Periaxonal Space Research Articles

Modified multi-layered model of temperature dependent motor nerve axons

PURPOSE: Extensive sets of simulations are realized in this paper to gain insight into the problems of impulse propagation in temperature dependent motor nerve axons. The effect of temperature on the impulse threshold currents and action potential parameters (velocity, amplitude) is shown in simulated mammalian (rabbit) and human motor nerve fibres. MATERIAL AND METHODS: Using our multi-layered model of human motor nerve fibre, temperature coefficients Q10's are involved for the conductivities of axoplasm, periaxonal space and for ionic currents (nodal and internodal). The standard temperature for our model is 37ºC. The temperature increases from 20º to 60ºC. RESULTS: The essential and very important result is that the simulated conduction block at 48º and 45ºC in the rabbit and human motor nerve axons, respectively, is extremely sensitive to the rate constants of nodal sodium (PNA), fast potassium (PKf) and slow (PKs) potassium maximum permeabilities. The temperature dependence of conduction velocity over a range of 20-42ºC and 20-40ºC for the rabbit and human motor nerve axons, respectively, is not linear or exponential. Due to this, a polynomial function of degree 2 (transfer standard parabola), which relates velocity to temperature, provides an accurate fit of the data. CONCLUSION: Our modified multi-layered model used in this study is first ever in which the simulated temperature dependent velocities and conduction block in the rabbit and human motor axons are in good agreement with the experimental values. Scripta Scientifica Medica 2013; 45(3): 36-41.

Internodal mechanism of pathological afterdischarges in myelinated axons

Recent optical recordings of transmembrane potentials in the axons of pyramidal neurons have shown that the internodal action potentials (APs) predicted in our previous studies do exist. These novel processes are not well understood. In this study we aim to clarify electrical phenomena in peripheral myelinated axons (MAs). We used a multi-cable Hodgkin-Huxley-type model to simulate MAs with potassium channels that were either normal or inhibited along a short region of the internodal membrane. A brief stimulus was applied to the first node. We demonstrated peculiarities in the internodal APs induced by a saltatory AP: They existed across internodal membranes, were detectable in periaxonal space but not in intracellular space, propagated continuously, collided near the mid-internodes, and produced internodal sources of afterdischarges. These results highlight the importance of the MA internodal regions as new therapeutic targets for avoiding afterdischarges provoked by reduced axonal fast potassium channel expression.

Abnormal myelinogenesis in the central nervous system of the VF mutant rat with recoverable tremor

The vacuole formation (VF) rat is an autosomal recessive myelin mutant characterized by generalized tremor and vacuole formation within the myelin of the central nervous system (CNS). The tremor is prominent between 4 and 8 weeks of age but thereafter gradually improves. The present study, aimed at producing a detailed description of how the principal lesions evolve, revealed that the VF rat has abnormal vacuoles in the periaxonal space in the spinal cord and that these vacuoles decreased concurrently with the loss of tremor. However, a degree of hypomyelination persisted throughout the CNS after the resolution of periaxonal vacuoles. In situ hybridization for proteolipid protein (PLP) demonstrated that the number of small, round PLP-positive oligodendrocytes increased in the spinal cord of VF rats between 10 and 20 weeks of age, suggesting a disruption of oligodendrocytes' maturation. Activated astrocytes and microglia were also found in the VF rats from 4 weeks of age. Our results indicate that the causative genetic defect of the VF rat is likely to be involved in the formation and maintenance of the CNS myelin.

Intra-axonal calcium changes after axotomy in wild-type and slow Wallerian degeneration axons

Calcium accumulation induces the breakdown of cytoskeleton and axonal fragmentation in the late stages of Wallerian degeneration. In the early stages there is no evidence for any long-lasting, extensive increase in intra-axonal calcium but there does appear to be some redistribution. We hypothesized that changes in calcium distribution could have an early regulatory role in axonal degeneration in addition to the late executionary role of calcium. Schmidt–Lanterman clefts (SLCs), which allow exchange of metabolites and ions between the periaxonal and extracellular space, are likely to have an increased role when axon segments are separated from the cell body, so we used the oxalate-pyroantimonate method to study calcium at SLCs in distal stumps of transected wild-type and slow Wallerian degeneration (WldS) mutant sciatic nerves, in which Wallerian degeneration is greatly delayed. In wild-type nerves most SLCs show a step gradient of calcium distribution, which is lost at around 20% of SLCs within 3mm of the lesion site by 4–24h after nerve transection. To investigate further the association with Wallerian degeneration, we studied nerves from WldS rats. The step gradient of calcium distribution in WldS is absent in around 20% of the intact nerves beneath SLCs but 4–24h following injury, calcium distribution in transected axons remained similar to that in uninjured nerves. We then used calcium indicators to study influx and buffering of calcium in injured neurites in primary culture. Calcium penetration and the early calcium increase in this system were indistinguishable between WldS and wild-type axons. However, a significant difference was observed during the following hours, when calcium increased in wild-type neurites but not in WldS neurites. We conclude that there is little relationship between calcium distribution and the early stages of Wallerian degeneration at the time points studied in vivo or in vitro but that WldS neurites fail to show a later calcium rise that could be a cause or consequence of the later stages of Wallerian degeneration.

Pathological Findings in a Case of Equine T-Cell Lymphoma Associated with Ataxia

A 15-year-old Dutch Warmblood gelding suddenly developed incoordination and hindlimb stumbling. The horse had a history of eyelid lymphoma. Necropsy revealed yellow-white or dark reddish-brown masses adhering to the outer surface of the spinal dura mater from the first cervical vertebra to the seventh thoracic vertebra. The spinal cord close to the first cervical vertebra and the seventh thoracic vertebra was markedly compressed by the masses filling the epidural space. The masses were also observed in the larynx, eyelids, and adipose-rich tissues, including the joints and orbits. They appeared similar in shape. The mandibular, retropharyngeal, axillary, superficial inguinal, deep inguinal, and lateral iliac lymph nodes were solid and enlarged. Histologically, the masses were composed of small or medium-sized lymphocyte-like tumor cells, but atypical cells and mitotic figures were rare. There were moderate infiltrations of macrophages and multinucleated giant cells, which were occasionally ingesting the surrounding tumor cells. Immunohistochemically, the tumor cells were classified as T-cell-derived cells. Throughout the spinal cord, enlargement or loss of nerve axons, dilation of periaxonal spaces, and macrophage infiltration into periaxonal spaces were observed, mainly in the ventral funiculus. Spinal cord compression by the tumor mass was suggested as a cause of the locomotive dysfunction. This is the first report of equine lymphoma with ataxia located from the proximal cervical to middle thoracic dura mater and in joint cavities.

Chronic motor axonal neuropathy

The identification of a distinct subgroup of patients within the spectrum of lower motor neuron syndromes (LMNS) is crucial as some are potentially treatable. We describe the clinical and neuropathological characteristics of a patient presenting with a rapidly progressive LMNS associated with high titers of anti-GM1 antibodies, leading to respiratory failure within 10 months. Histopathological study of a biopsy of a obturator nerve motor branch demonstrated a predominantly axonal motor neuropathy, while electron microscopy analysis localized macrophages located within the periaxonal space. Immunohistochemistry demonstrated deposits of complement activation products (C3i) and immunoglobulins (IgM) on nerve fibers. The patient's clinical, immunological and pathological findings are consistent with a diagnosis of a chronic motor axonal neuropathy (CMAN), likely of immune-mediated origin.

Paranodal permeability in “myelin mutants”

Fluorescent dextran tracers of varying sizes have been used to assess paranodal permeability in myelinated sciatic nerve fibers from control and three "myelin mutant" mice, Caspr-null, cst-null, and shaking. We demonstrate that in all of these the paranode is permeable to small tracers (3 kDa and 10 kDa), which penetrate most fibers, and to larger tracers (40 kDa and 70 kDa), which penetrate far fewer fibers and move shorter distances over longer periods of time. Despite gross diminution in transverse bands (TBs) in the Caspr-null and cst-null mice, the permeability of their paranodal junctions is equivalent to that in controls. Thus, deficiency of TBs in these mutants does not increase the permeability of their paranodal junctions to the dextrans we used, moving from the perinodal space through the paranode to the internodal periaxonal space. In addition, we show that the shaking mice, which have thinner myelin and shorter paranodes, show increased permeability to the same tracers despite the presence of TBs. We conclude that the extent of penetration of these tracers does not depend on the presence or absence of TBs but does depend on the length of the paranode and, in turn, on the length of "pathway 3," the helical extracellular pathway that passes through the paranode parallel to the lateral edge of the myelin sheath.

Multiple functions of the paranodal junction of myelinated nerve fibers

Myelin sheaths include an extraordinary structure, the "paranodal axoglial junction" (PNJ), which attaches the sheath to the axon at each end of each myelin segment. Its size is enormous and its structure unique. Here we review past and current studies showing that this junction can serve multiple functions in maintaining reliable saltatory conduction. The present evidence points to three functions in particular. 1) It seals the myelin sheath to the axon to prevent major shunting of nodal action currents beneath the myelin sheath while still leaving a narrow channel interconnecting the internodal periaxonal space with the perinodal space. This pathway represents a potential route through which juxtaparanodal and internodal channels can influence nodal activity and through which nutrients, such as glucose, and other metabolites can diffuse to and from the internodal periaxonal space. 2) It serves as a mechanism for maintaining discrete, differentiated axolemmal domains at and around the node of Ranvier by acting as a barrier to the lateral movement of ion channel complexes within the axolemma, thus concentrating voltage-gated sodium channels at the node and segregating fast voltage-gated potassium channels to the juxtaparanode under the myelin sheath. 3) It attaches the myelin sheath to the axon on either side of the node and can thus maintain nodal dimensions in the face of mechanical stresses associated with stretch or other local factors that might cause disjunction. It is therefore the likely means for maintaining constancy of nodal surface area and electrical parameters essential for consistency in conduction.

A possible mechanism of repetitive firing of myelinated axon

A unique mechanism is proposed according to which processes within the internodal axolemma are responsible for repetitive activation of myelinated axon with deficit of internodal potassium conductance. A numerical simulation of activity in axon with 21 nodes was performed. The axon was represented by cables for axoplasmic and periaxonal spaces. Accumulation and diffusion of ions were taken into account. Fine segmentation of each internode (338 segments) allowed simulation of internodal activation in response to a normal saltatorial action potential initiated by a short stimulus. The internodal membrane without potassium conductance experienced considerable depolarization. This resulted in formation of a transition zone and significant currents that caused repetitive activation of the internode and neighbor node. Decline of periaxonal sodium concentration during the spike production or lowering of sodium channel density decreased the sodium currents. As a result, the interspike intervals increased up to cessation of the burst. The cessation was reversible.

NMNAT expression in mitochondrial matrix delays Wallerian degeneration

Calcium accumulation induces the breakdown of cytoskeleton and axonal fragmentation in the late stages of Wallerian degeneration. In the early stages there is no evidence for any long-lasting, extensive increase in intra-axonal calcium but there does appear to be some redistribution. We hypothesized that changes in calcium distribution could have an early regulatory role in axonal degeneration in addition to the late executionary role of calcium. Schmidt–Lanterman clefts (SLCs), which allow exchange of metabolites and ions between the periaxonal and extracellular space, are likely to have an increased role when axon segments are separated from the cell body, so we used the oxalate-pyroantimonate method to study calcium at SLCs in distal stumps of transected wild-type and slow Wallerian degeneration (WldS) mutant sciatic nerves, in which Wallerian degeneration is greatly delayed. In wild-type nerves most SLCs show a step gradient of calcium distribution, which is lost at around 20% of SLCs within 3 mm of the lesion site by 4–24 h after nerve transection. To investigate further the association with Wallerian degeneration, we studied nerves from WldS rats. The step gradient of calcium distribution in WldS is absent in around 20% of the intact nerves beneath SLCs but 4–24 h following injury, calcium distribution in transected axons remained similar to that in uninjured nerves. We then used calcium indicators to study influx and buffering of calcium in injured neurites in primary culture. Calcium penetration and the early calcium increase in this system were indistinguishable between WldS and wild-type axons. However, a significant difference was observed during the following hours, when calcium increased in wild-type neurites but not in WldS neurites. We conclude that there is little relationship between calcium distribution and the early stages of Wallerian degeneration at the time points studied in vivo or in vitro but that WldS neurites fail to show a later calcium rise that could be a cause or consequence of the later stages of Wallerian degeneration.

Spinal cord dysmyelination caused by an antiproteolipid protein IgM antibody: Implications for the mechanism of central nervous system myelin formation

Antiglycolipid IgM antibodies are known to induce formation of "wide spaced" or "expanded" myelin, a distinctive form of dysmyelination characterized by a repeat period approximately two or three times normal, which is seen also in diseases, including multiple sclerosis. To determine whether an antibody directed against a myelin protein would cause equivalent pathology, we implanted O10 hybridoma cells into the spinal cord of adult or juvenile rats. O10 produces an IgM directed against PLP, the major protein of CNS myelin. Subsequent examination of the cords showed focal demyelination and remyelination. In addition, however, some juvenile cords, but none of the adult cords, displayed wide-spaced myelin with lamellae separated by an extracellular material comprising elements consistent with IgM molecules in appearance. Wide spacing tended to involve the outer layers of the sheath and in some cases alternated with normally spaced lamellae. A feature not seen previously consists of multiple expanded myelin lamellae in one sector of a sheath continuous with normally spaced lamellae in another, resulting in variation in sheath thickness around the axonal circumference. This uneven distribution of wide-spaced lamellae is most simply explained based on incorporation of IgM molecules into immature sheaths during myelin formation and implies a model of CNS myelinogenesis more complex than simple spiraling. The periaxonal space never displays widening of this kind, but the interface with adjacent myelin sheaths or oligodendrocytes may. Thus, wide spacing appears to require that IgM molecules bridge between two PLP-containing membranes and does not reflect the mere presence of immunoglobulin within the extracellular space.

Submyelin potassium accumulation may functionally block subsets of local axons during deep brain stimulation: a modeling study

Deep brain stimulation has been used for over a decade to relieve the symptoms of Parkinson's disease, although its mechanism of action remains poorly understood. To better understand the direct effects of DBS on central neurons, a computational model of a myelinated axon has been constructed which includes the effects of K+ accumulation within the peri-axonal space. Using best estimates of anatomic and electrogenic model parameters for in vivo STN axons, the model predicts a functional block along the axon due to K+ accumulation in the submyelin space. The functional block occurs for a range of model parameters: high stimulation frequencies (>130 Hz); high extracellular K+ concentrations (>3 × 10−3 M); low maximum Na+/K+ ATPase current densities (<0.026 A m−2); low diffusion coefficients for K+ diffusion out of the submyelin space (<2.4 × 10−9 m2 s−1); small periaxonal space widths of the myelin attachment sections (<2.7 × 10−9 m) and perinodal/internodal sections (<8.4 × 10−9 m). These results suggest that therapeutic DBS of the STN likely results in a functional block for many STN axons, although a subset of STN axons may also be activated at the stimulating frequency.

Injury of learning and memory induced by brain aging and the relationship with the changes of myelin structure

目的 探讨脑老化时认知功能改变、髓鞘结构和成分变化及其之间关系.方法使用穿梭箱、电镜和免疫组织化学技术,观察了老年大鼠学习、记忆维持功能、髓鞘和轴突的超微结构改变,以及髓鞘碱性蛋白表达变化.结果在训练第1天至第6天,老年大鼠主动逃避反应率(AAR)、总逃避反应(GAR)率较青年组显著减慢(P <0.01),逃避潜伏时(LER)较青年组延长(P <0.01),训练第6天青年组GAR达到(89.0±6.6)%,老年只有(40.5±6.8)%,青年组LER为(3.16±0.47)s,老年组为(6.10±0.71)s.在记忆功能维持上,训练后第5天和第10天,老年组大鼠AAR、GAR很快降低,LER明显升高,较青年组差异有显著性(P <0.01).老年大鼠髓鞘异常为轴突周围间隙扩大,髓鞘致密层分离,部分板层结构破坏,髓鞘内形成大泡,轴突直径增加,轴突内含有吞噬小泡、致密颗粒和巨大线粒体,少突胶质细胞电子密度增加.老年大鼠髓鞘碱性蛋白(MBP)表达下降.结论老年大鼠学习及记忆维持能力降低,老化可以导致轴突及髓鞘的结构变化,MBP表达降低,髓鞘结构和成分改变可能参与老化时学习和记忆损害。

Major myelin protein gene (P0) mutation causes a novel form of axonal degeneration

Mutations in the major peripheral nervous system (PNS) myelin protein, myelin protein zero (MPZ), cause Charcot-Marie-Tooth Disease type 1B (CMT1B), typically thought of as a demyelinating peripheral neuropathy. Certain MPZ mutations, however, cause adult onset neuropathy with minimal demyelination but pronounced axonal degeneration. Mechanism(s) for this phenotype are unknown. We performed an autopsy of a 73-year-old woman with a late-onset neuropathy caused by an H10P MPZ mutation whose nerve conduction studies suggested severe axonal loss but no demyelination. The autopsy demonstrated axonal loss and reorganization of the molecular architecture of the axolemma. Segmental demyelination was negligible. In addition, we identified focal nerve enlargements containing MPZ and ubiquitin either in the inner myelin intralaminar and/or periaxonal space that separates axons from myelinating Schwann cells. Taken together, these data confirmed that a mutation in MPZ can cause axonal neuropathy, in the absence of segmental demyelination, thus uncoupling the two pathological processes. More important, it also provided potential molecular mechanisms as to how the axonal degeneration occurred: either by disruption of glial-axon interaction by protein aggregates or by alterations in the molecular architecture of internodes and paranodes. This report represents the first study in which the molecular basis of axonal degeneration in the late-onset CMT1B has been explored in human tissue.

Internodal sodium channels ensure active processes under myelin manifesting in depolarizing afterpotentials

The current opinion about processes in myelinated axon is that action potential saltatorially propagates between nodes of Ranvier and passively charges internodal axolemma thus causing depolarizing afterpotentials (DAP). Demyelination blocks the conduction that gives additional argument in favor of hypothesis that internode is not able to be activated by the existing internodal sodium channels. The results of our modeling study shows that, when periaxonal space is sufficiently narrow, saltatorial action potential is able to activate internodes. Low density of internodal sodium channels is sufficient to generate active internodal waves that slowly propagate from nodes towards corresponding midinternodes where they collide. The periaxonal width that stops internodal wave propagation (about 400 nm) is significantly larger than the highest value of the physiological range for this parameter (30 nm). Internodal activation is directly manifested as transmembrane internodal potential or as a full-sized action potential in periaxonal space where it can hardly be detected, and only as a small deflection in intracellular space. However, changes in the periaxonal potential cause transmyelin currents that lead to significant DAP. The shape and amplitude of DAP depends on myelin parameters and densities of internodal channels. Several technical parameters affect the results of calculations. Internodal spatial segmentation has to be sufficiently fine (at most 20 μm) for the model to be able to simulate internodal activation. We employ 338 internodal segments as compared with up to 21 used in previous models. Ionic accumulation together with related diffusive and electrical processes alter the calculated DAP amplitude. Inclusion of these processes in calculations demands such increase in the total number of segments that the numerical methods used up to now become unapplicable. To overcome the problem, an iterative implicit approach is proposed. It reduces a matrix of general type in multi-cable models to tridiagonal one and accelerates calculations considerably.

Delayed-onset temporary auditory threshold shift following head blow in guinea pigs

This study attempts to investigate the development of sensorineural hearing loss following a head blow without skull fracture in association with physiological and histopathologic changes in an experimental animal model. With the head in a freely movable position, albino guinea pigs were given a single blow to the occipital region by a head blow device. At 1, 7, and 14 days after the blow, the animals’ auditory brainstem response (ABR) and cochlear microphonics (CM) were examined, and both the temporal bone and brain stem were observed by light and electron microscopy. The ABR threshold was unchanged at day 1, was significantly increased at day 7, and was fully recovered at day 14. The I–V and I–II interpeak latencies were significantly prolonged at days 1 and 7, and wave I latency was significantly prolonged at day 7 only. These latencies were recovered to normal limits at day 14. On the other hand, no significant change in CM versus the control group was observed at any point in the measurements. Histopathologically, no abnormal finding was seen at the light microscopic level. However, at the electron microscopic level, there were some injuries to the eighth nerve. At day 1, the lamellar structure of the myelin sheath was irregular, and the periaxonal space was expanded; at day 7, the myelin sheath was disintegrated. At day 14, however, these changes were partially reversed. These results suggest that sensorineural hearing loss following a head blow in this model is attributed to dysfunction of the eighth nerve rather than to cochlear impairment.

Acute motor axonal neuropathy rabbit model: immune attack on nerve root axons.

Macrophages in the periaxonal space and surrounding intact myelin sheath are the most prominent pathological feature of acute motor axonal neuropathy (AMAN). We describe this characteristic in nerve roots from paralyzed rabbits immunized with bovine brain ganglioside or GM1. IgG was deposited on nerve root axons. Distal nerve conduction was preserved, and late F wave components were absent during the acute phase. Initial lesions were located mainly on nerve root axons, as in human AMAN. This study thus provides supportive evidence that the rabbits constitute a model of AMAN.

Differential expression of matrix metalloproteinases in monkey eyes with experimental glaucoma or optic nerve transection

Extracellular matrix (ECM) remodeling after neuronal injury and reactive gliosis is carried out by activation of matrix metalloproteinases (MMPs) regulated by their tissue inhibitors (TIMPs). In glaucoma, there is a loss of retinal ganglion cells and extensive ECM remodeling (cupping) at the level of the optic nerve head, frequently associated with elevated intraocular pressure. To determine whether ECM remodeling in the glaucomatous optic nerve head occurs in response to loss of axons or to elevated intraocular pressure we compared the patterns of MMP and TIMP expression in the eyes of monkeys with laser-induced glaucoma or with optic nerve transection. MT1-MMP and MMP1 expression was markedly increased in reactive astrocytes in optic nerve heads with experimental glaucoma but not in the optic nerve head of transected eyes. In normal control eyes retinal ganglion cells expressed MMP2, TIMP1 and TIMP2 constitutively, and the proteins were detected in their axons. At the site of transection, MT1-MMP, MMP1, MMP2, TIMP1 and TIMP2 were expressed by reactive astrocytes. Inflammatory cells, fibroblasts and reactive astrocytes at the transected site expressed MMP3 and MMP9, which were undetectable in the retina and optic nerve head in any condition. Constitutive expression of MMP2, TIMP1 and TIMP2 in retinal ganglion cells suggests a role in maintenance of synaptic integrity and plasticity and maintenance of the periaxonal space. Increased MMP1 and MT1-MMP1 expression in the glaucomatous optic nerve head is specific to tissue remodeling due to elevated intraocular pressure and not secondary to loss of axons.

Some characteristics of periaxonal space in myelinated nerve fibers.

In [8] it was found that there is potassium ions accumulation in the periaxonal space of the giant axon of the squid between the excitable membrane and Schwann cell. This accumulation is considered to explain the prolonged negative afterpotential (n.a.p.) with a time constant ( τ ) of 30‐100 ms. Earlier, a prolonged n.a.p. was described in the whole nerve trunk of myelinated nerve fibers [7, 12]. H. Meves [13, 14] found that the n.a.p. of a single node of Ranvier of myelinated nerve fibers lasts for only 1‐3 ms, i.e., 1.5 - 2 orders of magnitude shorter than the whole nerve trunk n.a.p. (about 100 ms). Using these results and some calculations (where the potassium efflux per impulse was taken equal to the one of the squid [11]), Meves [14] concluded that there was no potassium ions accumulation in myelinated nerve fibers. Potassium ions diffuse away freely to the external fluid, which resulted in the absence of a prolonged n.a.p and the related superexcitable phase. As the results of the experiments and conclusions based on them by Meves are in contradiction with classical data on the whole nerve trunk, we decided to find out whether the absence of a prolonged n.a.p. is connected with the distraction of the diffusion barrier, separating the periaxonal space from the external fluid during the dissection of a single fiber. We found considerable differences between the duration of the n.a.p. of the node of Ranvier of isolated and intact nerve fibers; evaluated the net potassium efflux during the action potential, thickness of the periaxonal space, and permeability of the diffusion barrier separating the space from external fluid both experimentally and using a mathematical model [9]. All our experiments (more than 100) were performed on a single node of Ranvier of isolated nerve fibers dissected using the classical method of I. Tasaki [15], which we called an “open” node of Ranvier

The Myelin Vacuolation (mv) Rat with a Null Mutation in the Attractin Gene

We recently found a spontaneous tremor mutant in an outbred colony of Sprague-Dawley rats. The tremor behavior was exhibited from around 3 weeks of age and inherited as an autosomal recessive trait. The mutant rats had variously sized vacuoles in the neuropil and white matter throughout the central nervous system, especially in the brain stem, cerebellum, and spinal cord. Ultrastructurally these vacuoles mainly consisted of splitting of myelin lamella both in the periaxonal and intermyelinic spaces. Linkage analysis using intercross progeny between the myelin vacuolation (mv) rat, named after the pathologic characteristics, and normal control rat strains showed that the mv phenotypes were cosegregated with polymorphic markers adjacent to the Atrn (Attractin, formerly zi [zitter]) locus on rat chromosome 3. A test for allelism suggested that the mv mutation was a new allele in Atrn. In comparison with a marked decrease of Atrnzi/Arnzi, Northern blot analysis revealed no expression of Atrn mRNA in the brain of the mv rats. Finally, a genomic deletion including exon 1 of the mv rats was detected by genomic and sequence analyses. Discovery of the rat null mutation Atrnmv, different from Atrnzi, provides a new animal model for studying the functions of the attractin protein.