Introduction: Sensory conflicts in the vestibular system lead to motion sickness of which space motion sickness (SMS) is a special case. SMS affects up to 70% of the astronauts during the first 3 days in space. The search for effective countermeasures has led to several nonpharmacological and pharmacological approaches. The current study focuses on the effects of lorazepam (1 mg), meclizine (25 mg), promethazine (25 mg), and scopolamine (0.4 mg) on the vestibular system, with special focus on the canal and otolith functions separately. Methods: The study had a placebo-controlled, single blind, repeated measures design. Sixteen healthy volunteers were subjected to a total of 7 test sessions, the first and last being without intake of medication. Semicircular canal function was evaluated by means of electronystagmography and otolith function with unilateral centrifugation. The horizontal semicircular canal function was characterized by the vestibulo-ocular reflex (VOR) gain measured during earth vertical axis rotation as well as the total caloric response. The function of the utricles was represented by the utricular sensitivity, reflecting the ocular counter roll relative to the virtual induced head tilt. Results: Promethazine significantly decreased the semicircular canal and utricular parameters. Both scopolamine and lorazepam caused only a decrease in the utricular sensitivity, whereas meclizine only decreased the semicircular canal-induced VOR gain. Discussion: The results show that the drugs affected different areas of the vestibular system and that the effects can thus be attributed to the specific pharmacological properties of each drug. Meclizine, as an antihistaminergic and weak anticholinergic drug, only affected the VOR gain, suggesting a central action on the medial vestibular nucleus. The same site of action is suggested for the anticholinergic scopolamine since acetylcholine receptors are present and utricular fibers terminate here. The global vestibular suppression caused by promethazine is probably a consequence of its anticholinergic, antihistaminergic, and antidopaminergic properties. Based on the fact that lorazepam increased the affinity of gamma-aminobutyric acid (GABA) for the GABAA-receptor and its effects on the utriculi, the site of action seems to be the lateral vestibular nucleus. Conclusion: Meclizine, scopolamine, and lorazepam selectively suppress specific parts of the vestibular system. Selective suppression of different parts of the vestibular system may be more beneficial for alleviating (space) motion sickness than general suppressive agents. Additionally, this knowledge may help the clinician in his therapeutic management of patients with either semicircular canal or otolith dysfunction.

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