Twenty-four hour growth hormone and leptin secretion in active postpubertal adolescent girls: impact of fitness, fatness, and age at menarche

CONTEXT

GH is strongly related to body composition, physical activity, and pubertal progression. Adolescent girls decrease physical activity during puberty, whereas their weight increases. Because leptin is a good index of energy balance in active young women, we hypothesized that leptin is related to GH secretion in this population while taking into account fitness, fatness, and age at menarche.

METHODS

We measured body composition and maximal oxygen consumption (VO(2)max) in 37 postpubertal adolescent girls aged 16-21 yr. GH was sampled every 10 min and leptin hourly for 24 h. We first analyzed 6-h time blocks by repeated measures for GH and leptin, with body mass index (BMI), percent body fat, and VO(2)max as covariates for the entire group and a lean subgroup. The deconvolution method was used to characterize GH pulsatility from individual time points.

RESULTS

GH varied through the day (P < 0.0001), with the highest concentrations overnight. BMI, percent body fat, and VO(2)max were related to GH concentrations in the entire group, whereas leptin predicted GH in the entire group as well as the lean subgroup of girls. Higher leptin was related to lower GH concentrations (P = 0.011), regardless of time. A log leptin level increase by 1 unit decreased GH by 27%. Pulsatility characteristics showed a 1-yr increase of age at menarche increasing total GH input by 20% (P = 0.0035) independently from BMI.

CONCLUSION

In postpubertal adolescent girls, leptin is related to GH concentration across the lean to overweight BMI spectrum. GH pulsatile secretion was greater in girls with later age at menarche.

Does exercise affect female hamster gonadotropins by reducing estradiol negative feedback?

Exercise stimulates reproductive function in hamsters exposed to short-day photoperiod (SDP) in contrast to its inhibitory effects in women and rats. SDP inhibits hamster reproduction in part by increasing the sensitivity of the hypothalamo-pituitary-gonadal axis (HPGA) to the negative feedback of gonadal steroids. To determine whether EX facilitates reproduction in female hamsters by affecting this mechanism, we examined the influence of estradiol (E2) on basal LH and FSH concentrations in exercising and sedentary hamsters maintained on long-day photoperiod (LD 14:10, LDP) or SDP (LD 8:16). In the LDP, serum LH and FSH were unaffected or reduced by exercise in ovariectomized (OVX) nonhormone-replaced hamsters, and LH was increased after tonic E2 replacement compared to sedentary controls. In the SDP, serum LH and FSH were significantly higher in OVX exercising than in sedentary hamsters, whether the exercisers were injected with a high dose of E2 or not. Thus, the effects of exercise on basal gonadotropin secretion in female hamsters appear to depend on the level of estradiol negative feedback (ENF). When this feedback is low (LDP OVX condition), exercise is either ineffective or inhibitory. When the ENF is increased by exposure to SDP and/or by treatment with E2, exercise has a stimulatory effect on basal gonadotropin secretion. Exercise may stimulate hamster gonadotropin secretion by reducing the ENF either by lowering the sensitivity of the HPGA to steroid negative feedback or by other means.

The effects of exercise on growth

The way in which exercise influences statural, hypertrophic and reparative growth is examined from the perspective of the human lifespan. Statural growth depends on a neuroendocrine programme which channels nutrient energy towards increments in lean body mass. Exercise can facilitate statural growth and is a necessary stimulus for reparative growth through its stimulatory effects on secretion of growth hormone (GH) and other anabolic hormones. An exercise-associated increase in GH secretion is a response to acute or prolonged exercise-induced fuel shortage that directs metabolism towards utilisation of lipids and promotes growth. Exercise can transiently block the expression of statural growth by competitively removing the necessary nutritional support for growth. Statural growth retardation can be corrected by catch-up growth, but stunting may also be permanent (depending on the timing and magnitude of the energy drain). Hypertrophic growth is less dependent on hormonal and nutritional support than statural growth, and exercise provides the necessary mechanical stress for growth and remodelling of the musculoskeletal system. Excessive mechanical strain may suppress hypertrophic growth. The intermittent nature of exercise provides temporal organisation that is necessary for the normal operation of cellular growth process. Exercise by pregnant women does not appear to influence fetal growth. Evaluation of the effect of exercise on growth of children and adolescents is complicated by nonrandom selection of individuals for participation in organised sports, and by lack of information on the magnitude of exercise-induced energy drain. Exercise is essential for regulation of body composition in adulthood. It provides mechanical and metabolic stimuli that are necessary for hypertrophy of the musculoskeletal system and increased GH secretion for reparative growth.

Effect of voluntary exercise and food restriction in response to lipopolysaccharide in hamsters

We tested the hypothesis that voluntary running and moderate food restriction alter the acute phase response (APR), one index of nonspecific immune function. Hamsters were kept sedentary or permitted to run and were fed ad libitum or had food restricted for 20 days and were then injected intraperitoneally with saline or lipopolysaccharide (LPS). Fever and circulating interleukin-6, serum amyloid A (SAA), serum iron, and cortisol were measured by biotelemetry, B-9 cell growth assay, indirect enzyme-linked immunosorbent assay, colorimetric analysis, and radioimmunoassay, respectively. The febrile temperature; hypoferremia; and elevation of circulating interleukin-6, SAA, and cortisol after LPS injection were not altered by exercise. Because baseline temperatures were elevated in the exercised hamsters, the change in temperature in response to LPS was less than it was in the sedentary hamsters. Food restriction significantly decreased SAA and elevated cortisol after LPS injection and depressed the absolute temperature to which the core temperature rose in response to LPS in one trial but not in another. Because food restriction depressed baseline temperatures, it also affected the change in temperature after LPS injection. The hypoferremic response to LPS was inhibited in hamsters that were both food restricted and permitted to run. We conclude that exercise does not enhance the APR to a low dose of LPS, whereas food restriction and the combination of exercise and food restriction depress some portions of the APR in hamsters.

Effect of exercise and food restriction on selected markers of the acute phase response in hamsters

Acute aerobic exercise has been shown to elicit physiological changes characteristic of the acute phase response (APR), a nonspecific host defense response. Regular evocation of these changes may prime the immune system to improve resistance to disease. Because food deprivation is associated with an impaired APR, food restriction may prevent these beneficial changes. We tested the hypotheses that voluntary exercise elicits an APR and that food restriction modifies this response in four groups of hamsters: ad libitum-fed sedentary, ad libitum-fed exercised, food-restricted sedentary, and food-restricted exercised. Five variables altered during an APR were examined: core temperature, serum iron, serum interleukin-6, serum amyloid A, and serum glucocorticoids measured by biotelemetry, colorimetric analysis, B-9 cell growth assay, indirect enzyme-linked immunosorbent assay, and radioimmunoassay, respectively. Blood was drawn during the hamsters' inactive period after 19-20 days of access to running wheels. Resting core temperature was elevated by exercise and depressed by food restriction (P < 0.01). Iron was depressed by food restriction (P < 0.01). Cortisol, but not corticosterone, was elevated by food restriction (P < 0.001). There were no significant differences among groups in interleukin-6 (P > 0.49) or serum amyloid A (P > 0.29). We conclude that there is little evidence that voluntary exercise or exercise combined with food restriction causes an APR in hamsters.

Control by light of the temperature rhythm in food-restricted hamsters

To establish the relative importance of light and food in the control of core temperature (Tc) rhythm in food-restricted hamsters, mature female hamsters maintained in 14L:10D lighting were fed restricted amounts of food at the onset of light (n = 6) or at the onset of dark (n = 6) and were compared to ad lib-fed animals. After 21-25 days of this entrainment, light stimulus was shifted by 12 h, and animals were kept in shifted lighting for another 13 days. Food restriction led to a 0.6 degree decrease in the mean Tc, which was expressed entirely during the day in night-fed hamsters and was evenly divided between day and night in day-fed animals. Thus, Tc and general activity rhythms maintained the entrainment to light under both dietary conditions, with peak values for all occurring during the early night. During 13 days following the 12-h shift in lighting, Tc and activity rhythms shifted in all animals, regardless of nutritional status, from entrainment to preceding lighting, through double rhythm frequency, indicating entrainment to preceding as well as current lighting, to entrainment to current lighting. Thus, in food-restricted hamsters, light stimulus rather than predictable timing of food prevails as the entrainer of Tc and activity rhythms.

Neurohumoral mediation of exercise-induced growth

To assess exercise effects on growth, other variables modulating growth need to be taken into account. Endogenous control of growth proceeds from local actions of growth factors and dependence on nutrition abundance through guidance by growth hormone (GH) and other anabolic hormones to neuroendocrine suppression of growth. Nutrient abundance controls the reparative growth of lean body mass in adulthood by coupling it to anabolic endocrine reflexes. Growth is blocked when catabolic endocrine reflexes govern energy expenditure. The relationship between exercise intensity and growth is nonlinear. Growth is an intermittent process. Its expression and stimulation are dependent on ultradian and circadian rhythms of energy metabolism and neurohumoral release. High-resistance exercise selectively stimulates growth of the musculoskeletal system through expression of growth factor genes in the challenged tissues and without the GH guidance or abundant nutritional support. Habitual endurance exercise stimulates reparative growth of lean body mass through the neuroendocrine adaptations including increased pulsatile GH secretion. These also facilitate oxidative utilization of storage lipids thereby contributing to the regulation of body composition in adulthood. In the absence of sufficient high-resistance and endurance exercise regulation of adult body mass is impaired: excess LBM is lost during energy deficit, and excess fat accumulates during energy surplus.

Tumor necrosis factor is not involved in exercise-induced elevation in core temperature

Female Sprague-Dawley rats (12:12-h light-dark photoperiod) with access to running wheels have an elevated body temperature (BT) both during exercise (nighttime) and nonexercise periods (daytime). We studied whether the exercise-induced increase in BT is modulated by the release of the cytokine, tumor necrosis factor-alpha (TNF). Two weeks after the onset of exercise, nighttime temperatures of exercising rats were elevated approximately 0.5 degree C compared with preexercise values (P = 0.006). By 3 wk after the onset of exercise, daytime temperatures had increased 0.3 degree C (P = 0.03) above control levels. To confirm that endogenously produced TNF can modulate fever in female rats, we injected six rats with antiserum to TNF (300 microliters/rat) and six rats with control serum 24 h before intraperitoneal injection of lipopolysaccharide (50 micrograms/kg). As occurred in earlier studies on male rats, antiserum-treated female rats had significantly enhanced fevers (P = 0.017). To determine whether endogenously produced TNF was involved in modulating the daytime and nighttime increases in BT, antiserum to TNF (300 microliters/rat, n = 7) or control serum (300 microliters/rat, n = 5) was injected intraperitoneally in exercising rats. Neither injection of antiserum nor control serum had any effect on daytime or nighttime BTs. Because BTs of exercising female rats injected with antiserum against TNF were not affected, we conclude that TNF is not responsible for modulating their exercise-induced rise in BT.

Role of prostaglandins in exercise-induced core temperature elevation in female Sprague-Dawley rats

Female Sprague-Dawley rats (12:12-h photoperiod; body temperature, BT, measured with biotelemetry) with access to running wheels for 6 wk have an elevated BT (compared with rats with no access to exercise wheels, i.e, sedentary) both during the period of voluntary exercise (nighttime) (0.5 degree C, P = 0.0001) and the nonexercise period (daytime) (0.3 degree C, P = 0.002). To determine whether prostaglandin (PG) E was responsible for any portion of this daytime rise in BT, we injected a dose of sodium salicylate (300 mg/kg), which was shown to produce complete antipyresis in rats injected with lipopolysaccharide (LPS), into exercised and sedentary rats 4 h after the onset of the lights-on period. The injections of sodium salicylate led to a fall in body temperature in both the exercised and sedentary rats of similar amounts (-0.88 degree C vs. -0.61 degree C at 2 h postinjection, P = 0.59). We conclude that the increase in daytime BT of exercised female rats is not mediated by prostaglandins.

Stimulation by voluntary exercise of adrenal glucocorticoid secretion in mature female hamsters

The possibility that habitual voluntary running induces a chronic change in adrenal glucocorticoid synthesis and secretion was examined in freely running mature female hamsters, in whom this behavior accelerates growth, reduces body fat levels, and elevates core temperature. Hamsters were free to run on horizontal discs or in vertical wheels between 32 and 80 days, in 14L:10D or in 10L:14D photoperiods, and at the end of this period, corticosterone and cortisol steroidogenesis and serial plasma corticosterone concentrations during day and night were used as measures of the chronic stimulation of adrenal cortical activity. Habitual voluntary running significantly increased steroidogenesis of both glucocorticoids and plasma corticosterone concentrations and alone accounted for all the variance in enhanced synthesis and secretion of corticosterone. Acute exercise and/or the nocturnal phase of circadian period enhanced the chronic stimulatory effects of exercise on cortisol. Despite its voluntary and apparently stress-free nature, running induces chronic increases in basal glucocorticoid secretion in mature female hamsters. Putative oversecretion of corticotropin releasing factor in freely running hamsters could account for increased steroidogenesis, acceleration of growth, reduced body fat levels, and core temperature elevation.

Activity disc and cage for continuous measurement of running activity and core temperature in hamsters

We describe a design for the modular horizontal activity disc and tandem cages suitable for continuous monitoring of spontaneous running and of core temperatures in golden hamsters. An acrylic disc is equipped with a short brass axle. It is mounted inside a brass rotation sleeve at a 15 degrees angle off the horizontal plane. The disc module fits firmly inside either half of the tandem cage when activity measurements are needed. Easy removal allows for alternative use of cages. Minor modifications of disc dimensions and of disc base permit the use of activity modules with juvenile hamsters. The short distance between disc surface and cage floor permits continuous measurement of core temperature as well as running activity.

Body temperature rhythm and response to pyrogen in exercising and sedentary hamsters

In this study, we tested the hypothesis that daily voluntary exercise results in a chronic elevation in core temperature in the female golden hamster. Temperature and activity were measured by biotelemetry. Hamsters ran 6-7 km per night (12:12 L:D) when permitted access to wheels. No running occurred during the light periods. During the 3rd wk of running, temperatures of exercising hamsters were significantly elevated by 0.5 degree C (P less than 0.001) during the dark period and by 0.3 degree C (P less than 0.003) during the light period compared with sedentary hamsters. Cessation of running removed the difference between groups, and resumption of running restored it. Both the injection of endotoxin and the psychological stress of cage switch resulted in similar peak temperatures in exercising and sedentary hamsters despite higher pre-treatment temperatures in the exercise group. We interpret these results to support the hypothesis that regular exercise may cause an upward resetting of the set-point for body temperature.

Hippocampal serotonin mediates hypoactivity in dietarily obese hamsters: a possible manifestation of aging?

To determine whether endogenous opiates mediate hyperactivity in food restricted hamsters and serotonergic fibers innervating the hippocampus mediate hypoactivity in obese hamsters, food restriction and high-fat diet supplementation were used to produce significant body fat changes (8 vs. 21%). The levels and pattern of spontaneous running were examined after IP saline or naloxone HCl (20 mg/kg) and following the infusion of vehicle and 5,7-dihydroxytryptamine creatine sulfate (4 micrograms/2 microliters) into rostromedial septum of mature female hamsters. Septum-medial preoptic area (POA), hippocampus, hypothalamus, and cortex were dissected from the three groups as well as from two additional groups of hamsters receiving vehicle or neurotoxin. Concentrations of serotonin, norepinephrine, and dopamine were measured in these tissues by HPLC method. Fat-fed hamsters were hypoactive relative to food-restricted hamsters. Naloxone had no significant effect on running behavior. Serotonin neurotoxin increased the running activity of fat-fed hamsters to the level displayed by control hamsters by increasing the number of runs, the total activity level, the speed of running and by decreasing the duration of pauses. Neurotoxin led to selective deletion of serotonin in the hippocampus (77%) and parietal cortex (50%). Serotonergic fibers innervating the hippocampus thus appear to mediate the hypoactivity that is induced by dietary obesity in mature hamsters. Since serotonin mediates some other manifestations of aging, and slow weight increases characterize mid-portion of hamster life span, we hypothesize that serotonergic mediation of hypoactivity is another manifestation of aging.

Exercise inhibits reproductive quiescence induced by exogenous melatonin in hamsters

Voluntary exercise inhibits the reproductive regression associated with a short photoperiod in male or female hamsters. The question addressed by the present study was whether exercise would also attenuate the reproductive regression associated with injection of exogenous melatonin. In male hamsters exercise inhibited the testicular regression, decline in gonadotropin secretion, and reduction in testosterone release associated with two daily injections (15 micrograms) of melatonin in pinealectomized hamsters on long days. After the reproductive system of the sedentary melatonin group had regressed, one-half of these hamsters were placed in cages with exercise wheels. Access to the exercise wheels stimulated testicular recrudescence and restored gonadotropin secretion to levels found in vehicle-injected hamsters. Sedentary female hamsters injected with melatonin tended to go into a state of constant diestrus associated with daily afternoon increases in serum luteinizing hormone, whereas most exercising hamsters injected with melatonin generally continued having regular estrous cycles with proestrus luteinizing-hormone surges. The ability of exercise to inhibit the effect of exogenous melatonin in pinealectomized hamsters suggests that exercise acts, at least in part, by mechanisms other than altering melatonin secretion.

Group housing accelerates growth and induces obesity in adult hamsters

To determine whether variable body heat loss may influence control of growth and regulation of body fat content in hamsters, mature female hamsters were housed for 7-14 wk in one of three conditions: individually in metal cages (n = 6), individually in plastic boxes with bedding (n = 6), or communally (6 per box) in plastic boxes with bedding. Their resting metabolic rate (RMR) was measured individually or in pairs of two between days 65 and 75. When thermal properties of shelter varied alone, singly housed hamsters regulated energy balance fairly accurately by compensatory changes in food intake, and their rate of growth was unaffected. In contrast, group housing induced acceleration of growth and obesity without hyperphagia, was associated with an acute inhibition of RMR in hamsters tested in pairs, and was associated with a chronic inhibition of RMR in hamsters tested individually. We conclude that conspecific body contact in mature group-housed hamsters accelerates somatic growth and increases fat deposition. Energy for this anabolism is derived, in part, from reduced RMR and, in part, from a slight increase in food consumption.

Exercise-induced hyperphagia in the hamster is associated with elevated plasma somatostatin-like immunoreactivity

Syrian golden hamsters when allowed free access to food and an exercise wheel will run long distances and develop hyperphagia and accelerated linear body growth with high circulating levels of growth hormone and insulin. Somatostatin, a widely distributed brain-gut neurohormonal peptide, modulates nutrient absorption and may regulate food intake. To examine the role of circulating plasma somatostatin-like immunoreactivity (SRIF-LI; pg/ml) in exercise induced hyperphagia 4 groups of animals were studied; an unrestricted exercise group (279.0 +/- 107.7, n = 10); a sedentary group (121.1 +/- 40.8, n = 8); a restricted exercise group (107.7 +/- 12.4, n = 6); and a restricted no exercise group (115.5 +/- 45.9, n = 9). Thus, the unrestricted exercise group has a significantly elevated SRIF-LI concentration (P less than 0.01) while there was no difference between the other 3 groups. The elevation of plasma SRIF-LI in the unrestricted exercise group may represent a response to modulate increased nutrient entry in this group or may represent an incompletely effective satiety signal.

Rostromedial septal area controls pulsatile growth hormone release in the golden hamster

Limbic forebrain inhibits growth and growth hormone (GH) secretion in mature golden hamsters as shown by acceleration of growth and increases in serum GH concentrations following the electrolytic lesions of septum, transection of the hippocampus and surgical separation of these two regions. The growth-inhibitory function of this circuit is most probably mediated by somatostatinergic (SRIF) neurons. Such lesions induce hypoactivity possibly due to damage to endogenous opiatergic (EOP) neurons. EOP neurons facilitate spontaneous running in hamsters and mediate exercise-induced acceleration of growth and GH pulses. The coincidence of hypoactivity and growth acceleration after such lesions suggested the coexistence of SRIF and EOP fibers within the growth-inhibitory limbic forebrain circuit which control the rate of growth in mature hamsters by the variable inhibition of SRIF neurons by the EOP neurons. This hypothesis posits that accelerated growth is due to increased GH pulse frequency, and hypoactivity due to damage to EOP neurons, and was tested in this study by measuring pulsatile GH release (and as a measure of specificity, pulsatile prolactin release) in the presence and in the absence of opiate-receptor blocker naloxone in 21 female hamsters which sustained electrocoagulative lesions of rostromedial septum and 30 hamsters subjected to control surgery. Lesions doubled GH but not PRL pulse frequency, neither of which was affected by naloxone. Results support the hypothesis that opiatergic neurons facilitate pulsatile GH release by inhibiting the action of somatostatin neurons.

Physiological, metabolic and running characteristics of senescent female hamsters

Metabolic, endocrine and biochemical variables of female hamsters were studied to define age-related alterations in their characteristics. Endurance performance, aerobic capacity and metabolic fuels and hormones were measured and compared in young-adult (2-3 months) and old (16-17 months) hamsters to determine if the storage of substrates used in endurance exercise is altered with age, and if so, its effect on performance. The results indicate that storage of metabolic fuels are not appreciably altered with age, although voluntary running is markedly reduced. Other factors are likely involved in the hypoactivity of senescent female hamsters.

The effects of age on substrate depletion and hormonal responses during submaximal exercise in hamsters

Senescent hamsters display a marked reduction in volume of voluntary running. The purpose of this study was to determine whether age differences exist in the pattern of fuel utilization during submaximal exercise, which may account for the reduction in voluntary running. Further, we determined the effects of age on muscle oxidative capacity to assess its relationship to endurance performance in senescent hamsters. Depletion of carbohydrate and lipid content of skeletal muscle and liver, and changes in blood concentration of various hormones and substrates during one hour of exercise at 60 percent of VO2 max served to assess age effects on utilization of metabolic substrates. Exercise produced equivalent depletion of muscle glycogen and similar rise in plasma free fatty acids in young and old hamsters. No exercise effects on skeletal muscle triglyceride concentration or on plasma glycerol, glucagon or catecholamine concentrations were noted. With palmitoyl carnitine as substrate (but not with pyruvate) State 3 respiration of cardiac and skeletal muscle homogenates was lower in old compared to young hamsters. Although old hamsters have a reduced capacity to oxidize lipids in vitro, few age differences in fuel use are evident in vivo during submaximal exercise. Thus, these minor age differences in substrate utilization do not likely account for the substantial reduction in the levels of spontaneous running in senescent hamsters.

Ontogenesis of corticotropes and lactotropes in situ in the pituitary gland of the hamster. An immunohistochemical study

The development of corticotropes and lactotropes was investigated in the golden Syrian hamster using an anti-porcine ACTH antiserum and a homologous anti-hamster PRL antiserum. Oval corticotropes were first visible in the ventral region of the pars distalis at 13 days of gestation. By the end of gestation, corticotropes were found throughout the pars distalis and in the pars intermedia. Corticotropes in the pars distalis of postnatal hamsters were either round or irregularly-shaped, often appearing in clusters. Throughout development, corticotropes often appeared to be surrounding other cells. Scarce, very small lactotropes were first observed in the pars distalis of hamsters on the first postnatal day. The number of these cells, which were either round or polyhedral, increased dramatically between 4 and 20 days of postnatal life. These observations indicate that the sequence of appearance of corticotropes and lactotropes in the hamster is similar to that in other species and that lactotropes are confined to the pars distalis of postnatal hamsters.

Alteration of pulsatile growth hormone secretion by growth-inducing exercise: involvement of endogenous opiates and somatostatin

Our aim was to examine whether 1) growth-inducing exercise altered the pulsatile pattern of GH release, 2) endogenous opiates mediated these changes, and 3) endogenous somatostatin (SRIF) suppressed pulsatile GH secretion in slowly growing sedentary hamsters. To that end, mature female hamsters were either exposed to 2 weeks of voluntary running or prevented from engaging in running activity. Intracardiac catheters were inserted to allow blood collection every 20 min during 6 h. GH and PRL secretory patterns were determined with homologous RIAs for hamster hormones after administration of saline, naloxone, or anti-SRIF serum. It was found that exercise accelerated somatic growth. GH pulse frequency and amplitude were significantly higher in rapidly growing hamsters. The rate of growth was inversely related to basal trough values of GH and PRL. Opiate receptor blockade reduced GH pulse frequency and amplitude to below sedentary values and did not affect PRL secretion. Anti-SRIF serum increased GH pulse frequency and amplitude in naloxone- as well as in saline-injected animals. Our results are consistent with the conclusions that 1) rapid growth, spontaneous or induced by exercise, is associated with increased GH pulse frequency and amplitude and decreased GH and PRL baseline concentrations in hamsters; 2) endogenous SRIF inhibits pulsatile GH secretion in slowly growing hamsters; and 3) endogenous opiates mediate increases in GH pulse frequency and amplitude in rapidly growing hamsters by antagonizing the endogenous SRIF action.

Recovery from energy deficit in golden hamsters

Hamsters feeding at greater than 2-h intermeal intervals (IMI) lose weight but recover from weight losses without hyperphagia if they are allowed to feed at 2-h IMIs (Am. J. Physiol. 236 (Endocrinol. Metab. Gastrointest. Physiol. 5): E105-E112, 1979). To determine the relative importance of changes in energy expenditure and fat synthesis in their energy regulation, measurements were made of resting metabolic rate, respiration by brown adipose tissue (BAT), locomotion, fecal energy content, and insulin and hepatic lipogenic enzyme responses to feeding in underweight hamsters allowed to feed at 2- or 5-h IMIs. Energy deficit suppressed the resting metabolic rate and general locomotor activity and increased the activity of fatty acid synthetase (FAS). Heat production by BAT increased in underweight hamsters. Increase in IMIs blocked the postprandial insulin release, reduced plasma insulin concentration and FAS activity, and increased malic enzyme activity. Thus ad libitum feeding hamsters recover from energy deficit by reducing energy expenditure, whereas failure to add additional meals and impaired insulin and changed lipogenic responses to feeding produce energy deficits in infrequently feeding hamsters.

Regulation of testicular prolactin and luteinizing hormone receptors in golden hamsters

Iodinated ovine PRL (oPRL) was used to detect the presence of PRL receptors in hamster testes. Binding to these receptors was specific for PRL and was saturable and incompletely reversible. Testicular PRL receptors possessed a single class of independent, high affinity binding sites (Ka = 2.4 X 10(10) M-1). The ability of PRL to regulate testicular PRL and LH receptors was measured in experiments wherein mature hamsters were injected daily for 10 days with 600 micrograms bromocriptine, 600 micrograms bromocriptine plus 250 micrograms oPRL, or injection vehicles. Bromocriptine injections, which caused significant reductions in serum PRL, produced commensurate decreases in concentrations (femtomoles per mg protein) of testicular LH and PRL receptors, without affecting binding of PRL to adrenal or liver tissue preparations. Decreases in testicular PRL binding were not always statistically significant, but were consistently measured. The administration of oPRL with bromocriptine significantly increased both concentration and total content (femtomoles per paired testes) of testicular LH receptors and PRL-binding sites above those measured in bromocriptine-injected and control hamsters. Liver PRL-binding sites were, likewise, increased above values measured in controls and bromocriptine-injected hamsters by this latter treatment, which had no significant effect on adrenal PRL-binding sites. Neither bromocriptine nor bromocriptine plus oPRL had any effect on the affinity (Ka) of testicular PRL receptors for labeled oPRL. Serum testosterone and FSH concentrations were unaltered by treatment, thereby suggesting that effects on receptors did not occur secondary to changes in these hormones, but, rather, represent direct effects of PRL. An additional experiment in prepubertal male hamsters (aged 14-30 days) demonstrated that bromocriptine injections for 17 days significantly reduced the total content of testicular PRL receptors. In vitro experiments eliminated the possibility of direct effects of bromocriptine on testicular receptors. The data demonstrate the possibility that PRL is required for maintenance of normal concentrations of testicular LH/hCG and PRL receptors in adult and immature golden hamsters. An increase in putative serum anti-oPRL antibodies was measured in oPRL-injected hamsters. Since such antibodies could contribute, to an unknown degree, to PRL binding measured in tissue preparations, the present data indicate only that increased testicular binding occurs subsequent to injection of heterologous PRL, without demonstrating that this binding is indeed due to increased PRL receptors.

A role for somatostatin in the control of hamster growth

Concentrations of somatostatin-like immunoreactivity (SRIF-LI) were measured in cerebral cortex, hippocampus, septum-POA, median eminence, gastric antrum, fundus and pancreas in adult female hamsters to determine whether changes in somatostatin could be related to increased growth hormone (GH) secretion and somatic growth that follow bilateral transections of hippocampus (n = 18; 17 controls). In addition, choline acetyltransferase (CAT) activity was measured in the four brain regions in hippocampectomized (n = 10) and control hamsters (n = 10) to gain insight into the relationship between these two neurotransmitters. Hippocampal transections induced: significant acceleration of somatic growth; increased serum GH concentrations; increased concentrations of SRIF-LI in septum-POA and gastric antrum; reduced concentrations of SRIF-LI in hippocampus and pancreas; and reduced CAT activity in the hippocampus. These results suggest that somatostatinergic and cholinergic projections to hippocampus via fornix suppress GH and somatic growth in adult hamsters and that reduced release of SRIF-LI in the gastric antrum may contribute to the acceleration of somatic growth through facilitated nutrient digestion and entry.

Testicular prolactin receptors and serum growth hormone in golden hamsters: effects of photoperiod and time of day

Investigations were conducted to determine effects of exposure to short photoperiod--with its accompanying reductions in serum prolactin (Prl) concentrations--for various durations on testicular Prl receptors. An additional study investigated the possibility of nyctohemeral fluctuations in testicular Prl receptors and serum growth hormone (GH) concentrations and their alteration by photoperiod. After 10 and 28 days of exposure to a short photoperiod consisting of 5 h of light and 19 h darkness (5L:19D) (and prior to changes in testicular weight), there were progressive and significant reductions in the concentration of testicular Prl receptors (fmol/mg protein) when compared with long-photoperiod controls (14L:10D). After 12 weeks of 5L:19D, when testicular weights were dramatically decreased, Prl receptor concentration was reduced to 39% of long-photoperiod controls in one study, without alteration of affinity of Prl receptors for their labeled ligand. When measured at 6-h intervals in hamsters on 14L:10D, and on 5L:19D for 12 weeks, there were no significant changes in concentration or total content (fmol/testes) of testicular Prl receptors throughout the day. Although serum GH concentrations fluctuated markedly in hamsters on both photoperiods, no definitive nyctohemeral patterns were detected. These data provide indirect evidence for the ability of Prl to regulate its own testicular receptors, and demonstrate that diurnal fluctuations in testicular sensitivity to injected Prl are not a consequence of changes in Prl receptors. The data also suggest the absence of effects of photoperiod on serum GH concentrations in male golden hamsters.

Exercise reverses photoperiodic anestrus in golden hamsters

In contrast to the inhibitory effects of endurance exercise on reproductive function in women athletes, voluntary running induces estrous cyclicity in anestrous female hamsters maintained in the nonstimulatory short-day photoperiod. Increased concentrations of circulating prolactin (Prl) seen in these animals are not responsible for the reversal of photoperiodic anestrus as demonstrated by experimental manipulations of prolactin secretion. Voluntary running also facilitates growth hormone (GH) release and somatic growth in hamsters independently of the photoperiod. Thus, it appears that endurance exercise can have facilitatory as well as inhibitory effects on the reproductive function in female mammals.

Basis for the hypoactivity that accompanies rapid weight gain in hamsters

The pattern of hypoactivity that accompanies rapid weight gain following septal lesions in hamsters was characterized. Lesioned hamsters displayed reduced levels of running, shorter and slower running bouts, and longer pauses. We examined whether this hypoactivity was due to reassignment of metabolic fuels from supporting physical activity to anabolism, or due to reduced capacity of running to induce psychomotor arousal and mobilize metabolic fuels. Septal lesions were associated with increased rate of ponderal growth and higher titers of circulating growth hormone and insulin. No difference in concentrations of muscle and liver glycogen, percentage of body fat, or the capacity of muscle homogenates to oxidize substrates were identified. Lesioned hamsters ran as fast and as long as control animals on electrical-shock reinforced treadmill, but were unable to generate as much heat in response to injection (0.8 mg/kg) of norepinephrine. We concluded that hypoactivity that accompanies rapid weight gain in hamsters results either from a reduced capacity of running to induce psychomotor arousal and provide incentives that normally motivate that behavior, or from a failure of running to mobilize metabolic fuels at a rate necessary to sustain normal running speed and duration, and not from reduced availability of metabolic fuels or reduced muscle capacity to oxidize metabolic substrates.

Opiate-receptor blockade reduces voluntary running but not self-stimulation in hamsters

Naltrexone HCl, a long-acting opiate receptor blocker was administered to female hamsters at two doses, 10 and 20 mg/kg, IP prior to 12 hr of nocturnal running or every 12 hr during access to hypothalamic self-stimulation to determine whether endogenous opiates played a role in either of these two motivated behaviors. Naltrexone suppressed total running activity and speed, and caused an increase in pause time but did not affect the rate of hypothalamic self-stimulation. Furthermore, weight gain was unaffected by four weeks of self-stimulation but was accelerated during two weeks of voluntary running. Thus stimulation of endogenous opiate receptors helps support high levels of voluntary running but is not involved in initiation of running or in maintenance of intracranial self-stimulation in female hamsters. Furthermore, the association of opiate receptor stimulation and increased somatic growth with voluntary running but not with self-stimulation suggests a possible facilitatory role for endogenous opiates in acceleration of growth by exercise.

Hamster growth hormone. Species specificity and physiological changes in blood and pituitary concentrations as measured by a homologous radioimmunoassay

A specific and sensitive homologous radioimmunoassay (RIA) for determination of golden hamster growth hormone (GH) is described and compared to a heterologous hamster GH RIA. Using the homologous system, cross-reactivity experiments between golden hamster GH on one hand, and pituitary extracts from two other hamster species, and purified GHs from several rodent and mammalian species, suggested that hamster GH differs from other rodent and mammalian GHs to a greater extent than rat GH. Using the homologous method, we have determined that, in the golden hamster, diurnal plasma fluctuations have a mean interpeak interval of 70-80 min, that serum GH concentrations are affected by nyctohemeral differences in lighting, by ether vapors, by animal's gender, and by starvation and refeeding. Basal GH concentrations were not influenced by exposure to different photoperiods or by removal of the gonads. A heterologous hamster GH RIA, which utilizes a radioiodinated rat GH and monkey antihamster GH serum, represents the most advantageous method for the measurement of hamster GH because of its high sensitivity (0.3 ng/ml), low limit of detection (46 pg/ml), high antiserum binding to iodinated rat GH, and the ability to measure rat and mouse GHs in addition to hamster GH. Immunological differences between hamster, rodent, and other mammalian GHs are diminished when heterologous, rather than homologous, tracer is used.

Metabolic alterations induced by voluntary exercise and discontinuation of exercise in hamsters

A study was conducted to determine the possible biochemical mechanisms responsible for the rapid increase in body weight and body fat during the early phase of exercise-retirement. Thirty-two adult female hamsters were allotted to four groups of eight each. One group served as sedentary controls. Other groups had access to voluntary disc-running during a 35-day period of the 76-day experimental period and were retired from running for 0, 8, and 41 days, respectively, before the termination of the experiment. Disc-running reduced body fat and body weight. Discontinuation of running led to a fast gain of body fat and body weight. Disc-running decreased serum triacylglycerol levels but had no effect on serum cholesterol levels. Serum high-density lipoprotein-cholesterol concentration was significantly elevated at 8 days after retirement but was not changed during exercise or at 41 days after retirement. Exercise or discontinuation of exercise had no effect on serum T-3 or T-4 levels. The rate of fatty acid synthesis and the activities of malic enzyme and fatty acid synthetase in the liver were increased during running and during the early phase of retirement. Liver glucose-6-phosphate dehydrogenase activity was increased at 8 days after discontinuation of running. Disc-running enhanced food intake which was not reduced to the sedentary level until about 10 days after retirement, suggesting a delay in adaptation to reduced energy expenditure. Results of the present study suggest that both the delay in food intake adaptation and the enhanced lipogenic activity play a role in the rapid gain of body weight and body fat during the early phase of exercise-retirement.

Hamster prolactin: physiological changes in blood and pituitary concentrations as measured by a homologous radioimmunoassay

A specific and sensitive homologous radioimmunoassay (RIA) for determination of hamster prolactin is described and compared to heterologous hamster prolactin RIA and to the homologous and heterologous RIAs for rat prolactin. With this method, we have determined that, in the golden hamster, diurnal plasma prolactin fluctuations have a mean interpeak interval of 1.5 h, that serum prolactin concentrations are influenced by day length, by ether vapors, by animal's gender, and by refeeding following a fast but not by neurosurgical procedures which increase somatic growth and serum GH concentration in adult hamsters. Variations in day length influenced pituitary prolactin content and concentration, and pituitary removal abolished prolactin secretory responses to ether stress. A heterologous RIA for hamster prolactin which utilizes radioiodinated rat prolactin and rabbit antihamster prolactin serum (RK1-15) represents the most advantageous method for the measurement of hamster prolactin because of its high sensitivity (0.14 ng/ml), high antiserum binding to iodinated rat prolactin, and the ability to measure both hamster and rat prolactin.

Somatic, endocrine, and serum lipid changes during detraining in adult hamsters

Effects of abrupt discontinuation of chronic exercise on body composition and serum lipid, insulin, and glucagon concentrations were examined in adult female hamsters. Thirty-six hamsters (100 to 120 g) were randomly allotted to two groups of 18 each for an 84-day study. One group served as controls and were sedentary throughout the experimental period; another group had access to voluntary running on horizontal discs during the first 42 days of the experimental period. Six hamsters from each group were killed at the end of the exercise period and at 12 and 42 days after retirement. Results showed that hamsters engaged in high levels of voluntary activity increased food intake by about 10 to 20% and this effect persisted for about 10 days after retirement. Voluntary running resulted in a 60% reduction in body fat content and a 30% decrease in serum triglyceride levels. Exercise was also associated with an increase in body cholesterol level, a decrease in glucagon concentration, and a suggestive increase in serum insulin level. Increased food consumption and changes in serum insulin and glucagon may reflect compensatory adjustments to increased energy expenditure of exercise. Discontinuation of exercise resulted in a reversal of exercise effects on body fat, body cholesterol, and serum triglyceride levels.

Contribution of growth, fatness, and activity to weight disturbance after septohypothalamic cuts in adult hamsters

The mechanism responsible for weight stability in adult hamsters was investigated by (a) transecting the dorsoventrally oriented nerve pathways between the septal area and hypothalamus (SH cuts) and (b) partitioning the observed increases in the rate of weight gain into three contributory components: changes in somatic growth, in body fatness, and in energy expended as voluntary activity on horizontal disks. Between 60% and 70% of the weight increase after SH cuts was due to acquisition of lean body mass, and 30%-40% of weight increase consisted of excess body fat. After SH cuts, serum growth hormone and insulin concentrations were increased on Day 14, food intake was increased between Day 2 and Day 42, skeletal lengths were greater on Day 77, and voluntary activity levels were 84% lower on Days 10-45, relative to control hamsters. It is concluded that dorsoventrally oriented nerve pathways in the septal area are involved in the control of growth, maintenance of body fat reserves, and voluntary activity and that they contribute to the maintenance of stable body weight in adult hamsters.

Role of limbic system in the control of hamster growth

Rostral septal lesions accelerate somatic growth in adult hamsters. This study tested the hypothesis that this effect results from damage to fibers of passage by observing the effects of transections of septohippocampal and septohypothalamic connections on growth. We attempted to identify these fibers further by (a) measuring spectrofluorometrically changes in the monoamine concentrations in hippocampus, cerebral cortex, corpus striatum, and diencephalon, (b) staining the degenerating axons after septal lesions and the two cuts, and (c) examining the correspondence between such damage and the acceleration of growth. Both knife cuts accelerated somatic growth and were associated (as well as septal lesions) with significant depletions of serotonin (-27 to -57%) and norepinephrine (-27 to -60%) in the hippocampus, with less consistent depletions of these monoamines in the cerebral cortex, and with no changes in regional dopamine content. All three procedures were associated with degeneration in the hippocampal formation and its fiber systems. Thus, fibers interconnecting hippocampus and brainstem, and passing through septum, exert tonic suppression over somatic growth in adult hamsters.

Physiological and behavioral responses to starvation in the golden hamster

Physiological and behavioral responses of adult hamsters to starvation were studied by measuring food intake, weight recovery, serum concentrations of glucose, insulin, free fatty acids and beta-hydroxybutyrate, and ketonuria in animals subjected to different weight losses, diets, and durations of fast. Hamsters were debilitated by fasts longer than 12 h or leading to greater than 20% weight loss. Hamsters' feeding patterns were unmodified by fasts ranging between 5 and 12 h and showed no circadian periodicity. Hamsters predominantly recovered from weight losses without increasing their food consumption (unless they were offered a diet of pellets and seeds) and without changing their meal patterns, at a rate of weight gain proportional to the magnitude of preceding weight loss if provided with uninterrupted access to food. By 8 h of fast, blood metabolites were indicative of mobilization of body fat. Hamsters are thus behaviorally unresponsive to duration of fast, but compensate physiologically for weight losses with proportional increases in the rate of weight gain.

Increased serum growth hormone and somatic growth in adult hamsters with hippocampal transections

Somatic, endocrine, and behavioral correlates of growth and levels of voluntary running activity were measured in adult hamsters with hippocampal transections (HIPPO cuts) or in controls with transections of overlying cortex. Significant increase in serum concentration of growth hormone (GH) and decrease in pituitary concentration of GH were measured in HIPPO hamsters with a homologous radioimmunoassay method for hamster GH. HIPPO hamsters had increased: serum insulin concentration in fed state, food consumption, ponderal and linear growth, and percentage of body fat, and decreased levels of voluntary activity. Similarities between growth acceleration after HIPPO cuts and lesions of rostral medial septum suggest that fibers interconnecting, or passing through, the hippocampal formation and septum inhibit growth in adult hamsters.

Increased serum growth hormone and somatic growth in exercising adult hamsters

In freely feeding adult hamsters, voluntary exercise induces accelerated somatic growth and increased food consumption that last through several days of retirement. We examined the effects of exercise on serum concentrations of growth hormone (GH) and insulin during ad libitum or restricted intake of food. Serum insulin and GH concentrations were measured by radioimmunoassays in exercising, retired, or sedentary hamsters during ad libitum or restricted intake of food. Linear growth was delayed in food-restricted, exercising hamsters until they were retired and unlimited food consumption was allowed. Serum GH concentrations were increased during exercise and after retirement; serum insulin concentrations were increased only after retirement during both dietary regimes. In food-restricted hamsters, endocrine changes were noted after 4 h of feeding but not after a 14-h fast. We conclude that 1) in adult hamsters voluntary exercise leads to increased secretion of GH even when ingested nutrients are insufficient to support increased growth, and 2) increased secretion of insulin is not related directly to exercise.

A heterologous radioimmunoassay method for measurement of hamster growth hormone

A heterologous radioimmunoassay (RIA) for determination of hamster growth hormone (GH) is described which involves the use of radioiodinated rat GH (NIAMDD Rat GH-I-2), antiserum to rat GH (NIAMDD Anti-Rat-GHS-3), a standard hamster anterior pituitary homogenate (SHAP), as well as purified hamster GH (AFP-1595-B). This RIA was shown to be a reliable, sensitive, and specific method for measurement of hamster GH. With this method we have determined the effects of stress and hypophysectomy on serum GH concentrations as well as changes in serum GH concentration, and pituitary GH content and concentration as a function of hamster age. In contrast to responses noted in rats, the stress of ether anesthesia and i.c. puncture caused significant increases in serum GH. A decrease in the serum GH concentration was observed in hamsters at the age 2 to 16 days. Pituitary GH content and concentration increased steadily between 9 and 120 days of age.

The role of the septal area in the neuroendocrine control of growth in the adult golden hamster

Somatic, endocrine and behavioral correlates of growth were measured in intact and hypophysectomized adult hamsters with or without electrolytic damage to the rostral septal area. Septal (SEP) lesions significantly increased somatic growth rate, food consumption and serum concentration of growth hormone (GH) and insulin. Pituitary GH content and concentration were reduced in lesioned hamsters. None of these changes were found in hypophysectomized hamsters with SEP lesions. SEP lesions did not affect the percentage of body fat. These data suggest that the septum or adjacent fibers of passage inhibit growth in adult hamsters.