Extracellular potassium activity and axonal conduction in spinal cord of the myelin-deficient mutant rat

The role of intraoperative neurophysiological monitoring in intramedullary spinal cord tumor surgery

Intramedullary tumors are a class of central nervous system tumors with an incidence of 2 to 4%. As they are located very deep and frequently cause postoperative neurological complications, surgical resection is difficult. In recent years, many surgeons have performed electrophysiological monitoring to effectively reduce the occurrence of postoperative neurological complications. Modern electrophysiological monitoring technology has advanced considerably, leading to the development of many monitoring methods, such as SSEPs, MEPs, DCM, and EMG, to monitor intramedullary tumors. However, electrophysiological monitoring in tumor resection is still being studied. In this article, we discussed the different monitoring methods and their role in monitoring intramedullary tumors by reviewing previous studies. Intratumorally tumors need to be monitored for a summary of the condition of the patient. Only by using various monitoring methods flexibly and through clear communication between surgeons and neurophysiological experts can good decisions be made during surgery and positive surgical results be achieved.

Intraoperative Neurophysiological Monitoring for Craniovertebral Junction Surgery

Craniovertebral junction (CVJ) surgery encompasses a wide spectrum of neurosurgical procedures ranging from transoral approaches for CVJ bone anomalies to surgery for intramedullary tumours. Intraoperative neurophysiological monitoring (IONM) has been increasingly used in recent years because of its ability to prevent neurological complications during surgery. In CVJ surgery the risk of neurological injuries is related first to the positioning of the patient and then to the surgical procedure. Application of IONM during the positioning of the patient permits fast recognition of impending causes of neurological injury. During surgery, continuous IONM permits real-time assessment of the functional integrity of the spinal tracts and provides useful feedback during surgical manoeuvres. The applications of IONM are mainly related to intradural procedures, but wider application of these techniques during surgery for CVJ instability and degenerative disorders has recently been described, leading also to better understanding of the pathophysiology of spinal cord injuries. In this paper we review and discuss the principal IONM techniques used during surgery around the CVJ.

Diagnostic Utility of Intraoperative Neurophysiological Monitoring for Intramedullary Spinal Cord Tumors: Systematic Review and Meta-Analysis

STUDY DESIGN

Systematic review and meta-analysis.

OBJECTIVE

The aim of this study was to systematically evaluate the diagnostic utility of intraoperative neurophysiological monitoring (IONM) for detecting postoperative injury in resection of intramedullary spinal cord tumors (IMSCT).

SUMMARY OF BACKGROUND DATA

Surgical management of IMSCT can involve key neurological and vascular structures. IONM aims to assess the functional integrity of susceptible elements in real time. The diagnostic value of IONM for ISMCT has not been systematically evaluated.

METHODS

We performed a systematic review of the PubMed and MEDLINE databases for studies investigating the use of IONM for IMSCT and conducted a meta-analysis of diagnostic capability.

RESULTS

Our search produced 257 citations. After application of exclusion criteria, 21 studies remained, 10 American Academy of Neurology grade III and 11 American Academy of Neurology grade IV. We found that a strong pooled mean sensitivity of 90% [95% confidence interval (CI), 84-94] and a weaker pooled mean specificity of 82% (95% CI, 70-90) for motor-evoked potential (MEP) recording changes. Somatosensory-evoked potential (SSEP) recording changes yielded pooled sensitivity of 85% (95% CI, 75-91) and pooled specificity of 72% (95% CI, 57-83). The pooled diagnostic odds ratio for MEP was 55.7 (95% CI, 26.3-119.1) and 14.3 (95% CI, 5.47-37.3) for SSEP. Bivariate analysis yielded summary receiver operative characteristic curves with area under the curve of 91.8% for MEPs and 86.3% for SSEPs.

CONCLUSIONS

MEPs and SSEPs appear to be more sensitive than specific for detection of postoperative injury. Patients with perioperative neurological deficits are 56 times more likely to have had changes in MEPs during the procedure. We observed considerable variability in alarm criteria and interventions in response to IONM changes, indicating the need for prospective studies capable of defining standardized alarm criteria and responses.

Structural bases for central nervous system malfunction in the quaking mouse: dysmyelination in a potential model of schizophrenia

The dysmyelinating mouse mutant quaking (qk) is thought to be a model of schizophrenia based on diminution of CNS myelin (Andreone et al., 2007) and downregulation of the Qk gene (Haroutunian et al., 2006) in the brains of schizophrenic patients. The purpose of this study was to identify specific structural defects in the qk mouse CNS that could compromise physiologic function and that in humans might account for some of the cognitive defects characteristic of schizophrenia. Ultrastructural analysis of qk mouse CNS myelinated fibers shows abnormalities in nodal, internodal, and paranodal regions, including marked variation in myelin thickness among neighboring fibers, spotty disruption of paranodal junctions, abnormal distribution of nodal and paranodal ion channel complexes, generalized thinning and incompactness of myelin, and on many axonal profiles complete absence of myelin. These structural defects are likely to cause abnormalities in conduction velocity, synchrony of activation, temporal ordering of signals, and other physiological parameters. We conclude that the structural abnormalities described are likely to be responsible for significant functional impairment both in the qk mouse CNS and in the human CNS with comparable myelin pathology.

Combined motor and somatosensory evoked potential monitoring for intramedullary spinal cord tumor surgery: correlation of clinical and neurophysiological data in 17 consecutive procedures

The primary objective of neurophysiologic monitoring during surgery is to prevent permanent neurological sequelae. We prospectively evaluated whether the combined use of somatosensory- and motor-evoked potential (SEP/MEP) for intramedullary spinal cord tumor (IMSCT) surgery may be beneficial. Combined SEP/MEP monitoring was attempted in 20 consecutive procedures for IMSCT operations. Trains of transcranial electric stimulation over the motor cortex were used to elicit MEPs from limb target muscles. The tibial and median nerves were stimulated to record SEP. The operation was paused or the surgical strategy was modified in every case of significant SEP/MEP changes. Combined SEP/MEP recording was successfully achieved in 17 of 20 (85%) operations. In 3 of 17 operations, SEP and MEP were stable, and all patients remained neurologically intact after surgery. Significant MEP changes were recorded in 12 operations (70%). In 7 of these 12 operations, MEP recovered to some extent after surgical intervention, and these patients showed no neurological changes. In the remaining 5 operations, MEP did not recover and the patients had a transient (n = 2) or a permanent (n = 1) motor deficit. Significant SEP changes with stable MEP were related to a transient hypesthesia. Combined SEP/MEP monitoring provided higher sensitivity, and higher positive and negative predictive value than single-modality techniques. Detection of MEP changes and adjustment of surgical strategy may prevent irreversible pyramidal tract damage.

Surgery for intramedullary spinal cord tumors: the role of intraoperative (neurophysiological) monitoring

In spite of advancements in neuro-imaging and microsurgical techniques, surgery for intramedullary spinal cord tumors (ISCT) remains a challenging task. The rationale for using intraoperative neurophysiological monitoring (IOM) is in keeping with the goal of maximizing tumor resection and minimizing neurological morbidity. For many years, before the advent of motor evoked potentials (MEPs), only somatosensory evoked potentials (SEPs) were monitored. However, SEPs are not aimed to reflect the functional integrity of motor pathways and, nowadays, the combined used of SEPs and MEPs in ISCT surgery is almost mandatory because of the possibility to selectively injury either the somatosensory or the motor pathways. This paper is aimed to review our perspective in the field of IOM during ISCT surgery and to discuss it in the light of other intraoperative neurophysiologic strategies that have recently appeared in the literature with regards to ISCT surgery. Besides standard cortical SEP monitoring after peripheral stimulation, both muscle (mMEPs) and epidural MEPs (D-wave) are monitored after transcranial electrical stimulation (TES). Given the dorsal approach to the spinal cord, SEPs must be monitored continuously during the incision of the dorsal midline. When the surgeon starts to work on the cleavage plane between tumor and spinal cord, attention must be paid to MEPs. During tumor removal, we alternatively monitor D-wave and mMEPs, sustaining the stimulation during the most critical steps of the procedure. D-waves, obtained through a single pulse TES technique, allow a semi-quantitative assessment of the functional integrity of the cortico-spinal tracts and represent the strongest predictor of motor outcome. Whenever evoked potentials deteriorate, temporarily stop surgery, warm saline irrigation and improved blood perfusion have proved useful for promoting recovery, Most of intraoperative neurophysiological derangements are reversible and therefore IOM is able to prevent more than merely predict neurological injury. In our opinion combining mMEPs and D-wave monitoring, when available, is the gold standard for ISCT surgery because it supports a more aggressive surgery in the attempt to achieve a complete tumor removal. If quantitative (threshold or waveform dependent) mMEPs criteria only are used to stop surgery, this likely impacts unfavorably on the rate of tumor removal.

Genetic and physiological evidence that oligodendrocyte gap junctions contribute to spatial buffering of potassium released during neuronal activity

Mice lacking the K+ channel Kir4.1 or both connexin32 (Cx32) and Cx47 exhibit myelin-associated vacuoles, raising the possibility that oligodendrocytes, and the connexins they express, contribute to recycling the K+ evolved during neuronal activity. To study this possibility, we first examined the effect of neuronal activity on the appearance of vacuoles in mice lacking both Cx32 and Cx47. The size and number of myelin vacuoles was dramatically increased when axonal activity was increased, by either a natural stimulus (eye opening) or pharmacological treatment. Conversely, myelin vacuoles were dramatically reduced when axonal activity was suppressed. Second, we used genetic complementation to test for a relationship between the function of Kir4.1 and oligodendrocyte connexins. In a Cx32-null background, haploinsufficiency of either Cx47 or Kir4.1 did not affect myelin, but double heterozygotes developed vacuoles, consistent with the idea that oligodendrocyte connexins and Kir4.1 function in a common pathway. Together, these results implicate oligodendrocytes and their connexins as having critical roles in the buffering of K+ released during neuronal activity.

Motor evoked potential monitoring for spinal cord and brain stem surgery

Intraoperative Neurophysiology (ION) has established itself as one of the means by which modern neurosurgery can improve surgical results while minimizing morbidity. The advent of motor evoked potential (MEP) monitoring represents a landmark in this recent progress. ION consists of monitoring (the continuous "on-line" assessment of the functional integrity of neural pathways) and mapping (the functional identification and preservation of anatomically ambiguous nervous tissue) techniques. In this chapter we have attempted to critically review the evolution of MEP use during monitoring and mapping techniques for neurosurgical procedures in the brainstem and the spinal cord, providing the neurophysiological theoretical background and practical aspects of clinical applications. According to the experience from our and other groups involved in ION, we suggest the following: 1) ION is mandatory whenever neurological complications are expected as predicted by a known pathophysiological mechanism. It is therefore advisable to perform ION when dealing with brain stem and intramedullary spinal cord lesions. 2) MEP monitoring after transcranial electrical stimulation is today a feasible and reliable technique for use under general anesthesia. MEP monitoring is the most appropriate technique to assess the functional integrity of descending motor pathways in the brainstem and, foremost, in the spinal cord. 3) Mapping of the corticospinal tract at the level of the cerebral peduncle as well as mapping of the VII, IX-X and XII cranial nerve motor nuclei on the floor of the fourth ventricle is of great value with which to identify "safe entry zones" into the brainstem. 4) Other techniques, although safe and feasible, still lack rigorous validation in terms of prognostic value and correlation with the postoperative neurological outcome. These techniques include mapping of the corticospinal tract within the spinal cord and monitoring of the corticobulbar tracts. These techniques, however, are expected to open new perspectives in the near future.

The Department of Neurosurgery at Seoul National University: past, present, and future

The Department of Neurosurgery at Seoul National University College of Medicine is one of the oldest neurosurgical departments in Korea, and it is a center of academic leadership in neurosurgery. In September 1957, the department was established by Bo Sung Sim, and it has produced many leaders of neurosurgery in Korea. Chairmen Bo Sung Sim, Kil Soo Choi, Dae Hee Han, and Byung-Kyu Cho each brought special skills and talents to the development of the department. The current and fifth chair, Hyun Jib Kim, assumed the chairmanship in July 2000. The department comprises 11 full-time faculty members, 5 fellows, and 14 residents. More than 1,700 neurosurgical procedures are performed annually in four operating theaters. A gamma knife was installed in 1997, and approximately 200 gamma knife procedures are performed each year. In addition to clinical activities, research and education for graduate and postgraduate students are also particular strengths of the department. This article traces the clinical, academic, and scientific development of the department, its present activities, and its future direction.

The pathophysiology of multiple sclerosis: the mechanisms underlying the production of symptoms and the natural history of the disease

The pathophysiology of multiple sclerosis is reviewed, with emphasis on the axonal conduction properties underlying the production of symptoms, and the course of the disease. The major cause of the negative symptoms during relapses (e.g. paralysis, blindness and numbness) is conduction block, caused largely by demyelination and inflammation, and possibly by defects in synaptic transmission and putative circulating blocking factors. Recovery from symptoms during remissions is due mainly to the restoration of axonal function, either by remyelination, the resolution of inflammation, or the restoration of conduction to axons which persist in the demyelinated state. Conduction in the latter axons shows a number of deficits, particularly with regard to the conduction of trains of impulses and these contribute to weakness and sensory problems. The mechanisms underlying the sensitivity of symptoms to changes in body temperature (Uhthoff's phenomenon) are discussed. The origin of 'positive' symptoms, such as tingling sensations, are described, including the generation of ectopic trains and bursts of impulses, ephaptic interactions between axons and/or neurons, the triggering of additional, spurious impulses by the transmission of normal impulses, the mechanosensitivity of axons underlying movement-induced sensations (e.g. Lhermitte's phenomenon) and pain. The clinical course of the disease is discussed, together with its relationship to the evolution of lesions as revealed by magnetic resonance imaging and spectroscopy. The earliest detectable event in the development of most new lesions is a breakdown of the blood-brain barrier in association with inflammation. Inflammation resolves after approximately one month, at which time there is an improvement in the symptoms. Demyelination occurs during the inflammatory phase of the lesion. An important mechanism determining persistent neurological deficit is axonal degeneration, although persistent conduction block arising from the failure of repair mechanisms probably also contributes.

X-Linked Developmental Defects of Myelination: From Mouse Mutants to Human Genetic Diseases

Molecular cloning of the major myelin-specific genes and a systematic analysis of mouse mutants have led to the identification of molecular defects in human genetic diseases that affect myelination. In the central nervous system, Pelizaeus-Merzbacher disease (PMD) and X-linked spastic paraplegia (SPG-2) are clinically distinct with respect to the severity of motor dysfunction but involve the same gene for myelin proteolipid protein (PLP). Spontaneous mouse mutants of the PLP gene, such as jimpy and rumpshaker, provide faithful models of these human diseases and allow a detailed analysis of PLP dysfunction. Hypomyelination in jimpy and, presumably, in PMD is largely the result of abnormally increased oligodendrocyte death and a lack of terminal differentiation. In rumpshaker, a model for X-linked spastic paraplegia, myelinating oligodendrocytes appear normal in number but fail to assemble myelin correctly. Recently, PLP-transgenic mice have provided experimental evidence that increasing the normal PLP gene dosage (e.g., by a gene duplication) is by itself sufficient to cause PMD. The latter is strikingly similar to the peripheral neuropathy Charcot-Marie-Tooth disease frequently associated with a duplication of the myelin protein gene PMP-22.

Oligodendrocyte survival and function in the long-lived strain of the myelin deficient rat

This study has examined cellular and molecular aspects of glial cell function in a newly described long-lived myelin deficient rat mutant. In contrast to the shorter-lived mutants which died at 25-30 days, the longer-lived mutant rats lived to 75-80 days of age. Despite living longer, these mutants had a similar frequency of seizures to their younger counterparts. In the spinal cord and optic nerves of the older mutants, myelinated fibres in similar numbers to those seen in the younger myelin deficient rats were present. However, the total glial cell numbers were markedly reduced with few remaining normal appearing oligodendrocytes, and very few microglia compared to the younger mutants. In addition, little or no cell death or division was seen in the longer-lived rats. However, there was some evidence of ongoing myelination and the persistence of immature oligodendrocytes or their progenitors in the older mutant. There was some continued myelin gene expression, although this was at much reduced levels compared to normal, with proteolipid protein and myelin basic protein being most affected. In situ hybridization analysis for proteolipid protein mRNA showed that few proteolipid protein expressing oligodendrocytes remained in the 70-80-day-old mutant. Polymerase chain reaction analysis of exon 3 of the long-lived mutant revealed the same point mutation as described in the younger myelin deficient rat.

Effect of 4-aminopyridine in acute spinal cord injury

BACKGROUND

The demyelination process has been proven to be an important factor contributing to long-term sensory and motor impairments after spinal cord injury (SCI). The loss of myelin promotes exposure of K+ channels in internodal region of the damaged myelinated axons leading to K+ efflux into the neurons with subsequent blockage of action potentials. The potassium channel blocker 4-aminopyridine (4-AP) has been effective in restoring some sensory and motor impairment in incomplete SCI patients. The effect of this compound given immediately after an acute injury is not known. The objective of this study was to determine if blockage of K+ ions efflux immediately after an acute SCI would improve neuronal conduction in this model of injury.

METHODS

Cortical somatosensory evoked potentials (SSEPs) were recorded before and after a weight-induced compression injury of 120 grams, and were monitored up to 5 hours postinjury. A randomized treatment was initiated with administration of either vehicle or 4-AP. All 4-AP treatments were given as intravenous bolus injections of 1.0, 0.5, and 0.3 mg/kg at 1, 2, and 3 hours after the trauma.

RESULTS

The SSEPs were abolished immediately after the injury in all control and treated animals. Both groups showed spontaneous recovery of the SSEPs at the rate of 44.5% for the 4-AP treated and nontreated groups at the second hour postinjury. This recovery rate remained the same for both groups at the end of the experiments.

CONCLUSIONS

Based on the recovery of the SSEPs, our data indicate that early administration of 4-AP lacks any beneficial effect on axonal function during acute stage of spinal cord injury.

Effects of cerebellar lesions on tonic seizures, tremor and lifespan in myelin-deficient rats

In common with other dysmyelinating mutants, the myelin-deficient rat displays an action tremor and tonic seizures culminating in the death of the animals at approximately 23-26 days. We find that deep lesions of the cerebellar vermis alleviate the manifestations of the myelin deficiency significantly. Such lesions introduced at 20 days or later eliminate both tremor and seizures for periods up to 10 days. Lifespan is prolonged to nearly 30 days, on average, and to 35 days in some cases. Shallow lesions of the vermis or lateral lobe lesions have relatively little effect. Based on these observations we suggest that the cerebellum contributes not only to the action tremor but also to the tonic seizures characteristic of central myelin deficiency. Spontaneous activity originating in myelin-deficient fiber tracts may be carried to the cerebellum and processed there to produce a highly amplified and/or synchronized output to broad areas of the neuraxis. Deep lesions of the vermis presumably interfere with cerebellar output and compromise the cerebellar contribution to the seizures. Tonic seizures and other 'paroxysmal attacks' also occur commonly in human demyelinating diseases including multiple sclerosis [11]. Manipulation of cerebellar output offers a potential approach to the control of such spontaneous activity.

Conduction properties of central demyelinated and remyelinated axons, and their relation to symptom production in demyelinating disorders

The conduction properties of central demyelinated and remyelinated axons are discussed, and related to the expression of symptoms in central demyelinating disease. The mechanisms underlying the block and restoration of conduction in segmentally demyelinated axons are described, together with the range of deficits expressed by the conducting axons. These abnormalities are related to clinical relapses and remissions, and to the phenomena of weakness, fatigue, the temperature sensitivity of symptoms, and the generation of 'positive' symptoms (e.g. Uhthoff's and Lhermitte's symptoms). The potential role of circulating 'blocking factors' in the symptomatology of central demyelinating disease is examined, and some approaches are advanced for the symptomatic therapy of such diseases.

Conduction properties of spinal cord axons in the myelin-deficient rat mutant

Spinal cords of myelin-deficient and normal age-matched (control) rats were removed and their conduction and pharmacological properties studied in an in vitro brain slice chamber. The conduction velocity of the myelin-deficient dorsal column axons was reduced to about 25% of control axons; however, the amyelinated myelin-deficient axons displayed refractory periods and the ability to sustain high-frequency action potential discharge similar to that of dorsal column axons in control rats. Pharmacological results suggest that the myelin-deficient dorsal column axons qualitatively express a normal complement of ion channels and receptors. The demonstration of a normal representation of channels and receptors on these axons supports the proposal that the oligodendrocyte, and not the axon, is the site of the primary defect in the myelin-deficient rat mutant. It is concluded that, unlike acutely demyelinated axons which display marked frequency-dependent conduction block, amyelinated axons of the myelin-deficient rat spinal cord develop compensatory mechanisms to stabilize action potential conduction.

GABA-sensitivity of dorsal column axons: an in vitro comparison between adult and neonatal rat spinal cords

In neonatal rat spinal cord, conduction in the dorsal column is reversibly depressed by GABA. We compared the GABA-sensitivity of dorsal columns in neonate versus adult rats, using in vitro isolated dorsal column preparations. The extracellular compound action potential evoked by submaximal stimuli was recorded with a glass micropipette. GABA (10(-4)-10(-3) M) reversibly depressed the compound action potential of both neonatal and adult rat dorsal columns. The GABA-induced reduction of dorsal column compound action potential amplitudes was blocked by the GABAA antagonist picrotoxin (10(-3) M) and mimicked by the GABAA agonist isoguvacine (10(-4-10(-3) M). The compound action potential reduction by GABA was far less pronounced on adult dorsal columns. The reduction of compound action potential amplitudes by isoguvacine (10(-4)-10(-3) M) was also significantly less in adult dorsal columns. These data suggest that GABAA receptors may play a role in extrasynaptic modulation of spinal long tract conduction in an age-dependent manner.

Randomized double pulse stimulation for assessing stimulus frequency-dependent conduction in injured spinal and peripheral axons

Injury compromises the ability of axons to conduct action potentials at high frequencies. To study stimulus frequency-dependent conduction in injured spinal and peripheral axons, we developed a new stimulation paradigm which applies trains of double pulses at 5 Hz and randomly varied interpulse intervals of 3, 4, 5, 8, 10, 30, 50, and 80 msec. In each double pulse, the first pulse was used to condition the response activated by the second test pulse. Responses elicited by double pulses with 80 msec intervals served as controls. The L5 dorsal root was stimulated to activate dorsal column and dorsal root compound action potentials in pentobarbital anesthetized rats. To injure the spinal cord, we compressed the cord stepwise (0.25 mm every 5 min) until action potential conduction across the compression site was abolished and then decompressed the spinal cord 10 min later. Before injury, conditioning pulses applied 3-80 msec before the test pulses did not alter dorsal column responses except for a slight amplitude augmentation at 20 msec interpulse intervals (mean +/- S. E., + 4.2 +/- 0.8%, P less than 0.02) compared to controls. Injury had 3 effects on the responses. First, it significantly reduced response amplitudes and increased response latencies at 3-5 msec interpulse intervals, i.e., responses activated with 3 msec intervals were 26.0 +/- 7.4% (P less than 0.002, paired t test, n = 6) smaller and had 108 +/- 45 microseconds (P less than 0.04) longer latency than control responses. Second, response amplitude increases at 20 msec interpulse intervals (9.0 +/- 0.7%, P less than 0.0001) significantly exceeded those observed before injury (P less than 0.02, paired t test). Third, injury accentuated response amplitude declines during the stimulus train, most prominently at 80 msec intervals. Spinal cord injury did not affect the dorsal root responses. L5 root compression injury depressed dorsal root action potentials at 3-5 msec interpulse intervals (36.9 +/- 8.4%, n = 4, P less than 0.0001) but had no other effect on the responses. Our data indicate that randomized double pulse evoked potentials are sensitive detectors of acute axonal dysfunction and can be used to quantify stimulus frequency-dependent conduction deficits in injured central and peripheral axons.

GABAA receptors modulate axonal conduction in dorsal columns of neonatal rat spinal cord

gamma-Aminobutyric acid (GABA) can influence conduction in a number of axonal preparations from the peripheral and central nervous system. In the spinal cord, the excitability of primary afferent terminals has long been known to be affected by GABA. Whether conduction in the long fiber tracts of the spinal cord can be similarly modulated is unknown. Since GABA causes a pronounced depression of excitability in preparations of unmyelinated axons, and myelination is incomplete in the neonatal rat, we tested whether GABA can modulate conduction in the dorsal columns of 10-17-day-old rats. Experiments were performed in vitro, on isolated dorsal column segments (n = 18). The extracellular compound action potential evoked by submaximal stimuli was recorded with a glass micropipette positioned 0.5-2.0 mm from a stimulating electrode. At concentrations of 10(-4) - 10(-3) M, GABA decreased excitability, reversibly depressing the compound action potential amplitude, and increasing the latency by 47 +/- 11% and 22 +/- 9% (mean +/- S.E.M., n = 5, 10(-3) M), respectively. These effects were blocked by picrotoxin and mimicked by isoguvacine (10(-4) M), which decreased the compound action potential amplitude by 44 +/- 10% and increased the latency by 9 +/- 4% (n = 5). Lower concentrations of these agents caused a modest increase in excitability. At 10(-5) M, GABA increased the compound action potential amplitude by 14 +/- 2% and decreased the latency by 3 +/- 2% (n = 5). Our results demonstrate that functional GABAA receptors are present in neonatal dorsal columns.(ABSTRACT TRUNCATED AT 250 WORDS)

Axolemmal abnormalities in myelin mutants

Evidence is reviewed that the paranodal axoglial junction plays important roles in the differentiation and function of myelinated axons. In myelin-deficient axons, ion flux across the axolemma is greater than that in myelinated fibers because a larger proportion of the axolemma is active during continuous, as opposed to saltatory, conduction. In addition, older myelin-deficient rats that have developed spontaneous seizures display small foci of node-like E-face particle accumulations in CNS axons as well as more diffuse regions of increased particle density and number. Assuming that the E-face particles represent sodium channels, such regions could underlie high sodium current density during activity, low threshold for excitation, and increased extracellular potassium accumulation. Depending on the degree of spontaneous channel opening, they could also represent sites of spontaneous generation of activity. The appearance of seizures and their gradual increase in frequency and severity could represent an increase in the number of such regions. In addition, diminution in the dimensions of the extracellular space during maturation would result in increased extracellular resistance, which, together with increasing axonal diameter, would tend to increase the likelihood of ephaptic interaction among neighboring axons as well as the likelihood of extracellular potassium rises to levels that could cause spontaneous activity.

Anoxic injury of mammalian central white matter: decreased susceptibility in myelin-deficient optic nerve

The rat optic nerve, a typical central nervous system white matter tract, rapidly loses excitability when it is exposed to anoxia and is irreversibly damaged by prolonged anoxia. Neonatal optic nerve is extremely resistant to anoxia-induced dysfunction and injury; the adult pattern of response to anoxia appears between 10 and 20 days postnatal, that is, during the period of oligodendroglial proliferation and myelination. To test the hypothesis that myelination, or associated events, confer anoxic susceptibility on developing white matter, we analyzed the effects of anoxia on the myelin-deficient (md) strain of rat. Acutely isolated optic nerves from 19- to 21-day-old md rats and control optic nerves from unaffected male littermates were maintained in vitro at 37 degrees C, and exposed to a standard 60-minute period of anoxia. The supramaximal compound action potential was recorded and amplitude of the compound action potential, expressed as % of amplitude before anoxic exposure, was determined. The compound action potential was nearly abolished within 3 to 6 minutes after onset of anoxia in control optic nerves, while optic nerves from md rats displayed a slower decrease in compound action potential amplitude during anoxia, with a distinct action potential present even after 60 minutes of anoxia. Optic nerves from md rats showed significantly greater recovery of compound action potential (71 +/- 25%) than did control optic nerves (33 +/- 21%; p less than 0.02) after 60 minutes of anoxia. These findings support the hypothesis that myelination, or changes associated with it, may be important in the development of anoxic susceptibility in central white matter.