Effects of Dopaminergic Drugs, Occlusal Disharmonies, and Chronic Stress on Non-functional Masticatory Activity in the Rat, Assessed by Incisal Attrition

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Abstract
Observational methods and the recording of nonspecific jaw movements or masticatory muscle activity have been used to evaluate oral parafunctional movements in animal models of bruxism. In this study, we have used a new approach in which the non-functional masticatory activity in the rat was assessed by the measurement of incisal attrition, with the aim of investigating the role of diverse factors involved in the etiology of bruxism. We quantified the attrition rate weekly by making superficial notches in the lower incisors and measuring the distances to the incisor edges. Repeated stimulation of the dopaminergic system with apomorphine led to an enhancement of the non-functional masticatory activity (p < 0.0001). The severity of the apomorphine-induced oral behavior was positively correlated (rs = 0.69, p < 0.01) with an increase in the incisal attrition rate (20.9%, p < 0.0001). Apomorphine-induced non-functional masticatory activity was strongly enhanced by the placement of an acrylic cap on both lower incisors (306%, p < 0.0001), but not by the cutting of a lower incisor. Repeated cocaine administration also increased the attrition rate (22.5%, p < 0.0001). However, neither chronic blockade of dopaminergic receptors with haloperidol, nor its withdrawal, modified attrition. In addition, since emotional disturbances are considered to be causal factors of bruxism, we tested whether experimental stress might accelerate tooth wear. Exposure to two different chronic stress regimes did not induce significant changes in incisal attrition. Moreover, exposure to chronic stress after the withdrawal of chronic haloperidol treatment did not alter attrition either. These results partially support the role of the central dopaminergic system in bruxism and suggest that stress, in general, may not be a relevant factor in tooth wear.