Nitric oxide activates ATP-sensitive potassium channels in mammalian sensory neurons: action by direct S-nitrosylation

Background

ATP-sensitive potassium (K_ATP) channels in neurons regulate excitability, neurotransmitter release and mediate protection from cell-death. Furthermore, activation of K_ATP channels is suppressed in DRG neurons after painful-like nerve injury. NO-dependent mechanisms modulate both K_ATP channels and participate in the pathophysiology and pharmacology of neuropathic pain. Therefore, we investigated NO modulation of K_ATP channels in control and axotomized DRG neurons.

Results

Cell-attached and cell-free recordings of K_ATP currents in large DRG neurons from control rats (sham surgery, SS) revealed activation of K_ATP channels by NO exogenously released by the NO donor SNAP, through decreased sensitivity to [ATP]i.

This NO-induced K_ATP channel activation was not altered in ganglia from animals that demonstrated sustained hyperalgesia-type response to nociceptive stimulation following spinal nerve ligation. However, baseline opening of K_ATP channels and their activation induced by metabolic inhibition was suppressed by axotomy. Failure to block the NO-mediated amplification of K_ATP currents with specific inhibitors of sGC and PKG indicated that the classical sGC/cGMP/PKG signaling pathway was not involved in the activation by SNAP. NO-induced activation of K_ATP channels remained intact in cell-free patches, was reversed by DTT, a thiol-reducing agent, and prevented by NEM, a thiol-alkylating agent. Other findings indicated that the mechanisms by which NO activates K_ATP channels involve direct S-nitrosylation of cysteine residues in the SUR1 subunit. Specifically, current through recombinant wild-type SUR1/Kir6.2 channels expressed in COS7 cells was activated by NO, but channels formed only from truncated isoform Kir6.2 subunits without SUR1 subunits were insensitive to NO. Further, mutagenesis of SUR1 indicated that NO-induced K_ATP channel activation involves interaction of NO with residues in the NBD1 of the SUR1 subunit.

Conclusion

NO activates K_ATP channels in large DRG neurons via direct S-nitrosylation of cysteine residues in the SUR1 subunit. The capacity of NO to activate K_ATP channels via this mechanism remains intact even after spinal nerve ligation, thus providing opportunities for selective pharmacological enhancement of K_ATP current even after decrease of this current by painful-like nerve injury.

Opposing effects of spinal nerve ligation on calcium-activated potassium currents in axotomized and adjacent mammalian primary afferent neurons

Calcium-activated potassium channels regulate AHP and excitability in neurons. Since we have previously shown that axotomy decreases I(Ca) in DRG neurons, we investigated the association between I(Ca) and K((Ca)) currents in control medium-sized (30-39 microM) neurons, as well as axotomized L5 or adjacent L4 DRG neurons from hyperalgesic rats following L5 SNL. Currents in response to AP waveform voltage commands were recorded first in Tyrode's solution and sequentially after: 1) blocking Na(+) current with NMDG and TTX; 2) addition of K((Ca)) blockers with a combination of apamin 1 microM, iberiotoxin 200 nM, and clotrimazole 500 nM; 3) blocking remaining K(+) current with the addition of 4-AP, TEA-Cl, and glibenclamide; and 4) blocking I(Ca) with cadmium. In separate experiments, currents were evoked (HP -60 mV, 200 ms square command pulses from -100 to +50 mV) while ensuring high levels of activation of I(K(Ca)) by clamping cytosolic Ca(2+) concentration with pipette solution in which Ca(2+) was buffered to 1 microM. This revealed I(K(Ca)) with components sensitive to apamin, clotrimazole and iberiotoxin. SNL decreases total I(K(Ca)) in axotomized (L5) neurons, but increases total I(K(Ca)) in adjacent (L4) DRG neurons. All I(K(Ca)) subtypes are decreased by axotomy, but iberiotoxin-sensitive and clotrimazole-sensitive current densities are increased in adjacent L4 neurons after SNL. In an additional set of experiments we found that small-sized control DRG neurons also expressed iberiotoxin-sensitive currents, which are reduced in both axotomized (L5) and adjacent (L4) neurons.

CONCLUSIONS

Axotomy decreases I(K(Ca)) due to a direct effect on K((Ca)) channels. Axotomy-induced loss of I(Ca) may further potentiate current reduction. This reduction in I(K(Ca)) may contribute to elevated excitability after axotomy. Adjacent neurons (L4 after SNL) exhibit increased I(K(Ca)) current.

Painful Peripheral Nerve Injury Decreases Calcium Current in Axotomized Sensory Neurons

Background

Reports of Ca(2+) current I(Ca) loss after injury to peripheral sensory neurons do not discriminate between axotomized and spared neurons. The spinal nerve ligation model separates axotomized from spared neurons innervating the same site. The authors hypothesized that I(Ca) loss is a result of neuronal injury, so they compared axotomized L5 dorsal root ganglion neurons to spared L4 neurons, as well as neurons from rats undergoing skin incision alone.

Methods

After behavioral testing, dissociated neurons from L4 and L5 dorsal root ganglia were studied in both current and voltage patch clamp modes. The biophysical consequence of I(Ca) loss on the action potential was confirmed using selective I(Ca) antagonists. Data were grouped into small, medium, and large cells for comparison.

Results

Reduced I(Ca) was predominantly a consequence of axotomy (L5 after spinal nerve ligation) and was most evident in small and medium neurons. ICa losses were associated with action potential prolongation in small and medium cells, whereas the amplitude and duration of after hyperpolarization was reduced in medium and large neurons. Blockade with Ca(2+) channel antagonists showed that action potential prolongation and after hyperpolarization diminution were alike, attributable to the loss of I(Ca).

Conclusion

Axotomy is required for I(Ca) loss. I(Ca) loss correlated with changes in the biophysical properties of sensory neuron membranes during action potential generation, which were due to I(Ca) loss leading to decreased outward Ca(2+)-sensitive K currents. Taken together, these results suggest that neuropathic pain may be mediated, in part, by loss of I(Ca) and the cellular processes dependent on Ca(2+).

Distinct membrane effects of spinal nerve ligation on injured and adjacent dorsal root ganglion neurons in rats

BACKGROUND

Painful peripheral nerve injury results in disordered sensory neuron function that contributes to the pathogenesis of neuropathic pain. However, the relative roles of neurons with transected axons versus intact adjacent neurons have not been resolved. An essential first step is identification of electrophysiologic changes in these two neuronal populations after partial nerve damage.

METHODS

Twenty days after spinal nerve ligation (SNL), intracellular recordings were obtained from axotomized fifth lumbar (L5) dorsal root ganglion neurons and adjacent, intact L4 neurons, as well as from control neurons and others subjected to sham-SNL surgery.

RESULTS

Pronounced electrophysiologic changes were seen only in L5 neurons after SNL. Both Aalpha/beta and Adelta neuron types showed increased action potential duration, decreased afterhyperpolarization amplitude and duration, and decreased current threshold for action potential initiation. Aalpha/beta neurons showed resting membrane potential depolarization, and increased repetitive firing during sustained depolarization developed in Adelta neurons. The afterhyperpolarization duration in neurons with C fibers shortened after axotomy. In contrast to the axotomized L5 neurons, neighboring L4 neurons showed no changes in action potential duration, afterhyperpolarization dimensions, or excitability after SNL. Depolarization rate (dV/dt) increased after SNL in L4 Aalpha/beta and Adelta neurons but decreased in L5 neurons. Time-dependent rectification during hyperpolarizing current injection (sag) was greater after SNL in Aalpha/beta L4 neurons compared with L5. Sham-SNL surgery produced only a decreased input resistance in Aalpha/beta neurons and a decreased conduction velocity in medium-sized cells. In the L5 ganglion after axotomy, a novel set of neurons, consisting of 24% of the myelinated population, exhibited long action potential durations despite myelinated neuron conduction velocities, particularly depolarized resting membrane potential, low depolarization rate, and absence of sag.

CONCLUSIONS

These findings indicate that nerve injury-induced electrical instability is restricted to axotomized neurons and is absent in adjacent intact neurons.

Beta-escin diminishes voltage-gated calcium current rundown in perforated patch-clamp recordings from rat primary afferent neurons

Perforated patch recordings of neuronal calcium currents (I(Ca)) with amphotericin B or nystatin reduce dialysis of intracellular constituents and current rundown, but can be difficult and frequently unsuccessful. We investigated the saponin beta-escin as a putative ionophore for perforated patch I(Ca) recordings in acutely dissociated, rat dorsal root ganglion neurons. I(Ca) was recorded in time-course studies after including either beta-escin (50 microM), or amphotericin B (240 microg/ml) as perforating ionophores in the internal pipette solution, in comparison to standard ruptured-patch technique, using suction. Perforated patches were allowed to take place spontaneously. The percentage loss of I(Ca) per min (within the first 20 min) was significantly less after beta-escin (0.0518%) (n = 18), versus either amphotericin (1.82%) (n = 12) or standard patch (4.52%) (n = 7), (P < 0.001). The slope of the rundown after linear fit was also less after beta-escin (P < 0.001). Minimal "steady-state" access resistance (R(a)) of 6.6 +/- 1.6 MOmega was achieved within 7.1 +/- 9.3 min following perforation with beta-escin, 7.9 +/- 3.5 MOmega within 44 =/- 14 min after amphotericin B, and 6.8 +/- 1.9 MOmega with standard patch (P < 0.05 for R(a), and P < 0.01 for permeabilization time, respectively). Success rates were 59% with beta-escin versus 27% with amphotericin. Leak >10% of peak I(Ca) was present in 25% of cells after beta-escin versus 20% after amphotericin, and 12% after standard technique. Perforated patches using beta-escin were stable for 15-60 min. We conclude that beta-escin may be used as an alternative ionophore for perforated patch-clamp studies in neurons, and results in minimal rundown that can facilitate long-term recordings of I(Ca). Limited rundown may be due to better preservation of cytosolic ATP content.

Detection of neuropathic pain in a rat model of peripheral nerve injury

BACKGROUND

Behavioral criteria that confirm neuropathic pain in animal injury models are undefined. Therefore, the authors sought clinically relevant measures that distinguish pain behavior of rats with peripheral nerve injury from those with sham injury.

METHODS

The authors examined mechanical and thermal sensory sensitivity, comparing responses at baseline to responses after spinal nerve ligation (SNL group), sham nerve injury (sham group), or skin incision alone (control group).

RESULTS

Substantial variance was evident in all sensory tests at baseline. After surgery, tests using brush, cold, or heat stimulation showed minimal distinctions between surgical groups. Postsurgical thresholds for flexion withdrawal from mechanical stimulation with von Frey fibers were decreased bilaterally in SNL and sham groups. In contrast, the probability of a complex hyperalgesia-type response with prolonged elevation, shaking, or licking of the paw was selectively increased on the ipsilateral side in the SNL group. Nonetheless, the effect of SNL on behavior was inconsistent, regardless of the sensory test. The behavioral measure that best distinguishes between SNL and sham groups and thereby best identifies animals with successful SNL-induced neuropathic pain is increased ipsilateral postsurgical probability of a hyperalgesia-type response to noxious mechanical stimulation. Using receiver operating characteristics analysis, mechanical hyperalgesia identifies a local SNL effect in approximately 60% of animals when specificity is required to be 90% or higher.

CONCLUSIONS

Simple withdrawal from von Frey tactile stimulation, although frequently used, is not a valid measure of peripheral nerve injury pain in rats, whereas a complex hyperalgesic-type response is a specific neuropathy-induced behavior.

Painful neuropathy alters the effect of gabapentin on sensory neuron excitability in rats

BACKGROUND

Pain following peripheral nerve injury is associated with increased excitability of sensory neurons. Gabapentin (GBP), a novel anticonvulsant with an uncertain mechanism of action, is an effective treatment for neuropathic pain. We therefore investigated the effect of GBP on dorsal root ganglion (DRG) neurons from normal rats and those with painful peripheral nerve injury.

METHODS

Dorsal root ganglions were excised from rats with neuropathic pain behaviour following chronic constriction injury (CCI) of the sciatic nerve, and from normal rats. Intercellular recordings were made from myelinated sensory neuron somata using a microelectrode technique from DRGs bathed in artificial CSF with or without GBP (100 microM).

RESULTS

Compared with normal neurons, injury decreased the refractory interval (RI) for repeat action potential (AP) generation increased the number of APs during sustained depolarization, and shortened the after hyperpolarization following an AP. In normal neurons, GBP decreased the RI and increased the AP number during sustained depolarization. In an opposite fashion, the result of GBP application to injured neurons was a decreased number of APs during depolarization and no change in RI. In injured neurons only, GBP increased the time-to-peak for AP depolarization.

CONCLUSIONS

Nerve injury by CCI is associated with increased sensory neuron excitability, associated with a decreased AHP. In normal peripheral sensory neurons, GBP has pro-excitatory effects, whereas GBP decreases excitability in injured neurons, possibly on the basis of altered sodium channel function.

Loss of T-type calcium current in sensory neurons of rats with neuropathic pain

BACKGROUND

Pathophysiology in the primary sensory neuron may contribute to chronic neuropathic pain. Ca channels play a central role in neuronal processes, and sensory neurons are rich in low-voltage-activated calcium channels (LVACCs). However, the physiologic function of these channels is unknown. Their possible role in rebound burst firing makes them a candidate for increased excitability after neuropathic injury.

METHODS

This study uses pharmacological methods to isolate LVACC in cells from the dorsal root ganglia of neuropathic and sham-operated rats, including the blockade of high-voltage-activated Ca channels with fluoride and selective toxins. LVACCs were examined with conventional whole cell patch clamp electrophysiology techniques.

RESULTS

After chronic constriction injury of the peripheral axon, LVACC was significantly reduced compared to sham rats as shown by a 60% reduction in peak current density and an 80% reduction in total calcium influx. A depolarizing shift in the voltage dependence of activation and an increase in the rate of deactivation and inactivation appear to cause this reduction of LVACC. Either Ni2+ or mibefradil, blockers of LVACC, applied in the bath to normal dorsal root ganglion cells during current clamp significantly and reversibly increased excitability.

CONCLUSIONS

These results suggest that loss of LVACC may contribute to decreased spike frequency adaptation and increased excitability after injury to sensory neurons. Through decreased Ca2+ influx, the cell becomes less stable and more likely to initiate or transmit bursts of action potentials. Consequently, modulation of Ca2+ currents at the dorsal root ganglion may be a potential method of therapeutic intervention.

Painful neuropathy decreases membrane calcium current in mammalian primary afferent neurons

Hyperexcitability of the primary afferent neuron leads to neuropathic pain following injury to peripheral axons. Changes in calcium channel function of sensory neurons following injury have not been directly examined at the channel level, even though calcium is a primary second messenger-regulating neuronal function. We compared calcium currents (I(Ca)) in 101 acutely isolated dorsal root ganglion neurons from 31 rats with neuropathic pain following chronic constriction injury (CCI) of the sciatic nerve, to cells from 25 rats with normal sensory function following sham surgery. Cells projecting to the sciatic nerve were identified with a fluorescent label applied at the CCI site. Membrane function was determined using patch-clamp techniques in current clamp mode, and in voltage-clamp mode using solutions and conditions designed to isolate I(Ca). Somata of peripheral sensory neurons from hyperalgesic rats demonstrated decreased I(Ca). Peak calcium channel current density was diminished by injury from 3.06+/-0.30 pS/pF to 2. 22+/-0.26 pS/pF in medium neurons, and from 3.93+/-0.38 pS/pF to 2. 99+/-0.40 pS/pF in large neurons. Under these voltage and pharmacologic conditions, medium-sized neuropathic cells lacked obvious T-type calcium currents which were present in 25% of medium-sized cells from control animals. Altered Ca(2+) signalling in injured sensory neurons may contribute to hyperexcitability leading to neuropathic pain.

The mechanism of alpha2-adrenergic inhibition of sympathetic ganglionic transmission

Alpha2-adrenergic agonists produce analgesia and reduce hemodynamic stress through central and peripheral mechanisms, but the effect of adrenergic agonists on pre- and postganglionic sites has not yet been clarified. In this study, we examined the effects of dexmedetomidine (DMT), an alpha2-agonist, on neural conduction and neurotransmitter release in sympathetic ganglia. The stellate ganglia from 48 mongrel dogs were isolated, desheathed, and superfused with Krebs' solution. Compound action potentials were evoked, and chromatography was used to detect acetylcholine released by preganglionic stimulation in the presence or absence of DMT. To further elucidate the mechanism of alpha2 effects, DMT was applied in combination with the alpha2-antagonist atipamezole (AT) or the imidazoline antagonist idazoxan (ID). In other experiments, DMT was applied in the presence of exogenous nicotinic stimulation with 1,1-dimethyl-4-phenylpiperazinium iodide or muscarinic stimulation with (+)cis-dioxolane. DMT dose-dependently inhibited synaptic transmission with a 50% effective dose of 71.6 (26.0-174.3) microM. Neurotransmitter release was reduced 25% by 70 microM DMT during low-frequency (0.4 Hz) stimulation, but this effect was abolished at higher frequency (5 Hz) stimulation. AT but not ID blocked the inhibitory action of DMT. DMT inhibited the excitatory postsynaptic response to exogenous muscarinic stimulation but not nicotinic stimulation. These results indicate that alpha2-receptor activation depresses ganglionic transmission through postsynaptic inhibition of muscarinic stimulation, although reduction of neurotransmitter release through a presynaptic autofeedback mechanism is also involved.

IMPLICATIONS

This article provides novel insights into the mechanism of drug action of alpha2-receptor agonists in the sympathetic ganglia of dogs by directly measuring the relative contribution of pre- and postganglionic receptors. Our study indicates that the central sympatholytic effects of alpha2-adrenoceptor stimulation are augmented by peripheral inhibition of ganglionic transmission.

Effects of the optical isomers of verapamil on electrophysiological properties of the heart in conscious dogs

We compared the cumulative dose-response relations of verapamil (0.1, 0.2 and 0.4 mg kg(-1)) in different R/S enantiomer ratios (100/0, 90/10, 80/20, 50/50 and 20/80) on the electrophysiological and hemodynamic characteristics of the heart using the conscious dogs. A reduction of mean arterial pressure occurred with 20R/80S producing a 3-times greater decrease than 100R/0S, but an increase in heart rate occurred with 20R/80S producing a 9-times greater increase than 100R/0S. Increased heart rate was concurrent with decreased mean arterial pressure most prevalent with a higher ratio of S-isomer that produced a greater reduction in mean arterial pressure and increase in heart rate at lower overall verapamil doses. Atrio-ventricular conduction time increased 3-5 min after each infusion, with 20R/80S producing a 4-times greater effect than 100R/0S. These results indicate that the peripheral and cardiac electrophysiologic properties of various nonracemic verapamil mixtures are mainly attributable to the concentration of S-isomer.

Direct comparative effects of isoflurane and desflurane on sympathetic ganglionic transmission

Although the sympathetic ganglion is an important site of peripheral regulation, few studies have examined the effect of anesthetics on synaptic transmission. In the present study we compared the actions of desflurane with those of isoflurane on synaptic transmission and neurotransmitter release in the stellate ganglion. In the electrophysiologic group, 14 stellate ganglia were isolated from adult mongrel dogs after halothane anesthesia, desheathed, and superfused with Krebs' solution. Compound action potentials (CAP) were induced by supramaximal stimulation of the preganglionic T3-ramus at a low frequency of 0.4 Hz and were recorded from the postganglionic ventral ansa subclaviae. Each ganglion was exposed to two levels of anesthetics (equivalent to 1 and 2 minimum alveolar anesthetic concentration [MAC]), followed by an anesthetic-free washout period. While equianesthetic concentrations of isoflurane and desflurane caused essentially equipotent suppression of ganglionic transmission, desflurane was more efficacious than isoflurane, both with respect to the onset of and recovery from the inhibition of synaptic activity. In the electrochemical group, 25 ganglia were exposed to both anesthetics at a high concentration (equivalent to between 1.82 and 1.95 MAC) during maximal and submaximal current stimulations, and the release of actylcholine (ACh) in the superfusate was measured with liquid chromatography. Although desflurane and isoflurane caused a significant depression of CAP, neither anesthetic inhibited the release of ACh in the superfusate at either maximal or submaximal current stimulations. These results indicate that the suppression of ganglionic activity is equipotent for both anesthetics based on equivalent MAC values, but that desflurane is more efficacious than isoflurane with respect to onset and recovery at the higher concentrations of anesthetics.(ABSTRACT TRUNCATED AT 250 WORDS)

Does isoflurane alter mesenteric venous capacitance in the intact rabbit?

Volatile anesthetics act at a number of sites to alter cardiovascular function and the response of the cardiovascular system to barostatic reflexes. We examined the effects of isoflurane on reflex regulation of mesenteric venous capacitance vessels. To determine whether isoflurane alters mesenteric venous capacitance, continuous direct observations of mesenteric vein diameter, intravenous pressure, and mesenteric sympathetic efferent nerve activity (SENA) were made in 31 chloralose-anesthetized New Zealand white rabbits. Simultaneous measurements were obtained for aortic pressure and heart rate. The responses to changes in baroreceptor activation by means of either bilateral carotid occlusion (BCO) or aortic nerve stimulation (ANS) were studied in one group of 18 rabbits, while the response to direct electric activation by means of celiac ganglion stimulation (CGS) was studied in another group of 13 rabbits. In both groups, isoflurane vapor was administered at levels of 0.75% or 1.5%, and superfused isoflurane was administered directly to the vessel in doses of either 3% or 5% equilibrated with physiologic salt solution. Anesthetic levels were verified by mass spectrometry for expired gas and by gas chromatography for blood and superfusate levels. Inhaled isoflurane reduced hemodynamic variables and SENA in a dose-dependent fashion, but these same variables were unaffected by superfused isoflurane. One and one-half percent inhaled isoflurane abolished all reflex responses to baroreceptor stimulation in mesenteric capacitance veins and in SENA, but superfused isoflurane produced no corresponding attenuation of reflex responses to baroreceptor stimulation. Neither inhaled nor superfused isoflurane suppressed the reflex venoconstriction in response to CGS. Both inhaled and superfused isoflurane dilated the baseline vein diameter before stimulation. These results indicate that isoflurane dose increase the diameter of mesenteric venous capacitance vessels and inhibits reflex-induced constriction of mesenteric veins, whereas mesenteric sympathetic efferent nerve activity decreases and the reflex responses to activation of the carotid sinus and aortic baroreceptors are attenuated by inhaled isoflurane. The mechanism of this action appears to be primarily through the inhibition of central or peripheral sympathetic ganglionic transmission of barostatic control.

The inhibitory action of halothane on reflex constriction in mesenteric capacitance veins

Potent inhalational anesthetics depress autonomic reflex responses at multiple sites. Most studies emphasize cardiac chronotropic changes and changes in systemic blood pressure. Recently, active reflex venoconstriction of 500-1,000 microns O.D. mesenteric veins has been demonstrated. In the current study, the effects of halothane on the reflex responses of similar mesenteric veins were measured. Mesenteric vein diameter and intravenous pressure were measured in 500-1,000 microns O.D. veins from the mesentery of segments of terminal ileum externalized in situ from 27 New Zealand white rabbits anesthetized with alpha-chloralose. Mean arterial pressure was measured with femoral arterial cannulation, and heart rate was determined from the arterial pressure signal. In a separate group of six animals, sympathetic efferent nerve activity was measured from a postganglionic splanchnic nerve. Reflex venoconstriction and increases in mean arterial pressure and mesenteric vein pressure in response to bilateral carotid occlusion were attenuated by 0.5% and 1% inhaled halothane but not by superfusate equilibrated with 3% halothane. Decreases in mesenteric vein diameter and increases in mesenteric vein pressure in response to celiac ganglion stimulation were unaffected by both 0.5% inhaled halothane and superfusate equilibrated with 5% halothane. The bilateral carotid occlusion reflex-mediated increase in sympathetic efferent nerve activity was depressed by both 0.5% and 1% inhaled halothane. The effect of inhaled halothane on prestimulation baseline vein diameter was inconsistent. Superfusate equilibrated with 5% but not 3% halothane caused baseline venodilation. These results suggest a mechanism whereby control of venous tone is inhibited by halothane proximal to the postganglionic neuron. This could involve central or ganglionic inhibition.