Thierry Raclot

Selective mobilization of fatty acids from adipose tissue triacylglycerols.

Adipose tissue triacylglycerols represent the main storage of a wide spectrum of fatty acids differing by molecular structure. The release of individual fatty acids from adipose tissue is selective according to carbon chain length and unsaturation degree in vitro and in vivo in animal studies and also in humans. The mechanism of selective fatty acid mobilization from white fat cells is not known. Lipolysis is widely reported to work at a lipid-water interface where only small amounts of substrate are available. A preferential hydrolysis of a small triacylglycerol fraction enriched in certain triacylglycerol molecular species at the lipid-water interface and enzymological properties of hormone-sensitive lipase could explain the selective mobilization of fatty acids from fat cells. This selectivity could affect the individual fatty acid supply to tissues.

Hypothalamic gene expression in long-term fasted rats: relationship with body fat.

Many hypothalamic neuropeptides are involved in the regulation of energy homeostasis and feeding behavior. We have investigated whether and to what extent neuropeptide Y (NPY), agouti-related protein (AGRP), melanin-concentrating hormone (MCH), and prepro-orexin (prepro-OX) as well as pro-opiomelanocortin (POMC) and cocaine and amphetamine-regulated transcript (CART) mRNA levels are affected in rat hypothalamus. An experimental model of long-term fasting rat characterized by three metabolic phases from changes in lipid and protein utilization was used. Except for prepro-OX and compared to fed group, starvation induced an increase in the orexigenic gene expressions that was much more marked in phase 3 (by 2.5-, 8.1-, and 13.5-fold for MCH, AGRP, and NPY, respectively) than in phase 2 (by about 1.5-2.2-fold as an average) of fasting. AGRP and NPY mRNA levels were inversely related to body fat content. Anorexigenic gene expression was only slightly affected at both fasting stages. We conclude that the regulation of NPY and AGRP gene expression is primarily involved during late fasting and could mediate the concomitant enhanced drive for refeeding.

FORAGING STRATEGY OF KING PENGUINS (APTENODYTES PATAGONICUS) DURING SUMMER AT THE CROZET ISLANDS

The foraging ecology of King Penguins (Aptenodytes patagonicus) at Pos- session Island, Crozet Archipelago, was studied between January and March 1993 and between February and March 1994. Diving activity and position of the birds were deter- mined via externally attached logging units. Feeding behavior was detected using stomach temperature sensors. All calculated positions of the birds occurred south of Possession Island. The area most frequented was situated 250 km south of the island (48.5? S), where birds stayed for 40 h during a foraging trip, on average. The overall swim speed differed with time into the foraging trip, being lowest during the central part and highest (10 km/h) during the last night at sea, when birds were returning to the colony. Diving activities showed a clear diurnal pattern, with maximum dive depth being dependent on ambient light levels. During the night, only shallow dives 85% of the feeding events occurred during daylight and at twilight. Mean food mass ingested was calculated to be -2350 g/d, with the highest amounts ingested during the central and penultimate days of the foraging trip. Foraging success (grams ingested per unit time underwater) was also highest (>370 g/h) at this time. The relationships between foraging strategy, inter- and intraspecific competition, and King Penguin body mass are discussed.

Should I stay or should I go? Hormonal control of nest abandonment in a long-lived bird, the Adélie penguin.

According to life-history theory, long-lived birds should favor their survival over the current reproductive attempt, when breeding becomes too costly. In seabirds, incubation is often associated with spontaneous long-term fasting. Below a threshold in body reserves, hormonal and metabolic shift characteristics of a switch from lipid to protein utilization (phase III, PIII) occur. These metabolic changes are paralleled by nest abandonment and stimulation of refeeding behavior. Parental behavior is then under control of two hormones with opposite effects: corticosterone (CORT) and prolactin which stimulate foraging and incubation behavior, respectively. The aim of this study was to determine the respective role of these two hormones in nest abandonment by Adélie penguins. To this end, plasma hormone levels were measured before egg-laying and at departure from the colony (i.e. when birds were relieved by their partner or abandoned their nest), and related to nutritional state and incubation success. We found that males abandoning their nest in PIII presented high CORT levels and low prolactin levels. Interestingly, males which presented high plasma levels of prolactin in PIII did not abandon. We show that although CORT is the first hormone to be affected by prolonged energy constraints, the combined effects of high CORT and low prolactin levels are necessary for parents to favor self-maintenance and abandon the nest. We provide insights into time-course changes of the endocrine profile as PIII proceeds and report that reaching proteolytic late fasting is not sufficient to induce nest abandonment in a long-lived bird.

Selectivity of fatty acids on lipid metabolism and gene expression.

Triacylglycerols represent the main form of storage for a wide spectrum of fatty acids. Their utilization first involves mobilization from adipose tissue through lipolysis. The release of individual fatty acids from adipose tissue is selective in vitro and in vivo in animal studies and also in human subjects. Generally, fatty acids are more readily mobilized from fat cells when they are short-chain and unsaturated. This selectivity could affect the storage of individual fatty acids in adipose tissue, and their subsequent supply to tissues. The nature of the dietary fats could affect lipid homeostasis and body fat deposition. Dietary fish oil influences adipose tissue development in a site-specific manner as a function of diet and feeding period. A diet high in n-3 polyunsaturated fatty acids (PUFA) results in a preferential partitioning of ingested energy towards oxidation at the expense of storage. Fatty acids are important mediators of gene expression in the liver. Indeed, genes encoding both glycolytic and lipogenic enzymes and key metabolic enzymes involved in fatty acid oxidation are regulated by dietary PUFA. White adipose tissue could also be a target for PUFA control of gene expression. The treatment of pre-adipose cells by fatty acids induces the expression of numerous genes that encode proteins involved in fatty acid metabolism. The mechanisms of PUFA-mediated repression of gene expression in adipocytes seem to be different, at least partly, from those described in liver. Tissue-specific and site-specific factors are possibly involved in the specific effect of PUFA on gene expression, although other mechanisms cannot be excluded.

Exogenous corticosterone and nest abandonment: A study in a long-lived bird, the Adélie penguin

Breeding individuals enter an emergency life-history stage when their body reserves reach a minimum threshold. Consequently, they redirect current activity toward survival, leading to egg abandonment in birds. Corticosterone (CORT) is known to promote this stage. How and to what extent CORT triggers egg abandonment when breeding is associated with prolonged fasting, however, requires further investigation. We manipulated free-living male Adélie penguins with CORT-pellets before their laying period. We then examined their behavioral response with respect to nest abandonment in parallel with their prolactin levels (regulating parental care), and the subsequent effects of treatment on breeding success in relieved birds. Exogenous CORT triggered nest abandonment in 60% of the treated penguins ~14 days after treatment and induced a concomitant decline in prolactin levels. Interestingly, prolactin levels in treated penguins that did not abandon their nest were higher at the point of implantation and also after being relieved by females, when compared with abandoning penguins. Among successful birds, the treatment did not affect the number of chicks, nor the brood mass. Our results show the involvement of CORT in the decision-making process regarding egg abandonment in Adélie penguins when incubation is associated with a natural long fast. However, we suggest that CORT alone is not sufficient to trigger nest abandonment but that 1) prolactin levels need to reach a low threshold value, and 2) a rise in proteolysis (i.e. utilization of protein as main energy substrate) seems also to be required.

Positional distribution on n−3 fatty acids in triacylglycerols from rat adipose tissue during fish oil feeding

The present study was designed to investigate the metabolism of the n−3 olyunsaturated fatty acids (PUFA) in adipose tissue and its dependence upon dietary factors. Changes in the positional distribution of the fatty acids in triacylglycerols from retroperitoneal adipose tissue were studied as a function of time on rats fed for 4 wk a diet enriched with fish oil. The stereospecific analysis of triacylglycerols was based on random formation ofrac-1,2-diacylglycerols by Grignard degradation. This was followed by synthesis ofrac-phosphatidic acids and treatment with phospholipase A2. In the triacylglycerols of the fish oil diet, 57% of the total n−3 fatty acids were in position 3,i.e., two-thirds of 22∶5n−3 and 22∶5n−3 were esterified insn-3 position, whereas 22∶6n−3 was equally distributed in positions 2 and 3. After 4 wk of feeding fish oil, the fatty acid composition of adipose tissue triacylglycerols reached a steady state. Half of the n−3 fatty acids were found in position 3, namely 75% of 22∶5n−3, 50% of 20∶5n−3 and 18∶4n−3 and 45% of 22∶6n−3, the latter being equally distributed in positions 2 and 3. This pattern of distribution resembled that found in triacylglycerols of the fish oil diet, except for a higher proportion of 20∶5n−3 in adipose tissue in position 1 at the expense of position 3. Throughout the 4-wk period of fish oil feeding, the distribution pattern of minor n−3 fatty acids (18∶4n−3 and 22∶5n−3) in adipose tissue triacylglycerols remained unchanged. On the other hand, at the onset of fish oil feeding, 20∶5n−3 and 22∶6n−3 became concentrated in position 3, but thereafter 20∶5n−3 was progressively incorporated into position 1 and 22∶6n−3 into position 2. We thus conclude that n−3 fatty acids are differentially esterified in triacylglycerols of white adipose tissue. Despite the complex sequence of hydrolysis and acylation steps involved, the positional distribution of n−3 fatty acids was found to be similar in both the fish oil diet and the stored fat, in contrast to what was observed for nonessential fatty acids.

Body mass and clutch size may modulate prolactin and corticosterone levels in eiders.

Altered body condition, increased incubation costs, and egg loss are important proximate factors modulating bird parental behavior, since they inform the adult about its remaining chances of survival or about the expected current reproductive success. Hormonal changes should reflect internal or external stimuli, since corticosterone levels (inducing nest abandonment) are known to increase while body condition deteriorates, and prolactin levels (stimulating incubation) decrease following egg predation. However, in a capital incubator that based its investment on available body reserves and naturally lost about half of its body mass during incubation, corticosterone should be maintained at a low threshold to avoid protein mobilization for energy supply. This study focused on the regulation of corticosterone and prolactin release in such birds during incubation, when facing egg manipulation (control, reduced, or increased) or a stressful event. Blood samples were taken before and after clutch manipulation and at hatching. Corticosterone levels were determined before and after 30 min of captivity. Female eiders exhibited a high hypothalamic‐pituitary‐adrenal sensitivity, plasma concentration of corticosterone being increased by four‐ to fivefold following 30 min of captivity. The adrenocortical response was not modified by body mass loss but was higher in birds for which clutch size was increased. In the same way, females did not show different prolactin levels among the experimental groups. However, when incubation started, prolactin levels were correlated to body mass, suggesting that nest attendance is programmed in relation to the female initial body condition. Moreover, due to an artifactual impact of bird manipulation, increased baseline corticosterone was associated with a prolactin decrease in the control group. These data suggest that, in eiders, body mass and clutch size modification can modulate prolactin and corticosterone levels, which cross‐regulate each other in order to finely control incubation behavior.

Phytohemagglutinin Response and Immunoglobulin Index Decrease during Incubation Fasting in Female Common Eiders

To maximize their fitness, long‐lived species face trade‐offs between survival and reproduction. The cost of reproduction, which is defined as the negative effect of current parental investment on chances of adult survival and future reproduction, may affect immune function, possibly through hormonal changes. In this study, components of acquired immunity and plasma corticosterone levels of female eiders (Somateria mollissima) have been measured throughout the incubation period as a function of clutch size. These precocial birds lay up to six eggs and fast completely during incubation. Birds were sampled early and late in the incubation period, with clutches ranging from one to four eggs. T‐cell‐mediated immune response and humoral immunity were assessed by phytohemagglutinin (PHA) skin tests (a challenging method) and measurements of serum immunoglobulins (a monitoring method), respectively. During incubation, responses to PHA injection and immunoglobulin index significantly decreased, by about 40% and 25%, respectively. These observed decreases occurred independently of the number of eggs laid by the females. Corticosterone did not vary significantly during incubation, whatever the clutch size. We conclude that female eiders seem to reallocate their resources from immune function to reproductive effort independently of clutch size or corticosterone levels.

Relationships between metabolic status, corticosterone secretion and maintenance of innate and adaptive humoral immunities in fasted re-fed mallards.

SUMMARY The prolonged exposure of birds to environmental stressors known to affect energy status and glucocorticoid secretion may have several physiological consequences including a decrease in immunocompetence, further compromising the survival of individuals. However, the relationships between these parameters remain poorly understood. To this end, changes in body energy content, plasma corticosterone, adaptive (total plasma immunoglobulin Y; IgY) and innate (natural antibodies; NAbs) immune systems were assessed in female mallards (Anas plathyrhynchos) throughout prolonged fasts of different intensities and subsequent re-feeding. Plasma IgY and NAb scores were decreased by 36% and 50%, respectively, during phase II of fasting (protein-sparing phase) and by up to 40% and 80%, respectively, during phase III (protein-wasting phase), indicating a selective regulation of immune function. These results are consistent with the hypothesis of a trade-off between immune function and other energy-demanding activities. However, despite full repletion of fuel reserves and NAbs, only 76% of initial IgY levels were recovered, further supporting a trade-off between innate and adaptive branches of immunity. Although fasting induced significant increases in corticosterone levels to up to 6 times higher than baseline levels during phase III, baseline levels were recovered within 1 day of re-feeding. Our data do not support the hypothesis of a direct regulation of immunocompetence by corticosterone, at least during periods of energy repletion. Finally, the mismatch between the kinetics of body fuels and the two arms of the immune system during fasting and re-feeding suggests that variations in immune system components do not strictly covary with body mass under fluctuating food conditions.