies.²⁰ The Yerkes group seems to have demonstrated that increased neonatal testosterone is necessary for normal rhesus monkey sexual development, but, because these monkeys' sexual physiology differs from humans', the human situation remains unresolved.

Although the laboratory may offer the appearance of "control," these experiments cannot control for at least two factors that profoundly undermine their applicability to humans: the complex series of events involved in normal sexual development, while often superficially similar between species, remain species-specific in their details; and inherent laboratory stress2' altering cortisol and other hormone levels^22-24 that, in turn, interact with myriad other hormones, including the sex hormones.

Paper 5: GnRH Agonist Effects on Late Sexual Development²⁵

This paper expands on the previously described report (paper 4).¹⁸ Mann and colleagues studied GnRH-agonist treatment on adult sexual development, skeletal growth, and the integrity of the reproductive hormonal system in rhesus monkeys. They found that early-life GnRH agonist treatment delays puberty in captive monkeys, induces several skeletal alterations including shorter stature, and permanently alters hormonal responses to certain neurotransmitters. As discussed in the Introduction, none of these studies can test hypotheses about human reproductive systems. They may suggest new hypotheses, but human clinical material (e.g., cryptorchidism) also can and does perform this function. The major function of studies of this kind is to merely dramatize hypotheses already derived from human clinical investigation.


Paper 6: Bone Loss²⁶

Mann and colleagues have attempted to create a nonhuman primate model of osteoporosis (bone loss) from aging or ovary removal (oophorectomy). They have noted that there are non-primate models of hormonal effects on bone loss, but "each of the models has the distinct disadvantage of not being a primate model."²⁶ For example, aging dogs lose bone in a manner that resembles human osteoporosis, but the two processes still differ in important respects.²⁷ Since all animal "models" have some attributes that accord with the human condition and others that differ, it is not at all apparent that nonhuman primates make "better" models for such studies. Aged female monkeys do suffer a form of osteoporosis, but many researchers have not favored studying aged females because it is expensive to maintain monkeys until old age.²⁷ As an alternative, Yerkes researchers have chosen to use young monkeys to determine whether "medical oophorectomy" with GnRH-agomst results in an osteoporosis-like condition. A GnRH agonist did reduce bone density in the monkeys, and the researchers claimed to have created what "may be an excellent model for postmenopausal bone loss or bone reduction resulting from medical oophorectomy. The GnRH agonist-treated monkey has the potential to be developed as a model for type I postmenopausal osteoporosis."²⁶ While this model superficially resembles several different human conditions that exhibit bone loss, there is no reason to expect it to be an "excellent" model for any of them. Even subtle differences between species in bone metabolism and function, aging rates and effects, and hormone effects undermine the models' applicability to humans. For example, bone functions do differ between species, reflecting posture and day-to-day activities, and this surely influences bone loss under different experimental conditions.


2. Effects of Oral Contraceptives

Ronald Nadler and colleagues have studied the effects of oral contraceptives (OCs) on chimpanzee sex hormone levels and sexual behavior, justifying such research on the grounds that there is a "close biological relationship between chimpanzees and humans, especially with respect to endogenous sex hormones and sexual behavior."²⁸ For example, they found that one OC reduced sexual activity between chimpanzee pairs, which they claimed is "relevant to women." However, normal chimpanzee sexual behavior differs markedly from that of humans. For example, human females are sexually receptive throughout the estrous cycle, unlike chimpanzees. Furthermore, Nadler and colleagues paired chimpanzees, an artificial manipulation with an unknown effect on chimpanzee sexual behavior. While it may be true, as Nadler et al. comment, that there is "ambiguity surrounding the effects of OCs on the sexuality of humans," there is no reason to expect that these chimpanzee studies will in any way illuminate this area of inquiry.

In a similar study, Nadler and colleagues evaluated the effects of various OCs on female chimpanzee hormone levels and anogenital swelling. According to them, "These effects of oral contraceptives on female genital tissues are relevant to our laboratory studies ... and could also have some implications for women taking oral contraceptives."²⁹ However, chimpanzee genital tissues are much more responsive than analogous human genitalia to changes in sex hormone levels.²⁹ While female genital swelling influences sexual attractiveness to chimpanzee males, it is not reasonable to assume that human males are attracted to human females this way. Different habits between humans and chimpanzees also speak against the relevance of genital swelling. The fact that women are usually clothed does not seem to decrease a man's libidinous desire (and it may increase it). The quadrupedal position of chimpanzees tends to expose female genitalia for male inspection, whereas the human female's bipedal stance, even when naked, tends to conceal her genitals. Finally, according to Desmond Morris, it is the female buttocks, not her external genitalia, that attracts the sexual interest of men, presumably reminiscent of a hominid mating position. Advertisements to attract the sexual interest of men feature buttocks and shapes which approximate buttocks, not external genitalia.

The little variability of genital tissue swelling in women make it an unlikely trigger for human sexual behavior. Therefore, the suggested possible implications for women taking OCs is very doubtful.

Nadler and co-workers have commented that chimpanzee sexual behavior may be better than human studies for determining sexual effects of OCs because "the chimpanzee ... bears a close biological relationship to the human and is free of the self-consciousness that contributes to the difficulty in studying human sexual behavior."²⁹ As discussed above, the sexual biology and behavior of chimpanzees actual differs markedly from that in humans. Second, removing psychological and psychosocial components from sexual behavior is, again, a reductionistic method of eliminating a vital component from a human behavior in order to study its individual physiological components "scientifically" in nonhuman primates. It is like studying a dead animal in order to study the phenomenon of life. Third, the notion that chimpanzees lack self-consciousness (which Nadler and colleagues substantiate with a 1939 reference) contradicts a large volume of modern ethological literature.^29a While animal researchers often treat nonhumans as if they lacked reflective thought, such a stance is untenable in light of modern research.


3. Female Sexual Maturation^30,31

Mark Wilson and colleagues have reported on internal and external "triggers" of puberty and ovulation in rhesus monkey females housed outdoors. The researchers have utilized hypotheses derived from human-patient studies, a common sequence of events in biomedical discovery. For example, the notion that increasing levels of growth hormone (GH) and body weight trigger puberty (mediated by several hormones) derive from human studies.^32,33 The researchers did, in fact, acknowledge that their working hypothesis "was derived from observations that puberty is delayed in GH-deficient children and [later³⁴ in] rats and that it may be initiated with GH replacement therapy."³¹ According to the authors' account, the evident impetus for this research was that "experimental confirmation" in monkeys and rats had not yet been obtained,³¹ a rationale repeatedly, but inappropriately, used to justify animal research. Although some of their experimental findings did accord with certain previous human clinical observations, the researchers did not (and could not) clarify the initiating events of puberty and ovulation in human women. On the contrary, human patient-derived information has helped to deepen the researchers' understanding of the peculiarities of rhesus monkey reproduction.


4. Electroejaculation Experiments

Regarding research to enhance fertility of paraplegic men, Yerkes officials have written:

A method of restoring fertility and possibly sexual function in men with spinal cord injuries has been generated by research with chimpanzees ... The electrostimulation techniques, developed at the Yerkes Center for obtaining sperm from anesthetized chimps for artificial insemination or fertility studies are the basis of the technology that enables spinal cord injured human males to become fathers. Current research focuses on methods to improve semen collection and quality, the latter of which is less than normal in spinal cord injured males.¹³

Contrary to Yerkes' claim, electroejaculation had been performed long before the nonhuman primate work, as early as l948,³⁵ and the first pregnancy from sperm derived by this method was reported in l975.³⁶ At Yerkes, electroejaculation techniques were developed to provide semen for its artificial breeding programs. A cylindrical probe is inserted into the rectum of a male nonhuman primate and an electric current that passes through the probe causes ejaculation. In the 1970s and 1980s, Kenneth Gould and others utilized various sizes of rectal probes to electroejaculate chimpanzees, rhesus monkeys, orangutans, and gorillas.^37,38

For paraplegic men, the Yerkes group has developed a specialized electroejaculation devise, using computerized control over voltage and pulse duration.³⁹ Their design has been guided not by their primate studies but by their clinical observations,^40,41 which "have outlined the principles of design of rectal probes, electrostimulation equipment and reduction of stimulus hazards by appropriate electronic safeguards."³⁹

Electroejaculation has also been used as part of program to preserve gametes (eggs and sperm), purportedly to assist preservation of ape species and to enhance "biomedical science and research ultimately benefiting man."⁴² While Gould does not claim that this effort directly benefits humans, his cautions about application of data between species reflects the inter-species differences that undermine all animal research:

The work so far conducted using nonhuman primates has shown how significant small differences in gamete physiology may be and how important further work in this area is for the continued management of endangered primates.⁴²

Like many animal researchers, Gould's approach to the inherent difficulties associated with animal research is not to reconsider the animal-research paradigm, but rather to do more research.


5. Fertility Control in Women

Yerkes officials have told the public:

Dr. Kenneth Gould ... discovered a novel, non-hormonal approach to female fertility control. His discovery resulted from his studies of normal changes in cervical secretions that occur during the menstrual cycle and that contribute to the penetrability of the cervical mucus. In laboratory in vitro studies and in primates, iron applications were found to change mucus from the ovulatory state to the impenetrable state. This finding holds significant potential for the development of a non-hormonal contraceptive.¹³

Gould's research, dating back to the 1970s, has addressed the idea, earlier developed by clinical research gynecologists,⁴³ that the concentrations of charged particles and/or pH of the cervical mucus can affect fertility by altering sperm penetration." To "test" this hypothesis, Gould's group utilized bonnet monkey cervical mucus mixed with sperm from rhesus monkeys (because "no male bonnet monkeys were available") in vitro. They showed that positively charged ions, including iron, appeared to alter the ability of sperm to penetrate cervical mucus. However, all in vitro systems are potentially undermined by their artificial environments outside the body. This particular system is additionally suspect because it involves penetration of sperm from one species through the cervical mucus of another, and is therefore confounded by at least four methodological errors: 1) artificial, stress-inducing laboratory environment that may influence sperm and cervical mucus production, 2) artificial, in vitro environment that may not simulate the environment of living animals, 3) mixture of interspecies biological material, and 4) inappropriate interspecies generalization. To date, positively charged ions have not been used as contraceptives in humans.

In contrast to the nonhuman primate studies, Gould's related work with women on the role of cervical mucus pH in affecting fertility has had direct clinical relevance. During a six year period, he and co-workers advised 93 women whose infertility was associated with acidic cervical mucus to douche with sodium bicarbonate (an alkaline solution with high pH), and their conception rates seemed to increase.⁴⁵

In a later, related publication, Gould's group again used their in vitro system, this time utilizing cervical mucus from monkeys and women.⁴⁶ They asserted that all in vitro and in vivo evidence corroborates the notion that positively charged ions inhibit sperm penetration and negatively charged ions enhance it. However, when Gould and Ansari commented on "in vivo" studies7 they were referring to their studies on sodium bicarbonate douching in women,⁴⁵ not other primates.


Conclusions

As in other areas of research at Yerkes, the reproductive physiology experiments have tended, at best, to dramatize theories already derived from human clinical investigation. Development of the reproductive system and reproduction itself involve complex interactions of several hormones. While there are certainly similarities between primate species in hormone structures and functions, even subtle differences profoundly undermine inter-species extrapolations.


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