J. F. Leatherland

Circadian rhythms and feeding time in fishes

SynopsisAlthough several studies have described effects of meal frequency and timing of meals on growth performance and body composition of different species of fishes, the mechanisms by which such variables influence the energy partitioning processes is not known. They may interact with the natural feeding rhythm of the fish, or with various behavioural and physiological parameters that exhibit ‘circadian-like’ patterns; however, in most cases, the endogenous character of such rhythms is not clear. The time of feeding, perse, can act as a Zeitgeber and override the effect of the light/dark alternation. Fish that are fed always at the same time of day show typical pre-feeding activity. Blood levels of some nutrients and hormones also show peaks or troughs at the time of feeding, but whether these are pre-feeding or post-prandial events is not clear. These results from fish are compared with similar studies with mammals. The existence and location of an endogenous multi-oscillator system is also discussed.

Environmental physiology of the teleostean thyroid gland: a review

SynopsisRecent studies of thyroid hormone function are reviewed as they relate to the environmental physiology of teleost fish. In addition, reports dealing with the apparent interdependence of thyroid gland function with that of other endocrine glands are discussed with emphasis on the interrelated endocrine response associated with changing physiological status of teleosts.Seasonal changes in thyroid gland activity are described in several species. Although seasonal alterations in apparent thyroid status are concomitant with changes in ambient temperature, photoperiod and/or gonadal status, their biological significance is not fully understood and direct relationships are for the most part, not proven. Similarly, most reports of thyroid involvement in gonadal development or maturation are based on indirect evidence of the relationship. The exception to this is a study in immature or hypophysectomized goldfish in which thyroxine (T4) was shown to promote ovarian development and maturation, possibly acting collateralistically or synergistically with gonadotropin. Even in this study it is not clear whether the T4 effect is a direct action on the ovarian tissue or an indirect action via the regulation of metabolites necessary for gonad metabolism. Integumentary silvering and retinal porphyropsin formation in salmonids are stimulated by administration of T4 or thyroid extracts. Administration of T4 or triiodothyronine (T3) enhances skeletal and somatic growth in some teleostean species, although the effect on somatic growth is most pronounced when these hormones act synergistically with somatotropin (STH) or androgens. The growth-promoting effects of T4 and T3 may be linked to their apparent involvement in lipid, carbohydrate, protein and vitamin metabolism. alterations in apparent thyroid activity concomitant with changes in ambient temperature have been reported (for example correlated with seasonal ambient thermal changes), although there are marked contradictions in data presented by different investigators. Reported temperature-related effects on thyroid function are probably secondary responses of thyroid metabolism to altered temperatures. Evidence of a direct rate of thyroid hormones in the regulation of migration (and associated behavioural modifications), salmonid smoltification, oxygen consumption, and osmotic or ionic regulation although highly suggestive in a number of areas is inconclusive and requires further critical experimental evaluation.The pituitary control (by thyrotropin) of thyroid secretion of T4 is convincingly shown in several teleosts, and evidence of an inhibitory hypothalamic control of thyrotrop activity is highly suggestive in some species. A thyrotropic effect of somatotropin preparations is well established in several teleostean species; the effect does not appear to be related to contamination of the somatotropin preparations with thyrotropin, and may be an important consideration in explaining the apparently related involvement of T4 (or T3) and somatotropin in growth and metabolism. The apparent thyrotropic property of some gonadotropin preparations, shown in several teleostean species, requires further investigation before the doubts regarding hormone preparation purity can be satisfied. Recent studies of effects of prolactin on thyroid function are highly suggestive of an inhibitory role of prolactin in peripheral monodeiodination of T4 to T3 which secondarily affects thyroid activity in some species. There is no evidence of a direct involvement of corticotropin, melanotropin or fractions of these molecules on thyroid function in teleosts. Moreover, the little evidence in support of a role of gonadal or adrenocortical steroids in thyroid control is either often contradictory or indirect and needs to be evaluated further.Interlake epizootiological studies of thyroid dysfunction in Great Lakes salmonids provide substantive evidence for the presence of a ubiquitous waterborne goitrogen(s) in the Great Lakes environs. The nature of the goitrogen(s), whether naturally-occurring or a man-introduced toxicant, remains to be determined but the possible existence of waterborne goitrogens in ‘natural’ water systems and their possible effects on experimental studies of teleostean thyroid function have to be evaluated further. If goitrogens are a common component of aquatic environments their presence could explain some of the data discrepancy among different groups of investigators, and could account for some of the apparent seasonal change in teleost thyroid physiology.

High stocking density alters the energy metabolism of brook charr, Salvelinus fontinalis

Brook charr reared for 30 days at a high stocking density (120 kg/m3) had significantly lower final body weight and reduced food consumption when compared to fish reared at a lower stocking density (30 kg/m3). There was no effect of stocking density on plasma cortisol and T3 concentrations, while plasma T4 concentration was lowered in fish held at high stocking density. Plasma glucose levels, liver glycogen content and hepatosomatic index were significantly lower in fish reared at high stocking density, while plasma protein and free fatty acid (FFA) levels were not significantly different between groups. There were significantly higher activities of hepatic phosphofructokinase, fructose biphosphatase, 3-hydroxyacyl CoA dehydrogenase, glycerol kinase and glycerol-3-phosphate dehydrogenase in fish held at high stocking density compared with those at low stocking density. Hepatic hexokinase and glucose-6-phosphate dehydrogenase activities were significantly lower in fish at high stocking density compared with those at low stocking density, while there were no differences in pyruvate kinase, glucose-6-phosphatase, phosphoenolpyruvate carboxykinase, glutamate dehydrogenase, glutamate pyruvate transaminase and glutamate oxaloacetate transaminase between the groups. These results suggest that high stocking density has the effect of mobilizing triglyceride sources, promoting gluconeogenesis from glycerol, but has little effect on protein metabolism.

Cortisol-induced changes in some aspects of the intermediary metabolism of Salvelinus fontinalis

Cortisol was administered to brook charr (Salvelinus fontinalis) in the form of slow-release intraperitoneal implants (a) to investigate the effect of chronic cortisol stimulation (up to 60 days) on various aspects of intermediary metabolism, and (b) to determine whether such cortisol-induced changes were comparable to those seen in chronically fasted charr. Except for fish sampled at 1 and 6 h after implantation, there was no consistent increase in the plasma cortisol levels of the cortisol-implanted animals. Nevertheless, there were significant treatment effects (particularly after 60 days) on certain metabolite levels, and key hepatic enzymes, including a lowering of plasma glucose and hepatic glycogen concentrations, increased activities of FBPase, G6PDH, GK, and G3PDH, and reduced activities of PFK. When taken together these changes are indicative of a direct or indirect gluconeogenic action of cortisol, in which metabolites other than amino acids (possibly glycerol) are utilized as substrates. These metabolic changes differed from those found in food-deprived brook charr which appeared to use proteins, as well as lipids, as energy sources.

Circadian rhythm of plasma prolactin, growth hormone, glucose and free fatty acid in juvenile kokanee salmon, Oncorhynchus nerka

Abstract 1. 1. Plasma levels of prolactin and growth hormone in Oncorhynchus nerka show a marked circadian rhythm. 2. 2. High levels of both hormones are found during the dark phase of the photoperiod (03.00 hours to 06.00 hours for prolactin and 24.00 hours to 03.00 hours for growth hormone). 3. 3. Low values for prolactin were found between 12.00 hours and 16.00 hours and for growth hormone at 15.00 hours. 4. 4. A second “surge” of growth hormone may also be present at 12.00 hours. 5. 5. Plasma FFA levels were high during the morning hours (09.00 hours to 12.00 hours). Plasma glucose levels did not vary significantly at any time period. 6. 6. The possible involvement of prolactin and/or growth hormone in lipid mobilization in O. nerka is discussed.

Social dominance and interrenal cell activity in rainbow trout, Salmo gairdneri (Pisces, Salmonidae)

SynopsisThe relationship between standing in a dominance hierarchy and physiological stress was studied in rainbow trout. Individual fish were assigned relative dominance ranks, based on behavioral observations in a large, simulated stream tank. These ranks were compared to histometric measures of interrenal cell activity. Fish, isolated individually in the stream tank had significantly lower levels of interrenal activity than fish from the crowded holding tank. Groups of fish in the stream tank formed stable, linear dominance hierarchies. Interrenal activity correlated inversely with dominance rank, with the exception that top ranking fish had higher activity than expected. Possible cause and effect relationships are discussed.

Temporal changes in plasma thyroid hormone, growth hormone and free fatty acid concentrations, and hepatic 5'-monodeiodinase activity, lipid and protein content during chronic fasting and re-feeding in rainbow trout (Oncorhynchus mykiss).

Temporal changes in growth, plasma thyroid hormone, cortisol, growth hormone (GH) and non-esterified fatty acid (NEFA) concentrations, hepatic T3 content and hepatic 5′-monodeiodinase activity were measured in rainbow trout (Oncorhynchus mykiss) subjected to a sustained fast for up to eight weeks, and during a four-week re-feeding period. The purpose of the study was to examine aspects of the endocrine control of energy partitioning processes characteristic of short-term (acute; fasting) and long-term (chronic; starvation) food-deprivation states in fish, and to explore the role of the thyroid hormones, cortisol and GH in the energy repartitioning that takes place during an acute anabolic (re-feeding) state following chronic food deprivation.Differences in growth rate between fed and fasted groups were evident after two weeks, but significant weight loss by the fasted groups was not evident until between four and six weeks into the fast. Hepatosomatic indices (HSIs) were significantly reduced in the fasted fish within seven days, and as early as two days in one study; recovery of the HSI in fasted fish was evident within three days of re-feeding. Liver protein content (expressed as % wet weight) was consistently depressed in the fasted fish in only one of the three studies. Liver total lipid content (expressed as % wet weight) was depressed in the fasted fish within two days of food deprivation. Because of the rapid and sustained decrease in the HSI of fasted fish, the hepatic total protein and lipid reserves, when considered on a body weight basis, were markedly lowered within the first few days of the fast. Plasma GH concentrations exhibited a bi-modal pattern of change, with a transient fall in levels, followed by a sustained increase in fasted fish. The indicators of interrenal activity were suggestive of a depressed pituitary-interrenal axis in fasted animals; plasma cortisol levels were elevated to levels of fed animals within one day of re-feeding. The indicators of thyroid hormone economy (plasma thyroid hormone levels, liver triiodothyronine content, hepatic 5′-monodeiodinase (MD) activity, thyroid epithelial cell height) were similarly indicative of a depressed pituitary-thyroid axis in fasted animals, with recovery to levels of the fed animals within one week. Despite the compensatory changes in accumulation of reserves (as indicated by a compensatory increase in HSI), there were no apparent compensatory changes in any of the endocrine parameters evident during the re-feeding period.

Does oral 3,5,3 ′-triiodo-l-thyronine affect dietary glucose utilization and plasma insulin levels in rainbow trout (Salmo gairdneri)?

A factorial experiment was conducted to determine the effect and interaction of dietary carbohydrate level and triiodo-L-thyronine (T3) supplementation on the growth, physiological response and plasma insulin and cortisol levels of rainbow trout. The oral administration of T3 significantly increased the growth, protein efficiency ratio and feed efficiency of trout, indicating an increased protein and perhaps energy utilization in these fish. However, T, administration did not significantly increase the utilization of dietary glucose as an energy source by the trout. Similarly, the administration of T3 did not significantly affect plasma insulin levels in either the fed or the fasted trout. Plasma insulin levels were significantly higher in fed trout reared on the non-T3 supplemented high carbohydrate diet in comparison to trout reared on the low carbohydrate diets. This indicates that increased dietary carbohydrate stimulates increased insulin secretion in the trout. Therefore, although rainbow trout are not insulin-deficient, they can still be considered a diabetic-like animal due to their poor glucose tolerance. Plasma cortisol levels were not affected by diet composition and altered plasma glucose levels.

Circadian pattern of hepatosomatic index, liver glycogen and lipid content, plasma non-esterified fatty acid, glucose, T3, T4, growth hormone and cortisol concentrations in Oncorhynchus mykiss held under different photoperiod regimes and fed using demand-feeders

The circadian patterns of several tissue and plasma metabolites, and several plasma hormone concentrations are described in rainbow trout (Oncorhynchus mykiss) that were held in groups under three different photoperiod regimes, and given free access to a demand-feeder. Regardless of photoperiod regime, all the measured parameters showed significant diel rhythms that appeared to be synchronized by dawn; dawn was represented by the concomitant onset of both light and feeding. The diel increases in hepatic glycogen content, and plasma T4 and cortisol concentrations were in phase with the main period of feeding activity, whereas the peaks in plasma T3 and glucose concentrations that may also be triggered by feeding activity, were delayed by several hours. The peaks in hepatosomatic index, plasma non-esterified fatty acids and plasma growth hormone concentrations were 180° out of phase with the main period of feeding activity, and associated with periods of hypophagia and low activity.

Plasma growth hormone levels in fed and fasted rainbow trout (Oncorhynchus mykiss) are decreased following handling stress.

Plasma growth hormone concentrations of rainbow trout,Oncorhynchus mykiss, fasted for six weeks, were significantly (p < 0.01) higher than in comparable fed animals; in the fasted fish, the levels fell progressively following acute stress (by displacing the fish within their home aquarium), with significant differences from pre-stressed fish evident between one and thirty-two hours after application of the stressor. Plasma growth hormone concentrations also fell significantly in the fed group, but differences were evident only between two and eight hours after stressor application.Plasma cortisol concentrations in pre- and post-stressed fed and fasted fish were similar. There was a bimodal response to stressor application in both groups, with significantly higher values relative to the pre-stressed sample evident one and eight hours after disturbance, but not after two, four or thirty-two hours. The changes in plasma cortisol levels between the initial (09:00h) sample and the sample taken eight hours later resembles the diet pattern seen in trout given access to self-demand feeders.Plasma glucose concentrations in pre-stressed fed animals were higher than in pre-stressed fasted fish. This relationship was also evident between one and four hours and thirty-two hours after stressor application. The post-stress rise in plasma glucose concentration was evident between one and four hours in the fed group, and between four and eight hours in fasted fish.The diel changes in plasma growth hormone and glucose concentrations could not be attributed to normal circadian patterns, and there was no apparent correlation between changes in plasma growth hormone and cortisol concentrations. There was a significant inverse correlation between plasma glucose and growth hormone concentrations when the total data set were analyzed, but these correlations were not apparent when the treatment groups were analyzed separately.