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Endocrinology Vol. 145, No. 11 5252-5258
Copyright © 2004 by The Endocrine Society

Triiodothyronine Stimulates Food Intake via the Hypothalamic Ventromedial Nucleus Independent of Changes in Energy Expenditure

Wing May Kong, Niamh M. Martin, Kirsty L. Smith, James V. Gardiner, Ian P. Connoley, David A. Stephens, Waljit S. Dhillo, Mohammad A. Ghatei, Caroline J. Small and Stephen R. Bloom

Department of Metabolic Medicine (W.M.K., N.M.M., K.L.S., J.V.G., W.S.D., M.A.G., C.J.S., S.R.B.), Imperial College Faculty of Medicine at Hammersmith Campus, London W12 0NN, United Kingdom; Department of Physiology (I.P.C.), St. George’s Hospital Medical School, University of London, London SW17 0RE, United Kingdom; and Department of Mathematics (D.A.S.), Statistics Group, Imperial College, South Kensington Campus, London SW7 3AZ, United Kingdom

Address all correspondence and requests for reprints to: Professor S. R. Bloom, Department of Metabolic Medicine, Imperial College Faculty of Medicine at Hammersmith Campus, Du Cane Road, London W12 0NN, United Kingdom. E-mail: s.bloom{at}imperial.ac.uk.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Increased food intake is characteristic of hyperthyroidism, although this is presumed to compensate for a state of negative energy balance. However, here we show that the thyroid hormone T3 directly stimulates feeding at the level of the hypothalamus. Peripheral administration of T3 doubled food intake in ad libitum-fed rats over 2 h and induced expression of the immediate early gene, early growth response-1, in the hypothalamic ventromedial nucleus (VMN), whereas maintaining plasma-free T3 levels within the normal range. T3-induced feeding occurred without altering energy expenditure or locomotion. Injection of T3 directly into the VMN produced a 4-fold increase in food intake in the first hour. The majority of T3 in the brain is reported to be produced by tissue-specific conversion of T4 to T3 by the enzyme type 2 iodothyronine deiodinase (D2). Hypothalamic D2 mRNA expression showed a diurnal variation, with a peak in the nocturnal feeding phase. Hypothalamic D2 mRNA levels also increased after a 12- and 24-h fast, suggesting that local production of T3 may play a role in this T3 feeding circuit. Thus, we propose a novel hypothalamic feeding circuit in which T3, from the peripheral circulation or produced by local conversion, stimulates food intake via the VMN.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
IN HYPERTHYROIDISM, ELEVATED PLASMA levels of thyroid hormones increase energy expenditure and decrease body weight (1). It is widely assumed that the characteristic increased appetite of hyperthyroidism is compensatory for this state of negative energy balance. Interestingly, about 5–10% of hyperthyroid individuals have a sufficiently increased appetite to gain weight despite the catabolic thyrotoxic process (2), suggesting that thyroid hormones may directly stimulate feeding.

T3 is the biologically active thyroid hormone. Tissue T3 concentrations are controlled at the cellular level and may not reflect plasma thyroid hormone concentrations (3). Thus, within the rat central nervous system (CNS), physiological levels of T3 are largely dependent on cellular uptake and intracellular deiodination of T4 to T3 by type 2 iodothyronine deiodinase (D2) (4, 5). Within the hypothalamus, D2 mRNA (6, 7) and activity (8) are concentrated in the periventricular region of the third ventricle, the arcuate nucleus (ARC) and median eminence. D2 mRNA is localized to tanycytes, specialized ependymal cells lining the third ventricle (9). Tanycytes have long cytoplasmic processes projecting to several hypothalamic nuclei, including the ARC and the ventromedial nucleus (VMN) (10). The function of this local hypothalamic T3 production is unknown.

The hypothalamus plays an essential role in the regulation of energy homeostasis, integrating signals from other areas of the CNS and the periphery. Several hypothalamic nuclei have been implicated in the regulation of food intake and energy balance including the ARC, paraventricular nucleus (PVN), and VMN. Within the ARC two important neuronal populations have been identified: appetite inhibiting proopiomelanocortin (POMC)-expressing neurons and appetite-stimulating neuropeptide Y (NPY) and agouti-related protein (AgRP)-coexpressing neurons (11). Both of these neuronal populations project to the PVN. The VMN was labeled as a satiety center more than 50 yr ago when studies demonstrated that lesioning the VMN resulted in hyperphagia and weight gain (12). However, more recent studies suggest that the role of the VMN in appetite regulation is more complex. The VMN receives and sends out extensive projections to other regions of the hypothalamus including the PVN and dorsomedial hypothalamus and may modulate the release of orexigenic signals from these hypothalamic nuclei (13).

The effects of overt hyperthyroidism and hypothyroidism on energy homeostasis and appetite have been well established in rodents and man (14, 15, 16). However, such states are associated with marked effects on behavior and metabolism. Therefore, it is not possible to infer from these studies a physiological role for thyroid hormones in the regulation of food intake. To investigate a role for T3 in the physiological regulation of food intake, we studied the effects of peripheral and CNS administration of T3, using doses of T3 that did not elevate plasma free T3 (fT3) levels outside the normal range (referred to as low-dose T3 for the remainder of the paper). We examined the effects of low-dose T3 on food intake, energy expenditure, and behavior. In addition, we studied diurnal variation and the effect of short-term fasting on hypothalamic D2 mRNA expression.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Animal care and maintenance
Male Wistar rats (ICSM, London, UK) (200–250 g) aged 7–8 wk were maintained under standardized barrier conditions (21–23 C, lights on 0700–1900 h) and fed ad libitum RM1 diet (SDS Ltd., Witham, UK) unless described otherwise. All animal procedures were conducted under the British Home Office Animals (Scientific Procedures) Act (1986). All injections were administered in the early light phase (0700–0900 h).

T3 preparation for peripheral administration
For peripheral (sc) injections, T3 was prepared as T3 (Sigma, Dorset, UK) dissolved in absolute ethanol and emulsified in safflower oil (1:10) (volume 0.1 ml). Controls received vehicle (emulsion alone). Before all studies, animals received two sham sc saline injections.

Peripheral administration of T3 and food intake
In the acute study of the effects of T3 on food intake, animals received either sc T3 (1.1, 2.3, or 4.5 nmol/kg) or sc vehicle (n = 12 per group). Food was weighed at 2, 4, 8, and 24 h post injection. In a separate study, animals received sc T3 (4.5 nmol/kg) or vehicle (controls) and were killed 2 h post injection by decapitation for collection of trunk blood as described (17). In the chronic study, animals (n = 12 per group) were injected with sc T3 (4.5, 9, or 75 nmol/kg) or sc vehicle daily for 5 d. On d 5 trunk blood was collected. Brains were removed and hypothalami dissected out and snap frozen for subsequent measurement of neuropeptide mRNA expression by RNase protection assay (RPA) (described below). Interscapular brown adipose tissue (BAT) and epididymal white adipose tissue were also collected and then weighed and frozen. BAT uncoupling protein 1 (UCP-1) mRNA expression was measured by RPA.

RIA
Plasma TSH levels were assayed using methods and reagents (kindly provided by A. Parlow, National Institute of Diabetes and Digestive and Kidney Diseases, National Hormone and Pituitary Program, Torrance, CA) as previously described (17). Plasma leptin (Linco Research, St. Charles, MO), fT3 and free T4 (fT4) (Diagnostic Products Corp., Los Angeles, CA) were measured using commercial RIAs following the manufacturer’s instructions.

Behavioral study
Animals received a sc injection of T3 (4.5 nmol/kg) or vehicle at time 0 (n = 16 per group). Behavioral patterns were monitored continuously from 30 to 120 min by observers blinded to the experimental treatment. Behavior was classified into three different categories [adapted from Abbott et al. (18)]: feeding; active nonfeeding behavior (drinking, grooming, burrowing, rearing, locomotion); and inactive nonfeeding behavior (sleeping and still). These methods have previously been used to demonstrate abnormal behaviors after CNS administration of peptides (18). During the analysis, each rat was observed for 12 sec every 5 min. This 12-sec period was subdivided into three and the behavior of the rat during each section of the time period scored (816 total observations per rat).

Oxygen consumption (VO2) studies
Indirect calorimetry was used to measure VO2 as an indirect measurement of energy expenditure as previously described (19). VO2 was determined in closed circuit respirometers maintained at thermoneutral temperature for rats (29 C). Animals were acclimatized to the calorimetry chamber for 2 h before injection (n = 8 per group) and injected with T3 or vehicle at time 0. VO2 was measured for 240 min after treatment. To study the acute effects of T3 on VO2, calorimetry was performed after a single sc injection of 4.5 nmol/kg T3 or vehicle. This was repeated after a single ip injection of the ß3-adrenoceptor agonist BRL 35135 (40 µg/kg) as a positive control (20). To study the effects of chronic T3 administration on VO2, animals received 5 d of once-daily sc injections of T3 (4.5 or 75 nmol/kg) or vehicle, and calorimetry was performed on d 5.

RPA
Total RNA was extracted from hypothalami and BAT collected from animals after a single sc T3 injection or 5 d of once-daily T3 injections (4.5 nmol/kg) (n = 12 per group) using Tri-Reagent (Helena Biosciences, Sunderland, UK) following the manufacturer’s protocol. Hypothalamic AgRP, POMC, NPY, and D2 (all 5 µg) and BAT UCP-1 (0.25 µg) mRNA were quantified by RPA (21) (RPA III kit, Ambion Inc., Austin, TX) using in-house probes (accession no.: UCP-1 M11814, AgRP U89484, POMC NM_139326, NPY NM_012614, and D2 NM_031720). Rat ß-actin was used as an internal control (Ambion). RNA was hybridized overnight and separated on a 5% polyacrylamide gel. The dried gel was exposed to a PhosphorImager screen overnight and protected RNA hybrids quantified using ImageQuant software (Molecular Dynamics, Sunnyvale, CA). For each neuropeptide, the ratio of the OD of the band of neuropeptide mRNA to that of ß-actin was calculated and expressed in relative units (RU) (22).

Immunocytochemistry (ICC)
Early growth response (Egr)-1 immunoreactivity (IR) was measured by ICC in paraffin-embedded, paraformaldehyde-fixed male Wistar rat brains collected 2 h after sc administration 4.5 nmol/kg T3 or vehicle (23). Nonspecific binding was blocked using normal donkey serum and sections incubated in rabbit anti Egr-1 antibody (Santa Cruz Biotechnology, Santa-Cruz, CA) diluted 1:1000 at 4 C overnight. Slides were incubated for 30 min in biotinylated donkey antirabbit secondary antibody (1:50) (Jackson ImmunoResearch Laboratories Inc., West Grove, PA) followed by a 30-min incubation in ABC-horseradish peroxidase (1:100) (Dako Cytomation, Glostrup, Denmark). The antigen-antibody complex was visualized with 3,3'-diaminobenzidine in 0.01% hydrogen peroxide. Every fifth section was stained with hematoxylin and eosin to allow identification of the relevant hypothalamic nuclei. Egr-1-IR was counted by an observer blinded to the experimental treatment using an Eclipse E800 microscope (Nikon, Tokyo, Japan) and Image Pro-Plus software (version 4.5, MediaCybernetics, Silver Spring, MD).

Intranuclear cannulation and injection of T3
Animal surgical procedures and handling were carried out as previously described (18). Animals were anesthetized by ip injection of a mixture of Ketalar (ketamine HCl 60 mg/kg; Parke-Davis, Pontypool, UK) and Rompun (xylazine 12 mg/kg; Bayer UK Ltd., Bury St. Edmonds, UK) and placed on a stereotaxic frame (David Kopf Instruments, Tujunga, CA). Permanent 26-gauge stainless steel guide cannulae (Plastics One Inc., Roanoke, VA) were stereotactically placed into the hypothalamic VMN or ARC as previously described (24). Due to its low solubility, T3 was dissolved in 5% ethanol. We have previously shown that CNS injection of up to 70% ethanol does not produce behavioral abnormalities (25). Animals received 0.5, 1, 5, and 50 pmol T3 or vehicle administered in 1 µl. Food was weighed at 1, 2, 4, 8, and 24 h post injection. Cannula placement was verified at the end of the study by the injection of black ink (26). Data from an animal were excluded if its injection site extended more than 0.2 mm outside the intended hypothalamic injection site or if any ink was detected in the cerebral ventricular system.

Measurement of D2 mRNA expression
For the fasting study, hypothalami were dissected and collected from rats either fed ad libitum (control) or fasted for 12 or 24 h (n = 10 per group). For the diurnal variation study, rats were killed at nine time points (n = 10 per group) throughout a 24-h period. Hypothalamic D2 mRNA levels were measured using RPA (as described).

Statistical analysis
Statistical analyses were carried out in collaboration with David Stephens (Department of Mathematics, Imperial College, London, UK). Values are presented as the mean ± SEM unless otherwise stated. Behavioral data are presented as percent of total observations ± estimated SE. A Bayesian analysis was used to compare the relative probabilities of each behavioral profile in each group. For VO2 studies, groups of data were compared using a two-way analysis of covariance (S-plus, Seattle, WA). For Egr-1 studies (data expressed as median values with interquartile ranges), a Wilcoxon nonparametric test was used. In the remaining studies, comparisons were made using ANOVA, with post hoc Fisher’s least significant difference method (Systat, Evanston, IL). Normal ranges (mean ± 2 SD) for thyroid hormones were calculated within our laboratory from euthyroid control rats. Polynomial regression analysis was used for studying D2 mRNA diurnal rhythm (version 2.03, SigmaStat, Chicago, IL). P < 0.05 was considered significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Acute effects of peripheral T3 on food intake, plasma hormones, and behavior
Two hours post sc injection of T3 (4.5 nmol/kg), food intake increased by 140% [1.2 ± 0.3 (T3) vs. 0.5 ± 0.1 g (control), P < 0.05] (Fig. 1AGo). This stimulatory effect persisted for 8 h [5–8 h: 1.3 ± 0.4 (T3) vs. 0.6 ± 0.1 g (control), P < 0.05] (Fig. 1BGo). Lower doses of T3 had no effect on feeding. Twenty-four hours post injection, there was no difference in food intake between T3-treated and control animals.



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  		FIG. 1. Peripheral administration of T3 increases food intake. Ad libitum-fed rats received a single sc injection of T3 at the doses indicated (nanomoles per kilogram) or vehicle (V; control) (n = 12/group). Food intake A was 0–2 h and B was 5–8 h post injection.*, P < 0.05 vs. control. Results are mean ± SEM.

 
Plasma leptin levels were unchanged 2 h after T3 treatment (4.5 nmol/kg) [2.8 ± 0.2 (T3) vs. 2.9 ± 0.3 ng/ml (control)]. Plasma fT3 was 46% higher in the T3-treated group, compared with controls [3.5 ± 0.2 (T3) vs. 2.4 ± 0.2 pmol/liter (control), P < 0.001] yet remained within the normal range (1.2–3.6 pmol/liter). There was no difference in plasma fT4 [15.3 ± 0.6 (T3) vs. 15.7 ± 0.5 pmol/liter (control)] or TSH [2.9 ± 0.3 (T3) vs. 3.5 ± 0.4 ng/ml (control)] in T3-treated animals, compared with controls.

T3-treated (4.5 nmol/kg) animals spent over twice as much time feeding than control animals [5.3 ± 0.8 (T3) vs. 2.2 ± 0.5% total observations (control), P < 0.005]. Notably, there were no significant differences in active nonfeeding behaviors (including locomotion) between the two groups [15.6 ± 1.3 (T3) vs. 15.3 ± 1.3% total observations (control)]. No adverse behaviors were observed at any time.

Chronic effects of peripheral T3 on food intake, body weight, and body adiposity
T3 was administered (sc) once daily for 5 d at a dose of 4.5, 9, or 75 nmol/kg. Mean daily food intake on d 5 was significantly greater in the 4.5 nmol/kg T3 and 9 nmol/kg T3 groups [30.1 ± 0.4 4.5 nmol/kg (T3), 29.1 ± 0.7 (9 nmol/kg) and 27.3 ± 0.4 g (control), P < 0.005, 4.5 nmol/kg T3 vs. control, P < 0.05, 9 nmol/kg T3 vs. control] (Fig. 2Go A). the dose 4.5 nmol/kg T3 also significantly increased cumulative food intake [d 0–5: 116.0 ± 1.7 (T3) vs. 107.6 ± 1.9 g (control), P < 0.05] (Fig. 2BGo). The highest dose of T3, 75 nmol/kg, did not increase feeding. Consistent with the increased food intake, cumulative weight gain was 30% greater in the 4.5 nmol/kg T3 group, although this did not reach statistical significance [d 5: 23.3 ± 1.6 (T3) vs. 18.2 ± 2.1 g (control), P = 0.1] (Fig. 2CGo).



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  		FIG. 2. Chronic peripheral administration of T3 increases food intake and body weight. Ad libitum-fed rats were injected once daily with sc T3 at the doses indicated (nanomoles per kilogram) or vehicle (V; control) for 5 d (n = 12/group). Food intake and body weight were measured daily. A, Mean daily food intake. B, Cumulative food intake. C, Cumulative body weight gain. A, B, and C show values on d 5 of the study. {dagger}, P < 0.005 and *, P < 0.05 vs. control. Results are mean ± SEM.

 
The effects of chronic administration of T3 on body adiposity, plasma leptin, and thyroid hormones are shown in Table 1Go. Chronic treatment with 4.5 nmol/kg T3 also increased plasma fT3 by 40%, although this remained within the normal range. Plasma TSH was lowered by 20% (P < 0.05), compared with controls, but again remained within the normal range. Although 9 nmol/kg T3 had some stimulatory effects on feeding, this was associated with suppression of the thyroid axis (Table 1Go). The highest dose of T3, 75 nmol/kg, produced a 5-fold increase in plasma fT3 (P < 0.0005), with marked suppression of TSH consistent with significant thyrotoxicosis. Chronic administration of 4.5 mol/kg T3 did not alter plasma leptin, interscapular BAT mass, or interscapular BAT UCP-1 mRNA levels [20.0 ± 2.3 (T3) vs. 19.0 ± 2.1 RU (control)].


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  		TABLE 1. Chronic T3 treatment: plasma thyroid hormones, body adiposity, and hypothalamic neuropeptide expression

 
Effects of acute and chronic peripheral administration of T3 on energy expenditure
After a single injection of T3 (4.5 nmol/kg), VO2 was unchanged compared with control animals (Fig. 3AGo). However, an ip injection of the ß3-adrenoreceptor agonist BRL 35135 significantly increased VO2 as previously reported (20) (Fig. 3BGo). Similarly, once-daily injection of 4.5 nmol/kg T3 for 5 d did not alter d 5 VO2 (Fig. 3CGo). After chronic daily administration of 75 nmol/kg T3, basal VO2 had significantly increased [d 5: 18.7 ± 0.5 (T3) vs. 15.8 ± 0.5 ml/kg0.75·min (control), P < 0.005], and it was further increased 30 min after T3 injection [21.1 ± 0.4 (T3) vs. 16.5 ± 0.7 ml/kg0.75·min (control), P < 0.0005] (Fig. 3DGo). This elevation in VO2 remained evident for the duration of VO2 measurement (240 min).



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  		FIG. 3. Peripheral T3 (4.5 nmol/kg) stimulates feeding but does not alter energy expenditure. VO2 at thermoneutrality (29 C) after (A) a single sc injection of 4.5 nmol/kg T3 (open circles) or vehicle (filled circles) or (B) BRL 35135 (open squares) or vehicle (filled squares); or (C) five once-daily injections of 4.5 nmol/kg T3 (open circles) or vehicle (filled circles) and (D) 75 nmol/kg T3 (open triangles) or vehicle (filled triangles). Animals were acclimatized to the chambers for 120 min before injection with T3 or vehicle at time 0 (n = 8/group). Results are mean VO2± SEM for each 30-min period *, P < 0.05, {dagger}, P < 0.005, and {ddagger}, P < 0.0005 vs. control.

 
Effects of acute and chronic peripheral T3 treatment on hypothalamic neuropeptide mRNA expression
After a single injection of T3 (4.5 nmol/kg), hypothalamic POMC mRNA, AgRP mRNA, and NPY mRNA levels were unchanged, compared with control animals (data not shown). This was also true of 5 d of daily administration of T3 (4.5, 9, 75 nmol/kg) (Table 1Go).

Determination of neuronal activation after peripheral T3 injection
Within the CNS, T3 induces expression of the Egr family of transcription factors (27, 28) yet may inhibit c-fos expression (29, 30). Therefore, to investigate a potential hypothalamic site of action for peripheral T3, Egr-1-IR was examined. Peripheral T3 injection was associated with a significant increase in the number of cells positive for Egr-1 in the VMN [median value (interquartile range): 1080 (879:1282) (T3) vs. 642 (620:664) immunoreactive cells (control), P < 0.05 (Fig. 4Go, A and B)]. No change in Egr-1-IR was detected within the ARC [511 (367:656) (T3) vs. 399 (378:420) immunoreactive cells (control)]. No Egr-1-IR was observed in the PVN post-T3 administration. See supplemental data, published on The Endocrine Society’s Journals Online web site at http://endo.endojournals.org, for representative images.



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  		FIG. 4. Peripheral T3 (4.5 nmol/kg) administration induces Egr-1-IR in the VMN and injection of T3 into the VMN stimulates feeding. Representative example (bregma, –2.56 mm) of Egr-1-IR in response to sc injection of vehicle (A) and T3 (4.5 nmol/kg) (B), x100 magnification. 3V, Third ventricle; dotted line delineates the VMN. C, Food intake in ad libitum-fed rats 1 h post injection of T3 [at the doses indicated (picomoles)] or vehicle (V; control) into the VMN (n = 11–14/group). Results are mean ± SEM. *, P < 0.05 and {ddagger}, P < 0.0001 vs. control.

 
Effects of intranuclear administration of T3 on food intake
In view of the findings from these ICC studies, we studied the effects of injection of T3 (0.5, 1, 5, or 50 pmol) directly into the VMN. All doses of T3 significantly increased food intake 1 h post injection (Fig. 4CGo). Fifty picomoles T3 produced a 4-fold stimulation in feeding [0–1 h: 3.1 ± 0.5 50 pmol (T3) vs. 0.8 ± 0.2 g (control), P < 0.0001]. This stimulatory effect on food intake was short lived. Food intake between 1–2 and 2–4 h post injection was unchanged, compared with controls. In contrast, intra-ARC injection of T3 did not alter food intake at any dose (P = 0.3).

Hypothalamic D2 mRNA expression

Effect of fasting
In animals fasted for 12 and 24 h, hypothalamic D2 mRNA expression was increased by approximately 50%, compared with fed controls [5.9 ± 0.6 (12-h fast) vs. 4.0 ± 0.5 RU (fed), P < 0.05, 6.1 ± 0.9 (24-h fast) vs. 4.0 ± 0.5 RU (fed), P < 0.05] (Fig. 5AGo).



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  		FIG. 5. Hypothalamic D2 mRNA increases with fasting and has a diurnal variation. A, Hypothalamic D2 mRNA expression in rats fasted for 12 and 24 h, compared with ad libitum-fed rat (n = 10/group). *, P < 0.05 vs. fed group. B, Hypothalamic D2 mRNA expression over a 24-h period (n = 10/group). Black line denotes duration of dark phase. *, P < 0.05 vs. D2 mRNA expression at all other light phase time points. Results are mean ± SEM.

 
Diurnal variation
Hypothalamic D2 mRNA expression exhibited a diurnal rhythm (Fig. 5BGo), reaching a nadir at 1500 h, 8 h after lights on. Subsequently, D2 mRNA expression progressively increased, with maximal levels at 2300 h, 4 h after lights out. This peak D2 mRNA expression was significantly greater than all other light-phase time points (P < 0.05). A 3-fold change in D2 mRNA expression was observed through the 24-h period. Trend analysis by polynomial fit revealed a cubic relationship between D2 mRNA expression and time of day (P = 0.001).


   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
We have demonstrated a novel role for T3 in the stimulation of food intake. This effect was seen using T3 doses substantially lower than those used by other groups (14, 31). In our study, plasma fT3 levels were increased by approximately 40% but remained within the normal range after acute and chronic peripheral administration of 4.5 nmol/kg T3. In contrast, the higher doses of T3 produced up to a 5-fold increase in plasma fT3 and were associated with a marked suppression of plasma fT4 and TSH. Supraphysiological levels of T3 are likely to result in metabolic and behavioral changes that would make the effects on food intake difficult to interpret. Therefore, in our experiments studying the effects of low-dose T3 on feeding, we were careful to adjust the dose of T3 to maintain fT3 levels within the normal range.

In our studies, we peripherally administered T3, the biologically active thyroid hormone, which readily crosses the blood-brain barrier (32, 33). Our results support a direct effect of T3 on feeding because this stimulation occurred in the absence of changes in energy expenditure, behavior, or plasma leptin. The chronic orexigenic effect of T3 is unlikely to be a compensatory response to weight loss because our data showed that the dose of T3 that stimulated feeding (4.5 nmol/kg) was actually associated with a trend toward increased weight gain. The well-characterized effects of thyroid hormones are mediated by nuclear hormone receptors via activation of gene transcription and occur over a period of hours to days (34). However, a number of thyroid hormone effects occur more rapidly and are independent of the cell nucleus (34, 35). These nongenomic effects of thyroid hormones have been described in a variety of tissues (36, 37, 38, 39). The rapid increase in feeding after T3 injection observed in the current study suggests such a nongenomic effect.

In rodents, BAT is the major site of adaptive thermogenesis due to the expression of uncoupling proteins. UCP-1 is exclusive to BAT, and thyroid hormones are permissive for UCP-1 expression (40). We found that peripheral administration of low-dose T3 for 5 d increased food intake without any alteration in BAT weight or UCP-1 mRNA levels. This suggests that the orexigenic effect of T3 is not secondary to changes in adaptive thermogenesis. Consistent with this, energy expenditure, measured as VO2, was unchanged after both acute and chronic peripheral administration of low-dose T3. Thyrotoxicosis is associated with a significant increase in physical activity, and this alteration in energy balance may contribute to the characteristic increased food intake. However, we found that administration of low-dose T3 increased feeding behavior without altering locomotor activity.

Our studies demonstrate a role for the VMN in the feeding response to T3. Although early studies (12) demonstrated that lesioning of the VMN produces hyperphagia and obesity, more recent work (41, 42) suggests a more complex role for the VMN in appetite regulation. The VMN has projections to a number of hypothalamic nuclei involved in appetite regulation (43), and disruption of the VMN alters the expression of NPY mRNA in the ARC and NPY peptide in the PVN (44). However, it is unlikely that either the ARC or PVN is the primary site of action for the stimulatory feeding effects of T3 because peripheral T3 did not increase Egr-1-IR in either of these nuclei. In addition, injection of T3 into the ARC, unlike the VMN, did not affect feeding. The observed increase in food intake produced by T3 may not be mediated by well-characterized neuropeptide regulators of feeding, such as POMC, AgRP, or NPY, because their hypothalamic mRNA expression was unaltered after chronic T3 administration. However, changes in peptide synthesis and release may occur in the absence of altered mRNA expression. Other molecules involved in the regulation of food intake, such as dopamine (45), serotonin (45), and brain-derived neurotrophic factor (46), are expressed in the VMN, and their expression is altered by nutritional status. Therefore, the possible involvement of such neuromodulators of feeding cannot be excluded.

Rats display a diurnal pattern in feeding, and many neuropeptides involved in the regulation of food intake show a similar diurnal pattern in expression, i.e. AgRP (47), NPY (48), and POMC (48). Campos-Barros et al. (49) have shown that in rats, hypothalamic T3 concentrations peak just before the onset of the dark phase when feeding begins and hypothalamic D2 activity is maximal in the middark phase. Consistent with this we demonstrated a diurnal variation in hypothalamic D2 mRNA, with levels greatest 4 h into the dark phase. A previous study (6) has shown that hypothalamic D2 mRNA expression increases after a prolonged, 72-h fast. However, this duration of fasting was associated with pronounced suppression of the thyroid axis. We have shown that hypothalamic D2 mRNA expression is increased after only 12 h of fasting, which is more relevant to the daily regulation of food intake. This increase in hypothalamic D2 mRNA levels with fasting may increase hypothalamic T3 levels and hence stimulate appetite. One way to investigate this would be to inhibit the action of D2, for example with reverse T3, and examine the effects of this inhibition on food intake.

Studies suggest that T4 is taken up by tanycytes from the cerebrospinal fluid and capillaries and converted by D2 to the active hormone T3 for use in various hypothalamic regions (50). Therefore, T3 may be transported, possibly by tanycytes, to the VMN to stimulate food intake. These data have led us to propose a hypothalamic circuit involving T3, which is regulated by daily energy requirements. The source of T3 may be the peripheral circulation or locally derived in the hypothalamus by deiodination of T4 to T3 by D2.

Previously the role of thyroid hormones in energy balance has been largely confined to pathological states, such as hyperthyroidism. The effects of thyroid hormones on appetite have been presumed to be secondary to increased metabolism. Here we propose a novel role for T3 in the regulation of daily food intake via the hypothalamic VMN.


   Footnotes
 
This work was supported by Medical Research Council (MRC) program Grant G7811974 (to S.R.B., M.A.G., J.V.G., and C.J.S.). W.M.K. and N.M.M. are Wellcome Trust Clinical Training Fellows. K.L.S. is supported by a MRC PhD studentship. W.S.D. is supported by a Department of Health Clinician Scientist Fellowship.

W.M.K., N.M.M., and K.L.S. contributed equally to this work.

Abbreviations: AgRP, Agouti-related protein; ARC, arcuate nucleus; BAT, brown adipose tissue; CNS, central nervous system; D2, type 2 iodothyronine deiodinase; Egr, early growth response gene; fT3, free T3; fT4, free T4; ICC, immunocytochemistry; IR, immunoreactivity; NPY, neuropeptide Y; POMC, proopiomelanocortin; PVN, paraventricular nucleus; RPA, ribonuclease protection assay; RU, relative unit; UCP-1, uncoupling protein 1; VMN, ventromedial nucleus; VO2, oxygen consumption.

Received April 28, 2004.

Accepted for publication July 26, 2004.


   References
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 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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