KNDy neurons as the GnRH pulse generator: Recent studies in ruminants.

This review considers three aspects of recent work on the role of KNDy neurons in GnRH pulse generation in ruminants. First, work on basic mechanisms of pulse generation includes several tests of this hypothesis, all of which support it, and evidence that Kiss1r-containing neurons form a positive feedback circuit with the KNDy neural network that strengthen the activity of this network. The second section on pathways mediating external inputs focuses on the influence of nutrition and photoperiod, and describes the evidence supporting roles for proopiomelanocortin (POMC) and agouti-related peptide (AgRP) afferents to KNDy cells in each of these. Finally, we review studies exploring the potential applications of manipulating signaling by kisspeptin, and the other KNDy peptides, to control reproductive function in domestic animals and conclude that, although these approaches show some promise, they do not have major advantages over current practices at this time.

Evidence That Agouti-Related Peptide May Directly Regulate Kisspeptin Neurons in Male Sheep.

Agouti-related peptide (AgRP) neurons, which relay information from peripheral metabolic signals, may constitute a key central regulator of reproduction. Given that AgRP inhibits luteinizing hormone (LH) secretion and that nutritional suppression of LH elicits an increase in AgRP while suppressing kisspeptin expression in the arcuate nucleus (ARC) of the hypothalamus, we sought to examine the degree to which AgRP could directly regulate ARC kisspeptin neurons. Hypothalamic tissue was collected from four castrated male sheep (10 months of age) and processed for the detection of protein (AgRP input to kisspeptin neurons) using immunohistochemistry and mRNA for melanocortin 3 and 4 receptors (MC3R; MC4R) in kisspeptin neurons using RNAscope. Immunohistochemical analysis revealed that the majority of ARC kisspeptin neurons are contacted by presumptive AgRP terminals. RNAscope analysis revealed that nearly two thirds of the ARC kisspeptin neurons express mRNA for MC3R, while a small percentage (<10%) colocalize MC4R. Taken together, this data provides neuroanatomical evidence for a direct link between orexigenic AgRP neurons and reproductively critical kisspeptin neurons in the sheep, and builds upon our current understanding of the central link between energy balance and reproduction.

Undernutrition reduces kisspeptin and neurokinin B expression in castrated male sheep.

Undernutrition impairs reproductive success through suppression of gonadotropin-releasing hormone (GnRH), and subsequently luteinizing hormone (LH), secretion. Given that kisspeptin and neurokinin B (NKB) neurons in the arcuate nucleus (ARC) of the hypothalamus are thought to play key stimulatory roles in the generation of GnRH/LH pulses, we hypothesized that feed restriction would reduce the ARC mRNA abundance and protein expression of kisspeptin and NKB in young, male sheep. Fourteen wethers (castrated male sheep five months of age) were either fed to maintain (FM; n = 6) pre-study body weight or feed-restricted (FR; n = 8) to lose 20% of pre-study body weight over 13 weeks. Throughout the study, weekly blood samples were collected and assessed for LH concentration using RIA. At Week 13 of the experiment, animals were killed, heads were perfused with 4% paraformaldehyde, and brain tissue containing the hypothalamus was collected, sectioned, and processed for detection of mRNA (RNAscope) and protein (immunohistochemistry) for kisspeptin and NKB. Mean LH was significantly lower and LH inter-pulse interval was significantly higher in FR wethers compared to FM wethers at the end of the experiment (Week 13). RNAscope analysis revealed significantly fewer cells expressing mRNA for kisspeptin and NKB in FR wethers compared to FM controls, and immunohistochemical analysis revealed significantly fewer immunopositive kisspeptin and NKB cells in FR wethers compared to FM wethers. Taken together, this data supports the idea that long-term feed restriction regulates GnRH/LH secretion through central suppression of kisspeptin and NKB in male sheep. LAY SUMMARY: While undernutrition is known to impair reproduction at the level of the brain, the components responsible for this in the brain remain to be fully understood. Using male sheep we examined the effect of undernutrition on two stimulatory molecules in the brain critical for reproduction: kisspeptin and neurokinin B. Feed restriction for several weeks resulted in decreased luteinizing hormone in the blood indicating reproductive function was suppressed. In addition, undernutrition also reduced both kisspeptin and neurokinin B levels within a region of the brain involved in reproduction, the hypothalamus. Given that they have stimulatory roles in reproduction, we believe that undernutrition acts in the brain to reduce kisspeptin and neurokinin B levels leading to the reduction in luteinizing hormone secretion. In summary, long-term undernutrition inhibits reproductive function in sheep through suppression of kisspeptin and neurokinin B within the brain.

Early-Age Running Enhances Activity of Adult-Born Dentate Granule Neurons Following Learning in Rats.

Cognitive reserve, the brain's capacity to draw on enriching experiences during youth, is believed to protect against memory loss associated with a decline in hippocampal function, as seen in normal aging and neurodegenerative disease. Adult neurogenesis has been suggested as a specific mechanism involved in cognitive (or neurogenic) reserve. The first objective of this study was to compare learning-related neuronal activity in adult-born versus developmentally born hippocampal neurons in juvenile male rats that had engaged in extensive running activity during early development or reared in a standard laboratory environment. The second objective was to investigate the long-term effect of exercise in rats on learning and memory of a contextual fear (CF) response later in adulthood. These aims address the important question as to whether exercise in early life is sufficient to build a reserve that protects against the process of cognitive aging. The results reveal a long-term effect of early running on adult-born dentate granule neurons and a special role for adult-born neurons in contextual memory, in a manner that is consistent with the neurogenic reserve hypothesis.

Environmental enrichment protects against cognitive impairment following chemotherapy in an animal model.

Clinical studies indicate that up to 70% of cancer patients who receive chemotherapy experience cognitive impairment. The present study investigated environmental enrichment as a protective factor against the adverse effects of anticancer drugs on cognitive and biological processes in an animal model. Adult rats were housed in group cages with environmental stimulation or in standard cages for 3 months, before receiving 3 weekly injections of methotrexate + 5-fluorouracil, or equal volumes of saline. Rats were then administered tests of learning and memory that are sensitive to hippocampal or frontal lobe dysfunction. The relationship between cognitive performance and hippocampal neurogenesis was examined through sensitive time-dependent measures of neuronal maturation. Chemotherapy-treated rats in the standard environment were impaired on tests of spatial memory, nonmatching-to-sample (NMTS) rule learning, and delayed-NMTS. Chemotherapy-treated rats in the enriched environment performed at or near normal levels. The performance of the chemotherapy groups on the hippocampus-sensitive, spatial memory and delayed-NMTS tests correlated with neurogenesis levels. The results show that environmental enrichment can reduce the risk of chemotherapy-induced cognitive impairment, in part by promoting neuronal differentiation and growth during cell maturation. As well, they point to the importance of lifestyle factors in treating or preventing adverse effects of anticancer drugs on cognitive function. (PsycINFO Database Record

Homeostatic regulation of adult hippocampal neurogenesis in aging rats: long-term effects of early exercise.

Adult neurogenesis is highly responsive to environmental and physiological factors. The majority of studies to date have examined short-term consequences of enhancing or blocking neurogenesis but long-term changes remain less well understood. Current evidence for age-related declines in neurogenesis warrant further investigation into these long-term changes. In this report we address the hypothesis that early life experience, such as a period of voluntary running in juvenile rats, can alter properties of adult neurogenesis for the remainder of the animal's life. The results indicate that the number of proliferating and differentiating neuronal precursors is not altered in runners beyond the initial weeks post-running, suggesting homeostatic regulation of these processes. However, the rate of neuronal maturation and survival during a 4 week period after cell division was enhanced up to 11 months of age (the end of the study period). This study is the first to show that a transient period of physical activity at a young age promotes changes in neurogenesis that persist over the long-term, which is important for our understanding of the modulation of neurogenesis by exercise with age. Functional integration of adult-born neurons within the hippocampus that resist homeostatic regulation with aging, rather than the absolute number of adult-born neurons, may be an essential feature of adult neurogenesis that promotes the maintenance of neural plasticity in old age.

KNDy (kisspeptin/neurokinin B/dynorphin) neurons are activated during both pulsatile and surge secretion of LH in the ewe.

KNDy (kisspeptin/neurokinin B/dynorphin) neurons of the arcuate nucleus (ARC) appear to mediate the negative feedback actions of estradiol and are thought to be key regulators of pulsatile LH secretion. In the ewe, KNDy neurons may also be involved with the positive feedback actions of estradiol (E(2)) to induce the LH surge, but the role of kisspeptin neurons in the preoptic area (POA) remains unclear. The goal of this study was to identify which population(s) of kisspeptin neurons is (are) activated during the LH surge and in response to the removal of E(2)-negative feedback, using Fos as an index of neuronal activation. Dual-label immunocytochemistry for kisspeptin and Fos was performed on sections containing the ARC and POA from ewes during the luteal phase of the estrous cycle, or before or after the onset of the LH surge (experiment 1), and from ovary-intact, short-term (24 h) and long-term (>30 d) ovariectomized (OVX) ewes in anestrus (experiment 2). The percentage of kisspeptin neurons expressing Fos in both the ARC and POA was significantly higher during the LH surge. In contrast, the percentage of kisspeptin/Fos colocalization was significantly increased in the ARC, but not POA, after both short- and long-term E(2) withdrawal. Thus, POA kisspeptin neurons in the sheep are activated during, and appear to contribute to, E(2)-positive feedback, whereas ARC kisspeptin (KNDy) neurons are activated during both surge and pulsatile modes of secretion and likely play a role in mediating both positive and negative feedback actions of E(2) on GnRH secretion in the ewe.

Anatomy of the kisspeptin neural network in mammals.

Kisspeptin has been recognized as a key regulator of GnRH secretion during puberty and adulthood, conveying the feedback influence of endogenous gonadal steroids onto the GnRH system. Understanding the functional roles of this peptide depends on knowledge of the anatomical framework in which it acts, including the location of kisspeptin-expressing cells in the brain and their connections. In this paper, we review current data on the anatomy of the kisspeptin neuronal network, including its colocalization with gonadal steroid hormone receptors, anatomical sites of interaction with the GnRH system, and recent evidence of neurochemical heterogeneity among different kisspeptin neuronal populations. Evidence to date suggests that kisspeptin cells in mammals comprise an interconnected network, with reciprocal connections both within and between separate cell populations, and with GnRH neurons. At the same time, there is more functional and anatomical heterogeneity in this system than originally thought, and many unanswered questions remain concerning anatomical relationships of kisspeptin neurons with other neuroendocrine and neural systems in the brain.