Ion regulation in invertebrates: molecular and integrative aspects

The subject of ion regulation in invertebrates is discussed, using a variety of invertebrate model species and approaches that range from the whole-organism level to tissue, subcellular, and molecular levels to illustrate the future direction of the field. These organisms inhabit a variety of aquatic, freshwater, and terrestrial environments, showing specific adaptations to each environment. This overview discusses mechanisms of metal detoxification and the presence of Cl-ATPase in marine organisms to avoid excess intracellular Cl(-); Ca(2+) regulation and endocrine aspects of adaptations to transitional (semiterrestrial) environments; adaptations to Ca(2+)-poor freshwater, particularly the reabsorption of Ca(2+) through specific transporters found in the urine; and finally, ionoregulatory mechanisms for life on land, such as Ca(2+) conservation during molting in isopods and the presence of K(+) channels in insect Malpighian tubules. Convergent mechanisms for dealing with similar problems in dissimilar habitats are discussed, taking into consideration that invertebrates will continue to serve as model systems for the evolution of ionoregulation in different habitats.     

Implications of leptin in neuroendocrine regulation of male reproduction

Obesity is a major health problem contributing to increased subfertility in males, as well as increased morbidity for diseases related to a decline in testosterone production with aging. Leptin is a hormone produced by adipose tissue whose production increases with the amount of body fat. Several studies have supported a relationship between increased leptin production and regulation of reproductive function. Indeed, leptin acts at all levels of the hypothalamus–pituitary–gonadal (HPG) axis in males. However, most of the obese individuals become insensitive to increased endogenous leptin production and develop a functional leptin resistance. This deregulation of leptin signaling might result in abnormal endocrine and reproductive functions. Altered leptin dynamics may contribute to male infertility in different ways, leading to hypogonadism. These include leptin resistance or leptin insufficiency at the hypothalamus and leptin modulation of testicular physiology. In this review, we address the mechanisms of action of leptin at different levels of the HPG axis. Moreover, the influences of leptin on steroidogenesis and spermatogenesis, as well as seasonal variations of leptin's action on male reproduction are discussed.     

Bone endocrine regulation of energy metabolism and male reproduction

Usually vertebrate physiology is studied within the confined limits of a given organ, if not cell type. This approach has progressively changed with the emergence of mouse genetics that has rejuvenated the concept of a whole body study of physiology. A vivid example of how mouse genetics has profoundly affected our understanding of physiology is skeleton physiology. A genetic approach to bone physiology revealed that bone via osteocalcin, an osteoblast-secreted molecule, is a true endocrine organ regulating energy metabolism and male reproduction. This ongoing body of work that takes bone out of its traditional roles is connecting it to a growing number of peripheral organs. These novel important hormonal connections between bone, energy metabolism and reproduction underscore the concept of functional dependence in physiology and the importance of genetic approaches to identify novel endocrine regulations.     

Biological and clinical research on male reproduction and fertility regulation

Biological and clinical research on male reproduction and fertility regulation carried at the National Institute of Health and Family Welfare over the past 17 years has been highlighted in this review. Areas of research covered pertains to hormones in relation to sperm maturation and transport; fertilizing ability of spermatozoa under different experimental conditions; agents producing functional sterility; seasonal variations in primate reproduction; male infertility including semen biochemistry, differential diagnosis between obstructive and non-obstructive azoospermia and hormone therapy; vasectomy, reversible vasocclusion and vasanastomosis; and the use of cyproterone acetate and testosterone enanthate in male rhesus monkey and human volunteers for reversible contraception.     

Photoperiodic regulation of reproduction in the male silver fox (Vulpes vulpes)

Six silver fox males were exposed to short days (6L:18D) from February, when the testes were fully developed, until June 1986 (Group 6L). Eight males maintained in natural daylight served as controls (Group N). Histological sections from the testes of 2 males in Group 6L killed in June indicated full spermatogenic activity. Three blue fox vixens inseminated the following year with semen collected and frozen in June from 3 males in Group 6L failed to produce litters. One possible explanation for the reproductive failures could have been that the high environmental temperatures in June influenced semen quality. There was no significant difference (P greater than 0.05) in LH release in response to GnRH stimulation in June, but testosterone response to LH release was significantly higher (P less than 0.01) in animals subjected to a restricted photoperiod, demonstrating that testicular testosterone production was maintained longer than in control animals. Two males in Group 6L were retained in 6L:18D from June until December 1986 and then exposed to natural daylight until the end of the study in May 1987 (Group 6L:6L:N). These males started to shed their winter coat and showed clinical signs of testicular regression in December, i.e. after approximately 11 months exposure to 6L:18D. The 2 remaining males in Group 6L were moved to cages with natural daylight in June 1986, where they were kept until the end of the experiment (Group 6L: N:N). These males displayed testicular regression soon after the change in photoperiod but maintained their capacity for testicular redevelopment during the following breeding season.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonality of reproduction in sheep and its control by photoperiod

Seasonality of the reproductive cycle in sheep is a general phenomenon for mid-latitude breeds. The proximal part (breeding season) and also partially distal part (end of gestation and beginning of lactation) of this cycle is controlled by photoperiod, whatever the form of light regimens. Data are presented which indicate that male and female do not necessarily have the same photoperiodic sensitivity. Gonadal stimulation in the ram starts 1.5-2 months earlier than in the ewe under annual variations. Photoperiod controls the reproductive cycle by the intermediary of the hypothalamo-pituitary axis. There are both a steroid-independent and a steroid-dependent effect of light, depending on both decreasing and increasing daylength in mid-latitudes. Data are also presented which support Bunning's hypothesis on photoperiodic time measurement in mammals. Sheep measure photoperiodic time by using a circadian rhythm of photosensitivity. Daylength is not measured by the total duration of exposure to light but by the illumination of two special set points during the day, one of them entraining the circadian rhythm of photosensitivity and the other inducing or not inducing a physiological response if it is coincident, or not coincident, with photoinducible phase of that rhythm. A photoinducible phase has been found for prolactin secretion, and perhaps also for LH secretion. Melatonin secretion is used by sheep for measuring daylength. However, that secretion disappears during two set points during the day, thus raising the possibility of using alternatively melatonin and light pulse for controlling the reproductive cycle in sheep.     

Transferrin types and reproduction in sheep

Transferrin types were determined for flocks of Finnish Landrace, Clun Forest, Soay and Merino sheep and gene frequencies were calculated. Analysis of ratios of transferrin types in segregating matings of Finnish Landrace and Clun Forest revealed a significant excess of heterozygotes in matings of heterozygous rams with heterozygous and with homozygous ewes. In Finnish Landrace, matings of sheep homozygous for Tf ^c to those heterozygous for Tf ^C gave a significant excess of homozygous male lambs and heterozygous female lambs. Finnish Landrace ewes of transferrin type BD had smaller litters than ewes of other types.     

Leptin and metabolic control of reproduction

Leptin treatment prevents the effects of fasting on reproductive processes in a variety of species. The mechanisms that underlie these effects have not been elucidated. Progress in this area of research might be facilitated by viewing reproductive processes in relation to mechanisms that maintain fuel homeostasis. Reproduction, food intake, and fuel partitioning can be viewed as homeostatic responses controlled by a sensory system that monitors metabolic signals. These signals are generated by changes in intracellular metabolic fuel availability and oxidation rather than by changes in the amount of body fat or by changes in any aspect of body composition. Leptin might be viewed as either a mediator or as a modulator of the intracellular metabolic signal. Consistent with its purported action as a mediator of the metabolic signal, leptin synthesis and secretion are influenced acutely by changes in metabolic fuel availability, and these changes might lead to changes in reproductive function. The effects of leptin treatment on reproduction are blocked by treatments that inhibit intracellular fuel oxidation. Metabolic signals that inhibit reproduction in leptin-treated animals might act via neural pathways that are independent of leptin's action. Alternatively, both leptin and metabolic inhibitors might interact at the level of intracellular fuel oxidation. In keeping with the possibility that leptin modulates the metabolic signal, leptin treatment increases fuel availability, uptake, and oxidation in particular tissues. Leptin might affect reproduction indirectly by altering fuel oxidation or other peripheral processes such as gastric emptying. Reproductive processes are among the most energetically expensive in the female repertoire. Because leptin increases energy expenditure while simultaneously inhibiting energy intake, it may have limited use as a long-term treatment for infertility.     

Photoperiod and water temperature regulation of seasonal reproduction in male round stingrays (Urobatis halleri)

This study characterizes the seasonal reproductive cycle of male round stingrays (Urobatis halleri) in Seal Beach, California. Mature round stingrays were collected monthly by beach seine near the San Gabriel River outfall from August 2004–September 2006, and rays were assessed for gametogenesis and steroid hormone levels. Male round stingrays exhibit a seasonal pattern of increased gonadosomatic index (GSI), spermatogenesis, and production of testosterone (T) and 11-ketotestosterone (11-KT). Based on GSI, the male reproductive cycle was broken into three distinct phases. TUNEL positive staining was only observed in the Sertoli cells of mature spermatocysts during the degenerative testicular phase, suggesting that Sertoli cell death potentially plays a role in testicular degeneration and the regulation of sperm release. GSI, T, and 11-KT were all inversely correlated with daylength, while only T was inversely correlated with temperature. Captive male round stingrays subjected to water temperatures of 25 °C showed a significant decrease in plasma testosterone concentrations, but the same males exposed to ambient water temperatures (18 °–20 °C) exhibited T concentrations observed in wild male round stingrays during the recrudescent phase. Together, these findings suggest that temperature plays an important role in the regulation of testosterone, and may serve as an ultimate cue for reproduction in male round stingrays.     

Welfare aspects of the transport by road of sheep and pigs

That transport can be stressful to pigs and sheep has been inferred from behavioral and physiological measurements. Duration of journey is more likely to cause stress in pigs than in sheep, but loading and the start of travel are stressful to both. Vibration, related to vehicle design, and the jolting, shocks, and sudden impacts caused by road conditions and manner of driving, are probably of more importance than noise. Ventilation and stocking density can modulate the effects of ambient temperature, which may influence meat quality in pigs. Deprivation of food and water does not appear to stress sheep unduly, but this can become a serious welfare concern in the case of pigs. However, most journeys undertaken by pigs in the European Union are relatively short. In sheep, transport may follow soon after other stressful experiences such as weaning, shearing, handling, or marketing; the interaction of these factors in causing cumulative stress has not been studied. In pigs, fighting after the mixing of previously unacquainted animals is well known to be a welfare issue that can compromise meat quality. Considering the public interest in livestock transport and the large amounts of money involved, surprisingly little research has been done in the area, and more is urgently needed for legislation and welfare codes to be soundly based on scientific knowledge.     

Orexin gene expression and regulation by photoperiod in the sheep hypothalamus

Hypothalamic orexin gene expression has not been reported in the ruminant. Here, we describe the localization of preproorexin mRNA in the ovine lateral hypothalamic area (LHA) and zona incerta (ZI) using in situ hybridization. The hypothalamic localization of the orexin gene expression was similar in sheep to rodent models. Since appetite in sheep is seasonally (photoperiodically) regulated, we compared the amounts of preproorexin mRNA between long- (LD) and short-day (SD) photoperiods in both freely feeding (food intake is 20% higher in LD than SD) and food-restricted sheep (50% liveweight maintenance for 11 weeks). Gene expression was higher in SDs than in LDs but was not affected by chronic food restriction. In a second study, hypothalamic orexin gene expression in castrate sheep was not affected by a 4-day fast, irrespective of gonadal steroid (estradiol) replacement, and was not affected by the gonadal steroid per se. The results demonstrate the sensitivity of orexin gene expression to photoperiod, but up-regulation occurs in SDs when the appetite is characteristically low and no sensitivity to imposed changes in food intake. This supports the concept that orexins may not have a primary role in appetite regulation and correction of negative energy balance but since the sheep breed only in SDs, their role in seasonal reproductive activation deserves further study.     

Identification of nuclear receptors involved in regulation of male reproduction in the red flour beetle, Tribolium castaneum

Nineteen canonical and two Knirps-like family nuclear receptors (NRs) were identified in the genome of Tribolium castaneum. The current study was conducted to identify NRs involved in regulation of male reproduction. RNA interference (RNAi)-aided knockdown in the expression of genes coding for all 21 NRs showed that reduction in the levels of 11 NRs (E75, E78, FTZ-F1, HR38, HR4, Knirps-like, HNF4, Tailless, HR51, Dsf and HR39) in the male beetles caused more than 50% reduction in the eggs laid by the female beetles mated with RNAi male beetles. Among these 11 NRs that are required for male reproduction, knockdown in the expression of genes coding for E78 and HR39 in the male beetles resulted in a reduction in the number of sperm produced and transferred to the female when compared to the sperms produced and transferred by the control male beetles injected with bacterial malE dsRNA. In contrast, knockdown in the expression of genes coding for E75 and HR38 caused a reduction in the size of male accessory glands (MAG), the amount of protein produced by the MAG and the expression of genes coding for accessory gland proteins. These data suggest that NRs such as E78 and HR39 regulate sperm production and their transfer to the females and the other NRs such as E75 and HR38 regulate the development of MAG and the production of accessory gland proteins.     

Endocrine regulation of male fertility by the skeleton

Interactions between bone and the reproductive system have until now been thought to be limited to the regulation of bone remodeling by the gonads. We now show that, in males, bone acts as a regulator of fertility. Using coculture assays, we demonstrate that osteoblasts are able to induce testosterone production by the testes, though they fail to influence estrogen production by the ovaries. Analyses of cell-specific loss- and gain-of-function models reveal that the osteoblast-derived hormone osteocalcin performs this endocrine function. By binding to a G protein-coupled receptor expressed in the Leydig cells of the testes, osteocalcin regulates in a CREB-dependent manner the expression of enzymes that is required for testosterone synthesis, promoting germ cell survival. This study expands the physiological repertoire of osteocalcin and provides the first evidence that the skeleton is an endocrine regulator of reproduction.     

Study of LH response to GnRH in the young male as a criterion of genetic merit for female reproduction in sheep

A high and a low response line in sheep were selected on the basis of the mean concentration of LH in 10-week-old Finn-Dorset ram lambs after an i.v. injection of 5 micrograms GnRH. After 8 male generations the mean LH response of the high line was more than 5-fold that of the low line and the heritability of the selected trait was estimated at 0.44 +/- 0.015. Highly significant line differences in mean LH response to GnRH were also found in males at 20 weeks of age and females at 10 and 20 weeks of age and the genetic correlations between the four LH response traits appear to be close to unity. Large line differences in the mean FSH response to GnRH were also found in both males and females at 10 and 20 weeks of age. Selection had little effect on the physical characteristics of lambs. High-response line ewes entering their first breeding season at about 7 months of age showed oestrus earlier in the season and had higher ovulation rates and numbers of lambs born per ewe lambing than did low-response line ewes. In the second breeding season, at about 19 months of age, the only line difference was a higher ovulation rate early in the breeding season in high-line ewes. It is suggested that these changes may be mediated by a more rapid response in high-line ewes to increased GnRH stimulation at puberty or at the beginning of the breeding season.     

Weaning and metabolic regulation in the rat

Premature weaning of rats to high carbohydrate diets causes a variety of short- and long-term changes in lipid metabolism, but the mechanisms involved are unclear. It is likely that interaction of diet with certain emerging hormonal control patterns during weaning might condition metabolic control and (or) subsequent adaptations in the adult organism. This implies that the adaptive responses of infant animals to diet may differ from those of the mature organism. For example, premature weaning leads to early appearance of rat liver malic enzyme (ME), even when fat supplies as much as 65% of the dietary energy; the same diet suppresses ME activity in 45-day-old rats. The levels of plasma glucagon and thyroid hormones are elevated during the weaning period. Several studies have shown that triiodothyronine evokes hepatic ME in suckling rats. Conversely, glucagon infusion into prematurely weaned rats suppresses the early appearance of the enzyme. Premature weaning, regardless of fat intake, leads to a rapid decline in plasma glucagon levels. Since glucagon is known to antagonize the actions of triiodothyronine on liver ME, the interaction of diet with glucagon and thyroid hormones is conceivably part of the mechanism responsible for the early appearance of hepatic malic enzyme, whereby the decline of plasma glucagon permits triiodothyronine to act on liver ME. Insulin probably exerts a permissive action subsequently. The manner in which these events relate to the long-term consequences of premature weaning is unknown.