Effects of Levonantradol on Habituation of Startle in Rats

Levonantradol, a nonopioid analgetic related to cannabinoids, produced a dose-dependent impairment of the habituation of startle response in rats. Startle was elicited by air-puff stimuli and monitored in stabilimeters by a computer. Habituation was assessed by presenting the same tactile stimulus repeatedly at 15-second intervals. While a dose of 0.1 mg/kg reduced startle on all trials, doses ranging from 0.01 to 0.03 mg/kg had no effect on the initial level of reactivity to the first several stimuli but did impair the habituation of startle. This effect was not attributable to the time course of the drug's action and was also found with acoustic stimuli. The relatively inactive enantiomer of levonantradol was ineffective at 0.1 mg/kg. Thus, startle appears to be a sensitive measure of the effects of levonantradol in animals. Since comparable phenomena can be demonstrated in humans by measuring the blink reflex component of startle, direct comparisons can potentially be made across species.