The Metabolic Syndrome Is Associated with Reduced ...

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The Journal of Clinical Endocrinology & Metabolism 91(2):718 –721 Copyright © 2006 by The Endocrine Society doi: 10.1210/jc.2005-1654

BRIEF REPORT The Metabolic Syndrome Is Associated with Reduced Central Serotonergic Responsivity in Healthy Community Volunteers Matthew F. Muldoon, Rachel H. Mackey, Mary T. Korytkowski, Janine D. Flory, Bruce G. Pollock, and Stephen B. Manuck Divisions of Clinical Pharmacology (M.F.M.) and Endocrinology (M.T.K.), Departments of Medicine and Epidemiology (R.H.M.), Graduate School of Public Health, and Behavioral Physiology Laboratory, Department of Psychology (S.B.M.), University of Pittsburgh, Pittsburgh, Pennsylvania 15260; Department of Psychiatry, Mount Sinai School of Medicine (J.D.F.), New York, New York 10029; and Rotman Research Institute, University of Toronto (B.G.P.), Toronto, Ontario, Canada M6A 2E1 Context: The pathobiology of the metabolic syndrome remains unclear. The central nervous system is likely to be involved via regulation of eating, physical activity, blood pressure, and metabolism.

abetes Federation (IDF) criteria. Insulin resistance was estimated by homeostasis model assessment.

Design, Setting, Participants: This was a cross-sectional study of 345 healthy community volunteers, aged 30 –55 yr, not taking medications for hypertension, lipid disorders, or diabetes.

Results: Compared with other individuals, persons meeting either NCEP or IDF criteria for the metabolic syndrome had lower mean prolactin responses (P ⬍ 0.05 for both). Using logistic regression, a decrease in prolactin AUC of 1 SD (⫺13.6 ng/ml䡠h) more than doubled the odds of having the metabolic syndrome (NCEP criteria: odds ratio, 2.38; 95% confidence interval, 1.14 – 4.97; P ⫽ 0.02; IDF criteria: odds ratio, 2.80; 95% confidence interval, 1.48 –5.30; P ⫽ 0.002). Finally, the prolactin AUC was negatively associated with insulin resistance (␤ ⫽ ⫺0.03, P ⫽ 0.02).

Outcome Measures: Central serotonergic responsivity was assessed with the iv citalopram challenge test. The serum prolactin area under the curve (AUC) over 150 min was calculated, and all analyses were adjusted for age, sex, plasma citalopram concentration, and baseline prolactin. The metabolic syndrome was defined according to the National Cholesterol Education Program (NCEP) and International Di-

Conclusions: Corroborating previous evidence, the metabolic syndrome was associated with diminished brain serotonergic activity as reflected in a comparative blunting of the prolactin response to a selective serotonergic challenge. This association may have implications for the etiology, prevention, and treatment of the metabolic syndrome. (J Clin Endocrinol Metab 91: 718 –721, 2006)

Objective: The objective of this study was to test the hypothesis that low central serotonergic activity is associated with the metabolic syndrome.

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Previously, we reported preliminary evidence that individuals with the metabolic syndrome exhibit lower CNS serotonergic responsivity than subjects without the metabolic syndrome (4). Interpretation of that study is limited by use of a convenience sample in which hypercholesterolemia was overrepresented and by the absence of measured waist circumference, a key indicator of central obesity and a core component of the metabolic syndrome. Also, serotonergic function was indexed by challenge with oral fenfluramine, which lacks optimal serotonergic specificity and is no longer available for clinical use. The current investigation sought to confirm and extend the association between central serotonin and the metabolic syndrome in a de novo sample of community volunteers not selected with respect to serum lipid concentration and in whom central serotonergic responsivity was assessed by the prolactin response to iv administered citalopram, a highly selective serotonin reuptake inhibitor.

HE METABOLIC SYNDROME is a multifactorial disorder encompassing abdominal obesity, altered glucose and lipid metabolism, and elevated blood pressure. The pathological basis for the co-occurrence of these diverse abnormalities remains unknown. Because the central nervous system (CNS) broadly coordinates body homeostasis via autonomic and neuroendocrine pathways and by regulation of diet and physical activity, the brain may figure prominently in the development of the metabolic syndrome (1, 2). For example, neurons releasing the neurotransmitter, serotonin (5-hydroxytryptamine; 5-HT), modulate a range of behaviors and autonomic functions relevant to metabolism and blood pressure regulation (3). First Published Online November 22, 2005 Abbreviations: AUC, Area under the curve; CNS, central nervous system; HOMA-IR, homeostasis model assessment of insulin resistance; 5-HT, 5-hydroxytryptamine; IDF, International Diabetes Federation; NCEP, National Cholesterol Education Program. JCEM is published monthly by The Endocrine Society (http://www. endo-society.org), the foremost professional society serving the endocrine community.

Subjects and Methods Subjects were participants in the University of Pittsburgh’s Adult and Human Behavior Project and were recruited from Allegheny County,

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Muldoon et al. • Metabolic Syndrome and Brain Serotonin

Pennsylvania, via mailed brochures. All were community volunteers, 30 –55 yr of age. Exclusion criteria included clinical history of atherosclerotic disease, cancer diagnosis or treatment within the past year, chronic liver or kidney disease, as well as use of cardiovascular, lipidlowering, diabetic, glucocorticoid, weight loss, or psychotropic medications. Women currently experiencing age-related menstrual period irregularities were also excluded. Of 345 subjects who completed protocol, 328 were included in analyses. Ten were excluded because they experienced adverse reactions during the citalopram challenge that confound interpretation of hormonal responses (vomiting, vasovagal syncope, or both), four because baseline prolactin levels were greater than 40 ng/ml, two because measurements of plasma citalopram concentrations were missing, and one outlier whose prolactin response exceeded that sample mean by more than 10 sd. The protocol was approved by the University of Pittsburgh institutional review board, and subjects gave informed consent.

Risk factor assessments Subjects arrived at the project office between 0730 and 1030 h after a 12-h overnight fast. After subjects rested in the seated position for at least 10 min, a trained staff member obtained two blood pressure measurements from the right arm using a mercury sphygmomanometer and a regular, large, or extra large adult cuff, according to the subject’s arm circumference. The two readings were averaged. Then, phlebotomy was performed along with measurement of height, weight, waist circumference at the umbilicus, and lean body mass (bioelectrical impedance body composition analyzer, Tanita Corp. of America, Inc., Arlington Heights, IL). Determinations of standard serum lipids, glucose, and insulin were performed by the Heinz Nutrition Laboratory, University of Pittsburgh Graduate School of Public Health, as previously described (5). Insulin resistance was estimated from the homeostasis model assessment (HOMA-IR) ⫽ serum insulin (␮IU/ml) ⫻ fasting blood glucose (mmol/ liter)/22.5 (6). The metabolic syndrome was defined by the criteria of both the National Cholesterol Education Program (NCEP) (7) and the International Diabetes Federation (IDF) (8). The NCEP metabolic syndrome is defined as three or more of the following criteria: 1) fasting serum glucose of 110 mg/dl or higher, 2) serum triglycerides of 150 mg/dl or higher, 3) serum HDL cholesterol less than 40 mg/dl in men or less than 50 mg/dl in women, 4) blood pressure of 130/85 mm Hg or higher, and 5) waist circumference greater than 102 cm in men or greater than 88 cm in women. According to the IDF, the metabolic syndrome is present in subjects with central obesity (defined as a waist circumference of at least 94 cm in men or 80 cm in women) and meeting at least two of the following criteria: 1) fasting serum triglycerides of 150 mg/dl or higher, 2) HDL cholesterol less than 40 mg/dl in men or less than 50 mg/dl in women, 3) blood pressure of 130/85 mm Hg or higher, and 4) fasting glucose of 100 mg/dl or higher.

Citalopram challenge test Central serotonergic responsivity was measured as the change in serum prolactin concentration after the administration of citalopram. Citalopram increases serotonergic neurotransmission by highly selective inhibition of serotonin reuptake. Stimulation of hypothalamic serotonin receptors promotes the pituitary release of prolactin, such that the increase in circulating prolactin concentration induced by citalopram provides an index of serotonergic responsivity in the hypothalamicpituitary axis (9). Citalopram-induced prolactin responses are dose dependent (10) and correlate with prolactin responses to fenfluramine (11). Participants reported to the University of Pittsburgh’s General Clinical Research Center between 1300 and 1500 h after a 2-h fast. Testing was conducted in the afternoon to minimize the influence of circadian variation on prolactin levels. Premenopausal women were scheduled during the early follicular phase (i.e. between 3 and 9 d after the onset of menses). An iv catheter was inserted into each forearm, one for blood sampling and one for drug infusion. After a 30-min adaptation period, blood samples for baseline prolactin were drawn at 5 and 1 min before citalopram infusion. Subjects then received citalopram by infusion pump over 30 min at a dose of 0.33 mg/kg lean body mass. Subsequent blood samples for prolactin determinations were obtained at 30, 45, 60,

J Clin Endocrinol Metab, February 2006, 91(2):718 –721

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75, 90, 105, 120, and 150 min after initiation of the drug infusion. Samples for citalopram concentration were obtained at 30, 45, 90, and 150 min after initiation of the drug infusion. All samples were centrifuged, separated, and stored at ⫺70 C until analysis. Methods for determining serum prolactin and plasma citalopram concentrations have been described previously (10).

Statistical analysis The citalopram-induced prolactin response area under the curve (AUC), expressed as nanograms per milliliter per hour, was calculated by trapezoidal integration, using prolactin concentrations measured from 0 –150 min after infusion Time-integrated citalopram exposure (citalopram AUC) was calculated in an analogous fashion from plasma citalopram concentrations. Age, sex, citalopram AUC, and baseline prolactin were used as covariates in all analyses. Analysis of covariance was used to calculate the adjusted mean prolactin AUC levels for participants with vs. those without the metabolic syndrome. Logistic regression models were used to quantify the odds of having metabolic syndrome associated with a 1 sd decrease in prolactin AUC. Finally, we used multiple linear regression models to evaluate whether lower prolactin AUC was associated with higher insulin resistance, as estimated by log(HOMA-IR). Potential sex interactions were evaluated for all models.

Results

Characteristics of the study participants are summarized in Table 1. In general, subjects were normotensive with normal lipid profiles, but tended to be overweight and to have TABLE 1. Subject characteristics Characteristics

Valuea

No. of subjects Age (yr) Sex, no. (%) Male Female Race, no. (%) White Black Other Waist circumference (cm) Body mass index (kg/m2) Systolic blood pressure (mm Hg) Diastolic blood pressure (mm Hg) Total cholesterol (mg/dl) HDL cholesterol (mg/dl) LDL cholesterol (mg/dl) Triglycerides (mg/dl) Median Range Glucose (mg/dl) Median Range Insulin (mU/liter) Median Range HOMA-IR Median Range Citalopram AUC (ng/ml䡠h) Baseline prolactin (ng/ml) Median Range Prolactin AUC (ng/ml䡠h) Median Range

328 44.4 ⫾ 6.8

Value in SI units

184 (56) 144 (44) 264 (80) 59 (18) 5 (2) 92.4 ⫾ 14.7 27.3 ⫾ 5.4 116.5 ⫾ 12.2 76.3 ⫾ 8.6 199.1 ⫾ 34.3 52.8 ⫾ 14.9 122.5 ⫾ 31.0

5.15 ⫾ 0.89 1.37 ⫾ 0.39 3.17 ⫾ 0.80

96 39 –980

1.08 0.44 –11.06

95 59 –291

5.27 3.27–16.15

11.8 1.9 – 45.2 2.7 0.4 –16.7 56.8 ⫾ 14.0 9.5 2.4 –36.7 26.9 6.2– 89.1

HDL, High-density lipoprotein; LDL, low-density lipoprotein; SI, Syste`me International. a Values are the mean ⫾ SD unless indicated otherwise.


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mildly elevated fasting glucose concentration. Six (2%) were diabetic (fasting glucose, ⬎125 mg/dl). According to the NCEP criteria, 12% (n ⫽ 39) of this community-based sample had the metabolic syndrome. Based on the IDF definition, the disorder was present in 23% (n ⫽ 74) of subjects. Citalopram exposure (citalopram AUC) did not vary with body mass index. Compared with those without the metabolic syndrome, the adjusted mean prolactin response was significantly lower among subjects with the metabolic syndrome, whether defined by NCEP (P ⫽ 0.02) or IDF (P ⫽ 0.001) criteria (Fig. 1). In addition, we found that a decrease in prolactin AUC of 1 sd (⫺13.6 ng/ml䡠h) more than doubled the odds of having the metabolic syndrome (for NCEP criteria: odds ratio, 2.38; 95% confidence interval, 1.14 – 4.97; P⫽ 0.02; for IDF criteria: odds ratio, 2.80; 95% confidence interval, 1.48 –5.30; P ⫽ 0.002). Model fit was adequate, as assessed by Hosmer-Lemeshow criteria. These associations persisted upon adjustment for body weight and did not differ by gender. Finally, a lower prolactin response was associated with a higher insulin resistance, estimated by HOMA-IR (␤ ⫽ ⫺0.03; P ⫽ 0.02). Discussion

Corroborating previous evidence, this investigation found that both the metabolic syndrome and estimated insulin resistance are associated with blunted central serotonergic responsivity. Central serotonergic function was measured by the prolactin response to the selective serotonin reuptake inhibitor, citalopram, in 328 generally healthy, community volunteers. Previously, we found in a smaller and somewhat less representative sample that individuals with the metabolic syndrome had blunted prolactin responses to fenfluramine, a serotonin-releasing agent (4). Horacek et al. (12) also reported that prolactin responses to fenfluramine varied inversely with insulin sensitivity measured by euglycemic clamp in 19 healthy volunteers. The principal cell bodies of the 5-HT system are con-

FIG. 1. Mean citalopram-induced prolactin responses in subjects with and without the metabolic syndrome. Values are adjusted for age, gender, baseline prolactin concentrations, and plasma citalopram concentrations.


centrated in the Raphe´ nuclei of the brainstem and project diffusely to the cerebral cortex, hypothalamus, and major autonomic nuclei. The family of 5-HT receptor subtypes is the largest of all known neurotransmitters, adding substantially to the complexity of serotonergic neuropharmacology. In animal and human studies, drugs that either promote or inhibit serotonergic neurotransmission generally or stimulate or antagonize select 5-HT receptor subtypes can alter mood, eating behavior, locomotor activity, body temperature, and body weight and can raise or lower blood pressure (3). Mice lacking the 5-HT2C receptor eat excessively, gain weight, develop insulin resistance during adulthood, and serve as a useful model of human obesity (13). The selective 5-HT reuptake inhibitors and releasing agents improve insulin sensitivity (14 –16). In contrast, the atypical antipsychotic agents inhibit serotonergic neurotransmission and often induce insulin resistance and diabetes (17). The assessment of central serotonergic function based upon neuroendocrine responses to pharmacological probes, although indirect, has the advantages of being minimally invasive and generally well tolerated. The specificity of the tests is supported by evidence that the prolactin response is inhibited by 5-HT receptor antagonists and, in rats, by lesioning of the Raphe´ nuclei (18, 19). As previously reported, prolactin responses to fenfluramine and citalopram in humans correlate with one another, are dose-related, and appear to reflect a stable characteristic of individuals (10, 11). Also, our adjustment for any covariation in baseline prolactin concentration tends to exclude variability in dopaminergic and lactotroph function as an explanation for individual differences in challenge-induced responsivity. Finally, variability in citalopram exposure was addressed by iv administration (to maximize bioavailability), body size-adjusted dosing, and statistical adjustment of drug concentrations. Although we hypothesize that altered central serotonergic function is causally related to the metabolic syndrome, our data are cross-sectional and only help to establish that blunted serotonergic responsivity is observed in individuals with the metabolic syndrome and insulin resistance. Altered serotonergic function may simply be another abnormality induced by an as yet undiscovered cause of the metabolic syndrome or may even be a result of insulin resistance. Indeed, observations in animal models suggest that diabetes can disturb CNS serotonin synthesis and decrease receptor affinity (20). The now-replicated association between serotonergic responsivity and the metabolic syndrome invites additional research. Longitudinal studies could determine whether blunted central serotonergic function precedes the development of the metabolic syndrome. Additionally, laboratory as well as clinical research could explore the autonomic, neuroendocrine, or behavioral factors mediating such a relationship and also assess whether certain pharmacological manipulations altering brain serotonergic activity affect one or more components of the metabolic syndrome.



Acknowledgments Received July 25, 2005. Accepted November 14, 2005. Address all correspondence and requests for reprints to: Dr. Matthew F. Muldoon, Behavioral Physiology Laboratory, 506 OEH, 4015 O’Hara Street, University of Pittsburgh, Pittsburgh, Pennsylvania 15260. E-mail: [email protected]. This work was supported by National Institutes of Health Grants PO1-HL-40962, K24-MH-065416 and MO1-RR-000056.

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JCEM is published monthly by The Endocrine Society (http://www.endo-society.org), the foremost professional society serving the endocrine community.
