Monoaminergic neuronal changes in orexin deficient mice

Abstract

Orexin knockout (KO) mice and orexin/ataxin-3 mice (which have a different pathophysiological background in orexin deficiency) exhibit a phenotype that is similar to human narcolepsy. Although the interactions between the monoaminergic and orexinergic systems are not entirely clear, indirect monoamine-receptor agonists (especially psychostimulants) may contribute to the treatment of narcolepsy. The present study was designed to investigate the interaction between brain orexinergic and monoaminergic neurons as measured by the status of monoaminergic systems and monoamine-related behaviors using orexin-deficient mice. Previous studies have shown that a reduction of monoaminergic tone is related to wakefulness. In the present study, locomotor activity in a novel environment and dopamine turnover was significantly decreased in orexin-deficient mice compared to WT mice, which suggests that psychostimulants may be useful for maintaining wakefulness in orexin deficiency. We also examined the effects of orexin deficiency on psychostimulant-induced hyperlocomotion. The hyperlocomotion induced by methamphetamine and methylphenidate was lower, whereas that induced by MDMA was higher in orexin KO mice compared to WT mice. The sensitivities against psychostimulants in orexin/ataxin-3 mice differed from those in orexin KO mice. These results indicate that the effectiveness of each psychostimulant, which is closely related to its monoaminergic function, was influenced by orexin deficiency itself as well as by the different pathophysiological background in orexin deficiency.

Introduction

Excessive daytime sleepiness has been recognized to have negative consequences. There are numerous causes of such sleepiness, including intrinsic sleep disorders like narcolepsy (Banerjee et al., 2004). Monoamine neurons show robust changes across sleep-wake states; monoamines are more critically involved in regulating the characteristics of wakefulness (Miller et al., 1983). Psychostimulants (e.g. methamphetamine and methylphenidate) that increase the level of monoamines within the synaptic cleft can be regarded as drugs that treat narcolepsy and produce behavioral activation accompanied by an increase in arousal, motor activity, and alertness (Banerjee et al., 2004). Furthermore, the cataplexy associated with narcolepsy can be managed with tricyclic antidepressants or selective serotonin reuptake inhibitors (Thorpy, 2007). Thus, monoaminergic systems are closely related to sleep-wake states.

Orexin is a neuropeptide that is exclusively expressed in the lateral hypothalamic area, a region of the brain that is classically implicated in sleep-wake states. Recently, several reports have stated that dysfunction of the orexin system was observed in human narcolepsy (Mignot et al., 2002, Nishino et al., 2000, Peyron et al., 2000, Thannickal et al., 2000). Murine models of narcolepsy, generated by knocking out (KO) the gene for orexin in mice, exhibit a phenotype that is remarkably similar to human narcolepsy–cataplexy: difficulty maintaining wakefulness (sleep-wake state fragmentation), intrusion of REM sleep into wakefulness resembling cataplexy, and increased REM sleep during the active phase (Chemelli et al., 1999).

Orexin-containing neurons project to practically all brain regions and monoaminergic nuclei, including raphe nuclei (5-HT) locus coeruleus (norepinephrine), and the ventral tegmental area (dopamine) (Peyron et al., 1998, Date et al., 1999, Nambu et al., 1999). Therefore, the activation of orexin-containing neurons leads to the modification of monoaminergic neurons. Thus, the function of monoamine neurotransmitter systems might be altered in narcolepsy. However, to our knowledge, there is little information available on the status of monoaminergic systems or the effects of possible pharmacological agents against narcolepsy in orexin KO mice (Narita et al., 2006). In this study, we found evidence of altered brain monoaminergic neuronal functions, and that psychostimulant-induced behaviors were altered in orexin KO mice compared with WT mice. These results not only provide insight into an aspect of the pathology of narcolepsy, but may also provide a basis for medical treatment.

The induction of postnatal death in orexin-producing neurons (orexin/ataxin-3 mice) also showed a phenotype that was remarkably similar to that in human narcolepsy (Hara et al., 2001). Human narcolepsy shows metabolic abnormalities, and the metabolic abnormalities (e.g. change in energy homeostasis) in orexin/ataxin-3 mice seem more severe than those in orexin KO mice (Chemelli et al., 1999, Hara et al., 2001). Orexinergic neurons may coexpress other modulatory factors, and these coexpressed modulatory factors were also postnatally ablated in orexin/ataxin-3 mice (Hara et al., 2001). Therefore, it is possible that the phenotype of behaviors induced by monoamine-related compounds in orexin/ataxin-3 mice might differ according to the differential pathophysiological background in orexin deficiency. Therefore, we also compared monoamine-related behaviors in orexin KO mice and orexin/ataxin-3 mice.