μ- and κ-opioid receptors selectively reduce the same transient components of high-threshold calcium current in rat dorsal root ganglion sensory neurons

Abstract

Whole-cell patch-clamp recordings were used to examine the regulation of voltage-dependent calcium channels by μ- and κ-opioid receptors in acutely isolated rat dorsal root ganglion (DRG) sensory neurons. Agonists selective for either μ- (Tyr-Pro-NMePhe-D-Pro-NH2, PLO17) or κ-opioid receptors (dynorphin A, U69,593) inhibited high-threshold calcium currents in a reversible and naloxone-sensitive manner, whereas administration of D- Pen2,5-enkephalin, a δ-selective agonist, was without effect. However, none of the opioids reduced low-threshold T-type currents. The inhibitory effects of PLO17 were blocked by the irreversible μ-opioid antagonist β- funaltrexamine but not the κ-opioid antagonist nor-binaltorphimine, while responses to κ-opioid agonists showed the opposite pattern of antagonist sensitivity. In addition, many cells responded to both PLO17 and dynorphin A (or U69,593), and in these neurons the inhibitory response to one agonist was occluded when tested in the presence of the other. These data suggest that μ- and κ-opioid receptors are coexpressed on at least some DRG neurons and appear to be functionally coupled to a common pool of calcium channels. Both rapidly inactivating (transient) and sustained components of high-threshold current, arising from pharmacologically distinct types of calcium channels, were identified in our neurons. Activation of μ-opioid receptors selectively reduced the transient component of currents evoked at +10 mV from V(h) = -80 mV, while sparing the sustained component. The transient component was irreversibly blocked by the N-type channel antagonist ω-conotoxin GVIA (ω- CgTx), and in one-half of the neurons there was a concomitant loss of the response to PLO17. In the remaining neurons, PLO17 continued to reduce a small fraction of ω-CgTx-insensitive current and subsequent administration of the L-type channel blocker nifedipine in saturating concentrations failed to reduce the opioid-induced inhibitory effect. These data demonstrate that μ-opioid receptors are negatively coupled to several pharmacologically distinct types of calcium channels in DRG sensory neurons, one that was blocked by ω-CgTx and thus likely to be N-type, and a second that was resistant to blockade by N- and L-type channel blockers.