The role of the nigrostriatal dopaminergic pathway in the sequential progress

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The masticatory sequence is a whole set of movements from ingestion to swallowing. The masticatory sequence could be divided into three recognized stages (stage I, IIa, IIb) in the rabbit based on the form of jaw movements. It is, however, still unclear which neural mechanism is involved in the transition of the masticatory stages. The aim of this project was to clarify the role of the nigrostriatal dopaminergic pathway in the sequential progress during mastication.We obtained the findings in this study as follows ;1. We examined the alteration of dopamine (DA) concentration in the extracellular space of the putamen during 5 consecutive minutes of mastication in the awake rabbit, using the microdialysis. Analysis of the microdialysate samples indicated that the DA concentration in the putamen increased significantly during mastication, compared with resting the period.2. We studied the effect of bilateral 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopaminergic pathway on feeding behavior in freely-moving rabbits by means of video and electromyogram (EMG) of masticatory muscles. We observed that the duration of intake behavior, in which the rabbit inserted its mouth into the pellet-box and seemed to pick up pellets, was shortened after the lesions. Further, the number of intake behaviors for consuming 1 g of pellets was increased. During intake behavior, masseteric EMG activities varied in the control rabbits, whereas this variation was smaller in the lesioned subjects. This suggests that the behaviors of picking up the pellets and chewing them were intermingled during intake behavior in the control subjects. This effect was observed only when the dopaminergic pathway from the substantia nigra to the ventral part of the putamen was interrupted, and not when the dorsal part was disrupted.These results suggested that the nigrostriatal dopaminergic pathway might play a role in the sequential progress during mastication.