Partial denervation of the whiskerpad in adult mice: altered patterns of metabolic activity in barrel cortex

Deprivation-induced dendritic shrinkage might be oppositely affected by the expression of wild-type and mutated human amyloid precursor protein

The physiological role of the amyloid precursor protein (APP) and its proteolytic fragments in the brain is associated with neuronal survival, neurite outgrowth, synaptic formation, and neuronal plasticity. However, malregulation of APP processing leads to disordered balance of fragments, which may results in opposite, degenerative neuronal effects. In the present study, we analyzed in vivo effects of the expression of wild-type or mutated human APP on afferent deprivation-induced changes of dendritic morphology. After vibrissectomy, expression of wild-type human APP prevented diameter shrinkage of dendritic segments as well as dendritic rarefaction of apical arbors. In contrast, mutant human APP expression exacerbated degenerative changes of deprived barrel neurons. Degradation of apical arbors was especially pronounced. Results demonstrate for the first time opposite effects of the expression of wild-type and mutated human APP on deprivation-induced dendritic restructuring in vivo.

Metabolic barrel representations with various patterns of neonatal whisker deafferentation in rats

With various patterns of whisker deafferentation, C3 whisker stimulation produced divergently shaped metabolic barrel representations in layer IV of the primary somatosensory cortex. Whisker deafferentation results in functional and structural reorganization of the barrels in the primary somatosensory cortex. The present study examines the alteration of the metabolic barrel representations in layer IV with various configurations of selective whisker deafferentation in neonates, using [14C]2-deoxyglucose autoradiography. The deafferentation was produced by unilateral ablation of whiskers, leaving certain follicles intact. Configurations of intact follicles included: (I) row C follicles; (II) B3, C3, and D3 follicles; (III) B3, B4, C3, and C4 follicles; (IV) C2, C3, D2, and D3 follicles. The metabolic C3 barrel representations in layer IV after the deafferentations were found to have expanded only toward the barrel sites in which the corresponding whiskers were ablated, with no expansion toward the neighboring barrels. Expansion toward row D was significantly more pronounced than expansion toward row B, and expansion toward the C2 barrel was significantly more pronounced than expansion toward the C4 barrel. From these results, it can be inferred that asymmetric intrinsic structural connections are reflected in the functional metabolic barrel representation under the condition of neural plasticity in the barrel cortex following whisker deafferentation.

Cortical origin of functional recovery in the somatosensory cortex of the adult mouse after thalamic lesion

To study the degree and time course of the functional recovery in the somatosensory cortex (SI) after an excitotoxic lesion in the adult mouse thalamus, metabolic activity was determined in SI at various times points post-lesion. Immediately after the lesion, metabolic activity in the thalamically deafferented part of SI was at its lowest value but increased progressively at subsequent time points. This was seen in all cortical layers; however, layers I and Vb recovered more rapidly than layers II, III, IV, Va and VI. Removal of the mystacial whiskers corresponding to the deafferented area, 5 weeks after cortical recovery, produced a subsequent 32% drop in metabolic activity, demonstrating peripheral sensory activation of this part of the cortex. Tracing experiments revealed that the deafferented cortex did not receive a novel thalamic input but that cortico-cortical and contralateral barrel cortex projections to this area were reinforced. We conclude that the cortical functional recovery after a thalamic lesion is, at least partially, due to modified cortico-cortical and callosal projections to the deafferented cortical area.

Reduced plasticity of cortical whisker representation in adult tenascin-C-deficient mice after vibrissectomy

The effect of the extracellular matrix recognition molecule tenascin-C on cerebral plasticity induced by vibrissectomy was investigated with 2-deoxyglucose (2DG) brain mapping in tenascin-C-deficient mice. Unilateral vibrissectomy sparing row C of vibrissae was performed in young adult mice. Two months later, cortical representations of spared row C vibrissae and control row C on the other side of the snout were visualized by [(14)C]2DG autoradiography. In both wild-type and tenascin-C-deficient mice, cortical representation of the spared row was expanded in all layers of the barrel cortex. However, the effect was significantly more extensive in wild-type animals than in the mutant. Elimination of tenascin-C by genetic manipulation thus reduces the effect of vibrissectomy observed in the somatosensory cortex. No increase in number of fibres in the vibrissal nerve of spared vibrissae was seen, and occurrence of additional nerve to the spared follicle was very rare. Thus, in tenascin-C-deficient mice functional plasticity seems to be impaired within the CNS.

Altered synapse formation in the adult somatosensory cortex of brain-derived neurotrophic factor heterozygote mice

Increased sensory stimulation in the adult whisker-to-barrel pathway induces the expression of BDNF as well as synapse formation in cortical layer IV. Here, we investigated whether BDNF plays a role in the alterations of connectivity between neurons by analyzing the ultrastructure of the BDNF heterozygote mouse, characterized by a reduced level of BDNF expression. Using serial section electron microscopy, we measured synapse density, spine morphology, and synaptic vesicle distribution to show that mice with a reduced level of BDNF have a barrel neuropil that is indistinguishable from wild-type controls. After 24 hr of whisker stimulation, however, there is no indication of synapse formation in the heterozygous mouse. Whereas the balance between excitatory and inhibitory synapses is modified in the controls, it remains constant in the heterozygotes. The distribution of synaptic vesicles in excitatory synapses is the same in heterozygous and wild-type mice and is not influenced by the stimulation paradigm. Spine volume, however, is unchanged by stimulation in the wild-type animals, but does increase significantly in the heterozygous animal. These results provide evidence that, in vivo, BDNF plays an important role in the structural rearrangement of adult cortical circuitry as a consequence of an increased sensory input.

Carpal tunnel syndrome modifies sensory hand cortical somatotopy: a MEG study

The adult somatosensory system has shown reorganizational abilities at cortical and subcortical levels after peripheral nerve lesions. In the present study the effects of carpal tunnel syndrome (CTS) are investigated as reflected on the somatotopy of the primary cortical hand representation. Position and intensity of cortical sources activated by the separate electrical stimulation of median nerve and Digits 1, 3, and 5 of both affected and non-affected hands are evaluated by magnetoencephalographic (MEG) technique. Correlation of MEG results with patient-, physician- and neurophysiological-oriented evaluations of CTS was carried out. Patients showed changes in cortical hand somatotopy in strict relationship to self-referred assessment of symptoms and hand disability in daily activities, including: 1) a more extended representation of the affected hand when paresthesias prevailed; and 2) a more restricted representation due to lateral shift of the little finger was observed when pain symptoms dominated the clinical picture. Contralateral to the side of CTS, the cortical sources activated by Digit 5-stimulation appeared significantly enhanced with respect to contralateral ones from non-affected hand. When comparing the amplitude of peripheral sensory nerve action potentials of median and ulnar nerves to that of cortical responses (i.e., ECD strengths of M20 and M30 components after stimulation of Digits 3 and 5), a significant selective amplification of M30 with respect to M20 and sensory nerve action potential (SNAP) appeared during Digit 3 stimulation compared to that observed for Digit 5. This has been interpreted as a central magnification mechanism in brain responsiveness, possibly revealing a safety factor enabling sensory perception despite the small peripheral signal due to nerve trunk dysfunction. Hum.

Partial denervation of the whiskerpad in adult mice: pattern and origin of reinnervation

We studied sensory organ reinnervation after nerve transection in the mouse whisker-to-barrel pathway. In one set of adult mice, we determined at light microscopy level the number of fibres reaching the caudal whisker follicles 5, 15, 20, 60, 100 days and 1 year after transection of the sensory nerve of row C. Regenerated fibres were first detected 15 days post lesionem (p.l.) and myelin first observed at 20 days. Between 60 and 100 days, the number of fibres stayed at approximately 80% of the values obtained in control animals. At that time, myelinated fibres reached only 58% of their number in controls. At the electron microscopy level, these fibres differ from control ones by a smaller fibre diameter. The innervation of follicles of adjacent rows was not modified, indicating that follicular reinnervation is row specific. We checked this feature by injecting in another set of mice the denervated follicles and the adjacent ones with distinct retrograde tracers 45 days and 1 year after nerve transection. The percentage of double-labelled neurons in the Gasserian ganglion did not increase in experimental animals. This confirms the absence of colonization of intact follicles by regenerating fibres and indicates that reinnervation of the whisker follicles takes place by regeneration of the degenerated axons without collateral reinnervation. The companion paper describes the pattern of activation of the barrel cortex relative to the present findings.