Insulin Secretion in Hypothalamic Obesity: Diurnal Variation and the Effect of Naloxone

Insulin Secretion ,in Hypothalamic Obesity: Diurnal Variation and the Effect of Naloxone *Amy Lee, George A. Bray

Abstract

This paper has tested the hypothesis that patients with hypothalamic obesity have altered mechanisms controlling insulin secretion when compared to obese patients without hypothalamic injury. Fasting glucose and insulin values were significantly higher in the morning than in the afternoon in the six control obese patients, but there was no diurnal difference in the six patients with hypothalamic obesity (n=6). The control obese subjects showed a diurnal variation in glucose-stimulated insulin secretion, whereas the patients with hypothalamic obesity did not, suggesting that hypothalamic injury had destroyed diurnal rhythms. Naloxone, an opioid antagonist, acutely suppressed fasting insulin in the six patients with essential obesity but had little effect on fasting insulin in the three patients with hypothalamic obesity or in five normal-weight controls. Naloxone increased insulin sensitivity in the obese control patients, but did not affect either insulin secretion or insulin sensitivity in patients with hypothalamic obesity or in normal weight subjects. Our results sup- port the conclusion that hypothalamic obesity dis- rupts diurnal rhythms, with the suggestion that opioid peptides affect insulin secretion differently in patients with essential obesity as compared to normal weight subjects or those with hypothalamic obesity. (OBESITY RESEARCH 1993;1:449-458)

Introduction Hypothalamic obesity in human beings is a rare

syndrome (5,6), and most of our knowledge about its pathophysiology has come from experimental animals (7,8,22,35). Among the prominent features of this syn-

drome are hyperphagia, hyperinsulinemia, disturbances in the function of the autonomic nervous system, and disturbances in diumal rhythms. The experimental data, corroborated by the clinical findings, show reduced activity of the sympathetic nervous system and enhanced activity of the parasympathetic nervous system (7,8,22). These two derangements are thought to play a major role in the hyperinsulinemia of this syndrome (7,8,22,35). Blunted diurnal rhythm for plasma cortisol and for food intake are also observed in the hypothalamic obese animals (12).

Animal experiments (23,27,31,33) suggest that diurnal variations of food intake, pancreatic insulin secretion, and cortisol secretion are regulated by neural activity projecting from the circadian oscillator in the superchiasmatic nucleus (SCN) to the hypothalamus (31-33). In the present study we have examined the hypothesis that patients with the syndrome of hypothalamic obesity have lost their diurnal rhythm for insulin secretion.

In addition to the modulation of insulin secretion by the autonomic nervous system, recent reports have suggested that increased P-endorphin and/or hypersensitivi- ty to P-endorphin may play a role in regulation of pancreatic insulin secretion (13,15,17,19,28) and in the development of obesity. Individuals with essential obesity (15,16) are more sensitive to the suppression of insulin by an opioid antagonist, naloxone, than are lean controls. Opioid antagonists have been reported to restore insulin sensitivity in obese patients with the polycystic ovary syndrome and the syndrome of acan- thosis nigricans associated with severe insulin resistance (17). We have thus tested the hypothesis that hypothalamic obesity altered the release of insulin in response to blockade of opioid receptors by naloxone as compared to patients with essential obesity and lean controls.

Submitted for publication June 10,1993. Accepted for publication in final form April 26, 1993. From *Department of Medicine, University of Southern California L.A. County-USC Medical Center, Los Angeles. CA, and Pennington Biomedical Research Center. Baton Rouge, LA. Reprint requests to Dr. Bray, Pennington Biomedical Research Center, 6400 Perkins Road, Baton Rouge, LA 70808. Copyright 01993 NAASO.

Methods and Procedures Subjects

All subjects were admitted either as outpatients or inpatients to the Clinical Research Center of the University of Southern California. The protocols were

OBESITY RESEARCH Vol. 1 No. 6 Nov. 1993 449

Insulin Secretion in Hypothalamic Obesity, Lee et al.

approved by the Institutional Review Board. Informed consent was obtained after the nature and the potential risks of these studies were explained. The diagnosis of hypothalamic obesity was based on criteria described previously (5). These consisted of the following: 1) anatomical evidence of damage involving the hypothalamus: either a hypothalamic tumor, a pituitary tumor extending to the ventromedial region of the hypothalamus, or hypothalamic injury resulting from a surgical intervention demonstrated on a CT scan and/or an MRI study; 2) a history of rapid weight gain of substantial magnitude within a year following the onset of the CNS disorder or after an intracranial operation; and 3) clinical symptoms of a space-occupying lesion or endocrine dysfunction. Subjects with essential obesity were those with no evidence of hypothalamic dysfunction.

Carbohydrate intake was 250 g/day or more for at least two days before and during the entire study period. A weight-maintaining diet was prescribed with 50% carbohydrate, 30% fat, and 20% protein containing 25 kcalkg/day. Body weight changed by no more than 1 kg during the study. If carbohydrate intake was estimated to be less than 250 g/day, the patients were supple- mented to this level. The last meal before each test con- tained approximately 50 g of carbohydrate. Subjects with essential obesity were instructed not to take any medications for at least 48 hours prior to the test. Patients with hypothalamic obesity who had associated pituitary lesions were given replacement therapy of thy- roid hormone, hydrocortisone (or prednisone), or vaso- pressin (DDAVP) or combined premarin-provera regi- men by their referring physician were maintained on these same medications during the entire period of investigation. All premenopausal females with essential obesity were studied in the preovulatory phase of their menstrual cycle. There was no family history of diabetes or impaired glucose tolerance except patient A (Table 1) with hypothalamic obesity who had a slight increase in postprandial glucose level on two occasions.

For the study of diurnal insulin secretion, each subject underwent an intravenous glucose tolerance test (IVGTT) on two occasions, including one morning IVGTT (AM) and an afternoon IVGTT (PM) performed on a separate day. The AM and PM tests were performed in random order with an inter-test interval of at least 48 hours. The AM testing started between 0730 and 0830, and the PM testing started between 1730 and 1830, following a period of 10 to 11 hours with no food.

To examine the acute effect of naloxone on insulin secretion, five normal weight subjects, six patients with essential obesity and the three patients with hypothalamic obesity who volunteered, received 5 mg of naloxone (15) infused intravenously over 5 minutes with blood samples taken at -15, -10 & -5 minutes as baseline val-

ues, followed by samples taken at 5, 10, 15, 20, 30, 40, 50, 60, 75, 90 and 120 minutes. After one week of intramuscular treatment with naloxone, 1.6 mg twice a day (15), subjects were fasted overnight before the IVGTT and the oral glucose tolerance test (OGTT) were repeated one hour after the last intramuscular dose of naloxone. The IVGTT and OGTT were performed randomly on two separate days with an inter-test interval of 24 hours, for both the baseline period (before naloxone), and again the follow-up period (during naloxone treatment).

Procedures Intravenous glucose tolerance test: Subjects

remained resting throughout the 3 hour IVGTT. A 19- gauge scalp vein needle was inserted into an antecubital vein and the IV line was kept open with an infusion of 0.15 M NaCl containing heparin 100 U/ml. Three baseline samples were drawn at -15, -10, and -5 minutes before the administration of intravenous glucose, 0.33 g/kg over one minute starting at time 0, and 5 mgkg of intravenous tolbutamide (Upjohn Co, Kalamazoo, MI) given over 60 seconds starting at 18.5 minutes. Blood samples were obtained at 2, 4, 6, 8, 10, 12, 14, 16, 20, 30,40, 50, 60, 70, 80, 90, 100, 120, 140, 160, and 180 minutes after completion of the glucose infusion (3).

Oral glucose tolerance test: For the OGTT, three baseline samples were obtained at -15, -10 and -5 minutes before ingestion of 75 g of glucose in 225 ml of lemon-flavored solution at time 0. Blood samples were then taken at 15, 30,45,60, 90, 120, 150, and 180 minutes. For studies of IVGTT and OGTT, 2-3 ml of blood were collected in ice-cold tubes for measurement of the concentration of insulin and glucose. All samples were spun immediately and the serum was separated and kept frozen until assayed. Glucose was measured on the autoanalyzer in the core laboratory of the Clinical Research Center. Insulin was measured by radioim- munoassay (RIA) using a double antibody technique (30).

Calculations and Statistics For IVGTT, the computer program of minimal

model approach (MINMOD) developed by Bergman et al (3) was used to estimate the insulin sensitivity index (SI) and glucose effectiveness (SG). MINMOD ana- lyzes the IVGTT data and provides the values for the parameters SI and SG. SI represents the insulin-dependent increase in the glucose disappearance rate and measures the effect of insulin to accelerate glucose uptake in the liver and the peripheral tissue. SG measures the effect of glucose itself in enhancing its own disappearance, independent of an increment in plasma insulin.

Insulin responses were computed as the areas above

450 OBESITY RESEARCH Vol. 1 No. 6 Nov. 1993


baseline values for both the IVGlT and OGTT. Insulin areas between 0 and 10 minutes (1st phase), between 10 and 180 minutes (2nd phase), and between 0 and 180 minutes (total response) were used to quantitate insulin responses to either IV or oral glucose stimulation tests. All data were expressed as mean & SEM. Two-way or three-way analysis of variance (ANOVA) with one repeated measure, or Student's ?-test, was used for statistical analysis. Rank sum test was applied for non- parametric data. Statistical comparisons of each parameter were made either between the morning (AM) and afternoon (PM) tests or studies of the pre-naloxone and the post-naloxone treatment studies, within the group or among the groups of essential obesity, hypothalamic obesity, and/or the normal weight subjects.

Results Clinical Characteristics

The clinical profile of control obese subjects with essential obesity and the patients with hypothalamic

obesity is shown in Table 1. Controls for the study consisted of six women with essential obesity, age 26 to 44 years. A total of six patients with hypothalamic obesity (five females and one male, aged between 18 and 43 years) participated in the study of diurnal insulin variation.

The study with naloxone included the three female patients with hypothalamic obesity who volunteered (Table 1, D-F) along with six age-matched women with essential obesity age 30 and 46 (BMI: 38 & 3.4 kg/m2), and five age-matched normal-weight women (BMI: 26 f 0.4 kg/m2) with ages ranging from 22 to 49 years. The mean body weight of the group with essential obesity was slightly, but not significantly, higher than the group with hypothalamic obesity (BMI: 35 f 1.5).

Studies on Diurnal Insulin Secretion Within group comparison of fasting glucose showed

significantly higher values in the morning than in the

Table 1: Comparisons of fasting plasma glucose (mg/dl) and insulin pU/ml) in the morning (AM) and the afternoon (PM) and clinical characteristics in subjects with hypothalamic obese syndrome (HO) (n=6) and essential obesity

(EO) (n=6). ~

Essential Obesity

Subject Age Ht

(cm)

1. 26 150 2. 29 168 3. 35 164 4. 40 160 5. 42 173 6. 44 152

Mean 36 161 *SEM k3.0 3.6

Wt

(kg)

89.1 87.3 85 78.2 94.5 69.1

86.8 f2.2

40 31 32 31 32 38

34 f1.6

_ _ _ _ _ p< .05 _ _ _ _ _

Glucose AM

95 91 93 90

110 92

95 f 3 .O

Fasting PM

79 90 80 82 88 86

84 33.8

p< .05 _ _ _ _ _

Insulin AM

13.0 13.9 8.8

10.0 7.3

15.8

11.5 f l . 3

PM

12.9 10.5 8.3 5 .O 6.2

12.7

9.3 f1.4

Hypothalamic Syndrome

Subject Age Ht (cm)

A. 24 152 B. 18 168 C. 31 169 D. 43 152 E. ' 22 167 F. 40 183

Mean 30 165 fSEM k4.1 4.3

Wt

(kg)

80.9 94.5

104.8 74.5

104.5 112.3

95.2 3.1

Fasting Glucose BMI AM

(ks/m2)

35 138 34 81 37 75 33 87 38 79 34 85

Fasting Insulin PM AM

128 21.2 77 9 .o 86 4.9 86 6.8 92 20.9 90 4.5

PM

28.1 8.0 8.7 5.8 9.9 4.0

10.8 S . 6


insulin Secretion in Hypothalamic Obesity, Lee et al.

Table 2 Comparisons of diurnal glucose stimulated insulin release calculated as insulin areas above basal levels, and diurnal insulin sensitivity (SI) and glucose effectiveness (SG) estimated by MINMOD among subjects with

essential obesity (n=6) and hypothalamic obesity (n=6).

Essential Obesity

Subject Insulin Areas Above Basal Levels 0-180 min. 0-10 min. 10-180 min.

AM PM AM PM AM PM 1. 3158 1462 774 212 2384 1250 2. 7503 6776 1429 784 6074 5992 3. 3787 2426 749 486 3038 1940 4. 9187 5443 828 640 8359 4803 5. 4911 4107 380 752 4531 3355 6. 18660 15570 2485 2223 16175 13347

Mean 7868 5964 1108 850 6760 5115 fSEM f2352 f2077 f308 f288 f2079 f1796

P<.02 NS Pe.03

SI

AM PM 4.2 5.8 1 .o 1.3 3.6 3.1 1.1 2.1 3.7 4.6 0.8 0.7

2.4 2.9 f.6 f.8

NS

SG

AM PM 2.6 1.9 1.6 1.3 4.1 2.5 1.8 0.1 1.5 2.1 1.5 1.6

2.2 1.6 f .4 f .3

NS

Hypothalamic Obesity

Subject Insulin Areas Above Basal Levels 0-180 min. 0-10 min. 10-180 min.

AM PM AM PM AM PM A. 9222 9047 12 63 9209 8985 B. 1695 4538 200 200 1495 4338 C. 4019 5798 77 198 3942 5600 D. 1667 1307 299 44 1368 1263 E. 6844 13002 212 117 6632 12886 F. 4866 3367 544 476 4322 2981

SI SG

AM PM AM PM 0.6 0.6 0.2 0.7 9.4 3.2 1.7 1.3 4.8 4.7 1.7 1.5

10.4 7.8 1.4 1.8 1.5 1 .o 1.5 1.5 4.7 7.0 3.8 3.3

Mean 4719 6177 224 183 4495 5994 5.2 4.1 1.7 1.7 +SEM f1208 f1725 f76 f64 f1237 f1744 f1.6 f1.2 fo.5 fo.4

NS NS NS NS NS

Units: Integrated insulin area pU/ml*min SI SG 10-2 x min -1

10-4 x min -1 (pulml)

afternoon (95 ~t 3.0 vs. 84 ~t 1.8, Pc .05) in the group with essential obesity, whereas no diurnal difference was found in the hypothalamic obese group (F=7.81, P> .05) (Table 1). Similarly, within group fasting insulin levels were significantly higher in the morning in the essential obese group (11.5 k 1.3 vs. 9.3 k 1.4, Pc .05), but not in the hypothalamic obese group (Table 1). Comparison between these two groups of subjects showed no significant differences in either the fasting glucose or fasting insulin concentrations in either the morning or the afternoon.

Data on the response to the IVGlT tests are shown in Table 2. For both study groups, 2nd phase insulin secretion was significantly greater (F=17.28, Pc .002) than acute phase secretion for both the morning and

afternoon tests. Insulin responses in the morning and afternoon were significantly different (F=6.84, P c .005) between the essential obese group and hypothalamic obese group. In the group with essential obesity, the insulin area during the IVGTT was significantly greater (Figure 1) in the morning (AM) than the afternoon (PM) for both the total (0-180 min.) (P c .02), and the 2nd phase (10-180 minutes) (Pc .03) insulin secretion, and slightly higher in the acute phase response in the morning tests. In contrast, the group with hypothalamic obesity showed no significant differences at any time (Table 2, Figure 2). Between group comparisons showed that the acute phase insulin secretion in the morning was significantly greater (Pc .05) in the group with essential obesity.

452 OBESITY RESEARCH Voi. 1 No. 6 Nov. 1993


12.m

1o.m

r-l T

4 l T 1 -

Figure 1: Diurnal Variations in Insulin Secretion in Obese Controls. Total insulin release (0-180 min), 1st phase (0-10 min), and 2nd phase (10-180 min) insulin releases @U/mL x min) in response to intravenous glucose administration in subjects with essential obesity are shown. Data are mean f SEM for 6 subjects (* p <.02. ** P< .03).

The estimated insulin sensitivity index (SI lom4 min-l (uU/mL)) and the effect of glucose to enhance its own disposal (glucose effectiveness or SG 1 0 - h n - l ) in both groups of obese subjects are presented in Table 2. The formulas 1) and 2) were used for calculation of SI and SG (21).

= (Pl-X)G(t)-PlGb dt

Told (0-180 mi") 1s (0-10 mi") 2 3 (10-180 mi") Hyplhalamic Obesity

Figure 2: Diurnal Variations in Insulin Secretion in Patients with Hypothalamic Obesity. Total insulin release (0-180 min), 1st phase (0-10 min), and 2nd phase (10-180 min) insulin releases (pU/mL x min) in response to intravenous glucose administration in patients with hypothalamic obesity are shown. Data are mean f SEM for 6 patients.

r i T

Figure 3:Diurnal Insulin Sensitivity. Insulin sensitivity (SI, x10-4 pU/mUmin) in the morning (AM) and the afternoon (PM) is shown for subjects with essential obesity (n=6) and patients with hypothalamic obesity (n=6). Data are mean f

2, dt = PZX(t)+P3I(t)

G(t) and I(t) represent the time courses of glucose and insulin in plasma after the glucose injection. Gb is the basal glucose value. X(t) is the variable which describes the insulin effect on net glucose disappearance. Insulin sensitivity index (SI) is calculated as the ratio between P3 and P2, whereas the parameter of P1 describes the "glucose effectiveness" SG (3).

In the patients with essential obesity, neither SI (Figure 3) nor SG showed a significant diurnal difference, and the means went in opposite directions. Similarly, there was no significant diurnal variation in either the insulin sensitivity index (SI) (Fig. 3) or glucose effectiveness (SG) in patients with hypothalamic obesity. The mean value of SI (5.2 & 1.6) in the patients with hypothalamic obesity was twice that of the weight- matched essential obese subjects (2.4 * .6) (Table l), but this difference was not statistically significant (Table 2). The estimated values of SG showed no significant difference between the two groups of subjects (Table 2).

Effect of Naloxone After the first intravenous bolus of naloxone (5

mg), plasma insulin concentrations decreased slowly but steadily from the baseline in both the normal-weight group and in the patients with hypothalamic obesity. In the patients with essential obesity the drop in fasting insulin was more rapid and averaged 6.5 pU/ml with a nadir between 30 and 40 minutes after giving the nalox-


Insulin Secretion in Hypofhalamic Obesity, Lee et al.

r5

Figure 4:Effect of acute administration of naloxone to lean and obese subjects on plasma insulin concentrations. Insulin concentrations in groups of normal weight (n=5), essential obese (n=6), and hypothalamic obese subjects (n=3) are presented.as mean f SEM for 120 min.

one. The integrated decrease in insulin values from baseline during the 120-minute period after IV naloxone was significantly greater in the group with essential obesity (-474 f 128.7 (pU/mL) min) than the normal weight group (-101.1f26.7 @U/mL) min) or the three patients with hypothalamic obesity (-1 19k22.8 (pU/mL) min) (P< .05) (Figure 4).

Plasma glucose levels fell at each time interval after naloxone in the group with essential obesity (Figure 5). In the patients with hypothalamic obesity and in the normal-weight subjects glucose values rose after naloxone. The integrated glucose response was negative for the group with essential obesity [-142.4 f 4.3 (mg/dL) min], but was positive at [324 f 79.6 (mg/dL)] in the hypothalamic obese group and positive at [333 If: 90.2 (mg/dL) min] in the normal-weight group (Figure 5).

Baseline insulin concentration was reduced (insulin: 13.of4.1 vs. 11.6k3.9 pU/ml; Pc .05), and plasma glucose was slightly decreased (glucose: 92.7k2.4 vs. 88.7k2.2 mg/dL; P=.06) in the essential obese group, and there were no corresponding changes in the hypothalamic obese or the normal-weight group after the naloxone treatment.

After 7 days of treatment with naloxone there were significant differences between the essential obese group and the normal-weight group (F=12.79, Pc .01) in the response to intravenous glucose. As shown in Table 3, for both the baseline and post-naloxone studies,

-10

40 Bo 120 mlnules

Figure 5: Effect of acute administration of naloxone on plasma glucose concentrations in lean and obese subjects. Plasma glucose is presented for normal weight (n=5), essential obese (n=6), and hypothalamic obese subjects (n=3) as mean f SEM for 120 min.

insulin secretion in response to IV glucose was significantly greater between 10-180 minutes than between 0- 10 min in the study groups (F=9.80, P< .02). Total insuIin secretion (0-180 minutes) before naloxone treatment was significantly higher in the group with essential obesity (P< .05) than the normal-weight group (Table 3). In the essential obese group, total insulin release and 2nd phase insuIin secretion during the IVGTT tests were significantly reduced after seven days of intramuscular treatment with naloxone (total response: 12631 f 3740 vs. 8446 f 2830 @U/mL) min, Pc .05; 2nd phase: 11637 f 3585 vs. 7611 f 2637 (pU/mL) min, Pc .05). In the normal-weight group, no significant differences were detected at any time (Table 3). In the hypothalamic obese group, the observed values for insulin secretion during the IVG?T tests showed no significant effect of chronic naloxone treatment (Table 3). There was no significant change in insulin sensitivity (SI) after chronic intramuscular treatment with naloxone in the normal weight group (Table 3). The baseline insulin sensitivity (SI) before naloxone treatment in the normal-weight group was significantly higher than in the essential obese group (5.7 st .7 vs. 1.7 i- .5 lo4 min-' (pU/mL), Pc .01). Insulin sensitivity in the group with essential obesity was consistently higher after treatment with naloxone (pre-naloxone: 1.7 f .5 vs. post-naloxone: 3.3 f 1.3 lo4 min-l (uU/mL), Pc .05) (Table 3). SI values remained unchanged after treatment with naloxone in


Insulin Secretion in Hypothalamic Obesity, Lee et at.

Table 3: Effect of naloxone on insulin secretion

1. Essential Obesity Subject Insulin Responses

0-180 0-10 Naloxone Naloxone - + +

1) 5503 2101 1064 408 2) 18660 14081 2485 2035 3) 14696 7632 490 579 4) 5119 4013 481 534 5 ) 27092 19490 1139 1128 6) 4716 3357 308 326

Mean 12631 8446 995 835 fSEM f3740 f2830 3328 f266

Pc.05 ns

10-180 Naloxone

4439 1693 16175 12046 14206 7053 4638 3479

25953 18362 4408 3031

- +

11637 7611 f3585 f2637

P<.O5

SI

Naloxone

2.4 8.6 0.8 1.1 0.6 1.0 3.2 5.1 0.4 0.6 2.8 3.6

+

1.7 3.3 39.5 f1.3

P<.05

SG

Naloxone

2.4 3.4 1.5 1.9 2.0 2.9 2.2 1.6 2.8 1.7 1.1 1.8

+

2.0 2.2 f.3 f .3

ns

2. Hypothalamic Obesity Subject Insulin Responses SI SG

0-180 0-10 10-180 Naloxone Naloxone Naloxone Naloxone Naloxone - 4 . + + + +

E) 1667 2625 299 267 1368 2358 10.4 4.3 1.4 1.4 4322 4032 4.7 4.1 3.8 2.0

D> 6844 8674 212 487 6632 8187 1.5 1.9 1.5 1.5

F) 4866 4153 544 121

Mean 4459 5151 352 292 4107 4859 5.5 3.4 2.2 1.6 fSEM f1508 f1816 339 f106 f1523 f1733 f2.6 f.8 f.8 f .2

3. Normal Weight Group Subject Insulin Responses

0-180 0-10 Naloxone Naloxone

+ + 1) 1568 1119 224 219 2) 4995 6212 240 1292 3) 1735 1216 148 428 4) 3627 5153 810 1449 5 ) 2537 6709 125 477

10-180 Naloxone

1344 900 4755 4920 1587 788 2817 3684 2412 6232

+

SI

Naloxone

7.3 6.5 4.6 2.4 7.2 9.0 3.5 2.3 6.0 3.5

+

SG

Naloxone

2.6 3.4 2.4 2.1 6.4 3.8 2.0 2.5 1.7 3.4

+

Mean 2892 4082 309 773 2583 3305 5.7 4.7 3.0 3.0 fSEM f640f1216 f127 f249 f605 f1083 f.7 f1.3 f.9 f .4

ns ns ns ns ns

Insulin sensitivity (SI) and glucose effectiveness (SG) were estimated from MINMOD. Insulin areas calculated from areas about basal levels before naloxone (NAL), (NAL(-)), and during naloxone (NU(+)) treatment in subjects with essential obesity (n=6), and hypothalamic obesity (n=3).



lntergrated Insulin Area

(uU/ml) min

0 tsmti*1 o w q 40,000

30,000 f \ A N m l W e i @ t 0 H~p~lhlunicObniq

20.000 j -

Pre-Naloaone Post-Nalorone

Figure 6: Effect of treatment with naloxone for 7 days on total integrated insulin secretion in lean and obese subjects. The data before treatment is shown on the left, and the data after treatment on the right. Lean subjects are shown as A - - A . Patients with essential obesity as - and patients with hypothalamic obesity as o----0 . the hypothalamic obese group. There were no significant effects of treatment with naloxone on glucose effectiveness (SG) in any of the study groups, and no differences in the SG values between the study groups (Table 3).

Treatment with naloxone significantly attenuated the effect of oral glucose on insulin secretion. Total insulin secretion (0-180 minutes) after oral glucose ingestion was significantly lower after chronic naloxone treatment in the essential obese group (pre-naloxone: 14547 f 4676 vs. post-naloxone: 8666 f 3394 @U/mL) min, P< .05) than the other groups (Figure 6). The insulin secretion in response to oral glucose ingestion was low in the patients with hypothalamic obesity and showed a variable response to treatment with naloxone (pre-naloxone 5974 f 1354 vs. post-naloxone (7401 f 974 (pU/mL) min) (Figure 6). In the normal-weight group, the integrated insulin responses to oral glucose ingestion decreased in four of five subjects, but this change was not significant (9977 f 3114 @U/mL) minutes, pre-naloxone, vs. 7062 i 1945 (pU/mL) minutes, post-naloxone).

Discussion The hypothalamus plays a central role in the regula-

tion of body rhythms (32). Destruction of the suprachiasmatic nucleus (SCN) eliminates the diurnal rhythm for

food intake in rats, suggesting that this nucleus may be the oscillator for diurnal rhythms (31). Insulin injection into the SCN either stimulates or depresses sympathetic activity depending on the time of day when it is injected (33). Fibers from neurons in the SCN project to the hypothalamus from whence messages are transmitted to the pancreatic islet cells that can modulate insulin secretion (27). Damage to the ventromedial hypothalamic nucleus dampens corticosteroid and food intake rhythms (12,23), probably by interfering with the control of the ventromedial hypothalamus by interrupting connections from the SCN. In patients with hypothalamic obesity, this can be manifested by the loss of the regular patterns of eating and sleeping (5,6). The present studies suggest that, as in rodents, the hypothalamic centers in man may also modulate a diurnal control over insulin secretion which is lost in individuals with hypothalamic obesity.

The ventromedial hypothalamic nucleus and the adjacent arcuate nucleus are areas which control growth hormone secretion (29). Destruction of these hypothalamic areas in patients with hypothalamic obesity is often associated with impaired growth hormone secretion, including abnormal secretory patterns or frank stunting due to growth hormone deficiency (5). Nocturnal growth hormone secretion has been suggested as the underlying mechanism of the “Dawn Phenomenon” (28- 30), defined as an increase in insulin requirement or insulin secretion in the early morning hours needed to maintain normoglycemia in diabetic or normal individuals. Thus growth hormone might also be involved in the diurnal rhythm for insulin secretion. Treatment with oral methscopolamine, a cholinergic blocking agent which inhibits the sleep-related rise in growth hormone secretion, however, does not modify the diurnal differences in insulin secretion (25). Thus, the loss of growth hormone secretion in patients with hypothalamic obesity is not likely to explain their loss of diurnal rhythm in insulin secretion.

Both obese and nonobese individuals have impaired glucose tolerance in the afternoon as compared to the morning (1 1,20,21.25,26,37). Reduction in insulin secretion in response to glucose is thought to be the primary mechanism for this diurnal variation (11,20,21,25,26,37). The finding in the present study that baseline glucose and insulin levels after an overnight fast were higher than those in the afternoon among the control subjects with essential obesity is in agreement with a previous study (26). However, in the patients with hypothalamic obesity the diurnal periodici- ty of insulin and glucose concentrations was completely lost. No statistically significant diurnal effect on SI or SG was seen in this study, possibly due to the smaller number of subjects. The lack of a statistically significant diurnal rhythm in insulin sensitivity in the essential



obese subjects in this study as compared to previous ones xiuy be due to the small number of subjects.

The patients with essential obesity had a tendency toward higher values of SG (glucose effectiveness) in the morning, which is similar to those reported in the normal-weight subjects (26). In contrast to the normal- weight and the control obese subjects, no difference of SG between morning and afternoon (Table 2) could be detected in the patients with hypothalamic obesity, suggesting that the regulation of the diurnal rhythm for glucose effectiveness may also be mediated through the hypothalamus.

There was no difference in the integrated insulin secretion during either the study of diurnal variation or the study with naloxone between the groups with essential obesity and the group with hypothalamic obesity. The fact that patients with the hypothalamic obesity did not secrete more insulin than the control obese subjects could be due to the fact the patients were in the Clinical Research Center and were prescribed a weight-maintaining hospital diet. In previous studies (5) a higher insulin secretion was only demonstrated when the two groups of obese patients were fasted.

Endogenous opioid peptides may be involved in obesity and in the control of pancreatic insulin secretion (9,16). Obesity is associated with increased concentrations of plasma P-endorphin or P-lipotropin (17,18,28). Elevated plasma levels of P-endorphin have been found to correlate with body weight in obese women with hir- sutism (18). Givens et al (17) reported significantly reduced plasma insulin response and plasma insulin@- cose ratio after acute naloxone infusion and following five days of treatment with a low dose of nalmefene, a long-acting opiate antagonist. Similarly, Giugliano et al (15) reported a marked suppression in the basal insulin secretion after an acute naloxone injection (5 mg) in obese but not in lean subjects. They also demonstrated significantly reduced glucose-stimulated insulin response after intramuscular naloxone treatment without a major change in body weight, suggesting that decreased food intake by opioid antagonists may play some role in this response. In the present study naloxone decreased insulin levels in the patients with essential obesity, but not in the other groups. This suggests that opioid receptor activation may be involved in mod- ulating insulin release in essential obesity and this would be in harmony with the high circulating levels of P-endorphin. The lack of response to naloxone in patients with hypothalamic obesity suggests that opi- oids, including P-endorphin are not elevated and are not playing a role in the hyperinsulinemia of these patients.

Glucose concentrations were unchanged by naloxone (15,17), suggesting that blockade of opioid receptors improved tissue sensitivity to insulin. This finding

is in agreement with the diminution in insulin secretion and reduced insulin resistance after naloxone treatment. An earlier study of Kyriakides et al (24) showed that injection of low doses of naloxone decreased food intake in two of three patients with the hder-Willi syndrome. A recent study by Zipf et al on ten children with the Prader-Willi syndrome (38), however, failed to find an effect of naloxone on food consumption. Using a different protocol Atkinson (1) showed a dose-dependent decrease in food intake with naloxone treatment of patients with essential obesity, associated with slight, but not significant, decrease in insulin response to stimulation by a meal, suggesting that the effects of naloxone on food intake and on pancreatic insulin secretion may be mediated by different mechanisms. Data from the three hypothalamic obese patients showed minimal- ly reduced, totally unchanged or paradoxical response of insulin secretion to either the acute naloxone infusion or chronic naloxone treatment, which were in the opposite direction to those of our control obese subjects. Unfortunately only three of the original six patients with hypothalamic obesity were available for these studies. The failure of naloxone to modify insulin secretion in these three patients indicates that the decreased sensitivity of insulin secretion to the action of opioid antagonists in the patients with hypothalamic obesity may be due to a functional disruption of the neural pathways to the pancreatic islets. Thus, the loss of the opioid receptor control of insulin secretion by neural connections to the pancreas may involve endorphin-containing fibers. It is of interest that the patients with hypothalamic obesity responded similarly to the nonobese controls. This may imply that the hyperphagia of hypothalamic obesity produces a different pattern of response in the autonomic nervous system than in the control obese patients.

Acknowledgments The authors thank the nurses of the USC Clinical

Research Center for their help. Ms. Cutrer and Ms. Hodges provided essential secretarial service for which we are grateful.

Supported in part by the Clinical Research Center Branch of NIH (RR-43).

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Insulin Secretion in Hypothalamic Obesity: Diurnal Variation and the Effect of Naloxone

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