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.     

Unilateral striatal dopamine depletion: time-dependent effects on cortical function and behavioural correlates

Previously, we showed that unilateral blockade of D1 dopamine receptors in the striatum inhibits immediate-early gene expression bilaterally throughout large parts of the cortex, including sensory-evoked expression in the barrel cortex. To further investigate this dopamine regulation of cortical function, we examined the effects of dopamine depletion on cortical gene regulation and behavioural correlates. Two days after unilateral infusion of 6-hydroxydopamine into the midbrain, rats displayed a (to some degree) bilateral reduction in cortical zif 268 expression that was more pronounced on the lesioned side. This decrease was found across motor, somatosensory, insular and piriform, but not cingulate, cortex, similar to the effects of blockade of striatal D1 receptors. Furthermore, whisker stimulation-evoked c-fos and zif 268 expression in the barrel cortex ipsilateral to the lesion was also attenuated by acute dopamine depletion. These cortical deficits were accompanied by a breakdown of spontaneous behaviours in an open-field test. In contrast, 21 days after dopamine depletion, both basal and sensory-evoked gene expression in the cortex were near-normal. This cortical recovery was paralleled by recovery in locomotion and in sensory-guided behaviour (scanning) related to the hemisphere contralateral to the lesion, but not in scanning by the dopamine-depleted hemisphere. Our results suggest that striatal dopamine exerts a widespread facilitatory influence on cortical function that is necessary, but not sufficient, for normal behaviour. Moreover, the mechanisms mediating this cortical facilitation appear to be subject to substantial neuroplasticity after dopamine perturbation.     

Metabolic alterations in rat somatosensory cortex following unilateral vibrissal removal

Local cerebral metabolic rates for glucose were studied by [14C]-2-deoxyglucose autoradiography in adult rats following acute and chronic unilateral deafferentation, with particular attention to the barrel field regions of the primary somatosensory cortex. Deafferentation was produced by permanently removing all of the large whiskers (vibrissae) on one side of the face. Data from experimental animals were then compared to data from sham-operated controls at 1, 5, 10, 15, 30, and 60 days after deafferentation. The rate of glucose utilization was maximally depressed at day 1 in the deafferented barrel field. After that, there was a progressive recovery of glucose utilization toward control levels at each subsequent time point. In contrast, glucose utilization in the barrel field associated with the intact set of whiskers increased by day 5 and remained elevated throughout the duration of the experiment. Similar patterns of altered cerebral metabolism were observed following unilateral infraorbital nerve transection. These results demonstrate that interference with normal somatosensory input causes a transient decrease in glucose metabolism of the contralateral cortical barrel-field and, in addition, causes long-term increments in glucose metabolism in the ipsilateral cortical barrel field--a structure not normally influenced by acute manipulation.     

PSD95 protein level rises in murine somatosensory cortex after sensory training

Proteins of the postsynaptic density are implicated in mechanisms of synaptic plasticity. We examined involvement of PSD95 and alphaCaMkII in learning-dependent plastic changes of representational maps in somatosensory cortex of mice. The barrel cortex of mice was examined following a 3 day long classical conditioning training, in which activation of facial vibrissae was linked to an aversive stimulus. This procedure produced expansion of cortical representations of vibrissae involved in the training. In subcellular fraction enriched in postsynaptic densities from the barrel cortex, it was estimated by Western blotting that the level of PSD95 increased after the training by about 50%, while the level of CaMkII remained unchanged. The results indicate involvement of PSD95 in learning-dependent cortical plasticity.     

Postnatal growth of intrinsic connections in mouse barrel cortex.

Surprisingly little is known about the development of connections within a functional area of the cerebral cortex. We examined the postnatal growth of connections in mouse barrel cortex during the second and third weeks after birth, coinciding with the period of rapid synaptogenesis that occurs just after the barrels first form. A barrel is a group of neurons in layer 4 of somatosensory cortex that is part of a cortical column. Each whisker/barrel column is linked anatomically and functionally to a homotopic whisker on the contralateral face. Radial groups of cortical neurons were labeled with the neuronal tracer biotinylated dextran amine in mice ranging in age from postnatal day 8 (P8; P0 is the date of birth) to adulthood. The spatial distributions of retrogradely labeled neurons in different laminae were analyzed. The barrel map in layer 4 was used as a template to compare quantitative data from different animals and to account for substantial changes in barrel and barrel field size during development. Intrinsic projections 1) innervate increasingly more distant targets within barrel cortex up to 3 weeks of age; 2) continue to form in targets after 3 weeks, effectively strengthening existing connections; 3) follow a timetable for growth that is layer-specific; 4) link more distant barrel columns in layer 4 from neurons that are found preferentially in the barrel side and the septa between barrels; and 5) form over the shortest distances between the barrel columns. These data indicate that intrinsic connections in mouse barrel cortex develop by the progressive addition of neuronal connections rather than by sculpting preliminary connections. We describe statistically significant changes in connectivity during development that may be applied to model and assess the development of connections after a variety of experimental perturbations, such as to the environment and/or the genome.     

Vibrissa-related behavior in mice: transient effect of ablation of the barrel cortex

Knowing that the mystacial vibrissae are an important part of the tactile sensory apparatus of rodents, we investigated the role of the barrel cortex - the endstation of the pathway between whiskerpad and cerebral cortex - in mouse behavior. We tested 15 female adult mice 2 and 10 weeks after both unilateral ablation of the barrel cortex and removal of the vibrissae on the same side in order to assess acute as well as transient effects of the cortical lesion. Two kinds of behavioral tests were performed on animals permanently provided with opaque lenses: one involved a passive stimulation of the vibrissae; the other was the 'gap-crossing' test which required the animal's active use of the vibrissae. Lesioned subjects did not show a deficit during passive stimulation of the vibrissae. On the contrary, there was a deficit during the gap-crossing test 2 weeks after the ablation of the barrel cortex. The deficit partly disappeared when the subjects were tested 10 weeks later. The results show that in mice, the barrel cortex is involved in the performance of complex behavioral tasks. The recovery of function could be due to changes in strategies to solve the gap-crossing test and/or to physical changes in neuronal circuitry. In either case, the results are relevant for the interpretation of cortical transplantation models using the whisker-to-barrel pathway.     

Activity-dependent regulation of glutamic acid decarboxylase in the rat barrel cortex: effects of neonatal versus adult sensory deprivation.

Immunocytochemical techniques were used to study the effects of tactual deprivation on glutamic acid decarboxylase (GAD) containing neurons in rat somatosensory barrel cortex. In normal rats GAD immunoreactive neurons and puncta are present in all laminae, with dense patches of GAD immunoreactive puncta centered on the barrels in lamina IV. Trimming whiskers of adult rats leads to a reversible decrease of GAD immunoreactivity in barrels corresponding to trimmed hairs. Intensity of GAD staining also is reversibly altered in supragranular laminae of nondeprived barrel columns flanked by deprived barrels. This indicates that GAD levels in the barrel cortex ordinarily fluctuate with changes in sensory input. By contrast, animals whose whiskers are trimmed from birth have normal GAD staining in both deprived and nondeprived barrels. Moreover, if trimmed whiskers of neonatally deprived animals are allowed to grow to normal lengths and are retrimmed later in adulthood GAD staining is not affected. Thus early tactual deprivation disrupts mechanisms that permit modulation of transmitter enzyme levels in cortical neurons following changes in sensory experience.     

Rapid cortical reorganisation and improved sensitivity of the hand following cutaneous anaesthesia of the forearm

The cortical representation of various body parts constantly changes based on the pattern of afferent nerve impulses. As peripheral nerve injury results in a cortical and subcortical reorganisation this has been suggested as one explanation for the poor clinical outcome seen after peripheral nerve repair in humans. Cutaneous anaesthesia of the forearm in healthy subjects and in patients with nerve injuries results in rapid improvement of hand sensitivity. The mechanism behind the improvement is probably based on a rapid cortical and subcortical reorganisation. The aim of this work was to study cortical changes following temporary cutaneous forearm anaesthesia. Ten healthy volunteers participated in the study. Twenty grams of a local anaesthetic cream (EMLA^®) was applied to the volar aspect of the right forearm. Functional magnetic resonance imaging was performed during sensory stimulation of all fingers of the right hand before and during cutaneous forearm anaesthesia. Sensitivity was also clinically assessed before and during forearm anaesthesia. A group analysis of functional magnetic resonance image data showed that, during anaesthesia, the hand area in the contralateral primary somatosensory cortex expanded cranially over the anaesthetised forearm area. Clinically right hand sensitivity in the volunteers improved during forearm anaesthesia. No significant changes were seen in the left hand. The clinically improved hand sensitivity following forearm anaesthesia is probably based on a rapid expansion of the hand area in the primary somatosensory cortex which presumably results in more nerve cells being made available for the hand in the primary somatosensory cortex.     

Increase of GAP-43 expression following kainic acid injection into whisker barrel cortex

Plasticity after microinjection of kainic acid (KA) into the adult rat whisker barrel cortex was investigated with immunohistochemical staining of phosphorylated growth-associated protein (GAP)-43. After mapping the barrel cortex with the technique of intrinsic signal optical imaging, a small volume of KA was injected into one barrel. Rats were sacrificed at 2 days, 3 days, 1 week, and 6 weeks after lesioning. GAP-43 staining demonstrated intense immunoreactivity (IR) at the injected barrel which spread to the inter-barrel septa and the surrounding barrels. Elevated IR of GAP-43 was visible 2 days after KA injection, and increased gradually at least 6 weeks following the lesion. This model has the possibility of offering a simple and reliable tool for studying cortical plasticity.     

Critical period-dependent alterations of the transient body image in the rodent cerebral cortex

The present study demonstrates that the boundary patterns of cell surface-associated molecules detected with lectins in the barrel cortex of neonatal rodents are altered, as are the boundary patterns of cortical glia, following perturbation of large vibrissae in the contralateral mystacial face pad. The alterations in the transiently expressed molecular patterns of lectin-receptors provide data that are consistent with the idea that the periphery plays a prominent role in the establishment of functional cytoarchitecture in the developing cortex. The data are also consistent, however, with the notion that factors intrinsic to the cerebrum, such as the immature cortical glial cells, are of considerable importance in this respect and a direct or indirect interaction of thalamocortical afferents with glial cells in the somatosensory cortex of the neonate are indicated. It is suggested therefore that a critical period in early barrel development, a time in which the cortical neuronal architecture is malleable in response to altered afferent input, is directly related to the presence of these cellular and molecular boundaries. The transient barrel boundaries, it is argued, are the morphological and molecular substrates that form the physical basis of the critical period.