Quantitative single-cell differences in mu-opioid receptor mRNA distinguish myelinated and unmyelinated nociceptors.

A remarkable feature of opioids is that they inhibit pain that persists from previous injuries without eliminating either the initial pain of a new injury or the protective reflexes triggered by it. Here we ask whether selective expression of the mu-opioid receptor (MOR) gene in primary nociceptors (pain-sensing neurons) might contribute to this aspect of opioid specificity. We quantified single-cell levels of MOR mRNA and measured opioid inhibition of Ca channels on identified nociceptors and low-threshold mechanosensors (non-nociceptors) isolated from rats. Negligibly few non-nociceptors express MOR mRNA, thereby rendering nonpain sensations insensitive to opioids. Nearly half of nociceptors of all size classes also fail to express MOR mRNA or to respond to opioids. Among the opioid-responsive nociceptors, a gene dose-response relationship exists such that maximal opioid inhibition occurs when the MOR mRNA concentration of a cell is >15 pm. Almost all large, myelinated nociceptors express MOR mRNA below this level, whereas small, unmyelinated nociceptors are likely to express above it. Because myelinated nociceptors mediate anti-nociceptive reflexes, the data suggest that fine control of the MOR mRNA level contributes to a complex neural trait: the ability of opioids to suppress persistent pain without preventing response to a new injury.