Plasma total ghrelin and leptin levels in human narcolepsy and matched healthy controls: basal concentrations and response to sodium oxybate

Body Weight and Metabolic Rate Changes in Narcolepsy: Current Knowledge and Future Directions

Narcolepsy is a known auto-immune disease that presents mainly in the teenage years with irresistible sleep attacks. Patients with narcolepsy, especially NT1, have been found to have a high prevalence of obesity and other metabolic derangements. This narrative review aimed to address the relationship between narcolepsy and changes in weight and metabolic rate, and discuss potential mechanisms for weight gain and metabolic changes and future research agendas on this topic. This article will provide a balanced, up-to-date critical review of the current literature, and delineate areas for future research, in order to understand the pathophysiological metabolic changes in narcolepsy. Articles using predefined keywords were searched for in PubMed and Google Scholar databases, with predefined inclusion and exclusion criteria. Compared to controls, patients with narcolepsy are more likely to be obese and have higher BMIs and waist circumferences. According to recent research, weight gain in narcolepsy patients may be higher during the disease's outset. The precise mechanisms causing this weight gain remains unknown. The available information, albeit limited, does not support differences in basal or resting metabolic rates between patients with narcolepsy and controls, other than during the time of disease onset. The evidence supporting the role of orexin in weight gain in humans with narcolepsy is still controversial, in the literature. Furthermore, the available data did not show any appreciable alterations in the levels of CSF melanin-concentrating hormone, plasma and CSF leptin, or serum growth hormone, in relation to weight gain. Other mechanisms have been proposed, including a reduction in sympathetic tone, hormonal changes, changes in eating behavior and physical activity, and genetic predisposition. The association between increased body mass index and narcolepsy is well-recognized; however, the relationship between narcolepsy and other metabolic measures, such as body fat/muscle distribution and metabolic rate independent of BMI, is not well documented, and the available evidence is inconsistent. Future longitudinal studies with larger sample sizes are needed to assess BMR in patients with narcolepsy under a standard protocol at the outset of narcolepsy, with regular follow-up.

Precocious puberty in narcolepsy type 1: Orexin loss and/or neuroinflammation, which is to blame?

Narcolepsy type 1 (NT1) is a rare neurological sleep disorder triggered by postnatal loss of the orexin/hypocretin neuropeptides. Overweight/obesity and precocious puberty are highly prevalent comorbidities of NT1, with a close temporal correlation with disease onset, suggesting a common origin. However, the underlying mechanisms remain unknown and merit further investigation. The main question we address in this review is whether the occurrence of precocious puberty in NT1 is due to the lack of orexin/hypocretin or rather to a wider hypothalamic dysfunction in the context of neuroinflammation, which is likely to accompany the disease given its autoimmune origins. Our analysis suggests that the suspected generalized neuroinflammation of the hypothalamus in NT1 would tend to delay puberty rather than hastening it. In contrast, that the brutal loss of orexin/hypocretin would favor an early reactivation of gonadotropin-releasing hormone (GnRH) secretion during the prepubertal period in vulnerable children, leading to early puberty onset. Orexin/hypocretin replacement could thus be envisaged as a potential treatment for precocious puberty in NT1. Additionally, we put forward an alternative hypothesis regarding the concomitant occurrence of sleepiness, weight gain and early puberty in NT1.

Metabolic profile in patients with narcolepsy: a systematic review and meta-analysis

Narcolepsy, a sleep disorder characterized by loss of hypocretin neurons, has been associated with metabolic disturbances. Although the metabolic alterations in narcolepsy patients are widely investigated in the literature, the results are controversial. We performed a systematic search of literature to identify metabolic profiling studies in narcolepsy patients. A total of 48 studies were included in the meta-analysis. Narcolepsy patients exhibited higher prevalence of obesity (log OR = 0.93 [0.73-1.13], P < 0.001), diabetes mellitus (log OR = 0.64 [0.34, 0.94], P < 0.001), hypertension (log OR = 0.33 [0.11, 0.55], P < 0.001), and dyslipidemia (log OR = 1.19 [0.60, 1.77], P < 0.001) compared with non-narcoleptic controls. Narcolepsy was associated with higher BMI (SMD = 0.50 [0.32-0.68], P < 0.001), waist circumference (MD = 8.61 [2.03-15.19], P = 0.01), and plasma insulin (SMD = 0.61 [0.14-1.09], P = 0.01). Levels of fasting blood glucose (SMD = -0.25 [-0.61,0.10], P = 0.15), BMR-RMR (SMD = -0.17 [-0.52-0.18], P = 0.34), systolic blood pressure (SMD = 0.29 [-0.39-0.97], P = 0.40), diastolic blood pressure (SMD = 0.39 [-0.62, 1.40], P = 0.45), CSF melanin-concentrating hormone (MD = 5.56 [-30.79-41.91], P = 0.76), serum growth hormone (SMD = 7.84 [-7.90-23.57], P = 0.33), as well as plasma and CSF leptin (SMD = 0.10 [-1.32-1.51], P = 0.89 and MD = 0.01 [-0.02-0.04], P = 0.56, respectively) did not significantly differ between narcolepsy patients and controls. These findings necessitate early screening of metabolic alterations and cardiovascular risk factors in narcolepsy patients to reduce the morbidity and mortality rates.

Sleep disorders and the hypothalamus

As early as the 1920s, pathological studies of encephalitis lethargica allowed Von Economo to correctly identify hypothalamic damage as crucial for the profound associated sleep-related symptoms that helped define the condition. Only over the last 3 decades, however, has the key role of the hypothalamus in sleep-wake regulation become increasingly recognized. As a consequence, a close relation between abnormal sleep symptomatology and hypothalamic pathology is now widely accepted for a variety of medical disorders. Narcolepsy is discussed in some detail as the cardinal primary sleep disorder that is caused directly and specifically by hypothalamic pathology, most notably destruction of hypocretin (orexin)-containing neurons. Thereafter, various conditions are described that most likely result from hypothalamic damage, in part at least, producing a clinical picture resembling (symptomatic) narcolepsy. Kleine-Levin syndrome is a rare primary sleep disorder with intermittent symptoms, highly suggestive of hypothalamic involvement but probably reflecting a wider pathophysiology. ROHHAD (rapid-onset obesity with hypothalamic dysfunction, hypoventilation, and autonomic dysregulation) and Prader-Willi syndrome are also covered as hypothalamic syndromes with prominent sleep-related symptoms. Finally, sleep issues in several endocrine disorders are briefly discussed.

Associations of plasma hypocretin-1 with metabolic and reproductive health: Two systematic reviews of clinical studies

The hypocretin system consists of two peptides hypocretin-1 and hypocretin-2 (HCRT1 and HCRT2). Hypocretin-containing neurons are located in the posterior and lateral hypothalamus, and have widespread projections throughout the brain and spinal cord. In addition to its presence in the cerebrospinal fluid (CSF), peripheral HCRT1 has been detected in plasma. Robust experimental evidence demonstrates functions of hypothalamic-originated HCRT1 in regulation of multiple biological systems related to sleep-wake states, energy homeostasis and endocrine function. In contrast, HCRT1 studies with human participants are limited by the necessarily invasive assessment of CSF HCRT1 to patients with underlying morbidity. Regulation by HCRT1 of energy homeostasis and reproduction in animals suggests similar regulation in humans and prompts these two systematic reviews. These reviews translate prior experimental findings from animal studies to humans and examine associations between HCRT1 and: 1) metabolic risk factors; 2) reproductive function in men, women and children. A total of 21 studies and six studies met the inclusion criteria for the two searches, respectively. Research question, study design, study population, assessments of HCRT1, reproductive, cardiometabolic data and main findings were extracted. Associations between HCRT1, metabolic and reproductive function are inconsistent. Limitations of studies and future research directions are outlined.

Neuropsychiatric Correlates of Narcolepsy

PURPOSE OF REVIEW

The complex nature of narcolepsy symptoms, along with the use of stimulants and anticataplectic medications, poses diagnostic difficulties in terms of underlying neuropsychiatric comorbidities. This study reviews recent evidence for the association between narcolepsy and neuropsychiatric disorders. We also critically analyze studies that have addressed the neuropsychiatric correlates of patients with narcolepsy, with a discussion of the possible pathophysiological mechanisms linking narcolepsy and neuropsychiatric disorders.

RECENT FINDINGS

Neuropsychiatric manifestations are common among patients with narcolepsy as narcolepsy and some neuropsychiatric disorders share common clinical features. This may create challenges in making the correct diagnosis, and hence result in a delay in starting appropriate treatment. Comorbid neuropsychiatric manifestations in patients with narcolepsy include depression, anxiety, psychosis, rapid eye movement (REM) sleep behavior disorder, and cognitive impairment. Although hypocretin deficiency has been proposed as a pathophysiological mechanism underlying both narcolepsy and neuropsychiatric disorders, further research is necessary to identify the exact mechanisms. Narcolepsy patients often manifest comorbid neuropsychiatric symptoms, which makes the diagnosis difficult. Therefore, it is essential to address neuropsychiatric symptoms in the clinical care of patients with narcolepsy.

Role of Brown Adipose Tissue in Adiposity Associated With Narcolepsy Type 1

Narcolepsy type 1 is a neurological sleep-wake disorder caused by the destruction of orexin (hypocretin)-producing neurons. These neurons are particularly located in the lateral hypothalamus and have widespread projections throughout the brain, where they are involved, e.g., in the regulation of the sleep-wake cycle and appetite. Interestingly, a higher prevalence of obesity has been reported in patients with narcolepsy type 1 compared to healthy controls, despite a normal to decreased food intake and comparable physical activity. This suggests the involvement of tissues implicated in total energy expenditure, including skeletal muscle, liver, white adipose tissue (WAT), and brown adipose tissue (BAT). Recent evidence from pre-clinical studies with orexin knock-out mice demonstrates a crucial role for the orexin system in the functionality of brown adipose tissue (BAT), probably through multiple pathways. Since BAT is a highly metabolically active organ that combusts fatty acids and glucose toward heat, thereby contributing to energy metabolism, this raises the question of whether BAT plays a role in the development of obesity and related metabolic diseases in narcolepsy type 1. BAT is densely innervated by the sympathetic nervous system that activates BAT, for instance, following cold exposure. The sympathetic outflow toward BAT is mainly mediated by the dorsomedial, ventromedial, arcuate, and paraventricular nuclei in the hypothalamus. This review focuses on the current knowledge on the role of the orexin system in the control of energy balance, with specific focus on BAT metabolism and adiposity in both preclinical and clinical studies.

Decreased body mass index during treatment with sodium oxybate in narcolepsy type 1

Narcolepsy type 1 is characterised by an increase in body weight after disease onset, frequently leading to obesity. It was suggested that this weight gain may be counteracted by treatment with sodium oxybate. We here provide longitudinal body mass index data of patients with narcolepsy type 1 after starting treatment with sodium oxybate, compared with patients in whom treatment with modafinil was initiated. Eighty-one individuals with narcolepsy type 1 fulfilled the entry criteria for this retrospective study: 59 had newly started treatment with sodium oxybate and 22 had newly started modafinil. Gender-specific differences between both treatment groups were compared using Student's t tests and mixed effect modeling. Patients using sodium oxybate lost weight, with a mean body mass index decrease of 2.56 kg/m2 between the first and last measurement (women; p = .001) and 0.84 kg/m2 (men; p = .006). Patients using modafinil, however, gained weight, with a mean body mass index increase of 0.57 kg/m2 (women; p = .033) and 0.67 kg/m2 (men; p = .122). Medication (p = .006) and baseline body mass index (p = .032) were predictors for body mass index decrease. In conclusion, treatment with sodium oxybate is associated with a body mass index reduction in narcolepsy type 1, whereas modafinil treatment is not. This effect is most pronounced in those who already have a higher baseline body mass index.

Relationship between cerebrospinal fluid concentrations of orexin A/hypocretin-1 and body composition in humans

The hypothalamic neuropeptide orexin A (hypocretin-1) is a key signal in sleep/wake regulation and promotes food intake. We investigated the relationship between cerebrospinal fluid orexin A concentrations and body composition in non-narcoleptic human subjects with a wide range of body weight to gain insight into the role of orexin A in human metabolism. We collected cerebrospinal fluid and blood samples and measured body composition by bioelectric impedance analysis in 36 subjects (16 women and 20 men) with body mass indices between 16.24 and 38.10 kg/m² and an age range of 19-80 years. Bivariate Pearson correlations and stepwise multiple regressions were calculated to determine associations between orexin A and body composition as well as biometric variables. Concentrations of orexin A in cerebrospinal fluid averaged 315.6 ± 6.0 pg/ml, were comparable between sexes (p > 0.15) and unrelated to age (p > 0.66); they appeared slightly reduced in overweight/obese compared to normal-weight subjects (p = .07). Orexin A concentrations decreased with body weight (r = -0.38, p = .0229) and fat-free mass (r = -0.39, p = .0173) but were not linked to body fat mass (p > 0.24). They were inversely related to total body water (r = -0.39, p = .0174) as well as intracellular (r = -0.41, p = .0139) and extracellular water (r = -0.35, p = .0341). Intracellular water was the only factor independently associated with cerebrospinal fluid orexin A concentrations (p = .0139). We conclude that cerebrospinal fluid orexin A concentrations do not display associations with body adiposity, but are inversely related to intracellular water content. These cross-sectional findings suggest a link between orexin A signaling and the regulation of water homeostasis in humans.

Aberrant Food Choices after Satiation in Human Orexin-Deficient Narcolepsy Type 1

STUDY OBJECTIVES

Besides influencing vigilance, orexin neurotransmission serves a variety of functions, including reward, motivation, and appetite regulation. As obesity is an important symptom in orexin-deficient narcolepsy, we explored the effects of satiety on food-related choices and spontaneous snack intake in patients with narcolepsy type 1 (n = 24) compared with healthy matched controls (n = 19). In additional analyses, we also included patients with idiopathic hypersomnia (n = 14) to assess sleepiness-related influences.

METHODS

Participants were first trained on a choice task to earn salty and sweet snacks. Next, one of the snack outcomes was devalued by having participants consume it until satiation (i.e., sensory-specific satiety). We then measured the selective reduction in choices for the devalued snack outcome. Finally, we assessed the number of calories that participants consumed spontaneously from ad libitum available snacks afterwards.

RESULTS

After satiety, all participants reported reduced hunger and less wanting for the devalued snack. However, while controls and idiopathic hypersomnia patients chose the devalued snack less often in the choice task, patients with narcolepsy still chose the devalued snack as often as before satiety. Subsequently, narcolepsy patients spontaneously consumed almost 4 times more calories during ad libitum snack intake.

CONCLUSIONS

We show that the manipulation of food-specific satiety has reduced effects on food choices and caloric intake in narcolepsy type 1 patients. These mechanisms may contribute to their obesity, and suggest an important functional role for orexin in human eating behavior.

CLINICAL TRIALS REGISTRATION

Study registered at Netherlands Trial Register. URL: www.trialregister.nl. Trial ID: NTR4508.

The behavioural profile of gamma-hydroxybutyrate, gamma-butyrolactone and 1,4-butanediol in humans

Gamma-hydroxybutyrate (GHB) is a putative neurotransmitter, a drug of abuse, and a medical treatment for narcolepsy and other neuropsychiatric disorders. Its precursors gamma-butyrolactone (GBL) and 1,4-butanediol (1,4-BD) are endogenously converted to GHB and thereby exert their psychobehavioural effects. In humans, GHB has a wide spectrum of properties ranging from stimulation and euphoria in lower doses, to sedation, deep sleep, and coma after ingestion of high doses. However, behavioural studies in healthy volunteers remain scarce and are usually limited to psychomotor performance testing. Most available data arise from either qualitative studies with illicit users or clinical trials examining therapeutic properties of GHB (then usually termed sodium oxybate). Here, we present an overview of the behavioural effects of GHB, GBL, and 1,4-BD in these three populations. GHB and its precursors strongly influence behaviours related to core human autonomic functions such as control of food intake, sexual behaviour, and sleep-wake regulation. These effects are instrumentalised by illicit users and clinically utilised in neuropsychiatric disorders such as narcolepsy, fibromyalgia, and binge-eating syndrome. Considering the industry withdrawal from psychopharmacology development, repurposing of drugs according to their behavioural and clinical profiles has gained increasing relevance. As such, GHB seems to be an attractive candidate as an experimental therapeutic in depression.

Challenges in the development of therapeutics for narcolepsy

Narcolepsy is a neurological disorder that afflicts 1 in 2000 individuals and is characterized by excessive daytime sleepiness and cataplexy-a sudden loss of muscle tone triggered by positive emotions. Features of narcolepsy include dysregulation of arousal state boundaries as well as autonomic and metabolic disturbances. Disruption of neurotransmission through the hypocretin/orexin (Hcrt) system, usually by degeneration of the HCRT-producing neurons in the posterior hypothalamus, results in narcolepsy. The cause of Hcrt neurodegeneration is unknown but thought to be related to autoimmune processes. Current treatments for narcolepsy are symptomatic, including wake-promoting therapeutics that increase presynaptic dopamine release and anticataplectic agents that activate monoaminergic neurotransmission. Sodium oxybate is the only medication approved by the US Food and Drug Administration that alleviates both sleep/wake disturbances and cataplexy. Development of therapeutics for narcolepsy has been challenged by historical misunderstanding of the disease, its many disparate symptoms and, until recently, its unknown etiology. Animal models have been essential to elucidating the neuropathology underlying narcolepsy. These models have also aided understanding the neurobiology of the Hcrt system, mechanisms of cataplexy, and the pharmacology of narcolepsy medications. Transgenic rodent models will be critical in the development of novel therapeutics for the treatment of narcolepsy, particularly efforts directed to overcome challenges in the development of hypocretin replacement therapy.

Central and peripheral metabolic changes induced by gamma-hydroxybutyrate

STUDY OBJECTIVES

Gamma-hydroxybutyrate (GHB) was originally introduced as an anesthetic but was first abused by bodybuilders and then became a recreational or club drug.1 Sodium salt of GHB is currently used for the treatment of cataplexy in patients with narcolepsy. The mode of action and metabolism of GHB is not well understood. GHB stimulates growth hormone release in humans and induces weight loss in treated patients, suggesting an unexplored metabolic effect. In different experiments the effect of GHB administration on central (cerebral cortex) and peripheral (liver) biochemical processes involved in the metabolism of the drug, as well as the effects of the drug on metabolism, were evaluated in mice.

DESIGN

C57BL/6J, gamma-aminobutyric acid B (GABAB) knockout and obese (ob/ob) mice were acutely or chronically treated with GHB at 300 mg/kg.

MEASUREMENTS AND RESULTS

Respiratory ratio decreased under GHB treatment, independent of food intake, suggesting a shift in energy substrate from carbohydrates to lipids. GHB-treated C57BL/6J and GABAB null mice but not ob/ob mice gained less weight than matched controls. GHB dramatically increased the corticosterone level but did not affect growth hormone or prolactin. Metabolome profiling showed that an acute high dose of GHB did not increase the brain GABA level. In the brain and the liver, GHB was metabolized into succinic semialdehyde by hydroxyacid-oxoacid transhydrogenase. Chronic administration decreased glutamate, s-adenosylhomocysteine, and oxidized gluthathione, and increased omega-3 fatty acids.

CONCLUSIONS

Our findings indicate large central and peripheral metabolic changes induced by GHB with important relevance to its therapeutic use.

Gender differences between hypocretin/orexin knockout and wild type mice: age, body weight, body composition, metabolic markers, leptin and insulin resistance

Female hypocretin knockout (Hcrt KO) mice have increased body weight despite decreased food intake compared to wild type (WT) mice. In order to understand the nature of the increased body weight, we carried out a detailed study of Hcrt KO and WT, male, and female mice. Female KO mice showed consistently higher body weight than WT mice, from 4 to 20 months (20-60%). Fat, muscle, and free fluid levels were all significantly higher in adult (7-9 months) as well as old (18-20 months) female KO mice compared to age-matched WT mice. Old male KO mice showed significantly higher fat content (150%) compared to age-matched WT mice, but no significant change in body weight. Respiratory quotient (-19%) and metabolic rates (-14%) were significantly lower in KO mice compared to WT mice, regardless of gender or age. Female KO mice had significantly higher serum leptin levels (191%) than WT mice at 18-20 months, but no difference between male mice were observed. Conversely, insulin resistance was significantly higher in both male (73%) and female (93%) KO mice compared to age- and sex-matched WT mice. We conclude that absence of the Hcrt peptide has gender-specific effects. In contrast, Hcrt-ataxin mice and human narcoleptics, with loss of the whole Hcrt cell, show weight gain in both sexes.

Narcolepsy: a clinical review

Despite the classic tetrad of clinical features that typify it, narcolepsy remains much under-diagnosed, in part, because of the wide spectrum of clinical phenotypes, but also because of its insidious onset, usually in a young person. The median time to diagnosis from first symptoms remains very long, around 10 years in the UK. Conversely, in the specialist setting, it is likely over-diagnosed, largely because of failure to exclude other causes of hypersomnia. There is an over-reliance on a biological marker of the condition, the multiple sleep latency test (MSLT), which, like many tests, has a significant false-positive and false-negative rate. This review aims to discuss some of the difficulties in achieving a diagnosis, interpretation of investigations, differential diagnosis, and appropriate management of patients with narcolepsy.