Gerald A. Lincoln

Seasonal breeding: nature's contraceptive.

Publisher Summary This chapter focuses on the seasonal control of fertility. It discusses two aspects in detail—the way in which changes in the secretion of luteinizing hormone-releasing hormone by the hypothalamus controls the activity of the pituitary and testis, and the way changes in day length influence the activity of the hypothalamus. The downstream events evoked by the hypothalamus are relatively easy to understand, whereas the mechanisms involved in the photoperiodic control are very complex and largely unresolved. While seasonal changes in temperature, rainfall, and food availability are the factors of the environment that dictate survival of adults and young—and are thus ultimately responsible for dictating the timing of the birth season—these are not necessarily the factors used as cues by the animals to regulate their reproductive endocrinology. This is because it is necessary to anticipate the timing of birth by dictating the timing of conception, as the duration of gestation in mammals is usually fixed. As accurate timing of conception is important, animals tend to become reliant on cues from the environment, which are the best predictors of the time of year. The seasonal cycle highlights the seasonal changes in daylight length, rutting behavior, testicular diameter, sexual skin flush, and concentrations of plasma follicle-stimulating hormone, prolactin, and testosterone in a group of rams throughout the year.

A novel mammalian receptor for the evolutionarily conserved type II GnRH.

Mammalian gonadotropin-releasing hormone (GnRH I: pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2) stimulates pituitary gonadotropin secretion, which in turn stimulates the gonads. Whereas a hypothalamic form of GnRH of variable structure (designated type I) had been shown to regulate reproduction through a cognate type I receptor, it has recently become evident that most vertebrates have one or two other forms of GnRH. One of these, designated type II GnRH (GnRH II: pGlu-His-Ser-His-Gly-Trp-Tyr-Pro-Gly-NH2), is conserved from fish to man and is widely distributed in the brain, suggesting important neuromodulatory functions such as regulating K+ channels and stimulating sexual arousal. We now report the cloning of a type II GnRH receptor from marmoset cDNA. The receptor has only 41% identity with the type I receptor and, unlike the type I receptor, has a carboxyl-terminal tail. The receptor is highly selective for GnRH II. As with the type I receptor, it couples to Gαq/11 and also activates extracellular signal-regulated kinase (ERK1/2) but differs in activating p38 mitogen activated protein (MAP) kinase. The type II receptor is more widely distributed than the type I receptor and is expressed throughout the brain, including areas associated with sexual arousal, and in diverse non-neural and reproductive tissues, suggesting a variety of functions. Surprisingly, the type II receptor is expressed in the majority of gonadotropes. The presence of two GnRH receptors in gonadotropes, together with the differences in their signaling, suggests different roles in gonadotrope functioning.

Ancestral TSH Mechanism Signals Summer in a Photoperiodic Mammal

In mammals, day-length-sensitive (photoperiodic) seasonal breeding cycles depend on the pineal hormone melatonin, which modulates secretion of reproductive hormones by the anterior pituitary gland [1]. It is thought that melatonin acts in the hypothalamus to control reproduction through the release of neurosecretory signals into the pituitary portal blood supply, where they act on pituitary endocrine cells [2]. Contrastingly, we show here that during the reproductive response of Soay sheep exposed to summer day lengths, the reverse applies: Melatonin acts directly on anterior-pituitary cells, and these then relay the photoperiodic message back into the hypothalamus to control neuroendocrine output. The switch to long days causes melatonin-responsive cells in the pars tuberalis (PT) of the anterior pituitary to increase production of thyrotrophin (TSH). This acts locally on TSH-receptor-expressing cells in the adjacent mediobasal hypothalamus, leading to increased expression of type II thyroid hormone deiodinase (DIO2). DIO2 initiates the summer response by increasing hypothalamic tri-iodothyronine (T3) levels. These data and recent findings in quail [3] indicate that the TSH-expressing cells of the PT play an ancestral role in seasonal reproductive control in vertebrates. In mammals this provides the missing link between the pineal melatonin signal and thyroid-dependent seasonal biology.

Temporal expression of seven clock genes in the suprachiasmatic nucleus and the pars tuberalis of the sheep: Evidence for an internal coincidence timer

The 24-h expression of seven clock genes (Bmal1, Clock, Per1, Per2, Cry1, Cry2, and CK1ɛ) was assayed by in situ hybridization in the suprachiasmatic nucleus (SCN) and the pars tuberalis (PT) of the pituitary gland, collected every 4 h throughout 24 h, from female Soay sheep kept under long (16-h light/8-h dark) or short (8-h light/16-h dark) photoperiods. Locomotor activity was diurnal, inversely related to melatonin secretion, and prolactin levels were increased under long days. All clock genes were expressed in the ovine SCN and PT. In the SCN, there was a 24-h rhythm in Clock expression, in parallel with Bmal1, in antiphase with cycles in Per1 and Per2; there was low-amplitude oscillation of Cry1 and Cry2. The waveform of only Per1 and Per2 expression was affected by photoperiod, with extended elevated expression under long days. In the PT, the high-amplitude 24-h cycles in the expression of Bmal1, Clock, Per1, Per2, Cry1, and Cry2, but not CK1ɛ, were influenced by photoperiod. Per1 and Per2 peaked during the day, whereas Cry1 and Cry2 peaked early in the night. Hence, photoperiod via melatonin had a marked effect on the phase relationship between Per/Cry genes in the PT. This supports the conclusion that an ”external coincidence model“ best explains the way photoperiod affects the waveform of clock gene expression in the SCN, the central pacemaker, whereas an ”internal coincidence model“ best explains the way melatonin affects the phasing of clock gene expression in the PT to mediate the photoperiodic control of a summer or winter physiology.

Characterizing a mammalian circannual pacemaker

Many species express endogenous cycles in physiology and behavior that allow anticipation of the seasons. The anatomical and cellular bases of these circannual rhythms have not been defined. Here, we provide strong evidence using an in vivo Soay sheep model that the circannual regulation of prolactin secretion, and its associated biology, derive from a pituitary-based timing mechanism. Circannual rhythm generation is seen as the product of the interaction between melatonin-regulated timer cells and adjacent prolactin-secreting cells, which together function as an intrapituitary “pacemaker-slave” timer system. These new insights open the way for a molecular analysis of long-term timing mechanisms.

RFamide-Related Peptide and its Cognate Receptor in the Sheep: cDNA Cloning, mRNA Distribution in the Hypothalamus and the Effect of Photoperiod

Photoperiodic responses enable animals to adapt their physiology to predictable patterns of seasonal environmental change. In mammals, this depends on pineal melatonin secretion and effects in the hypothalamus, but the cellular and molecular substrates of its action are poorly understood. The recent identification of a mammalian orthologue of the avian gonadotrophin‐inhibitory hormone gene has led to interest in its possible involvement in seasonal breeding. In long‐day breeding Syrian hamsters, hypothalamic RFamide‐related peptide (RFRP) expression is increased by exposure to long photoperiod. Because, opposite to hamsters, sheep are short‐day breeders, we predicted that a conserved role in mammalian reproductive activation would decrease RFRP expression in sheep under a long photoperiod. We cloned the ovine RFRP cDNA and examined its expression pattern in Soay sheep acclimated to a 16 : 8 h or 8 : 16 h light /dark cycle (LP and SP, respectively). RFRP was expressed widely in the sheep hypothalamus and increased modestly overall with exposure to LP. Interestingly, RFRP expression in the ependymal cells surrounding the base of the third ventricle was highly photoperiodic, with levels being undetectable in animals held on SP but consistently high under LP. These data are inconsistent with a conserved reproductive role for RFRP across mammals. Additionally, we cloned the ovine homologue of the cognate RFRP receptor, rfr‐2 (NPFF1) and found localised expression in the suprachiasmatic nuclei and in the pars tuberalis. Taken together, these data strengthen the emerging view that interplay between ependymal cells and the pars tuberalis might be important for the seasonal timing system.

Hypothalamic pulse generators

Publisher Summary This chapter describes pulsatile hormone secretion and analyzes the mechanisms underlying the neural organization of this phenomenon in context of oxytocin and luteinizing hormone releasing hormone (LHRH) secretion. It also discusses five concepts— (1) oxytocin and LHRH are both released from the hypothalamus in pulses superimposed upon a continuous or intermittent low level of secretion; these two modes of secretion can produce separate actions, or one might govern the response to the other; (2) the synchronous generation of action potentials at a very fast rate within a population of peptidergic neurones provides the neural substrate for the release of a hormone pulse; (3) the control of interpulse interval is determined by events within the brain, though the interval generator may not reside within the neurons that secrete oxytocin or LHRH; (4) amplitude modulation of pulsatile secretion can relate to different levels of electrical activation within the brain and/or to differences in the responsiveness of the target tissues that transduce the pulsatile signal; and (5) opioid peptides inhibit the secretion of both oxytocin and LHRH. This involves an inhibition of stimulus-secretion coupling within the nerve terminals, and possibly an inhibition of synaptically mediated events that impinge upon the cell bodies of the peptidergic neurons within the hypothalamus.

Clock Genes and the Long-Term Regulation of Prolactin Secretion: Evidence for a Photoperiod/Circannual Timer in the Pars Tuberalis

Prolactin secretion is regulated by photoperiod through changes in the 24‐h melatonin profile and displays circannual rhythmicity under constant photoperiod. These two processes appear to occur principally within the pituitary gland, controlled by the pars tuberalis. This is evident because: (i) hypothalamic‐pituitary disconnected (HPD) sheep show marked changes in prolactin secretion in response to switches in photoperiod and manipulations of melatonin, similar to brain‐intact controls; (ii) HPD sheep also show photoperiod‐specific, long‐term cycles in prolactin secretion under constant long or short days, with the timing maintained even when prolactin secretion is blocked for 2–3 months; and (iii) pars tuberalis cells, but not lactotrophs, express high concentrations of melatonin (MT1) receptor, and exhibit a duration‐sensitive, cAMP‐dependant, inhibitory response to physiological concentrations of melatonin. This suggests the existence of an intrinsic, reversible photoperiod‐circannual timer in pars tuberalis cells. A full complement of clock genes (Bmal1, Clock, Per1, Per2, Cry1 and Cry2) are expressed in the ovine pars tuberalis, and undergo 24‐h cyclical expression as observed in a cell autonomous, circadian clock. Activation of Per genes occurs in the early day (melatonin off‐set), while activation of Cry genes occurs in the early night (melatonin on‐set). This temporal association is evident under both long and short days, thus the Per–Cry interval varies directly with photoperiod. Because, PER : CRY, protein : protein interactions affect stability, nuclear entry and gene transcription based on rodent data, the change in phasing of Per/Cry expression provides a potential mechanism for decoding the long day/short day melatonin signal. A speculative, but testable, extension of this hypothesis is that intrinsically regulated changes in the phase of Per/Cry rhythms, regulates both photorefractoriness and the generation of circannual rhythms in prolactin secretion.

Disrupted seasonal biology impacts health, food security and ecosystems

The rhythm of life on earth is shaped by seasonal changes in the environment. Plants and animals show profound annual cycles in physiology, health, morphology, behaviour and demography in response to environmental cues. Seasonal biology impacts ecosystems and agriculture, with consequences for humans and biodiversity. Human populations show robust annual rhythms in health and well-being, and the birth month can have lasting effects that persist throughout life. This review emphasizes the need for a better understanding of seasonal biology against the backdrop of its rapidly progressing disruption through climate change, human lifestyles and other anthropogenic impact. Climate change is modifying annual rhythms to which numerous organisms have adapted, with potential consequences for industries relating to health, ecosystems and food security. Disconcertingly, human lifestyles under artificial conditions of eternal summer provide the most extreme example for disconnect from natural seasons, making humans vulnerable to increased morbidity and mortality. In this review, we introduce scenarios of seasonal disruption, highlight key aspects of seasonal biology and summarize from biomedical, anthropological, veterinary, agricultural and environmental perspectives the recent evidence for seasonal desynchronization between environmental factors and internal rhythms. Because annual rhythms are pervasive across biological systems, they provide a common framework for trans-disciplinary research.

Identification of Melatonin-Regulated Genes in the Ovine Pituitary Pars Tuberalis, a Target Site for Seasonal Hormone Control

The pars tuberalis (PT) of the pituitary gland expresses a high density of melatonin (MEL) receptors and is believed to regulate seasonal physiology by decoding changes in nocturnal melatonin secretion. Circadian clock genes are known to be expressed in the PT in response to the decline (Per1) and onset (Cry1) of MEL secretion, but to date little is known of other molecular changes in this key MEL target site. To identify transcriptional pathways that may be involved in the diurnal and photoperiod-transduction mechanism, we performed a whole genome transcriptome analysis using PT RNA isolated from sheep culled at three time points over the 24-h cycle under either long or short photoperiods. Our results reveal 153 transcripts where expression differs between photoperiods at the light-dark transition and 54 transcripts where expression level was more globally altered by photoperiod (all time points combined). Cry1 induction at night was associated with up-regulation of genes coding for NeuroD1 (neurogenic differentiation factor 1), Pbef / Nampt (nicotinamide phosphoribosyltransferase), Hif1alpha (hypoxia-inducible factor-1alpha), and Kcnq5 (K+ channel) and down-regulation of Rorbeta, a key clock gene regulator. Using in situ hybridization, we confirmed day-night differences in expression for Pbef / Nampt, NeuroD1, and Rorbeta in the PT. Treatment of sheep with MEL increased PT expression for Cry1, Pbef / Nampt, NeuroD1, and Hif1alpha, but not Kcnq5. Our data thus reveal a cluster of Cry1-associated genes that are acutely responsive to MEL and novel transcriptional pathways involved in MEL action in the PT.