PubMed Health⌕ Search

Biomedical subjects

E F Barrett

Publications and source records attributed to E F Barrett.

At least 19 recordsLinked to original sources
PubMed2001

Bone morphogenetic proteins (BMP6 and BMP7) enhance the protective effect of neurotrophins on cultured septal cholinergic neurons during hypoglycemia.

The effects of two bone morphogenetic proteins (BMP6, BMP7), alone and in combination with neurotrophins, were tested on cultures of embryonic day 15 rat septum. A week-long exposure to BMP6 or BMP7 in the optimal concentration range of 2-5 n M increased the activity of choline acetyltransferase (ChAT) by 1.6-2-fold, in both septal and combined septal-hippocampal cultures. The increase in ChAT activity reached significance after 4 days and continued to increase over an 11-day exposure. Under control culture conditions neither BMP significantly altered the number of cholinergic neurons, and BMP effects on ChAT activity were less than linearly additive with those of nerve growth factor. The effects of BMPs and BMP + neurotrophin combinations were also assayed under two stress conditions: low-density culture and hypoglycemia. In low-density cultures BMPs and BMP + neurotrophin combinations preserved ChAT activity more effectively than neurotrophins alone. During 24 h hypoglycemic stress, BMPs alone did not preserve ChAT activity, but BMP + neurotrophin combinations preserved ChAT activity much more effectively than neurotrophins alone. These results demonstrate that BMP6 and BMP7 enhance ChAT activity under control and low-density stress conditions, and that during a hypoglycemic stress their trophic effect requires and complements that exerted by neurotrophins.

Animals↗
PubMed2000

Stimulation-evoked increases in cytosolic [Ca(2+)] in mouse motor nerve terminals are limited by mitochondrial uptake and are temperature-dependent.

Increases in cytosolic [Ca(2+)] evoked by trains of action potentials (20-100 Hz) were recorded from mouse and lizard motor nerve terminals filled with a low-affinity fluorescent indicator, Oregon Green BAPTA 5N. In mouse terminals at near-physiological temperatures (30-38 degrees C), trains of action potentials at 25-100 Hz elicited increases in cytosolic [Ca(2+)] that stabilized at plateau levels that increased with stimulation frequency. Depolarization of mitochondria with carbonylcyanide m-chlorophenylhydrazone (CCCP) or antimycin A1 caused cytosolic [Ca(2+)] to rise to much higher levels during stimulation. Thus, mitochondrial Ca(2+) uptake contributes importantly to limiting the rise of cytosolic [Ca(2+)] during repetitive stimulation. In mouse terminals, the stimulation-induced increase in cytosolic [Ca(2+)] was highly temperature-dependent over the range 18-38 degrees C, with greater increases at lower temperatures. At the lower temperatures, application of CCCP continued to depolarize mitochondria but produced a much smaller increase in the cytosolic [Ca(2+)] transient evoked by repetitive stimulation. This result suggests that the larger amplitude of the stimulation-induced cytosolic [Ca(2+)] transient at lower temperatures was attributable in part to reduced mitochondrial Ca(2+) uptake. In contrast, the stimulation-induced increases in cytosolic [Ca(2+)] measured in lizard motor terminals showed little or no temperature-dependence over the range 18-33 degrees C.

Action Potentials↗
PubMed2000

Brief exposure to neurotrophins produces a calcium-dependent increase in choline acetyltransferase activity in cultured rat septal neurons.

We demonstrate that brief (30-min) exposure of cultured embryonic rat septal neurons to neurotrophins (NTs) increases choline acetyltransferase (ChAT) activity by 20-50% for all tested NTs (nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4, each at 100 ng/ml). The increase in ChAT activity was first detected 12 h after NT exposure, persisted at least 48 h, and was not mediated by increased neuronal survival or action-potential activity. Under some conditions, the response to brief NT exposure was as great as that produced by continuous exposure. NT stimulation of ChAT activity was inhibited by inhibitors of p75- and Trk kinase-mediated signaling, by removal of extracellular Ca2+ during the period of NT exposure, and by buffering intracellular Ca2+ with BAPTA. Application of nerve growth factor and brain-derived neurotrophic factor transiently increased [Ca2+] within a subpopulation of neurons. NT stimulation of ChAT activity was not affected significantly by cyclic AMP agonists or antagonists. These findings suggest that brief exposure to NTs can have a long-lasting effect on cholinergic transmission, and that this effect requires Ca2+, but not cyclic AMP.

Animals↗
PubMed1998

Evidence that mitochondria buffer physiological Ca2+ loads in lizard motor nerve terminals.

1. Changes in cytosolic and mitochondrial [Ca2+] produced by brief trains of action potentials were measured in motor nerve terminals using a rapidly scanning confocal microscope. Cytosolic [Ca2+] was measured using ionophoretically injected Oregon Green BAPTA 5N (OG-5N). Mitochondrial [Ca2+] was measured using rhod-2, bath loaded as dihydrorhod-2. 2. In response to 100-250 stimuli at 25-100 Hz the average cytosolic [Ca2+] showed an initial rapid increase followed by a much slower rate of increase. Mitochondrial [Ca2+] showed no detectable increase during the first fifteen to twenty stimuli, but after this initial delay also showed an initially rapid rise followed by a slower rate of increase. The onset of the increase in mitochondrial [Ca2+] coincided with the slowing of the rate of rise of cytosolic [Ca2+]. The peak levels of cytosolic and mitochondrial [Ca2+] both increased with increasing frequencies of stimulation. 3. When stimulation terminated, the initial rate of decay of cytosolic [Ca2+] was much more rapid than that of mitochondrial [Ca2+]. 4. After addition of carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 1-2 microM) to dissipate the proton electrochemical gradient across the mitochondrial membrane, cytosolic [Ca2+] rose rapidly throughout the stimulus train, reaching levels much higher than normal. CCCP inhibited the increase in mitochondrial [Ca2+]. 5. These results suggest that mitochondrial uptake of Ca2+ contributes importantly to buffering presynaptic cytosolic [Ca2+] during normal neuromuscular transmission.

Action Potentials↗
PubMed1997

Stimulation-induced changes in [Ca2+] in lizard motor nerve terminals.

1. Motor axons were injected ionophoretically with one of five Ca(2+)-sensitive dyes (fluo-3, Calcium Green-2, Calcium Green-5N, fluo-3FF and Oregon Green BAPTA-5N). Changes in fluorescence (delta F/Frest) within motor terminal boutons following a single action potential and brief stimulus trains were monitored with high temporal resolution using a confocal microscope. 2. Stimulation-induced increases in delta F/Frest were confined primarily to boutons, with roughly uniform increases in all the boutons of a terminal. The increase in delta F/Frest began prior to, and decayed more slowly than, the endplate potential (EPP) recorded in the underlying muscle fibre. delta F/Frest was graded with bath [Ca2+]. Both delta F/Frest and the EPP were reduced, but not eliminated, by omega-conotoxin GVIA (5-10 microM). 3. For dyes with lower affinity for Ca2+ (e.g. Oregon Green BAPTA-5N, Kd approximately 60 microM) stimulation-induced increases in delta F/Frest were measured in the presence of the K+ channel blocker 3,4-diaminopyridine (3,4-DAP, 100 microM). During brief stimulus trains (4 at 50 Hz) in 3,4-DAP, the EPP exhibited profound depression, but the fluorescence increase associated with each stimulus showed little decrement, suggesting that depression was not mediated by a reduction in Ca2+ entry. 4. For dyes with a higher affinity for Ca2+ (e.g. fluo-3, Kd approximately 0.5-1 microM) stimulation-induced increases in delta F/Frest could also be measured in normal physiological saline. Increases in delta F/Frest were much greater with 3,4-DAP present, but the amplitude decreased with successive stimuli due to partial dye saturation. 5. Calculations suggested that following a single action potential the average [Ca2+] within a bouton increased by up to 150 nM in normal saline and 940 nM in 3,4-DAP. With low affinity dyes the delta F/Frest measured near the membrane had a higher peak amplitude and a faster early decay than that measured in the centre of the bouton, suggesting that substantial spatial [Ca2+] gradients exist within boutons for at least 15 ms following stimulation.

Action Potentials↗
PubMed1997

Spatiotemporal gradients of intra-axonal [Na+] after transection and resealing in lizard peripheral myelinated axons.

1. Post-transection changes in intracellular Na+ ([Na+]i) were measured in lizard peripheral axons ionophoretically injected with the Na(+)-sensitive ratiometric dye, sodium-binding benzofuran isophthalate (SBFI). 2. Following axonal transection in physiological saline [Na+]i increased to more than 100 mM in a region that quickly extended hundreds of micrometers from the transection site. This post-transection increase in [Na+]i was similar when the bath contained 5 microM tetrodotoxin, but was absent in Na(+)-free solution. Depolarization of uncut axons in 50 mM K+ produced little or no elevation of [Na+]i until veratridine was added. These results suggest that the post-transection increase in [Na+]i was due mainly to Na+ entry via the cut end, rather than via depolarization-activated Na+ channels. 3. The spatiotemporal profile of the post-transection increase in [Na+]i could be accounted for by movement of Na+ from the cut end with an apparent diffusion coefficient of 1.3 x 10(-5) cm2 s-1. 4. [Na+]i began to decline toward resting levels by 20 +/- 15 min (mean +/- S.D.) post-transection, except in regions of the axon within 160 +/- 85 microns of the transection site, where [Na+]i remained high. The boundary between axonal regions in which [Na+]i did or did not recover probably defines a locus of resealing of the axonal membrane. 5. [Na+]i returned to resting values within about 1 h after resealing, even in axonal regions where the normal transmembrane [Na+] gradient had completely dissipated. The recovery of [Na+]i was faster and reached lower levels than expected by diffusional redistribution of Na+ along the axon. Partial recovery occurred even in an isolated internode, indicating that the internodal axolemma can actively extrude Na+.

Animals↗
PubMed1996

Neurotrophin effects on survival and expression of cholinergic properties in cultured rat septal neurons under normal and stress conditions.

These studies tested the hypothesis that survival-promoting effects of neurotrophins on basal forebrain cholinergic neurons are enhanced under stress. Septal neurons from embryonic day 14-15 rats exposed for 10-14 d to neurotrophin [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or neurotrophin-4 (NT-4), each at 100 ng/ml] showed a two- to threefold increase in choline acetyltransferase (ChAT) activity, with little evidence of synergistic interactions. Neurotrophins produced no significant increase in the survival of total or acetylcholinesterase (AChE)-positive neurons at moderate plating density (1200-1600 cells/mm2). However, with very low plating densities (2-28 cells/mm2) BDNF, NT-3, and NT-4 (but not NGF) increased total neuronal survival, and BDNF increased survival of AChE-positive neurons. NGF and BDNF enhanced ChAT activity and survival of cholinergic neurons after a 24 hr hypoglycemic stress, even when added 1 hr after stress onset. All four tested neurotrophins increased total neuronal survival after hypoglycemic stress. These results suggest that neurotrophins are important for preservation of central cholinergic function under stress conditions, with different neurotrophins protecting against different stresses. The stress-associated survival-promoting effects of neurotrophins were not limited to the cholinergic subpopulation.

Animals↗
PubMed1995

Electrical and morphological factors influencing the depolarizing after-potential in rat and lizard myelinated axons.

1. Intra-axonal recording and electron microscopy were applied to intramuscular myelinated axons in lizards and rats to investigate factors that influence the amplitude and time course of the depolarizing after-potential. 2. Depolarizing after-potentials in lizard axons had larger peak amplitudes and longer half-decay times than those recorded in rat axons (mean values 10 mV, 35 ms in lizard; 3 mV, 11 ms in rat). These differences were not due to differences in temperature, resting potential or action potential amplitude or duration. 3. For a given axon diameter, the myelin sheath in lizard fibres was thinner and had fewer wraps than in rat fibres. There was no significant difference in myelin periodicity. Calculations suggest that the thinner myelin sheath accounts for < 30% of the difference between depolarizing after-potential amplitudes recorded in lizard and rat axons. 4. Consistent with a passive charging model for the depolarizing after-potential, the half-time of the passive voltage transient following intra-axonal injection of current was shorter in rat than in lizard axons. 5. Aminopyridines prolonged the falling phase of the action potential and increased the amplitude of the depolarizing after-potential in both types of axon. 6. During repetitive stimulation the depolarizing after-potentials following successive action potentials exhibited little or no summation. Axonal input conductance in the interspike interval increased during the train. 7. These findings suggest that the amplitude and time course of the depolarizing after-potential are influenced not only by the passive properties of the axon and myelin sheath, but also by persisting activation of axolemmal K+ channels following action potentials.

Action Potentials↗
PubMed1993

Reversibility of nerve growth factor's enhancement of choline acetyltransferase activity in cultured embryonic rat septum.

Choline acetyltransferase (ChAT) activity and survival of acetylcholinesterase (AChE)-positive neurons were measured in low-density cultures of embryonic (Day 14-15) rat septum exposed to various sequences of nerve growth factor (NGF) exposure and deprivation for up to 7 weeks in vitro. Most septal cultures grown 4-5 weeks with no exogenous NGF (including exposure to monoclonal or polyclonal antibodies against NGF) retained both a basal ChAT activity and the ability to increase ChAT activity in response to subsequently added NGF. When cultures were exposed to NGF (7S, 0.75 nM) for 2-3 weeks and then deprived of NGF for 2 weeks, ChAT activity fell gradually, but the number of AChE-positive neurons remained unchanged, and in many cases ChAT activity could be restored by subsequent re-exposure to NGF. Thus NGF's enhancement of ChAT activity in embryonic septal neurons in vitro is largely reversible and is not mediated by differential survival of cholinergic neurons.

Animals↗
PubMed1993

Activation of internodal potassium conductance in rat myelinated axons.

1. Voltage changes associated with currents crossing the internodal axolemma were monitored using a microelectrode inserted into the myelin sheath (peri-internodal region) of rat phrenic nerve fibres. This microelectrode was also used to change the potential and the ionic environment in the peri-internodal region. 2. Following stimulation of the proximal nerve trunk, the peri-internodal electrode recorded a positive-going action potential whose amplitude increased (up to 75 mV) with increasing depth of microelectrode penetration into the myelin. The resting potential recorded by the peri-internodal electrode remained within 4 mV of bath ground. 3. Confocal imaging of fibres injected peri-internodally with the fluorescent dye Lucifer Yellow revealed a staining pattern consistent with spread of dye throughout the myelin sheath of the injected internode. 4. After ionophoresis of K+ (but not Na+) into the peri-internodal region, the action potential was followed by a prolonged negative potential (PNP) lasting hundreds of milliseconds to several seconds. The duration of the PNP increased as the frequency of stimulation decreased. PNPs could also be evoked by sub-threshold depolarization of the internodal axolemma with peri-internodally applied current pulses. In the absence of action potentials or applied depolarization PNPs sometimes appeared spontaneously. 5. Peri-internodal application of Rb+ also produced evoked and spontaneous PNPs. These PNPs had longer durations (up to 20 s) than those recorded from K(+)-loaded internodes. 6. Spontaneous action potentials sometimes appeared during the onset of the PNP, suggesting that PNPs are associated with depolarization of the underlying axon. 7. Passage of current pulses during the PNP demonstrated that the PNP is associated with an increased conductance of the pathway linking the peri-internodal recording site to the bath. At least part of this conductance increase occurs across the internodal axolemma, since peri-internodally recorded action potentials evoked during the PNP had larger amplitudes than those evoked before or after the PNP. 8. PNPs were suppressed by tetraethylammonium (TEA, 10-20 mM) and by 4-aminopyridine (1 mM). 9. These results suggest that the PNPs recorded in K(+)- or Rb(+)-loaded myelin sheaths are produced by a regenerative K+ or Rb+ current that enters the internodal axolemma via K+ channels opened by action potentials or subthreshold depolarizations. 10. When normal extracellular [K+] was preserved (by using Na+ rather than K+ salts in the peri-internodal electrode), action potentials recorded within the myelin sheath were instead followed by a brief, positive after-potential that was inhibited by TEA.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1993

Posttetanic hyperpolarization produced by electrogenic Na(+)-K+ pump in lizard axons impaled near their motor terminals.

1. The hyperpolarization that follows tetanic stimulation was recorded intra-axonally from the internodal region of intramuscular myelinated motor axons. 2. The peak amplitude of the posttetanic hyperpolarization (PTH) that followed stimulation at 20-100 Hz for < or = 35 s increased with increasing train duration, reaching a maximum of 22 mV. PTH decayed over a time course that increased from tens to hundreds of seconds with increasing train duration. For a given frequency of stimulation the time integral of PTH was proportional to the number of stimuli in the train, averaging 3-4 mV.s per action potential. 3. Ouabain (0.1-1 mM) and cyanide (1 mM) depolarized the resting potential and abolished PTH. Tetanic stimulation in ouabain was followed by a slowly decaying depolarization (probably due to extra-axonal K+ accumulation) whose magnitude and duration increased as the duration of the train increased. 4. Axonal input resistance showed no consistent change during PTH in normal solution but increased during PTH in the presence of 3 mM Cs+ (which blocks axonal inward rectifier currents). 5. PTH was abolished when bath Na+ was replaced by Li+ or choline. PTH persisted after removal of bath Ca2+ and addition of 2 mM Mn2+. 6. Removal of bath K+ abolished the PTH recorded after brief stimulus trains and greatly reduced the duration of PTH recorded after longer stimulus trains. 7. A brief application of 10 mM K+, which normally depolarizes axons, produced a ouabain-sensitive hyperpolarization in axons bathed in K(+)-free solution. 8. These observations suggest that in these myelinated axons PTH is produced mainly by activation of an electrogenic Na(+)-K(+)-ATPase, rather than by changes in K+ permeability or transmembrane [K+] gradients. This conclusion is supported by calculations showing agreement between estimates of Na+ efflux/impulse based on PTH measurements and estimates of Na+ influx/impulse based on nodal voltage-clamp measurements. Pump activity also appears to contribute to the resting potential. 9. The stimulus intensity required to initiate a propagating action potential increased during PTH but decreased during the posttetanic depolarization recorded in ouabain. Thus changes in axonal excitability after tetanic stimulation correlate with changes in the posttetanic membrane potential. 10. Action potentials that propagated during PTH had a larger peak amplitude and were followed by a larger and longer depolarizing afterpotential than action potentials elicited at the resting potential. This enhancement of the depolarizing afterpotential is consistent with previous reports of an increased superexcitable period after action potentials evoked during PTH.

Animals↗
PubMed1992

Evidence that action potentials activate an internodal potassium conductance in lizard myelinated axons.

1. We have studied action potentials and after-potentials evoked in the internodal region of visualized lizard intramuscular nerve fibres by stimulation of the proximal nerve trunk. Voltage recordings were obtained using microelectrodes inserted into the axon (intra-axonal) or into the layers of myelin (peri-internodal), with the goal of studying conditions required to activate internodal K+ currents. 2. Peri-internodal recordings made using K2SO4-, KCl- or NaCl-filled electrodes exhibited a negligible resting potential (less than 2 mV), but showed action potentials with peak amplitudes of up to 78 mV and a duration less than or equal to that of the intra-axonally recorded action potential. 3. Following ionophoretic application of potassium from a peri-internodal microelectrode, the peri-internodal action potential was followed by a prolonged (hundreds of milliseconds) negative plateau. This plateau was not seen following peri-internodal ionophoresis of sodium. The prolonged negative potential (PNP) was confined to the K(+)-injected internode: it could be recorded by a second peri-internodal microelectrode inserted into the same internode, but not into an adjacent internode. 4. The peri-internodally recorded PNP was accompanied by an equally prolonged intra-axonal depolarizing after-potential, and by an increase in the conductance of the internodal axolemma. However, the K+ ionophoresis that produced the PNP had little or no detectable effect on the intra-axonally or peri-internodally recorded resting potential or action potential. These findings suggest that the PNP is generated by an inward current across the axolemma of the K(+)-injected internode, through channels opened following the action potential. 5. Following peri-internodal K+ ionophoresis a PNP could also be evoked by passage of depolarizing current pulses through an intra-axonal electrode or by passage of negative current pulses through an electrode in the K(+)-filled peri-internodal region. The threshold for evoking a PNP was less than the threshold for evoking an action potential, and the PNP persisted in 10 microM-tetrodotoxin. Thus the PNP is evoked by depolarization of the axolemma rather than by Na+ influx. 6. The PNP was reversibly blocked by tetraethylammonium (TEA, 2-10 mM), but was not blocked by 100 microM-3,4-diaminopyridine or 5 mM-4-aminopyridine.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1990

Evidence for two calcium-dependent potassium conductances in lizard motor nerve terminals.

Action potentials and afterpotentials were recorded with a microelectrode inserted into lizard motor axons within a few millimeters of their motor terminals. In the presence of 1 mM 4-aminopyridine (4-AP), the duration of the action potential recorded near motor terminals was Ca-sensitive: repolarization was more rapid when bath [Ca] was elevated, and became slower when bath [Ca] was removed or when 0.1-1 mM Mn was added. Repolarization was also slowed following addition of 3-10 nM charybdotoxin or 100-300 microM tetraethylammonium (TEA) to the bath, and following intra-axonal injection of the Ca buffer BAPTA. These results, in agreement with published extracellular recordings, indicate that the motor nerve terminal membrane contains rapidly activating, Ca-activated K channels. When these (and other) K channels were blocked by 10 mM TEA, the action potential recorded near motor terminals was followed by Ca-dependent depolarizing afterpotentials, followed in turn by a slow hyperpolarizing afterpotential (h.a.p.) that lasted several seconds. This slow h.a.p. was also Ca-sensitive: it became larger with increasing bath [Ca] and was abolished by removal of bath [Ca] and by addition of 1 mM Mn. Intra-axonal injection of BAPTA reduced the amplitude of the slow h.a.p., and prolonged injections promoted repetitive discharge. The slow h.a.p. following single action potentials was observed in 100 microM ouabain and in K-free solutions and thus is pharmacologically distinct from the hyperpolarization that follows tetanic stimulation. The slow h.a.p. was selectively inhibited by 100 nM apamin, but persisted in 100 nM charybdotoxin. This afterpotential was enhanced by 0.1-1 mM 4-AP and by the dihydropyridine Bay K 8644 (0.1-1 microM). These results suggest that the slow h.a.p. in lizard motor nerve terminals is mediated by Ca-activated K channels that can be activated near the resting potential and are pharmacologically distinct from the Ca-activated K channels that contribute to action potential repolarization. The slow h.a.p. was enhanced by 0.1-1 mM caffeine and inhibited by 100 microM procaine, raising the possibility that this afterpotential may be activated not only by Ca entering via the plasma membrane, but also by Ca released from intra-terminal stores.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1989

Calcium-dependent depolarizations originating in lizard motor nerve terminals.

Action potentials and afterpotentials were recorded via a microelectrode inserted into motor axons innervating the lizard ceratomandibularis muscle. Intra-axonal injection of Lucifer yellow dye indicated that these axons innervate multiple (at least 6-25) motor terminals. In 10 mM tetraethylammonium (TEA), the action potential was followed by a sequence of afterpotentials, whose amplitude and duration increased with increasing proximity to motor nerve terminals. In axons impaled within 1 mm of their most distal terminals, these afterpotentials included a depolarizing plateau (mean amplitude and duration: 32 mV, 24 msec) and a subsequent smaller depolarization that decayed over a time course of several hundred milliseconds. These depolarizing afterpotentials were Ca dependent: They increased with increasing bath [Ca] and were abolished by low [Ca]-high [Mg] solutions, by omega-conotoxin (GVIA, 1 microM), by addition of Cd (1 microM) or Mn (0.3-1 mM) to the bath, and by selective perfusion of Cd over the terminal region. In [Ca]-free solutions the afterpotentials were restored by selective perfusion of Ca over the terminal region but not by Ca applied to the more proximal nerve trunk. When Na influx was eliminated by 1-10 microM tetrodotoxin or by substitution of TEA for bath Na, passage of depolarizing current into the axon evoked prolonged depolarizing afterpotentials that were blocked by Mn. Bay K 8644 (0.1-1 microM), a dihydropyridine that prolongs the opening of certain calcium channels, enhanced mainly the slower component of the depolarizing afterpotential. Nimodipine (0.1-1 microM), a dihydropyridine that favors the closed state of some calcium channels, shortened the plateau phase of the depolarizing afterpotential. Another antagonist dihydropyridine, nitrendipine (0.1-1 microM), had little or no effect on the depolarizing afterpotential but did antagonize the actions of Bay K 8644. These results suggest that the intraaxonally recorded Ca-dependent afterpotentials are caused by electrotonic spread of depolarizations produced by calcium influx into that axon's terminals and that some motor nerve terminal calcium channels are sensitive to certain dihydropyridines.

Animals↗
PubMed1988

Effects of tetraethylammonium on the depolarizing after-potential and passive properties of lizard myelinated axons.

1. Intra-axonal recordings were obtained from myelinated axons innervating a lizard skeletal muscle. 2. Bath application of tetraethylammonium (TEA, 1-10 mM) depolarized the resting potential, prolonged the action potential and increased the amplitude and duration of the ensuing passive depolarizing after-potential (DAP) in a dose-dependent and reversible manner. TEA increased the axonal input resistance and the slow time constant of the passive voltage response, not only in depolarized axons, but also in resting and hyperpolarized axons. The resting input resistance was tripled in 10 mM-TEA. 3. TEA's effects on the resting potential and action potential usually approached a steady state within 5 min, whereas TEA's effects on input resistance and on the amplitude and time course of the DAP increased progressively for 10-15 min or more, and persisted for 10-15 min after removal of TEA from the bath. 4. 4-Aminopyridine (4-AP, 1 mM), which prolonged the action potential by about the same extent as 10 mM-TEA, did not depolarize the resting potential or increase the resting input resistance, and produced a much smaller increase in DAP time course than 10 mM-TEA. Gallamine (1 mM) had effects more similar to those of TEA. 5. These results suggest that the resting input conductance and DAP time course in lizard motor axons are controlled in part by K+ channels that are blocked by TEA and gallamine, but not by 4-AP. The slow development of the TEA-induced increase in input resistance and DAP time course suggests that some of these channels are located in paranodal or internodal axolemma. 6. In TEA and gallamine additional depolarizing potentials were superimposed on the falling phase of the action potential and on the passive DAP. These superimposed potentials were abolished by 1 mM-Mn2+, and were probably caused by Ca2+ influx into motor terminals.

4-Aminopyridine↗
PubMed1982

Intracellular recording from vertebrate myelinated axons: mechanism of the depolarizing afterpotential.

1. Electrophysiological techniques are described which allow intracellular recording from peripheral myelinated axons of lizards and frogs for up to several hours. The sciatic and intramuscular axons studied here have resting potentials of -60 to -80 mV and action potentials (evoked by stimulation of the proximal nerve trunk) of 50-90 mV. They show a prominent depolarizing afterpotential (d.a.p.), which is present both in isolated axons and in axons still attached to their peripheral terminals. This d.a.p. has a peak amplitude of 5-20 mV at the resting potential, and decays with a half-time of 20-100 msec.2. The peak amplitude of the d.a.p. is voltage-sensitive, increasing to up to 26 mV with membrane hyperpolarization. The d.a.p. disappears as the axon is depolarized to -60 to -45 mV, and does not appear to reverse with further depolarization.3. The d.a.p. is not reduced when bath Ca is replaced by 2-10 mm divalent Mn or Ni. The d.a.p. is not reversed when axons depleted of Cl (by prolonged exposure to Cl-deficient, SO(4)-enriched solutions) are bathed in Cl-rich solutions. These results suggest that the d.a.p. is not mediated by a conductance change specific for Ca or Cl ions. Partial substitution of tetramethylammonium for bath Na, or addition of 10(-5)m-tetrodotoxin to the normal bathing solution, reduces the amplitude of both the action potential and the d.a.p.4. The amplitude of the d.a.p. is not sensitive to bath [K] over the range 1-7.5 mm, provided that all measurements are made at the same holding potential. This result argues that the d.a.p. is not mediated by accumulation of K outside the active axon.5. Treatments expected to inhibit the Na-K exchange pump (cooling from 25 to 10 degrees C, or 0.15 mm-ouabain) do not enlarge or prolong the d.a.p., although they do abolish a slower hyperpolarizing afterpotential seen following repetitive stimulation.6. The passive voltage response of the axon to small injected pulses of depolarizing or hyperpolarizing current shows a prominent, slowly decaying component with a time course similar to that of the d.a.p. Depolarizing current reduces the input resistance of the axon, and increases the rate of decay of both the passive voltage response and the d.a.p. There is a slight conductance increase during the peak of the d.a.p., but the same conductance increase can be produced by a comparable passive depolarization.7. We conclude that the d.a.p. is due mainly to a passive capacitative current, probably resulting from discharge of the internodal axonal membrane capacitance through a resistive current pathway beneath or through the myelin sheath. We suggest that this slow capacitative discharge becomes evident as soon as most of the nodal ionic channels activated during the action potential have closed. An electrical model of the myelinated axon that incorporates the postulated internodal leakage pathway can account both for the prolonged d.a.p. recorded inside the axon, and for the potential profile recorded extra-axonally in or near the internodal periaxonal space.

Action Potentials↗