Duncan S. MacKenzie

Nutrient regulation of endocrine function in fish

Abstract Hormones play a central role in the regulation of growth and nutrient utilization in fish. Consequently, fish endocrine systems are sensitive to alterations in nutrient intake. Procedures routinely employed in the development of diets and feeding protocols for cultured fish have pronounced effects on endocrine systems. We review the evidence that alterations in ration level (including food restriction and food deprivation), diet composition, photoperiod, and feeding time influence the most intensively-studied fish metabolic hormones: thyroid hormones, pancreatic hormones, and hormones of the growth hormone–insulin-like growth factor axis. Whereas effects of these dietary manipulations on total circulating hormone levels are commonly examined, nutrient intake may also influence hormone transport in blood, activation in peripheral tissues, receptor binding, and neuroendocrine pathways regulating hormone secretion. Information on the cellular and molecular mechanisms through which nutrients influence endocrine systems is still needed. Significant new information about the regulation of endocrine function can be derived from nutritional studies currently employed in aquaculture for the development of diets. Additional information on the influence of nutrients on endocrine function is essential for the design and interpretation of hormone supplementation studies, and should eventually allow development of feeding strategies which promote anabolic hormone production.

Seasonal changes in thyroid and reproductive steroid hormones in female channel catfish (Ictalurus punctatus) in pond culture

Abstract Mature female channel catfish in pond culture in Texas were sampled monthly throughout 1 year to characterize changes occurring in ovarian histology and plasma concentrations of reproductive steroid and thyroid hormones. Oocyte growth, measured by oocyte diameter and gonadosomatic index, was initiated in the fall and continued until the time of spawning in May. Plasma estradiol and testosterone were lowest immediately after spawning and increased in the fall in association with the initiation of vitellogenesis. Both hormones showed significant increases in February; estradiol then remained elevated in prespawning animals, whereas testosterone returned to levels found in the fall. Increased plasma estradiol was accompanied by oocyte growth and preceded increases in plasma alkali-labile protein phosphorus, an indicator of circulating vitellogenin, supporting a role for estradiol in regulating vitellogenesis in channel catfish. A significant increase in the concentrations of both thyroxine and triiodothyronine occurred in February, coincident with the steroid hormone peak, indicating that endocrine interactions between thyroid activity and reproduction may be occurring at this time. A second peak in thyroid hormones occurred in the summer, unrelated to reproductive state and coincident with the period of maximum water temperature and weight gain, suggesting that in this species thyroid activation may also accompany periods of maximum anabolic activity.

Effects of dietary sodium chloride on red drum juveniles in waters of various salinities.

Abstract Three 8-week feeding trials were conducted to determine the effects of supplemental dietary salts on growth and osmoregulation of juvenile red drums (Sciaenops ocellatus) at various water salinities. In each experiment, sodium chloride was added to a basal diet composed of semipurified and practical ingredients; the basal diet provided intrinsic levels of 0.80% Cl, 0.84% K, and 0.92% Na on a dry-matter basis. In fresh water, red drums fed the basal diet supplemented with NaCI at 2% had significantly (P < 0.05) greater weight gain and feed efficiency than fish given the basal diet alone. In brackish water (6‰ salinity), fish given the diet supplemented with NaCI at 2% had weight gains that were numerically but not significantly (P = 0.076) greater than gains offish fed the basal diet. In full-strength artificial seawater (35‰), fish fed diets supplemented with NaCI at 2 and 10% had weight gains not significantly (P = 0.23) different from those of fish fed the basal diet, and those fed the diet sup...

Thyrotropin in teleost fish

Thyrotropin (TSH), a pituitary glycoprotein hormone that stimulates the thyroid gland, has been cloned and sequenced from over a dozen teleost fish species. Although TSH is established as a primary driver of systemic thyroid status in mammals, its importance in the regulation of fish thyroid function is still uncertain. We review recent studies indicating that TSH structure is highly conserved across species representing six teleost families. These studies have found TSH messenger RNA consistently expressed in teleost pituitary tissue, although ectopic expression, particularly in gonads, has also been observed. They have also provided evidence for negative feedback inhibition of TSH expression by thyroid hormones, as well as stimulation by hypothalamic peptides. Descriptive studies have found increased TSHbeta expression associated with life history events thought to be promoted by thyroid hormones. These results, coupled with the discovery of a G-protein coupled TSH receptor in several teleost species, supports an active and conserved role for TSH in the regulation of teleost thyroid function. The relative importance of central pathways in regulating thyroid hormone provision to targets and the identity of a proposed thyrotropin-inhibiting factor in teleost fish are still unanswered questions whose resolution will be facilitated by development of methods to measure circulating TSH and its secretion from the pituitary gland.

Seasonal changes in the reproductive condition and body composition of free-ranging red drum, Sciaenops ocellatus

Abstract Adult red drum ( Sciaenops ocellatus ) from a wild, autumn (fall) spawning population were studied over a 1-year period to evaluate seasonal changes in body composition in comparison with cultured red drum. Each month, female and male fish were captured and blood sampled. Standard length and weight were measured, and gonads, liver, intraperitoneal fat (IPF), and a sample of muscle tissue were collected from each fish. Gonadosomatic index (GSI), hepatosomatic index (HSI), IPF ratio, protein and lipid composition of muscle and liver tissues, and lipid class composition of liver samples were determined for each fish. All tissue indices exhibited a similar seasonal cycle in both sexes, with highest HSI in spring (March–April) and maximal IPF ratio in May, followed in September and October by minimal values for HSI and IPF ratio. Whereas GSI was low, gonadal histology demonstrated activation of spermatogenesis and oocyte development in July and August. Liver composition varied dramatically throughout the year. Liver lipid content ranged from 7.4% of wet weight in November to 30.2% in August, with triglycerides (TG) being the most abundant component at all times. Liver protein was more stable, ranging from 11.5% in August to 16.3% in September. Muscle composition was relatively constant, with muscle crude protein ranging from 20.5% to 25.6% of wet weight and muscle lipid ranging from 0.4% to 2.2% of wet weight. These data indicate that red drum utilize the liver as a major depot for lipid. Depletion of maximal lipid reserves from liver and IPF in late summer indicates that lipid stored during active spring and summer feeding supplies energy for reproduction which is mobilized rapidly (within 1 month) in this fall-spawning species. Body composition of wild fish is similar to that of laboratory cultured red drum.

SIMULATED HIBERNATION OF SEA TURTLES IN THE LABORATORY : I. FEEDING, BREATHING FREQUENCY, BLOOD PH, AND BLOOD GASES

Captive immature green (Chelonia mydas) and Kemps ridley (Lepidochelys kempi) sea turtles were examined to determine if a hibernation-like state could be induced under controlled conditions. Both species demonstrated that they are able to acclimate to cold temperatures behaviorally. However, the two species appeared to respond differently to decreasing temperature. Whereas the green turtles tolerated the onset of cold water temperatures by reducing swimming activity, the ridleys became very agitated and active as they were exposed to temperatures below 20 degrees C. Nevertheless, both species displayed semi-dormant behavior at temperatures below 15 degrees C, coming to the surface to breathe periodically at intervals of up to three hours. At low temperatures, venous blood pO2 and pCO2 decreased, whereas venous blood pH increased. Feeding also decreased as either species was exposed to cold temperature: greens (at 15 degrees C) and ridleys (at 20 degrees C) decreased food consumption to 50% of control levels, and ceased feeding below 15 degrees C. Thus, these species tolerated temperature drops and the associated hypophagia. They did not exhibit cold-stunning behavior, as has been observed in wild sea turtles exposed to rapid temperature drops, or prolonged periods of hibernation-like dormancy, as has been proposed for wild sea turtles during cold winter months.

OSMOREGULATION IN WILD AND CAPTIVE WEST INDIAN MANATEES (TRICHECHUS MANATUS)

The Ability of West Indian Manatees (Trichechus manatus latirostris and Trichechus manatus manatus) to inhabit both fresh water and marine habitats presents an interesting model to study osmoregulation in sirenians. Blood samples were analysed from manatees held in fresh‐and saltwater and from wild animals captured in fresh‐, and saltwater and from wild animals captured in fresh‐, brackish, and saltwater for concentrations of aldosterone, arginine vasopressin, plasma renin activity, Na+, K+, Cl−, and osmolality. Two separate experiments were also conducted on captive animals to evaluate osmoregulatory reponses to acute saltwater exposure and freshwater deprivation. Spurious differences were observed inplasman electrolyte and osmolality among the captive and wild groups. Wild brackish water animals exhibited the highest vasopressin concentrations, while wild freshwater manatees had the highest aldosterone levels. A significant correlation between mean vasopressin and osmolality was demonstrated for captive and wild animals. When freshwater animals were acutely exposed to saltwater, osmolality, Na+, and Cl− increased 5.5%, 8.0%, and 14%, repectively, while aldosterone decreased 82.6% Salwater animals deprived of freshwater exhibited an almost twofold increase in aldosterone during the deprivation period and a foufold decrease when freshwater was again provided. Whithing this gruop, osmolality increased significantly by 3.4% over the course of the study; however, electrolytes did not change. The lack of consitent difference in electrolyte and osmolality among wild and captive group suggests that manatees are good osmoregulators regardless of the environment. The high aldosterone levels in wild freshwater animals may indicte a need to conserve Na+, while the high vasoperssin levels in wild brackish‐water mantees suggest an antidiuretic state to conserve water. Vasopressin levels appear to be osmotically mediated in manatees as in other mammals.

Basal and stress-induced corticosterone levels in olive ridley sea turtles (Lepidochelys olivacea) in relation to their mass nesting behavior.

Adrenocortical responsiveness to turning stress was examined in wild, reproductively-active olive ridley sea turtles (Lepidochelys olivacea) in relation to their mass nesting (arribada) behavior. We hypothesized that the high sensitivity threshold (HST) observed in ovipositing sea turtles is associated with a diminished sensitivity of the hypothalamo-pituitary-adrenal (HPA) axis to stressful stimuli in arribada females. We tested this hypothesis by determining whether arribada females exhibited an increased activation threshold of the HPA axis to an imposed stressor (turning stress). Mean basal corticosterone (B) and glucose levels were below 1.0 ng/ml and 60 mg/dl, respectively. Basal B remained unchanged throughout a 24-hr period in basking females. Most animals responded to turning stress with elevated mean B levels (up to 6.5 ng/ml after 6 hr) and no increase in circulating glucose. Nearly 50% of females (and none of the males) were refractory to the stimulation. Males exhibited the most rapid response, with B levels significantly elevated by 20 min over basal levels. Among females, arribada and solitary nesters exhibited a slower rate of response than basking, non-nesting animals. These results demonstrate that olive ridleys exhibit stress-induced changes in circulating B which are slower than those observed in most reptilian and in mammalian, avian, and piscine species. Furthermore, the presence of refractory females and the relatively slower increase in B in arribada and solitary nesters indicate a hyporesponsiveness of the HPA axis to turning stress in nesting olive ridleys. The hyporesponsiveness may be part of a mechanism to facilitate arribada nesting. J. Exp. Zool. 284:652-662, 1999.

Increased levels of arginine vasotocin and neurophysin during nesting in sea turtles

Arginine vasotocin (AVT) and neurophysin (NP) levels were measured by radioimmunoassay in two species of sea turtle, the olive ridley, Lepidochelys olivacea, and the loggerhead, Caretta caretta, during the brief period of nesting and oviposition. In both species, AVT was low in animals which were not reproductively active. AVT was also low at the time animals emerged from the surf to nest, but increased significantly during oviposition and then declined as the animals returned to the water. NP increased in concert with AVT, also reaching highest levels during oviposition. In both species, however, NP levels remained elevated over prenesting levels at the time of return to the water. These findings are consistent with the hypothesis that an AVT-neurophysin complex is released from the neurohypophysis during nesting, and that AVT is a physiological regulator of oviducal contractions in sea turtles.

Chemotaxis of Escherichia coli to Norepinephrine (NE) Requires Conversion of NE to 3,4-Dihydroxymandelic Acid

Norepinephrine (NE), the primary neurotransmitter of the sympathetic nervous system, has been reported to be a chemoattractant for enterohemorrhagic Escherichia coli (EHEC). Here we show that nonpathogenic E. coli K-12 grown in the presence of 2 μM NE is also attracted to NE. Growth with NE induces transcription of genes encoding the tyramine oxidase, TynA, and the aromatic aldehyde dehydrogenase, FeaB, whose respective activities can, in principle, convert NE to 3,4-dihydroxymandelic acid (DHMA). Our results indicate that the apparent attractant response to NE is in fact chemotaxis to DHMA, which was found to be a strong attractant for E. coli. Only strains of E. coli K-12 that produce TynA and FeaB exhibited an attractant response to NE. We demonstrate that DHMA is sensed by the serine chemoreceptor Tsr and that the chemotaxis response requires an intact serine-binding site. The threshold concentration for detection is ≤5 nM DHMA, and the response is inhibited at DHMA concentrations above 50 μM. Cells producing a heterodimeric Tsr receptor containing only one functional serine-binding site still respond like the wild type to low concentrations of DHMA, but their response persists at higher concentrations. We propose that chemotaxis to DHMA generated from NE by bacteria that have already colonized the intestinal epithelium may recruit E. coli and other enteric bacteria that possess a Tsr-like receptor to preferred sites of infection.