بررسی سیتوکین و مداخله غدد درون ریز در حمله خواب

Narcolepsy is a disabling neurological sleep disorder characterized by excessive daytime sleepiness and abnormal REM sleep manifestations. Recently, the role of cytokines and growth hormone in the regulation of sleep and narcolepsy has been considered, and data suggest that proinflammatory cytokines may be involved in sleep and narcoleptic symptoms. Serum and clinical data were obtained from the Stanford Center for Narcolepsy Research for 39 Narcoleptics (22 Females, 17 Males, age 39 ± 14.9) and 40 controls (13 Females, 27 Males, age 46 ± 17.9). Plasma levels of TNF-α, IL-6, and human growth hormone (hGH) were measured by ELISA. TNF-α and IL-6 were significantly increased in narcoleptic subjects compared to controls (p=.001). Interestingly, hGH was significantly increased in narcoleptic subjects (p<.0001). There was also a significant difference in the epworth sleepiness scale (ESS) (17.7 ± 4.6 vs. 5.5 ± 3.2, p<.0001). These data indicate that narcoleptics, relative to controls, had higher serum levels of TNF-α, IL-6, and hGH. These data suggest that the dysregulation of sleep observed in narcoleptics correlates with the immune and endocrine dysregulation seen in these subjects, and the observed changes may in fact contribute to the higher likelihood of disturbed sleep and/or increased incidence of infection. Additional work is required to fully characterize connections between cytokines and narcoleptic symptomatology.

Narcolepsy is a neurological condition that is characterized by excessive daytime sleepiness and a triad of disassociated REM sleep—cataplexy, hypnagogic hallucinations, and sleep paralysis. Narcolepsy has an HLA association, DQB1*0602, in over 95% of its patients (Mignot et al., 1995; Okun et al., 2002), in addition to a strong association with absent or abnormally low levels of the neuropeptide hypocretin (Mignot et al., 2002; Nishino et al., 2001). These data maintain that this disorder may have an autoimmune origination. However, no evidence has been obtained to confirm this hypothesis. The impact of low or absent hypocretin for narcoleptics may be due to the one of the functions of this neuropeptide. Hypocretin appears to stabilize, rather than generate vigilance states ( Overeem et al., 2001). This may explain why narcoleptics experience urges to sleep during the day and have pronounced sleep fragmentation at night. With improper hypocretin levels, the narcoleptic is unable to maintain a consistent sleep-wake pattern. Related to the possibility that narcolepsy may have an autoimmune component is recent work addressing a possible role of cytokines in the regulation of sleep and sleep disorders. Several inflammatory cytokines, including TNF-α and IL-1, have been shown to play a role in the regulation of sleep (Krueger and Majde, 1995; Moldofsky, 1995), while IL-6 has been shown to augment fatigue and sleepiness (Opp and Imeri, 1999). These and other studies linking cytokines to the regulation of sleep and sleepiness have led to speculation that these mediators may play a role in the pathogenesis of narcolepsy, and some studies have suggested that proinflammatory cytokines may be involved in sleep disorders. For example, Vgontzas and colleagues (1997) evaluated plasma cytokine levels in patients with obstructive sleep apnea, idiopathic hypersomnia, and narcolepsy. The concentrations of TNFα in plasma were elevated in apneics and narcoleptics compared to controls. Additional studies (Vgontzas et al., 1999, Vgontzas et al., 2002 and Vgontzas et al., 2003) have shown elevations and circadian shifts in IL-6 and TNF α, in subjects who are sleep restricted either experimentally or naturally via insomnia. Moreover, although neither IL-1β nor IL-6 was significantly elevated in narcoleptics, IL-6 was markedly elevated in the group of sleep apneics who exhibited obesity (Vgontzas et al., 1997). The relationship between IL-6 and obesity is interesting since narcoleptics tend to be heavier than controls (Dahmen et al., 2001; Hong et al., 2002; Kok et al., 2003; Nishino et al., 2001; Okun et al., 2002). In addition, narcoleptics have been reported to have disturbances in metabolism and food intake as a result of depleted hypocretin (Schuld et al., 2000). These factors may contribute to the elevated obesity levels reported in narcoleptics (Dahmen et al., 2001; Hong et al., 2002; Kok et al., 2003; Nishino et al., 2001; Okun et al., 2002). Interestingly, increased BMI has been related to higher levels of circulating proinflammatory cytokines (Esposito et al., 2003; Vgontzas et al., 1997) and sleepiness (Vgontzas et al., 1998), further suggesting that BMI may be related to proinflammatory cytokines levels and potentially, narcoleptic symptoms. Finally, the possibility that the TNF-α gene is coupled with the development of narcolepsy has been purported by Hohjoh and colleagues (Hohjoh et al., 1999) and is based on the aforementioned work (Vgontzas et al., 1997). They found an increase in the frequency of certain TNF alleles in patients with narcolepsy than in controls. Although not definitive, it leaves open the possibility that certain cytokines may be involved in the development of narcolepsy. Other researchers have suggested that the observed changes in proinflammatory cytokines seen in sleep disordered patients are not merely the result of impaired T-cell function, but may, in fact, represent a more subtle interaction between immune function and the regulation of sleep patterns. For example, Hinze-Selch et al. (1998) assessed cytokine levels of IL-1β, IL-1ra, IL-2, IL-6 TNF-α, and TNF-β in plasma and in mitogen-stimulated monocytes and lymphocytes in narcoleptics and HLA matched controls. They only found elevated secretion of IL-6 from LPS stimulated monocytes compared to controls. The authors conclude that there are no major T-cell functional abnormalities in narcoleptics, but the elevation in IL-6 may play a role in the REM sleep associated symptoms of narcolepsy since IL-6 promotes the growth of cholinergic neurons (Hinze-Selch et al., 1998). Animal data strongly implicates a relationship between pro-inflammatory cytokines, particularly IL-1, TNF-α, and IL-6, and sleep modulation (Hogan et al., 2003; Krueger et al., 1998; Krueger and Majde, 2003; Takahashi et al., 1999). Both TNF and IL-1 are known to be somnogenic and have been shown to augment non-rapid eye movement sleep (NREMS) when administered exogenously in rats (Krueger et al., 1998; Krueger et al., 2001; Takahashi et al., 1999). The inclination to extend this theory to humans has not been irrefutably confirmed. Administration of IL-1 or TNF on human sleep reveals species specificity and a dependence on factors such as substance concentration, time, and route of administration, while in animals more consistency in outcomes is observed (Marshall and Born, 2002). Recent data on IL-6 administration suggest that the sleep modulating properties of IL-6 may only occur during times of illness (Hogan et al., 2003). However, IL-6 does alter NREMS in rats after central administration (Hogan et al., 2003) and in humans after subcutaneous administration (Spath-Schwalbe et al., 1998). Extrapolation of the effects that cytokines have on human sleep has occurred via in vitro measurements, for example serum levels of IL-1β, TNF-α, and IL-6 or cytokine production following lymphocyte stimulation. It must be acknowledged that the most informative animal protocols are prohibited in human research and indirect measurements of the affects of exogenous administration of cytokines in humans in the only means currently available. The comprehension about human growth hormone (hGH), growth hormone releasing hormone (GHRH) and their role in sleep in both animals and humans in more straightforward (Marshall and Born, 2002). GHRH has been documented to be the best sleep-promoting substance and has been shown to promote NREMS in both animals and humans (Krueger and Majde, 2003; Obal and Krueger, 2001; Steiger, 2003). Few studies have assessed HGH levels in patients with narcolepsy. However, a recent study (Overeem et al., 2003) found that hGH secretion in narcoleptics was dispersed differently than in matched controls over a 24-h period. Refuting expectations, the basal production of hGH was similar in both groups (Overeem et al., 2003). The previous conclusions of these early studies (Besset et al., 1979; Clark et al., 1979) suggest that hGH secretion is blunted in narcolepsy. The methodology of these studies and the time period in which they were conducted imparts skeptical acceptance of the results. One study (Clark et al., 1979) showed hGH concentrations remain stable after administration of L-DOPA. The other two studies (Besset et al., 1979; Higuchi et al., 1979) concentrated on the sleep onset secretion of HGH as their determining factor rather than the entire 24-h period. Knowing that sleep is distributed throughout the 24-h period for narcoleptics, rather than in a concise 8-h period, hGH levels should be altered to some degree from normal subjects due to the alterations in SWS patterns and production. Although at this point few studies have directly addressed the role of proinflammatory cytokines in sleep disorders, especially narcolepsy, existing studies suggest that certain sleep associated cytokine-producing genes may predispose a person to increased susceptibility to develop narcolepsy (Hohjoh et al., 2001). This study attempted to add to the current literature by assessing serum levels of IL-6, TNF-α, and hGH in narcoleptics and controls. Since fragmentation of sleep patterns in narcoleptics is a common feature, several hypotheses were generated. First, we anticipated elevated levels of IL-6 based on previous data and that IL-6 is primarily secreted during stages 1 and 2 and REM sleep. Since sleep is fragmented with a decrease in SWS and REM sleep, an increase in stages 1 and 2 would account for the elevated IL-6 levels. Second, we anticipated elevated levels of TNF-α again based on previous data, and its association with sleep. Finally, since hGH is secreted in a pulsatile fashion with SWS, and since narcoleptics display fragmented sleep patterns, we expected possible alterations in hGH levels for the narcoleptics.

Narcoleptic subjects had significantly higher serum levels of TNF-α, IL-6, and hGH relative to controls. There were no differences seen between genders on any of the pro-inflammatory cytokines measures, so the two groups were collapsed during analysis for TNF-α and IL-6. However, gender differences were noted on hGH levels. Women had higher levels of hGH than men. Despite the significant differences reported in this study, a primary concern with these data is the one time-point sampling of the blood. Conducting secondary analysis often predisposes the data to considerable critique, quite justifiably. Future studies should not only include larger samples to overcome the possible power limitation, but 24-h blood sampling to ensure accurate assessment of circadian rhythmicity of these cytokines. Notwithstanding, the present data suggest that indeed, alterations in circulating proinflammatory cytokines are related to narcoleptic status, and are consistent with previous data (Vgontzas et al., 1997) that cytokine levels are elevated in patients with disorders of excessive daytime sleepiness. The clinical data present a different picture. Despite several previous reports that BMI is higher in narcoleptics compared to controls (Dahmen et al., 2001; Hong et al., 2002; Kok et al., 2003; Nishino et al., 2001; Okun et al., 2002), this study did not observe this outcome. A possible explanation may lie within the controls themselves: they consisted of primarily family members. Family members often have similar eating behaviors, hereditary characteristics and environmental cues. These factors may have influenced the BMI results seen in the controls. If the controls had consisted of non-family members, the difference may have been more pronounced. Various behaviors and habits, including medication usage and smoking, have been noted to influence cytokine levels (Ryder et al., 2002; Szuster-Ciesielska et al., 2003). Although smoking information was not available for either group, medication usage was available for the narcoleptic group. With regards to medication usage, differences were observed between stimulant users and non-stimulant users in TNF-α, and for those who used anti-depressants in hGH. Narcoleptics who used stimulants had higher TNF-α levels, but no differences were noted between IL-6 and hGH. Those who used anti-depressants had lower levels of hGH than those who did not use them. The type of stimulant and/or anti-depressant used was not available, but would be an important variable to consider in the future. Serum cytokines follow a circadian rhythm that is altered during sleep deprivation (Irwin, 2002; Redwine et al., 2000; Vgontzas and Chrousos, 2002; Vgontzas et al., 1999; Vgontzas et al., 2003) and in disorders of daytime sleepiness (Vgontzas et al., 1997). People with insomnia show a major disruption in the TNF secretion compared to controls and an alteration of IL-6 secretion peak from nighttime to daytime (Vgontzas et al., 2002). Narcoleptics exhibit insomnia type symptoms with fragmented nighttime sleep. Twenty-four hour sampling may reveal similar findings in a narcoleptic sample. Proper production of cytokines is essential to a healthy immune response. Proinflammatory cytokine production and release are crucial in the function and regulation of peripheral blood mononuclear cells. They are also “secreted by macrophages in response to infectious challenge and play a key role in the differentiation, maturation, and proliferation of T and B cells” (Redwine et al., 2000). Finally, IL-6 is known to be a powerful stimulator of the HPA axis (Vgontzas and Chrousos, 2002). Future studies should ascertain data about illnesses in narcoleptics and controls to see if the alterations in cytokine levels have any clinical relevance. One might anticipate a chronic activation of the HPA axis, for example through elevated levels of IL-6, could lead to immunosuppression in the subject. The present data have several interesting implications. First, the dysregulation of sleep observed in narcoleptics correlates with the immune and endocrine dysregulation seen in these subjects. Narcoleptics have a disrupted and fragmented 24-h sleep cycle, which is thought to stem from the depletion of the neuropeptide, hypocretin (Mignot et al., 2002). Frequent daytime naps can intrude upon the narcoleptic, while frequent awakenings can plague the nighttime hours. This persistent disruption of sleep could potentially be a primary explanatory variable in the altered cytokine levels seen in this and other studies (Irwin, 2002; Krueger and Majde, 1995; Redwine et al., 2000) (Vgontzas et al., 1997 and Vgontzas et al., 2003). There is data suggesting that partial sleep deprivation staves off the release of nocturnal IL-6 (Redwine et al., 2000), while others have shown that IL-1β and/or TNF-α release is disrupted following sleep deprivation (Heiser et al., 2001; Uthgenannt et al., 1995). However, other data exists contradicting this hypothesis, stating that sleep loss and sleep restriction are associated with elevated levels of IL-6 and TNF-α ; and people with elevated daytime sleepiness have similar elevated cytokine levels as well (Vgontzas and Chrousos, 2002; Vgontzas et al., 1999; Vgontzas et al., 2003). The data from this study confirms the latter hypothesis. Krueger et al., 1998 and Krueger et al., 2001 reports that inhibition of either IL-1 or TNF may inhibit spontaneous sleep, while stimulation of endogenous production of these cytokines enhances NREMS. If this hypothesis holds, the elevated TNF-α levels seen in this and other studies (Vgontzas et al., 1997) may partially explain the sudden sleep attacks narcoleptics experience. Moreover, the observed changes in cytokine levels may in fact contribute to the higher likelihood of disturbed sleep and/or increased incidence of infection. Irwin (2002) has reported that even partial sleep deprivation can produce a significant reduction in cellular immunity. However, it must be noted that all the cited studies have assessed peripheral blood cytokine measurements. This does not clearly inform the researcher as to where the cytokines originated from, only that they are present in serum. The cytokine and hGH alterations may be explanatory variables for the diverse variations seen in narcoleptic sleep patterns. It is well established that narcoleptics have fragmented sleep architecture with non-normal SWS patterns in the first third of the night, often accompanied by frequent daytime naps. The shortened REM latency may contribute to the altered levels of cytokines and hGH observed since IL-6 is secreted during stages 1 and 2, and REM sleep, and hGH secretion is associated with SWS. hGH secretion is stimulated not only by GHRH, but also by IL-6 (Marshall and Born, 2002; Redwine et al., 2000). In addition, deep sleep has been shown to enhance activity of the hGH axis (Vgontzas et al., 1999). In attempting to explain the extreme levels of hGH, a possible hypothesis may stem from the additional SWS acquired via the frequent daytime naps taken by narcoleptics. It is possible that narcoleptic episodes could be associated with increased SWS secretions of hGH as the attainment of sleep is spread throughout the 24-h period rather than obtained in the consolidated 8-h nighttime cycle. Hence, the distribution and timing of hGH secretion over the 24-h period in narcoleptics may be the more important measure to assess, rather than overall levels. Taken together, this study indicates that proinflammatory cytokines and hGH may be involved in the development of or the exacerbation of narcolepsy. As the role of the immune system in sleep becomes clearer, a better interpretation will be available to assess why narcoleptics have altered cytokine and hGH levels. Future studies should be conducted to fully characterize the sources, patterns, and significance of alterations in patterns of proinflammatory cytokines and hGH in narcolepsy. In addition, future studies should include detailed assessment of sleep patterns and health status to account for potential cumulative deleterious effects of sleep deprivation upon the immune system. Even though the number of individuals who develop narcolepsy appears small in comparison to other sleep disorders such as sleep apnea, the present data provide further support for the use of narcolepsy as a model for understanding how chronic sleep disruption/fragmentation can impact the functioning of the immune system.