“Resistant” Channels Reluctantly Reveal Their Roles

tical pathfinding. Thus, axons from the VL, which nor-domain protein control of the expression of EphA recep-mally project to the rostral cortex (motor cortex), target tor and ephrin-A ligands in motor neurons and limb mes-instead the parietal cortex (somatosensory cortex; cau-enchymal cells, respectively. It will be of interest to see dal to the motor cortex) in the absence of ephrin-A5 and whether a bHLH transcription factor can also control EphA4. This defect is observed with a low frequency in the expression of EphA receptors. ephrin-A5 (33%), EphA4 (12.5%), and EphA7 (50%) sin-Another intriguing issue derived from these studies is gle mutants, supporting the view that several EphA re-the mechanism guiding corticothalamic axons. Is the ceptors and ephrin-A ligands are involved in vivo in topography of corticothalamic projections also con-setting up the topographic mapping of thalamocortical trolled by the ventral telencephalon or by the reciprocal axons. Finally, analysis of thalamocortical connectivity thalamocortical axons? Additional studies are required in ephrin-A5/EphA4 double mutants at the time when to reevaluate previous models on the light of the recent thalamic axons are invading the ventral telencephalon findings. Finally, it is clear that there is much work to showed that the topographic sorting of thalamocortical be done on the field of intra-areal topography in the axons is already shifted in this intermediate region— cortex; future studies should be directed to increase our additional evidence supporting the role of the ventral knowledge on the role of ephrins in this process as well telencephalon in this process. as to test the potential role of other guidance cues. ephrins Contribute to the Formation of the Cortical Somatotopic Map In addition to their role in establishing the topographic Oscar Marín organization of thalamic projections to distinct cortical Developmental Neurobiology Unit regions, Dufour et al. (2003) investigated whether ephrin-Institute of Neurosciences A/EphA interactions are required for topographic map-University Miguel Herná ndez and CSIC ping within an individual cortical area. Ephrins have been 03550 San Juan de Alicante previously shown to play a role in the formation of other Spain intra-areal topographic maps (Wilkinson, 2001), but their implication in cortical mapping has remained elusive. projections (inter-areal topography) and within individ-thalamocortical mapping is established. For example, several questions are raised regarding the role of Ngn2 in specifying rostral thalamic axon responsiveness to ventral telencephalic cues. What are the downstream effectors of Ngn2 in the dorsal thalamus? Is Ngn2 suffi-" Resistant " Channels cient …