Excitable Membrane Properties Research Articles

Immunohistological and Ultrastructural Study of the Inflammatory Response to Perforated Polyimide Cortical Implants: Mechanisms Underlying Deterioration of Electrophysiological Recording Quality.

The deterioration of field potential (FP) recording quality and yield by in vivo multielectrode arrays (MEA) within days to weeks of implantation severely limits progress in basic and applied brain research. The prevailing hypothesis is that implantation of MEA platforms initiate and perpetuate inflammatory processes which culminate in the formation of scar tissue (the foreign body response, FBR) around the implant. The FBR leads to progressive degradation of the recording qualities by displacing neurons away from the electrode surfaces, increasing the resistance between neurons (current source) and the sensing pads and by reducing the neurons’ excitable membrane properties and functional synaptic connectivity through the release of pro-inflammatory cytokines. Meticulous attempts to causally relate the cellular composition, cell density, and electrical properties of the FBR have failed to unequivocally correlate the deterioration of recording quality with the histological severity of the FBR. Based on confocal and electron microscope analysis of thin sections of polyimide based MEA implants along with the surrounding brain tissue at different points in time after implantation, we propose that abrupt FP amplitude attenuation occurs at the implant/brain-parenchyma junction as a result of high seal resistance insulation formed by adhering microglia to the implant surfaces. In contrast to the prevailing hypothesis, that FP decrease occurs across the encapsulating scar of the implanted MEA, this mechanism potentially explains why no correlations have been found between the dimensions and density of the FBR and the recording quality. Recognizing that the seal resistance formed by adhering-microglia to the implant constitutes a downstream element undermining extracellular FP recordings, suggests that approaches to mitigate the formation of the insulating glial could lead to improved recording quality and yield.

Serum ionic dysequilibria in clinical opioid dependence: Cross-sectional and longitudinal studies.

Despite an increasing awareness that the activity of excitable membranes is determined by the underlying ionic gradients across them, and their importance in drug dependency, we were not able to identify any reports of comparing the electrolyte composition of opioid-dependent and non-addicted controls. Linear regression was used to compare clinical pathology blood results taken from 2699 opioid-dependent patients (ODP) and 5307 medical control (MC) patients on a total of 21,734 occasions for the period 1995-2015. The presence of a hepatitis C antibody test was used to separate OPD and MC patients. The mean age among ODP and MC was 33.51 ± 0.16 and 37.99 ± 0.23 years, respectively ( p < 0.0001). The groups were 71.5% and 54.2% male ( p < 0.0001). Drug use in this cohort has been reported previously. Analysis of sodium, haemoglobin and albumin were used to exclude marked effects of haemodilution/haemoconcentration. Repeated measures linear regression against age and time showed depressed levels of bicarbonate ( p < 0.0001) and potassium ( p < 0.05) and elevated levels of chloride ( p < 0.025) and anions ( p < 0.01) in ODP in both sexes. Multiple regression in mixed-effects models showed that these effects were all worse in females ( p = 0.0001). This data shows that opioid dependence is associated with significant changes in chloride, potassium, bicarbonate and anions in both sexes, and worse in females. This likely has implications for the electrophysiological properties of excitable membranes. It is consistent with the reported impairment of potassium-chloride exchangers in opioid dependence. Explication of the mechanisms responsible must await further studies.

Spinal neurons require Islet1 for subtype-specific differentiation of electrical excitability.

BackgroundIn the spinal cord, stereotypic patterns of transcription factor expression uniquely identify neuronal subtypes. These transcription factors function combinatorially to regulate gene expression. Consequently, a single transcription factor may regulate divergent development programs by participation in different combinatorial codes. One such factor, the LIM-homeodomain transcription factor Islet1, is expressed in the vertebrate spinal cord. In mouse, chick and zebrafish, motor and sensory neurons require Islet1 for specification of biochemical and morphological signatures. Little is known, however, about the role that Islet1 might play for development of electrical membrane properties in vertebrates. Here we test for a role of Islet1 in differentiation of excitable membrane properties of zebrafish spinal neurons.ResultsWe focus our studies on the role of Islet1 in two populations of early born zebrafish spinal neurons: ventral caudal primary motor neurons (CaPs) and dorsal sensory Rohon-Beard cells (RBs). We take advantage of transgenic lines that express green fluorescent protein (GFP) to identify CaPs, RBs and several classes of interneurons for electrophysiological study. Upon knock-down of Islet1, cells occupying CaP-like and RB-like positions continue to express GFP. With respect to voltage-dependent currents, CaP-like and RB-like neurons have novel repertoires that distinguish them from control CaPs and RBs, and, in some respects, resemble those of neighboring interneurons. The action potentials fired by CaP-like and RB-like neurons also have significantly different properties compared to those elicited from control CaPs and RBs.ConclusionsOverall, our findings suggest that, for both ventral motor and dorsal sensory neurons, Islet1 directs differentiation programs that ultimately specify electrical membrane as well as morphological properties that act together to sculpt neuron identity.

Patterned expression of ion channel genes in mouse dorsal raphe nucleus determined with the Allen Mouse Brain Atlas

The dorsal raphe nucleus (DR) is the major source of serotonin (5-hydroxytryptamine, 5-HT) in the forebrain and dysfunction of this midbrain structure is implicated in affective disorders. The DR is composed of several types of 5-HT and non-5-HT neurons and their excitable-membrane properties are heterogeneous and overlapping. In order to understand how these properties may be generated, we examined the mRNA expression patterns of voltage- and ligand-gated ion channels in the DR using the Allen Mouse Brain Atlas. Since DR cytoarchitecture is organized with respect to the midline, we sought to identify genes that were expressed in a pattern with respect to the midline, either enriched or depleted, rather than those that were homogenously expressed throughout the DR. Less than 10% of the screened genes for voltage-gated ion channels showed patterned expression within the DR. Identified genes included voltage-gated sodium channel beta subunits, potassium channels, P/Q-, N-type calcium channels, as well as the alpha2/delta-1 calcium channel. Several voltage-gated chloride channels were also identified, although these may function within intracellular compartments. Of the ligand-gated ion channels examined, 20% showed patterned expression. These consisted primarily of glutamate and GABA-A receptor subunits. The identified genes likely contribute to unique excitable properties of different groups of neurons in the DR and may include novel pharmacologic targets for affective disorders.

Exposing Gaps in Linear Radiofrequency Lesions

The termination of atrial fibrillation (AF) by radiofrequency ablation in the left atrium is undeniable evidence that the pathophysiology underlying AF can be ameliorated. The cornerstone of this therapy, pulmonary vein isolation, can be achieved acutely in virtually 100% patients.1,2 Recurrent AF is the bane of this procedure, however, and is most commonly attributed to recovery of pulmonary vein conduction through the ablation lines.2 Return of conduction across ablation lines to the mitral isthmus and the left atrial roof contribute to postablation arrhythmias as well. Intensive investigation of sophisticated ablation systems have failed to yield consistently durable ablation lines. Efforts to map and eliminate conductive gaps at the time of ablation have not resolved this problem. Gaps in linear ablation lines for AF ablation are a reality of current clinical practice. As a result, the single procedure success rate for AF ablation is constrained. It appears that a major advance in ablation and/or mapping technologies is needed to improve the state of catheter ablation for AF. Article see p 279 Despite the clinical problems they produce, gaps within ablation lines represent fascinating examples of myocardial electrophysiology. A brief review is relevant to placing new studies in perspective. The animal model of the myocardial gap was introduced almost 100 years ago by Garrey3 to study fibrillatory conduction patterns over an isthmus of myocardial tissue. With the advent of linear surgical and catheter-based ablation for AF, the model has been used extensively to reveal the complex conduction behavior associated with discontinuous linear lesions.4–10 Direct experimentation has benefited from computer modeling to provide deeper insights into gap conduction.11,12 A description of gap conduction invokes all 3 determinants of myocardial impulse propagation: (1) passive tissue electric properties, (2) excitable membrane properties, and (3) …

Calpain Inhibitors Alter the Excitable Membrane Properties of CulturedAplysiaNeurons

The calpain superfamily of calcium-dependent papain-like cysteine proteases constitutes highly conserved proteases that function to posttranslationally modify substrates by partial proteolysis. Calpains are known to proteolyze >100 substrates that lack strong sequence homology. Consequently, the calpain superfamily has been implicated in playing a central role in diverse physiological and pathological processes. Investigation of the physiological functions of calpains, on the one hand, and the need to develop pharmacological reagents to inhibit calpain-mediated pathological processes, on the other hand, led to the development of numerous calpain inhibitors. Using cultured Aplysia neurons and voltage-clamp analysis, we report here that the calpain inhibitors calpeptin, MG132, and the calpain inhibitor XII inhibit voltage-gated potassium conductance and moderately reduce the sodium conductance. These consequently lead to spike broadening and increased calcium influx. Such alterations of the excitable membrane properties may alter the normal patterns of neuronal and muscle electrical activities and thus should be taken into account when evaluating the effects of calpain inhibitors as protective/therapeutic drugs and as research tools.

Calcium-mediated spine stem restructuring

A spine is a protrusion from the dendritic (or somatic) surface of a neuron. In recent experiments, caffeine-induced calcium released from internal stores was shown to cause elongation of dendritic spine stems in slice cultures. Still another experiment indicates that glutamate-induced increases in calcium may cause spine stem shortening. Harris draws a schematic model to explain these seemingly conflicting results, indicating that a small amount of activity may increase free calcium within the spines and cause spine stem elongation, but an excessive amount of activity may increase intraspine calcium beyond a critical level and cause spine stem shortening (see [1, Figure 2]). This paper develops a mathematical model for a fixed population of spines along the dendrite, each with a dynamic structure and calcium level. The system is integrated over time and space to observe an interdependent relationship between calcium, morphology and chemical/electrical activity. Results of simulation qualitatively capture phenomena observed in recent experiments and exhibit periodic oscillations in potential when the spines have excitable membrane properties by allowing spine structure to transition through threshold geometries for generation of action potentials in a bidirectional manner. As in recent experiments, a variety of chemical and structural profiles emerge, depending on membrane properties, patterns of synaptic input, and initial conditions considered.

Electrophysiological Basics of Neuromuscular Stimulation

BACKGROUND: Since neuromonitoring of motor nerve function makes use of the electrical excitability of nerve and muscle tissue, this article provides a short overview of the basic electrophysiological properties of excitable membranes. METHODS: Relevant recent knowledge of the distribution of functional ion channels in the mammalian myelinated axon and in the muscle is outlined. Its implications for electrical stimulation and recording of motor unit potentials are discussed. RESULTS: Electrical stimulation of motor nerves induces compound action potentials in the supplied muscle that may be used to identify a nerve and to control its integrity; evaluation of waveform, amplitude and latency of the muscle potential allows assessing details of functional impairment. CONCLUSIONS: During surgery, recording conditions have to be adapted to surgical needs, and erroneous results might therefore be facilitated. An understanding of the neurophysiological basics helps to detect and exclude possible sources of error.

Selective regulation of xSlo splice variants during Xenopus embryogenesis.

Calcium-activated potassium channels regulate excitability of the adult nervous system. In contrast, little is known about the contribution of calcium-activated potassium channels to excitability of the embryonic nervous system when electrical membrane properties and intracellular calcium levels show dramatic changes. Embryonic Xenopus spinal neurons exhibit a well-characterized developmental program of excitability that involves several different currents including calcium-activated ones. Here, we show that a molecular determinant of calcium-activated potassium channels, xSlo, is expressed during Xenopus embryogenesis even prior to differentiation of excitable tissues. Five different xSlo variants are expressed in embryonic tissues as a consequence of alternative exon usage at a single splice site. One of these variants, xSlo59, is neural-specific, and its expression is limited to late stages of neuronal differentiation. However, expression of the four other variants occurs in both muscle and neurons at all stages of development examined. Electrophysiological analysis of recombinant xSlo channels reveals that the xSlo59 exon serves as a gain-of-function module and allows physiologically relevant levels of membrane potential and intracellular calcium to activate effectively the resultant channel. These results suggest that xSlo59 channels play a unique role in sculpting the excitable membrane properties of Xenopus spinal neurons.

DENDRITIC INTEGRATION IN A RECURRENT NETWORK

Classical nonlinear cable theory is appropriate for the unmyelinated axonal membrane because voltage-dependent ion channels are densly distributed, but dendrites with a sparse density distribution of voltage-dependent ion channels show "weakly" excitable membrane properties. Therefore, a model for "weakly" active dendrites is presented by introducing voltage-dependent ion channels at discrete locations along the dendritic cable. This provides an alternative representation for the investigation of regenerative potentials in dendrites in order to explore how active dendrites influence synaptic integration. As an example, we consider a two-neuron recurrent network of biophysically distinct conductance-based model neurons with discrete clusters of persistent sodium channels. Analytical solutions, expressed in terms of a Volterra series expansion for the voltage in response to a suprathreshold input current at the soma of one neuron, are obtained to investigate dendritic spikes, and the effect of backpropagation on distal dendritic spike-like potentials.

Modeling Temporal Behavior of Postnatal Cat Retinal Ganglion Cells

During development, mammalian retinal ganglion cells (RGCs) go through marked ontogenetic changes with respect to their excitable membrane properties. Voltage-clamp studies conducted in our laboratory have shown that the amplitude, voltage-dependence and kinetics of activation and inactivation (where present) of Na+, K+and Ca2+conductances all exhibit developmental changes during a time when the firing patterns of mammalian ganglion cells shift from being transient to being predominantly sustained in nature. In order to better understand the contribution of each conductance to the generation of spikes and spiking patterns, we have developed a model based on our experimental data. For simplicity, we have initially used experimental data obtained from postnatal ganglion cells. At this age the ontogenetic changes observed in the characteristics of the various ionic currents are complete. Utilizing the methods adopted by Hodgkin and Huxley for the giant squid axon, we have determined rate equations for the activation and inactivation properties of theIA , IK dr, INa, ICa L,ICa N , and Ileakcurrents in postnatal cat RGCs. Combining these with a simplified model of the calcium-activated potassium current (IKCa), we have solved and analysed the resulting differential equations. While spikes and spiking patterns resembling experimental data could be obtained from a model in which [Ca2+i] was averaged across the whole cell, more accurate simulations were obtained when the diffusion of intracellular Ca2+was modeled spatially. The resulting spatial calcium gradients were more effective in gating IKCa, and our simulations more accurately matched the recorded amplitude and shape of individual spikes as well as the frequency of maintained discharges observed in mammalian postnatal RGCs.

Dual Video Microscopic Imaging of Membrane Potential and Cytosolic Calcium of Immunoidentified Embryonic Rat Cortical Cells

Membrane potential (MP) and cytosolic Ca2+ (Ca2+c) constitute important components involved in the physiological regulation of a myriad of cell functions in eukaryotic organisms. In particular, during development of the central nervous system, both properties are thought to be important in the regulation of cell cycle, cell migration, cell differentiation, cell–cell communication, and naturally occurring cell death. However, obtaining insight into the precise relationship between these two parameters of cell function is relatively limited either by technical difficulties inherent in using electrical recordings of membrane properties in conjunction with optical imaging of single cells or by employing optical imaging of either one or another property alone. Here, we describe in detail a novel strategy to record changes in both MP and Ca2+c from many intact single cells in a noninvasive manner using digital video microscopy. This method involves double-loading the cells with voltage- and calcium-sensitive fluorescent indicator dyes, green oxonol, and fura-2, which can be sequentially excited with a mercury arc lamp filtered at appropriate wavelengths and their resulting emissions can be captured with an intensified charged-coupled device camera at 1-s intervals. As an example of the utility of dual-recording strategy, we present data on a distinct functional expression of excitable membrane and cytoplasmic calcium properties in proliferating and differentiating embryonic rat cerebral cortical cells.

Astrocytes regulate the developmental appearance of GABAergic and glutamatergic postsynaptic currents in cultured embryonic rat spinal neurons.

The effects of astrocytes on the emergence of synaptic transients and excitable membrane properties in cultured, embryonic, rat ventral spinal neurons were studied with electrical and optical recording techniques. Neurons on astrocytes had significantly longer neurites and an accelerated rate of growth in surface membrane during the initial 24 h in culture compared to neurons on poly-D-lysine (PDL). GABAergic (GABA, gamma-aminobutyric acid) and glutamatergic transients appeared spontaneously in co-cultured neurons by 24 h. GABAergic quanta did not appear in neurons on PDL until 4 days in culture, and glutamatergic transients did not emerge until 7 days in culture. Astrocyte-conditioned medium (ACM) partially mimicked the effects of direct astrocytic contact. GABAergic transients appeared by 2 days, and glutamatergic signals by 4 days in neurons on PDL exposed to ACM. All of the spontaneous, synaptic-like transients were eliminated by tetrodotoxin or Ca2+o-free saline, implicating voltage-dependent cation channels in their generation. Astrocytes immediately and significantly increased the density of voltage-dependent Na+ currents compared to neurons on PDL, but by the end of 24 h, Na+ current densities were identical. Electrophysiological and optical recording revealed comparable densities of high-voltage-activated (HVA) Ca2+ currents on both co-cultured neurons and neurons on PDL throughout the first week. However, neurons on astrocytes had significantly greater contributions of P/Q-type currents and lesser contributions of L-type currents beginning at 24 h and continuing for 7 days. The contribution of N-type current was significantly more in co-cultured neurons only at 24 h. Thus, in vitro, astrocytes help to differentiate specific excitable membrane properties in spinal neurons, along with GABAergic and glutamatergic forms of synaptic transmission.

Ion channel activity drives ion channel expression.

Ion channel activity drives ion channel expression.

A mathematical model of neural information processing at the cellular level

The basis for this neuronal model is that the properties of excitable membranes are controlled by biochemical reactions occurring in the nerve cells. The kinetics of these supposed chemical reactions is described by a set of first order differential equations. We considered the effect of regulation of the properties of sodium channels. This allows the simulation of the changes in the neuron's electrical activity parameters occurring during learning, associated with its excitability. The neuronal model exhibits different excitability after the learning procedure relative to the different input signals that corresponds to the experimental data.

A CNS Catecholaminergic Cell Line Expresses Voltage-gated Currents

CATH.a is a central nervous system (CNS) catecholaminergic cell line derived from a transgenic mouse carrying the SV40 T antigen oncogene under the transcriptional control of regulatory elements from the rat tyrosine hydroxylase gene (Suri et al., 1993). CATH.a cells express several differentiated neuronal characteristics including medium and light chain neurofilament proteins, synaptophysin, tyrosine hydroxylase, and dopamine beta-hydroxylase; they synthesize dopamine and norepinephrine. Conversely, they do not express glial-specific fibrillary acidic protein. To establish definitively that CATH.a cells are of neuronal origin, we characterized the repertoire of voltage-gated inward currents expressed by CATH.a cells. Such inward currents are necessary for neuronal excitability. We report that all CATH.a cells possess a tetrodotoxin-sensitive sodium current (peak amplitude = 590 +/- 319 pA) and 68% possess a high voltage-activated calcium current (peak amplitude = 175 +/- 67 pA). Pharmacological analyses suggest that individual cells express varying levels of L- and N-type calcium current, but no P-type current. In addition, in 55% of the cells with a calcium current, about a half of this current is resistant to selective antagonists for L- and N-type currents, suggesting that another calcium current exists in these CATH.a cells which is not L-, N-, or P-type. The heterogeneous pattern of current detected persisted in several CATH. a subclones, suggesting that factors other than genetic variability influence current expression. The demonstration that CATH.a cells express these currents indicates that they have excitable membrane properties characteristic of neurons. Although many peripheral nervous system (PNS) cell lines exist, very few CNS cell lines with differentiated neuronal properties exist. Since the CATH.a cells can be grown continuously in large amounts, they may be useful for purifying, characterizing, and/or cloning various neuronal-specific molecules and thereby may add to our understanding of CNS catecholaminergic neurons.

Properties of K+ Conductances in Cat Retinal Ganglion Cells During the Period of Activity‐mediated Refinements in Retinofugal Pathways

During ontogeny retinal ganglion cells manifest pronounced changes in excitable membrane properties. To further our understanding of the ionic conductances underlying such functional changes, the whole-cell voltage-clamp variation of the patch-clamp technique was used to record potassium currents in 220 ganglion cells dissociated from cat retinas ranging in age from embryonic day 31 to postnatal day 10. Potassium currents were isolated by blocking voltage-gated Na+ and Ca2+ currents with tetrodoxin (TTX) and CoCl2 respectively and were characterized by their pharmacology, kinetics and voltage-dependence of activation and inactivation. In all cases, a combination of three currents accounted for the total outward calcium-independent K+ current: (i) a steady linear conductance; (ii) a voltage-gated transient current, IA, and (iii) a voltage-gated sustained current, IK. Both voltage-gated currents were affected by the application of 4-aminopyridine and tetraethylammonia (TEA): IA showed a greater sensitivity to 4-aminopyridine, while IK was more sensitive to TEA. Both voltage-gated currents were present throughout the developmental period examined; however, the percentage of retinal ganglion cells (RGCs) expressing IA showed a marked decline from 82% at E31 to 45% at postnatal ages. During this developmental period there was an increase in the density of the two voltage-gated and the linear conductance. Additionally, with maturation, significantly slower inactivation kinetics were observed for IK. These findings, and our previous results dealing with maturational changes in the TTX-sensitive voltage-gated Na current, are related to the generation of excitability in developing retinal ganglion cells. Furthermore, the presence of cells with and without transient K+ conductance throughout development suggests that the different spiking patterns observed in RGC classes may be partially due to differences in their membrane properties.

The Survival of Transected Axonal Segments of Cultured Aplysia Neurons is Prolonged by Contact with Intact Nerve Cells

Axonal segments transected from their cell body in vivo commonly undergo degeneration within 3-4 days (Wallerian degeneration). In lower vertebrates and invertebrates, however, some transected axonal segments survive for long periods ranging between 30 and 200 days. To circumvent the technical complications of studying the mechanisms underlying long-term survival of transected axons in vivo, we developed an in vitro system. We found previously that isolated axonal segments of cultured Aplysia neurons preserved their morphological integrity for an average duration of 7.6 days (range 2-14 days) and maintain their passive and excitable membrane properties. This survival occurred in the absence of de novo protein synthesis. In the present study we examined the influence of homologous neurons on the survival of transected axonal segments. We found that the average survival time of transected axons was doubled when co-cultured in physical contact with intact homologous neurons (average 15.3 days, range 2-27 days). During this period, the transected axons extended neurites, maintained normal passive and excitable membrane properties, formed electrotonic junctions with the intact neurons and maintained normal free intracellular Ca2+ levels. Consistent with these observations, electron micrographs of the transected axon revealed that the cytoskeletal elements of the axon appeared normal even 20 days after transection. In contrast, the mitochondria and smooth endoplasmic reticulum appeared damaged. As the prolonged survival was conditional on physical contact between the transected axon and the surrounding intact neurons, we suggest that the prolongation of survival time is promoted by the direct transfer of material from the intact neurons to the transected axon. However, co-culture of transected axons with homologous neurons did not fully mimic in vivo conditions, in which transected axons can survive for several months.

Bioferroelectricity: Modeling the transitions of the sodium channel

Abstract A thermodynamic model for a phase transition in sodium channels between a ferroelectric “closed” phase and an ion-conducting “open” phase is discussed. This model implies Curie-Weiss behavior in the dielectric properties of excitable membranes, and this is consistent with data from squid axon. The question as to whether a ferroelectric phase as small as a macromolecule of 270 kD can exist is analyzed. A molecular interpretation of the transitions is given in terms of helix-coil transitions. “True relations may exist between ferroelectricity, the formation of liquid crystals, and the generation of electric impulses in nerves and muscles.”—A. R. von Hippel, 1970

Prenatal development of excitability in cat retinal ganglion cells: action potentials and sodium currents

The development of precise retinofugal projections is dependent on activity-mediated events, but as yet nothing is known about the ontogeny of excitable membrane properties in retinal ganglion cells (RGCs). In order to begin to understand how functional maturity is attained in these neurons, whole-cell patch-clamp recordings were obtained from acutely dissociated RGCs of fetal and postnatal timed-pregnant cats. Current-clamp recordings revealed a pronounced developmental increase in the proportion of RGCs capable of generating action potentials. At embryonic day 30 (E30), 5 weeks before birth and during a time when RGCs are still being generated, electrical stimulation elicited spikes in only a third of the cells. None of these neurons were capable of multiple discharges in response to maintained depolarization. The proportion of spiking neurons increased during ontogeny, such that by E55 all RGCs could be induced to generate action potentials, with the majority manifesting repetitive spiking patterns. Application of tetrodotoxin abolished spike activity of all fetal RGCs, indicating that sodium-mediated action potentials are present very early in development. At the same time, voltage-clamp recordings revealed significant ontogenetic modifications in several key properties of the sodium currents (INa). These were (1) a twofold increase in Na current densities; (2) a shift in the voltage dependence of both activation and steady state inactivation: with maturity, sodium currents activate at more negative potentials, while steady state inactivation of INa occurs at less negative potentials; and (3) a decrease in decay time constants of the Na current, at membrane potentials negative to -15 mV. These developmental changes were largely restricted to the period of axon ingrowth (E30-E38), suggesting that maturation of INa is not the limiting factor for the onset of activity-dependent restructuring of retinofugal projections.