REVERSIBLE CERVICAL HEMISPINALIZATION OF THE RAT SPINAL-CORD BY A COOLING DEVICE

Differentiation between traumatic and activity dependent plasticity in the CNS has been a challenge to neuroscientists in the past. We descr ibe a cooling device that allows reversible block of the inspiratory d rive to phrenic motoneurons without injury to the spinal cord at the C 2 level. Thus, this experimental approach can be used to differentiate between the plasticity induced by blockade of synaptic activity in th e phrenic nucleus from the trauma-induced plasticity caused by a C2 sp inal cord hemisection which would also interrupt descending inspirator y drive. Complete block of axon transmission of the respiratory pathwa ys running unilaterally in the ventral as well as in the lateral funic ulus was achieved by approximation of a cold probe to the ventral surf ace of the spinal cord. The spinal cord surface temperature was lowere d to 7 degrees C. The temperature was maintained by a cold recirculate d alcohol system. The efficacy of the reversible block was assessed by bilateral continuous EMG activity recording from the hemidiaphragms i psilateral and contralateral to the cold application. Quantitative ana lysis of the EMG; hemidiaphragmatic signals was performed in two sham- operated control (no cold application) and an experimental (cold appli cation) group of Sprague-Dawley rats. The control groups were employed to confirm that the surgical exposure of the cord and/or the chronic placement of the probe and the administration of IV dopamine given to maintain stable blood pressure did not affect respiration. No signific ant change occurred in EMG hemidiaphragmatic activity in control anima ls. The descending pathway from the rVRG; to the phrenic nucleus was c ompletely and continuously blocked for 4 h in all four experimental an imals as demonstrated by abolition of the EMG: hemidiaphragmatic signa l ipsilateral to cold block. In all experimental animals hemidiaphragm atic activity returned when the cold block was removed, The recovered EMG activity was significantly higher than pre-block values. Interesti ngly EMG activity contralateral to the block did not change significan tly from control values after the block was removed, but was significa ntly enhanced during cold block. The present results suggest that cold block provides a means of studying activity-dependent plasticity in t he respiratory pathways of the spinal cord. (C) 1996 Academic Press, I nc.