Naloxone inhibition of postprandial growth hormone releasing hormone-induced growth hormone release in obesity

Intimate associations between the endogenous opiate systems and the growth hormone-releasing hormone system in the human hypothalamus

Although it is a general consensus that opioids modulate growth, the mechanism of this phenomenon is largely unknown. Since endogenous opiates use the same receptor family as morphine, these peptides may be one of the key regulators of growth in humans by impacting growth hormone (GH) secretion, either directly, or indirectly, via growth hormone-releasing hormone (GHRH) release. However, the exact mechanism of this regulation has not been elucidated yet. In the present study we identified close juxtapositions between the enkephalinergic/endorphinergic/dynorphinergic axonal varicosities and GHRH-immunoreactive (IR) perikarya in the human hypothalamus. Due to the long post mortem period electron microscopy could not be utilized to detect the presence of synapses between the enkephalinergic/endorphinergic/dynorphinergic and GHRH neurons. Therefore, we used light microscopic double-label immunocytochemistry to identify putative juxtapositions between these systems. Our findings revealed that the majority of the GHRH-IR perikarya formed intimate associations with enkephalinergic axonal varicosities in the infundibular nucleus/median eminence, while endorphinergic-GHRH juxtapositions were much less frequent. In contrast, no significant dynorphinergic-GHRH associations were detected. The density of the abutting enkephalinergic fibers on the surface of the GHRH perikarya suggests that these juxtapositions may be functional synapses and may represent the morphological substrate of the impact of enkephalin on growth. The small number of GHRH neurons innervated by the endorphin and dynorphin systems indicates significant differences between the regulatory roles of endogenous opiates on growth in humans.

Effect of the opioid blockade on the feeding-induced growth hormone response to growth hormone-releasing hormone in women with polycystic ovary syndrome

OBJECTIVE

To investigate the effect of naloxone, an opioid receptor antagonist, on the release of growth hormone (GH) induced by the growth hormone-releasing hormone (GHRH) in normal-weight and obese women with PCOS in relation to feeding.

DESIGN

Prospective clinical study.

SETTING

Academic research center.

PATIENT(S)

Seventeen women with PCOS (10 who were normal weight and 7 who were obese) and 14 control women (7 who were normal weight and 7 who were obese).

INTERVENTION(S)

A GHRH test (50 microg i.v.) and, on a different day, a GHRH test during a naloxone infusion (1.6 mg/h) during fasting. The same tests were repeated after a standard meal.

MAIN OUTCOME MEASURE(S)

GH response to GHRH (expressed as the area under the curve [AUC]) in different experimental conditions.

RESULT(S)

All normal-weight women showed a significantly higher AUC-GH compared with obese women in the fasting state. Normal-weight controls had a decrease in GH response to GHRH after feeding, and naloxone did not reverse the decrease. In obese controls, feeding increased the GH response but naloxone induced a decrease in the AUC. In fasting, normal-weight women with PCOS, naloxone significantly decreased the AUC-GH; in these patients, food intake induced an inhibition of GH response to GHRH, reversed by naloxone infusion. In obese PCOS patients, GH levels did not increase significantly after GHRH stimulation, either in the fasting state or after a meal, and naloxone did not affect these responses.

CONCLUSION(S)

Factors other than obesity and insulin may be involved in disruption of GH secretion in women with PCOS.

Effect of feeding on growth hormone response to growth hormone-releasing hormone in polycystic ovarian syndrome: relation with body weight and hyperinsulinism

The plasma growth hormone (GH) response to direct stimulation with growth hormone-releasing hormone (GHRH) before and after a standard meal was investigated in 14 polycystic ovarian syndrome (PCOS) subjects. Data were compared with those obtained from 14 healthy normovulatory matched patients. All women underwent an oral glucose tolerance test (OGTT) (75 g) and basal plasma hormone concentrations were evaluated. On a different day all subjects had a GHRH test (50 microg GHRH) both before and after lunch randomly. In obese PCOS subjects the GH response to GHRH was blunted after a meal, while in obese control patients there was an enhanced response of GH to GHRH after a meal. Normal control subjects showed an inhibition of the GH response after feeding and lean PCOS subjects showed a trend toward an augmented GHRH related secretion after a meal significantly higher than normal controls (P < 0.05) but not significantly higher than the pre-prandial response. In conclusion, the data indicate in PCOS a derangement of GH secretion related to food ingestion; in particular obese PCOS patients did not exhibit any change of GH response after a meal compared with the paradoxical response observed in obese controls. Several other factors beyond body mass index and hyperinsulinism could be involved in these pathophysiological events.

Effect of opioid blockade on insulin and growth hormone (GH) secretion in patients with polycystic ovary syndrome: the heterogeneity of impaired GH secretion is related to both obesity and hyperinsulinism

OBJECTIVE

To investigate the involvement of opioid tone, obesity, and hyperinsulinemia in GH secretion in women with polycystic ovary syndrome (PCOS).

DESIGN

Controlled clinical study.

SETTING

Catholic University of Sacred Heart School of Medicine in Rome, Italy.

PATIENT(S)

Twenty-two patients with PCOS and 14 healthy, normally ovulating volunteers, matched for age and body mass index.

INTERVENTION(S)

Patients underwent a GH-releasing hormone (GHRH) test and an oral glucose tolerance test before and after 4-5 weeks of treatment with 50 mg/d of naltrexone.

MAIN OUTCOME MEASURE(S)

Serum concentrations of GH, insulin, glucose, steroids, and gonadotropins, as well as the GH area under the curve (AUC-GH) and the insulin area under the curve (AUC-I), were measured before and after naltrexone treatment.

RESULT(S)

In patients with PCOS, the administration of naltrexone increased the GH response to the GHRH test without interfering with the insulin response to the oral glucose tolerance test. However, the GH response to the GHRH test was improved significantly only in lean patients with PCOS, whereas obese patients with PCOS did not show any improvement in GH secretion. In obese control subjects, the treatment reduced plasma basal insulin concentrations and increased the AUC-GH, whereas in lean control subjects, the treatment reduced the GHRH-induced response. In normoinsulinemic patients with PCOS, the GH response to the GHRH test increased significantly after treatment, whereas the AUC-I was not affected. In hyperinsulinemic patients with PCOS, treatment with naltrexone significantly reduced the AUC-I, whereas the AUC-GH increased only in lean hyperinsulinemic patients with PCOS.

CONCLUSION(S)

Naltrexone treatment improves GHRH-induced GH secretion in patients with PCOS. However, this GH response is heterogeneously represented in relation to both obesity and hyperinsulinism.

Impact of long-term naltrexone treatment on growth hormone and insulin secretion in hyperandrogenic and normal obese patients

The growth hormone (GH) response to stimulation tests is impaired in obesity. Moreover, obese patients exhibit a "paradoxical" increase of GH to GH-releasing hormone (GHRH) stimulation after food ingestion; this paradoxical response is reversed by naloxone infusion. On the other hand, beta-endorphin seems to exert profound effects on insulin release. Recent studies also demonstrated an impairment of GH response to several stimuli in polycystic ovary syndrome (PCOS), a condition associated with obesity, hyperinsulinism, and insulin resistance. Chronic inhibition of opioid tone by the opioid antagonist naltrexone (NTX) is able to reduce the insulin response to an oral glucose tolerance test (OGTT) in hyperinsulinemic PCOS patients. Since insulin and GH may reciprocally influence their secretion and the opioid system may have a role in the pathogenesis of hyperinsulinemia and reduced GH secretion, we have explored the involvement of these neuroendocrine mechanisms in essential obesity and in obesity associated with hyperandrogenism by a long-term treatment with an opiate antagonist. We tested seven obese patients affected by PCOS, seven matched women with essential obesity (EO), and five non-obese control subjects. All patients, in the follicular phase, underwent an OGTT (75 g) and basal hormone assay. Two days later, patients were subjected to a GHRH test. The patients then had 4 weeks of treatment with NTX 50 mg/d. Following continuation of the treatment, OGTT and GHRH tests were repeated. Insulin-like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3) plasma concentrations were also determined in the basal condition before and after NTX treatment. NTX treatment reduced fasting insulin levels in patients with EO (P < .05) and restored a normal GH response to GHRH without affecting IGF-1 and IGFBP-3 levels. In PCOS subjects, NTX reduced the insulin response to a glucose load and failed to modify the blunted GH response to GHRH. Our data suggest a significant difference in opioid system function in PCOS and EO subjects, indicating a particular form of obesity in PCOS. The opiate antagonist treatment in EO may act through the reduction of negative insulin feedback on GH secretion. In PCOS patients, the failure to improve GH secretion in obese hyperandrogenized patients may be related to a high opioidergic tone or to the inhibitory predominance of other neurotransmitters.

Opioid dysregulation in anorexia nervosa: naloxone effects on preprandial and postprandial growth hormone response to growth hormone-releasing hormone

Previously, we have shown that in the opposite extremes of nutritional status, obesity and anorexia nervosa (AN), growth hormone (GH) response to growth hormone-releasing hormone (GH-RH) is not inhibited by the ingestion of a normal 800-cal meal consumed at lunch time (1 PM), which is at variance with results in normal subjects. However, in obese patients the postprandial increase in GH response to GH-RH is inhibited by an infusion of naloxone (NAL). In this study we have tested anorectic patients, performing the following tests at 1 PM: GH-RH test (50 micrograms IV) or, in a different day session, NAL (1.6 mg/h, starting 30 minutes before GH-RH) + GH-RH test (50 micrograms IV). The tests were performed in the following three different experimental conditions: (1) short-term fasting studies (lasting from breakfast), (2) long-term fasting studies (from midnight of the day before) and (3) postprandial studies (after a standard meal consumed 1 hour before the test). In AN, the GH response to GH-RH was not influenced by NAL infusion at 1 PM, in both short- and long-term fasting studies (short-term fasting: peak values after GH-RH alone, 26.5 +/- 6.5 ng/mL, during NAL, 28.0 +/- 3.3 ng/mL; long-term fasting: peak values after GH-RH alone, 32.2 +/- 6.8 ng/mL, during NAL, 30.6 +/- 4.0 ng/mL). A partial NAL-inhibitory effect was instead observed in postprandial studies, as evidenced by the calculation of areas under the curve ([AUCs] 1,662.1 +/- 90.0 after GH-RH alone v 1,090.5 +/- 245.4 ng/mL/h during NAL).(ABSTRACT TRUNCATED AT 250 WORDS)

Improving medical approaches to primary CNS malignancies--retinoid therapy and more

Successfully inducing differentiation in ectodermal diseases, retinoids harbour considerable therapeutic potential in the treatment of neuroectodermal-neuroepithelial malignancies. The principal tissue retinoid, retinoic acid, can be potently upregulated in vivo by a relatively specific catabolic inhibitor, R75251 (liarozole). Both substances have been given orally over 2 years in addition to standard treatment, and have been well tolerated. Corresponding closely to plasma retinoid levels, cutaneous side effects facilitate individual dosing. We evaluate this adjuvant retinoid approach and additional efforts to improve therapy of primary CNS malignancies, including the topical administration of retinoids in gamma linolenic acid.

Naloxone does not modify fenfluramine-induced prolactin increase in obese patients

OBJECTIVE

To evaluate whether the supposed physiological interaction between serotoninergic and opioidergic pathways in the modulation of PRL release is preserved in human obesity, a pathological condition in which these two systems are greatly impaired.

DESIGN

According to a single-blind randomized procedure, three tests were performed: test A (oral placebo + saline infusion over 5 hours), test B (fenfluramine, a well known serotoninergic drug, 60 mg orally + saline infusion over 5 hours) and test C (fenfluramine at the same dose + naloxone, an opiate receptor antagonist, infusion over 5 hours at a dose of 3 mg/h).

PATIENTS

Ten obese women (body mass index 34.4 +/- 2.3 kg/m2, mean +/- SE) and ten normal-weight sex and age-matched subjects (body mass index 22.3 +/- 2.4 kg/m2) volunteered for the study.

MEASUREMENTS

At each test, blood samples for PRL determination were collected in basal conditions (time 0) and every hour for 5 hours. Plasma PRL was determined by radioimmunoassay.

RESULTS

In controls, naloxone significantly reduced the clear-cut PRL increase induced by fenfluramine. In obese patients, serotoninergic stimulation caused an increment in PRL levels similar to that in the controls, but opioid receptor blockade by naloxone did not affect this response.

CONCLUSIONS

These findings confirm that there is a physiological relationship between the serotoninergic and the opioidergic systems in the control of PRL secretion and show that this interaction is not present in obese subjects. Our data provide indirect proof of the functional impairment of these two systems in human obesity.

Increase by naloxone of arginine vasopressin and oxytocin responses to insulin-induced hypoglycemia in obese men

The present study was carried out to establish whether the low arginine vasopressin (AVP) and oxytocin (OT) responses to insulin-induced hypoglycemia observed in obese men was due to alteration of the opioid control of posterior pituitary function. For this purpose, the AVP and OT releasing effect of insulin (0.15 IU/kg bw)--induced hypoglycemia was tested in eight normal weight men and in 10 age-matched obese subjects, without and with the previous treatment with the specific opioid receptor antagonist naloxone (3 mg in an iv bolus). In a control study, naloxone was given alone to the same subjects. Obese men showed similar basal glucose, AVP and OT levels, which remained unmodified after treatment with naloxone alone. Insulin induced a similar decrement of blood glucose levels in all subjects, with a nadir at 30 min. Plasma levels of AVP and OT rose strikingly in normal and obese subjects with mean peak responses at 30 min for AVP and at 45 min for OT. However, both AVP and OT responses were significantly lower in obese than in control subjects. Pretreatment with naloxone did not modify the AVP and OT responses to hypoglycemia in normal weight subjects, whereas it significantly enhanced both hormonal responses in obese subjects. In the presence of naloxone normal controls and obese subjects showed similar responses of both AVP and OT to hypoglycemia. These data indicate that an abnormal activity of endogenous opioids might account for the hypothalamic posterior pituitary dysfunction, which is responsible for the low AVP and OT responses to insulin-induced hypoglycemia in obesity.