High voltage-activated Ca ²⁺ channel currents in isolectin B ₄-positive and -negative small dorsal root ganglion neurons of rats

Voltage-gated Ca ²⁺ channels in the primary sensory neurons are important for neurotransmitter release and regulation of nociceptive transmission. Although multiple classes of Ca ²⁺ channels are expressed in the dorsal root ganglion (DRG) neurons, little is known about the difference in the specific channel subtypes among the different types of DRG neurons. In this study, we determined the possible difference in high voltage-activated Ca ²⁺ channel currents between isolectin B ₄ (IB ₄)-positive and IB ₄-negative small-sized (15-30 μm) DRG neurons. Rat DRG neurons were acutely isolated and labeled with IB ₄ conjugated to a fluorescent dye. Whole-cell patch clamp recordings of barium currents flowing through calcium channels were performed on neurons with and without IB ₄. The peak current density of voltage-gated Ca ²⁺ currents was not significantly different between IB ₄-positive and IB ₄-negative neurons. Also, both nimodipine and ω-agatoxin IVA produced similar inhibitory effects on Ca ²⁺ currents in these two types of neurons. However, block of N-type Ca ²⁺ channels with ω-conotoxin GVIA produced a significantly greater reduction of Ca ²⁺ currents in IB ₄-positive than IB ₄-negative neurons. Furthermore, the IB ₄-positive neurons had a significantly smaller residual Ca ²⁺ currents than IB ₄-negative neurons. These data suggest that a higher density of N-type Ca ²⁺ channels is present in IB ₄-positive than IB ₄-negative small-sized DRG neurons. This differential expression of the subtypes of high voltage-activated Ca ²⁺ channels may contribute to the different function of these two classes of nociceptive neurons.