Different voltage-dependent potassium conductances regulate action potential repolarization and excitability in frog myelinated axon

Effects of acetazolamide and 4-aminoprydine on the responses of deflationary slowly adapting pulmonary stretch receptors to CO2 inhalation in the rat

The inhibitory effect of CO(2) on deflationary slowly adapting pulmonary stretch receptors (deflationary SARs) was investigated before and after administration of acetazolamide, a carbonic anhydrase (CA) inhibitor, or 4-aminopyridine (4-AP), a K(+) channel blocker, in anesthetized, artificially ventilated rats after unilateral vagotomy. CO(2) inhalation (maximum tracheal CO(2) concentration ranging from 9 to 12%) for approximately 60 s decreased the impulse activity of deflationary SARs but had no significant effect on tracheal pressure (P(T)) as an index of bronchomotor tone. Acetazolamide treatment (20 mg/kg) diminished the inhibitory response of deflationary SARs to CO(2) inhalation. 4-AP (0.7 and 2.0 mg/kg) dose-dependently attenuated the decrease in deflationary SAR activity induced by CO(2) inhalation. When comparing the maximum attenuation due to 4-AP (2.0 mg/kg) and acetazolamide (20 mg/kg) in CO(2)-induced deflationary SAR inhibition, blockade of K(+) channels had a more pronounced effect. These results suggest that inhibition of deflationary SARs by CO(2) inhalation may be largely mediated by the stimulating action of 4-AP-sensitive K(+) currents in the nerve terminals of the receptors.

Intra-axonal recording from large sensory myelinated axons: demonstration of impaired membrane conductances in early experimental diabetes

AIM/HYPOTHESIS

Diabetic neuropathy is accompanied by a range of positive (paresthaesia, dysesthaesia, pain) and negative (hypesthaesia, anesthaesia) neurological symptoms suggesting widespread alterations in axonal excitability. The nature and the mechanisms underlying these alterations in axonal excitability are not well understood. The aim of this study was to examine the extent of changes in membrane properties of an identified neuronal structure-the large myelinated sensory axons in early experimental diabetes in rats.

METHODS

Intra-axonal microelectrode recordings from large sensory myelinated axons from the isolated sural nerve in short-term streptozotocin-induced diabetic rats were used to study membrane properties using standard current-clamp technique.

RESULTS

In addition to decreased conduction velocity we found several differences in physiological properties of sensory axons from diabetic rats: decreased resting membrane potential, decreased single action potential amplitude associated with slower rate of rise and decrease in inward rectification associated with slight alteration in outwardly rectifying conductances indicating impaired potassium conductances.

CONCLUSION/INTERPRETATION

These results extend previous indirect evidence that potassium and sodium ionic conductances, most notably the inward rectifier (IR, I(h)), are altered in large sensory axons of diabetic rats. The depression of IR could underly clinical neurological findings in diabetic patients.

Excitatory mechanism of deflationary slowly adapting pulmonary stretch receptors in the rat lung

The excitatory responses of deflationary slowly adapting pulmonary stretch receptor (SAR) activity to lung deflation ranging from approximately -15 to -25 cm of H(2)O for approximately 5 s were examined before and after administration of flecainide, a Na(+) channel blocker, and K(+) channel blockers, such as 4-aminopyridine (4-AP) and tetraethylammonium (TEA). The experiments were performed in anesthetized, artificially ventilated rats after unilateral vagotomy. The deflationary SARs increased their activity during lung deflation and its effect became more pronounced by increasing the degree of negative pressure. During lung deflation the average values for the deflationary SAR adaptation index (AI) were below 40%. Intravenous administration of veratridine (50 microg/kg), an Na(+) channel opener, stimulated deflationary SAR activity: one maintained excitatory activity mainly during deflation and the other receptors showed a tonic discharge during both deflation and inflation. Despite the difference in deflationary SAR firing patterns after veratridine administration, flecainide treatment (6.0 mg/kg) blocked veratridine-induced deflationary SAR stimulation and also caused strong inhibition of the excitatory responses of deflationary SARs to lung deflation. Under these conditions, the average values for deflationary SAR AI were over 90%. The responses of deflationary SARs and deflationary SAR AI to lung deflation were not significantly altered by pretreatment with either 4-AP (0.7 and 2.0 mg/kg) or TEA (2.0 and 6.0 mg/kg). These results suggest that the excitatory effect of lung deflation on deflationary SAR activity is mediated by the activation of flecainide-sensitive Na(+) channels on the nerve terminals of deflationary SARs.

Effects of potassium channel and Na+-Ca2+ exchange blockers on the responses of slowly adapting pulmonary stretch receptors to hyperinflation in flecainide-treated rats

1. The effects of K(+) channel blockers, such as 4-aminopyridine (4-AP) and tetraethylammonium (TEA), and a reverse-mode Na(+)-Ca(2+) exchange blocker, 2-[2-[4-(4-nitrobenzyloxyl) phenyl] ethyl] isothiourea methanesulphonate (KB-R7943), on the responses of slowly adapting pulmonary stretch receptor activity to hyperinflation (inflation volume=3 tidal volumes) were investigated in anaesthetized, artificially ventilated, unilaterally vagotomized rats after pretreatment with a Na(+) channel blocker flecainide. The administration of flecainide (9 mg kg(-1)) at a dose greater than that which abolished 50 microg kg(-1) veratridine-induced SAR stimulation also inhibited hyperinflation-induced stimulation of SARs. 2. In flecainide-treated animals, administration of 4-AP (0.7 and 2 mg kg(-1)) stimulated SAR activity during normal inflation and also caused a partial blockade of hyperinflation-induced SAR inhibition. 3. The discharges of SARs during normal inflation in flecainide-treated animals were not significantly altered by administration of either TEA (2 and 7 mg kg(-1)) or KB-R7943 (1 and 3 mg kg(-1)), but both K(+) channel and Na(+)-Ca(2+) exchange blockers partially attenuated hyperinflation-induced SAR inhibition. 4. These results suggest that hyperinflation-induced SAR inhibition in the presence of flecainide (9 mg kg(-1)) involves the activation of several K(+) conductance pathways.

Subthreshold membrane potential oscillations of type A neurons in injured DRG

An abundance of subthreshold membrane potential oscillations (SMPOs) at resting potential was observed in the neurons of chronically compressed dorsal root ganglia (DRG) using intracellular recording in vivo. Out of 386 neurons, 63 type A neurons displayed SMPOs. Three types of SMPOs were distinguished based on their characterizations of oscillation: (1) A regular pattern of SMPO emerged consistently with a mean frequency of 86 Hz and mean amplitudes of 3.3 mV. (2) A spindle-like pattern of SMPO was denominated by a spindle alteration of its amplitude. (3) An irregular pattern of SMPO had no rule on its change of amplitude and frequency. Compared with normal DRG neurons and injured DRG neurons but without SMPO, the injured DRG neurons with SMPO had the lowest spike rheobase, in accordance with the detection of spike accommodation. No significant differences among the three groups can be found in either membrane potential or input resistance. Further observation showed that the spontaneous discharge of hyperexcitable neurons usually occurred on the depolarizing phase of oscillations. In addition, the regular pattern of SMPO was based on the period and integer multiple patterns of spontaneous discharges. The spindle-like pattern of SMPO contributed to spontaneous bursting discharge. The irregular pattern of SMPO had a striking relation with irregular spontaneous discharge. The results show that neurons with SMPO in injured DRG have a higher excitability than those without SMPO, and that the SMPO underlie the patterns of spontaneous discharges, suggesting that SMPO is the basic electrophysiological change of hyperexcitable neurons.

Electrophysiological properties of axons in mice lacking neurofilament subunit genes: disparity between conduction velocity and axon diameter in absence of NF-H

Neurofilament proteins (NFs) are made by co-polymerization of three intermediate filament proteins, NF-L, NF-M and NF-H and constitute the most abundant cytoskeletal element in large myelinated axons. NFs have a well-established role as intrinsic determinants of axon caliber with all the functional implications, but the role of each individual NF subunit is much less clear. The aim of our study was to examine functional properties of large myelinated axons with altered morphology from mice bearing a targeted disruption of each NF genes (NF-L -/-, NF-M-/- and NF-H -/- mice). Membrane properties, action potentials and single axon refractory period were measured in isolated sciatic nerves in vitro, using intra-axonal microelectrode recording in conjunction with current-clamp technique. Some results were obtained from whole nerves by sucrose-gap recording. The NF-knockout mice showed several deficits in physiological properties of low-threshold fibers. In keeping with smaller axon diameter, the conduction velocity was significantly decreased in NF-L -/- and NF-M -/- transgenic animals (control, 39.9+/-1.8 m/s, NF-M -/-; 23.5+/-1. 4 m/s, and NF-L-/-; 12.0+/-0.7 m/s, mean+/-S.E.M.; intra-axonal recording; similar ratios obtained by sucrose-gap recording; 22-26 degrees C). However, in spite of their preserved caliber, large myelinated axons in NF-H -/- mice also showed a significant decrease in conduction velocity (22.8+/-1.0 m/s, mean+/-S.E.M.). Although action potential amplitudes, duration and shape did not differ between control axons and transgenic animals, the refractory period was prolonged in NF-H -/- and NF-M -/- animals. Intracellular injections of 200 ms depolarizing and hyperpolarizing currents revealed outward and inward rectification in all animal groups. In comparison to control animals, NF-H -/- mice expressed a significant decrease in outward rectification. Potassium channel blockers (4AP and TEA) and cesium ions were able to block outward and inward rectification in all myelinated axons in qualitatively the same manner. These results suggest that NF-H may have a specific role in modulating ion channel functions in large myelinated fibers.

Higher sensitivity of CA1 synapses to aglycemia in streptozotocin-diabetic rats is age-dependent

We studied conduction velocity in peripheral nerves and the block of synaptic transmission produced by lack of glucose in hippocampal slices from 4- and 12-month-old streptozotocin-induced diabetic rats and their age-matched controls. In sural nerves of young and old diabetic rats, the conduction velocity was reduced by 30-35%. In slices from young diabetics, CA1 synaptic transmission was more sensitive to aglycemia than in control slices. However, all slices from older rats showed comparable increases in CA1 synaptic sensitivity to aglycemia. We conclude that the cerebral adaptation to diabetic hyperglycemia apparent in the hippocampus of young rats is masked in older rats by an age-dependent increase in sensitivity to lack of glucose.

Abnormal distributions of potassium channels in human neuromas

Twenty-seven human traumatic neuromas were examined immunocytochemically using highly specific antibodies directed against the voltage-gated delayed-rectifier potassium channel, Kv1.1. Normal sural nerves from six of the above-noted patients served as control specimens. Additionally, nine of the neuromas and two of the sural nerves were immunostained for voltage-gated calcium channels using an antibody that reacts with a wide spectrum of calcium channels. Normal myelinated fibers showed Kv1.1 specific immunoreactivity only at the juxtaparanodal regions. In contrast, within the neuromas approximately 30% of the myelinated fibers exhibited Kv1.1 specific immunoreactivity in dense patches along internodal axonal regions. The clustering of Kv1.1 channels along myelin-ensheathed internodal segments of axon was highly specific for the neuromas, and was never seen in normal nerve. Specific calcium channel immunoreactivity was not detectable in either the neuromas or sural nerves. Taken together with prior studies on sodium channels, these results suggest that selective and specific mechanisms control the distribution of ion channels within neuromas. Further investigation of ion channel changes within neuromas should provide a better understanding of the abnormal axonal hyperexcitability that frequently develops after nerve injury.

Changes in pharmacological sensitivity of the spinal cord to potassium channel blockers following acute spinal cord injury

In this investigation we studied changes in the pharmacological sensitivity of dorsal column white matter to a variety of K+ channel blockers, including 4-aminopyridine (4-AP), following acute spinal cord injury (SCI) in vitro using a modified aneurysm clip. Compound action potentials (CAPs) were recorded extracellularly with microelectrodes and by the sucrose gap recording technique. With acute trauma, injured axons showed significantly enhanced sensitivity to 4-AP in comparison to uninjured controls as early as 10 min following injury. Microelectrode derived field potential recordings showed a significantly greater increase in a delayed positive component (P2) of the CAP at both 1 and 5 mM 4-AP in injured as compared to noninjured axons. Sucrose gap recordings showed an increase in CAP area and amplitude of injured axons with 1 mM 4-AP at 22 degrees C. The relative improvement in CAP area and amplitude with 4-AP was even more pronounced (P < 0.05) at higher temperatures (37 degrees C). As shown by sucrose gap, 4-AP also caused a delay in repolarization of the CAP and depolarization of the resting membrane potential of acutely injured axons. TEA (0.1 mM and 10 mM), when infused alone and with CsCl (10 mM), produced similar effects on injured and intact axons. In conclusion, the results of this study show an altered sensitivity of the spinal cord to 4-AP following acute SCI. In contrast, TEA and CsCl exhibit no difference in their effects on low frequency axonal conduction between injured and noninjured axons. The data suggest that acute traumatic myelin disruption following SCI causes axonal dysfunction partly due to abnormal activation of 4-AP-sensitive 'fast' K+ channels.

Evidence for a sodium-dependent potassium conductance in frog myelinated axon

After blockade of the voltage-dependent potassium conductances by intracellular application of 4-aminopyridine and tetraethylammonium in frog myelinated axons, a set of brief (0.1 ms) intracellular depolarizing pulses or a long (200 ms) depolarizing pulse evoked a train of action potentials. Under both experimental conditions a hyperpolarizing afterpotential appeared (duration 367 ms +/- 34, mean +/- S.E., n = 15). The purpose of this study was to investigate the properties of this hyperpolarizing afterpotential. It was found that the hyperpolarizing afterpotential increases in amplitude with: (1) the number of sodium-dependent action potentials; (2) action potential broadening (following potassium channels blockade); and (3) the level of depolarization during a current step. Application of tetrodotoxin prevented the activation of the hyperpolarizing afterpotential by any of the above stimuli. The hyperpolarizing afterpotential was unaffected by: (1) 8-acetyl-strophanthidin, an agent that poisons the electrogenic pumping in the axon; (2) blocking calcium influx with extracellular 10 mM magnesium or 2 mM manganese; and (3) buffering of the intracellular calcium, using EGTA in the recording microelectrode. Extracellular application of tetraethylammonium, but not 4-aminopyridine, reduced the hyperpolarizing afterpotential. The hyperpolarizing afterpotential reversed at >> -92 mV. Increasing the external potassium concentration from 2 to 10 mM shifted the reversal potential +14.5 mV, indicating that the hyperpolarizing afterpotential is a potassium mediated conductance.(ABSTRACT TRUNCATED AT 250 WORDS)