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Keywords:

  • adrenocorticotrophic hormone;
  • cortisol;
  • gender;
  • ghrelin;
  • growth hormone;
  • sleep endocrinology

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Ghrelin, an endogenous ligand of the growth hormone (GH) secretagogue receptor, stimulates sleep, appetite and weight gain as well as the secretion of GH, adrenocorticotropic hormone (ACTH), cortisol in humans and rodents. The interaction between nocturnal ghrelin levels, sleep EEG and the secretion of these hormones was not investigated systematically so far. Furthermore conflicting data exist on gender differences in nocturnal ghrelin secretion. We examined simultaneously sleep EEG and the nocturnal levels of ghrelin, GH, ACTH and cortisol in young and middle-aged normal human subjects (eight males, eight females). A significant interaction between gender and the course of ghrelin concentration was observed to the interval between 20:00 and 23:00 hours. In males a continuous increase of ghrelin levels before sleep onset was found. In females, however, a rise of ghrelin during the night was missed. We found a trend suggesting a lower time spent in stage I sleep in subjects with high nocturnal ghrelin levels. Other systematic interactions between plasma ghrelin, sleep EEG and other hormones were not found. No peak in plasma ghrelin levels resembling the GH surge was observed. We suggest that under naturalistic conditions plasma ghrelin levels show no distinct interaction with sleep.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Ghrelin is a peptide hormone acting at the growth hormone (GH) secretagogue (GHS) receptor. It was isolated from the stomach and CNS of rats and humans (Kojima et al., 1999). The substance participates in the regulation of endocrine activity, energy balance and sleep. Since the sleeping period is a time of considerable activity in various endocrine systems, we were interested in the sleep-related pattern of ghrelin levels and its interaction with sleep EEG and other hormones. Given the fact that levels of leptin, which appears to counteract the influence of ghrelin on the energy balance, differ distinctly between women and men, we also investigated gender differences in nocturnal ghrelin concentrations (Antonijevic et al., 1998; Saad et al., 1997).

Besides GH-releasing hormone (GHRH) and somatostatin, ghrelin appears to be another endogenous factor in the regulation of GH release. After ghrelin administration, GH secretion was enhanced in rats (Kojima et al., 1999) and humans at daytime (Peino et al., 2000) and around sleep onset (Weikel et al., 2003). Obviously ghrelin shares the capacity of GHRH to stimulate GH. A synergism between these peptides is suggested in this action, whereas they were also shown to be partly independent from each other (Horvath et al., 2001). GHRH and ghrelin differ in their effect on the hypothalamo-pituitary-adrenocortical (HPA) system. In male subjects after repetitive i.v. administration of GHRH around sleep onset HPA hormones are blunted (Antonijevic et al., 2000; Steiger et al., 1992), whereas after ghrelin in an analogue protocol cortisol and adrenocorticotrophic hormone (ACTH) are increased in young males (Weikel et al., 2003). Similarly cortisol and ACTH are elevated in humans after ghrelin at daytime (Arvat et al., 2001). We suggest that ghrelin may act as an interface between the HPA and the hypothalamo-pituitary-somatotrophic (HPS) systems (Weikel et al., 2003).

The key role of ghrelin in the energy balance is well documented (Horvath et al., 2001). After ghrelin food intake in rodents (Tschöp et al., 2000; Wren et al., 2001b) and humans (Wren et al., 2001a), appetite in humans (Schmid et al., 2005; Wren et al., 2001a) and body weight in rodents (Tschöp et al., 2000) increase. Ghrelin levels are blunted in obese patients (Tschöp et al., 2001) and are elevated in patients with anorexia (Otto et al., 2001). Weight gain during antidepressive drug treatment is related to a decrease of ghrelin levels (Steiger et al., 2003). It is thought that a reciprocal interaction exists between the orexigenic ghrelin and the anorexigenic leptin (Horvath et al., 2001).

Additionally, ghrelin promotes sleep. Non-REM sleep increased after systemic ghrelin in mice (Obál et al., 2003). Similarly, after repetitive i.v. ghrelin slow wave sleep (SWS) increases in young normal males (Weikel et al., 2003). These data show that ghrelin shares the sleep-promoting effect of GHRH (Obál and Krueger, 2002). In mice, intact GHRH receptors show to be the prerequisite for this effect (Obál et al., 2003).

In humans, the sleep EEG and the nocturnal secretion of various hormones show distinct patterns. During the first half of the night the GH peak and the major portion of SWS preponderate, whereas HPA activity is low. In contrast during the second half of the night the amounts of GH and SWS are low and the major amounts of cortisol, ACTH and rapid eye movement (REM) sleep occur. There is good evidence that neuropeptides are common regulators of the neurophysiological and the endocrine components of sleep. It is thought that GHRH promotes SWS and GH around sleep onset, which is followed by corticotropin-releasing hormone in the early morning hours (Steiger, 2003). Leptin also shows variations throughout the night with a maximum between 00:00 and 04:00 hours (Licinio et al., 1997). Leptin levels are about threefold higher in females than in males (Antonijevic et al., 1998; Deuschle et al., 1996; Saad et al., 1997).

The knowledge accumulated on ghrelin so far points to its role in the regulation of rhythms. This view is further supported by the observation that ghrelin containing neurons are located adjacent to the third ventricle between the dorsal, central, paraventricular and arcuate hypothalamic nuclei. It is claimed that the space occupied by ghrelin immunoreactive cells overlaps the hypothalamic projections from the suprachiasmatic nucleus and the central lateral geniculate body of the thalamus. The authors hypothesised that central ghrelin mediates circadian information from these sites of the hypothalamus (Cowley et al., 2003).

Little is known about the sleep-related levels of ghrelin. Cummings et al. (2001) investigated ghrelin levels in humans (nine women, one man, 29–63 years old) over 24 h. Specimens were collected at 30-min intervals from 08:00 to 21:00 hours, then hourly until 08:00 hours the next morning. These authors reported increases of ghrelin before meals and another peak around midnight resembling the nocturnal GH peak. Ghrelin fell to trough levels within 1 h after eating. Intermeal ghrelin levels are found to rise throughout the day to zenith at 01:00 hours, then falling overnight to a nadir at 09:00 hours. Dzaja et al. (2004) compared intra-individually during 24-h ghrelin concentrations assessed hourly in young normal male subjects who were in supine position under two conditions. They either slept at night or were sleep deprived.

In contrast to Cummings et al. (2001), in the sleeping condition there is no continuous rise throughout the day, but a distinct increase in ghrelin concentration at 01:00 hours is observed which is followed by a steady decline until the morning. Ghrelin secretion is blunted, however, during sleep deprivation and ghrelin levels increase only slightly until the early morning. From their data, Dzaja et al. (2004) suggested that sleep is a stimulus for ghrelin release, whereas Cummings et al. (2001) hypothesised that ghrelin displays a diurnal rhythm which is positively regulated by fluxes in the overall energy balance. In rats, no significant correlation between GH and ghrelin circulating levels is found, whereas mean interpeak intervals and pulse frequencies are close for the two hormones (Tolle et al., 2002). In another study in rats, plasma ghrelin displays diurnal rhythms with a peak preceding the major daily feeding peak after dark onset (Bodosi et al., 2004). Two recent studies report relationship between sleep duration and ghrelin levels. Spiegel et al. (2004) compared nights with sleep restriction and nights with sleep extension in normal male controls. Sleep restriction is associated with increases in ghrelin levels during daytime from 09:00 to 21:00 hours. Similarly, in a sample of 1024 volunteers an association between short sleep and elevated morning ghrelin levels is found (Taheri et al., 2004).

There exists no simultaneous investigation of sleep EEG and nocturnal levels of ghrelin and hormones which may interact with its secretion, e.g. GH and HPA hormones employing a sampling rate of specimens below 1 h. Finally, studies on gender differences in ghrelin activity show conflicting results. In healthy and obese children morning ghrelin levels are independent of gender (Bellone et al., 2002). In 60 adult men and women of widely varying ages and weights (Purnell et al., 2003) and in healthy elderly women and men (Rigamonti et al., 2002) ghrelin levels do not differ between sexes. Espelund et al. (2005) sampled ghrelin concentrations every 3 h from 12 to 84 h of fasting. They describe diurnal changes with a nadir in the morning, peak levels in the afternoon and a gradual decline during the night. Ghrelin levels remain higher in women than in men throughout the study. Glucose load decreases ghrelin levels in adult subjects. This response is modulated by sex, in that levels are higher in females (Greenman et al., 2004). In young rats stomach ghrelin mRNA levels are not gender dependent (Gualillo et al., 2001). In contrast, in older female mice ghrelin mRNA is higher than in males (Liu et al., 2002).

We wanted to investigate interactions between ghrelin, sleep EEG and other hormones and to elucidate gender differences in these variables. Therefore, we examined sleep EEG and the sleep-related levels of ghrelin, GH, ACTH and cortisol in young and middle-aged normal controls of both sexes.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Subjects

The subjects were 16 healthy volunteers, eight males (mean age ± SD: 28.5 ± 8.2 years, range 19–46) and eight females (30.8 ± 8.3 years, range 22–49). Age distribution did not differ between male and female subjects (P = 0.593). All subjects were free of any psychiatric or medical conditions. The body mass index (BMI) of the male subjects (23.0 ± 0.7) was significantly higher (P < 0.05) than the BMI of the female subjects (20.5 ± 2.1). All women had regular menstrual cycles and were not taking hormonal contraceptives. They were examined in the follicular phase (between days 4 and 6).

The experiment was approved by the Ethics Committee for Human Experiments at the University of Munich. After the purpose of the study had been explained to the subjects, all of them gave their informed consent according to the tenets of the declaration of Helsinki. They underwent extensive psychiatric, physical and laboratory (hematology, virology, clinical chemistry, endocrinology, EEG and electrocardiographic) examinations. Individuals with a personal or family history of psychiatric disorders or a recent stressful life event were excluded, as were shift workers and persons who had recently made a transmeridian flight.

Subjects were interviewed about their usual sleep habits and were included in the study, when these were similar to the bed times (23:00 lights off, 07:00 lights on) in the sleep laboratory. Further, they were asked to keep this bed time habits in the week before the experimental procedure.

Other exclusion factors were abuse of drugs, nicotine (more than two cigarettes per day), alcohol and caffeine. Consumption of alcohol was not allowed throughout the study period starting 1 week prior to the adaptation night. Caffeine was restricted to 200 ml coffee in the morning. The subjects had standardised meals at 08:00, 12:00 and 18:00 hours with a macronutrient composition about 50% carbohydrates, 20% proteins and 30% fat. The median energy intake per day was about 30 kcal kg1 bodyweight). It was ensured that there was no history of drug treatment during the previous 3 months.

Experimental procedure

The experimental sessions consisted of two successive nights in our sleep laboratory. The first night served for adaptation to the laboratory setting. On the second night, an indwelling i.v. catheter was inserted at 19:30 hours and connected to plastic tubing that ran through a soundproof lock into the adjacent room. This allowed repeated blood collection in the adjacent laboratory without disturbing the study subject. All subjects were in a supine position from 20:00 hours. Blood samples were collected every 30 min between 20:00 and 22:00 hours and every 20 min between 22:00 and 07:00 hours. The subjects were observed on a television screen in the adjacent room.

Sleep EEG recordings

Electrodes for polysomnographic recordings (Comlab 32 Digital Sleep Lab, Brainlab V 3.3 Software, Schwarzer GmbH, Munich, Germany) were fixed between 21:00 and 22:00 hours. The subjects were not allowed to sleep until the lights were turned off at 23:00 hours. Polysomnographic recordings were obtained from 23:00 to 07:00 hours and consisted of two EEGs (C3–A2, C4–A1; time constant 0.3 s, low-pass filtering 70 Hz), vertical and horizontal electro-oculograms (EOG) and electromyogram (EMG).

Conventional sleep EEG analysis

Sleep stages [wakefulness, stages I–IV sleep (stages III and IV sleep is SWS) and REM sleep] were scored visually in all subjects off-line according to conventional criteria (Rechtschaffen and Kales, 1968) by a rater who was unaware of the aim of the study. Calculations of sleep parameters included time in bed (TIB), sleep onset latency (time between lights off and the first occurrence of stage II sleep), SWS latency (interval between sleep onset and the first 30-s epoch including stage III sleep), REM latency (interval between sleep onset and the first 30-s epoch including REM sleep), number of awakenings and the time spent in the different sleep stages with reference to TIB. Sleep-EEG parameters were analysed for the TIB (480 min; identical to the total night) and the both halves of the night (each 240 min).

Endocrine analysis

Plasma ghrelin (Phoenix, Belmont, CA, USA; intra- and interassay coefficients of variation <13%), plasma ACTH (Ria Kit J125; Nichols Institute, San Juan Capistrano, CA, USA; intra- and interassay coefficients of variation <8%) and cortisol (Ria Kit J125; ICN Biomedicals, Carson, CA, USA; intra- and interassay coefficients of variation <7%) concentrations were measured by radio-immunoassay; GH (Advantage; Nichols Institute, San Juan Capistrano, CA, USA; intra- and interassay coefficients of variation <10%) concentrations were determined by chemiluminescence. Random samples for each hormone were analysed in duplicate. According to standard procedures for time series the remaining specimens were analysed only once.

Statistical analysis

Areas under the plasma concentration curves (AUC) were calculated for the intervals 20:00–23:00 (evening), 23:00–03:00 (first half of the night), 03:00–07:00 (second half of the night) and 23:00–07:00 hours (total night) by applying the trapezoid rule (Yeh and Kwan, 1978). AUC scores were log-transformed to reduce the skewness of the distribution. Additionally, hormonal maximum concentrations were identified as the individually highest concentration of all samples collected during the evening (20:00–23:00 hours), during the first (23:00–03:00 hours) and second half (03:00–07:00 hours) of the night and during the total night (23:00–07:00 hours).

For correlation analyses the Kendall Tau (b) coefficients were applied. Kendall Tau (b) is a non-parametric correlation coefficient similar to Spearman's rho, but regarded as superior especially when tied values occur. In case of a significant association a partial correlation controlling for gender was additionally calculated to rule out that gender effects interfere with the result of the non-parametric analysis. Differences between male and female subjects were evaluated by anova. The level of significance was set to P = 0.05. Mean values and standard deviations are reported.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Associations between age or BMI and ghrelin levels

We did not observe any significant associations between age or BMI and ghrelin concentrations (data not shown). A parametric partial correlation analysis controlling for gender did not reveal significant associations either.

Gender effects on ghrelin levels

Gender effects in ghrelin levels were assessed by AUC and maximum values were evaluated by means of an anova. We observed a trend suggesting higher AUC ghrelin concentrations between 20:00 and 23:00 hours in female subjects (P = 0.075), which was significant when maximum values during the interval are considered. The results are presented in Table 1. No gender effects were observed for the ghrelin secretion between 23:00 and 07:00 hours.

Table 1.  Gender effects on ghrelin baseline and overnight secretion assessed by AUC and peak values
 Male subjects (n = 8)Female subjects (n = 8) P
  1. AUC values are log-transformed; P-values indicate effects of gender in a univariate analysis of variance. The values are expressed as mean (SD).

  2. AUC, areas under the plasma concentration curves.

Ghrelin AUC
20:00–23:0010.70 (0.62)11.37 (0.77)0.075
23:00–03:0011.38 (0.46)11.73 (0.62)0.215
03:00–07:0011.45 (0.31)11.68 (0.52)0.307
23:00–07:0012.11 (0.38)12.40 (0.57)0.253
Ghrelin maximum levels (pg ml−1)
20:00–23:00346 (186)734 (418)0.031
23:00–03:00536 (251)736 (453)0.294
03:00–07:00546 (233)638 (322)0.527
23:00–07:00628 (279)740 (451)0.558

Furthermore, we conducted a repeated measures analysis with the single ghrelin concentrations as a within-subjects factor and gender as a between-subjects factor. The mean nocturnal course of the ghrelin concentrations is depicted separately for male and female subjects in Fig. 1a.

image

Figure 1. (a–d) Nocturnal ghrelin, GH, ACTH and cortisol secretion in male (dark line with triangles) and female (light line with circles) subjects. Means and standard errors of the mean (SEM) are presented. ACTH levels were not determined in samples collected before lights were switched off (23:00 hours).

Download figure to PowerPoint

We observed a significant interaction between gender and the course of the ghrelin secretion (F31,434 = 2.92; P = 0.025, Greenhouse–Geisser corrected) suggesting a nocturnal increase in male subjects (indicated by the positive 95% confidence interval for the mean slope ranging from 73 to 417) and stable concentrations in female subjects (indicated by the slope 95% confidence interval around zero; −403 to 131). This interaction effect can be considered as a large effect according to the effect size measure (partial η2 = 0.17).

Fig. 1b–d displays overnight secretion of GH, ACTH and cortisol in male and female subjects. We found significantly higher maximum GH scores between 20:00 and 23:00 hours in female subjects (P = 0.046). This effect could not be shown for the AUC (P = 0.115). Although GH secretion appears to be higher in male subjects during the first half of the night, a statistically significant effect was not detected (AUC: P = 0.208; maximum scores: P = 0.247), presumably due to the large interindividual variability of the GH secretion (as indicated by the standard errors).

Gender effects on sleep

Besides a trend towards higher latency until stage IV sleep and a higher amount of stage I sleep in female subjects, we did not observe gender effects on polysomnographic sleep parameter (Table 2a). When the first and second halves of the night are separately analysed we could replicate the higher amount of shallow stage I sleep in women (Table 2b). Effect size measures (partial eta-squared) suggest a large effect of gender on stage I sleep (total night: partial η2 = 0.41; 1st half: partial η2 = 0.29; 2nd half: partial η2 = 0.27).

Table 2.  Gender effects on (a) total night sleep parameters and (b) sleep separately for the first and the second half of the night
 Male subjects (n = 8)Female subjects (n = 8) P
  1. The values are expressed as mean (SD).

  2. REM, rapid eye movement.

(a)
 Time in bed482.6 (4.8)484.0 (4.5)0.565
 Sleep period time449.7 (17.9)430.6 (47.1)0.302
 Total sleep time411.7 (35.5)413.9 (54.9)0.926
 Sleep efficiency85.3 (7.3)85.5 (11.4)0.963
 Sleep onset latency32.1 (15.4)48.1 (41.3)0.321
 Stage III latency24.3 (35.7)26.8 (28.8)0.883
 Stage IV latency9.9 (6.3)33.6 (31.3)0.054
 REM latency89.3 (65.5)77.1 (19.0)0.621
 Time in stage I (min)27.8 (6.6)16.3 (8.2)0.008
 Time in stage II (min)229.1 (40.5)226.8 (38.1)0.910
 Time in stage III (min)30.4 (10.6)33.3 (11.0)0.603
 Time in stage IV (min)36.1 (25.2)41.6 (27.0)0.680
 Time in REM (min)81.6 (18.5)88.0 (28.1)0.601
 Time awake (min)42.3 (39.6)21.1 (18.8)0.194
(b)
First half of the night
 Time in stage I (min)8.7 (5.6)3.6 (2.2)0.032
 Time in stage II (min)101.1 (18.9)92.0 (25.9)0.438
 Time in stage III (min)17.8 (8.4)22.1 (7.8)0.311
 Time in stage IV (min)27.2 (19.2)30.6 (22.8)0.749
 Time in REM (min)28.1 (15.7)33.6 (14.0)0.472
 Time awake (min)25.1 (31.4)10.6 (19.2)0.286
Second half of the night
 Time in stage I (min)19.1 (3.7)12.7 (7.1)0.040
 Time in stage II (min)128.0 (26.0)134.8 (18.4)0.555
 Time in stage III (min)12.6 (9.3)11.2 (7.5)0.749
 Time in stage IV (min)8.9 (9.7)10.9 (12.6)0.719
 Time in REM (min)53.5 (10.7)54.4 (18.3)0.909
 Time awake (min)17.2 (11.4)10.4 (5.7)0.155

Associations between ghrelin secretion and sleep

We observed a trend (P = 0.095) suggesting a lower time spent in stage I sleep during the total night in subjects with high ghrelin AUC scores during this interval (r = −0.31; P = 0.095), which achieves significance when only the second half of the night is considered (r = −0.42;P = 0.024). Interestingly, we observed a significant gender effect suggesting less stage I sleep in the second half of the night in female subjects (P = 0.040). However, the negative association between ghrelin secretion and time in stage I in the second half of the night remained significant when a partial correlation was applied controlling for the effects of gender (AUC: r = −0.55, P = 0.030; Peak: r = −0.54, P = 0.036). No further associations were found.

Associations between GH secretion and sleep

We additionally calculated the associations between GH secretion and sleep parameters. We found a negative association between the amount of REM sleep and low GH secretion during the total night (AUC: r = −0.39, P = 0.038; maximum: r = −0.41, P = 0.030), which could be confirmed after controlling for the effects of gender (AUC: r = −0.61, P = 0.015; peak: r = −0.58, P = 0.022). We observed a trend towards a positive association between the total night amount of SWS (stage III + stage IV) and GH maximum values (r = 0.34, P = 0.071), which could not be replicated after gender effects were controlled. We repeated this analysis separately for both genders and found a significant association in male subjects (r = 0.41, P = 0.030) but not in female subjects (r = 0.30, P = 0.315), suggesting a more pronounced association in males.

Associations between ghrelin, GH, ACTH and cortisol concentrations

Besides a positive association between ACTH and cortisol during the first half of the night (AUC: r = 0.38, P = 0.038) we did not observe any correlations between ghrelin, GH, ACTH and cortisol, neither for the total nor for the two halves of the night (data not shown).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The major findings of our study are (i) higher ghrelin levels in young and middle aged normal females than in males during the interval between 20:00 and 23:00 hours, but the lack of significant gender differences between 23:00 and 07:00 hours, (ii) a negative correlation between time spent in stage I sleep and ghrelin levels during the second half of the night and (iii) the absence of other interactions between nocturnal ghrelin levels and sleep EEG, GH, ACTH and cortisol secretion.

During the investigated interval from 20:00 to 07:00 hours mean ghrelin levels in females did not display major variations. In males, however, the lowest mean value was found at 20:00 hours. It was followed by an arbitrarily continuous increase until 01:00 hours. Later on the values in males remained, by trend, lower than in females. Similar to the females, the following course in males remained widely stable except for a small peak at 04:20 hours in the males. No major nocturnal peak of ghrelin levels was detectable either in females or males. This finding corresponds to the individual inspection of the timing of the ghrelin maxima, which revealed a widely spread distribution throughout the night. A sleep-dependent rise of ghrelin around midnight as observed in males by Dzaja et al. (2004) was absent. In contrast, ghrelin levels in females showed, from the start of the observation period at 20:00 hours, similar values to those at midnight and beyond, up to the last sampling at 07:00 hours. Whereas ghrelin levels in males started to increase already at 20:00 hours, i.e. 3 h before lights were switched off and subjects were allowed to sleep, we did not observe a sleep-related increase. One may argue that the observed gender differences can be explained by differences in BMI between male and female subjects. However, all subjects were within a normal weight range and we did not observe any significant association between the individual BMI and ghrelin levels.

The discrepancy between our observation and the report by Dzaja et al. (2004) may be explained by methodological differences. In the latter study subjects were supine for 24 h, whereas our subjects performed their normal activities until 19:00 hours. The pattern found in our sample resembles that reported by Cummings et al. (2001). These authors, however, investigated mainly females, who were older and had a higher BMI than our sample. Sleep EEG was not recorded in their study. Our finding that ghrelin levels in males already increase prior to sleep onset resembles our previous report that GH levels in a subgroup of healthy young men increased before sleep onset (Steiger et al., 1987). Otherwise it must be considered that a postprandial rise after dinner at 18:00 hours of the ghrelin levels (Cummings et al., 2001) in males may play a role, whereas females had their meals at the same time without a change of ghrelin levels before sleep onset. It appears possible, however, that depending on the time of the day changes of ghrelin levels reflect different physiological roles of the peptide. The sharp increases before meals may promote hunger and food intake, whereas the nocturnal rise observed in our study in males and in previous reports (Cummings et al., 2001, Dzaja et al., 2004) maybe related to sleep. It is also possible that depending on the threshold higher concentrations induce hunger. This may be the mechanism of night-eating disorder. In a single patient with this disorder very high nocturnal ghrelin levels were observed (Rosenhagen et al., 2005). Since we started to examine ghrelin levels on 20:00 hours, when values in females did not differ from those later on during the night, it remains unclear whether we missed an increase prior to 20:00 hours, or whether major fluctuations were absent in the females of our sample throughout the 24 h.

A limitation of our study might be that ghrelin levels appear to be affected by sleep duration. The night adaptation can be associated with reduced total sleep time where elevated ghrelin levels are reported (Spiegel et al., 2004; Taheri et al., 2004). This may influence the ghrelin levels during the examination night.

As summarised in the Introduction only a few studies have addressed gender differences in ghrelin activity and have reported conflicting results. Furthermore, an influence of sexual steroids on morning ghrelin levels has been suggested recently. In male patients with hypogonadism ghrelin concentration was lower than in controls. After testosterone replacement therapy ghrelin levels increased in the patients (Pagotto et al., 2003). A study in obese female patients with and without polycystic ovary syndrome showed a negative correlation between ghrelin and androstendione levels (Pagotto et al., 2002). Leptin is thought to oppose the action of ghrelin on the energy balance. In women, leptin levels are about three times higher than in men (Antonijevic et al., 1998; Deuschle et al. 1996; Saad et al., 1997). Given these observations we expected a major influence of gender on ghrelin levels. This was restricted to the evening hours, however, in our study.

The negative correlation between stage I sleep, which corresponds to shallow sleep and ghrelin levels is compatible with its role as a sleep promoting factor. Exogenous ghrelin enhanced non-REM sleep in mice (Obál et al., 2003) and humans (Weikel et al., 2003). Intact GHRH receptors have been determined to be the prerequisite for this effect in mice (Obál et al., 2003). Both GHRH and ghrelin promote sleep and stimulate GH (Obál and Krueger, 2002). A partly synergistic action of both peptides in the release of GH has been suggested (Horvath et al., 2001). Similarly it appears likely that ghrelin and GHRH are cofactors in sleep regulation. A coincidence of the highest amounts of GH and SWS is well established. As expected in our study, there was a correlation between SWS and GH secretion (Van Cauter and Copinschi, 2000). In the rat GHRH synthesis and release peak at the time of highest sleep propensity at the onset of the light period (Bredow et al., 1996; Gardi et al., 1999). Therefore one might expect a peak of ghrelin overlapping the GH peak. This is also suggested by the observation of Cummings et al. (2001). Similarly Dzaja et al. (2004) reported a positive correlation between the nocturnal GH peak and ghrelin levels between 23:00 and 03:00 hours. In our study, however, a ghrelin peak around sleep onset was missing. This does not exclude an enhanced activity of central ghrelin around sleep onset and the GH peak. In rats, interpeak intervals and pulse frequencies were close for ghrelin and GH, whereas no significant correlation between these hormones was found (Tolle et al., 2002); Bodosi et al. (2004), however, reported a peak of ghrelin after dark onset in rats prior to their feeding peak. Probably the major surge of plasma ghrelin analysed in our study derived from the stomach (Ariyasu et al., 2001). The exact pattern of central ghrelin activity related to sleep remains to be elucidated by preclinical studies.

Our study suggests gender differences in the pattern of peripheral ghrelin levels in the interval between 20:00 and 23:00 hours in young and middle-aged healthy subjects. No major interaction between ghrelin and sleep EEG and between ghrelin and the other investigated hormones were found.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study was supported by a grant from the Deutsche Forschungsgemeinschaft (STE 485/5-3).

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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