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Abstract:
Complex accelerative stimuli can induce pica in rats as well as the treatment with poisons, which means eating of non-nutritive substances such as kaolin, in proportion to the severity of their sickness. For the purpose of using pica as an index of motion sickness in rats, we examined what kind of rotation was effective for inducing pica in rats with or without normal bilateral labyrinth functions. Clinically potent anti-motion sickness drugs, such as scopolamine, methamphetamine, diphenhydramine, were examined in reducing rotation-induced pica in rats. Rats ate more kaolin after double rotation with continuously changing acceleration, than after single rotation. Both the animals treated with anti-motion sickness drugs or labyrinthectomy ate less kaolin even after double rotation. Since the physiological and pharmacological mechanisms for inducing pica in rats were similar with those of motion sickness in humans, pica in rats should be an acceptable index of their motion sickness. In order to study neural mechanisms of motion sickness in rats, we examined the effects of an anti-cholinergic as a potent anti-motion sickness drug and cholinergics as an antagonistic drug treated during the 4th-7th day of rotation on both habituation to double rotation within daily rotations for 10-11 days, using pica as an index of motion sickness. Rats were separated into three groups according to their initial susceptibility, and rats with low susceptibility were omitted in these experiments. Scopolamine (TTS-scopolamine) as an anticholinergic facilitated habituation to motion, especially in rats with moderate susceptibility. Treatment of physostigmine suppressed residual habituation to motion sickness in rats, especially with moderate susceptibility, though neostigmine, peripherally acting anti-cholinesterase, had no effect. These results suggested that centrally acting acetylcholine play an important role in suppressing habituation of motion sickness. In conclusion, rats should be a convenient model for studying for motion sickness, as we examined one of the neural mechanisms in motion sickness using pica as an index.