Activation of a latent respiratory motor pathway by stimulation of neurons in the medullary chemoreceptor area of the rat.

Previous studies have demonstrated that during respiratory stress (hypercapnia and hypoxia), a latent crossed respiratory pathway can be activated to produce hemidiaphragm recovery following an ipsilateral C2 spinal cord hemisection. The present study investigates the effects of ventral medullary chemoreceptor area stimulation by microinjection of (1S,3R)-aminocyclopentanedicarboxylic acid (ACPD), a glutamate metabotropic receptor agonist, on activating the latent pathway following left C2 spinal cord hemisection in rats in which end-tidal CO2 was maintained at a constant level. Experiments were conducted on anesthetized, vagotomized, paralyzed, and artificially ventilated rats in which phrenic nerve activity was recorded bilaterally. Before drug injection, the phrenic nerve contralateral to hemisection showed vigorous respiratory-related activity, but the phrenic nerve ipsilateral to hemisection showed no discernible respiratory-related activity. ACPD (1-100 nl, 1 mM) was injected directly into the region of the retrotrapezoid nucleus (RTN), a known medullary chemoreceptor area. Microinjection of ACPD into the right RTN increased respiratory-related activity in the right phrenic nerve (contralateral to hemisection). ACPD (>5 nl, 1 mM) microinjection also significantly induced respiratory recovery in the phrenic nerve ipsilateral to hemisection in a dose-dependent manner. The present study indicates that respiratory recovery can be achieved by stimulation of respiratory circuitry without increasing CO2 levels.

Chronic hypoxia does not induce synaptic plasticity in the phrenic nucleus.

Interruption of the main descending respiratory drive to phrenic motoneurons by cold block or spinal cord hemisection results in morphological modifications of the ipsilateral phrenic nucleus in the rat. The modifications consist of an increase in the number of multiple synapses and dendrodendritic appositions and elongation of the asymmetric and symmetric synaptic active zones. Hemisection and hemispinalization by cold block cause not only "functional deafferentation" of the ipsilateral phrenic neurons (i.e., a loss of ipsilateral descending respiratory drive), but also an increase in the remaining contralateral descending respiratory drive. The contralateral respiratory pathways connect with phrenic motoneurons ipsilateral to cold block or hemisection by decussating collateral axons which cross the spinal cord midline below the hemisection/cold block site. Thus, the phrenic nucleus synaptic plasticity could possibly be induced by functional deafferentation or by an increase of the descending respiratory drive. To differentiate between these two possible inducers of the plasticity, we assessed the synaptic morphology of the phrenic nucleus of nonoperated rats exposed to 48 h of hypoxia in an atmosphere chamber. The hypoxia exposure produces an increased descending respiratory drive without functional deafferentation. The quantitative data extracted from electron micrographs of the phrenic nucleus from four experimental rats were compared with the data from four normal breathing animals. Phrenic nucleus morphometric analysis showed that there was no significant difference in the mean number of single synapses between the samples from control animals (141 +/- 12.12) and the experimental animals (156 +/- 26.73). Similarly, no significant difference was detected in the total number of synaptic active zones of control animals (178.25 +/- 11.13) and experimental animals (195.05 +/- 5.35). Furthermore, the length of synaptic active zones of asymmetrical synapses (0.21 +/- 0.024 micron) or symmetrical synapses (0.22 +/- 0.022 micron) did not change significantly compared to the synaptic active zone length in control animals (0.21 +/- 0.018 micron for asymmetrical and 0.21 +/- 0.010 micron for symmetrical). We conclude that no synaptic plasticity occurs in the phrenic nucleus without functional deafferentation in spite of an increase in descending respiratory drive. Therefore functional deafferentation may be the primary inducer of phrenic nucleus synaptic plasticity occurring after hemisection or cold block.

Morphological plasticity induced in the phrenic nucleus following cervical cold block of descending respiratory drive.

Morphological plasticity occurs in the phrenic nucleus within hours following an ipsilateral C2 spinal cord hemisection. The plasticity has been associated with the unmasking of a latent respiratory pathway (the crossed phrenic pathway) which allows recovery of the hemidiaphragm paralyzed by the hemisection during a reflex known as the crossed phrenic phenomenon. This study tests if the plasticity is induced by the generalized effects of spinal cord trauma or the more specific effect of interrupting the main descending respiratory drive to phrenic motoneurons. Electron microscopic quantitative morphometric analysis of the phrenic nucleus neuropil was carried out on four Sprague-Dawley rats (200-250 g) sacrificed 4 h following unilateral reversible cold block of the descending bulbospinal respiratory drive at the second cervical segment of the spinal cord (C2). The data from four sham-operated control animals were compared with those of the experimental group. The following morphological alterations were documented in cold block animals compared to controls: (1) a significant increase in the number of multiple synapses (i.e., terminals with synaptic active zones contacting two or more postsynaptic profiles in the same plane of section), (2) a significant increase in the number of dendrodendritic appositions, and (3) a significant increase in the length of symmetric and asymmetric synaptic active zones. The above changes are similar to the changes induced in the phrenic nucleus following C2 hemisection. We conclude therefore, that injury to the spinal cord is not a requirement for this type of morphological plasticity in the phrenic nucleus, but rather the induced changes are activity-dependent and are likely caused by the interruption of the descending bulbospinal respiratory drive to the phrenic nucleus.

Pathophysiological classification of human spinal cord ischemia.

Diagnosis of myelopathies of vascular origin is difficult and they are probably underdiagnosed at this time because of the lack of diagnostic tools. A recent report of a 58 year old patient who developed ASAS after an episode of cardiac arrest pointed out the importance of MRI and somatosensory evoked potentials (SEP) to support the diagnosis. MRI with T2 weighted imaging demonstrated diffuse signal abnormalities in both gray matter and surrounding white matter below T7. Furthermore, SEP latencies showed a delay between T6 and T7. Therefore, new technologies including MRI and SEP may improve the diagnosis of spinal cord ischemic injuries. A brief discussion of the normal blood supply of the human spinal cord is presented in this review followed by new, pathophysiologically based classifications of the clinical syndromes of vascular myelopathies. A complete description of the clinical syndromes related to vascular myelopathies is included. Vascular myelopathies were divided into acute and chronic syndromes depending on the time at which the pathophysiological events take place. Subsequently, the two major groups of vascular myelopathies were divided depending on the type of vascular damage, e.g., arterial, venous and/or mixed origin. Posttraumatic spinal cord ischemia is included in the present classification because it is generally considered to be a significant factor contributing to secondary damage following blunt trauma. Since several new diagnostic techniques are now available to characterize the pathology of spinal cord injury, physicians involved in the diagnosis and treatment of vascular myelopathies may find the new classification useful in correlating clinical presentation with subjacent pathology. Identification of the correct pathology should result in more accurate treatment approaches.

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 describe a cooling device that allows reversible block of the inspiratory drive to phrenic motoneurons without injury to the spinal cord at the C2 level. Thus, this experimental approach can be used to differentiate between the plasticity induced by blockade of synaptic activity in the phrenic nucleus from the trauma-induced plasticity caused by a C2 spinal cord hemisection which would also interrupt descending inspiratory drive. Complete block of axon transmission of the respiratory pathways running unilaterally in the ventral as well as in the lateral funiculus was achieved by approximation of a cold probe to the ventral surface of the spinal cord. The spinal cord surface temperature was lowered to 7 degrees C. The temperature was maintained by a cold recirculated alcohol system. The efficacy of the reversible block was assessed by bilateral continuous EMG activity recording from the hemidiaphragms ipsilateral and contralateral to the cold application. Quantitative analysis of the EMG hemidiaphragmatic signals was performed in two sham-operated control (no cold application) and an experimental (cold application) 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 significant change occurred in EMG hemidiaphragmatic activity in control animals. The descending pathway from the rVRG to the phrenic nucleus was completely and continuously blocked for 4 h in all four experimental animals as demonstrated by abolition of the EMG hemidiaphragmatic signal ipsilateral to cold block. In all experimental animals hemidiaphragmatic activity returned when the cold block was removed. The recovered EMG activity was significantly higher than pre-block values. Interestingly, EMG activity contralateral to the block did not change significantly from control values after the block was removed, but was significantly enhanced during cold block. The present results suggest that cold block provides a means of studying activity-dependent plasticity in the respiratory pathways of the spinal cord.

Skull base cerebrospinal fluid leakage control with a fibrin-based composite tissue adhesive.

Cerebrospinal fluid (CSF) leaks can be responsible for significant patient morbidity and mortality. While the majority of leaks induced after head trauma will seal without intervention, spontaneous or surgically-induced leaks often require operative repair. Many modifications on standard surgical technique are available for repair of CSF fistulae, but none assures adequate closure. We have studied the efficacy of a novel fibrin-based composite tissue adhesive (CTA) for closure of experimentally-induced CSF leaks in rats. Fistulae were created in two groups of animals. Two weeks after creation of the leaks, the animals were sacrificed and analyzed for persistence of leak. A 58% leakage rate was noted in the control group (n = 12), and no leaks were noted in the experimental group closed after application of CTA to the surgical defect followed by skin closure (n = 11). Comparing the control group to the experimental group, results were statistically significant (p = 0.015). These data suggest that CTA may be effective as an adjunct for the closure of CSF fistulae.