Suppression of vasoactive intestinal polypeptide in the suprachiasmatic nucleus leads to aging-like alterations in cAMP rhythms and activation of gonadotropin-releasing hormone neurons

Vasoactive intestinal peptide excites GnRH neurons via KCa3.1, a potential player in the slow afterhyperpolarization current

Vasoactive intestinal peptide (VIP) is an important component of the suprachiasmatic nucleus (SCN) which relays circadian information to neuronal populations, including GnRH neurons. Human and animal studies have shown an impact of disrupted daily rhythms (chronic shift work, temporal food restriction, clock gene disruption) on both male and female reproduction and fertility. To date, how VIP modulates GnRH neurons remains unknown. Calcium imaging and electrophysiology on primary GnRH neurons in explants and adult mouse brain slice, respectively, were used to address this question. We found VIP excites GnRH neurons via the VIP receptor, VPAC2. The downstream signaling pathway uses both Gs protein/adenylyl cyclase/protein kinase A (PKA) and phospholipase C/phosphatidylinositol 4,5-bisphosphate (PIP2) depletion. Furthermore, we identified a UCL2077-sensitive target, likely contributing to the slow afterhyperpolarization current (IAHP), as the PKA and PIP2 depletion target, and the KCa3.1 channel as a specific target. Thus, VIP/VPAC2 provides an example of Gs protein-coupled receptor-triggered excitation in GnRH neurons, modulating GnRH neurons likely via the slow IAHP. The possible identification of KCa3.1 in the GnRH neuron slow IAHP may provide a new therapeutical target for fertility treatments.

Circadian Biology and the Neurovascular Unit

Mammalian physiology and cellular function are subject to significant oscillations over the course of every 24-hour day. It is likely that these daily rhythms will affect function as well as mechanisms of disease in the central nervous system. In this review, we attempt to survey and synthesize emerging studies that investigate how circadian biology may influence the neurovascular unit. We examine how circadian clocks may operate in neural, glial, and vascular compartments, review how circadian mechanisms regulate cell-cell signaling, assess interactions with aging and vascular comorbidities, and finally ask whether and how circadian effects and disruptions in rhythms may influence the risk and progression of pathophysiology in cerebrovascular disease. Overcoming identified challenges and leveraging opportunities for future research might support the development of novel circadian-based treatments for stroke.

Circadian and kisspeptin regulation of the preovulatory surge

Fertility in mammals is ultimately controlled by a small population of neurons - the gonadotropin-releasing hormone (GnRH) neurons - located in the ventral forebrain. GnRH neurons control gonadal function through the release of GnRH, which in turn stimulates the secretion of the anterior pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In spontaneous ovulators, ovarian follicle maturation eventually stimulates, via sex steroid feedback, the mid-cycle surge in GnRH and LH secretion that causes ovulation. The GnRH/LH surge is initiated in many species just before the onset of activity through processes controlled by the central circadian clock, ensuring that the neuroendocrine control of ovulation and sex behavior are coordinated. This review aims to give an overview of anatomical and functional studies that collectively reveal some of the mechanisms through which the central circadian clock regulates GnRH neurons and their afferent circuits to drive the preovulatory surge.

Circle(s) of Life: The Circadian Clock from Birth to Death

Most lifeforms on earth use endogenous, so-called circadian clocks to adapt to 24-h cycles in environmental demands driven by the planet’s rotation around its axis. Interactions with the environment change over the course of a lifetime, and so does regulation of the circadian clock system. In this review, we summarize how circadian clocks develop in humans and experimental rodents during embryonic development, how they mature after birth and what changes occur during puberty, adolescence and with increasing age. Special emphasis is laid on the circadian regulation of reproductive systems as major organizers of life segments and life span. We discuss differences in sexes and outline potential areas for future research. Finally, potential options for medical applications of lifespan chronobiology are discussed.

Neuroendocrine mechanisms underlying estrogen positive feedback and the LH surge

A fundamental principle in reproductive neuroendocrinology is sex steroid feedback: steroid hormones secreted by the gonads circulate back to the brain to regulate the neural circuits governing the reproductive neuroendocrine axis. These regulatory feedback loops ultimately act to modulate gonadotropin-releasing hormone (GnRH) secretion, thereby affecting gonadotropin secretion from the anterior pituitary. In females, rising estradiol (E₂) during the middle of the menstrual (or estrous) cycle paradoxically "switch" from being inhibitory on GnRH secretion ("negative feedback") to stimulating GnRH release ("positive feedback"), resulting in a surge in GnRH secretion and a downstream LH surge that triggers ovulation. While upstream neural afferents of GnRH neurons, including kisspeptin neurons in the rostral hypothalamus, are proposed as critical loci of E₂ feedback action, the underlying mechanisms governing the shift between E₂ negative and positive feedback are still poorly understood. Indeed, the precise cell targets, neural signaling factors and receptors, hormonal pathways, and molecular mechanisms by which ovarian-derived E₂ indirectly stimulates GnRH surge secretion remain incompletely known. In many species, there is also a circadian component to the LH surge, restricting its occurrence to specific times of day, but how the circadian clock interacts with endocrine signals to ultimately time LH surge generation also remains a major gap in knowledge. Here, we focus on classic and recent data from rodent models and discuss the consensus knowledge of the neural players, including kisspeptin, the suprachiasmatic nucleus, and glia, as well as endocrine players, including estradiol and progesterone, in the complex regulation and generation of E₂-induced LH surges in females.

Vasoactive intestinal peptide exerts an excitatory effect on hypothalamic kisspeptin neurons during estrogen negative feedback

Hypothalamic kisspeptin neurons are the primary modulators of gonadotropin-releasing hormone (GnRH) neurons. It has been shown that circadian rhythms driven by the suprachiasmatic nucleus (SCN) contribute to GnRH secretion. Kisspeptin neurons are potential targets of SCN neurons due to reciprocal connections with the anteroventral periventricular and rostral periventricular nuclei (AVPV/PeN) and the arcuate nucleus of the hypothalamus (ARH). Vasoactive intestinal peptide (VIP), a notable SCN neurotransmitter, modulates GnRH secretion depending on serum estradiol levels, aging or time of the day. Considering that kisspeptin neurons may act as interneurons and mediate VIP's effects on the reproductive axis, we investigated the effects of VIP on hypothalamic kisspeptin neurons in female mice during estrogen negative feedback. Our findings indicate that VIP induces a TTX-independent depolarization of approximately 30% of AVPV/PeN kisspeptin neurons in gonad-intact (diestrus) and ovariectomized (OVX) mice. In the ARH, the percentage of kisspeptin neurons that were depolarized by VIP was even higher (approximately 90%). An intracerebroventricular infusion of VIP leds to an increased percentage of kisspeptin neurons expressing the phospho^Ser133 cAMP-response-element-binding protein (pCREB) in the AVPV/PeN. On the other hand, pCREB expression in ARH kisspeptin neurons was similar between saline- and VIP-injected mice. Thus, VIP can recruit different signaling pathways to modulate AVPV/PeN or ARH kisspeptin neurons, resulting in distinct cellular responses. The expression of VIP receptors (VPACR) was upregulated in the AVPV/PeN, but not in the ARH, of OVX mice compared to mice on diestrus and estradiol-primed OVX mice. Our findings indicate that VIP directly influences distinct cellular aspects of the AVPV/PeN and ARH kisspeptin neurons during estrogen negative feedback, possibly to influence pulsatile LH secretion.

A multi-level assessment of the bidirectional relationship between aging and the circadian clock

The daily temporal order of physiological processes and behavior contribute to the wellbeing of many organisms including humans. The central circadian clock, which coordinates the timing within our body, is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Like in other parts of the brain, aging impairs the SCN function, which in turn promotes the development and progression of aging-related diseases. We here review the impact of aging on the different levels of the circadian clock machinery-from molecules to organs-with a focus on the role of the SCN. We find that the molecular clock is less effected by aging compared to other cellular components of the clock. Proper rhythmic regulation of intracellular signaling, ion channels and neuronal excitability of SCN neurons are greatly disturbed in aging. This suggests a disconnection between the molecular clock and the electrophysiology of these cells. The neuronal network of the SCN is able to compensate for some of these cellular deficits. However, it still results in a clear reduction in the amplitude of the SCN electrical rhythm, suggesting a weakening of the output timing signal. Consequently, other brain areas and organs not only show aging-related deficits in their own local clocks, but also receive a weaker systemic timing signal. The negative spiral completes with the weakening of positive feedback from the periphery to the SCN. Consequently, chronotherapeutic interventions should aim at strengthening overall synchrony in the circadian system using life-style and/or pharmacological approaches.

Circadian Function in Multiple Cell Types Is Necessary for Proper Timing of the Preovulatory LH Surge

The timing of the preovulatory surge of luteinizing hormone (LH), which occurs on the evening of proestrus in female mice, is determined by the circadian system. The identity of cells that control the phase of the LH surge is unclear: evidence supports a role of arginine vasopressin (AVP) cells of the suprachiasmatic nucleus (SCN), but it is not known whether vasopressinergic neurons are necessary or sufficient to account for circadian control of ovulation. Among other cell types, evidence also suggests important roles of circadian function of kisspeptin cells of the anteroventral periventricular nucleus (AvPV) and gonadotropin-releasing hormone (GnRH) neurons of the preoptic area (POA), whose discharge is immediately responsible for the discharge of LH from the anterior pituitary. The present studies used an ovariectomized, estradiol-treated preparation to determine critical cell types whose clock function is critical to the timing of LH secretion. As expected, the LH surge occurred at or shortly after ZT12 in control mice. In further confirmation of circadian control, the surge was advanced by 2 h in tau mutant animals. The timing of the surge was altered to varying degrees by conditional deletion of Bmal1 in AVP^Cre, Kiss^CreBAC, and GnRH^CreBAC mice. Excision of the mutant Cnsk1e (tau) allele in AVP neurons resulted in a reversion of the surge to the ZT12. Conditional deletion of Bmal1 in Kiss1 or GnRH neurons had no noticeable effect on locomotor rhythms, but targeting of AVP neurons produced variable effects on circadian period that did not always correspond to changes in the phase of LH secretion. The results indicate that circadian function in multiple cell types is necessary for proper timing of the LH surge.

Molecular Mechanisms of Gonadotropin-Inhibitory Hormone (GnIH) Actions in Target Cells and Regulation of GnIH Expression

Since gonadotropin-inhibitory hormone (GnIH) was discovered in 2000 as the first hypothalamic neuropeptide that actively inhibits gonadotropin release, researches conducted for the last 18 years have demonstrated that GnIH acts as a pronounced negative regulator of reproduction. Inhibitory effect of GnIH on reproduction is mainly accomplished at hypothalamic-pituitary levels; gonadotropin-releasing hormone (GnRH) neurons and gonadotropes are major targets of GnIH action based on the morphological interaction with GnIH neuronal fibers and the distribution of GnIH receptor. Here, we review molecular studies mainly focusing on the signal transduction pathway of GnIH in target cells, GnRH neurons, and gonadotropes. The use of well-defined cellular model systems allows the mechanistic study of signaling pathway occurring in target cells by demonstrating the direct cause-and-effect relationship. The insights gained through studying molecular mechanism of GnIH action contribute to deeper understanding of the mechanism of how GnIH communicates with other neuronal signaling systems to control our reproductive function. Reproductive axis closely interacts with other endocrine systems, thus GnIH expression levels would be changed by adrenal and thyroid status. We also briefly review molecular studies investigating the regulatory mechanisms of GnIH expression to understand the role of GnIH as a mediator between adrenal, thyroid and gonadal axes.

Vasoactive Intestinal Peptide Excites GnRH Neurons in Male and Female Mice

A variety of external and internal factors modulate the activity of GnRH neurons to control fertility in mammals. A direct, vasoactive intestinal peptide (VIP)-mediated input to GnRH neurons originating from the suprachiasmatic nucleus is thought to relay circadian information within this network. In the present study, we examined the effects of VIP on GnRH neuron activity in male and female mice at different stages of the estrous cycle. We carried out cell-attached recordings in slices from GnRH-green fluorescent protein mice and calcium imaging in slices from a mouse line expressing the genetically encoded calcium indicator GCaMP3 selectively in GnRH neurons. We show that 50%-80% of GnRH neurons increase their firing rate in response to bath-applied VIP (1nM-1000nM) in both male and female mice and that this is accompanied by a robust increase in intracellular calcium concentrations. This effect is mediated directly at the GnRH neuron likely through activation of high-affinity VIP receptors. Because suprachiasmatic nucleus-derived timing cues trigger the preovulatory surge only on the afternoon of proestrus in female mice, we examined the effects of VIP during the estrous cycle at different times of day. VIP responsiveness in GnRH neurons did not vary significantly in diestrous and proestrous mice before or around the time of the expected preovulatory surge. These results indicate that the majority of GnRH neurons in male and female mice express functional VIP receptors and that the effects of VIP on GnRH neurons do not alter across the estrous cycle.

Evidence of a molecular clock in the ovine ovary and the influence of photoperiod

The influence of the central circadian clock on reproductive timing is well established. Much less is known about the role of peripheral oscillators such as those in the ovary. We investigated the influence of photoperiod and timing of the LH surge on expression of circadian clock genes and genes involved in steroidogenesis in ovine ovarian stroma. Seventy-two Suffolk cross ewes were divided into two groups, and their estrous cycles were synchronized. Progestagen sponge removal was staggered by 12 hours between the groups such that expected LH peak would occur midway through either the light or dark phase of the photoperiodic cycle. Four animals from each group were killed, and their ovaries were harvested beginning 36 hours after sponge removal, at 6-hour intervals for 48 hours. Blood was sampled every 3 hours for the period 24 to 48 hours after sponge removal to detect the LH surge. The interval to peak LH did not differ between the groups (36.2 ± 1.2 and 35.6 ± 1.1 hours, respectively). There was an interaction between group and the time of sponge removal on the expression of the core clock genes ARNTL, PER1, CRY1, CLOCK, and DBP (P < 0.01, P < 0.05, P < 0.01, P < 0.01, and P < 0.01, respectively). As no significant interaction between group and time of day was detected, the datasets were combined. Statistically significant rhythmic oscillation was observed for ARNTL, CLOCK, CRY1 (P < 0.01, respectively), PTGS2, DBP, PTGER2, and CYP17A1 (P < 0.05, respectively), confirming the existence of a time-sensitive functionality within the ovary, which may influence steroidogenesis and is independent of the ovulatory cycle.

A Computational Model of the Dendron of the GnRH Neuron

Gonadotropin-releasing hormone (GnRH) neurons have two major processes that have properties of both dendrites (they receive synaptic input from other neurons) and axons (they actively propagate action potentials to the synaptic terminal). These processes have thus been termed dendrons. We construct a stochastic spatiotemporal model of the dendron of the GnRH neuron, with the goal of studying how stochastic synaptic input along the length of the dendron affects the initiation and propagation of action potentials. We show (1) that synaptic inputs closer to the soma are effective controllers of action potential initiation and electrical bursting and (2) that although the effects on the amplitude and width of propagating action potentials are critically dependent on the timing and location of synaptic input addition, the effects remain small. We conclude that although stochastic synaptic input along the length of the dendron is likely to be a major determinant of action potential initiation, it is an unlikely mechanism for controlling whether or not action potentials reach the synaptic terminal. Thus, the role of synaptic inputs situated along the dendron a long way from the site of action potential initiation remains unclear. We also show that the actions of kisspeptin can result in significant modulation of the amount of calcium released by an action potential at the synaptic terminal. Furthermore, we show that the actions of kisspeptin are greatest when multiple effects operate together and that a kisspeptin-induced increase in firing rate is, by itself, less effective at increasing Ca2+ release than is a combination of an increased firing rate, an increase in Ca2+ influx, and an increase in inositol trisphosphate (IP3) production. We conclude that the inherent synergies in the various actions of kisspeptin make it a likely candidate for the precise control of Ca2+ transients at the synaptic terminal.

Disrupted reproduction, estrous cycle, and circadian rhythms in female mice deficient in vasoactive intestinal peptide

The female reproductive cycle is gated by the circadian timing system and may be vulnerable to disruptions in the circadian system. Prior work suggests that vasoactive intestinal peptide (VIP)-expressing neurons in the suprachiasmatic nucleus (SCN) are one pathway by which the circadian clock can influence the estrous cycle, but the impact of the loss of this peptide on reproduction has not been assessed. In the present study, we first examine the impact of the genetic loss of the neuropeptide VIP on the reproductive success of female mice. Significantly, mutant females produce about half the offspring of their wild-type sisters even when mated to the same males. We also find that VIP-deficient females exhibit a disrupted estrous cycle; that is, ovulation occurs less frequently and results in the release of fewer oocytes compared with controls. Circadian rhythms of wheel-running activity are disrupted in the female mutant mice, as is the spontaneous electrical activity of dorsal SCN neurons. On a molecular level, the VIP-deficient SCN tissue exhibits lower amplitude oscillations with altered phase relationships between the SCN and peripheral oscillators as measured by PER2-driven bioluminescence. The simplest explanation of our data is that the loss of VIP results in a weakened SCN oscillator, which reduces the synchronization of the female circadian system. These results clarify one of the mechanisms by which disruption of the circadian system reduces female reproductive success.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

The Interplay between Circadian System, Cholesterol Synthesis, and Steroidogenesis Affects Various Aspects of Female Reproduction

Circadian aspect of reproduction has gained much attention in recent years. In mammals, it is very important that the timing of greatest sexual motivation is in line with the highest fertility. Peripheral clocks have been found to reside also in reproductive organs, such as the uterus and ovary. The timing signal from the suprachiasmatic nucleus is suggested to be transmitted via hormonal and neural mechanisms, and could thus mediate circadian expression of target genes in these organs. In turn, estrogens from the ovary have been found to signal back to the hypothalamus, completing the feedback loop. In this review we will focus on the interplay between clock and estrogens. Estradiol has been directly linked with expression of Per1 and Per2 in the uterus. CLOCK, on the other hand, has been shown to alter estradiol signaling. We also present the idea that cholesterol could play a vital role in the regulation of reproduction. Cholesterol synthesis itself is circadially regulated and has been found to interfere with steroidogenesis in the ovary on the molecular level. This review presents a systems view on how the interplay between circadian clock, steroidogenesis, and cholesterol synthesis affect various aspects of mammalian reproduction.

Regulation of electrical bursting in a spatiotemporal model of a GnRH neuron

Gonadotropin-releasing hormone (GnRH) neurons are hypothalamic neurons that control the pulsatile release of GnRH that governs fertility and reproduction in mammals. The mechanisms underlying the pulsatile release of GnRH are not well understood. Some mathematical models have been developed previously to explain different aspects of these activities, such as the properties of burst action potential firing and their associated Ca(2+) transients. These previous studies were based on experimental recordings taken from the soma of GnRH neurons. However, some research groups have shown that the dendrites of GnRH neurons play very important roles. In particular, it is now known that the site of action potential initiation in these neurons is often in the dendrite, over 100 μm from the soma. This raises an important question. Since some of the mechanisms for controlling the burst length and interburst interval are located in the soma, how can electrical bursting be controlled when initiated at a site located some distance from these controlling mechanisms? In order to answer this question, we construct a spatio-temporal mathematical model that includes both the soma and the dendrite. Our model shows that the diffusion coefficient for the spread of electrical potentials in the dendrite is large enough to coordinate burst firing of action potentials when the initiation site is located at some distance from the soma.

Circadian regulation of kisspeptin in female reproductive functioning

Female reproductive functioning requires the precise temporal -organization of numerous neuroendocrine events by a master circadian brain clock located in the suprachiasmatic nucleus. Across species, including humans, disruptions to circadian timing result in pronounced deficits in ovulation and fecundity. The present chapter provides an overview of the circadian control of female reproduction, underscoring the significance of kisspeptin as a key locus of integration for circadian and steroidal signaling necessary for the initiation of ovulation.

The clock in the brain: neurons, glia, and networks in daily rhythms

The master coordinator of daily schedules in mammals, located in the ventral hypothalamus, is the suprachiasmatic nucleus (SCN). This relatively small population of neurons and glia generates circadian rhythms in physiology and behavior and synchronizes them to local time. Recent advances have begun to define the roles of specific cells and signals (e.g., peptides, amino acids, and purine derivatives) within this network that generate and synchronize daily rhythms. Here we focus on the best-studied signals between neurons and between glia in the mammalian circadian system with an emphasis on time-of-day pharmacology. Where possible, we highlight how commonly used drugs affect the circadian system.

Feedback actions of locomotor activity to the circadian clock

The phase of the mammalian circadian system can be entrained to a range of environmental stimuli, or zeitgebers, including food availability and light. Further, locomotor activity can act as an entraining signal and represents a mechanism for an endogenous behavior to feedback and influence subsequent circadian function. This process involves a number of nuclei distributed across the brain stem, thalamus, and hypothalamus and ultimately alters SCN electrical and molecular function to induce phase shifts in the master circadian pacemaker. Locomotor activity feedback to the circadian system is effective across both nocturnal and diurnal species, including humans, and has recently been shown to improve circadian function in a mouse model with a weakened circadian system. This raises the possibility that exercise may be useful as a noninvasive treatment in cases of human circadian dysfunction including aging, shift work, transmeridian travel, and the blind.

Circadian control of neuroendocrine circuits regulating female reproductive function

Female reproduction requires the precise temporal organization of interacting, estradiol-sensitive neural circuits that converge to optimally drive hypothalamo-pituitary-gonadal (HPG) axis functioning. In mammals, the master circadian pacemaker in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus coordinates reproductively relevant neuroendocrine events necessary to maximize reproductive success. Likewise, in species where periods of fertility are brief, circadian oversight of reproductive function ensures that estradiol-dependent increases in sexual motivation coincide with ovulation. Across species, including humans, disruptions to circadian timing (e.g., through rotating shift work, night shift work, poor sleep hygiene) lead to pronounced deficits in ovulation and fecundity. Despite the well-established roles for the circadian system in female reproductive functioning, the specific neural circuits and neurochemical mediators underlying these interactions are not fully understood. Most work to date has focused on the direct and indirect communication from the SCN to the gonadotropin-releasing hormone (GnRH) system in control of the preovulatory luteinizing hormone (LH) surge. However, the same clock genes underlying circadian rhythms at the cellular level in SCN cells are also common to target cell populations of the SCN, including the GnRH neuronal network. Exploring the means by which the master clock synergizes with subordinate clocks in GnRH cells and its upstream modulatory systems represents an exciting opportunity to further understand the role of endogenous timing systems in female reproduction. Herein we provide an overview of the state of knowledge regarding interactions between the circadian timing system and estradiol-sensitive neural circuits driving GnRH secretion and the preovulatory LH surge.

New insights into the control of pulsatile GnRH release: the role of Kiss1/neurokinin B neurons

Gonadotropin-releasing hormone (GnRH) is the ultimate output signal of an intricate network of neuroendocrine factors that, acting on the pituitary, trigger gonadotropin release. In turn, gonadotropins exert their trophic action on the gonads to stimulate the synthesis of sex steroids thus completing the gonadotropic axis through feedback regulatory mechanisms of GnRH release. These feedback loops are predominantly inhibitory in both sexes, leading to tonic pulsatile release of GnRH from puberty onward. However, in the female, rising levels of estradiol along the estrous cycle evoke an additional positive feedback that prompts a surge-like pattern of GnRH release prior to ovulation. Kisspeptins, secreted from hypothalamic Kiss1 neurons, are poised as major conduits to regulate this dual secretory pathway. Kiss1 neurons are diverse in origin, nature, and function, convening distinct neuronal populations in two main hypothalamic nuclei: the arcuate nucleus (ARC) and the anteroventral periventricular nucleus. Recent studies from our group and others point out Kiss1 neurons in the ARC as the plausible generator of GnRH pulses through a system of pulsatile kisspeptin release shaped by the coordinated action of neurokinin B (NKB) and dynorphin A (Dyn) that are co-expressed in Kiss1 neurons (so-called KNDy neurons). In this review, we aim to document the recent findings and working models directed toward the identification of the Kiss1-dependent mechanisms of GnRH release through a synoptic overview of the state-of-the-art in the field.

The adrenal peripheral clock: glucocorticoid and the circadian timing system

The mammalian circadian timing system is organized in a hierarchy, with the master clock residing in the suprachiasmatic nucleus (SCN) of the hypothalamus and subsidiary peripheral clocks in other brain regions as well as peripheral tissues. Since the local oscillators in most cells contain a similar molecular makeup to that in the central pacemaker, determining the role of the peripheral clocks in the regulation of rhythmic physiology and behavior is an important issue. Glucocorticoids (GCs) are a class of multi-functional adrenal steroid hormones, which exhibit a robust circadian rhythm, with a peak linked with the onset of the daily activity phase. It has long been believed that the production and secretion of GC is primarily governed through the hypothalamus-pituitary-adrenal (HPA) neuroendocrine axis in mammals. Growing evidence, however, strongly supports the notion that the periodicity of GC involves the integrated activity of multiple regulatory mechanisms related to circadian timing system along with the classical HPA neuroendocrine regulation. The adrenal-intrinsic oscillator as well as the central pacemaker plays a pivotal role in its rhythmicity. GC influences numerous biological processes, such as metabolic, cardiovascular, immune and even higher brain functions, and also acts as a resetting signal for the ubiquitous peripheral clocks, suggesting its importance in harmonizing circadian physiology and behavior. In this review, we will therefore focus on the recent advances in our understanding of the circadian regulation of adrenal GC and its functional relevance.

Temporal phase relation of circadian neural oscillations as the basis of testicular maturation in mice: a test of a coincidence model

To study the underlying mechanism of gonadal growth during the attainment of puberty and to test a coincidence model, 7 experimental groups of 2-week-old male mice, Mus musculus, were administered the serotonin precursor, 5-hydroxytryptophan, followed by the dopamine precursor, L-dihydroxyphenylalanine at hourly intervals of 6, 7, 8, 9, 10, 11 and 12 h (5 mg/100 g body weight per day for 13 days). At 11 days post-treatment, a suppression of gonadal activity was seen in the 7-h mice and a maximum suppression in the 8-h mice, along with a significantly increased degree of gonadal development in the 12-h mice, as compared with the controls. In addition to its known regulation of seasonal gonadal cycles, the relative position of two circadian neural oscillations may also affect the rate of gonadal development during the attainment of puberty in mice. Moreover, the present study provides an experimental paradigm to test the coincidence model of circadian oscillations.

Structural and functional changes in the suprachiasmatic nucleus following chronic circadian rhythm perturbation

Circadian rhythms, generated in the suprachiasmatic nucleus (SCN), are synchronized to the ambient light/dark (LD) cycle. Long-term disruptions in circadian rhythms are associated with many health problems. However, the underlying mechanisms for such pathologies are not well understood. In the present study, we utilized a chronic jet lag paradigm consisting of weekly 6 h phase shifts in the LD cycle to investigate the circadian responses in behavior and in the functioning of the SCN following long-term circadian perturbation, and to explore the duration and direction dependent changes of the SCN using rats subjected to weekly phase advances or delays. Wheel-running activity was monitored over four weekly phase advances. The nocturnal activity pattern was re-established by the end of each shift, and the rate for recovering the nocturnality appeared to accelerate following multiple shifts. SCN function was assessed by the expressions of the protein product of clock gene PER1 and of two putative SCN output signals, arginine vasopressin (AVP) and prokineticin2 (Pk2). At the end of the 4th weekly advance, the amplitude of the PER1 rhythm in the SCN decreased, and this reduction was more prominent in the dorsomedial SCN than in the ventrolateral SCN. The levels of AVP and Pk2 expression were also attenuated in the SCN and in targets of its efferent projections. Comparing rats subjected to four or eight shifts of either delay or advance, the results revealed that the responses of the SCN depended on both duration and direction of the shifts, such that the level of PER1 expression further decreased at the end of the 8th compared to the 4th phase advance, but did not change significantly following phase delays. Taken together, the results suggest that rhythm perturbation could compromise the time-keeping function of the SCN, which could contribute to the associated health issues.

Vasoactive intestinal polypeptide requires parallel changes in adenylate cyclase and phospholipase C to entrain circadian rhythms to a predictable phase

Circadian oscillations in the suprachiasmatic nucleus (SCN) depend on transcriptional repression by Period (PER)1 and PER2 proteins within single cells and on vasoactive intestinal polypeptide (VIP) signaling between cells. Because VIP is released by SCN neurons in a circadian pattern, and, after photic stimulation, it has been suggested to play a role in the synchronization to environmental light cycles. It is not known, however, if or how VIP entrains circadian gene expression or behavior. Here, we tested candidate signaling pathways required for VIP-mediated entrainment of SCN rhythms. We found that single applications of VIP reset PER2 rhythms in a time- and dose-dependent manner that differed from light. Unlike VIP-mediated signaling in other cell types, simultaneous antagonism of adenylate cyclase and phospholipase C activities was required to block the VIP-induced phase shifts of SCN rhythms. Consistent with this, VIP rapidly increased intracellular cAMP in most SCN neurons. Critically, daily VIP treatment entrained PER2 rhythms to a predicted phase angle within several days, depending on the concentration of VIP and the interval between VIP applications. We conclude that VIP entrains circadian timing among SCN neurons through rapid and parallel changes in adenylate cyclase and phospholipase C activities.

The effects of aging and chronic fluoxetine treatment on circadian rhythms and suprachiasmatic nucleus expression of neuropeptide genes and 5-HT1B receptors

Age-related changes in circadian rhythms, including attenuation of photic phase shifts, are associated with changes in the central pacemaker in the suprachiasmatic nucleus (SCN). Aging decreases expression of mRNA for vasoactive intestinal peptide (VIP), a key neuropeptide for rhythm generation and photic phase shifts, and increases expression of serotonin transporters and 5-HT(1B) receptors, whose activation inhibits these phase shifts. Here we describe studies in hamsters showing that aging decreases SCN expression of mRNA for gastrin-releasing peptide, which also modulates photic phase resetting. Because serotonin innervation trophically supports SCN VIP mRNA expression, and serotonin transporters decrease extracellular serotonin, we predicted that chronic administration of the serotonin-selective reuptake inhibitor, fluoxetine, would attenuate the age-related changes in SCN VIP mRNA expression and 5-HT(1B) receptors. In situ hybridization studies showed that fluoxetine treatment does not alter SCN VIP mRNA expression, in either age group, at zeitgeber time (ZT)6 or 13 (ZT12 corresponds to lights off). However, receptor autoradiographic studies showed that fluoxetine prevents the age-related increase in SCN 5-HT(1B) receptors at ZT6, and decreases SCN 5-HT(1B) receptors in both ages at ZT13. Therefore, aging effects on SCN VIP mRNA and SCN 5-HT(1B) receptors are differentially regulated; the age-related increase in serotonin transporter sites mediates the latter but not the former. The studies also showed that aging and chronic fluoxetine treatment decrease total daily wheel running without altering the phase of the circadian wheel running rhythm, in contrast to previous reports of phase resetting by acute fluoxetine treatment.

cAMP-regulated dynamics of the mammalian circadian clock

Previous molecular description of the mammalian timekeeping mechanism was based mainly on transcriptional/translational feedback loops (TTFLs). However, a recent experimental report challenges such a molecular architecture, showing that the cAMP signaling is an indispensable component of the mammalian circadian clock. In this paper, we develop a reduced mathematical model that characterizes the mammalian circadian network. The model with 8-state differential equations incorporates both TTFLs and cAMP-mediated feedback loop. In agreement with experimental observations, our results show that: (1) the model simulates sustained circadian (23.4-h periodic) oscillations in constant darkness and entrained circadian dynamics by light-dark cycles; (2) circadian rhythmicity is lost without cAMP signaling; (3) the system is resilient to large fluctuations in transcriptional rates; (4) it successfully simulates the phenotypes of Per1(-/-)/Per2(-/-) double-mutant mice and Bmal1(-/-) mutant mice. Our study implies that to understand the circadian pacemaking in suprachiasmatic nucleus neurons, the TTFLs should not be isolated from intracellular cAMP-dependent signaling.

The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges

Ovarian steroids normally exert homeostatic negative feedback on GnRH release. During sustained exposure to elevated estradiol in the late follicular phase of the reproductive cycle, however, the feedback action of estradiol switches to positive, inducing a surge of GnRH release from the brain, which signals the pituitary LH surge that triggers ovulation. In rodents, this switch appears dependent on a circadian signal that times the surge to a specific time of day (e.g., late afternoon in nocturnal species). Although the precise nature of this daily signal and the mechanism of the switch from negative to positive feedback have remained elusive, work in the past decade has provided much insight into the role of circadian/diurnal and estradiol-dependent signals in GnRH/LH surge regulation and timing. Here we review the current knowledge of the neurobiology of the GnRH surge, in particular the actions of estradiol on GnRH neurons and their synaptic afferents, the regulation of GnRH neurons by fast synaptic transmission mediated by the neurotransmitters gamma-aminobutyric acid and glutamate, and the host of excitatory and inhibitory neuromodulators including kisspeptin, vasoactive intestinal polypeptide, catecholamines, neurokinin B, and RFamide-related peptides, that appear essential for GnRH surge regulation, and ultimately ovulation and fertility.

Chronic stimulation of the hypothalamic vasoactive intestinal peptide receptor lengthens circadian period in mice and hamsters

Evidence suggests that circadian rhythms are regulated through diffusible signals generated by the suprachiasmatic nucleus (SCN). Vasoactive intestinal peptide (VIP) is located in SCN neurons positioned to receive photic input from the retinohypothalamic tract and transmit information to other SCN cells and adjacent hypothalamic areas. Studies using knockout mice indicate that VIP is essential for synchrony among SCN cells and for the expression of normal circadian rhythms. To test the hypothesis that VIP is also an SCN output signal, we recorded wheel-running activity rhythms in hamsters and continuously infused the VIP receptor agonist BAY 55-9837 in the third ventricle for 28 days. Unlike other candidate output signals, infusion of BAY 55-9837 did not affect activity levels. Instead, BAY 55-9837 lengthened the circadian period by 0.69 +/- 0.04 h (P < 0.0002 compared with controls). Period returned to baseline after infusions. We analyzed the effect of BAY 55-9837 on cultured SCN from PER2::LUC mice to determine if lengthening of the period by BAY 55-9837 is a direct effect on the SCN. Application of 10 muM BAY 55-9837 to SCN in culture lengthened the period of PER2 luciferase expression (24.73 +/- 0.24 h) compared with control SCN (23.57 +/- 0.26, P = 0.01). In addition, rhythm amplitude was significantly increased, consistent with increased synchronization of SCN neurons. The effect of BAY 55-9837 in vivo on period is similar to the effect of constant light. The present results suggest that VIP-VPAC2 signaling in the SCN may play two roles, synchronizing SCN neurons and setting the period of the SCN as a whole.

Hypothalamic insulin-like growth factor-I receptors are necessary for hormone-dependent luteinizing hormone surges: implications for female reproductive aging

Brain IGF-I receptors are required for maintenance of estrous cycles in young adult female rats. Circulating and hypothalamic IGF-I levels decrease with aging, suggesting a role for IGF-I in the onset of reproductive senescence. Therefore, the present study investigated potential mechanisms of action of brain IGF-I receptors in the regulation of LH surges in young adult and middle-aged rats. We continuously infused IGF-I, the selective IGF-I receptor antagonist JB-1, or vehicle into the third ventricle of ovariectomized young adult and middle-aged female rats primed with estradiol and progesterone. Pharmacological blockade of IGF-I receptors attenuated and delayed the LH surge in young adult rats, reminiscent of the LH surge pattern that heralds the onset of reproductive senescence in middle-aged female rats. Infusion of IGF-I alone had no effect on the LH surge but reversed JB-1 attenuation of the surge in young females. In middle-aged rats, infusion of low doses of IGF-I partially restored LH surge amplitude, and infusion of JB-1 completely obliterated the surge. Intraventricular infusion of IGF-I or JB-1 did not modify pituitary sensitivity to exogenous GnRH or GnRH peptide content in the anterior or mediobasal hypothalamus in either young or middle-aged rats. These findings support the hypothesis that brain IGF-I receptor signaling is necessary for GnRH neuron activation under estrogen-positive feedback conditions and that decreased brain IGF-I signaling in middle-aged females contributes, in part, to LH surge dysfunction by disrupting estradiol-sensitive processes that affect GnRH neuron activation and/or GnRH release.

Critical roles for fast synaptic transmission in mediating estradiol negative and positive feedback in the neural control of ovulation

A switch in the balance of estradiol feedback actions from negative to positive initiates the GnRH surge, triggering the LH surge that causes ovulation. Using an ovariectomized, estradiol-treated (OVX+E) mouse model that exhibits daily switches between negative in the morning and positive feedback in the evening, we investigated the roles of fast synaptic transmission in regulating GnRH neuron firing during negative and positive feedback. Targeted extracellular recordings were used to monitor activity of GnRH neurons from OVX+E and OVX mice in control solution or solution with antagonists to both ionotropic glutamate and gamma-aminobutyric acid receptors (blockade). Blockade had no effect on activity of OVX cells. In contrast, in OVX+E cells in the morning, blockade increased activity compared with control cells, whereas in the evening, blockade decreased activity. In vivo barbiturate sedation of OVX+E mice that blocked LH surge induction prevented the in vitro evening changes in firing rate and response to blockade. These observations suggest at least partial inversion of the negative-to-positive switch in estradiol feedback action and indicate that changes in fast synaptic transmission to GnRH neurons and within the network of cells presynaptic to GnRH neurons are critical for mediating estradiol negative and positive feedback actions on GnRH neurons. Fast synaptic transmission may also affect GnRH neuron activity indirectly through altering release of excitatory and inhibitory neuromodulators onto GnRH neurons at specific times of day. Fast synaptic transmission is thus critical for proper generation and timing of the GnRH surge.

Age affects spontaneous activity and depolarizing afterpotentials in isolated gonadotropin-releasing hormone neurons

Neuronal activity underlying the pulsatile secretion of GnRH remains poorly understood, as does the endogenous generation of such activity. It is clear that changes at the level of the hypothalamus are taking place during reproductive aging, yet virtually nothing is known about GnRH neuronal physiology in aging and postreproductive animals. In these studies, we performed cell-attached and whole-cell recordings in GnRH-enhanced green fluorescent protein neurons dissociated from young (3 months), middle-aged (10 months), and old (15-18 months) female mice. All mice were ovariectomized; half were estradiol replaced. Neurons from all ages fired spontaneously, most in a short-burst pattern that is characteristic of GnRH neuronal firing. Membrane characteristics were not affected by age. However, firing frequency was significantly reduced in neurons from old animals, as was spike patterning. The amplitude of the depolarizing afterpotential, evoked by a 200-msec current pulse, was significantly smaller in aged animals. In addition, inward whole-cell currents were reduced in estradiol-treated animals, although they were not significantly affected by age. Because depolarizing afterpotentials have been shown to contribute to prolonged discharges of activity after a very brief excitatory input, a decreased depolarizing afterpotential could lead to attenuated pulses in older animals. In addition, decreases in frequency and pattern generation could lead to improper information coding. Therefore, changes in the GnRH neuron during aging could lead to dysregulated activity, potentially resulting in the attenuated LH pulses observed in the transition to reproductive senescence.

Aging in the circadian system: considerations for health, disease prevention and longevity

The circadian system orchestrates internal physiology on a daily schedule to promote optimal health and maximize disease prevention. Chronic disruptions in circadian function are associated with an increase in a variety of disease states including, heart disease, ulcers and diabetes. With advanced age, the genes regulating circadian function at the cellular level become disorganized and the ability of the brain clock to entrain to local time diminishes. As a result, aged individuals exhibit a loss of temporal coordination among bodily systems, leading to deficits in homeostasis and sub-optimal functioning. Such disruptions in the circadian system appear to accelerate the aging process and contribute to senescence, with some systems being more vulnerable than others. This review explores aging-associated changes in circadian function and examines evidence linking such alterations to adverse health consequences in late life and promotion of the aging process.

Prenatal ethanol exposure alters core body temperature and corticosterone rhythms in adult male rats

Ethanol's effects on the developing brain include alterations in morphology and biochemistry of the hypothalamus. To examine the potential functional consequences of ethanol's interference with hypothalamic differentiation, we studied the long-term effects of prenatal ethanol exposure on basal circadian rhythms of core body temperature (CBT) and heart rate (HR). We also examined the late afternoon surge in corticosterone (CORT). Core body temperature and HR rhythms were studied in separate groups of animals at 4, 8, and 20 months of age. The normal late afternoon rise in plasma CORT was examined in freely moving male rats at 6 months of age via an indwelling right atrial cannula. Results showed that the CBT circadian rhythm exhibited an earlier rise after the nadir of the rhythm in fetal alcohol-exposed (FAE) males at all ages compared to controls. At 8 months of age, the amplitude of the CBT circadian rhythm in FAE males was significantly reduced to the level observed in controls at 20 months. No significant effects of prenatal ethanol exposure were observed on basal HR rhythm at any age. The diurnal rise in CORT secretion was blunted and prolonged in 6-month-old FAE males compared to controls. Both control groups exhibited a robust surge in CORT secretion around the onset of the dark phase of the light cycle, which peaked at 7:30 p.m. Whereas FAE males exhibited a linear rise beginning in mid afternoon, which peaked at 9:30 p.m. These results indicate that exposure to ethanol during the period of hypothalamic development can alter the long-term regulation of circadian rhythms in specific physiological systems.

Changes of expression of genes related to the activity of the gonadotrophin-releasing hormone pulse generator in young versus middle-aged male rats

In females, it is well established that changes in the expression of neurotransmitters and peptides regulating the activity of the gonadotrophin-releasing hormone (GnRH) pulse generator are altered during ageing. By contrast, little is known about whether those age-related changes also occur in males. Therefore, we designed an animal study with orchidectomised young and middle-aged male rats to investigate changes in luteinising hormone (LH) secretion profiles and changes in the mRNA expression of genes regulating the activity of the GnRH pulse generator. Our results demonstrate that middle-aged rats exhibit lower serum LH levels and relatively fewer LH pulses with attenuated amplitude compared to young animals. Furthermore, upon ageing, GnRH mRNA expression is up-regulated in the preoptic area and the septum where GnRH neurones reside. Analysis of mRNA levels of glutamate decarboxylase (GAD) enzymes revealed that GAD(65) and GAD(67) mRNA expression increased in the mediobasal hypothalamus (MBH) and that GAD(67) mRNA levels decreased in the suprachiasmatic nucleus. In addition, we observed an age-related increase of oestrogen receptor (ER)alpha mRNA in the MBH, and both ERalpha and ERbeta mRNA expression was up-regulated in the pituitary of middle-aged rats compared to young animals. Taken together, our data support the existence of a male 'andropause' that is, like the menopause in females, accompanied by changes in neurotransmitter and hormone receptor expression that are involved in regulating the function of the GnRH pulse generator.

Novel perspectives for progesterone in hormone replacement therapy, with special reference to the nervous system

The utility and safety of postmenopausal hormone replacement therapy has recently been put into question by large clinical trials. Their outcome has been extensively commented upon, but discussions have mainly been limited to the effects of estrogens. In fact, progestagens are generally only considered with respect to their usefulness in preventing estrogen stimulation of uterine hyperplasia and malignancy. In addition, various risks have been attributed to progestagens and their omission from hormone replacement therapy has been considered, but this may underestimate their potential benefits and therapeutic promises. A major reason for the controversial reputation of progestagens is that they are generally considered as a single class. Moreover, the term progesterone is often used as a generic one for the different types of both natural and synthetic progestagens. This is not appropriate because natural progesterone has properties very distinct from the synthetic progestins. Within the nervous system, the neuroprotective and promyelinating effects of progesterone are promising, not only for preventing but also for reversing age-dependent changes and dysfunctions. There is indeed strong evidence that the aging nervous system remains at least to some extent sensitive to these beneficial effects of progesterone. The actions of progesterone in peripheral target tissues including breast, blood vessels, and bones are less well understood, but there is evidence for the beneficial effects of progesterone. The variety of signaling mechanisms of progesterone offers exciting possibilities for the development of more selective, efficient, and safe progestagens. The recognition that progesterone is synthesized by neurons and glial cells requires a reevaluation of hormonal aging.

A molecular model for intercellular synchronization in the mammalian circadian clock

The mechanisms and consequences of synchrony among heterogeneous oscillators are poorly understood in biological systems. We present a multicellular, molecular model of the mammalian circadian clock that incorporates recent data implicating the neurotransmitter vasoactive intestinal polypeptide (VIP) as the key synchronizing agent. The model postulates that synchrony arises among circadian neurons because they release VIP rhythmically on a daily basis and in response to ambient light. Two basic cell types, intrinsically rhythmic pacemakers and damped oscillators, are assumed to arise from a distribution of Period gene transcription rates. Postsynaptic neurons show time-of-day dependent responses to VIP binding through a signaling cascade that activates Period mRNA transcription. The heterogeneous cell ensemble model self-synchronizes, entrains to ambient light-dark cycles, and desynchronizes in constant bright light or upon removal of VIP signaling. The degree of synchronicity observed depends on cell-specific features (e.g., mean and variability of parameters within the rhythm-generating loop), in addition to the more commonly studied effect of intercellular coupling strength. These simulations closely replicate experimental data and predict that heterogeneous oscillations (e.g., sustained, damped, and arrhythmic) arise from small differences in the molecular parameters between cells, that damped oscillators participate in entrainment and synchrony of the ensemble of cells, and that constant light desynchronizes oscillators by maximizing VIP release.

Neuromedin s as novel putative regulator of luteinizing hormone secretion

Neuromedin S (NMS), a 36 amino acid peptide structurally related to neuromedin U, was recently identified in rat brain as ligand for the G protein-coupled receptor FM4/TGR-1, also termed neuromedin U receptor type-2 (NMU2R). Central expression of NMS appears restricted to the suprachiasmatic nucleus, and NMS has been involved in the regulation of dark-light rhythms and suppression of food intake. Reproduction is known to be tightly regulated by metabolic and photoperiodic cues. Yet the potential contribution of NMS to the control of reproductive axis remains unexplored. We report herein analyses of hypothalamic expression of NMS and NMU2R genes, as well as LH responses to NMS, in different developmental and functional states of the female rat. Expression of NMS and NMU2R genes was detected at the hypothalamus along postnatal development, with significant fluctuations of their relative levels (maximum at prepubertal stage and adulthood). In adult females, hypothalamic expression of NMS (which was confined to suprachiasmatic nucleus) and NMU2R significantly varied during the estrous cycle (maximum at proestrus) and was lowered after ovariectomy and enhanced after progesterone supplementation. Central administration of NMS evoked modest LH secretory responses in pubertal and cyclic females at diestrus, whereas exaggerated LH secretory bursts were elicited by NMS at estrus and after short-term fasting. Conversely, NMS significantly decreased elevated LH concentrations of ovariectomized rats. In summary, we provide herein novel evidence for the ability of NMS to modulate LH secretion in the female rat. Moreover, hypothalamic expression of NMS and NMU2R genes appeared dependent on the functional state of the female reproductive axis. Our data are the first to disclose the potential implication of NMS in the regulation of gonadotropic axis, a function that may contribute to the integration of circadian rhythms, energy balance, and reproduction.

Vasoactive intestinal polypeptide regulates dynamic changes in astrocyte morphometry: impact on gonadotropin-releasing hormone neurons

Recent studies suggest that astrocytes modulate the GnRH-induced LH surge. In particular, we have shown that the surface area of astrocytes that ensheath GnRH neurons exhibits diurnal rhythms. Vasoactive intestinal polypeptide (VIP) influences numerous aspects of astrocyte function in multiple brain regions and is a neurotransmitter in the suprachiasmatic nucleus (SCN) that affects GnRH neurons. The goals of this study were to: 1) assess whether astrocytes that surround GnRH neurons express VIP receptors, 2) determine the effects VIP suppression in the SCN on the morphometry of astrocytes surrounding GnRH neurons, and 3) assess whether this effect mimics aging-like changes in surface area of astrocytes. Young rats were ovariectomized (d 0), implanted with cannulae into the SCN (d 5), injected with VIP antisense (antioligo) or random sequence oligonucleotides, implanted with capsules containing 17beta-estradiol dissolved in oil (d 7), and perfused at 0300, 1400, and 1800 h (d 9). Brains were processed for immunocytochemistry. Our results demonstrate that astrocytes in close apposition to GnRH neurons express VIP receptors. Antioligo treatment blocked diurnal rhythms in surface area of astrocytes ensheathing GnRH neurons. The absence of diurnal rhythms resembles observations in middle-aged rats. Together these findings suggest that the ability of the VIP-containing neurons in the SCN to relay diurnal information to GnRH neurons may be by influencing dynamic changes in the morphometry of astrocytes that surround GnRH neurons. Furthermore, the absence of a VIP rhythm in aging animals may lead to altered GnRH activity via astrocyte-dependent mechanisms.

Neuroendocrine control of reproductive aging: roles of GnRH neurons

The process of reproductive senescence in many female mammals, including humans, is characterized by a gradual transition from regular reproductive cycles to irregular cycles to eventual acyclicity, and ultimately a loss of fertility. In the present review, the role of the hypothalamic gonadotropin-releasing hormone (GnRH) neurons is considered in this context. GnRH neurons provide the primary driving force upon the other levels of the reproductive axis. With respect to aging, GnRH cells undergo changes in biosynthesis, processing and release of the GnRH decapeptide. GnRH neurons also exhibit morphologic and ultrastructural alterations that appear to underlie these biosynthetic properties. Thus, functional and morphologic changes in the GnRH neurosecretory system may play causal roles in the transition to acyclicity. In addition, GnRH neurons are regulated by numerous inputs from neurotransmitters, neuromodulators and glia. The relationship among GnRH cells and their inputs at the cell body (thereby affecting GnRH biosynthesis) and the neuroterminal (thereby affecting GnRH neurosecretion) is crucial to the function of the GnRH system, with age-related changes in these relationships contributing to the reproductive senescent process. Therefore, the aging hypothalamus is characterized by changes intrinsic to the GnRH cell, as well as its regulatory inputs, which summate to contribute to a loss of reproductive competence in aging females.

Minireview: timely ovulation: circadian regulation of the female hypothalamo-pituitary-gonadal axis

The preovulatory surge in the secretion of LH is timed by a neuroendocrine integrative mechanism that involves ovarian estradiol levels and the endogenous circadian system. Studies in female rats and hamsters have established that the clock in the hypothalamic suprachiasmatic nucleus has a preeminent role in setting the LH surge, and anatomical, physiological, and pharmacological data are revealing the responsible connections between suprachiasmatic nucleus neurons and GnRH and estradiol-receptive areas. Recent investigations show that GnRH and pituitary cells express circadian clock genes that might play a role in the release and reception of the GnRH signal. Analysis of the circadian regulation of the LH surge may provide a model for understanding how multiple neural oscillators function within other neuroendocrine axes.

Effects of Acute and Chronic Exposure to the Aryl Hydrocarbon Receptor Agonist 2,3,7,8-Tetrachlorodibenzo-p-Dioxin on the Transition to Reproductive Senescence in Female Sprague-Dawley Rats1

Activation of the aryl hydrocarbon receptor (AHR) can occur in polluted environments, either from smoking-related toxicants or from endogenous ligands. We tested whether acute or chronic exposure to the AHR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alters the transition to reproductive senescence in female Sprague-Dawley rats. In experiment 1, rats (n = 6 per experimental group) received a single dose of 0 or 10 mug/kg of TCDD orally (p.o.) on Postnatal Day 29. Vaginal cytology was monitored for 1 wk each month until rats were killed at 1 yr of age. The single prepubertal exposure to TCDD hastened the transition to reproductive senescence in female rats and was associated with delayed puberty, abnormal cyclicity, and premature reproductive senescence. In a second experiment, rats were exposed to TCDD chronically through weekly dosing (0, 50, or 200 ng kg(-1) wk(-1) p.o., n = 7 each dose) beginning in utero. Lifelong exposure to these lower doses of TCDD induced a dose- and time-dependent loss of normal cyclicity and significantly hastened the onset of the transition to reproductive senescence (P < 0.05). This premature transition to reproductive senescence was associated with prolonged estrous cycles and, at the highest dose of TCDD, persistent estrus or diestrus. The number and size of ovarian follicles were not altered by TCDD. Diestrous concentrations of LH in rats exposed chronically to TCDD were similar to those in controls, whereas progesterone tended to be elevated at both doses of the dioxin (P < 0.08). Serum FSH was elevated in the group exposed to 50 ng/kg of TCDD (P < 0.02), whereas estradiol was decreased at both doses of dioxin (P < 0.01). Data thus far support endocrine disruption rather than depletion of follicular reserves as a primary mechanism of the premature transition to reproductive senescence following activation of the AHR pathway by TCDD in female rats.

Colony sectorization of Metarhizium anisopliae is a sign of ageing

Spontaneous phenotypic degeneration resulting in sterile sectors is frequently observed when culturing filamentous fungi on artificial medium. Sterile sectors from two different strains of the insect pathogenic fungus Metarhizium anisopliae were investigated and found to contain reduced levels of cAMP and destruxins (insecticidal peptides). Microarray analysis using slides printed with 1730 clones showed that compared to wild-type, sterile sectors down-regulated 759 genes and upregulated 27 genes during growth in Sabouraud glucose broth or on insect cuticle. The differentially expressed genes are largely involved in cell metabolism (18.8 %), cell structure and function (13.6 %) and protein metabolism (8.8 %). Strong oxidative stress was demonstrated in sectorial cultures using the nitro blue tetrazolium assay and these cultures show other syndromes associated with ageing, including mitochondrial DNA alterations. However, genes involved in deoxidation and self-protection (e.g. heat-shock proteins, HSPs) were also upregulated. Further evidence of physiological adaptation by the degenerative sectorial cultures included cell-structure reorganization and the employment of additional signalling pathways. In spite of their very similar appearance, microarray analysis identified 181 genes differentially expressed between the two sectors, and the addition of exogenous cAMP only restored conidiation in one of them. Most of the differentially expressed genes were involved in catabolic or anabolic pathways, but the latter included genes for sporulation. Compared to the mammalian ageing process, sectorization in M. anisopliae showed many similarities, including similar patterns of cAMP production, oxidative stress responses and the involvement of HSPs. Thus, a common molecular machinery for ageing may exist throughout the eukaryotes.

Developmental and reproductive performance in circadian mutant mice

BACKGROUND

Genes underlying circadian rhythm generation are expressed in many tissues. We explore a role for circadian rhythms in the timing and efficacy of mouse reproduction and development using a genetic approach.

METHODS

We compare fecundity in Clock(Delta19) mutant mice (a dominant-negative protein essential for circadian rhythm activity) and in Vipr2-/- null mutant mice (affecting the generation and output of the circadian rhythm of the hypothalamic suprachiasmatic nucleus) with wild type (WT) litter mates under both a 12 h:12 h light:dark cycle and continuous darkness.

RESULTS

Uteri from Clock(Delta19) mice show no circadian rhythm and Vipr2-/- mice show a phase-advanced rhythm compared to WT uteri. In neither mutant line were homozygous or heterozygous fetuses lethal. Sexually mature adults of both mutant lines showed mildly reduced male in vivo (but not in vitro) fertility and irregular estrous cycles exacerbated by continuous darkness. However, pregnancy rates and neonatal litter sizes were not affected. The Clock(Delta19) mutant line was distinguishable from the Vipr2-/- null mutant line in showing more peri-natal delivery problems and very poor survival of offspring to weaning.

CONCLUSIONS

A fully functional central and peripheral circadian clock is not essential for reproduction and development to term, but has critical roles peri-natally and post-partum.