Excitation and inhibition jointly regulate cortical reorganization in adult rats

Abstract

The primary somatosensory cortex (SI) retains its capability for cortical reorganization after injury or differential use into adulthood. The plastic response of SI cells to peripheral stimulation is characterized by extension of cortical representations accompanied by changes of the receptive field size of neurons. We used intracortical microstimulation that is known to enforce local, intracortical synchronous activity, to induce cortical reorganization and applied immunohistochemical methods in the same individual animals to investigate how plasticity in the cortical topographic maps is linked to changes in the spatial layout of the inhibitory and excitatory neurotransmitter systems. The results reveal a differential spatiotemporal pattern of upregulation and downregulation of specific factors for an excitatory (glutamatergic) and an inhibitory (GABAergic) system, associated with changes of receptive field size and reorganization of the somatotopic map in the rat SI. Predominantly local mechanisms are the specific reduction of the calcium-binding protein parvalbumin in inhibitory neurons and the low expression of the activity marker c-Fos. Reorganization in the hindpaw representation and in the adjacent SI cortical areas (motor cortex and parietal cortex) is accompanied by a major increase of the excitatory transmitter glutamate and c-Fos. The spatial extent of the reorganization appears to be limited by an increase of glutamic acid decarboxylase and the inhibitory transmitter GABA. The local and medium-range net effects are excitatory and can facilitate receptive field enlargements and cortical map expansion. The longer-range increase of inhibition appears suited to limit these effects and to prevent neurons from pathological hyperexcitability.

Figure 1.

Experimental procedure. a, The ratunculus with the hindpaw representation and the recording sites (dots) with a representative receptive field. An enlargement of the rectangle in illustration a is represented in b. It shows the place of the hindpaw representation in the map, which was electrically stimulated. c, For the investigation of the immunolabeled cells, at least nine coronal sections in a rostrocaudal extent of 2 mm with intervals of 250 μm between the sections were analyzed for each marker. Each coronal section was divided into sampling areas (five on each hemisphere) with a dorsoventral extent from the pia mater to the white matter and with a mediolateral extent of 500 μm in layer IV. HP, Hindpaw; UZ, unresponsive zone; FP, forepaw; Barrel, barrel cortex; T, tail.

Figure 2.

Somatosensory mapping of the hindpaw representation before (a) and after (b) ICMS. L, Lateral; C, caudal. Dots indicate recording sites with the appending receptive field (indicated by the arrow). Bars show recording sites, which are unresponsive to tactile stimulation. Asterisks mark newly accrued receptive fields after ICMS (b). Receptive field size from both hemispheres of control (sham) and stimulated animals (c). Time course of receptive field changes before (pre ICMS) and after 2 h of electrical stimulation (post ICMS) over an interval of 10 h after stimulation compared with preconditions (d). *p < 0.05.

Figure 3.

Histological changes after 2 h of ICMS compared with the contralateral side. a, b, Changes at the stimulation site in c-Fos and in PV. A dark arrow indicates the electrode track. c, Increase of the excitatory system (glutamate) after 2 h of ICMS 1 mm lateral the ICMS electrode. The small right top image shows a magnification of stained pyramidal cell. d, e, Enhancement of inhibitory system after 2 h of ICMS in GABA and GAD 500 μm rostral the ICMS electrode. In the small windows of the GAD images at a higher magnification, a nonlabeled pyramidal cell is shown, which is surrounded by labeled GAD puncta. (For magnification, see scale bar in a).

Figure 4.

Summary of the changes after 2 h of ICMS are shown in first two columns. a, b, d, e, g, h, j, k, Color-coded representations of the histological changes after ICMS. The statistical differences are marked by * (significance level of 1%) or × (significance level of 5%). Arrows indicate the location of the electrodes during ICMS or without ICMS. Note that, in controls (sham), c-Fos-IR cells are below threshold for color coding (Tables 2–4). The third column (c, f, i, l) summarizes the changes of the different expression pattern and classifies the different zones. L, Lateral; M, medial.

Figure 5.

Optical density of glutamate (a), GABA (b), and GAD (c) at different time points after ICMS from the different zones (Fig. 4c,f,l). For comparison, see time course of changes in RF size (Fig. 2d). Because the OD of the immunoreactivity of animals differ, the data were standardized, to compare among animals. The OD of each individual measure was divided by the average value of all stained sections from the same animal and same marker. IZ, Inhibition zone.

Figure 6.

Cortical topography surrounding the ratunculus in primary somatosensory cortex (a). VC, Visual cortex; fl, forelimb; hl, hindlimb; SI, primary somatosensory cortex; MC, motor cortex; cg, cingulate cortex; OB, bulbus olfactorius; RAG, retrosplenial cortex; TE, temporal cortex; CR, cerebral cortex; SII, secondary somatosensory cortex; Ins, insular cortex; PIR, piriform cortex. Classification of the zones attributable to the specific changes of the transmitters and proteins immediately after two hours of ICMS and overlay these to the cortical representation (b). FP, Forepaw; HP, hindpaw; IZ, inhibition zone; UZ, unresponsive zone.