Mapping plasticity in the forepaw digit barrel subfield of rat brains using functional MRI

Abstract

The topographic organization of the forepaw barrel subfield in layer IV of rat primary somatosensory cortex (S1) is a good model for studying neural function and plasticity. The goal of this study was to test the feasibility of functional MRI (fMRI) to map the forepaw digit representations in the S1 of the rat and its plasticity after digit amputation. Three dimensional echo-planar imaging with 300 micron isotropic resolution at 11.7 T was used to achieve high signal-to-noise ratios and laminar layer resolution. By alternating electrical stimulation of the 2nd (D2) and 4th (D4) digits, functional activation in layer IV of the barrel subfields could be distinguished using a differential analysis. Furthermore, 2 and a half months after the amputation of the 3rd digit in baby rats, the overlapping area between D2 and D4 representations was increased. This indicates that the forepaw barrel subfield previously associated with the ablated digit is now associated with the representation of nearby digits, which is consistent with studies using electrophysiology and cytochrome oxidase staining.

Fig. 1

Regular (a) and differential (b) paradigms used in the correlation analysis. The activation of a digit was detected by comparing with the resting period (regular paradigm) or with the signal for the other digit (differential paradigm).

Fig. 2

Functional maps of D2 (red) and D4 (green) activations in two of the five rats (a, b) in the control group analyzed by regular (left 2 columns) and differential (right 2 columns) paradigms. The correlation threshold is 0.2. Two slice orientations were shown in each case: the left one is sagittal and the right one is horizontal. The slice locations were illustrated in the bottom row.

Fig. 3

Line profiles from the upper, middle, and deeper portions of the S1 were drawn on the correlation coefficient map of D2 activation in a rat. The peak correlation coefficient could be found in middle layers.

Fig. 4

Functional maps of D2 (red) and D4 (green) activations in two of the five rats (a, b) in the amputated group analyzed using the regular (left 2 columns) and differential (right 2 columns) paradigms. The correlation threshold is 0.2.

Fig. 5

The averaged correlation coefficient profiles of the D2 (solid line) and D4 (dotted line) activations from the control (a) and amputated (b) groups. The distance between the peaks of D2 and D4 activations in the digit-amputated rats was smaller than that in the normal rats. The stars indicate significant differences in correlation coefficients between the two digits. (*: p<0.05 and **: p<0.01)

Fig. 6

The averaged signal change profiles of the D2 (solid line) and D4 (dotted line) activations from the control group (a) with the amputated group (b). The stars indicate significant differences in the signal change with respect to other digit. (*: p<0.05 and **: p<0.01)