Hypothermic effects of hops could be antagonized with the competitive melatonin receptor antagonist luzindole

Flowers of Humulus lupulus L. (Cannabinaceae), commonly known as hops, are traditionally used to relief insomnia, anxiety, excitability and restlessness associated with tension headache and gastrointestinal spasms. However, little information is available about the underlying sleep inducing mechanism of hops. It has been shown previously that a combination of valerian and hops interacts with serotoninergic 5-HT₆ and melatoninergic ML₁ receptors [1]. Melatonin is known to have both hypnotic and hypothermic effects at physiological levels. Indeed, the hypnotic effect may be mediated via the hypothermic action [2].

The above considerations and the traditional use of hops as a sleep inducer prompted us to evaluate the hypothermic activity of hops extract (HE) in mice. In a dosage of 250mg/kg HE significantly decreased body temperature in male BL6/C57J mice (ΔT -1.1°C) 2h after oral administration. The effects of the plant extract were comparable to melatonin (50mg/kg; ΔT -0.8°C; 2h after i.p. injection). The hypothermic effects of both, melatonin and HE could be antagonized with the competitive melatonin receptor antagonist luzindole. Thus, these data suggest that the hypothermic effects of HE are mediated through activation of melatonin receptors.

References: 1. Abourashed, E.A. et al. (2004), Phytomedicine 11: 633–638. 2. Zemlan, F.P. (2005), J. Clin. Psychiatry 66: 384–390.