Slow-wave activity homeostasis in the somatosensory cortex after spinal cord injury.

The cortical reorganization after spinal cord injury (SCI) involves a series of physiological changes that drive the expansion of the intact cortical area to the deafferented cortical area. These changes have always been studied under a stimulus-response paradigm, which demonstrates that the deafferented cortex becomes more responsive to stimulation of body regions above the level of the lesion. However, less is known about how permanent large-scale deafferentation affects spontaneous activity in the somatosensory cortex, an important physiological feature related to the processing of peripheral inputs and perception. Here we studied the spontaneous activity at two sites of the somatosensory cortex, corresponding to forepaw and hindpaw, and at three different time points after SCI: acute SCI, one week post-SCI and chronic SCI (1-3 months after injury). Electrophysiological recordings from anesthetized rats were obtained in conditions of slow-wave activity in order to compare features of the neural populations in periods of cortical up-states. Our data demonstrate that acute SCI reduces the excitability of cortical neurons during up-states in both the forepaw and the hindpaw cortex. One week after SCI, the properties of cortical neurons were similar to those under control conditions, indicating a homeostatic plasticity. Finally, chronic SCI increased neural activity during up-states, while reduced up-state frequency in the cortex. We conclude that SCI induces different homeostatic changes in cortical slow-wave depending on the time after lesion. This temporal evolution of spontaneous activity could help better understand the cortical plasticity associated with acute or chronic SCI.

Increased responses in the somatosensory thalamus immediately after spinal cord injury.

Spinal cord injury (SCI) involves large-scale deafferentation of supraspinal structures in the somatosensory system, producing well-known long-term effects at the thalamo-cortical level. We recently showed that SCI provokes immediate changes in cortical spontaneous and evoked responses and here, we have performed a similar study to define the immediate changes produced in the thalamic ventro-postero-lateral nucleus (VPL) that are associated with the forepaw and hindpaw circuits. Extracellular electrophysiological recordings from the VPL reflected the spontaneous activity and the responses to peripheral electrical stimulation applied to the paws. Accordingly, the activity of the neuronal populations recorded at specific thalamic locations that correspond to the forepaw and hindpaw circuits was recorded under control conditions and immediately after thoracic SCI. The results demonstrate that peripheral inputs from both extremities overlap on neuronal populations in the somatosensory thalamus. In addition, they show that the responses of thalamic neurons to forepaw and hindpaw stimuli are increased immediately after SCI, in association with a specific decrease in spontaneous activity in the hindpaw locations. Finally, the increased thalamic responses after SCI have a state-dependent component in relation with cortical activity. Together, our results indicate that the thalamic changes occurring immediately after SCI could contribute to the cortical changes also detected immediately after such spinal lesions.