Inhibition of Ca2+ currents by a mu-opioid in a defined subset of rat sensory neurons.

Activation of the endogenous opioid system can suppress pain without affecting other sensations, but the cellular mechanism of this selectivity is unclear. The analgesia might be due to inhibitory synapses arranged only on neurons whose activity leads to pain sensations. Alternatively, opioids might be released broadly, with neurons involved in pain sensation being especially sensitive. Therefore, we asked whether different subsets of rat dorsal root ganglion (DRG) sensory neurons vary in their sensitivity to opioids. Dissociated neurons were subdivided according to the spinal laminae to which they likely had projected, and whether they had innervated muscle. Using the patch-clamp method, we measured the inhibition of Ca2+ current by DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol), a peptide that selectively activates the mu (morphine) receptor. We also investigated the presence of different types of Ca2+ channels. In DRG neurons chosen at random, Ca2+ currents were inhibited by DAGO to widely varying degrees, with an average inhibition of 38%. Ca2+ currents in neurons in a subset that projects to laminae I and II had a lower average inhibition, and unlike the randomly selected cells, the responses were predictable and tightly distributed about the mean. This indicates that the variability of opioid sensitivity among DRG neurons reflects the presence of different subsets of cells. Since neurons projecting to laminae I and II, the projection site of nociceptive neurons, did not show high opioid sensitivity, there is no evidence that nociceptive neurons have stronger responses to opioids. But a firm conclusion is impossible because projection site does not strictly define sensory modality.