Electrical stimulation of a forepaw digit increases the physiological representation of that digit in layer IV of SI cortex in rat

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

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.

Intracortical pathway involving dysgranular cortex conveys hindlimb inputs to S-I forelimb-stump representation of neonatally amputated rats

Reorganization of the primary somatosensory cortex (S-I) forelimb-stump representation of rats that sustained neonatal forelimb removal is characterized by the expression of hindlimb inputs that are revealed when cortical GABA receptors are pharmacologically blocked. Recent work has shown that the majority of these inputs are transmitted from the S-I hindlimb representation to the forelimb-stump field via an, as yet, unidentified pathway between these regions. In this study, we tested the possibility that hindlimb inputs to the S-I forelimb-stump representation of neonatally amputated rats are conveyed through an intracortical pathway between the S-I hindlimb and forelimb-stump representations that involves the intervening dysgranular cortex by transiently inactivating this area and evaluating the effect on hindlimb expression in the S-I forelimb-stump representation during GABA receptor blockade. Of 332 S-I forelimb-stump recording sites from six neonatally amputated rats, 68.3% expressed hindlimb inputs during GABA receptor blockade. Inactivation of dysgranular cortex with cobalt chloride (CoCl(2)) resulted in a significant decrease in the number of hindlimb responsive sites (9.5%, P < 0.001 vs. cortex during GABA receptor blockade before CoCl(2) treatment). Results were also compiled from S-I forelimb recording sites from three normal rats: 14.1% of 136 sites were responsive to the hindlimb during GABA receptor blockade, and all of these responses were abolished during inactivation of dysgranular cortex with CoCl(2) (P < 0.05). These results indicate that the S-I hindlimb representation transmits inputs to the forelimb-stump field of neonatally amputated rats through a polysynaptic intracortical pathway involving dysgranular cortex. Furthermore the findings from normal rats suggest that this pathway might reflect the amplification of a neuronal circuit normally present between the two representations.

Source of inappropriate receptive fields in cortical somatotopic maps from rats that sustained neonatal forelimb removal

Previously this laboratory demonstrated that forelimb removal at birth in rats results in the invasion of the cuneate nucleus by sciatic nerve axons and the development of cuneothalamic cells with receptive fields that include both the forelimb-stump and the hindlimb. However, unit-cluster recordings from primary somatosensory cortex (SI) of these animals revealed few sites in the forelimb-stump representation where responses to hindlimb stimulation also could be recorded. Recently we reported that hindlimb inputs to the SI forelimb-stump representation are suppressed functionally in neonatally amputated rats and that GABAergic inhibition is involved in this process. The present study was undertaken to assess the role that intracortical projections from the SI hindlimb representation may play in the functional reorganization of the SI forelimb-stump field in these animals. The SI forelimb-stump representation was mapped during gamma-aminobutyric acid (GABA)-receptor blockade, both before and after electrolytic destruction of the SI hindlimb representation. Analysis of eight amputated rats showed that 75.8% of 264 stump recording sites possessed hindlimb receptive fields before destruction of the SI hindlimb. After the lesions, significantly fewer sites (13.2% of 197) were responsive to hindlimb stimulation (P < 0.0001). Electrolytic destruction of the SI lower-jaw representation in four additional control rats with neonatal forelimb amputation did not significantly reduce the percentage of hindlimb-responsive sites in the SI stump field during GABA-receptor blockade (P = 0.98). Similar results were obtained from three manipulated rats in which the SI hindlimb representation was silenced temporarily with a local cobalt chloride injection. Analysis of response latencies to sciatic nerve stimulation in the hindlimb and forelimb-stump representations suggested that the intracortical pathway(s) mediating the hindlimb responses in the forelimb-stump field may be polysynaptic. The mean latency to sciatic nerve stimulation at responsive sites in the GABA-receptor blocked SI stump representation of neonatally amputated rats was significantly longer than that for recording sites in the hindlimb representation [26.3 +/- 8.1 (SD) ms vs. 10.8 +/- 2.4 ms, respectively, P < 0.0001]. These results suggest that hindlimb input to the SI forelimb-stump representation detected in GABA-blocked cortices of neonatally forelimb amputated rats originates primarily from the SI hindlimb representation.