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.