Modulators of calcium influx regulate membrane excitability in rat dorsal root ganglion neurons.
BACKGROUND
Chronic neuropathic pain resulting from neuronal damage remains difficult to treat, in part, because of incomplete understanding of underlying cellular mechanisms. We have previously shown that inward Ca2+ flux (I(Ca)) across the sensory neuron plasmalemma is decreased in a rodent model of chronic neuropathic pain, but the direct consequence of this loss of I(Ca) on function of the sensory neuron has not been defined. We therefore examined the extent to which altered membrane properties after nerve injury, especially increased excitability that may contribute to chronic pain, are attributable to diminished Ca2+ entry.


METHODS
Intracellular microelectrode measurements were obtained from A-type neurons of dorsal root ganglia excised from uninjured rats. Recording conditions were varied to suppress or promote I(Ca) while biophysical variables and excitability were determined.


RESULTS
Both lowered external bath Ca2+ concentration and blockade of I(Ca) with bath cadmium diminished the duration and area of the after-hyperpolarization (AHP), accompanied by decreased current threshold for action potential (AP) initiation and increased repetitive firing during sustained depolarization. Reciprocally, elevated bath Ca2+ increased the AHP and suppressed repetitive firing. Voltage sag during neuronal hyperpolarization, indicative of the cation-nonselective H-current, diminished with decreased bath Ca2+, cadmium application, or chelation of intracellular Ca2+. Additional recordings with selective blockers of I(Ca) subtypes showed that N-, P/Q, L-, and R-type currents each contribute to generation of the AHP and that blockade of any of these, and the T-type current, slows the AP upstroke, prolongs the AP duration, and (except for L-type current) decreases the current threshold for AP initiation.


CONCLUSIONS
Taken together, our findings show that suppression of I(Ca) decreases the AHP, reduces the hyperpolarization-induced voltage sag, and increases excitability in sensory neurons, replicating changes that follow peripheral nerve trauma. This suggests that the loss of I(Ca) previously demonstrated in injured sensory neurons contributes to their dysfunction and hyperexcitability, and may lead to neuropathic pain.
Lirk
Philipp
P
Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin, USA.
Poroli
Mark
M
Rigaud
Marcel
M
Fuchs
Andreas
A
Fillip
Patrick
P
Huang
Chun-Yuan
CY
Ljubkovic
Marko
M
Sapunar
Damir
D
Hogan
Quinn
Q
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D004492:Edetic Acid / Q000494:pharmacology; D004594:Electrophysiology; D005727:Ganglia, Spinal / Q000502:physiology*; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects*; D009475:Neurons, Afferent / Q000378:metabolism / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley
D002121:Calcium Channel Blockers; D002614:Chelating Agents; D002104:Cadmium; D004492:Edetic Acid; D002118:Calcium