Ca²⁺-dependent regulation of Ca²⁺ currents in rat primary afferent neurons: role of CaMKII and the effect of injury

Currents through voltage-gated Ca²⁺ channels (I(Ca)) may be regulated by cytoplasmic Ca²⁺ levels ([Ca²⁺](c)), producing Ca²⁺-dependent inactivation (CDI) or facilitation (CDF). Since I(Ca) regulates sensory neuron excitability, altered CDI or CDF could contribute to pain generation after peripheral nerve injury. We explored this by manipulating [Ca²⁺](c) while recording I(Ca) in rat sensory neurons. In uninjured neurons, elevating [Ca²⁺](c) with a conditioning prepulse (-15 mV, 2 s) inactivated I(Ca) measured during subsequent test pulses (-15 mV, 5 ms). This inactivation was Ca²⁺-dependent (CDI), since it was decreased with elimination of Ca²⁺ influx by depolarization to above the I(Ca) reversal potential, with high intracellular Ca²⁺ buffering (EGTA 10 mm or BAPTA 20 mm), and with substitution of Ba²⁺ for extracellular Ca²⁺, revealing a residual voltage-dependent inactivation. At longer latencies after conditioning (>6 s), I(Ca) recovered beyond baseline. This facilitation also proved to be Ca²⁺-dependent (CDF) using the protocols limiting cytoplasmic Ca²⁺ elevation. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) blockers applied by bath (KN-93, myristoyl-AIP) or expressed selectively in the sensory neurons (AIP) reduced CDF, unlike their inactive analogues. Protein kinase C inhibition (chelerythrine) had no effect. Selective blockade of N-type Ca²⁺ channels eliminated CDF, whereas L-type channel blockade had no effect. Following nerve injury, CDI was unaffected, but CDF was eliminated in axotomized neurons. Excitability of sensory neurons in intact ganglia from control animals was diminished after a similar conditioning pulse, but this regulation was eliminated by injury. These findings indicate that I(Ca) in sensory neurons is subject to both CDI and CDF, and that hyperexcitability following injury-induced loss of CDF may result from diminished CaMKII activity.