Monoamine metabolite concentrations in the cerebrospinal fluid of normal and narcoleptic dogs

Neurobiology of cataplexy

Cataplexy is the pathognomonic and the most striking symptom of narcolepsy. It has originally been, and still is now, widely considered as an abnormal manifestation of rapid eye movement (REM) sleep during wakefulness due to the typical muscle atonia. The neurocircuits of cataplexy, originally confined to the brainstem as those of REM sleep atonia, now include the hypothalamus, dorsal raphe (DR), amygdala and frontal cortex, and its neurochemistry originally focused on catecholamines and acetylcholine now extend to hypocretin (HCRT) and other neuromodulators. Here, we review the neuroanatomy and neurochemistry of cataplexy and propose that cataplexy is a distinct brain state that, despite similarities with REM sleep, involves cataplexy-specific features.

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.

Animal models of narcolepsy

Narcolepsy is a debilitating sleep disorder with excessive daytime sleepiness and cataplexy as its two major symptoms. Although this disease was first described about one century ago, an animal model was not available until the 1970s. With the establishment of the Stanford canine narcolepsy colony, researchers were able to conduct multiple neurochemical studies to explore the pathophysiology of this disease. It was concluded that there was an imbalance between monoaminergic and cholinergic systems in canine narcolepsy. In 1999, two independent studies revealed that orexin neurotransmission deficiency was pivotal to the development of narcolepsy with cataplexy. This scientific leap fueled the generation of several genetically engineered mouse and rat models of narcolepsy. To facilitate further research, it is imperative that researchers reach a consensus concerning the evaluation of narcoleptic behavioral and EEG phenomenology in these models.

Cerebrospinal Fluid

This chapter discusses the anatomy, functions, and biochemistry of cerebrospinal fluid (CSF). CSF has four major functions: physical support of neural structures, excretion and “sink” action, intracerebral transport, and control of the chemical environment of the central nervous system. CSF provides a “water jacket” of physical support and buoyancy. The CSF is protective because its volume changes reciprocally with changes in the volume of intracranial contents, particularly blood. Thus, the CSF protects the brain from changes in arterial and central venous pressure associated with posture, respiration, and exertion. Acute or chronic pathological changes in intracranial contents can be accommodated, to a point, by changes in the CSF volume. The direct transfer of brain metabolites into the CSF provides excretory function. This capacity is important because the brain lacks a lymphatic system. The lymphatic function of the CSF is also manifested in the removal of large proteins and cells, such as bacteria or blood cells, by bulk CSF absorption. The “sink” action of the CSF arises from the restricted access of water-soluble substances to the CSF and the low concentration of these solutes in the CSF.

CSF markers in sleep neurobiology

The cerebrospinal fluid has been used in the study of normal and pathological conditions of the central nervous system for more than a century. CSF analysis has also been applied to the study of sleep and its disorders but methodological aspects have often limited the results. The discovery of the hypocretin system (also known as orexin system) and its involvement in the pathophysiology of narcolepsy has opened a new field in the diagnosis of hypersomnia by CSF analysis and has revived the interest on this subject in sleep medicine. Older and new lines of research involving CSF measurement of hypocretin and other neurotransmitters in sleep and its disorders are reviewed.

N-acetylaspartic acid (NAA) and N-acetylaspartylglutamic acid (NAAG) in human ventricular, subarachnoid, and lumbar cerebrospinal fluid

N-Acetylaspartic and N-acetylaspartylglutamic acid concentrations in human ventricular, subarachnoid and lumbar cerebrospinal fluid were measured by combined gas chromatography-mass spectrometry using selected ion monitoring with deuterated internal standards. N-Acetylaspartate concentrations were in the range 55, 9, and 1 microM, respectively; N-acetylaspartylglutamate concentrations in the same fluids were in the range 8, 3 and 4 microM, respectively. There did not appear to be any difference in lumbar fluid concentrations of either compound between control subjects, schizophrenic patients, Alzheimer's disease patients and a pooled group of patients with neurological degeneration. Ventricular concentrations of both compounds were greatly increased in deceased patients suggesting that maintenance of their intracellular concentrations is probably energy dependent. The concentrations of these compounds in lumbar cerebrospinal fluid from living, and ventricular cerebrospinal fluid from deceased subjects were weakly correlated with one another. In lumbar fluid neither compound appeared to be correlated with age. Analysis of serially collected lumbar samples from two subjects showed a weak concentration gradient for both compounds. Neither antipsychotic medication nor the acid transport inhibitor probenecid had any effect on lumbar concentrations of either compound. Attempts to use anion exchange high pressure liquid chromatography with UV detection for measurement of the low concentrations of N-acetylaspartate found in cerebrospinal fluid from living subjects were unsuccessful.

Comparison of cerebrospinal fluid monoamine metabolite levels in dominant-aggressive and non-aggressive dogs

Aggression has been shown to be related to reduced serotonergic activity in humans and non-human primates, and in rodents. We now studied the relationship between cerebrospinal fluid (CSF) monoamine metabolites and canine aggression in 21 dominant-aggressive dogs (Canis familiaris) and 19 controls. The diagnosis of dominance-related aggression was based upon a history of biting family members in contexts associated with dominance challenges. Post-mortem CSF 5-HIAA, MHPG and HVA were measured by high-performance liquid chromatography using electrochemical detection. Concentrations of CSF 5-HIAA (P = 0.01) and HVA (P < 0.001) were lower in the aggressive group (median values: 5-HIAA 202.0 pmol/ml; HVA 318.0 pmol/ml) than in controls (5-HIAA 298.0 pmol/ml; HVA 552.0 pmol/ml). No differences were noted in CSF MHPG levels. Differences in 5-HIAA were maintained after controlling for breed and age of dogs, but HVA differences may have been breed-dependent. Lower levels of 5-HIAA (P = 0.02) and HVA (P = 0.04) were found in the subgroup of aggressive dogs with a history of biting without warning (5-HIAA 196.0 pmol/ml; HVA 302.0 pmol/ml) compared to dogs that warned (5-HIAA 244.0 pmol/ml; HVA 400.0 pmol/ml). This study suggests that reduced serotonergic function is associated with aggressive behavior and impaired impulse control in dogs, a finding that is consistent with observations in primates, and suggests that serotonin modulates aggressive behavior throughout mammals.

[123I]IBZM SPET analysis of dopamine D2 receptor occupancy in narcoleptic patients in the course of treatment

Elevated levels of central D2 dopamine receptors were found on postmortem examination in cases of human narcolepsy. In vivo investigations using positron emission tomography (PET) and single photon emission tomography (SPET) found no changes of D2 binding in the striatal structures. To investigate whether the elevated D2 receptors in postmortem investigations are due to long-term treatment effects, we applied 123I-labeled (S)-2-hydroxy-3-iodo-6-methoxy-([1-ethyl-2-pyrrolidinyl]methyl) benzamide (IBZM) ([123I]IBZM, a highly selective CNS D2 dopamine receptor ligand) and SPET in narcoleptic patients in the course of treatment with stimulants and/or antidepressants. Before treatment we found no changes in D2 binding in 10 patients (in comparison to 10 normal controls). After treatment (performed in five patients for 3 months) we found changes in D2 binding in four of them, indicating that the results of the postmortem studies could have been influenced by long-term medications. Human narcolepsy seems not to be related to a striatal D2 dopaminergic disturbance.

Familial occurrence of narcolepsy in miniature horses

In an investigation of 2 closely related Miniature Horses with a history of excessive sleepiness, depression and episodes of collapse, a diagnosis of narcolepsy was made on the basis of neurological examination and pharmacological testing. Further investigations included electroencephalographic examination (EEG), and analysis of protein content, cell count and monoamine metabolite concentrations of lumbosacral cerebrospinal fluid (CSF). There were no abnormalities noted in the EEGs, and no consistent changes in CSF neurotransmitter metabolites in the narcoleptic horses when compared with 3 normal, unrelated Miniature Horses and 2 related, clinically unaffected animals. The breeding background of the 2 affected horses was investigated and a limited survey of Miniature Horse breeders in North America was conducted. These investigations have shown that narcolepsy is a rare but distinct syndrome in the Miniature Horse, and that the cases described here appear to represent a familial occurrence of the disease.

Passage of MHPG from plasma to CSF in a non-human primate

In experimental protocols with humans and non-human primates, cerebrospinal fluid (CSF) concentrations of 3-methoxy-4-hydroxyphenylethylene glycol (MHPG), the predominant end-product of norepinephrine metabolism in the mammalian central nervous system (CNS), have been widely used as an index of the rate of CNS norepinephrine metabolism. However, an earlier investigation showed that there was slow but free exchange between plasma and CSF MHPG. To define more precisely the time-course of equilibration of plasma and CSF MHPG, we intravenously administered 100 micrograms/kg of [2H3]-MHPG to drug-naive squirrel monkeys. Measurements were made of the concentrations of [2H3]- and [1H]-MHPG in plasma and cervical CSF samples collected at time points from 10 min to 4 hr thereafter. The results indicated that neither plasma nor CSF concentrations of [1H]-MHPG changed during the course of the experiment, and that [2H3]-MHPG appeared in the CSF within 10 min of intravenous administration. The maximal plasma and CSF concentrations of [2H3]-MHPG were 7.6- and 2.3-fold higher than the respective concentrations of [1H]-MHPG. The plasma and CSF pools of [2H3]-MHPG reached concentration equilibrium within 30 min, and thereafter the temporal decline in concentration of [2H3]-MHPG was the same in plasma and CSF. These results demonstrate that MHPG rapidly crosses from plasma to CSF, and support the suggestion that this factor be included in any attempts to estimate norepinephrine turnover in the CNS from measurements of steady-state MHPG concentrations in CSF or plasma.

Intercorrelations among monoamine metabolite concentrations in human lumbar CSF are not due to a shared acid transport system

Intercorrelations among homovanillic acid (HVA), 5-hydroxyindoleacetic acid (5-HIAA), and 3-methoxy-hydroxy-phenylglycol (MHPG) concentrations in lumbar cerebrospinal fluid (CSF) were examined before and after blockade of the acid transport system by probenecid in 59 psychiatric inpatients. The three compounds remained intercorrelated despite acid transport blockade, suggesting that the common transport system does not account for their covariance. Other possibilities to explain the interrelationship among these compounds are discussed.

Neurochemical perspectives of the narcoleptic syndrome

Narcolepsy has been defined as a disorder of excessive sleep often associated with cataplexy, sleep paralysis and hypnagogic hallucinations. Although the pathophysiology of the narcoleptic syndrome is not well understood, derangement in the functions of CNS catecholamines and serotonin (5-HT) have been implicated. In the present paper we summarize evidence to suggest a role for the endogenous opioids in the regulation of normal sleep and in the pathophysiology of the narcoleptic syndrome.

Brain dopamine receptor levels elevated in canine narcolepsy

Concentrations of dopamine D2 receptors in discrete brain areas differed significantly between dogs with the genetically transmitted form of narcolepsy, and age- and breed-matched controls. D2 receptors were assayed and quantified with Scatchard analysis using [3H]spiperone. Receptor densities in the nucleus accumbens, rostral caudate, and amygdala were consistently higher in narcoleptic animals. In amygdala, dopamine receptor abnormalities were associated with elevated dopamine and 3,4-dihydroxyphenylacetic acid concentrations, but no change in 3-methoxytyramine or homovanillic acid concentrations. These data indicate mesolimbic system involvement in canine narcolepsy and point to impaired dopamine release as a possible etiologic factor.

Extractive acylation and mass spectrometric assay of 3-methoxytyramine, normetanephrine, and metanephrine in cerebrospinal fluid

The chemical analysis of 3-methoxytyramine, normetanephrine, and metanephrine in tissues, blood, and cerebrospinal fluid is complicated by the low levels in which they occur and the amphoteric nature of the functional groups, which hampers their isolation from aqueous media. In the present report, we describe a practical and simple method for the routine isolation and derivatization of 3-methoxytyramine, normetanephrine, and metanephrine in high yield from aqueous samples like cerebrospinal fluid. The metabolites are simultaneously derivatized with pentafluoropropionic anhydride and extracted into an organic solvent. After further treatment with pentafluoropropionic anhydride under anhydrous conditions, the samples are ready for GC/MS analysis. The method is able to exploit the theoretical maximal sensitivity available in the electron capture negative-ion chemical ionization mode without any apparent losses during extraction and derivatization, giving limits of detection in the low picogram range. Mean levels of free 3-methoxytyramine, normetanephrine, and metanephrine in human cerebrospinal fluid were 3.77, 5.20, and 0.40 pmol/ml. Assay of pools of squirrel monkey, human, and canine cerebrospinal fluid with and without previous enzymatic hydrolysis demonstrated that the three metabolites occur predominantly in a conjugated form.

The neurobiology of the narcoleptic syndrome

Relevant electroencephalographic, psychopharmacologic, and genetic research reports are described in support of a neurobiological explanation of the narcoleptic syndrome. Despite increased support in this realm, no single neurobiological theory has won unanimous approval among sleep researchers, which has led toward speculation that the condition may be heterogeneous in nature. A multifactorial perspective, including psychological as well as neurobiological influences, appears to be the most productive model for research. Future investigation of sleep disorders utilizing such a model may enhance the understanding of neurobiological correlates of behavioural disorders.

Measurement of tritiated norepinephrine metabolism in intact rat brain

A procedure for the study of NE metabolism in the intact rat brain is described. The method involves ventriculocisternal perfusion of the adult male rat with artificial CSF containing [3H]NE. Radioactivity in the perfusate associated with NE and its metabolites 3,4-dihydroxymandelic acid (DOMA), 3,4-dihydroxyphenylethyleneglycol (DHPG), 3-methoxy-4-hydroxymandelic acid (VMA), 3-methoxy-4-hydroxyphenylethyleneglycol (MHPG), and normetanephrine (NMN) is separated using high-performance liquid chromatography (HPLC). After 80 min the radioactivity in the perfusate reaches an apparent steady-state. Analysis of the steady-state samples shows higher activity in the fractions corresponding to DHPG and MHPG than in those corresponding to DOMA and VMA, confirming glycol formation as the major pathway of NE metabolism in rat brain. Pretreatment with an MAO inhibitor (tranylcypromine) results in a marked decrease in the deaminated metabolites DHPG and MHPG and a concurrent increase in NMN. The results indicate this to be a sensitive procedure for the in vivo determination of changes in NE metabolism.

Narcolepsy: biogenic amine deficits in an animal model

Concentrations of biogenic amine metabolites in discrete brain areas differed significantly between dogs with genetically transmitted narcolepsy and age- and breed-matched controls. Dopamine and 3,4-dihydroxyphenylacetic acid were consistently elevated in the brains of narcoleptic animals, while homovanillic acid was not. Narcoleptic animals consistently exhibited lower utilization of dopamine and higher intraneuronal degradation of dopamine but no uniform decrease in serotonin utilization. Hence neuropathology appears to be associated with genetically transmitted canine narcolepsy. The data indicate a nonglobal depression of dopamine utilization or turnover or both.