Modulation of rhythmogenic properties of trigeminal neurons contributing to the masticatory CPG

Increasing evidence suggests that the dorsal part of the principal sensory nucleus of the trigeminal nerve (NVsnpr) contains a significant core of the central pattern generator (CPG) circuitry required for mastication (Tsuboi et al., 2003). Like many trigeminal brainstem neurons, those of NVsnpr are rhythmically active in phase with fictive mastication in vivo (Tsuboi et al., 2003) and project directly to the trigeminal motoneurons (Kolta et al., 2000), but in contrast with the others, they are the only neurons with intrinsic bursting abilities (Sandler et al., 1998; Brocard et al., 2006) within the minimal area of the brainstem necessary to produce rhythmic activity in trigeminal nerves (Bourque and Kolta, 2001). Development of bursting in NVsnpr neurons closely follows the development of mastication. It is mediated by a persistent Na(+) current (I(NaP)) that is expressed only within a certain membrane potential range and that is modulated by the extracellular Ca(2+) concentration ([Ca(2+)](e)), the lower the concentration, the larger the magnitude of I(NaP). Under physiological [Ca(2+)](e), bursting can also be induced in vitro by repetitive electrical stimulation of the trigeminal sensory tract, which projects massively to NVsnpr or by local applications of N-methyl-d-aspartic acid. Both types of stimuli also depolarize glial cells recorded in NVsnpr and increase coupling between them. Glial cells play a determinant role in setting [Ca(2+)](e) and hence are in a key position to influence NVsnpr neuronal firing pattern.