Changes in Na(+) channel currents of rat dorsal root ganglion neurons following axotomy and axotomy-induced autotomy

Section of rat sciatic nerve (axotomy) increases the excitability of neurons in the L(4)-L(5) dorsal root ganglia (DRG). These changes are more pronounced in animals that exhibit a self-mutilatory behavior known as autotomy. We used whole cell recording to examine changes in the tetrodotoxin-sensitive (TTX-S) and the tetrodotoxin-resistant (TTX-R) components of sodium channel currents (I(Na)) that may contribute to axotomy-induced increases in excitability. Cells were initially divided on the basis of size into "large," "medium," and "small" groups. TTX-S I(Na) predominated in "large" cells, whereas TTX-R I(Na) predominated in some, but not all "small cells." "Small" cells were therefore subdivided into "small-slow" cells, which predominantly exhibited TTX-R I(Na) and "small fast" cells that exhibited more TTX-S I(Na). In contrast to results obtained in other laboratories, where slightly different experimental procedures were used, we found that axotomy increased TTX-R and/or TTX-S I(Na) and slowed inactivation. The effects were greatest in "small-slow" cells and least in "large" cells. The changes promoted by axotomy were expressed more clearly in animals that exhibited autotomy. Also, the presence of autotomy correlated with a shift in the properties of I(Na) in "large" rather than "small-slow," putative nociceptive cells. These trends parallel previous observations on axotomy-induced increases in excitability, spike height, and spike width that are also greatest in "small" cells and least in "large" cells. In addition, the presence of autotomy correlates with an increase in excitability of "large" rather than "small" cells. Increases in TTX-R and TTX-S I(Na) thus coincide with axotomy-induced increases in excitability and alterations in spike shape across the whole population of sensory neurons. Injury-induced changes of this type are likely associated with the onset of chronic pain in humans.     

Reduction in potassium currents in identified cutaneous afferent dorsal root ganglion neurons after axotomy.

Potassium currents have an important role in modulating neuronal excitability. We have investigated the effects of axotomy on three voltage-activated K(+) currents, one sustained and two transient, in cutaneous afferent dorsal root ganglion (DRG) neurons. Fourteen to 21 days after axotomy, L(4) and L(5) DRG neurons were acutely dissociated and were studied 2-8 h after plating. Whole cell patch-clamp recordings were obtained from identified cutaneous afferent neurons (46-50 microm diam); K(+) currents were isolated by blocking Na(+) and Ca(2+) currents with appropriate ion replacement and channel blockers. Separation of the current components was achieved on the basis of sensitivity to dendrotoxin or 4-aminopyridine and by the response to variation in conditioning voltage. Both control and injured neurons displayed qualitatively similar complex K(+) currents composed of distinct kinetic and pharmacological components. Three distinct K(+) current components, a sustained (I(K)) and two transient (I(A) and I(D)), were identified in variable proportions. However, total peak current was reduced by 52% in the axotomized cells when compared with control cells. Two current components were reduced after ligation, I(A) by 60%, I(K) by over 65%, compared with control cells. I(D) appeared unaffected after acute ligation. These results indicate a large reduction in overall K(+) current, resulting from reductions in I(K) and I(A), on large cutaneous afferent neurons after nerve ligation and have implications for excitability changes of injured primary afferent neurons.     

Increased Na+ and K+ currents in small mouse dorsal root ganglion neurons after ganglion compression

We investigated the effects of chronic compression (CCD) of the L3 and L4 dorsal root ganglion (DRG) on pain behavior in the mouse and on the electrophysiological properties of the small-diameter neuronal cell bodies in the intact ganglion. CCD is a model of human radicular pain produced by intraforaminal stenosis and other disorders affecting the DRG, spinal nerve, or root. On days 1, 3, 5, and 7 after the onset of compression, there was a significant decrease from preoperative values in the threshold mechanical force required to elicit a withdrawal of the foot ipsilateral to the CCD (tactile allodynia). Whole cell patch-clamp recordings were obtained, in vitro, from small-sized somata and, for the first time, in the intact DRG. Under current clamp, CCD neurons exhibited a significantly lower rheobase compared with controls. A few CCD but no control neurons exhibited spontaneous action potentials. CCD neurons showed an increase in the density of TTX-resistant and TTX-sensitive Na(+) current. CCD neurons also exhibited an enhanced density of voltage-dependent K(+) current, due to an increase in delayed rectifier K(+) current, without a change in the transient or "A" current. We conclude that CCD in the mouse produces a model of radicular pain, as we have previously demonstrated in the rat. While the role of enhanced K(+) current remains to be elucidated, we speculate that it represents a compensatory neuronal response to reduce ectopic or aberrant levels of neuronal activity produced by the injury.     

Chronic compression of mouse dorsal root ganglion alters voltage-gated sodium and potassium currents in medium-sized dorsal root ganglion neurons

Chronic compression (CCD) of the dorsal root ganglion (DRG) is a model of human radicular pain produced by intraforaminal stenosis and other disorders affecting the DRG, spinal nerve, or root. Previously, we examined electrophysiological changes in small-diameter lumbar level 3 (L3) and L4 DRG neurons treated with CCD; the present study extends these observations to medium-sized DRG neurons, which mediate additional sensory modalities, both nociceptive and non-nociceptive. Whole-cell patch-clamp recordings were obtained from medium-sized somata in the intact DRG in vitro. Compared with neurons from unoperated control animals, CCD neurons exhibited a decrease in the current threshold for action potential generation. In the CCD group, current densities of TTX-resistant and TTX-sensitive Na(+) current were increased, whereas the density of delayed rectifier voltage-dependent K(+) current was decreased. No change was observed in the transient or "A" current after CCD. We conclude that CCD in the mouse produces hyperexcitability in medium-sized DRG neurons, and the hyperexcitability is associated with an increased density of Na(+) current and a decreased density of delayed rectifier voltage-dependent K(+) current.     

Acetylcholinesterase inhibitor treatment delays recovery from axotomy in cultured dorsal root ganglion neurons

Summary We have previously reported that dorsal root ganglion neurons cultured in the presence of the highly specific, reversible acetylcholinesterase inhibitor 1,5-bis-(4-allyldimethylammoniumphenyl) pentan-3-one dibromide (BW284c51), showed significantly reduced neurite outgrowth and contained massive perikaryal inclusions of neurofilaments. In the present report we have more closely examined these changes in a time course study over a 21-day culture period using a combined morphological, immunocytochemical and enzymatic approach and additionally, describe, the effects of acetylcholinesterase inhibitor treatment on the state of neurofilament phosphorylation. Finally, we have examined the effects of co-administration of N⁶,2-0-dibutyryladenosine 35-cyclic monophosphate (dbcAMP) with BW284c51. At 1 day in culture, both control and treated cells displayed eccentrically located nuclei, numerous polysomes and perikaryal accumulations of neurofilaments which were immunoreactive with both phosphorylation- and nonphosphorylation-dependent neurofilament antibodies. These cytological changes, which are common features of the chromatolytic reaction following axotomyin vivo, rapidly resolved in the control neurons, where by 7 days in culture, the neurofilament accumulations had completely disappeared and neurite outgrowth was robust. In contrast, inhibitor-treated neurons retained the post-axotomy features up to 21 days and had significantly reduced neurite outgrowth. In addition, we have investigated a possible role of cyclic adenosine monophosphate (cAMP) in the recovery process since it has been shown to enhance neuritic outgrowth in cultured neurons. Our results demonstrate that the addition of dbcAMP, a membrane permeable analog of cAMP, significantly enhanced neuritic outgrowth and accelerated the recovery of BW284c51-treated dorsal root ganglion cells, as gauged by the disappearance of the axotomy-related cytological changes. Treatment with dbcAMP also increased acetylcholinesterase activity which has been positively correlated with neurite outgrowth bothin vivo andin vitro. Together, these observations suggest that acetylcholinesterase has a non-cholinolytic, neurotrophic role in neuronal regeneration and development.     

Voltage-dependent K+ currents in rat cardiac dorsal root ganglion neurons

We have assessed the expression and kinetics of voltage-gated K(+) currents in cardiac dorsal root ganglion (DRG) neurons in rats. The neurons were labelled by prior injection of a fluorescent tracer into the pericardial sack. Ninety-nine neurons were labelled: 24% small (diameter<30 microm), 66% medium-sized (diameter 30 microm>.48 microm) and 10% large (>48 microm) neurons. Current recordings were performed in small and medium-sized neurons. The kinetic and pharmacological properties of K(+) currents recorded in these two groups of neurons were identical and the results obtained from these neurons were pooled. Three types of K(+) currents were identified:a) I(As), slowly activating and slowly time-dependently inactivating current, with V(1/2) of activation -18 mV and current density at +30 mV equal to 164 pA/pF, V(1/2) of inactivation at -84 mV. b) I(Af) current, fast activating and fast time-dependently inactivating current, with V(1/2) of activation at two mV and current density at +30 mV equal to 180 pA/pF, V(1/2) of inactivation at -26 mV. At resting membrane potential I(As) was inactivated, whilst I(Af), available for activation. The I(As) currents recovered faster from inactivation than I(Af) current. 4-Aminopiridyne (4-AP) (10 mM) and tetraethylammonium (TEA) (100 mM) produced 98% and 92% reductions of I(Af) current, respectively and 27% and 66% of I(As) current, respectively. c) The I(K) current that did not inactivate over time. Its V(1/2) of activation was -11 mV and its current density equaled 67 pA/pF. This current was inhibited by 95% (100 mM) TEA, whilst 4-AP (10 mM) produced its 23% reduction. All three K(+) current components (I(As), I(Af) and I(K)) were present in every small and medium-sized cardiac DRG neuron.We suggest that at hyperpolarized membrane potentials the fast reactivating I(As) current limits the action potential firing rate of cardiac DRG neurons. At depolarised membrane potentials the I(Af) K(+) current, the reactivation of which is very slow, does not oppose the firing rate of cardiac DRG neurons.     

The modulation effects of BmK I, an alpha-like scorpion neurotoxin, on voltage-gated Na(+) currents in rat dorsal root ganglion neurons

The present study investigated the effects of BmK I, a Na(+) channel receptor site 3 modulator purified from the Buthus martensi Karsch (BmK) venom, on the voltage-gated sodium currents in dorsal root ganglion (DRG) neurons. Whole-cell patch-clamping was used to record the tetrodotoxin-sensitive (TTX-S) and tetrodotoxin-resistant (TTX-R) components of voltage-gated Na(+) currents in small DRG neurons. It was found that the inhibitory effect of BmK I on open-state inactivation of TTX-S Na(+) currents was stronger than that of TTX-R Na(+) currents. In addition, BmK I exhibited a selective enhancing effect on voltage-dependent activation of TTX-S currents, and an opposite effect on time-dependent activation of TTX-S and TTX-R Na(+) currents. The results suggested that the inhibitory effect of BmK I on open-state inactivation might contribute to the increase of peak TTX-S and TTX-R currents, and the enhancing effect of BmK I on time-dependent activation might also contribute to the increase of peak TTX-S currents. It was further suggested that a combined effect of BmK I including inhibiting the inactivation of TTX-S and TTX-R channels, accelerating activation and decreasing the activation threshold of TTX-S channels, might produce a hyperexcitability of small DRG neurons, and thus contribute to the BmK I-induced hyperalgesia.     

Altered expression of potassium channel subunit mRNA and alpha-dendrotoxin sensitivity of potassium currents in rat dorsal root ganglion neurons after axotomy

Previous studies have raised the possibility that a decrease in voltage-gated K+ currents may contribute to hyperexcitability of injured dorsal root ganglion (DRG) neurons and the emergence of neuropathic pain. We examined the effects of axotomy on mRNA levels for various Kv1 family subunits and voltage-gated K+ currents in L4-L5 DRG neurons from sham-operated and sciatic nerve-transected rats. RNase protection assay revealed that Kv1.1 and Kv 1.2 mRNAs are highly abundant while Kv1.3, Kv1.4, Kv1.5 and Kv1.6 mRNAs were detected at lower levels in L4-L5 DRGs from sham and intact rats. Axotomy significantly decreased Kv1.1, Kv1.2, Kv1.3 and Kv1.4 mRNA levels by approximately 35%, approximately 60%, approximately 40% and approximately 80%, respectively, but did not significantly change Kv1.5 or Kv1.6 mRNA levels. Patch clamp recordings revealed two types of K+ currents in small-sized L4-L5 DRG neurons: sustained delayed rectifier currents elicited from a -40 mV holding potential and slowly inactivating A-type currents that was additionally activated from a -120 mV holding potential. Axotomy decreased both types of K+ currents by 50-60% in injured DRG neurons. In addition, axotomy increased the alpha-dendrotoxin sensitivity of the delayed rectifier, but not slow A-type K+ currents in injured DRG neurons. These results suggest that Kv1.1 and Kv1.2 subunits are major components of voltage-gated K+ channels in L4-L5 DRG neurons and that the decreased expression of Kv1-family subunits significantly contributes to the reduction and altered kinetics of Kv current in axotomized neurons.     

Dihydropyridine block of voltage-dependent K currents in rat dorsal root ganglion neurons

The dihydropyridines nifedipine, nimodipine and Bay K 8644 are widely used as pharmacological tools to assess the contribution of L-type voltage-gated Ca²⁺ channels to a variety of neuronal processes including synaptic transmission, excitability and second messenger signaling. These compounds are still used in neuronal preparations despite evidence from cardiac tissue and heterologous expression systems that they block several voltage-dependent K⁺ (Kv) channels. Both because these compounds have been used to assess the relative contribution of L-type Ca²⁺ channels to several different processes in dorsal root ganglion (DRG) neurons and because a relatively wide variety of Kv channels present in other neuronal populations is present in DRG neurons, we determined the extent to which dihydropyridines block Kv currents in these neurons. Standard whole cell patch clamp techniques were used to study acutely disassociated adult rat DRG neurons. All three dihydropyridines tested blocked Kv currents in DRG neurons; IC₅₀ values (concentration resulting in an inhibition that is 50% of maximum) for nifedipine and nimodipine-induced block of sustained Kv currents were 14.5 and 6.6 μM, respectively. The magnitude of sustained current block was 44±1.6%, 60±2%, and 56±2.9% with 10 μM nifedipine, nimodipine and Bay K 8644, respectively. Current block was occluded by neither 4-aminopyridine (5 mM) nor tetraethylammonium (135 mM). Dihydropyridine-induced block of Kv currents was not associated with a shift in the voltage-dependence of current activation or inactivation, the recovery from inactivation, or voltage dependent block. However, there was a small use-dependence to the dihydropyridine-induced block. Our results suggest that several types of Kv channels in DRG neurons are blocked by mechanisms distinct from those underlying block of Kv channels in cardiac myocytes. Importantly, our results suggest that if investigators wish to explore the contribution of L-type Ca²⁺ channels to neuronal function, they should consider alternative strategies for the manipulation of these channels than the use of dihydropyridines.     

P2X receptor-mediated ionic currents in dorsal root ganglion neurons.

Nociceptive neurons in the dorsal root ganglia (DRG) are activated by extracellular ATP, implicating P2X receptors as potential mediators of painful stimuli. However, the P2X receptor subtype(s) underlying this activity remain in question. Using electrophysiological techniques, the effects of P2X receptor agonists and antagonists were examined on acutely dissociated adult rat lumbar DRG neurons. Putative P2X-expressing nociceptors were identified by labeling neurons with the lectin IB4. These neurons could be grouped into three categories based on response kinetics to extracellularly applied ATP. Some DRG responses (slow DRG) were relatively slowly activating, nondesensitizing, and activated by the ATP analogue alpha,beta-meATP. These responses resembled those recorded from 1321N1 cells expressing recombinant heteromultimeric rat P2X2/3 receptors. Other responses (fast DRG) were rapidly activating and desensitized almost completely during agonist application. These responses had properties similar to those recorded from 1321N1 cells expressing recombinant rat P2X3 receptors. A third group (mixed DRG) activated and desensitized rapidly (P2X3-like), but also had a slow, nondesensitizing component that functionally prolonged the current. Like the fast component, the slow component was activated by both ATP and alpha, beta-meATP and was blocked by the P2X antagonist TNP-ATP. But unlike the fast component, the slow component could follow high-frequency activation by agonist, and its amplitude was potentiated under acidic conditions. These characteristics most closely resemble those of rat P2X2/3 receptors. These data suggest that there are at least two populations of P2X receptors present on adult DRG nociceptive neurons, P2X3 and P2X2/3. These receptors are expressed either separately or together on individual neurons and may play a role in the processing of nociceptive information from the periphery to the spinal cord.     

Differential effects of bupivacaine and tetracaine on capsaicin-induced currents in dorsal root ganglion neurons

Capsaicin opens the TRPV1 channel, a cation channel that depolarizes and activates nociceptive neurons. Following this initial activation, neurons become desensitized to subsequent applications of capsaicin as well as to other noxious stimuli, a phenomenon attributed primarily to the entry of Ca2+ ions through the open TRPV1 channel. This ability of capsaicin to desensitize nociceptors has led to its use as an analgesic in the treatment of a variety of chronic pain states. Because treatment with capsaicin is initially quite painful, local anesthetics are sometimes used to block axonal conduction in nociceptive neurons and thus minimize pain. However, local anesthetics might also block TRPV1 and prevent the Ca2+ entry required for capsaicin-induced desensitization. We have studied the direct effect of local anesthetics on currents induced by capsaicin (1 microM) in acutely isolated rat dorsal root ganglion neurons using the whole cell patch clamp technique. At the highest concentration tested (1 mM), bupivacaine only moderately inhibited the capsaicin-induced current to 55 +/- 27% of control (mean +/- S.D.; n=12, p<0.01). Tetracaine (1 mM), on the other hand, enhanced the capsaicin-induced current to 151 +/- 34% of control (mean +/- S.D.; n=7, p<0.01). These results show that local anesthetics can be used to prevent the initial pain induced by application of capsaicin without abolishing, and perhaps even enhancing, its desensitizing actions.     

5HT increases excitability of nociceptor-like rat dorsal root ganglion neurons via cAMP-coupled TTX-resistant Na(+) channels.

The physiological effects of 5HT receptor coupling to TTX-resistant Na(+) current, and the signaling pathway involved, was studied in a nociceptor-like subpopulation of rat dorsal root ganglion (DRG) cells (type 2), which can be identified by expression of a low-threshold, slowly inactivating A-current. The 5HT-mediated increase in TTX-resistant Na(+) current in type 2 DRG cells was mimicked and occluded by 10 microM forskolin. Superfusion of type 2 DRG cells on the outside with 1 mM 8-bromo-cAMP or chlorophenylthio-cAMP (CPT-cAMP) increased the Na(+) current, but less than 5HT itself. However, perfusion of the cells inside with 2 mM CPT-cAMP strongly increased the amplitude of control Na(+) currents and completely occluded the effect of 5HT. Thus it appears that the signaling pathway includes cAMP. The phosphodiesterase inhibitor 3-isobutyl-L-methylxanthine (200 microM) also mimicked the effect of 5HT on Na(+) current, suggesting tonic adenylyl cyclase activity. 5HT reduced the amount of current required to evoke action potentials in type 2 DRG cells, suggesting that 5HT may lower the threshold for activation of nociceptor peripheral receptors. The above data suggest that serotonergic modulation of TTX-resistant Na(+) channels through a cAMP-dependent signaling pathway in nociceptors may participate in the generation of hyperalgesia.     

Calcium currents and transmitter output in cultured spinal cord and dorsal root ganglion neurons

The effects of repetitive activation upon voltage-dependent calcium currents (ICa) and transmitter release were studied in dissociated cell cultures of fetal mouse spinal cord and dorsal root ganglion. Sodium and potassium currents were suppressed with tetrodotoxin (TTX) and tetraethylammonium (TEA) ions, 4-aminopyridine (4-AP), and cesium sulfate. Calcium currents were compared under voltage clamp before and after a series of depolarizing clamp pulses in spinal cord (SC) and dorsal root ganglion (DRG) neurons. Repetitive activation resulted in an exponential decline in ICa, with the decrease in ICa being much more marked in DRG compared with SC neurons. Both voltage-dependent inactivation and inactivation related to the intracellular movement of Ca2+ appeared to be involved in the decrement in ICa with repetitive activation. A decrease in transmitter output occurred with repetitive activation in DRG neurons but not in SC neurons (either excitatory or inhibitory). DRG neuron synaptic boutons had fewer mitochondria than did the boutons of either excitatory or inhibitory of SC neurons. The decrement in both ICa and synaptic transmitter output in DRG neurons could last for prolonged periods (at least minutes) following repetitive activation. We hypothesize that this vulnerability of DRG neurons to repetitive activation may be related, at least in part, to a relative incapacity to maintain a low intracellular calcium ion concentration [Ca]i during periods of increased calcium ingress associated with excitation. Such an incapacity to buffer [Ca]i may be one mechanism leading to the inactive synapses seen in some studies in vitro and in vivo of synaptic transmission.     

Axotomy- and autotomy-induced changes in Ca2+ and K+ channel currents of rat dorsal root ganglion neurons

Sciatic nerve section (axotomy) increases the excitability of rat dorsal root ganglion (DRG) neurons. The changes in Ca2+ currents, K+ currents, Ca2+ sensitive K+ current, and hyperpolarization-activated cation current (I(H)) that may be associated with this effect were examined by whole cell recording. Axotomy affected the same conductances in all types of DRG neuron. In general, the largest changes were seen in "small" cells and the smallest changes were seen in "large" cells. High-voltage-activated Ca2+ channel current (HVA-I(Ba)) was reduced by axotomy. Although currents recorded in axotomized neurons exhibited increased inactivation, this did not account for all of the reduction in HVA-I(Ba). Activation kinetics were unchanged, and experiments with nifedipine and/or omega-conotoxin GVIA showed that there was no change in the percentage contribution of L-type, N-type, or "other" HVA-I(Ba) to the total current after axotomy. T-type (low-voltage-activated) I(Ba) was not affected by axotomy. Ca2+ sensitive K+ conductance (g(K,Ca)) appeared to be reduced, but when voltage protocols were adjusted to elicit similar amounts of Ca2+ influx into control and axotomized cells, I(K,Ca)(s) were unchanged. After axotomy, Cd2+ insensitive, steady-state K+ channel current, which primarily comprised delayed rectifier K+ current (I(K)), was reduced by about 60% in small, medium, and large cells. These data suggest that axotomy-induced increases in excitability are associated with decreases in I(K) and/or decreases in g(K,Ca) that are secondary to decreased Ca2+ influx. Because I(H) was reduced by axotomy, changes in this current do not contribute to increased excitability. The amplitude and inactivation of I(Ba) in all cell types was changed more profoundly in animals that exhibited self-mutilatory behavior (autotomy). The onset of this behavior corresponded with significant reduction in I(Ba) of large neurons. This finding supports the hypothesis that autotomy, that may be related to human neuropathic pain, is associated with changes in the properties of large myelinated sensory neurons.     

Similar electrophysiological changes in axotomized and neighboring intact dorsal root ganglion neurons

We investigated electrophysiological changes in chronically axotomized and neighboring intact dorsal root ganglion (DRG) neurons in rats after either a peripheral axotomy consisting of an L5 spinal nerve ligation (SNL) or a central axotomy produced by an L5 partial rhizotomy (PR). SNL produced lasting hyperalgesia to punctate indentation and tactile allodynia to innocuous stroking of the foot ipsilateral to the injury. PR produced ipsilateral hyperalgesia without allodynia with recovery by day 10. Intracellular recordings were obtained in vivo from the cell bodies (somata) of axotomized and intact DRG neurons, some with functionally identified peripheral receptive fields. PR produced only minor electrophysiological changes in both axotomized and intact somata in L5 DRG. In contrast, extensive changes were observed after SNL in large- and medium-sized, but not small-sized, somata of intact (L4) as well as axotomized (L5) DRG neurons. These changes included (in relation to sham values) higher input resistance, lower current and voltage thresholds, and action potentials with longer durations and slower rising and falling rates. The incidence of spontaneous activity, recorded extracellularly from dorsal root fibers in vitro, was significantly higher (in relation to sham) after SNL but not after PR, and occurred in myelinated but not unmyelinated fibers from both L4 (9.1%) and L5 (16.7%) DRGs. We hypothesize that the changes in the electrophysiological properties of axotomized and intact DRG neurons after SNL are produced by a mechanism associated with Wallerian degeneration and that the hyperexcitability of intact neurons may contribute to SNL-induced hyperalgesia and allodynia.     

Sulfur dioxide derivatives modulation of high-threshold calcium currents in rat dorsal root ganglion neurons

This study addressed the effect of sulfur dioxide (SO(2)) derivatives on high-voltage-activated calcium currents (HVA-I(Ca)) in somatic membrane of freshly isolated rat dorsal root ganglion (DRG) neurons by using the whole-cell configuration of patch-clamp technique. High-threshold Ca(2+) channels are highly expressed in small dorsal root ganglion neurons. SO(2) derivatives increased the amplitudes of calcium currents in a concentration-dependent and voltage-dependent manner. The 50% enhancement concentrations (EC(50)) of SO(2) derivatives on HVA-I(Ca) was about 0.4 microM. In addition, SO(2) derivatives significantly shifted the activation and inactivation curve in the depolarizing direction. Parameters for the fit of a Boltzmann equation to mean values for the activation were V(1/2)=-17.9+/-1.3 mV before and -12.5+/-1.1 mV after application 0.5 microM SO(2) derivatives 2 min (P<0.05). The half inactivation of HVA-I(Ca) was shifted 9.7 mV to positive direction (P<0.05). Furthermore, SO(2) derivatives significantly prolonged the slow constant of inactivation, slowed the fast recovery but markedly accelerated the slow recovery of HVA-I(Ca) from inactivation. From HP of -60 mV 0.5 microM SO(2) derivatives increased the amplitude of HVA-I(Ca) with a depolarizing voltage step to -10 mV about 54.0% in small DRG neurons but 33.3% in large DRG neurons. These results indicated a possible correlation between the change of calcium channels and SO(2) inhalation toxicity, which might cause periphery neurons abnormal regulation of nociceptive transmission via calcium channels.     

Radiation-induced apoptosis in dorsal root ganglion neurons

Ionizing radiation (IR) results in apoptosis in a number of actively proliferating or immature cell types. The effect of IR on rat dorsal root ganglion (DRG) neurons was examined in dissociated cell cultures. After exposure to IR, embryonic DRG neurons, established in cell culture for six days, underwent cell death in a manner that was dose-dependent, requiring a minimum of 8 to 16 Gy. Twenty-five per cent cell loss occurred in embryonic day 15 (E-15) neurons, grown in cell culture for 6 days (immature), and then treated with 24 Gy IR. In contrast, only 2% cell loss occurred in E-15 neurons maintained in culture for 21 days ("mature") and then treated with 24 Gy IR. Staining with a fluorescent DNA-binding dye demonstrated clumping of the nuclear chromatin typical of apoptosis. Initiation of the apoptosis occurred within 24 h after exposure to IR. Apoptosis was prevented by inhibition of protein synthesis with cycloheximide. Apoptosis induced by IR occurred more frequently in immature than in mature neurons. Immature DRG neurons have a lower concentration of intracellular calcium ([Ca2+]i) than mature neurons. Elevation of [Ca2+]i by exposure to a high extracellular potassium ion concentration (35 M) depolarizes the cell membrane with a resultant influx of calcium ions. The activation of programmed cell death after nerve growth factor (NGF) withdrawal is inversely correlated with [Ca2+]i in immature DRG neurons. When treated with high extracellular potassium, these immature neurons were resistant to IR exposure in a manner similar to that observed in mature neurons. These data suggest that [Ca2+]i modulates the apoptotic response of neurons after exposure to IR in a similar manner to that proposed by the Ca2+ setpoint hypothesis for control of NGF withdrawal-induced apoptosis.     

Ionic currents in the somatic membrane of rat dorsal root ganglion neurons —III. Potassium currents

Potassium transmembrane currents induced by membrane depolarization have been studied on isolated dorsal root ganglion neruons of 5–10 day-old rats using the voltage-clamp technique. The neurons were intracellularly dialysed with solutions containing a fixed amount of K⁺ ions, and the correspondence between the reversal potentials of the measured currents and the theoretical potassium equilibrium potential was determined. Sodium and calcium transmembrane currents were eliminated by replacement of Na⁺ ions in the extracellular solution and by introduction of fluoride into the cell.In all cells studied, the total potassium current could be separated into two components—fast and slow (I_K^f and I_K^sby changing the holding potential level. I_K^fwas inactivated comparatively fast obeying first-order kinetics. The dependence h_∝ (V) for this current was S-shaped with meanV₁₂ = ?75 mV. Therefore, this current could be almost completely switched off at holding potentials more positive than ?50 mV. On the contrary, the inactivation of I_K^s developed very slowly even at stronger depolarizing potential shifts. The mean activation time constants calculated on the basis of Hodgkin-Huxley model for potassium currents were 0.5 ms at zero testing potential for I_K^f and 40 ms at + 30 mV for I_K^s.The reversal potential for I_K^f determined from instantaneous current-voltage characteristics was close to the equilibrium potential for a potassium electrode. The reversal potential for I_K^s was shifted in the depolarizing direction by about 25 mV indicating lower selectivity of the corresponding channels.