Role of decreased sensory neuron membrane calcium currents in the genesis of neuropathic pain

Abstract

The pathogenesis of neuropathic pain is incompletely understood and treatments are often inadequate. Cytoplasmic Ca(2+) regulates numerous cellular processes in neurons. This review therefore examines the pathogenic contribution of altered inward Ca(2+) flux (I(Ca)) through voltage-gated Ca(2+) channels in sensory neurons after peripheral nerve injury. We reviewed studies that recorded membrane currents through intracellular and patch-clamp techniques, as well as intracellular Ca(2+) levels using fluorimetric indicators, and performed behavioral analysis of rodent nerve injury models. Following nerve injury by partial ligation, a response characterized by sustained lifting, shaking, and licking of the paw after sharp mechanical stimulation is a reliable indicator or neuropathic pain. Primary sensory neurons isolated from animals with this behavior show a decrease in high-voltage activated I(Ca) by approximately one third. Low voltage-activated I(Ca) is nearly eliminated by peripheral nerve injury. Loss of I(Ca) leads to decreased activation of Ca(2+)-activated K(+) currents, which are also directly reduced in traumatized neurons. As a result of these changes in membrane currents, membrane voltage recordings show increased action potential duration and diminished afterhyperpolarization. Excitability is elevated, as indicated by resting membrane potential depolarization and a decreased current threshold for action potential initiation. Traumatized nociceptive neurons develop increased repetitive firing during sustained depolarization after axotomy. Concurrently, cytoplasmic Ca(2+) transients are diminished. In conclusions, axotomized neurons, especially pain-conducting ones, develop instability and elevated excitability after peripheral injury. Treatment of neuronal I(Ca) loss at the level of injury of the dorsal root ganglion may provide a novel therapeutic pathway.

Figure 1

Neuropathic pain models involving peripheral nerve injury in the rat. (A) In chronic constriction injury (Bennett et al, 26), four chronic gut ligatures are placed around the distal sciatic nerve. They tighten and create inflammation with time, and additionally cause ischemia and axonal discontinuity. (B) The spinal nerve ligation model (Kim et al, 33) is produced by ligating and sectioning the fifth lumbar (L5) and L6 spinal nerves, so that the sciatic nerve is supplied only by the neurons of L4. This allows anatomic segregation of the axotomized (L5) and intact (L4) elements. Contributions to abnormal pain may arise from the axotomized neurons (a), or from degeneration of distal segments (b) that initiates inflammation and irritation of intact fibers (c).

Figure 2

Measurement of mechanical hyperalgesia. Touch of a 22-gauge spinal needle to the plantar surface of the hind paw may elicit a prolonged lifting, shaking, and chewing of the paw. This is rare in control animals and those having sham surgery to expose but not cut the spinal nerves. However, after spinal nerve ligation (SNL), there is an ipsilateral (circles) increase in incidence of this behavior when tested 4, 11, and 18 days after baseline (BL). # – different from baseline; * – different from contralateral (squares). From Hogan et al (35), with permission.

Figure 3

Inward high-voltage activated Ca²⁺ currents measured by patch-clamp recording in dissociated sensory neurons decrease after chronic constriction injury. (A) Sample currents elicited by square-wave voltage commands (bottom) are decreased in an injured neuron (middle) compared to a neuron from a control animal (top). (B) Current-voltage plot of average data for medium sized neurons shows a loss of peak current in injured neurons from animals with neuropathic pain. From Hogan et al (46), with permission.

Figure 4

Inward low-voltage activated Ca²⁺ currents measured by patch-clamp recording in dissociated sensory neurons decrease after chronic constriction injury. (A) Current-voltage plot of average data for medium sized-neurons shows a loss of peak current in injured neurons from animals with neuropathic pain, and particularly a loss of current at low voltages. (B) Presentation of a voltage command in the from of an action potential (top) produces current through low-voltage activated Ca²⁺ channels (bottom) that is substantially reduced in an injured neuron compared with a control neuron. From McCallum et al (49), with permission.

Figure 5

Action potential (AP) dimensions in control neurons and after spinal nerve ligation (SNL), by intracellular microelectrode recording from intact dorsal root ganglia. (A) Three sample recordings from control and from the fifth lumber (L5) ganglion after SNL show the loss of afterhyperpolarization and increase in AP duration after injury. CV – conduction velocity. Resting membrane potential is indicated by the dotted lines. (B) Average data for the incidence of repetitive firing during sustained depolarization of neurons in the control group (C), after sham injury (Sh), and in the L4 and L5 ganglia after SNL. Small numbers in the columns indicate number of neurons recorded. Brackets indicate significant differences by post hoc testing. From Sapunar et al (59), with permission.

Figure 6

Relationship of conduction velocity (CV) and action potential (AP) duration for sensory neurons recorded by intracellular microelectrode, in control neurons and fifth lumbar (L5) neurons after spinal nerve ligation (SNL). (A) Control neurons show an inverse relation between CV and AP duration. (B) After injury, there is general prolongation of AP duration, but also the appearance of a new population with very prolonged AP despite a CV characteristic of nociceptive C-type fibers that have CV less than 1.5m/s (vertical line). The horizontal line indicates AP duration of 2 ms. From Sapunar et al (59), with permission.

Figure 7

Consequence of loss on Ca²⁺ current on membrane function. (A) Recordings from a dissociated sensory neuron by the patch-clamp technique show the action potential (AP) voltage traces (top) produced by a current pulse (bottom) at baseline and after Ca²⁺ current is blocked by selective toxins. (B) Using the baseline AP trace as a voltage command (not shown), the maximal inward component of the total current is reduced, but the later outward current is also diminished, accounting for the prolongation of the AP and diminished afterhyperpolarization evident in the voltage traces. From McCallum et al (48), with permission.

Figure 8

Regulation of firing properties of sensory neurons by Ca²⁺ current. During intracellular recording from an intact dorsal root ganglion under baseline conditions (A) only a single action potential (AP) is triggered by injection of progressively larger depolarizing currents C, After lowering the Ca²⁺ concentration in the bath (B) with reciprocal elevation of Mg²⁺ concentration, multiple APs are initiated by identical current injection.

Figure 9

Influence of injury on neuronal excitability. In the normal condition, a depolarizing stimulus opens voltage-gated Ca²⁺ channels. The resulting of Ca²⁺ activates Ca²⁺-sensitive K⁺ channels, which regulate the afterhyperpolarization (AHP) of the action potential. By controlling the membrane potential and resistance, the AHP in turn regulates repetitive firing. After injury, decreased Ca²⁺ influx causes a diminished outward K⁺ current and less of AHP, such that the same stimulus results in repetitive firing.

Figure 10

Injury reduces resting level of cytoplasmic Ca²⁺ ([Ca²⁺]_c). Spinal nerve ligation injury reduces [Ca²⁺]_c in axotomized fifth lumbar (L5) neurons of both sizes, but only in large neurons from the adjacent L4 population. Small numbers in the bars indicate number of neurons. From Fuchs et al (83), with permission.

Figure 11

Schematic traces of Ca²⁺ transients indicated by fluorescence ratio of Fura-2 (R_340/380), summarizing injury-induced alterations. The bottom two panels show traces for axotomized neurons from the 5th lumbar (L5) ganglion after spinal nerve ligation, while the upper two panels show traces for neighboring neurons from the L4 ganglion. For both, the left panels show traces from large and capsaicin-insensitive neurons that convey non-nociceptive sensory information, while the right panels show traces from small and capsaicin-sensitive nociceptive neurons. In each case, the dotted line represents traces from control neurons and the solid line represents traces from neurons after injury.

Figure 12

Schematic diagram depicting the contribution of reduced sensory neuron Ca²⁺ current (I_Ca) to neuropathic pain. Injured nerve tissue distal to the spinal nerve ligation (SNL) undergoes Wallerian degeneration (dotted line), while neurons from the fifth lumbar (L5) dorsal root ganglion (DRG) are activated by movement, catecholamines, other algogenic agents and cross-excitation from adjacent intact neurons. Reduced I_Ca shortens afterhyperpolarizations (AHPs), which contributes to burst firing. Loss of I_Ca impairs natural signal filtering at the T-branch, where the stem axon splits into spinal nerve and dorsal root branches. Reduced I_Ca also prolongs action potential (AP) duration, which may increase excitatory synaptic transmission in the dorsal horn (DH). Spared nerves transmit activity evoked by stimulation in the receptive field, and these signals encounter DH neurons that are sensitized by L5 input. From McCallum et al (48), with permission.