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Physiological Regulation of Appetite
Behavioral Neurogenetics Laboratory, Ingestive Behavior Research Center and Department of Psychological Sciences, Purdue University, West Lafayette, Indiana 47907
Submitted 23 December 2003 ; accepted in final form 24 February 2004
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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neurotrophins; meal pattern; microstructural analysis; homeostasis
One of the most common forms of human obesity is middle-age obesity (42). Several mouse strains exhibit a similar obesity known as mature-onset obesity, first described some time ago in agouti mutant mice (Ay/a; 4). Importantly, for the present purposes, one of the features of this syndrome is its dependence on hyperphagia (4, 42). Additional characteristics of mature-onset obesity include increases in fat cell size, linear growth, and insulin and leptin levels, as well as insulin and leptin resistance, decreased glucose tolerance, and normal corticosterone levels. Pharmacological and gene manipulation studies have revealed that agouti obesity is mainly due to ectopic expression of agouti protein, which antagonizes melanocortin-4 receptors (MC4-Rs; 4). Thus MC4-R knockout mice, and to some extent proopiomelanocortin knockout mice, exhibit an obesity similar to that of agouti mice (4).
Ventromedial hypothalamus (VMH) neurons that express brain-derived neurotrophic factor (BDNF) were recently identified as acting downstream of MC4-Rs in the regulation of energy balance (46). Several findings are consistent with this hypothesis: 1) BDNF mutants exhibit mature-onset obesity, 2) they have reduced VMH BDNF levels in association with this obesity, and 3) they have normal expression levels of feeding neuropeptides/neurotransmitters found upstream of MC-Rs, including neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), melanin-concentrating hormone, orexin, agouti-related peptide, 
-melanocyte-stimulating hormone, thyrotropin releasing hormone, and serotonin (23, 27, 37). Moreover, the high-affinity receptor for BDNF (trkB) exhibited little colocalization with melanocyte-concentrating hormone or orexin in the lateral hypothalamus or with CART or NPY in the arcuate nucleus (46). These findings do not suggest BDNF to be the only downstream pathway mediating the effects of the hypothalamic melanocortin system on energy balance, nor do they imply BDNF pathways activated by melanocortin neurons are the only ones affecting food intake and body weight. Nevertheless, these findings do indicate that some BDNF-expressing neurons, including at a minimum those activated via MC4-Rs, are involved in energy balance. They also suggest these BDNF neurons comprise one of the most distal components identified to date that is utilized by melanocortins for this purpose. Therefore, VMH BDNF neurons provide one of the closest known links to downstream neural pathways that more directly control ingestion, and BDNF mutants may be valuable for identifying these pathways. Moreover, as pointed out by Kernie et al. (23), there are rare patients with WAGR (Wilms' tumor, aniridia, genitourinary anomalies, and mental retardation) contiguous gene syndrome who are obese and have chromosome deletions that include the BDNF locus, thus illustrating the potential of reduced BDNF levels to contribute to obesity in humans.
The initial characterization of obesity in BDNF mutant mice has revealed several features suggesting these mice to be a valuable model for progressing our understanding of the central nervous system (CNS) regulation of normal and disordered food intake. One of the most important of these characteristics is the significant contribution of hyperphagia to the development of obesity: metabolic alterations appear for the most part to be secondary to increased food intake and body weight (12, 27, 37). Therefore, BDNF mutants may provide a model for examining the central mechanisms underlying one type of hyperphagia, without the confounds produced by parallel metabolic alterations, as occur in some commonly studied genetic and hypothalamic obesities (3). Another important attribute of obesity in BDNF mutants is that it appears to be centrally mediated: it is a consequence of reduced BDNF levels in the CNS rather than the reduced levels in peripheral tissues (23, 37, 46). This focuses investigation of the mechanism underlying obesity within the brain rather than on BDNF expression in peripheral tissues, such as the adrenal glands, taste buds, and gastrointestinal (GI) tract, which could have played a significant role (26, 35, 43). The third significant characteristic of obesity in BDNF-deficient mice is that reduced BDNF levels appear to affect neuron maintenance or function rather than neural development (37). This is important because it directs investigation to activities of BDNF that occur in the mature nervous system.
Given the potential significance of BDNF-deficient obesity for improving our understanding of normal and disordered feeding, and the important role of hyperphagia in this obesity, we characterized several dimensions of the hyperphagia exhibited by BDNF-deficient mice to gain insight into the mechanism underlying their feeding disturbances. First, we identified meal pattern parameters altered in association with increased food intake (e.g., meal size, meal number). Second, we utilized microstructural analysis of individual meals in concert with the meal pattern findings to determine whether BDNF acts on neural systems that contribute to meal initiation and maintenance or to satiation and satiety (8, 40). The third dimension involved an examination of the effects of a high-fat diet on food intake, body weight, meal pattern, and meal microstructure. Enhanced hyperphagia in response to a moderate-fat or high-fat diet (e.g., 5), which is one of the hallmarks of mature-onset obesities and which correlates with human obesity (11), has not been examined in BDNF mutants (but see Ref. 46). The present characterization of feeding behavior of BDNF-deficient mice constitutes the first step in a strategy for identifying a neural mechanism that underlies hyperphagia.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Male BDNF heterozygous mutant mice (B6.129S4-Bdnftm1Jae; +/–) and their male wild-type littermates (B6.129S4; +/+), weighing 20–25 g and 
2 mo of age, were obtained from Jackson Laboratories (Bar Harbor, ME). Animals were housed individually, maintained on a 12:12-h light-dark schedule at 23°C, and given ad libitum access to tap water and pellets (Laboratory Rodent Diet 5001, PMI Nutrition International, St. Louis, MO). All procedures were conducted in accordance with Principles of Laboratory Animal Care (NIH publication No. 86–23, revised 1985) and American Association for Accreditation of Laboratory Animal Care guidelines, and the Purdue University Animal Care and Use Committee approved them.
Experiment 1: Feeding Behavior of BDNF Mutant and Wild-Type Mice Consuming a Balanced Diet
Diet. In experiment 1 a balanced diet was employed (20 mg dustless precision pellets, Bio-Serv, Frenchtown, NJ). The caloric distribution in the precision pellet diet is 22% protein, 66% carbohydrate, and 12% fat, with a caloric density of 3.623 kcal/g. This is comparable, in particular in percent fat, to the maintenance diet in which the distribution is 28% protein, 60% carbohydrate, and 12% fat, with a caloric density of 3.34 kcal/g.
Treatment groups. Experiment 1 examined the effects of the heterozygous BDNF mutation (Bdnf +/– vs. Bdnf +/+) on spontaneous intake of a nutritionally balanced diet in young mice (before development of obesity in the mutants; 3 mo of age) and in mature mice (after development of obesity in the mutants; 8–11 mo of age). This resulted in four groups: 1) wild-type, young, balanced diet (WT-YNG-BAL; n = 6); 2) mutant, young, balanced diet (MUT-YNG-BAL; n = 6); 3) wild-type, mature, balanced diet (WT-MAT-BAL; n = 12); and 4) mutant, mature, balanced diet (MUT-MAT-BAL; n = 12). Separate cohorts of animals were used in the young and mature groups. Half of the animals in each cohort were tested on the balanced diet for 2 wk, then returned to maintenance on standard chow for 2 wk, and then tested on a high-fat diet for 2 wk (see experiment 2 below). The order of the diet presentations was counterbalanced by testing the other half of each cohort on high-fat food first and on the balanced diet second. The order of presentation of the two diets had no effect on food intake (not shown). Therefore, data from the two balanced diet runs were pooled, and data from the two high-fat diet runs were pooled. This resulted in one young and one mature dataset for the balanced diet that comprised the data for experiment 1, whereas the parallel datasets for the high-fat diet provided the data for experiment 2 (see below). The larger subject numbers in the mature groups of experiments 1 and 2 resulted from replications of these studies in additional cohorts of mature animals. This was done to verify the different meal pattern changes observed in association with hyperphagia in mature mutants fed the balanced or the high-fat diet (see results). Because the results in the second runs replicated those of the initial runs (not shown), the results of the two runs were pooled for analysis.
Experimental protocol and apparatus. The balanced precision pellet diet was delivered using automated pellet dispensers and Graphic State software (v. 1.011; Coulbourn Instruments, Allentown, PA) as described previously (18). Mice were adapted to the test room and test cages for 1 wk before testing. During that week animals received three limited preexposures to the test diet to prevent neophobia at the start of testing, each consisting of 10 of the Bio-Serv precision pellets. Intake patterns were monitored 18 h each day, and animals were fasted the remaining 6 h, during which time diet and cage maintenance were performed and mice were weighed. Each daily session began at the start of the dark phase of the light cycle and extended 6 h into the light phase, and meal pattern data were collected for 14 consecutive days. This interval provided time for adaptation to the diets and apparatus followed by several days of stable intake patterns. Mutant and wild-type mice were always tested in parallel to control for any inadvertent variations in the testing conditions.
Data analysis and meal criteria. Strict criteria were used to define a meal: meal initiation was defined as a minimum of seven pellet removals with <20 min elapsing between responses. Once a meal was initiated, meal termination was defined as the onset of a 20-min interval with no intake. The criteria for meal onset (time interval between pellet removals and number of pellet removals) were determined by systematically varying them and examining the effect on meal number (n = 6). These data were used to identify the range of criteria that exhibited the greatest stability in estimates of meal numbers, and the specific set of criteria chosen was drawn from the middle of this range. These criteria were applied to the raw data using the Graphic State software to identify the times of onset and termination of each meal, which were used to calculate several meal parameters. An estimate of the number of calories consumed by each animal could be calculated as equal to the pellet number multiplied by pellet weight (20 mg) and then multiplied by the caloric density of the pellet diet (3.623 kcal/g). These were considered to be good estimates based on the observation that mice consumed all or almost all of each pellet, as evidenced by the minute amount of spillage present on cage floors.
Experiment 2: Feeding Behavior of BDNF Mutant and Wild-Type Mice Consuming a High-Fat Diet
Diet. In experiment 2 a high-fat diet was employed (Isocal, Mead Johnson, Evansville, IN). The caloric distribution of the Isocal diet is 13% protein, 50% carbohydrate, and 37% fat, with a caloric density of 1.05 kcal/g. We employed this diet because the precision pellets used in automated feeders in experiment 1 could not be readily fashioned with high concentrations of fat; such pellets have a pliable texture and disintegrate easily. Therefore, a high-fat diet in liquid form was chosen and a different apparatus (see below) was utilized to deliver the diet in this experiment compared with experiment 1.
Treatment groups. Experiment 2 examined the effects of the heterozygous BDNF mutation (Bdnf +/– vs. Bdnf +/+) on spontaneous intake of a high-fat diet in young mice (before the development of obesity in the mutants; 3 mo of age) and in mature mice (after obesity had developed in the mutants; 8–11 mo of age). This resulted in four groups: 1) wild-type, young, high-fat diet (WT-YNG-HIFAT; n = 6); 2) mutant, young, high-fat diet (MUT-YNG-HIFAT; n = 6); 3) wild-type, mature, high-fat diet (WT-MAT-HIFAT; n = 10); and 4) mutant, mature, high-fat diet (MUT-MAT-HIFAT; n = 10). One mouse was dropped from the WT-MAT-HIFAT group and one from the MUT-MAT-HIFAT group due to equipment failure that occurred after they had achieved stable intake, resulting in final group sizes of nine. The testing schedules of these groups were described above in the methods for experiment 1.
Experimental protocol and apparatus. All aspects of the protocol and apparatus were the same as for experiment 1 unless specified below. Isocal was delivered using automated liquid dippers in most experiments (Coulbourn Instruments) as described previously (18). Each time the animal activated the dipper they were presented with a 0.02-ml cup filled with Isocal for 4.5 s, which was sufficient time for mice to consume the entire 0.02 ml of Isocal. Additionally, experiments were repeated in a small number of animals (BDNF +/+, n = 2; BDNF +/–, n = 2) utilizing optical lickometers (Coulbourn Instruments). Mice were not given any preexposure to Isocal because it was not associated with neophobia.
Data analysis and meal criteria. All aspects of the data analysis and the meal criteria were the same as for experiment 1 unless specified below. The meal pattern parameters that utilized number of dipper activations for their calculation, which included total intake, meal size, and intake rate, were also determined using the total number of grams of Isocal consumed in each daily test session. Only the data derived from the total amount of Isocal consumed are presented in results. The rationale for presenting the data in this form was that we could not directly determine that all mice consumed the entire volume of diet in the dipper cup on each delivery. Thus it was more accurate to adjust the meal parameter calculations based on the estimate of the actual amount consumed rather than to assume that all the delivered Isocal was consumed. Calculation of the amount of Isocal consumed for each animal per day took into account evaporation of Isocal from the dipper trough. Evaporation was measured using one extra trough filled with Isocal that was placed on the same rack as the test cages and weighed at the same times as the other troughs, at the start and at the end of each session. The values of the meal pattern parameters derived from the amount of Isocal consumed did not differ significantly from the values for those same parameters determined directly from the number of dipper activations (not shown).
Statistical Analysis and Graphical Display of Data
Differences in each meal pattern parameter between BDNF mutants and wild-type mice were tested using repeated-measures ANOVA, and some additional comparisons were made using unpaired t-tests (Statistica, v5.0, StatSoft, Tulsa, OK). Values reported are means ± SE. For all statistical tests, P < 0.05 was required for statistical significance. Graphpad was used to construct all graphs (Graphpad Prism Version 3.0, Graphpad Software).
Microstructural analysis. The first meal of each daily test session (defined as spontaneous food intake during the first 30 min after mice gained access to the food at the start of the session) was subjected to microstructural analysis. We characterized intake rates during this meal because it is most analogous to the scheduled meals that have been the focus of the majority of previous microstructural studies. Davis and his colleagues developed this analysis and have used it extensively to characterize the effects of several gustatory, dietary, pharmacological, and surgical manipulations on the patterns of intake rate that occur over the course of a scheduled 30-min meal (8). The relevance of the Weibull function for analysis of spontaneous food intake is based on three parameters that can be estimated. The A parameter estimates the initial intake rate, the B parameter estimates the slope, or rate of decline in intake rate, and the C parameter estimates how much the curve deviates from an exponential, which is an indicator of the duration for which initial intake rates are maintained. The physiological significance of these parameters is that they provide information about the activities of two of the major peripheral inputs regulating food intake (8). First, initial intake rate is mainly influenced by the magnitude of oropharyngeal stimulation, which serves as an input that drives ingestive behavior. Second, the rate of decrease in intake rate, and the duration for which the initial intake rate is maintained, mainly represent the influence of post-oral factors, in particular negative-feedback signals activated by the accumulation of ingesta in the GI tract.
The average intake rate data from each animal was fit to the Weibull function y = Aexp[(–Bt)C]. The first minute of the meal was not included in this analysis because the computer program began collecting data for several animals simultaneously, whereas subsequently the experimenter took 30–60 s to successively remove a panel that blocked access to the feeders or dippers from each animal's cage, thus delaying initiation of intake for some animals relative to others. Also, 14% of the trials were excluded from the Weibull analysis because on a small subset of the eight trials/animal, some animals did not exhibit an early peak intake as indicated by the following criterion for mice fed the balanced diet in experiment 1. If intake was equal to 1 or 0 pellets during minute 2, the trial was dropped, whereas if intake was equal to 2 or more pellets, the trial was included. A similar set of criteria was applied to the trials of animals fed the high-fat diet in experiment 2, but an additional criterion was required because intake data were continuous. 1) If intake during minute 2 was <0.1 g, the trial was dropped, whereas trials with intake >0.15 g were included in analysis. 2) If intake during minute 2 fell between 0.1 and 0.15 g, the mean intake during minutes 2 and 3 was determined. If this mean was <2 SDs below the mean of the average intake of that animal for the same time period over all eight of their trials, this trial was dropped. These different criteria associated with the balanced diet and the high-fat diet utilized cutoffs that were similar in terms of numbers of calories consumed. The dropped trials were distributed relatively evenly across the groups (WT-YNG-BAL: 7/48; MUT-YNG-BAL: 5/48; WT-MAT-BAL: 17/96; MUT-MAT-BAL: 12/96; WT-YNG-HIFAT: 3/48; MUT-YNG-HIFAT: 4/48; WT-MAT-HIFAT: 10/72; MUT-MAT-HIFAT: 16/72). After these trials were eliminated, the curves of the intake rates averaged over the last 8 days of testing were fit by the Weibull function, and from these fits the Weibull parameters were calculated.
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Experiment 1: Feeding Behavior of BDNF Mutant and Wild-Type Mice Consuming a Balanced Diet
Food intake and body weight in mice consuming a balanced diet.
YOUNG BDNF MUTANTS CONSUMING THE BALANCED DIET DID NOT EXHIBIT HYPERPHAGIA OR OBESITY.
Tested on the balanced diet at 3 mo of age, the average body weight of MUT-YNG-BAL mice was slightly but not significantly greater than for WT-YNG-BAL mice [F(1,10) = 3.2, P = 0.104] (Fig. 1A), which suggests that significant obesity had not yet developed in these BDNF mutants. There was a trend toward increased daily food intake in the MUT-YNG-BAL group, but this difference was not significant (14.5% increase) [F(1,10) = 2.95, P = 0.12] (Fig. 1B). Estimates of the average number of calories consumed by WT-YNG-BAL and MUT-YNG-BAL groups calculated from the mean number of pellets eaten equal 14.61 and 16.73 kcal/day, respectively.
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Meal patterns in mice consuming a balanced diet.
YOUNG BDNF MUTANTS CONSUMING THE BALANCED DIET EXHIBITED NORMAL MEAL PATTERNS.
Almost all of the meal pattern parameters examined were similar in MUT-YNG-BAL and WT-YNG-BAL mice with one exception. In particular, the satiety ratio was moderately reduced in mutants (21% decrease) [F(1,10) = 6.5, P < 0.05] (Fig. 1C), suggesting that a given amount of food was not as effective at producing satiety in the mutants as it was in controls [satiety ratio is the ratio of meal size to the following intermeal interval (IMI)]. The lower satiety ratio was a consequence of nonsignificant differences in IMI (8% decrease in mutants) [F(1,10) = 2.94, P = 0.12], and meal size (13% increase in mutants) [F(1,10) = 1.65, P = 0.23]. Meal pattern data from experiments 1 and 2 that are not graphed are presented in Table 1.
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Microstructure of feeding behavior in mice consuming a balanced diet.
YOUNG BDNF MUTANTS CONSUMING THE BALANCED DIET SHOWED NO ALTERATIONS IN MICROSTRUCTURE.
Before the development of obesity, the intake rate curves for young mutants and controls were nearly overlapping. Consistent with this, there were no significant differences between mutants and controls in the Weibull parameters A [F(1,10) = 0.14, P = 0.72], B [F(1,10) = 4.01, P = 0.07], or C [F(1,10) = 1.47, P = 0.25]. Values of Weibull parameters calculated for experiments 1 and 2 are listed in Table 2.
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Food intake and body weight in mice consuming a high-fat diet.
YOUNG BDNF MUTANTS CONSUMING A HIGH-FAT DIET EXHIBIT HYPERPHAGIA.
Mice were maintained on a balanced diet before the start of experiment 2, at which time they were switched to a high-fat diet. Thus, as in experiment 1, body weights were similar in young BDNF mutants and wild-type mice [F(1,10) = 4.03, P = 0.073] (Fig. 3A), suggesting that significant obesity had not yet developed in the BDNF mutants. However, in contrast to experiment 1, and despite the lack of significant weight gain in MUT-YNG-HIFAT mice, their daily intake of the high-fat diet was increased compared with WT-YNG-HIFAT mice (28% increase) [F(1,10)=11.3, P < 0.01] (Fig. 3B). Estimates of the average number of calories consumed by WT-YNG-HIFAT and MUT-YNG-HIFAT mice calculated from the mean number of grams of high-fat diet consumed equal 13.97 and 17.85 kcal/day, respectively.
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Meal patterns in mice consuming a high-fat diet.
YOUNG BDNF MUTANTS CONSUMING THE HIGH-FAT DIET INCREASED MEAL SIZE TO SUPPORT HYPERPHAGIA.
In contrast to the mature mice tested on the balanced diet in experiment 1, the hyperphagia of the BDNF mutants tested with the high-fat diet before development of obesity (MUT-YNG-HIFAT group) was mediated mainly by a 70% increase in meal size relative to controls [F(1,10) = 20.3, P < 0.01] (Fig. 3C). Consistent with this, first meal size was larger for the mutants (42% increase) [F(1,10) = 6.36, P < 0.05] (Fig. 5A). The increase in meal size was due to increasing trends in average intake rate (15.6% increase) [F(1,10) = 3.06, P = 0.11] (Fig. 5C) and meal duration (53% increase) [F(1,10) = 2.5, P = 0.14] that approached significance. Also, the MUT-YNG-HIFAT mice exhibited a 21% decrease in meal frequency that may have been a compensatory response to their larger meals [F(1,10) = 5.6, P < 0.05] (Fig. 3D). Moreover, the larger average meal size of the MUT-YNG-HIFAT mice combined with similar IMIs for both groups [F(1,10) = 0.4, P = 0.52] resulted in the MUT-YNG-HIFAT mice having a 37% decrease in satiety ratio compared with WT-YNG-HIFAT mice [F(1,10) = 9.86, P < 0.05] (Fig. 5B).
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To examine the generality of these meal pattern findings that had been obtained using liquid dippers to deliver the high-fat diet, we repeated the experiments in a small number of naive mice using lickometers and analyzed the data from the last 8 days of the 2-wk test, during which intake patterns were stable. This experiment demonstrated similar effects on meal parameters as those obtained employing the liquid dippers. For example, MUT-YNG-HIFAT mice that were tested using the lickometer apparatus were hyperphagic (40% increase in intake; Fig. 4B), and this was mediated mainly by increased meal size compared with WT-YNG-HIFAT mice (WT-YNG-HIFAT: 0.97 ± 0.18 g/meal vs. MUT-YNG-HIFAT: 1.92 ± 0.23; 98% increase) [F(1,2) = 105.69, P < 0.01]. Also similar, there was a compensatory decrease in meal number for the mutants that approached significance (WT-YNG-HIFAT: 14.63 ± 1.94 meals/day vs. MUT-YNG-HIFAT: 10.38 ± 1.24; 30% decrease) [F(1,2) = 9.475, P = 0.09]. These results suggest that the present meal pattern findings obtained using liquid dippers are not apparatus specific. However, a more stringent validation will require testing larger group sizes with lickometers.
Microstructure of Feeding Behavior in Mice Consuming the High-Fat Diet
YOUNG BDNF MUTANTS CONSUMING THE HIGH-FAT DIET HAD ALTERED MICROSTRUCTURE.
The MUT-YNG-BAL mice ate at higher rates than WT-MAT-BAL mice during minutes 2–5 (23–48% increases in intake rate), whereas intake rates were similar for both groups during the remainder of the meal. Consistent with this pattern, the Weibull parameter A was increased by 30% in MUT-YNG-HIFAT mice relative to the WT-YNG-HIFAT group [A, F(1,10) = 5.72, P < 0.05]. In contrast, the B [F(1,10) = 4.29, P = 0.07] and C [F(1,10) = 0.38, P = 0.55] parameters were not different.
MATURE BDNF MUTANTS CONSUMING THE HIGH-FAT DIET HAD ALTERED MICROSTRUCTURE.
The MUT-MAT-HIFAT mice ate at higher rates than WT-MAT-HIFAT mice during minutes 2 and 3 (22 and 31% increases in intake rate, respectively), whereas intake rates were similar for both groups during the remainder of the meal. Consistent with this pattern, the Weibull parameter A was increased by 35% in the MUT-MAT-HIFAT mice [F(1,16)=4.83, P < 0.05], but there were no differences in the B [F(1,16) = 3.97, P = 0.064] or C [F(1,16) = 0.93, P = 0.35] parameters.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONGRANTSREFERENCES |
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Characteristics of Hyperphagia and Obesity in BDNF-Deficient Mice Fed a Balanced Diet
The two studies that continuously monitored daily food intake and body weight of BDNF mutants showed that hyperphagia and obesity develop gradually, with hyperphagia preceding obesity (23, 27). Consistent with these findings, in the present study young BDNF mutants fed a balanced diet did not show significant hyperphagia or obesity, whereas older, mature mutants did. However, the young BDNF mutants had significantly decreased satiety ratios, which suggests that their trends toward increased total intake and meal size may represent an early stage of hyperphagia. Interestingly, significant hyperphagia in mature BDNF mutants was mediated by increased meal frequency, which raises the possibility that factors involved in meal initiation were enhanced. Moreover, the size and duration of their meals were not different from controls, suggesting that regulation of individual meals was normal, and both groups had similar satiety ratios, indicating their similar-size meals were equally effective at producing satiety. The large contribution of increased meal number to the hyperphagia of mature BDNF mutants distinguishes their mature-onset obesity from most other rodent models of obesity, which involve overconsumption of a balanced diet mediated by increased meal size (e.g., leptin-deficient rodents with early-onset obesity; Refs. 24, 41). This distinction could imply that mature-onset obesity is mediated by a different underlying mechanism than these other obesities, although some disrupted components may be shared.
Mechanism Underlying Delay of Obesity Onset in BDNF-Deficient Mice
In the experimental design of the present study, data obtained from young and mature mice were not directly compared. Nevertheless, there were some suggestions in the present data of changes that might occur with aging that appeared instructive and warrant more direct investigation. For instance, comparison of feeding behavior of young vs. mature BDNF mutants and controls in the present study suggested the effects of reduced BDNF levels might interact with an independent process that changes with age to produce a delay in obesity onset. In particular, both mature wild types and mature mutants fed the balanced diet appeared to eat larger meals compared with their young counterparts (Table 1). Although this increase in meal size was small, its impact on total daily intake was substantial when considered in terms of the large number of meals the mice consumed each day. The wild-type mice compensated for this increased food consumption by decreasing meal number as they aged, whereas the mutants did not (meals/day: young wild types = 11.8, mature wild types = 9.9, young mutants = 11.5, mature mutants = 12). This adjustment maintained total daily food intake at similar levels in young and mature wild-type mice, whereas the failure of mature BDNF mutants to make this adjustment led to an increase in their total daily intake (wild types = 3% reduction in intake vs. mutants = 15% increase in intake, an overall 18% increase in mutants; Fig. 1B). The importance of understanding this increase in meal size with age is underscored by the finding that both controls and mutants fed the high-fat diet also appeared to increase meal size with age (Fig. 3C), and mature controls largely compensated for this increase by decreasing meal number (young vs. mature wild types = 15.3 vs. 13.5 meals/day). This suggests that the underlying mechanism is not specific to only one set of diet properties.
High-Fat Diet Alters Hyperphagic Meal Pattern of BDNF-Deficient Mice, but Not Total Energy Consumption
In marked contrast to the meal patterns associated with the balanced diet, BDNF mutants consuming the high-fat diet increased meal size, rather than meal number, to support hyperphagia. Moreover, both the young and mature mutants consuming the high-fat diet showed similar changes in meal parameters, including similar magnitude increases in meal size. This suggests the metabolic alterations associated with mature-onset obesity did not have a significant impact on these meal patterns.
The present finding of different hyperphagic meal patterns of BDNF mutants associated with each diet is unusual. The different meal patterns might be accounted for by the different physical and nutrient properties associated with the balanced and high-fat diets. For instance, as a result of their different properties, each diet could produce different patterns of oropharyngeal and GI feedback, which could result in different meal patterns. In particular, fat content and viscosity are two properties of the balanced and high-fat diets used that differed dramatically and could have differentially altered patterns of oropharyngeal and GI feedback. Solid high-fat diets have been shown to produce hyperphagia by increasing meal size in normal rats (15), similar to the effect of the liquid high-fat diet consumed by both young and mature BDNF mutants in the present study. This could imply that fat content is the most important diet property contributing to this pattern of increased meal size. However, it is also possible that the vastly different viscosities of the balanced and high-fat diets played a role. In particular, compared with a solid diet, a liquid diet will be processed more rapidly in the stomach, it will empty into the intestine more quickly, and it may exhibit faster transit through the intestine (1, 22, 30). In this manner, a liquid diet will not be as effective as a solid diet in activating mechanoreceptors that detect muscle stretch due to filling of the stomach and small intestine or that sense the muscle tension associated with peristaltic contractions of these organs (consistent with this, liquids have weaker effects on GI myoelectrical activity; 16, 25). Therefore, it would be possible for mice to consume a larger-than-normal liquid meal before activating sufficient negative-feedback signals to prevent further intake. One approach to investigating the potential contribution of variation in diet viscosity to the different meal pattern changes observed would be to directly compare meal patterns of BDNF mutants fed a solid high-fat diet or a liquid high-fat diet.
The different meal patterns associated with each diet resulted in different amounts of energy consumed per meal by mature BDNF mutants (balanced diet: 1.7 kcal/meal vs. high-fat diet: 2.4 kcal/meal). However, the high-fat diet did not increase the total daily intake of mature mutants beyond the amount consumed when they were fed the balanced diet. In fact, mature BDNF mutants ate almost exactly the same average number of total calories per day on each diet (balanced diet: 19.17 kcal/day vs. high-fat diet: 19.61 kcal/day). Similar flexibility of meal patterns in achieving a specific level of energy intake has previously been observed with different manipulations of feeding (e.g., 21). These results suggest that the regulation of meals must occur at least partially independently from regulation of total energy intake. Moreover, because wild-type mice in each of the age and diet groups consumed approximately 14–15 kcal/day, these results are consistent with the hypothesis that energy intake is regulated in both mutants and controls but that it is regulated at a higher energy level in the BDNF-deficient mice.
Increased Strength of Oropharyngeal Cues that Drive Intake May Underlie BDNF Mutants' Hyperphagia
There are two possible mechanisms that could contribute to the increased meal size exhibited by mutants consuming the high-fat diet (40): 1) enhancement of positive feedback activated by the sensory receptors that supply oropharyngeal structures, or 2) reduction of negative feedback arising from the sensory receptors that innervate the upper GI tract and oropharynx. However, only this first mechanism could account for the increased meal number associated with hyperphagia in BDNF mutants fed the balanced diet because their unaltered meal size and meal duration suggest negative feedback was operating normally. Thus we favored the hypothesis that only enhanced oropharyngeal positive feedback contributed to hyperphagia associated with both diets and investigated this further by examining meal microstructure. This analysis demonstrated that all significantly hyperphagic groups, which included mature mutants fed a balanced diet, and young and mature mutants fed high-fat food, exhibited higher intake rates during the first few minutes of a test meal compared with controls. In contrast, the one mutant treatment group that did not exhibit hyperphagia, the young BDNF mutants fed the balanced diet, had similar initial intake rates to wild types. Food intake rates at meal onset are most strongly influenced by oropharyngeal positive feedback, as food accumulation in the GI tract during this phase is not sufficient to strongly activate negative feedback (8, 40). Therefore, this correspondence between hyperphagia and higher intake rates at the start of the test meal could imply that a greater potency of the oropharyngeal positive feedback signals in the mutants contributed to the hyperphagia associated with both the balanced and high-fat diets.
Although reduced negative feedback could have contributed to the increased meal size observed on the high-fat diet, there was no strong evidence in the microstructure of the first daily meal for reduced potency of these signals in the mutants. For instance, reduced feedback would have been indicated by an increase in the duration of maintained initial intake rates or by a slower rate of decay of intake rates. However, for almost all comparisons, estimates of these parameters derived from the Weibull analysis were similar in mutants and controls. In one instance there was an increased rate of decay of intake rates in mutants (Weibull parameter B), which is indicative of increased rather than decreased negative feedback (discussed in results). Also consistent with normal functioning of negative-feedback signaling in BDNF-deficient mice, the suppression of 30-min intake of 12.5% glucose by exogenous cholecystokinin (CCK-8, 2 µg/kg) in these mutants (n = 6) was not different from suppression in wild types (n = 6). Percent suppression in wild types was 35 ± 4.8% and in BDNF mutants was 34 ± 2.9% based on four replications (M. M. Chi and E. A. Fox, unpublished observations).
Mechanism of Premature Hyperphagia in Young BDNF Mutants Fed a High-Fat Diet
One possible explanation for induction of premature hyperphagia by a high-fat diet in young BDNF mutants is that the enhancement of oropharyngeal positive feedback in BDNF mutants was not equivalent for all macronutrients but was stronger for fat. Consistent with this proposal, a large body of evidence has demonstrated that fat consumption can be regulated independent of other macronutrients. For example, food deprivation can selectively increase intake of fat vs. other macronutrients, and of high-fat diets vs. balanced or high-carbohydrate diets (38, 45). Also, lipid infusions into the duodenum have been shown to selectively alter taste signals in the parabrachial nucleus that are associated with energy intake (20). Moreover, similar lipid infusions produced a greater suppression of fat intake than of carbohydrate intake (17). One mechanism by which reduced BDNF levels could selectively enhance oropharyngeal positive feedback produced by high-fat foods might involve reduced sensitivity of these mice to neurotransmitters or hormones that inhibit fat intake such as serotonin (39) and enterostatin (14).
Effects of Reduced BDNF Levels on Sensory Systems Could Contribute to Altered Meal Patterns
In view of the finding that reduced BDNF levels in the adult CNS produce the hyperphagia and obesity of BDNF mutants (23, 37), it seems parsimonious to hypothesize, as discussed above, that the changes in feeding patterns demonstrated by BDNF mutants in the present study also resulted from BDNF deficiency in the CNS. Nevertheless, the possibility that the effects of reduced BDNF levels on specific sensory systems contributed to the altered feeding behavior must be considered. First, vagal sensory neurons in the nodose ganglion are reduced by 22% in heterozygous BDNF mutant mice (13). If any of these missing neurons were among those that mediate GI negative-feedback signaling, this loss could reduce negative feedback. However, most or all of these missing neurons normally innervate arterial beds, including the carotid body or sinus, and in heterozygous mutants this loss did not significantly affect cardiorespiratory function (2, 13). Moreover, the meal patterns and meal microstructure of BDNF mutants in the present study and the preliminary data on CCK suppression of their food intake suggested that GI negative feedback was operating normally. Second, BDNF deficiency is associated with reduced numbers of taste buds and of neurons that innervate them (e.g., neurons in the petrosal and geniculate ganglia) and could affect the function of adult taste buds or olfactory bulbs (9, 28, 35, 47). Although the consequences of any of these effects might be to alter oropharyngeal positive-feedback signals, such alterations are likely to decrease the potency of these signals, whereas the behavior of BDNF mutants suggested a relative increase. Moreover, similar to the suggestion of Xu et al. (46) in regard to trkB hypomorphs, the reduction in taste buds and their innervation could have decreased the ability of mutants to discriminate among different tastes or diets. However, the mutants appeared to successfully discriminate the balanced and high-fat diets in the present experiments. Thus, although the effects of reduced BDNF levels on sensory pathways could have contributed to the observed meal pattern and microstructure changes in BDNF mutants, it would be less parsimonious to attribute the bulk of these alterations to them as opposed to CNS BDNF pathways.
Potential Effects of Reduced BDNF Levels on Activity and Metabolism
Altered food intake appears to be the major cause of obesity in BDNF-deficient mice, whereas metabolic changes occur secondary to weight gain. However, BDNF mutants have exhibited several forms of altered activity compared with wild-type mice that could influence weight regulation (23, 27, 37). These included increases in intermale aggression, locomotor activity, activity in response to stressors, and anxiety levels. In addition, pharmacological studies have demonstrated that BDNF can directly alter metabolism. For instance, systemic injections of BDNF have had several physiological effects in obese, diabetic mice but not in lean or wild-type mice (32, 33, 44). These include reduction of hyperglycemia, improved glucose tolerance, as well as restoration of pancreatic insulin and glucagon levels, body temperature, and oxygen consumption. Also, intracerebroventricular injections reproduced some of these effects, suggesting the site of action of the systemic injections may have been in the CNS (33, 34). Therefore, it is conceivable that BDNF deficiency might produce effects in the opposite direction of those incurred by higher-than normal BDNF levels such as a reduction in oxygen consumption, a possibility that merits further investigation.
Perspectives
The results of the present study suggest that neural systems that transmit the oropharyngeal positive-feedback signals that drive food intake or that modulate their strength are candidate targets of neurons that express BDNF. Therefore, neurons in the orosensory regions of the nucleus of the solitary tract (NTS) and parabrachial nucleus, or their synapses with interneurons and motor neurons that control ingestion, are probably among the ultimate targets of BDNF neurons that influence intake. The BDNF neurons that have been most strongly implicated in mature-onset obesity are located in the adjacent caudal and dorsomedial regions of the VMH (23, 46). In the rat these VMH subnuclei project directly to the NTS, to the parabrachial nucleus via the periaqueductal gray (6), and to both these nuclei indirectly via basal forebrain regions, including nuclei of the amygdala and bed nucleus of the stria terminalis (6, 7, 10, 29). Thus it will be important to determine which of these pathways contain neurons that express BDNF and to establish whether those neurons modulate orosensory controls of food intake.
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Present address for M. Byerly: Dept. of Animal and Avian Science, Univ. of Maryland, College Park, MD 20742.
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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) exhibited a similar magnitude of hyperphagia relative to young wild types (
) throughout the 7 wk of testing and rapidly became obese (significant increase in body weight by the end of week 2; see RESULTS). Data plotted are means ± SE based on data averaged over 7-day blocks with the exception of the first point (block 0), which expresses the data on day 1 of the experiment.