Interviews: A collection of stories and perspectives about the past and future of GnRH research.

I commenced my research career in the 1970s, at a time when a variety of species were studied. In particular, a number of excellent laboratories throughout the world conducted research in domestic animals, which were large enough to allow serial measurement of hormones. Work in ungulates was seminal in deciphering the operation of the hypothalamic‐pituitary‐gonadal axis, especially in the female. This was because the relatively long estrous cycles of such species were more comparable to that of humans than the 4–5 day cycles of rodents. Furthermore, the endocrinology of the cycle differs between species, an example being the support of luteal function in rodents versus ungulates; in rats and mice, prolactin has an important role, whereas this is not the case in species such as sheep in which LH is the main factor driving luteal function. Importantly, the estrous cycle of rats and mice is tightly regulated by photoperiod, which is not the case in most other species. With the advent of transgenics, there was a shift towards work in mice, with a decline in work on other species. Sadly, this has led to a somewhat myopic view of how GnRH neurons are regulated, and the current literature rarely refers to work in nonrodent species. Accordingly, we are probably missing important facets of reproductive function. A classic example of how work in disparate species led to important findings is the story of mountain and prairie voles and the control of reproductive behavior. Although this is not directly linked to GnRH function, it serves to indicate that important discoveries may be made by working in a variety of species. Whilst knowledge has been vastly expanded by the use of rodent species and transgenics, work in other species could also contribute to our understanding of reproductive function in future. I do not think any of us really anticipated the enormous impact that sequencing the human genome and identifying mutations in several human disease conditions would have on the science in this area. These human genetic studies have demonstrated that, given the marked species specificity of reproduction, it was now possible to conduct primary GnRH experimentation in human disease models. This revolution has had a major impact on research in this area. That said, scientific truths in the area of GnRH that are derived from humans always require validation and experimental validation and testing in vitro and in vivo in appropriate animal and cellular models. So the importance of basic scientists in this area has only grown except that now they know they are working on relevant problems when they have originated from human‐based research. As a leading investigator in this field, Allan Herbison said in an after‐dinner speech in Spain at a GnRH meeting: “When I think of all the time I wasted applying virtually every chemical in the Merck Manual to GnRH neurons, all too little or to no avail, I now realize that I should have simply sat down and had a beer and conversation with the clinical investigators in this area who were doing genetic studies.” Pretty good quote to summarize what I had been telling him for some time. I would say that the evolution from systemic physiology, that relied on using RIAs and physiological replacement models that mimicked normal hormone levels to neuroscience that requires more sophisticated approaches. In 1969, I began an endocrine fellowship at the Royal Post Graduate Medical School in London. My project was to develop LH/FSH assays which required preparation of all reagents (purified human LH was a gift from Ann Hartree in Cambridge); antibody was produced in rabbits and gamma globulin for a second antibody was obtained by exsanguinating tuberculous Guinea pigs! The assay was one of the first in Europe able to measure normal human plasma values and we initiated studies of human gonadotropin physiology. In late 1970 imagine the excitement of a young fellow when I received a call from a representative of Hoechst (who had provided funding to Andrew Schally's research): “Would I like a supply of synthetic GnRH to use in human studies?” This timing was most propitious and allowed us to initiate a multitude of studies in humans—role of GnRH in normal human physiology, puberty, ovulatory cycles, delineation of steroid feedback, GnRH self‐priming action—studies which contributed to much of our present understanding of human physiology. Before the discovery of GnRH molecule, specific structures in the brain controlling reproductive function were called tonic (MBH) and phasic (preoptic area) centers. This was based on studies such as electrical stimulation, lesions and knife cuts of portions of the preoptic area and basal hypothalamus, as well as perinatal sex steroid administration examining morphological changes related to sexual differentiation of the brain. At that time, the preoptic area‐hypothalamus was viewed as a big black box. The discovery of GnRH molecule led to clear insights as to where neurons expressing GnRH were distributed. I remember that in the rat brain GnRH perikarya were found in the preoptic area, but in the MBH only GnRH fibers were seen. Why? A clear answer to this question was not made until the discovery of kisspeptin molecule (and perhaps the discovery of NKB molecule), which are also indispensable for GnRH/LH release and subsequent studies of neurons expressing kisspeptin in the MBH and AVPV in the preoptic area. In addition, findings in the sheep MBH, demonstrating the colocalization of kisspeptin, NKB, and dynorphin in a group of cells, were made that would be important for pulsatility of GnRH release.

We published a paper recently to show that glucagon like peptide‐1 is a potent stimulator of GnRH secretion, acting at the level of the median eminence. This raises a number of questions. First, how are gut secretions involved in the regulation of GnRH secretion in the wider sense? Second, the action of such a peptide at the level of the median eminence begs the question as to how it exerts its effect. It seems most probable that classical ligand binding is required but receptor expression within the median eminence is not apparent. The elegant descriptions of dendrons by the Herbison group are a major advance in our understanding of how GnRH secretion is controlled but much more needs to be done on mechanisms of control beyond the GnRH cell body. This should be a major area of focus in the future, bearing in mind that the projections into the neurosecretory zone of the median eminence differ between species. Work on larger animal species should proceed in parallel with work on rodent species. To date, almost all genetic studies in the GnRH field have been made in families affected by powerful single Mendelian genetic loci. However, with the availability of ever‐larger human population studies (e.g., the UK Biobank's 500 k cohort), it is now possible to determine the roles of GnRH‐related disease‐causing loci in the complex genetic trait genetic arena. These studies will now reveal an ever‐increasing list of GnRH‐related genes that are contributing to the skewed sex ratios that are present in many (if not most) common medical diseases. This new area should be a vast arena in which to examine the broader role of GnRH‐related genes in the complex genetic architectures of many common diseases. Thus, the future is rich with opportunities as (1) whole genome sequencing costs continue to drop reaching $100/person very soon; (2) whole genome sequencing becomes the test of choice in all newborns to determine the presence of all genetic loci that can potentially contribute to diseases at birth (already started in the UK); and the rapid ability to mine ever larger populations and databases efficiently becomes the norm rather than the exception. This future is bright indeed. I think one of the major remaining issues is distinguishing between neural inputs that are major regulators of GnRH secretion and those that can influence this system but are of minor physiological significance. This is a difficult problem because the relative importance of these inputs will vary depending on the external and internal environment. I personally think that a major challenge is the movement to the exclusive use of mice as a preclinical model. Given the major species differences in reproductive neuroendocrinology, there are clear questions about the relevance of this model to human reproductive function. Nevertheless, because of current sophisticated experimental techniques available with transgenic mice this trend appears to be accelerating and this obstacle is unlikely to be addressed. I believe the major unanswered question is what are the precise mechanisms of the GnRH pulse generation system and how is it controlled by the CNS? Is the KNDy neuron system the pulse generator or an intermediate messenger from higher signals which control other clock and cyclical functions in the body? How is this pulse generator system influenced by interactions with other physiological systems—reduced bodyweight, calorie balance and all serious illness result in a marked slowing or loss of GnRH pulses and particularly in females, the reproductive system is a bellwether for general health of the whole animal. Elucidation of this regulatory systems will be complex, involving both local and distant interactions within the CNS and delineation of their nature will probably require developing the abilities to measure central interactions between different physiological regulatory systems. This is a major challenge, but solutions would allow interventions in many disorders and our ability to monitor the GnRH signal peripherally may provide insights into intersystem CNS interactions, probably applicable to other regulatory systems. The neurobiology underlying the dampening of pulsatile GnRH release from infancy until the onset of puberty that guarantees the relative quiescence of the prepubertal gonad in boys and girls. Three questions must be answered. (1) What is the nature of the switch (off switch) that leads to a decrease in GnRH drive to the pituitary‐gonadal axis during infancy. (2) What are the cellular and molecular components of the neurobiological brake that appears to be imposed on the kisspeptin GnRH pulse generator in the infundibular nucleus during childhood and juvenile development. (3) Is the onset of puberty after a protracted delay following infancy triggered by throwing the “off switch” of infancy into reverse? A major obstacle to answering these questions is the lack of an appropriate nonhuman paradigm that is genetically tractable. The last part of the question is the most difficult to address, and I do not have a definitive answer—maybe a quantum leap in development of methodologies required to noninvasively and continuously monitor temporal changes in activity of, and between, neuronal nuclei and other defined regions of the primate brain! First, as I wrote in my article for this special issue, the neural substrates responsible for “central inhibition” of GnRH neurons prior to puberty onset in primates is a critically important question, yet a full answer is not yet available. Second, we do not yet know the entire composition of the GnRH pulse generator and how it works. KNDy neurons are one of the key components, but recent work indicates that nitric oxide also appears to be involved, and we have observed release of NPY is pulsatile and NPY pulses are synchronous with GnRH pulses. Additionally, GABA and glutamate neurons are intimately involved in the regulation of GnRH, but where are they placed in between KNDy and GnRH neurons? Third, the precise cellular and molecular mechanisms of GnRH neurodegeneration in the olfactory placode, as well as migration of surviving cells into the preoptic area and basal hypothalamus has been understudied. Because of this, there are few treatment tools for patients with idiopathic/congenital hypogonadotropic hypogonadism. Finally, during the adolescent period, major reorganization of the neural circuit of the brain occurs. This can be either steroid‐independent and/or steroid‐dependent, but proper timing of puberty onset, which is regulated by GnRH neurons, is one of the most important developmental events in human life. Presently, we have little knowledge between the start of GnRH release (onset of puberty) and the full maturation of the brain.

Not really. Because of our nearly exclusive focus on human genetics in the GnRH field so early on, virtually all the discoveries we made were guaranteed to be relevant to humans. Therefore, we did not waste any time on findings that are not relevant. Since my earliest days as a neuroendocrinologist, I have been interested in the mechanisms that mediate feedback actions of gonadal steroids on GnRH and LH release. At that time, the field was somewhat preoccupied with identifying the neurotransmitter cell groups that regulate GnRH release and characterizing the role that these neuronal groups might play in conveying the positive and/or negative feedback actions of estrogens and progesterone in females, and testosterone in males. The hunt for targets of estradiol became particularly intense with the publication by Donald Pfaff's group that found a paucity of estradiol‐concentrating GnRH neurons. Like many other laboratories, we completed many studies that implicated neuropeptide Y, opioid peptides, excitatory amino acids, etc., in steroid feedback mechanisms. I don't doubt that these neurotransmitters mediate some fraction of physiological signals to GnRH neurons, yet their importance in steroid feedback seems much diminished in light of the preeminent role that we now ascribe to kisspeptin neurons. Looking back on this work, I would have given more weight to the observations that only small fractions of some of these neurotransmitter cell groups express steroid receptors and focused more on other roles that these other neuronal groups may have played in physiological regulation of GnRH neurons. No, I would not have changed anything. I thought that, given the situation, there were always so many exciting projects. I enjoyed the process of new discoveries. [Correction added on 15 June 2022 after first online publication: Section 4.2 was shifted here as Section 6.2, and Section 6.2 was changed to Section 6.3.]

The peer review history for this article is available at https://publons.com/publon/10.1111/jne.13140 .

As an endpiece to this Special Issue celebrating the 50th anniversary of the discovery of GnRH, we asked a group of luminaries in this area of neuroendocrine research to consider the past and future of GnRH research. The following questions were not meant to cover all possibilities, but rather to provide an interesting range of perspectives, looking backward at 50 years of research and forward toward future challenges and opportunities in the field. I am grateful to the respondents, all of whom agreed to be identified informally, for their very interesting and illuminating answers to these questions and thank them for sharing their views and stories with us.

The author has nothing to conflict of interest.