Synaptic plasticity of 5-hydroxytryptamine-immunoreactive terminals in the phrenic nucleus following spinal cord injury: a quantitative electron microscopic analysis

The present study was conducted to examine the plasticity of 5-hydroxytryptamine (5-HT)-immunoreactive terminals in the rat phrenic nucleus following an ipsilateral C2 spinal cord hemisection and 30-day survival period. A retrograde horseradish peroxidase (HRP) labeling technique was used to identify the phrenic motoneurons at the electron microscopic (EM) level. After employing a pre-embedding immunocytochemical technique, the ultrastructural characteristics of 5-HT-immunoreactive terminals were qualitatively and then quantitatively analyzed with a computerized morphometric system before and after injury in separate groups of rats. The results indicated that the majority of the 5-HT-labeled terminals formed axodendritic contacts, but some 5-HT-labeled terminals made axosomatic contacts. 5-HT terminals were associated with either asymmetrical or symmetrical synapses, and some displayed postsynaptic dense bodies. Approximately 2% of the 5-HT terminals had dense-core vesicles. Although the total number of labeled and unlabeled terminals in the phrenic nucleus was reduced after hemisection, the number of 5-HT terminals in the hemisected group was greater than that of the control group. Moreover, the total number and length of asymmetrical and symmetrical synaptic active zones per 5-HT terminal were significantly greater after injury. Finally, the total number of 5-HT terminals with multiple synapses was significantly greater in the hemisected group as compared to controls. It is possible that 5-HT synaptic plasticity may be part of the morphological substrate for the unmasking of the latent crossed phrenic pathway which mediates recovery of the ipsilateral hemidiaphragm paralyzed by C2 spinal cord hemisection.

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.

Differential spine loss and regrowth of striatal neurons following multiple forms of deafferentation: a Golgi study

Golgi-Cox method and morphometric analyses were used to study the plasticity of striatal medium spiny I neurons in 6-month-old C57BL/6N mice after unilateral or bilateral lesion of the cerebral cortex or combined lesions of the ipsilateral cerebral cortex and intralaminar thalamus. In adult mouse, unilateral lesions of the cerebral cortex did not result in a net gain or loss of linear dendritic length in a randomly selected population of striatal medium spiny I neurons. In addition, there was a well-defined time course of striatal spine loss and replacement occurring after a unilateral cortical lesion. By day 3 postlesion the average 20-microm dendritic segment had lost 30% of the unlesioned control spine value, reached its nadir, lost 45.5%, at 10 days postlesion, and recovered to 80% of unlesioned control levels by 20 days postlesion. The recovery of spines was blocked by a secondary lesion on the contralateral cortex but not on the ipsilateral intralaminar thalamus. These data suggest that striatal medium spiny I neurons of adult mice have a remarkable capacity for plasticity and reactive synaptogenesis following a decortication. The recovery of spine density is primarily induced by axonal sprouting of survival homologous afferent fibers from the contralateral cortex.

Altered immunoreactivity for glial fibrillary acidic protein in astrocytes within 1 h after cervical spinal cord injury

One hour after a C2 spinal cord hemisection, there are changes in astrocyte morphology and increased glial fibrillary acidic protein immunoreactivity (GFAP IR) near the site of the lesion. Astrocytes adjacent to the lesion display thick, richly branched processes within 1/2 segment rostral and caudal to the level of hemisection. Astrocytes are also enlarged and immunoreactive in gray matter regions of the spinal cord as far caudal as C4 and rostrally to C1. Sham-operated controls, undergoing a laminectomy and durotomy at C2, but not spinal cord hemisection, also exhibit a strong astroglial reaction within 1 h from C1 to C4. However, controls undergoing only a C2 laminectomy do not demonstrate alterations in GFAP IR compared to non-operated controls. The results of this investigation suggest that spinal cord astrocytes are extremely sensitive to both major as well as minor alterations of their microenvironment. Rapid changes in astroglial morphology, as detected by altered GFAP IR, may play a role in changes in neuronal function following spinal cord injury.

Ultrastructural characteristics of glutamatergic and GABAergic terminals in cat lamina IX before and after spinal cord injury

The present study was designed to: 1) morphologically characterize cat glutamate and GABAergic synaptic terminals in lamina IX in the intact spinal cord at the electron microscopic level using postembedding immunochemical techniques and .2), begin an analysis of how the synaptic architecture of glutamate and GABAergic terminals changes after an ipsilateral spinal cord hemisection. The present study shows that glutamate immunoreactive terminals are characterized by a wide synaptic cleft, asymmetric synaptic membrane densities and spherical synaptic vesicles. Most of the glutamatergic terminals are presynaptic to small or medium size dendrites. In contrast, GABAergic terminals display typical pleomorphic synaptic vesicles, a narrow synaptic cleft and a symmetrical membrane density. Qualitative analysis indicated that 13-17 months after hemisection, the length of the synaptic active zones in both glutamatergic and GABAergic terminals ipsilateral to hemisection is longer than those observed in the terminals contralateral to hemisection orfin normal control cats. Furthermore, the perimeters of both dendrites and either glutamate or GABA immunoreactive terminals are longer on the hemisected side compared with those observed in the nonhemisected side of the spinal cord. The results are important for complete understanding of the mechanisms which underlie locomotor recovery in mammals following spinal cord injury.

The superimposed effects of chronic phrenicotomy and cervical spinal cord hemisection on synaptic cytoarchitecture in the rat phrenic nucleus

The present study was carried out to determine the effects of a combined peripheral phrenicotomy and rostral spinal cord hemisection on the synaptic architecture in the ipsilateral rat phrenic nucleus. Young adult female Sprague-Dawley rats were divided into a hemisection-alone and two hemisection-plus-phrenicotomy (HPP) groups. In all animals, DiI, a fluorescent carbocyanine dye was injected into the left hemidiaphragm to retrogradely label the ipsilateral phrenic motoneurons. In the HPP groups, left intrathoracic phrenicotomies were carried out at 2 and 4 weeks prior to sacrificing. Hemisection-alone animals were not subjected to phrenicotomy. In all animals, a left C2 spinal cord hemisection was performed 24 h prior to death. Quantitative morphometric analysis of the phrenic nucleus showed that the number of synapses contacting phrenic profiles is significantly less in the HPP (2 week) group as compared to the hemisection-alone group, but this number returns to a level not significantly different from the hemisection-alone value in the HPP (4 week) group. The results suggest that the transient change in the number of synapses might contribute to the differential expression of the crossed phrenic phenomenon documented in another group of animals subjected to the same surgical procedures. Furthermore, the different stages of glial reaction induced by phrenicotomy/spinal cord hemisection might underlie the change in synaptic number.

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.

Ultrastructural quantitative analysis of glutamatergic and GABAergic synaptic terminals in the phrenic nucleus after spinal cord injury

Quantitative analysis of electron microscopic postembedding immunochemically stained material indicates that 48% of all terminals in the rat phrenic nucleus are glutamatergic and 33% are gamma-aminobutyric acid (GABA)ergic. Three distinct types of glutamatergic terminals were observed in the rat phrenic nucleus: terminals characterized by large, loosely arranged spherical synaptic vesicles (SI) or small, compact spherical synaptic vesicles (Ss) and elongated terminals containing spherical synaptic vesicles with neurofilaments (NFs). All three types of glutamatergic terminals display asymmetrical synaptic membrane densities with postsynaptic dense bodies being present in some of the S-type terminals. The GABAergic immunoreactive terminals in the phrenic nucleus most closely resemble F-type terminals. They are characterized by flattened or pleomorphic synaptic vesicles and symmetric synaptic membrane densities. Among the 48% glutamatergic terminals, 27% are SI, 65% are Ss, and 8% are NFs, respectively. Significantly fewer glutamate, GABA, and unlabeled terminals per unit area are present in the phrenic nucleus 30 days after a C2 spinal cord hemisection as compared to nonhemisected controls. The average number of active zones per terminal, however, is greater in the hemisection group (1.45 +/- 0.03) than in the control group (1.34 +/- 0.03), with the active zones in the glutamate terminals mainly accounting for this difference. Moreover, the length of the active zones in the glutamate terminals was significantly longer in the hemisection group (0.37 +/- 0.013 microns) as compared to the controls (0.24 +/- 0.008 microns). In addition, the mean length of synaptic active zones in GABAergic terminals was also found to be longer in the hemisection group (0.36 +/- 0.022 microns) as compared to controls (0.28 +/- 0.014 microns). Finally, there is also a significantly higher ratio of synaptic active zones to the total number of glutamate-labeled terminals after injury (1.73 +/- 0.08) as compared to controls (1.41 +/- 0.04). The number of double/multiple synapses, the percentages of Sl, Ss, and NFs-type terminals, and the percentages of synaptic active zones contacting either distal dendrites or proximal dendrites/somata do not change significantly 30 days after injury. These results are important for a more complete understanding of the synaptic plasticity that occurs in the phrenic nucleus after spinal cord injury and to show how the plasticity may relate to the unmasking of latent bulbospinal respiratory connections which restore function to the hemidiaphragm paralyzed by an ipsilateral spinal cord hemisection.

Actions of systemic theophylline on hemidiaphragmatic recovery in rats following cervical spinal cord hemisection

This study assesses the effects of theophylline on enhancing phrenic nerve discharge and functional hemidiaphragmatic recovery after C2 spinal cord hemisection in adult female rats. There were three separate groups of spinal hemisected rats and one nonhemisected group studied. Twenty-four hours following C2 spinal hemisection, ipsilateral phrenic nerve activity was recorded under standardized, normoxic and then hypoxic conditions. After 30 min, theophylline was administered and the recordings were repeated in group 1 animals. In group 2, activity in both phrenic nerves was recorded simultaneously before and after drug administration. In a third group of rats, both ipsilateral phrenic nerve and hemidiaphragmatic activities were monitored before and after the drug. In control nonhemisected animals under standardized recording conditions, the effects of theophylline were quantitatively assessed by determining the mean area under integrated phrenic nerve discharge waveforms before and after drug administration. Generally, theophylline induced biphasic effects; i.e., at a low dose (15 mg/kg) it evoked excitation, while at a high dose (30 mg/kg) depression of respiratory activity predominated. In group 2 animals, respiratory activity was induced in the nerve ipsilateral to the hemisection and enhanced in the contralateral phrenic nerve for up to 3 h after a single standard dose of theophylline (15 mg/kg). Prior to drug administration, there was an absence of respiratory-related activity in both the phrenic nerve and hemidiaphragm ipsilateral to C2 spinal cord hemisection. A standard dose of theophylline, however, induced recovery of activity in both the phrenic nerve and the left hemidiaphragm ipsilateral to the hemisection in group 3 animals. In control (nonhemisected) animals, theophylline enhanced phrenic nerve activity, but decreased the duration of respiratory bursts. These results show for the first time that theophylline can activate latent respiratory motor pathways and thus restore the respiratory drive to phrenic motoneurons lost by spinal cord injury. Respiratory activity is not only reestablished in the phrenic nerve made quiescent by hemisection, but it is also enhanced in the contralateral phrenic nerve. The drug also restores function to the hemidiaphragm paralyzed by the spinal cord hemisection. The findings may have clinical relevance to human cases of cervical spinal cord injury in which respiratory function is compromised.

The superimposed effects of chronic phrenicotomy and cervical spinal cord hemisection on glial cytoarchitecture in the rat phrenic nucleus

This study was conducted to determine the effects of chronic phrenicotomy on spinal hemisection-induced morphological plasticity occurring in the phrenic nucleus. Young adult rats were divided into a hemisection-alone and two hemisection-plus-phrenicotomy (HPP) groups. HPP animals received a left phrenicotomy two or four weeks prior to sacrificing; whereas hemisection-alone animals did not. All animals received a left C2 spinal hemisection 24 hours prior to death. Quantitative morphometric analysis of the phrenic nucleus showed significant reductions in phrenic dendritic size and the number of dendrodendritic appositions in HPP (two week) animals and in the length of dendrodendritic appositions in HPP (four week) animals. Significant increases in microglial area fraction in HPP (two week) animals and in astroglia area fraction in HPP (four week) animals were also detected. The results suggest that the alterations in the spinal hemisection-induced dendrodendritic apposition formation is most likely influenced by the different stages of the glial reactions induced by the chronic phrenicotomy/spinal hemisection.

Quantitative assessment of the effect of chronic phrenicotomy on the induction of the crossed phrenic phenomenon

The present study was carried out to determine if chronic peripheral phrenicotomy has a functional influence on the plasticity that is normally demonstrated by phrenic motoneurons in the spinal cord following spinal cord injury. Young adult female Sprague-Dawley rats were divided into an experimental and a control group. Left intrathoracic phrenicotomies were carried out at 1, 2, 3, and 4 weeks prior to induction of the crossed phrenic phenomenon and crossed phrenic nerve activity recording in the experimental group. Control animals were not subjected to chronic phrenicotomy. In each animal the crossed phrenic phenomenon was induced by left C2 spinal cord hemisection and turning off the ventilator. The reflex-induced activity in the phrenic nerve ipsilateral to hemisection is defined as "crossed phrenic nerve activity." All animals were subjected to spinal cord hemisection 24 h before crossed phrenic nerve activity recording. The results showed that there is a transient but statistically significant depression of crossed phrenic nerve activity at 2 weeks postphrenicotomy and a recovery to the normal activity level at 4 weeks postphrenicotomy. One control experiment was carried out to assess the effects of phrenicotomy on respiratory activity that is normally present in the phrenic nerve (i.e., not reflex-induced). This "primary respiratory nerve activity" is different from crossed phrenic nerve activity in that the phrenic motoneurons are driven by different bulbospinal respiratory pathways. The results indicated a marked decrease in primary respiratory nerve activity at 1 week after phrenicotomy with no significant recovery by the 4th week after phrenicotomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Aging enhances synaptic efficacy in a latent motor pathway following spinal cord hemisection in adult rats

The effect of aging on the development of the neural circuitry mediating the crossed phrenic phenomenon (CPP) was investigated in anesthetized, vagotomized, artificially ventilated young adult (9-10 weeks old) and older adult (9-10 months old) rats. Cervical spinal cord hemisection rostral to the phrenic nucleus abolishes respiratory activity in the ipsilateral phrenic nerve. The respiratory activity can be restored by subjecting the animal to respiratory stress after spinal cord injury. The stress activates a normally latent respiratory motor pathway which mediates the functional recovery in the phrenic nerve ipsilateral to hemisection. This is the CPP and the recovered activity in the phrenic nerve is designated, "crossed phrenic activity." In the present study, blood pressure and end-tidal CO2 concentration were monitored. Crossed phrenic activity was induced by asphyxia (turning off the ventilator within 30 min following a left C2 spinal cord hemisection) and recorded from the left phrenic nerve. The mean area under the three largest successive integrated waveforms of the phrenic nerve bursts served as our quantitative assessment of the CPP. The results showed that the mean integrated area in the young rat group was 10.79 +/- 1.58 mm2. The mean integrated area in the older adult rats was significant greater (P < 0.05) at 41.58 +/- 10.05 mm2. Thus, there is almost a fourfold enhancement of crossed phrenic nerve activity that can be generated in older adult rats as compared to young adult rats. In addition, the CPP persists for significantly longer periods during progressive asphyxial hypoxia in the older adult rats than in the young adult rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural changes in the rat phrenic nucleus developing within 2 h after cervical spinal cord hemisection

The present study was conducted to describe the ultrastructural changes which occur in the young adult rat phrenic nucleus within 2 h after an ipsilateral C2 spinal cord hemisection. The main objective was to determine if there is a temporal relationship between specific ultrastructural changes in the phrenic nucleus and a significant augmentation of crossed phrenic nerve activity which occurs as early as 2 h after hemisection. Phrenic motoneurons were identified at electron microscopic levels by retrograde HRP labeling. Ultrastructural features in the phrenic nucleus of control and experimental rats were qualitatively analyzed and then quantitated. At 2 h posthemisection, there was a significant increase in the mean percentage of phrenic dendrodendritic appositions. In the control rats, 4.73 +/- 0.18% of phrenic dendrites were in apposition, and this percentage increased significantly to 8.58 +/- 0.54% at 2 h after injury. Furthermore, the mean lengths of asymmetrical and symmetrical synaptic active zones increased significantly at 2 h posthemisection from control lengths of 0.372 +/- 0.009 microns and 0.404 +/- 0.007 microns to 0.410 +/- 0.011 microns and 0.513 +/- 0.032 microns, respectively, in experimental rats. The phrenic nucleus is therefore capable of morphological plasticity as early as 2 h after spinal cord hemisection and this plasticity coincides temporally with the physiological augmentation of crossed phrenic nerve activity at 2 h. The data further suggest that these morphological changes may be part of the substrate for the unmasking of ineffective synapses during the crossed phrenic phenomenon.

Bulbospinal respiratory neurons are a source of double synapses onto phrenic motoneurons following cervical spinal cord hemisection in adult rats

The purpose of this study was to determine if the medullary neurons that provide the primary excitatory drive to phrenic motoneurons (i.e., rostral ventral respiratory group, rVRG) are a source of double synapse formation in the phrenic nucleus after spinal cord hemisection. The axons of rVRG neurons either ipsilateral or contralateral to the hemisection were labeled by injection of a mixture of HRP and WGA-HRP into the rostral ventral respiratory group. Phrenic motoneurons ipsilateral and caudal to the hemisection were labeled by the retrograde transport of HRP. The ultrastructural results indicated that after hemisection, rVRG neurons from both sides of the medulla formed labelled double synapses in the phrenic nucleus.

Identification of the axon pathways which mediate functional recovery of a paralyzed hemidiaphragm following spinal cord hemisection in the adult rat

Despite extensive neurophysiological work carried out to characterize the crossed phrenic phenomenon, relatively little is known about the morphological substrate of this reflex which restores function to a hemidiaphragm paralyzed by spinal cord injury. In the present study WGA-HRP was injected into normal and functionally recovered hemidiaphragm muscle in rats during the crossed phrenic phenomenon. The retrograde transynaptic transport characteristics of WGA-HRP was utilized to delineate the source of the neurons which mediate the crossed phrenic phenomenon. The results indicated that the neurons which drive phrenic motoneurons in spinal hemisected rats during the crossed phrenic phenomenon are located bilaterally in the rostral ventral respiratory group (rVRG) of the medulla. No transneuronal labeling of propriospinal neurons was noted in either normal or spinal-hemisected rats. Thus, propriospinal neurons do not relay respiratory drive to phrenic motoneurons. The neurons of the rVRG project monosynaptically to phrenic motoneurons. The present results suggest that both crossed and uncrossed bulbospinal pathways from the rVRG collateralize to both the left and right phrenic nucleic and functional recovery of a hemidiaphragm paralyzed by ipsilateral spinal cord hemisection is mediated by supraspinal neurons from both sides of the brain stem. These results are important to our complete understanding of the mechanisms which govern motor recovery in mammals following spinal cord injury.

Quantitative assessment of phrenic nerve functional recovery mediated by the crossed phrenic reflex at various time intervals after spinal cord injury

The present study was carried out to determine if augmentation of phrenic nerve activity during the crossed phrenic phenomenon temporally coincides with the morphological changes in the phrenic nucleus that we have observed in previous studies. This investigation consisted of two experiments in spinal cord hemisected young adult female Sprague-Dawley rats. Crossed phrenic activity was quantitatively assessed from the left phrenic nerve after bilateral vagotomy and sectioning of the right phrenic and accessory phrenic nerves. The first experiment involved serial recordings of crossed phrenic activity performed on each of 4 animals at hourly intervals ranging from 1 to 6 h after spinal cord hemisection. The second experiment consisted of single recordings from each of 24 animals at one of the following time intervals after hemisection: 1/2, 1, 2, 4, 12, and 24 h. Recording conditions were standardized at each recording session in both experiments by paralyzing the animals, regulating temperature and blood pressure, and controlling end tidal PCO2 with a volume ventilator. Crossed phrenic activity was induced by stopping the ventilator and quantitated by measuring the area under the integrated waveform of the largest respiratory burst. The results revealed a small, statistically insignificant increase in crossed phrenic activity at 1 h compared to the 30-min recordings. At 2 h there was a large, statistically significant increase in activity. Experiment one showed further increases from 3 to 6 h. The second experiment showed a smaller increase from 2 to 4 h and then maintained this level at 12 and 24 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Vestibular compensation after labyrinthectomy and vestibular neurectomy in cats

Labyrinthectomy and vestibular neurectomy are two ablative procedures used for definitive control of disabling vertigo. It is not known if vestibular compensation after labyrinthectomy and vestibular neurectomy differs. We have addressed this question by examining the pattern of recovery of the vestibular ocular reflex in cats after either labyrinthectomy or vestibular neurectomy. Temporal bone histologic examination confirmed the surgical lesion. Our results demonstrate a reduction of the long time constant of the vestibular ocular reflex in both groups of animals. Although gain of the vestibular ocular reflex recovered substantially, it never returned to control levels in either group. In general, animals that had undergone vestibular neurectomy demonstrated greater vestibular ocular reflex asymmetries than did labyrinthectomized animals. The recovery pattern of the vestibular ocular reflex indicates vestibular compensation is more rapid after labyrinthectomy than after vestibular neurectomy. We believe this result is related to survival of the vestibular nerve after labyrinthectomy, but not after vestibular neurectomy, suggesting that the vestibular nerve can contribute to the adaptive response after labyrinthectomy.

Decussation of bulbospinal respiratory axons at the level of the phrenic nuclei in adult rats: a possible substrate for the crossed phrenic phenomenon

The axonal trajectories of inspiratory bulbospinal neurons were examined after deposition of the anterograde neuronal tracer phaseolus vulgaris leucoagglutinin (PHA-L) into the rostral ventral respiratory group in rats. At the level of the phrenic nucleus, PHA-L-labeled bulbospinal axons crossed the midline of the spinal cord in both the anterior gray and the anterior white commissure. These spinally decussating neurons provide a possible anatomical substrate for the respiratory reflex known as the crossed phrenic phenomenon.

Ventral respiratory group projections to phrenic motoneurons: electron microscopic evidence for monosynaptic connections

The hypothesis that excitatory drive is transmitted monosynaptically from bulbospinal medullary respiratory neurons to spinal respiratory motoneurons was tested by an ultrastructural analysis of the phrenic motoneuronal pool in the rat. Combined anterograde labeling of the principal inspiratory bulbospinal neuron population (ventral respiratory group) and retrograde labeling of the phrenic motoneuron pool demonstrated the presence of labeled synaptic profiles, indicating that at least some bulbospinal inspiratory neurons make monosynaptic contacts with phrenic motoneurons. The synaptic boutons of ventral respiratory group neurons that were labeled in the phrenic nucleus had asymmetrical membrane densities at sites of synaptic contact with labeled phrenic somal or dendritic profiles, supporting the notion that this bulbospinal pathway has excitatory contacts with phrenic motoneurons. The morphological types of labeled boutons included three of the eight previously identified bouton types in the phrenic nucleus (Goshgarian and Rafols: Journal of Neurocytology 13:85-109, 1984), including the "S"-terminal, the "NFs"-terminal, and the "F"-terminal. There was no conclusive evidence of labeled double synapses, indicating that this type of synaptic contact is not common in the intact bulbospinal pathway.

Increased glial fibrillary acidic protein immunoreactivity in astrocytes within the lateral vestibular nucleus of the cat following labyrinthectomy and vestibular neurectomy

Unilateral vestibular injury evokes a characteristic pattern of acute disorganization of posture, locomotion, and eye movements. Following this acute stage, functional recovery occurs. In the present study, unilateral labyrinthectomy and vestibular neurectomy were performed in cats. The lateral vestibular nucleus (LVN) and vestibular nerve root entry zone on both sides of the brain stem were examined 24 hours, 3 days, and 8 weeks after operation by use of an immunochemical astrocyte marker, glial fibrillary acidic protein (GFAP). The results demonstrate extensive GFAP immunoreactivity within the ipsilateral nerve root following neurectomy, but not after labyrinthectomy. Prominent GFAP-immunoreactive astrocytic processes were detected in the LVN both ipsilateral and contralateral to neurectomy and labyrinthectomy. Within the ipsilateral LVN, the intensity of GFAP immunoreactivity was greater following neurectomy than after labyrinthectomy, but the pattern of GFAP reactivity remained similar. In the contralateral LVN, GFAP reactivity was noted exclusively in the dorsal-rostral region corresponding to the zone of cerebellar afferents to the LVN. The results of the present study suggest that reactive astroglia may play an important role in the mechanism that leads to vestibular compensation.

Chronic hypoxia causes morphological alterations in astroglia in the phrenic nucleus of young adult rats

The present study was carried out to determine if chronic hypoxia in noninjured rats causes morphological alterations in the phrenic nucleus similar to those caused by spinal cord hemisection rostral to the nucleus. Normal rats were subjected to chronic hypoxia in an atmosphere chamber for 48 h. A Bioquant system was used to quantitatively analyze electron micrographs through the phrenic nuclei of chronically hypoxic and normal rats. The results indicated that chronic hypoxia caused astroglial processes to retract from between adjacent dendrites, leading to a significant increase in phrenic dendrodendritic membrane apposition. Unlike spinal cord injury, however, chronic hypoxia did not induce synaptogenesis in the phrenic nucleus. Specifically, the mean length of phrenic dendrodendritic appositions increased significantly from a normal value of 1.42 +/- 0.09 to 1.82 +/- 0.11 microns in hypoxic animals. Moreover, the mean number of dendrodendritic appositions in the normal phrenic nucleus (16.75 +/- 1.11) increased markedly (24.00 +/- 3.54) in the phrenic nucleus of hypoxic rats. There was no significant difference between the mean number of double (17.75 +/- 1.03) or multiple (20.00 +/- 1.47) synapses in normal rats as compared to the mean number of double (18.50 +/- 3.10) or multiple (20.25 +/- 2.84) synapses in the phrenic nucleus of hypoxic animals. From the present results, it is concluded that the rapid synaptogenesis in the phrenic nucleus following spinal cord hemisection is induced by the generalized effects of spinal cord injury and astroglial process retraction does not necessarily lead to synapse formation. The physiological significance of the astroglial movement, however, needs elucidation by additional experiments.

Characterization of axonal injury produced by controlled cortical impact

Axonal injury and behavioral changes were evaluated 3-7 days after traumatic brain injury. Previous research from this laboratory demonstrated that clinical central nervous pathology is produced by dynamic brain compression using a stroke-constrained impactor. We wanted to determine if the technique also would produce diffuse axonal injury after recovery from the procedure. The experiments were performed at Wayne State University School of Medicine using aseptic techniques while assuring analgesic care. Impacts were performed at 4.3 m/sec or 8.0 m/sec, with congruent to 10% compression (2.5 mm). Extensive axonal injury was observed at 3 and 7 days postinjury using both velocity-compression combinations. Regions displaying axonal injury were the subcortical white matter, internal capsule, thalamic relay nuclei, midbrain, pons, and medulla. Axonal injury also was evident in the white matter of the cerebellar folia and the region of the deep cerebellar nuclei. Behavioral assessment showed functional coma lasting up to 36 h following 8.0 m/sec impacts, with impaired movement and control of the extremities over the duration of the postinjury monitoring time. These experiments confirm that the cortical impact model of traumatic brain injury mimics all aspects of traumatic brain injury in humans and can be used to investigate mechanisms of axonal damage and prolonged behavioral suppression.

Neuronal and glial changes in the rat phrenic nucleus occurring within hours after spinal cord injury

The present study describes specific morphological changes in the normal ultrastructure of the rat phrenic nucleus which occur within 4 hours after an ipsilateral spinal cord hemisection rostral to the nucleus. Phrenic neurons were identified at EM levels by retrograde HRP labeling. Ultrastructural features of the phrenic nucleus in uninjured animals and at 4 hours and 1, 2, and 4 days after injury were qualitatively analyzed and then quantitated with a computerized morphometric system. Our results indicated that by 4 hours posthemisection, there was a significant increase in the number of double synapses. Furthermore, the number of double synapses remained significantly higher than normal at all the other posthemisection periods. A significant increase in the length of dendrodendritic membrane appositions was also noted as early as 4 hours posthemisection. The mean normal appositional length of 1.42 +/- 0.09 microns increased to 1.89 +/- 0.12 microns at 4 hours and further increased to 2.20 +/- 0.20 microns by 1 day posthemisection. The increase in the length of membrane appositions was most likely due to an active retraction of astroglial processes from their normal position in between the dendrites. Although there was an increase in the mean length of the dendrodendritic appositions, the mean percentage of the appositions (expressed as the total number of appositions divided by the total number of dendrites in each sample) was not increased significantly over normal values during the early posthemisection periods. By 2 and 4 days posthemisection, however, the percentage of dendrodendritic appositions increased to significantly higher values than normal. Normally, 4.68 +/- 0.69% of the dendrites in the phrenic nucleus were found to be in apposition, and this number increased significantly to 7.27 +/- 1.06% by 2 days and 7.46 +/- 0.79% by 4 days posthemisection. At these later posthemisection periods, the mean length of the appositions decreased to levels which were no longer significantly higher than normal. A distribution analysis of the length of each dendrodendritic apposition in both the normal and spinal hemisected rats showed that there were more dendrodendritic appositions in the phrenic nucleus at the later posthemisection periods. It also showed that their mean length was decreased because many of the new appositions were relatively short. The above neuronal and glial alterations of the phrenic nucleus have never before been described as a response to injury of the mammalian spinal cord. Furthermore, the possibility that the above changes could represent the morphological substrate for the unmasking of functionally ineffective synapses in ou

Spinal cord localization and characterization of the neurons which give rise to the accessory phrenic nerve in the adult rat

This study describes the spinal cord location and morphology of the neurons which give rise to the accessory phrenic nerve in the rat. The results indicate that the cell bodies of the accessory phrenic nerve are a caudal extension of the phrenic nucleus. These cell bodies are located from cervical spinal cord levels C5 to upper C6 and comprise approximately 11% of the total phrenic motoneuron pool. The substantial phrenic contribution indicates the importance of the accessory phrenic nerve in both experimental and clinical manipulations of diaphragm innervation.