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J. Anim. Sci. 2005. 83:1023-1032
© 2005 American Society of Animal Science

ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION

Effects of short-term feed deprivation and melatonin implants on circadian patterns of leptin in the horse1

P. R. Buff*, C. D. Morrison{ddagger}, V. K. Ganjam{dagger} and D. H. Keisler*,2

* Departments of Animal Sciences and and {dagger} Biomedical Sciences and Veterinary Medicine and Surgery, University of Missouri, Columbia 65211; and and {ddagger} Pennington Biomedical Research Center, Baton Rouge, LA 70817


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Literature Cited
 
Leptin is a protein hormone produced by adipose tissue that influences hypothalamic mechanisms regulating appetite and energy balance. In species tested thus far, including horses, concentrations of leptin increase as animal fat mass increases. The variables and mechanisms that influence the secretion of leptin are not well known, nor is it known in equine species how the secretion of leptin is influenced by acute alterations in energy balance, circadian patterns, and/or reproductive competence. Our objectives were to determine in horses: 1) whether plasma concentrations of leptin are secreted in a circadian and/or a pulsatile pattern; 2) whether a 48-h period of feed restriction would alter plasma concentrations of leptin, growth hormone, or insulin; and 3) whether ovariectomy and/or a melatonin implant would affect leptin. In Exp. 1, mares exposed to ambient photoperiod of visible light (11 h, 33 min to 11 h, 38 min), received treatments consisting of a 48-h feed restriction (RES) or 48 h of alfalfa hay fed ad libitum (FED). Mares were maintained in a dry lot before sampling and were tethered to a rail during sampling. Analyses revealed that leptin was not secreted in a pulsatile manner, and that mean leptin concentrations were greater (P < 0.001) in FED vs. RES mares (17.20 ± 0.41 vs. 7.29 ± 0.41 ng/mL). Plasma growth hormone was pulsatile, and mean concentrations were greater in RES than FED mares (2.15 ± 0.31 vs. 1.08 ± 0.31 ng/mL; P = 0.05). Circadian patterns of leptin secretion were observed, but only in FED mares (15.39 ± 0.58 ng/mL for morning vs. 19.00 ± 0.58 ng/mL for evening; P < 0.001). In Exp. 2, mares that were ovariectomized or intact received either a s.c. melatonin implant or a sham implant. Thereafter, blood was sampled at weekly intervals at 1000 and 1700. Concentrations of leptin in samples collected at 1700 were greater (P < 0.001) than in those collected at 1000 (28.24 ± 1.7 vs. 22.07 ± 1.7 ng/mL). Neither ovariectomy nor chronic treatment with melatonin affected plasma concentrations of leptin or the circadian pattern of secretion. These data provide evidence that plasma leptin concentrations in the equine are sensitive to acute changes in nutritional status and vary in a circadian pattern that is sensitive to fasting but not to melatonin treatment or ovariectomy.

Key Words: Equine • Feed Deprivation • Leptin • Melatonin


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Literature Cited
 
A growing proportion of horses are obese due to poor nutritional management and sedentary lifestyles. Not withstanding these management issues, certain horses are prone to becoming obese (i.e., "easy keepers"), even when their contemporaries are not, making these horses difficult to manage. One concern with obesity in horses is that it predisposes and aggravates the condition of laminitis and other maladies. Recently, Johnson (2002)Go reviewed the equine metabolic syndrome and asserted that understanding the mechanisms involved in appetite regulation and feeding behavior should offer insight into preventing or managing obesity in horses.

When an animal is in a positive energy balance, circulating concentrations of the hormone leptin are increased (Schwartz et al., 2000Go). Leptin is a protein hormone produced by adipocytes, and investigations in several species, including horses, have demonstrated a positive relationship between blood concentrations of leptin and BCS (Delavaud et al., 2000Go; Estienne et al., 2000Go; Buff et al., 2002Go). Conversely, short-term feed restriction decreases blood concentrations of insulin (Ralston et al., 1979Go) and increases concentrations of GH in horses (Sticker et al., 1995Go). Each of these hormones affects appetite and body fat mass. Potentially confounding these issues is that fact that horses are seasonal animals (Ginther, 1992Go), and thus are sensitive to changes in photoperiod and melatonin. Studies in humans indicate that leptin concentrations may be influenced by circadian rhythms in addition to nutritional status (Langendonk et al., 1998Go; Saad et al., 1998Go). Therefore, our objectives were to determine in horses 1) whether peripheral concentrations of leptin exhibit a circadian and/or a pulsatile profile; 2) whether a 48-h feed restriction would alter peripheral concentrations of leptin, GH, and/or insulin; and 3) whether ovariectomy and/or treatment with melatonin would affect peripheral profiles of leptin.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Literature Cited
 
Animals and Procedures

The University of Missouri Animal Care and Use Committee approved all procedures with live animals. Test subjects for all experiments consisted of crossbred (Quarter Horse x Shetland or Welsh type) pony mares ranging in age from 4 to 14 yr, with a mean BW of 273.6 ± 17.9 kg. All mares were managed on a forage-based diet to maintain a BCS of 6 (Henneke et al., 1983Go) before and during experimentation.

Experiment 1

Eight seasonally anestrous (determined via rectal ultrasound of ovaries) crossbred pony mares were divided randomly into two groups (A and B) of four mares per group. Experimentation was conducted when the visible light ranged from 11 h, 33 min to 11 h, 38 min per day. During this period, sunrise and sunset ranged between 0704 to 0701 and 1742 to 1744, respectively. Mares were contained in a three-sided shed with the open side facing west. Before and during experimentation, horses were maintained at ambient light and temperature. A crossover design with two periods was employed, such that in Period 1, animals in Group A received ad libitum access to alfalfa hay (FED) for 48 h, and animals in Group B were feed-restricted by removing all feed sources (RES) for 48 h, with treatments being reversed in Period 2 (Figure 1Go). Water was freely available at all times to all mares, and alfalfa hay was freely available during FED treatment. The first 24 h of each treatment period was used as a transitional acclimation interval between the nutritional states. One day before treatment, the mares were fitted with two jugular cannulas to aid in the collection of blood samples. Blood samples were collected every 10 min during the second 24 h of each 48-h period (Figure 1Go). Nighttime samples were collected with the aid of flashlights, fitted with red lenses to minimize light exposure. Blood was collected (3 mL) into tubes containing 100 µL of 0.05 M EDTA. Blood samples were stored at 4°C for no more than 12 h and centrifuged at 3,000 x g for 25 min at 4°C, after which plasma samples were stored at –20°C until assayed.



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  		Figure 1. Mares were assigned to one of two treatments consisting of a 48-h feed restriction (RES) or fed alfalfa hay ad libitum for 48 h (FED). A crossover design encompassing two periods was used, such that in Period 1, half the ponies were RES and the other half FED, with treatments alternated in Period 2. The first 24 h of each treatment period was used as a transitional acclimation interval between the FED and RES states. Blood samples were collected every 10 min for 24 h.

 
Experiment 2

Seven chronically ovariectomized mares (ovariectomized 2 yr before the study) and eight ovary-intact pony mares were used for this experiment (initial BW = 273.6 ± 17.9 kg). The ovarian status of intact mares was determined by rectal ultrasound of the ovaries, and all mares were cycling normally at the beginning and end of the experiment. Mares were maintained as a single herd on fescue grass pasture with free access to water and salt for the duration of the experiment. Mares were held in a drylot on days when samples were collected and offered free-choice fescue hay that was harvested from the pasture 2 mo before experimentation.

Four mares from each group were implanted with 144-mg melatonin silo-polymer implants (Regulin, Sanofi, Watford, U.K.); the remaining mares received sham implants. Plasma samples were collected weekly during two intervals, beginning at 1000 and 1700. Samples were collected every 15 min for 1 h, resulting in four samples from each mare at 1000 and 1700. This sampling protocol was repeated weekly from July 7 through October 7, 2000. The BW of the mares was measured weekly following the 1000 sample. Blood was collected via jugular venipuncture using Vacutainer (Becton Dickinson, Franklin Lakes, NJ) tubes containing EDTA. Blood samples were kept on ice and centrifuged at 3,000 x g for 25 min at 4°C within 12 h of collection to collect the plasma, which was then stored at –20°C until assayed.

Hormone Analyses

Plasma samples were analyzed in duplicate with 200 µL for leptin with double-antibody RIA procedures previously validated for equine plasma (Buff et al., 2002Go). The intra- and interassay CV were <10%. Plasma concentrations of equine GH were determined as described by Thomas et al. (1998)Go, in duplicate, using a volume of 200 µL. The intra- and interassay CV were <10%. Plasma concentrations of insulin were measured in duplicate using a volume of 200 µL with a RIA kit (Diagnostic Products Corp., Los Angeles, CA), validated for use with equine plasma by Freestone et al. (1991)Go. The intra- and interassay CV were <10%.

Statistical Analyses

Experiment 1. To determine the effects of feed restriction on leptin, insulin, and GH, mean concentrations of hormones were determined in each mare in each 24-h period. Analysis of variance was performed using GLM procedures of SAS (v. 8, SAS Inst., Inc., Cary, NC). To determine effects of photoperiod, mean concentrations of leptin were grouped as A.M. and P.M. samples in each mare in each 24-h sampling interval. The A.M. samples were collected from 0800 to 1210, whereas P.M. samples were those collected from 2000 to 2400. Time periods were selected to occur 12 h apart and to encompass the periods of light and dark that follow the events of sunrise and sunset, respectively. Effects within the model included animal, treatment, replicate, photoperiod (A.M. vs. P.M.), and treatment x photoperiod. Pulse frequencies and pulse amplitude of leptin and GH were determined for each animal within each treatment group using the Cluster pulse analysis program (Veldhuis and Johnson, 1986Go). The criteria for determining pulsatile secretion of leptin and GH are outlined in Table 1Go, and were previously described by Veldhuis and Johnson (1988)Go. The GLM procedures of SAS were used to determine whether frequency and amplitude differences existed between RES and FED treatments. The model tested the effects of animal and treatment on pulse frequency and pulse amplitude, using the residual error as the error term.


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  		Table 1. Criteria for determining pulse secretion of leptin and growth hormonea
 
Experiment 2. Analyses were conducted to determine whether differences existed in concentrations of leptin due to melatonin treatment for each sampling period. Data from 1000 (A.M.) and 1700 (P.M.) were analyzed as separate analyses using a repeated-measures design using the Mixed procedure of SAS. Effects used in the model were treatment (melatonin vs. control), sample (time), ovarian status (intact vs. ovariectomized), and all interactions of the previous variables on plasma concentrations of leptin. The variable of sample was used in the repeated statement, and the error term used was animal within treatment x ovarian status. The compound symmetry heterogeneous covariance structure was determined by Akaike information criterion (AIC) and Schwarz Bayesian criterion (SBC) as providing the best fit for analyses of plasma concentrations of leptin (Littell et al., 1998). Least squares means were generated for treatment x sample and treatment x ovarian status. A similar analysis was performed to determine whether differences existed between A.M. and P.M. concentrations of leptin. Effects used in the model were treatment (melatonin vs. control), time (A.M. vs. P.M.), ovarian status, and all interactions of the previous variables on plasma concentrations of leptin. The variable of time was used in the repeated statement, and the error term used was animal within treatment x ovarian status. Covariance structure compound symmetry was determined by AIC and SBC as best fitting for analyses of plasma concentrations of leptin. Least squares means were generated for the variables of time and treatment x time. Effects of the variation associated with treatment, time (week), for the ovarian status, and all interactions on BW were analyzed as a repeated-measures design using mixed model procedures of SAS. Initial individual BW was used as a covariate due to initial mean BW difference between the intact and ovariectomized groups. The variable week was used in the repeated statement, and the error term consisted of individual within treatment x ovarian status. Covariance structure Toeplitz was determined by AIC and SBC as providing the best fit for analyses of BW. Least squares means were generated for treatment x time. To determine differences between A.M. and P.M. concentrations of leptin, we performed an ANOVA using the Mixed procedure of SAS. Effects used in the model were week and time (A.M. vs. P.M.) and the interactions of these variables on plasma concentrations of leptin. The variable week was used in the repeated statement, and the error term used was animal. The compound symmetry covariance structure was determined by AIC and SBC as providing the best fit for analyses of plasma concentrations of leptin. Least squares means were generated for week x time, and posttest comparisons were made between A.M. and P.M. within a given week.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Literature Cited
 
Experiment 1

Mean plasma concentrations of insulin were 17.98 ± 4.49 and 50.48 ± 4.49 µIU/mL for RES and FED mares, respectively (P < 0.001), indicating that the RES mares had lower plasma insulin concentrations. Feed deprivation lead to a decrease in mean plasma concentrations of leptin (7.29 ± 0.52 and 17.20 ± 0.52 ng/mL for RES and FED mares, respectively; Figure 2Go; P < 0.001). Additionally, a circadian pattern of leptin was detected in FED mares, with P.M. concentrations being greater than A.M. concentrations (19.00 ± 0.58 vs. 15.39 ± 0.58 ng/mL, respectively; Figure 2Go; P < 0.001); however, this pattern was not detected in RES mares (P = 0.89). We did not observe any rhythmic pulsatile characteristics of leptin in either FED or RES mares (Figure 3Go). Mean plasma concentrations of GH were 2.14 ± 0.31 vs. 1.07 ± 0.31 ng/mL in the RES vs. FED animals, respectively (P = 0.05). Rhythmic pulsatile characteristics of GH secretion were observed in both treatment groups (Figure 4Go). Additionally, differences in GH pulse frequency were observed, such that a greater number (P < 0.01) of pulses existed in the RES group (4.00 ± 0.40 pulses/24 h) than in the FED group (1.50 ± 0.40 pulses/24 h). A greater GH pulse amplitude (P = 0.05) was observed in the RES group (12.04 ± 1.51 ng/mL) than in the FED group (7.20 ± 1.51 ng/mL).



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  		Figure 2. Mean concentrations of leptin in mares fed alfalfa hay ad libitum for 48 h (FED) or feed-restricted for 48 h (RES). Samples collected from 0800 to 1210 were designated as A.M., and those collected from 2000 to 2400 as P.M. Mean plasma concentrations of leptin were less in RES vs. FED mares, P = 0.001. Mean plasma concentrations of leptin were greater in the P.M. than A.M. in FED mares, P = 0.001. No differences were observed between A.M. and P.M. concentrations of leptin in RES mares.

 


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  		Figure 3. Changes in plasma concentrations of leptin over time in two mares receiving alfalfa hay ad libitum (FED) or feed restricted (RES). Samples were collected every 10 min for 24 h. No differences in pulse frequency or pulse amplitude were observed. Pulse criteria: one data point for a peak; two data points for a nadir; and 5 ng/mL for a pulse minimum value. Samples collected between 0800 and 1210 were predetermined to be A.M. samples (between solid vertical lines) and between 2000 and 2400 were predetermined to be P.M. samples (between dashed vertical lines).

 


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  		Figure 4. Changes in plasma concentrations of GH over time in two mares receiving ad libitum alfalfa hay (FED) or feed-restricted (RES). Samples were collected every 10 min for 24 h. Pulse frequency was greater in the RES treatment (4.00 ± 0.40 pulses/24 h) than in the FED treatment (1.50 ± 0.40 pulses/24 h; P < 0.01). Pulse amplitude also was greater in the RES treatment (12.04 ± 1.51 ng/mL) than in the FED treatment (7.20 ± 1.51 ng/mL; P = 0.05. Pulse criteria: one data point for a peak; two data points for a nadir; and 5 ng/mL for a pulse minimum value.

 
Experiment 2

No differences in mean plasma concentrations of leptin were observed in ovariectomized vs. intact mares (P = 0.83), and there was no effect of chronic melatonin treatment on leptin concentrations or BW change in either ovariectomized or intact mares over the course of the experiment. Greater concentrations of leptin were observed in the P.M. sample than in the A.M. sample regardless of treatment (28.24 ± 1.7 vs 22.07 ± 1.7 ng/mL; P < 0.001), indicating that melatonin did not influence the circadian patterns of leptin secretion. As there were no effects of ovarian status and melatonin on leptin concentrations, we evaluated the data to determine differences in concentrations of leptin over time between A.M. and P.M. Results indicated that concentrations of leptin were greater in P.M. samples than in the A.M. samples over the duration of the experiment (Figure 5Go; P < 0.001).



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  		Figure 5. Mean plasma concentrations of leptin over time in samples collected at A.M. (1000) and P.M. (1700) sampling for all mares regardless of ovarian status or melatonin treatment. Data are least squares means. Concentrations of leptin were greater in the P.M. samples than in A.M. samples for the duration of the experiment, P < 0.001. Posttest comparisons of least squares means were made between A.M. and P.M. within a given week. Significance denoted by the following: {dagger}P < 0.10; *P < 0.05; **P < 0.01; ***P < 0.001. Jul, Aug, Sep, and Oct = July, August, September, and October, respectively.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Literature Cited
 
In recent years, there has been an emerging interest in equine obesity and related health issues potentially associated with equine obesity as reviewed in Johnson (2002)Go. We propose that equine obesity may be on the rise, as is obesity in the human population in the United States, where it is considered to be an epidemic (Mokdad et al., 2001Go). Many horses fall into the role of companion animals rather than work animals, and live a lifestyle that includes poor dietary management and lack of exercise. This lifestyle is compounded in individual horses known as "easy keepers," which have an increased propensity to develop obesity. The diets of these animals are difficult to manage, and they tend to gain BW even when fed a low-energy diet (NRC, 1989Go). The seasonally reproductive nature of the horse may partially control the regulation of adiposity. Reproductive events that coincide with an increase in photoperiod, such as maintenance of pregnancy, lactation, and stallion breeding activity, require a dramatic increase in nutrient availability (NRC, 1989Go). In addition to preparation for these events, horses increase consumption of feed to increase and maintain energy stores due to decreasing ambient temperature (Brody, 1945Go). Thus, it is possible that circadian variation in hormones associated with feeding and metabolism may underlie these effects. The aim of the current study was to better describe the patterns of leptin associated with nutritional and circadian status, with the potential that these observations will offer insight for developing feeding strategies for the obese horse.

Feed restriction of mares for 48 h suppressed plasma concentrations of leptin compared with mares receiving feed ad libitum, suggesting that leptin is not only sensitive to chronic levels of body adiposity (Buff et al., 2002Go), but also sensitive to more acute changes in nutritional status. Additionally, a significant nocturnal rise in leptin also was noted in mares receiving ad libitum feed, but this rise was absent in mares following a 48-h feed restriction.

The suppressive effect of a 48-h feed restriction corroborates previous unpublished work from our laboratory, in which pony mares were subjected to a 24-h feed restriction, and plasma concentrations of leptin tended to be lower during the final hour of the treatment. Other investigators (McManus and Fitzgerald, 2000Go) have reported similar findings with a 24-h restriction. However, no post hoc comparisons were made in that study to determine treatment differences between specific points in time. Our data agree with observations in humans, where a fast of 2.5 d resulted in lower peripheral concentrations of leptin (Bergendahl et al., 2000Go). We hypothesized that our inability to detect a significant effect after a 24-h feed restriction was due to the retention of feed in the equine hindgut and the slow rate of gut empting in the horse. Hintz and Loy (1973)Go determined that when horses were fed a meal consisting of 50% alfalfa hay combined with grain products, that less than 30% of the meal passed through the gut within 27 h. Consequently, we chose to determine the effects of a 48-h feed restriction on plasma concentrations of leptin, and in the current experiment, plasma leptin was decreased in response to feed restriction. Furthermore, the observation that P.M. plasma concentrations of leptin in pony mares were greater than A.M. values is similar to what has been observed in humans (Simon et al., 1998Go). In a recent study by Piccione et al. (2004)Go, significant suppression in concentrations of leptin in horses was observed when they were deprived of feed for more than 48 h, but in contrast to the results of the present study, a suppression of the circadian variation was not observed following feed deprivation. The differences between the findings of Piccione et al. (2004)Go and our results could be due to either management or breed differences. They used Thoroughbred mares that were housed in stalls and fed three meals daily compared with the crossbred pony mares in the present study that were maintained on pasture and ad libitum hay. The findings of Piccione et al. (2004)Go support our theory that horses need to be restricted from feed for more than 24 h to suppress concentrations of leptin. Therefore, leptin seems to be influenced by circadian patterns that were acutely sensitive to feed restriction in our group of pony mares, although the mechanism is unknown.

When evaluating concentrations of leptin, we found no evidence of a pulsatile pattern of secretion, which contrasts observations in humans (Licinio et al., 1997Go; Saad et al., 1998Go). Variation in concentrations of leptin occurred, but these were atypical of a pulse as characterized by the Cluster pulse analysis program, as outlined in Table 1Go. The criteria used included a constant CV of 5% for assay precision, a df = 1 for sample duplicates, a time period between samples of 10 min, two points for a nadir, one point for a peak, the t-statistic was set at a 5% false positive rate, and the minimum value for a pulse was 5 ng. That is, we observed increases and decreases in concentrations of leptin in all mares; however, these changes in concentrations did not fit the characteristics of a hormone pulse as defined by the Cluster program. Hormones that are secreted in pulses must also have a pulse generator that regulates the episodic secretion (Knobil, 1989Go), and to our knowledge, a leptin pulse generator has not been identified.

The effects of fasting on peripheral concentrations of insulin and GH in this study agree with results observed in lambs (Foster et al., 1989Go), pigs (Vandergrift et al., 1985Go), horses (Ralston et al., 1979Go; Sticker et al., 1995Go; 1996Go), and humans (Phillips, 1986Go), providing evidence that the 48-h feed restriction was physiologically significant and influenced a variety of endocrine systems. The mechanisms that dictate the nutritional consequences influencing the secretion of GH also are not completely understood. Interestingly, leptin receptors are expressed within both the hypothalamus and anterior pituitary gland, and studies in sheep (Nagatani et al., 2000Go; Henry et al., 2001Go; Morrison et al., 2001Go) and pigs (Barb et al., 1998Go) have shown that leptin increases GH secretion. In undernourished ewes, an increase in the expression of leptin receptor mRNA has been observed by Dyer et al. (1997)Go, which may be responsible for the increase in GH secretion. Leptin has been demonstrated to regulate secretion of GH in rats following an intracerebroventricular infusion of leptin antiserum, indicating the existence of hypothalamic regulatory mechanism by leptin (Carro et al., 1997Go).

Because we detected circadian changes in leptin, we next tested whether melatonin mediates this nocturnal increase in leptin. Melatonin production is acutely regulated by light, plays an intricate role in the seasonal variation of endocrine profiles in the equine, and is crucial for the seasonal onset of the ovulatory cycles in mares (Grubaugh et al., 1982Go). In obese and normal human subjects, nocturnal melatonin concentrations were decreased when individuals were subjected to a short-term fast (Rojdmark et al., 1989Go, 1992Go). Therefore, melatonin seemed a likely candidate to explore as a regulator of leptin and adiposity in equine. We chose to administer melatonin with an implant that was designed to provide a constant release of melatonin to the animal. This was a reliable method to ensure mares were exposed to elevated concentrations of melatonin during the experiment. Regulin implants have been previously used in horses and were demonstrated to increase peripheral concentrations of melatonin for 12 wk (Guerin, 1997Go). The current results demonstrate that when s.c. implants of melatonin were given to mares in the fall, peripheral concentrations of leptin were not affected. Results similar to the current findings were observed when lower doses of s.c. implants of melatonin were given to horses (Fitzgerald and McManus, 2000Go). When male rats were treated orally with melatonin, subsequent concentrations of leptin and visceral fat mass decreased significantly (Rasmussen et al., 1999Go; Wolden-Hanson et al., 2000Go). In contrast, when female minks were treated with s.c. implants of melatonin, peripheral concentrations of leptin increased (Mustonen et al., 2000Go). It is unknown why these responses differ among species. The possibility exists that differences could be due to a photoperiod effect on different species (Adam and Mercer, 2004Go). The methods of melatonin treatment in each of these studies differed, which could potentially affect the leptin response. In other studies, rodents were treated with melatonin in their drinking water (Rasmussen et al., 1999Go; Wolden-Hanson et al., 2000Go), whereas in the present studies, pony mares were treated with s.c. implants. We conclude that s.c. implants of melatonin do not affect peripheral concentrations of leptin, nor do they effect BW change in mares. Because melatonin has a distinct circadian pattern of secretion, it is possible that alterations in melatonin might mediate the circadian pattern of leptin.

In agreement with results from Exp. 1, there were greater concentrations of leptin in samples collected at 1700 than in those collected at 1000. The repeatability of this observation of leptin provides evidence of a circadian pattern of leptin secretion. Interestingly, the concentrations of leptin in Exp. 2 seemed to be greater than those in Exp. 1. We speculate that this difference may reflect a seasonal influence on leptin, as Exp. 1 was conducted in February, and Exp. 2 was conducted from July through October of the same year. These variations in leptin may be a result of seasonal sensitivity to leptin, causing resistance during the fall to allow an increase in adiposity, as reported in seasonal-breeding Siberian hamsters (Rousseau et al., 2002Go); however, the seasonal changes in adiposity observed in horses is not as great as the 40% decrease of BW reported in Djungarian hamsters (Steinlechner et al., 1983Go). The theory of seasonal sensitivity to leptin may be a key in understanding equine obesity. Horses that increase adiposity in the fall in preparation for production and environmental demands for energy may not use those energy stores under many modern management practices. Maintaining the energy stores throughout the year may further decrease the sensitivity to leptin and cause an even greater accumulation of adipose tissue in the fall.

In summary, results from the current experiments provide evidence that secretion of leptin is acutely sensitive to fasting in the equine, which suggests that circulating concentrations of leptin are sensitive to acute perturbations in energy balance. Additionally, these data provide evidence that leptin also is secreted in a circadian pattern, with concentrations increasing during the nighttime hours.


   Footnotes
 
1 The authors thank K. Barnhart, E. Berg, J. Demko, J. Hampton, B. McFadin, H. Monica, G. Perry, J. Seamen, E. Wells, and J. Witherspoon for their assistance. Back

2 Correspondence: 160 Anim. Sci. Res. Center (phone: 573-882-7267; fax: 573-882-6827; e-mail: keislerd{at}missouri.edu).

Received for publication October 26, 2004. Accepted for publication January 31, 2005.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Literature Cited
 


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