Research ReportValerian extract and valerenic acid are partial agonists of the 5-HT5a receptor in vitro
Introduction
Insomnia is considered the most frequent sleep complaint and affects nearly all populations throughout the world, particularly the elderly [28]. Menopausal women often suffer from sleep disturbances with frequent awakenings, likely due to disturbed expression of daily biological rhythms [4], [30]. Some of the more commonly used hypnotic drugs to treat insomnia are benzodiazepines, which are associated with adverse events such as tolerance, dependence, and morning sleepiness [28]. Many menopausal women prefer not to use benzodiazepines but use valerian preparations instead, and report that these products are well tolerated, effective for the treatment of insomnia, while not inducing impairment of vigilance or cognitive and psychomotor performance [17], [29], [30].
Extracts from the roots of valerian (Valeriana officinalis L., Valerianaceae) have long been used in alternative medicine for the treatment of insomnia and are the most well recognized herbal sedatives worldwide. Approximately 15 controlled clinical trials have assessed the efficacy of various valerian extracts, and the German Commission E has approved its use for the treatment of restlessness and sleeping disorders [4]. Numerous clinical trials support the use of valerian and have demonstrated an improvement in sleep latency and quality in both healthy volunteers and patients with sleep disorder [4], [29], [33]. Currently, there is no scientific agreement on the mechanism of action of valerian's sedating activity or the compounds responsible. Many potential mechanisms for the pharmacological activity of valerian have been proposed, including agonistic activities on the GABA, adenosine, barbiturate, and benzodiazepine receptors [34], [40], [46]. However, the concentrations used in these investigations were extremely high in the older reports and more recent reports put some of these data into question [12], [40].
Although serotonin is well known to modulate a variety of physiological and behavioral processes including sleep–wake cycles, and circadian rhythms, the effect of valerian on serotonin receptor binding has not been completely elucidated [9], [24]. Serotonin (5-HT, 5-hydroxytryptamine) impacts numerous sensory, motor, and cortical functions by activating multiple 5-HT receptor subtypes [9], [24], [35]. Abnormalities of these receptor systems have been implicated in many psychiatric disorders including anxiety, depression, as well as disorders of cognition, stress, and sleep [9], [28], [35]. Serotonin was initially thought to be a true neuromodulator of sleep because the destruction of 5-HT neurons of the raphe system or the inhibition of 5-HT synthesis with p-chlorophenylalanine induced severe insomnia that could be reversed by restoring 5-HT synthesis [2]. More recent experiments suggest that the release of 5-HT during the sleep–wakefulness cycle initiates a cascade of genomic events in some hypnogenic neurons located in the preoptic area and the neighboring suprachiasmatic nucleus including vasoactive intestinal polypeptide and GABAergic mechanism [1], [3], [9], [15], [28], [35].
Our previous work has shown that dichloromethane and petroleum ether extracts of valerian bind weakly (51% at 50 μg/ml) to the 5-HT2b receptor but have significant binding (80%) at the 5-HT5a receptor [12]. When compared with other serotonin receptors, little is known about the 5-HT5a receptor, but it is believed to be involved in circadian (sleep–wake) rhythms, anxiety, and explorative behavior [15], [18], [20], [42]. Distribution of the 5-HT5a receptor is widespread throughout the rat brain, but the receptor concentrations are particularly intense in the suprachiasmatic nucleus of the hypothalamus, the site of the biological clock that drives circadian rhythms. Although the involvement of the 5-HT5a receptors is a more recent hypothesis, support for the 5-HT5a-mediated function in the SCN has been shown by strong distinct immunoreactivity in three neural components of the circadian timing system—the intergeniculate leaflet, median raphe, and dorsal raphe nucleus, in addition to the SCN [42].
Considering that our previous work indicated that valerian extracts bind predominantly to the 5-HT5a receptor, the purpose of this work was to determine if the valerian extract acts as an agonist of this receptor. To this end, bioassay-guided fractionation of the DCM and PE extracts to isolate and identify the chemical constituents responsible for this activity was also performed.
Section snippets
Materials and reagents
All chemicals and reagents were purchased from Fisher (Hanover Park, IL) or Sigma (St. Louis, MO) unless otherwise indicated. All cell culture media were obtained from Life Technologies (Carlsbad, CA). FBS was acquired from Atlanta Biologicals (Norcross, GA). [3H]Lysergic acid diethylamide (LSD), [3H]hydroxytryptamine (5-HT), and [3H]-8-hydroxy-2-(di-N-propylamino)tetralin (8-OH-DPAT) were obtained from NEN Life Science Products (Boston, MA). 5-HT5a membranes were purchased from Perkin-Elmer
Results
Extracts of valerian (100 MeOH, 50% MeOH, DCM and PE) were initially screened to determine if these extracts contained any potential ligand(s) of the melatonin, serotonin, benzodiazepine, glutamate, and GABAA receptors, or serotonin transporter [12]. No significant binding was found for any of the extracts at the 5-HT1A, 1B, 2C, 3, 6, 7, melatonin, benzodiazepine, or glutamine receptors. Only the PE and the DCM extracts (50 μg/ml) were active in the 5-HT2b, 5-HT5a receptor assays, and the
Discussion
Valerian has been used as a sedative for the treatment of insomnia and restlessness since the second century [26], [27], [29], [33]. Currently, valerian is approved in Europe as a sedative by the German Commission E. Approximately 15 controlled clinical trials have been published to date and suggest that treatment with valerian extracts improves sleep structure and sleep perception of insomnia in healthy patients and in patients suffering from sleep disorders, without producing the traditional
Acknowledgments
This study was partly funded by NIH Grant P50 AT00155 jointly provided to the UIC/NIH Center for Botanical Dietary Supplements Research by the Office of Dietary Supplements, National Institute for General Medical Sciences, Office for Research on Women's Health, National Center for Complementary, and Alternative Medicine. The contents are solely the responsibility of the authors and do not necessarily represent the views of the funding agencies.
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