Role of IGF-1 in cortical plasticity and functional deficit induced by sensorimotor restriction.

In the adult rat, sensorimotor restriction by hindlimb unloading (HU) is known to induce impairments in motor behavior as well as a disorganization of somatosensory cortex (shrinkage of the cortical representation of the hindpaw, enlargement of the cutaneous receptive fields, decreased cutaneous sensibility threshold). Recently, our team has demonstrated that IGF-1 level was decreased in the somatosensory cortex of rats submitted to a 14-day period of HU. To determine whether IGF-1 is involved in these plastic mechanisms, a chronic cortical infusion of this substance was performed by means of osmotic minipump. When administered in control rats, IGF-1 affects the size of receptive fields and the cutaneous threshold, but has no effect on the somatotopic map. In addition, when injected during the whole HU period, IGF-1 is interestingly implied in cortical changes due to hypoactivity: the shrinkage of somatotopic representation of hindlimb is prevented, whereas the enlargement of receptive fields is reduced. IGF-1 has no effect on the increase in neuronal response to peripheral stimulation. We also explored the functional consequences of IGF-1 level restoration on tactile sensory discrimination. In HU rats, the percentage of paw withdrawal after a light tactile stimulation was decreased, whereas it was similar to control level in HU-IGF-1 rats. Taken together, the data clearly indicate that IGF-1 plays a key-role in cortical plastic mechanisms and in behavioral alterations induced by a decrease in sensorimotor activity.