Relationship between non-functional masticatory activity and central dopamine in stressed rats.

In humans, diurnal tooth-clenching and other oral stereotyped behaviour are associated with stress/anxiety. In rodents, gnawing/biting of objects is observed during exposure to stress. Both nigrostriatal and mesocortical dopaminergic systems are involved in the development of this coping behaviour. To clarify the relationship between central dopaminergic activity and stress-induced parafunctional masticatory behaviour, using microdialysis in vivo, we assessed the changes in extracellular dopamine concentrations in both prefrontal cortex and striatum of rats subjected to a mild tail pinch. The animals were divided into two groups according to the degree of non-functional masticatory activity (NFMA) displayed during exposure to tail pinch. In prefrontal cortex, rats which displayed severe NFMA showed a greater increase in extracellular dopamine concentration in relation to basal values (Emax=184±26%) than those which did not display this coping behaviour (Emax=139± 23%) (F(NFMA) [1,86]=3·97; P<0·05) (n=17). A positive association was also found between cortical dopamine maximal value from baseline and the degree of NFMA displayed (r=0·36; P<0·05) (n=17). There were no significant differences in the tail-pinch-induced striatal dopamine increase between both groups of rats (Emax=130±10%) (n=17). These results provide further evidence in support of prefrontal dopamine playing a relevant role in the expression of stress-induced masticatory coping behaviour.