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

MECHANISMS OF ENDOCRINOLOGY: Endocrinology of opioids

The use of opioids has grown substantially over the past two decades reaching the dimensions of a global epidemic. These drugs have effects on multiple levels of the endocrine system through mechanisms which are still not fully elucidated, and awareness of their endocrine sequelae is vital for all specialists prescribing or managing patients on them. Hypogonadism is the most well-recognised consequence of opioid use (prevalence 21–86%) which, however, may remain undiagnosed with potential adverse outcomes for the patients. Although less frequent, cortisol deficiency can also be found. Furthermore, there is a negative impact on bone health (with reduced bone mineral density and increased fracture risk) and occasionally hyperprolactinaemia, whereas the clinical significance of alterations in other hormones remains to be clarified. Discontinuation or reduction of the opioid and, in cases of chronic pain, consideration of alternative therapies for pain relief are potential management options. Hormonal replacement, especially when the above measures are not practically feasible, needs to be considered. Further studies are needed to clearly establish the prevalence of hormonal abnormalities with various regimes, doses and routes of opioids and to address reliably the long-term benefits and risks of hormonal treatment in patients on opioids. Until evidence-based, safe and cost-effective clinical guidelines become available, periodical assessment of the gonadal and adrenal function (particularly when relevant clinical manifestations are present) and evaluation of the bone health status are advised.

Somatostatin Agonist Pasireotide Inhibits Exercise-Stimulated Growth in the Male Siberian Hamster (Phodopus sungorus)

The Siberian hamster (Phodopus sungorus) is a seasonal mammal, exhibiting a suite of physiologically and behaviourally distinct traits dependent on the time of year and governed by changes in perceived day length (photoperiod). These attributes include significant weight loss, reduced food intake, gonadal atrophy and pelage change with short-day photoperiod as in winter. The central mechanisms driving seasonal phenotype change during winter are mediated by a reduced availability of hypothalamic triiodothyronine (T3), although the downstream mechanisms responsible for physiological and behavioural changes are yet to be fully clarified. With access to a running wheel (RW) in short photoperiod, Siberian hamsters that have undergone photoperiod-mediated weight loss over-ride photoperiod-drive for reduced body weight and regain weight similar to a hamster held in long days. These changes occur despite retaining the majority of hypothalamic gene expression profiles appropriate for short-day hamsters. Utilising the somatostatin agonist pasireotide, we recently provided evidence for an involvement of the growth hormone (GH) axis in the seasonal regulation of bodyweight. In the present study, we employed pasireotide to test for the possible involvement of the GH axis in RW-induced body weight regulation. Pasireotide successfully inhibited exercise-stimulated growth in short-day hamsters and this was accompanied by altered hypothalamic gene expression of key GH axis components. Our data provide support for an involvement of the GH axis in the RW response in Siberian hamsters.

Counterregulation of insulin by leptin as key component of autonomic regulation of body weight

A re-examination of the mechanism controlling eating, locomotion, and metabolism prompts formulation of a new explanatory model containing five features: a coordinating joint role of the (1) autonomic nervous system (ANS); (2) the suprachiasmatic (SCN) master clock in counterbalancing parasympathetic digestive and absorptive functions and feeding with sympathetic locomotor and thermogenic energy expenditure within a circadian framework; (3) interaction of the ANS/SCN command with brain substrates of reward encompassing dopaminergic projections to ventral striatum and limbic and cortical forebrain. These drive the nonhomeostatic feeding and locomotor motivated behaviors in interaction with circulating ghrelin and lateral hypothalamic neurons signaling through melanin concentrating hormone and orexin-hypocretin peptides; (4) counterregulation of insulin by leptin of both gastric and adipose tissue origin through: potentiation by leptin of cholecystokinin-mediated satiation, inhibition of insulin secretion, suppression of insulin lipogenesis by leptin lipolysis, and modulation of peripheral tissue and brain sensitivity to insulin action. Thus weight-loss induced hypoleptimia raises insulin sensitivity and promotes its parasympathetic anabolic actions while obesity-induced hyperleptinemia supresses insulin lipogenic action; and (5) inhibition by leptin of bone mineral accrual suggesting that leptin may contribute to the maintenance of stability of skeletal, lean-body, as well as adipose tissue masses.

Effect of exercise on photoperiod-regulated hypothalamic gene expression and peripheral hormones in the seasonal Dwarf Hamster Phodopus sungorus

The Siberian hamster (Phodopus sungorus) is a seasonal mammal responding to the annual cycle in photoperiod with anticipatory physiological adaptations. This includes a reduction in food intake and body weight during the autumn in anticipation of seasonally reduced food availability. In the laboratory, short-day induction of body weight loss can be reversed or prevented by voluntary exercise undertaken when a running wheel is introduced into the home cage. The mechanism by which exercise prevents or reverses body weight reduction is unknown, but one hypothesis is a reversal of short-day photoperiod induced gene expression changes in the hypothalamus that underpin body weight regulation. Alternatively, we postulate an exercise-related anabolic effect involving the growth hormone axis. To test these hypotheses we established photoperiod-running wheel experiments of 8 to 16 weeks duration assessing body weight, food intake, organ mass, lean and fat mass by magnetic resonance, circulating hormones FGF21 and insulin and hypothalamic gene expression. In response to running wheel activity, short-day housed hamsters increased body weight. Compared to short-day housed sedentary hamsters the body weight increase was accompanied by higher food intake, maintenance of tissue mass of key organs such as the liver, maintenance of lean and fat mass and hormonal profiles indicative of long day housed hamsters but there was no overall reversal of hypothalamic gene expression regulated by photoperiod. Therefore the mechanism by which activity induces body weight gain is likely to act largely independently of photoperiod regulated gene expression in the hypothalamus.

Synergistic effect of obesity and lipid ingestion in suppressing the growth hormone response to exercise in children

Diet plays an important role in modulating exercise responses, including activation of the growth hormone (GH)/insulin-like growth factor-I (IGF-1) axis. Obesity and fat ingestion were separately shown to reduce exercise GH responses, but their combined effect, especially important in children, has not been studied. We therefore measured the GH response to exercise [30-min intermittent cycling, ten 2-min bouts at ~80% maximal aerobic capacity (Vo(2max)), separated by 1-min rest], started 45 min after ingestion of a high-fat meal (HFM) in 16 healthy [controls; body mass index percentile (BMI%ile) 51 ± 7], and 19 obese (Ob, BMI%ile 97 ± 0.4) children. Samples were drawn at baseline (premeal), and at start, peak, and 30 min postexercise. In the Ob group, a marked ~75% suppression of the GH response (ng/ml) to exercise was observed (2.4 ± 0.6 vs. 10.6 ± 2.1, P < 0.001). This level of suppression was also significantly greater compared with age-, fitness-, and BMI-matched historical controls that had performed identical exercise in fasting conditions. Our data indicate that the reduction in the GH response to exercise, already present in obese children vs. healthy controls, is considerably amplified by ingestion of fat nutrients shortly before exercise, implying a potentially downstream negative impact on growth factor homeostasis and long-term modulation of physiological growth.

Opioid-induced biochemical alterations of the neuroendocrine axis

Exogenous opioids have been used for decades to palliate cancer-related pain and other cancer-related manifestations and, more recently, to treat patients with pain not related to oncologic disease. While the goal of opioid treatment is symptomatic relief and improved quality of life, these patients often suffer from adverse side effects, including endocrine system abnormalities, of which hypogonadism is the best known. Opioids may interact with other hypothalamic-pituitary pathways and endocrine end organs, and in most cases these interactions are subtle and the effects unclear. The long-term effects of these agents on the endocrine system are still largely unknown. This article discusses the various effects of opioid agents on the endocrine system and provides information that allows early recognition of side effects that may alter the quality of life of patients affected by pain, awareness of the potential complications in opioid addicts, and detection and treatment of side effects in participants of an opioid detoxification program.

The effects of opioids and opioid analogs on animal and human endocrine systems

Opioid abuse has increased in the last decade, primarily as a result of increased access to prescription opioids. Physicians are also increasingly administering opioid analgesics for noncancer chronic pain. Thus, knowledge of the long-term consequences of opioid use/abuse has important implications for fully evaluating the clinical usefulness of opioid medications. Many studies have examined the effect of opioids on the endocrine system; however, a systematic review of the endocrine actions of opioids in both humans and animals has, to our knowledge, not been published since 1984. Thus, we reviewed the literature on the effect of opioids on the endocrine system. We included both acute and chronic effects of opioids, with the majority of the studies done on the acute effects although chronic effects are more physiologically relevant. In humans and laboratory animals, opioids generally increase GH and prolactin and decrease LH, testosterone, estradiol, and oxytocin. In humans, opioids increase TSH, whereas in rodents, TSH is decreased. In both rodents and humans, the reports of effects of opioids on arginine vasopressin and ACTH are conflicting. Opioids act preferentially at different receptor sites leading to stimulatory or inhibitory effects on hormone release. Increasing opioid abuse primarily leads to hypogonadism but may also affect the secretion of other pituitary hormones. The potential consequences of hypogonadism include decreased libido and erectile dysfunction in men, oligomenorrhea or amenorrhea in women, and bone loss or infertility in both sexes. Opioids may increase or decrease food intake, depending on the type of opioid and the duration of action. Additionally, opioids may act through the sympathetic nervous system to cause hyperglycemia and impaired insulin secretion. In this review, recent information regarding endocrine disorders among opioid abusers is presented.

Endogenous anabolic hormone responses to endurance versus resistance exercise and training in women

Research in exercise endocrinology has flourished over the past few decades. In general, research examining short- and long-term hormone responses to endurance exercise preceded studies on resistance exercise, and research on women lagged behind research on men. Sufficient data are now available to allow a comparison of endogenous anabolic hormone responses to endurance versus resistance exercise and training in women. Circulating levels of testosterone, dehydroepiandrosterone, dehydroepiandrosterone sulphate, estradiol, growth hormone and cortisol have been shown to increase in response to an acute bout of endurance exercise in women. However, only growth hormone, estradiol and cortisol have been reported to increase following resistance exercise. Hormone changes following training, either endurance or resistance, have been variable, probably because of differences in experimental design and major differences in the length, intensity and volume of training programmes. With the notable exception of growth hormone, the anabolic hormones reviewed here appear to decline with endurance training. Resistance training has little effect on resting hormone levels, except insulin-like growth factor-I, which has been shown to increase following a training programme. These hormone changes potentially have both metabolic and hypertrophic implications, and future research needs to focus on the biological significance of these adaptations.

Growth hormone pulsatility profile characteristics following acute heavy resistance exercise

This investigation examined the hypothesis that acute heavy resistance exercise (AHRE) would increase overnight concentrations of circulating human growth hormone (hGH). Ten men (22 +/- 1 yr, 177 +/- 2 cm, 79 +/- 3 kg, 11 +/- 1% body fat) underwent two overnight blood draws sampled every 10 min from 1700 to 0600: a control and an AHRE condition. The AHRE was conducted from 1500 to 1700 and was a high-volume, multiset exercise bout. Three different immunoassays measured hGH concentrations: the Nichols immunoradiometric assay (Nichols IRMA), National Institute of Diabetes and Digestive and Kidney Diseases radioimmunoassay (NIDDK RIA), and the Diagnostic Systems Laboratory immunofunctional assay (DSL IFA). The Pulsar peak detection system was used to evaluate the pulsatility profile characteristics of hGH. Maximum hGH was lower in the exercise (10.7 microg/l) vs. the control (15.4 microg/l) condition. Mean pulse amplitude was lower in the exercise vs. control condition when measured by the Nichols IRMA and the DSL IFA. A differential pattern of release was also observed after exercise in which hGH was lower in the first half of sleep but higher in the second half. We conclude that AHRE does influence the temporal pattern of overnight hGH pulsatility. Additionally, because of the unique molecular basis of the DSL IFA, this influence does have biological relevance because functionally intact molecules are affected.

Peak oxygen uptake, muscle volume, and the growth hormone-insulin-like growth factor-I axis in adolescent males

PURPOSE

The growth effects of exercise appear to be mediated in part by central neuroendocrine control reflected in circulating levels of growth hormone (GH), insulin-like growth factor-I (IGF-I), and their binding proteins (BP). In previous studies positive correlations between peak VO2 and circulating IGF-I have been demonstrated. The relationship between peak oxygen uptake and these potential regulating factors has not been examined in adolescent males where patterns of GH pulsatility and levels of IGF-I are rapidly changing.

METHODS

Forty-three healthy adolescent males (age 16 +/- 0.7 yr, 70% at Tanner V) performed cycle ergometry to determine p oxygen uptake (peak VO2), and magnetic resonance images to determine the thigh muscle volume. Baseline blood samples were collected for GHBP, the extracellular portion of the GH tissue receptor (by ligand mediated immunofunctional assay), IGF-I (by RIA), and IGFBPs 1-5 (by RIA). Mean GH was determined from samples obtained every 20 min overnight.

RESULTS

Peak VO2/kg was positively correlated with mean overnight GH levels (r = 0.41, P < 0.005). Both peak VO2/kg and thigh muscle volume/kg were negatively correlated with GHBP (r = -0.33, P < 0.02) and IGFBP-4 (r = -0.52, P < 0.005). There were no correlations between peak VO2/kg and IGF-I or IGFBPs 1-3, and 5.

CONCLUSIONS

GH pulsatility is increased adolescent males who have higher peak VO2, but this did not translate into increases in IGF-I. We speculate that in the fitter males, lower GHBP levels may reduce hepatic sensitivity to GH. Thus, circulating IGF-I was unchanged despite higher mean GH in subjects with higher peak VO2. IGFBP-4 which is known to inhibit IGF-I was negatively correlated with peak VO2 leading, possibly, to increased IGF-I bioactivity. Fitness (as assessed by muscle mass and peak VO2) does modulate the GH-IGF-I axis, but not solely through circulating IGF-I; both GHBP and IGFBPs play important roles.

Diurnal, photoperiodic, and age-related changes in plasma growth hormone levels in the golden hamster

The golden hamster has been used extensively as an animal model for the study of both circadian and seasonal rhythms, and their regulation by the light-dark (LD) cycle. More recently, this species has been used to examine how the generation and entrainment of circadian rhythms are altered in advanced age. Recent studies in both humans and rodents indicate that age-related changes in the diurnal rhythm of pituitary growth hormone (GH) release may mediate some of the adverse effects of aging on a variety of physiological systems. As a first step in determining whether or not age-related changes in circulating GH levels are associated with changes in the regulation and/or expression of circadian rhythms, the effects of age on both the ultradian and diurnal patterns of plasma GH levels were determined in 3- to 22-month-old male hamsters that were bled every 15 min for a 24-hr period while entrained to an LD 14:10 light cycle. An additional study involving a similar blood collection protocol examined whether or not the length of the day is involved in the regulation of plasma GH levels. Although the frequency of pulsatile GH release did not change with advanced age, both the mean levels of GH per sample and the mean amplitude per pulse of GH were significantly elevated in 3- to 4-month-old animals, compared to animals that were 12-13, 15-16, or 21-22 months of age. In hamsters aged 3-4 and 12-13 months, there was an increase in both mean levels and the mean amplitude per pulse of GH, but not pulse frequency, during the night as compared to daytime values. No such diurnal rhythm was detected in the two groups of older animals. A clear diurnal rhythm in GH levels was also detected in animals maintained in a short-day (LD 6:18) cycle, and the mean levels of GH per sample were greater in hamsters maintained on short compared to long (LD 14:10) days. These results indicate that there are pronounced age-related changes in pituitary GH release in the hamster, and that both the time of day and the length of the day influence the pattern of GH release.

Recombinant growth hormone treatment of amyotrophic lateral sclerosis

Based on the known trophic effects of growth hormone (GH) on nerve and muscle 75 patients with ALS were treated for up to 18 months with synthetic human growth hormone (Protropin) or a placebo. The course of ALS was assessed serially using a quantitative (TQNE) neuromuscular and manual exam (MRC) and laboratory chemistries. Average insulin-related growth factor (IGF-I) values increased from 1.2 to 2.3 U/mL in the treated group. Surprisingly, serum insulin levels did not increase. Hyperglycemia was noted in only 2 patients of the 38 patients receiving hGH, and this resolved with cessation of treatment. Over the 12 months of treatment there were 11 deaths (6 controls, 5 treated). Survival analysis, performed approximately 12 months following cessation of treatment, did not reveal a difference between the treatment and placebo group. The TQNE scores declined inexorably in both the control and treated group. Retrospective analysis of the TQNE data indicated a poor prognosis for patients who lost arm strength early. A correlation between the TQNE and MRC scores was evident at early stages of motor unit loss, less so when muscle weakness was advanced.

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.

Intrahypothalamic Mu-, not Delta- or Kappa-Opioid Receptor Activation Causes Growth Hormone Secretion

Abstract The possible effects of opioid receptor agonists on growth hormone (GH)-releasing factor or somatostatin neurons were examined by measuring the effects of localized intracerebral injections of mu-, delta- and kappa-selective agonists on GH secretion. Serial GH concentrations were measured in plasma in unanaesthetized male rats chronically prepared with venous and intracerebral cannulae, before and after treatment with bilateral intracerebral injections of opioid agonists in the preoptic anterior hypothalamic area and medial basal hypothalamus. In the medial basal hypothalamus, injections of the mu-agonist DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) caused dose-responsive increases in GH, the maximally effective dose being 0.001 nmoles. Injection of 10,000-fold higher doses of the delta-agonist DPDPE ([D-Pen, D-Pen]enkephalin) and the kappa-agonist U50,488H were also effective in stimulating GH secretion. In the preoptic anterior hypothalamic area, DAGO caused dose-responsive increases in GH, the maximally effective dose being 0.01 nmoles. U50.488H was ineffective at 1,000-fold higher doses while DPDPE was effective at 100- to 1,000-fold higher doses. We conclude that hypothalamic mu-opioid receptor activation on or near somatostatin or GH-releasing factor neurons causes GH secretion. Opioids capable of acting on other opioid receptors may also stimulate GH secretion, though only at doses that seem likely to affect mu-receptors.

Effects of complex experience on somatic growth and organ development in rats

Rats kept in complex environments (EC) show an array of brain changes relative to animals housed individually (IC). These effects have been explained as due to (a) information storage, (b) chronic stress that causes brain damage, or (c) neuroendocrine effects on brain maturation. Complex experience also affects somatic growth and organ development, and these may be related to the EC/IC brain differences. We have compared somatic growth and internal organs of 315 weanling and adult rats with various histories. (a) Young EC rats showed slower skeletal and visceral growth, while many brain components expand. (b) Although thymus and spleen were lighter in young ECs, immunocompetence was nonsignificantly (p less than .07) higher than in ICs. (c) Somatic growth of adult rats was slow and not very responsive to experience, whereas studies have shown EC/IC brain effects similar to those in young rats. (d) Males had slightly greater EC/IC somatic and visceral differences. (e) The stress index, adrenal weight, varied across age and experience, so chronic stress can not explain EC/IC brain differences. Training paradigms show brain changes similar to those from complex experience, occurring specifically with learning and in brain regions using the information. Learning and memory, therefore remain the best explanation of the EC brain effects.

Pulsatile secretion pattern of growth hormone in turkeys: effects of age and sex

Male and female turkeys were cannulated through the jugular vein, and blood samples were withdrawn remotely at 10-min intervals for a period of 8 hr at two points in the post-hatch growth phase (4 and 14 weeks of age). Growth hormone (GH) concentration was determined for each sample by radioimmunoassay using a recombinant chicken growth hormone preparation as standard. Data were evaluated for age- and sex-related differences. Four-week-old male and female turkeys displayed a pulsatile pattern of GH secretion. Growth hormone secretory profile characteristics differed significantly between ages with regard to overall mean, number of peaks, amplitude of peaks, interval between peaks, baseline, and total GH detected. Male four-week-old turkeys had a peak amplitude significantly greater than that of females of the same age. Older (14-week-old) male turkeys demonstrated a significantly greater number of GH secretory peaks than females during the 8-hr sampling period; however, overall it did not appear that the older birds had an organized pattern of GH secretion above baseline levels.

Comparison of the effects of pulsatile and continuous TRH infusion on TSH release in men

Pulsatile secretion of hypothalamic releasing factors modulates the release of pituitary hormones. To compare the effects of pulsatile and continuous administration of TRH on TSH secretion, we studied six healthy euthyroid 20- to 38-year-old men by obtaining blood samples every 20 minutes for 12 hours (8 AM to 8 PM) during five days of study. TRH was administered according to the following schedule: day 1 (no TRH, control); day 2 and subsequent day 3 (20 micrograms IV bolus of TRH every 96 minutes); 6 to 17 days rest; then consecutive days 4 and 5 (continuous infusion of 20 micrograms TRH/96 minutes) for 12 hours on and 12 hours off. The highest mean serum TSH levels occurred on the first day of pulsatile TRH. Serum TSH on pulsatile days 1 and 2 and continuous day 1 was significantly greater than on the control day. Similarly, the mean TSH on each day of pulsatile TRH was greater than the mean TSH on the corresponding days of continuous TRH administration. The highest serum T4 and T3 levels were observed on pulsatile day 2, suggesting that the decrease in serum TSH on this day was due to thyroid hormone negative feedback at the pituitary. The mean T4 and T3 values on continuous day 1 and 2 did not differ significantly, suggesting that other factors, including "down-regulation" of the pituitary TRH receptors by the continuous TRH infusion may be involved in the further decline of TSH levels on continuous day 2. We conclude that pulsatile TRH infusion releases more TSH, T3, and T4 than the corresponding amount of TRH administered continuously.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal control of muscle growth

In muscle of whole animals, pituitary growth hormone, the thyroid hormones, and insulin are major growth-promoting hormones, and the glucocorticoids have significant catabolic actions. At the cellular level the primary anabolic hormones for cultured myoblasts are the somatomedins (insulin-like growth factors) and fibroblast growth factor. In these cells physiological concentrations of growth hormone, thyroid hormones, and insulin have no growth-promoting effect; some of the reported actions of insulin probably result from cross-reaction with the somatomedin receptor. Results with purified proteins do not support the view that mitogens block myoblast differentiation; transforming growth factor-beta and interferon are nonmitogenic proteins that inhibit differentiation, insulin-like growth factors are mitogens that stimulate differentiation, and fibroblast growth factor is the only purified mitogen that inhibits differentiation. At least six serum-free media have now been devised for the growth of various kinds of muscle cells under closely defined conditions.

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.

Opiate participation in thyroid hormone regulation in domestic fowl

Administration of morphine sulfate (at 5 and 50 mg/kg) to immature domestic fowl lowered the circulating thyroxine (T4) concentration in a dose- and time-dependent manner. The inhibitory effect of morphine on T4 secretion was mediated via opiate receptors, since it was completely blocked in the presence of naloxone (50 mg/kg). The possibility that T4 secretion may be under tonic opioid inhibition was suggested by a stimulatory effect of naloxone on plasma T4 concentrations. Morphine sulfate (50 mg/kg) enhanced plasma triiodothyronine concentrations, an effect that was completely suppressed in the presence of naloxone. These results demonstrate differential effects of opiates on T4 and T3 concentrations and suggest that opiates tonically inhibit T4 release and accelerate its rate of peripheral monodeiodination.

Opiate inhibition of growth hormone secretion in young chickens

Plasma concentrations of growth hormone (GH) were decreased following the intravenous administration of morphine sulfate. Maximum inhibition of GH secretion was observed 40 min after morphine sulfate challenge. At this time, doses of morphine sulfate (at 5 mg and 50 mg/kg) reduced the GH concentrations by 86 and 90%, respectively, in comparison with those in the vehicle-injected controls. An opiate antagonist, naloxone, had no stimulatory effect on basal GH concentrations, but attenuated the GH response to morphine. Neither morphine nor naloxone had any significant effect on circulating luteinizing hormone (LH) levels. These results indicate an inhibitory opiate pathway in the control of GH release and demonstrate effects on GH and LH secretion contrary to those observed in mammalian species.