Modulation of parallel fiber excitability by postsynaptically mediated changes in extracellular potassium.

Field potentials and extracellular potassium concentration ([K+]o) were simultaneously monitored in the molecular layer of the rat cerebellar cortex during stimulation of the parallel fibers. The synaptic field potential elicited by stimulation was reduced by several methods. Reduction of synaptic field potentials was accompanied by a marked increase in the excitability of the parallel fibers. This change in excitability was related to the degree of extracellular K+ accumulation associated with parallel fiber stimulation. These findings support the proposal that increases in [K+]o associated with activity in postsynaptic elements can modulate the excitability of presynaptic afferent fibers.

Retinal ganglion cells express a cGMP-gated cation conductance activatable by nitric oxide donors.

We have identified a putative cGMP-gated cation conductance in rat retinal ganglion cells. Both in situ hybridization and polymerase chain reaction amplification detected transcripts in ganglion cells that were highly homologous to the cGMP-gated cation channel expressed in rod photoreceptors. Whole-cell patch-clamp recordings detected a current stimulated by cGMP due to activation of nonselective cation channels. This current had a reversal potential near 0 mV, showed some outward rectification, and could be blocked by Cd2+. The current could also be activated by a phosphodiesterase inhibitor and the nitric oxide donors sodium nitroprusside and S-nitrosocysteine. We propose that nitric oxide released from an identified subpopulation of amacrine cells may activate this channel to modulate ganglion cell activity.

Demyelinating diseases and potential repair strategies.

Demyelination is associated with a number of neurological disorders including multiple sclerosis (MS), spinal cord injury and nerve compression. MS lesions often show axon loss and therefore reparative therapeutic goals include remyelination and neuroprotection of vulnerable axons. Experimental cellular transplantation has proven successful in a number of demyelination and injury models to remyelinate and improve functional outcome. Here we discuss the remyelination and neuroprotective potential of several myelin-forming cells types and their behavior in different demyelination and injury models. Better understanding of these models and current cell-based strategies for remyelination and neuroprotection offer exciting opportunities to develop strategies for clinical studies.

Neuroprotection by PlGF gene-modified human mesenchymal stem cells after cerebral ischaemia.

Intravenous delivery of mesenchymal stem cells (MSCs) prepared from adult bone marrow reduces infarction size and ameliorates functional deficits in rat cerebral ischaemia models. Placental growth factor (PlGF) is angiogenic to impaired non-neural tissue. To test the hypothesis that PlGF contributes to the therapeutic benefits of MSC delivery in cerebral ischaemia, we compared the efficacy of systemic delivery of human MSCs (hMSCs) and hMSCs transfected with a fibre-mutant F/RGD adenovirus vector with a PlGF gene (PlGF-hMSCs). A permanent middle cerebral artery occlusion (MCAO) was induced by intraluminal vascular occlusion with a microfilament. hMSCs and PlGF-hMSCs were intravenously injected into the rats 3 h after MCAO. Lesion size was assessed at 3 and 6 h, and 1, 3, 4 and 7 days using MR imaging and histology. Functional outcome was assessed using the limb placement test and the treadmill stress test. Both hMSCs and PlGF-hMSCs reduced lesion volume, induced angiogenesis and elicited functional improvement compared with the control sham group, but the effect was greater in the PlGF-hMSC group. Enzyme-linked immunosorbent assay of the infarcted hemisphere revealed an increase in PlGF in both hMSC groups, but a greater increase in the PlGF-hMSC group. These data support the hypothesis that PlGF contributes to neuroprotection and angiogenesis in cerebral ischaemia, and cellular delivery of PlGF to the brain can be achieved by intravenous delivery of hMSCs.

Xenotransplantation of transgenic pig olfactory ensheathing cells promotes axonal regeneration in rat spinal cord.

Here we describe transplantation of olfactory ensheathing cells (OECs) or Schwann cells derived from transgenic pigs expressing the human complement inhibitory protein, CD59 (hCD59), into transected dorsal column lesions of the spinal cord of the immunosuppressed rat to induce axonal regeneration. Non-transplanted lesion-controlled rats exhibited no impulse conduction across the transection site, whereas in animals receiving transgenic pig OECs or Schwann cells impulse conduction was restored across and beyond the lesion site for more than a centimeter. Cell labeling indicated that the donor cells migrated into the denervated host tract. Conduction velocity measurements showed that the regenerated axons conducted impulses faster than normal axons. By morphological analysis, the axons seemed thickly myelinated with a peripheral pattern of myelin expected from the donor cell type. These results indicate that xenotranplantation of myelin-forming cells from pigs genetically altered to reduce the hyperacute response in humans are able to induce elongative axonal regeneration and remyelination and restore impulse conduction across the transected spinal cord.

Ion channel organization of the myelinated fiber.

The myelinated axon provides a model in which it is possible to examine how various types of ion channels are incorporated into a membrane to form an excitable neuronal process. The available evidence now indicates that mammalian myelinated fibers contain a repertoire of physiologically active membrane molecules including at least four types of ion channels and an electrogenic Na+,K(+)-pump. Physiological properties of myelinated fibers reflect the distribution of these various types of channels and pumps, as well as interactions with myelinating Schwann cells in the PNS or oligodendrocytes in the CNS. A growing body of data also suggests a role for astrocytes and Schwann cells at nodes of Ranvier. This article reviews the current understanding of the ion channel organization of the mammalian myelinated fiber.

Transplantation of cryopreserved adult human Schwann cells enhances axonal conduction in demyelinated spinal cord.

Schwann cells derived from human sural nerve may provide a valuable source of tissue for a cell-based therapy in multiple sclerosis. However, it is essential to show that transplanted human Schwann cells can remyelinate axons in adult CNS and improve axonal conduction. Sections of sural nerve were removed from amputated legs of patients with vascular disease or diabetes, and Schwann cells were isolated and cryopreserved. Suspensions of reconstituted cells were transplanted into the X-irradiation/ethidium bromide lesioned dorsal columns of immunosuppressed Wistar rat. After 3-5 weeks of extensive remyelination, a typical Schwann cell pattern was observed in the lesion zone. Many cells in the lesion were immunopositive for an anti-human nuclei monoclonal antibody. The dorsal columns were removed and maintained in an in vitro recording chamber; the conduction properties were studied using field potential and intra-axonal recording techniques. The transplanted dorsal columns displayed improved conduction velocity and frequency-response properties, and action potentials conducted over a greater distance into the lesion, suggesting that conduction block was overcome. These data support the conclusion that transplantation of human Schwann cells results in functional remyelination of a dorsal column lesion.

I.V. infusion of brain-derived neurotrophic factor gene-modified human mesenchymal stem cells protects against injury in a cerebral ischemia model in adult rat.

I.V. delivery of mesenchymal stem cells prepared from adult bone marrow reduces infarction size and ameliorates functional deficits in rat cerebral ischemia models. Administration of the brain-derived neurotrophic factor to the infarction site has also been demonstrated to be neuroprotective. To test the hypothesis that brain-derived neurotrophic factor contributes to the therapeutic benefits of mesenchymal stem cell delivery, we compared the efficacy of systemic delivery of human mesenchymal stem cells and human mesenchymal stem cells transfected with a fiber-mutant F/RGD adenovirus vector with a brain-derived neurotrophic factor gene (brain-derived neurotrophic factor-human mesenchymal stem cells). A permanent middle cerebral artery occlusion was induced by intraluminal vascular occlusion with a microfilament. Human mesenchymal stem cells and brain-derived neurotrophic factor-human mesenchymal stem cells were i.v. injected into the rats 6 h after middle cerebral artery occlusion. Lesion size was assessed at 6 h, 1, 3 and 7 days using MR imaging, and histological methods. Functional outcome was assessed using the treadmill stress test. Both human mesenchymal stem cells and brain-derived neurotrophic factor-human mesenchymal stem cells reduced lesion volume and elicited functional improvement compared with the control sham group, but the effect was greater in the brain-derived neurotrophic factor-human mesenchymal stem cell group. ELISA analysis of the infarcted hemisphere revealed an increase in brain-derived neurotrophic factor in the human mesenchymal stem cell groups, but a greater increase in the brain-derived neurotrophic factor-human mesenchymal stem cell group. These data support the hypothesis that brain-derived neurotrophic factor contributes to neuroprotection in cerebral ischemia and cellular delivery of brain-derived neurotrophic factor can be achieved by i.v. delivery of human mesenchymal stem cells.

Mechanisms of enhancement of neurite regeneration in vitro following a conditioning sciatic nerve lesion.

To examine the mechanisms responsible for the more rapid nerve regeneration observed after a previous (conditioning) nerve injury, adult rats were subjected to a midthigh sciatic nerve transection by using one of three protocols designed to facilitate or restrict nerve regeneration: 1) ligation, in which transected axons were prevented from regenerating; 2) cut, in which transected axons were permitted to extend into peripheral target tissue but were separated from the denervated peripheral nerve stump; and 3) crush, in which axons could regenerate normally through the denervated distal nerve tract. The affected dorsal root ganglia (DRG) were subsequently removed, dissociated, and cultured for up to 3 days, and the timing of neurite initiation, rate of outgrowth, and arborization pattern of previously injured neurons were compared with control DRG. Our results indicate that conditioning lesions have at least four distinct and differentially regulated effects on neuronal morphogenesis: 1) conditioning lesions promote earlier neurite initiation, 2) prior nerve injury decreases the ability of neurons to extend long neurites following a second axotomy, 3) exposure to the environment of a denervated peripheral nerve stimulates greater initial rates of neurite outgrowth, and 4) conditioning lesions reduces initial neuritic branching frequency, resulting in straighter neurites whose growth cones extend further distances from their cell bodies. The primary effect of all conditioning lesions on cultured DRG neurons appeared to be to advance the timing of morphogenesis, resulting in conditioning-lesioned neurons that exhibited characteristics consistent with control neurons that had been cultured for an additional day or more. A secondary effect of conditioning lesions on neurite outgrowth rates was dependent on the local environment of the axons prior to culturing.

Different effects of 4-aminopyridine on sensory and motor fibers: pathogenesis of paresthesias.

Mammalian motor and sensory fibers respond differently to the potassium channel-blocking agent, 4-aminopyridine (4-AP). The action potentials of the motor fibers increase in duration after 4-AP, while the sensory fibers respond with bursts of action potentials after a single stimulus. These differences may account for the paresthesias reported by patients with multiple sclerosis following treatment with 4-AP.

Modulation of neuronal activity in the hippocampus by 5-hydroxytryptamine and 5-hydroxytryptamine1A selective drugs.

The interactions between 5-hydroxytryptamine (5-HT), 8-hydroxy-2-(N,N-dipropylamino)-tetralin (8-OH-DPAT), buspirone, 2-(4-(4-(2-pyrimidinyl)-1-piperazinyl)butyl)-1,2-benzisothiazol-3- (2H)one-1, 1-dioxide-hydrochloride (TVX Q 7821) and ketanserin, and putative 5-HT receptors were analyzed using both radioligand techniques and an in vitro hippocampal slice preparation. The potencies of the drugs were determined at 5-HT1A binding sites labelled by [3H]8-OH-DPAT in hippocampal membranes from the rat. The binding site had similar affinity for 5-HT, 8-OH-DPAT, buspirone and TVX Q 7821, whereas ketanserin was essentially inactive. Physiological effects of these drugs were also examined using an in vitro hippocampal slice preparation. With the exception of ketanserin, application of each drug to the bath modulated the amplitude of the field potential recorded in the pyramidal layer of CA1 evoked by stimulation of Schaffer collaterals. Application of micromolar concentrations of 5-HT produced an initial increase in the population spike followed by a return to near baseline levels within 5 min. By contrast, the amplitude of the population spike was reduced in a dose-dependent manner by micromolar concentrations of 8-OH-DPAT, buspirone and TVX Q 7821, beginning 5 min after application of drug. Ketanserin did not affect the amplitude of the population spike and it did not antagonize the effects of 5-HT, buspirone or TVX Q 7821. Neither buspirone nor 8-OH-DPAT altered the initial increase in population spike induced by 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Nerve growth factor maintains potassium conductance after nerve injury in adult cutaneous afferent dorsal root ganglion neurons.

Whole-cell patch-clamp techniques were used to study the effects of nerve growth factor on voltage-dependent potassium conductance in normal and axotomized identified large cutaneous afferent dorsal root ganglion neurons (48-50 micrometer diameter) many of which probably give rise to myelinated Abeta fibers. K-currents were isolated by blocking Na- and Ca-currents with appropriate ion replacement and channel blockers. Separation of current components was achieved on the basis of response to variation in conditioning voltage. Cutaneous afferents were labeled by the retrograde marker hydroxy-stilbamide (FluoroGold) which was injected into the skin of the foot. The sciatic nerve was either ligated or crushed with fine forceps five to seven days later. Neurons were dissociated 14-17 days after injury. The cut ends of the sciatic nerves were positioned into polyethylene tubes, which were connected to mini-osmotic pumps filled with either nerve growth factor or sterile saline. Control neurons displayed a prominent sustained K-current and the transient potassium currents "A" and "D". Nerve ligation, which blocks target reconnection resulted in near 50% reduction of total outward current; isolated sustained K-current and transient A-current were reduced by a comparable amount. Nerve crush, which allows regeneration to peripheral targets and exposure of the regenerating nerve to the distal nerve segment, resulted in a small reduction in sustained K-current but no reduction in transient A-current compared to controls. Levels of transient A-current and sustained K-current were maintained at control levels after nerve growth factor treatment. These results indicate that the large reduction in transient A-current, and in sustained K-current, observed in cutaneous afferent cell bodies after nerve ligation is prevented by application of nerve growth factor.

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.

The association of the supernormal period and the depolarizing afterpotential in myelinated frog and rat sciatic nerve.

Excitability properties of isolated frog and rat sciatic nerve fibers were examined using intra-axonal and sucrose-gap recording techniques. Paired stimulation experiments on rat myelinated fibers indicate that a small proportion (11%; n = 84) of these axons demonstrate decreased threshold indicative of a supernormal period. In contrast, 81% (n = 23) of frog axons displayed a supernormal period. A depolarizing afterpotential was observed in most of the rat and frog fibers having a supernormal period and the depolarizing afterpotential increased in magnitude and duration during hyperpolarization. In addition to whole nerve stimulation, a supernormal period could be induced by stimulation of a single axon via current passage through the recording microelectrode. Brief (2-5 ms) subthreshold depolarizing pulses were followed by a slowly decaying depolarization and a period of increased excitability that mimicked the supernormal period. A supernormal period was also observed in the whole nerve preparation using a sucrose-gap technique. The magnitude and duration of the supernormal period, as measured in the sucrose-gap, were greater for frog nerve than for rat nerve. Additionally, a larger postspike negativity, the extracellular equivalent of the intra-axonally observed depolarizing afterpotential, was present in sucrose-gap recordings for frog nerve than for rat nerve. The results indicate that the depolarizing afterpotential is an important determinant of the supernormal period, and that both the depolarizing afterpotential and supernormal period are more prominent in frog than in rat sciatic nerve.

Electrogenic pump (Na+/K(+)-ATPase) activity in rat optic nerve.

Rat optic nerves were studied in a sucrose gap chamber in order to study the origin of a late afterhyperpolarization that follows repetitive activity. The results provide evidence for electrogenic pump (Na+/K(+)-ATPase) activity in central nervous system myelinated axons and demonstrate an effect on axonal excitability. Repetitive stimulation (25-200 Hz; 200-5000 ms) led to a prolonged, temperature-dependent post-train afterhyperpolarization with duration up to about 40 s. The post-train afterhyperpolarization was blocked by the Na+/K(+)-ATPase blockers strophanthidin and ouabain, and the substitution of Li+ for Na+ in the test solution, which also blocks Na+/K(+)-ATPase. The peak amplitude of the post-train afterhyperpolarization was minimally changed by the potassium-channel blocker tetraethylammonium (10 mM), and the Ca2(+)-channel blocker CoCl2 (4 mM). Hyperpolarizing constant current did not reverse the afterhyperpolarization. The amplitude of the hyperpolarization was increased in the presence of the potassium-channel blocker 4-aminopyridine (1 mM). In the presence of 4-amino-pyridine, the post-train hyperpolarization was much reduced by strophanthidin, except for a residual early component lasting several hundred milliseconds which was blocked by the potassium-channel blocker tetraethylammonium. This finding indicates that after exposure to 4-aminopyridine, repetitive stimulation leads to activation of a tetraethylammonium-sensitive K(+)-channel that contributes during the first several hundred milliseconds to the post-train afterhyperpolarization. The amplitude of the compound action potential elicited by a single submaximal stimulus during the post-train hyperpolarization was smaller than that of the control response. The decrement in amplitude was not present under identical stimulation conditions when the post-train hyperpolarization was blocked by strophanthidin, indicating that the hyperpolarization associated with repetitive stimulation reduced excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Conduction of trans of impulses in uniform myelinated fibers: computed dependence on stimulus frequency.

The conduction of trains of action potentials in myelinated fibers was studied using computer simulations based on a modification of the Hodgkin-Huxley equations. Stimulation at short but regular interstimulus intervals caused some stimuli to fail to elicit propagated action potentials. Propagated impulse trains observed close to the stimulation site, elicited by high frequency stimulus trains, took the form of "clusters" of impulses, e.g. doublets or triplets. When these impulse trains were observed at distances farther from the stimulation site, interspike intervals were more uniform. For interstimulus intervals of less than 10 ms, distant intervals between impulses were relatively insensitive to the temporal patterning of impulses at the initiation zone and tended toward regular intervals corresponding to the average interstimulus intervals for propagated stimuli. This tendency toward uniform intervals between impulses was also observed for lower average frequency stimulus trains with irregular interstimulus intervals. Moreover, for the first two stimuli in a train, there was a very tendency toward impulse entrainment. These results indicate that intervals between impulses along unbranched myelinated axons are not fixed, but vary according to the site along the conduction pathway where they are observed. The tendency toward entrainment, and regularization of intervals, may represent a factor limiting the frequency with which interval-coded impulses are initiated.

Sodium currents of large (Abeta-type) adult cutaneous afferent dorsal root ganglion neurons display rapid recovery from inactivation before and after axotomy.

Voltage-dependent Na-currents were studied, using whole cell voltage clamp, in acutely dissociated, large (mostly Abeta-fiber type) cutaneous afferent dorsal root ganglia neurons (L(4) and L(5)) from the adult rat. Cells were dissociated 14-17 days after axotomy. Control and axotomized neurons were identified via the retrograde marker hydroxy-stilbamide (fluorogold) which was injected into the lateral and plantar region of the skin of the foot and were studied using whole cell patch clamp techniques within 12-20 h of dissociation and plating. Cells were dissociated 14-17 days after injury. Both control and axotomized neurons displayed complex Na-currents composed of components with distinct kinetic and pharmacological properties. The large (48-50 microm diameter) control cutaneous afferent neurons, many of which likely give rise to myelinated Abeta-fibers, exhibited Na-currents with both slow and fast inactivating kinetics. The fast inactivating current in large cutaneous afferent dorsal root ganglion neurons was tetrodotoxin-sensitive and recovered from inactivation approximately four-fold faster at -60 mV (P<0.001) and approximately six-fold faster at -70 mV (P<0.001) than the tetrodotoxin-sensitive current in small (<30 microm diameter) neurons. Further, while the tetrodotoxin-sensitive currents in smaller dorsal root ganglion neurons (mainly C-fiber type) reprime approximately four-fold faster following peripheral axotomy, repriming of the fast inactivating current in larger cutaneous afferent neurons was not significantly altered following axotomy. However, while 77% of control large neurons were observed to express the slower inactivating, tetrodotoxin-resistant current, only 45% of these large neurons did after axotomy. These results indicate that large adult cutaneous afferent dorsal root ganglion neurons (Abeta-type) express tetrodotoxin-sensitive Na-currents, which have much faster repriming than Na-currents in small (C-type) neurons, both before, and after axotomy. Like small neurons, the majority of large neurons downregulate the tetrodotoxin-resistant current following sciatic nerve section.

Expression of Jun, Fos, and ATF-2 proteins in axotomized explanted and cultured adult rat dorsal root ganglia.

The expression of c-Jun, JunB, JunD, c-Fos and ATF-2 transcription factors was studied in L4/L5 dorsal root ganglion neurons of adult rats, in order to determine the extent to know to which extend the expression of transcription factors in vitro parallels the pathophysiological expression in vivo. First, dorsal root ganglia were dissociated and cultured for up to 15 days in vitro (culture). Second, the dorsal root and the peripheral nerve fibres were transected close at the dorsal root ganglia, and the completely axotomized dorsal root ganglia were kept in artificial cerebrospinal fluid for up to 24 h. This procedure (explantation) preserves the intraganglionic morphology intact. Culture evoked a persistent expression of c-Jun and JunD in the majority of small neurons independent on neurite extension, In contrast, the number of large neurons with c-Jun decreased and with JunD increased with incubation time. JunB and c-Fos, which were also visible in the majority of neurons, strongly decreased with culture time in both small and large neurons. ATF-2 was visible in the vast majority of neurons and did not change during the observation period. Incubation with brain-derived neurotrophic factor for 15 days reduced JunB expression and raised c-Fos expression, but did not affect c-Jun or JunD labelings. Explantation of dorsal root ganglia evoked a dramatic and rapid induction of c-Jun in neurons located in the periphery of the ganglia, an area that showed prominent apoptosis as visualized by transferase dUTP nick end-labelling, followed by a delayed increase in neurons of the central parts of dorsal root ganglia. Expression of JunB showed a dramatic increase within 2 h in the whole ganglion, but disappeared within the following hours. JunD dropped from its basal levels within 4 h and was almost absent after 8 h. c-Fos did not appear until 6 h, when transferase dUTP nick end-labelling also became detectable, and remained visible in a rather small number of neurons. As with culture, incubation of explanted dorsal root ganglia with brain-derived neurotrophic factor prevented the initial rise in JunB, accelerated and enhanced c-Fos expression, but did not alter c-Jun and JunD expression. Immunoreactivity of ATF-2 declined or disappeared in those dorsal root ganglia compartments that showed a rise in c-Jun and transferase dUTP nick end-labelling. These findings demonstrate that inducible transcription factors such as Jun and Fos proteins are differentially expressed in adult neurons in vitro when compared to pathophysiological conditions in vivo such as nerve fibre transection (axotomy or rhizotomy). Moreover, the comparison between the explantation and culture experiments suggests that it is the complete axotomy of neurons that provokes those expression patterns found in neuronal cultures of adult neurons. The rapid and persisting expression of c-Jun during neurite extension and apoptosis points at the activation of a pivotal program that might be determined by the presence or absence of ATF-2 and that is involved in regeneration or degeneration.

Transient presence and functional interaction of endogenous GABA and GABAA receptors in developing rat optic nerve.

GABA (gamma-aminobutyric acid) is a major inhibitory synaptic neurotransmitter with widespread distribution in the central nervous system (CNS). GABA can also modulate axonal excitability by activation of GABAA receptors in CNS white matter regions where synapses and neuronal cell bodies are not present. Studies on cultured glia cells have revealed the synthesis of GABA in rat optic nerve O-2A progenitor cells that give rise to oligodendrocytes and type 2 astrocytes in vitro. We report here that: (i) GABA is detected by immuno-electron microscopy in intact rat optic nerve and is localized to glia and pre-myelinated axons during the first few weeks of postnatal development, but is markedly reduced or absent in the adult; and (ii) neonatal optic nerve is depolarized by GABAA receptor agonists or by the inhibition of GABA uptake. These results demonstrate the presence of functional GABAA receptors, and GABA uptake and release mechanisms in developing rat optic nerve, and suggest that excitability of developing axons can be modulated by endogenous neurotransmitter at non-synaptic sites.

Action potential conduction and sodium channel content in the optic nerve of the myelin-deficient rat.

Compound action potential (CAP) conduction and Na+ channel content were studied in optic nerves from control and myelin-deficient (md) rats. Action potential propagation was approximately five times slower in the md rat, but the action potentials propagated securely and had frequency-following and refractory properties equivalent to control myelinated axons. Tritium-labelled saxitoxin ([3H]-STX) binding in md optic nerve was approximately 30% greater, per wet mass of tissue, than in the control optic nerve. However, calculations of channel density per axon based on previously published anatomical data from md and control optic nerves (Dentinger et al. 1985) show an equivalent number of sodium channels per axon, with an average density of 10 channels micron-2 in md and 11 channels micron-2 in control optic nerve axons. The amplitude of the CAP in both control and md optic nerves was significantly attenuated by 50 nM TTX, precluding the possibility that TTX-insensitive channels are responsible for the action potential in myelinated or amyelinated axons. In addition, the amplitudes of voltage-activated Na+ currents in type I and type II astrocytes cultured from control and md optic nerves were similar, suggesting that the glial component of Na+ channels is not abnormal in the optic nerve of the md rat. These results suggest that myelination (or its absence) may not directly regulate the number of axonal Na+ channels.