Remodelling in the array of cell aggregates in somatotopic representation of the facial vibrissae through the trigeminal sensory system of the mouse

Glutamate transporters regulate lesion-induced plasticity in the developing somatosensory cortex

Glutamate transporters are involved in neural differentiation, neuronal survival, and synaptic transmission. In the present study, we examined glutamate transporter 1 (GLT1) expression in the neonatal somatosensory cortex of C57BL/6 mice, and pursued its role in somatosensory development by comparing barrel development between GLT1 knock-out and control mice. During the first few neonatal days, a critical period for barrels, GLT1 expression is strikingly upregulated in cortical astrocytes, whereas it was downregulated in neuronal elements to below the detection threshold. GLT1 knock-out neonates developed normally in terms of body growth, cortical histoarchitecture, barrel formation, and critical period termination. However, when row C whiskers were lesioned during the critical period, reduction of lesioned row C barrels and reciprocal expansion of intact row B/D barrels were both milder in GLT1 knock-out mice than in control littermates. Accordingly, the map plasticity index, calculated as (B + D)/2C, was significantly lowered in GLT1 knock-out mice. We also found that extracellular glutamate levels in the neonatal somatosensory cortex were significantly elevated in GLT1 knock-out mice. Diminished lesion-induced plasticity was further found in mutant mice lacking glutamate-aspartate transporter (GLAST), an astrocyte-specific glutamate transporter throughout development. Therefore, glutamate transporters regulate critical period plasticity by enhancing expansion of active barrels and shrinkage of inactive barrels. Because cortical contents of glutamate receptors and GLAST were unaltered in GLT1 knock-out mice, this action appears to be mediated, at least partly, by keeping the ambient glutamate level low. Considering an essential role of glutamate receptors in the formation of whisker-related thalamocortical synapse patterning, glutamate transporters thus facilitate their activity-dependent remodeling.

An in vitro electrophysiological and Co2+-uptake study on the effect of infraorbital nerve transection on the cortical and thalamic neuronal activity

Changes of neuronal membrane characteristics in somatosensory barrel cortex and barreloid thalamus were investigated in rats following unilateral transection of the infraorbital nerve. Kainate induced Co2+-uptake method and image analysis were used to assess the Ca2+ permeability of non-NMDA (N-methyl-D-aspartate) glutamate receptors. Changes in some biophysical parameters of the affected cortical neurons were also investigated by intracellular recording in slice experiments. The altered neuronal activity was measured on days 1, 5 and 14 after surgery. Kainate induced Co2+ uptake increased markedly reflecting enhanced Ca2+ permeability of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/KAIN)-type receptors. Changes were more pronounced in the cortex than in the thalamus and peaked on the first day following nerve transection. After that, parameters gradually returned to the normal level. However, a small enhancement was still detectable in the cortex at the end of the 2-week-long observation period. In parallel with the increased Co2+-uptake, moderate membrane potential changes, stronger spiking activity and enhanced excitability were characteristic for cortical neurons. The observed alterations in neuronal characteristics underlie the reorganization and regeneration processes following injuries or surgeries. We can conclude that immediate change of the receptive field in the barrel cortex following unilateral nerve transection is based on changes in biophysical parameters of the neurons. Altered peripheral activation evokes changes in the neuronal activity, thus providing opportunity for a quick synaptic rearrangement. AMPA/KAIN-type glutamate receptors have a decisive role in the regulation of these processes. This kind of synaptic plasticity is more significant in the cortex than in the thalamus.

Early establishment of lesion-insensitive mature barrelettes corresponding to upper lip vibrissae in developing mice

Vibrissae are tactile sense organs on the face of non-human mammals, and build up topographical representations in the brainstem trigeminal sensory nucleus called barrelettes. In the present study, we examined postnatal development of barrelettes corresponding to upper lip vibrissae by cytochrome oxidase (CO) histochemistry. At nuclear regions corresponding to upper lip vibrissae, a few segregated barrelettes first appeared at postnatal day 2 (P2), and segregation became clear for most upper lip barrelettes at P4. Compared with major barrelettes corresponding to mystacial vibrissae on the snout, the development of segregated pattern formation for upper lip barrelettes was retarded by 1-2 days. When vibrissa-related patterns were examined 5 days after infraorbital nerve transection, upper lip barrelettes became obscure in all mice lesioned at P1 and P2. Lesion-insensitive upper lip barrelettes first emerged in a few mice lesioned at P3 (33%), and the percentage attained 100% at P6. This temporal transition from lesion-sensitive to lesion-insensitive barrelettes was 3 days ahead of mystacial barrelettes. Therefore, upper lip barrelettes achieve rapid development within a narrow time frame during the first postnatal week. The early and rapid establishment of lesion-insensitive, mature barrelettes can be interpreted as suggesting the importance of oral sensory function in neonatal life.