Cellular and Molecular Mechanisms of Pain Control by Opiates

Abstract

Most prescribed analgesics take advantage a complex and dynamic opioid neurotransmission system endogenous to all mammals, capable of relieving the pain of a recent injury while leaving other sensations intact. At analgesic doses, the opiate morphine selectively activates the mu opioid receptor (MOR) and produces a similar pain block. The goal of section I of this dissertation is to explain this phenomenon. Using an MOR agonist called DAMGO, we measured the electrical inhibition caused by this opioid peptide in pain sensing (nociceptor) sensory neurons that we put into culture. This is our in vitro model for the block of pain transmission by opioids. We then correlated the size of this inhibition on individual nociceptors with the level of MOR expression as measured by quantitative single cell RT-PCR. Despite the obvious involvement of numerous proteins in the MOR signaling pathway, we found that the levels of MOR mRNA in nociceptors solely dictated their susceptibility to electrical modulation by DAMGO. We then asked if non-nociceptors expressed DAMGO responses or MOR, and found that they did not. While this may seem like a small surprise, that the opiate sensitivity of nociceptors is under the fine control of a molecule that is not expressed in non-nociceptors, this result supports a simple explanation of a complex behavior with a history of controversy. The selective block of pain by morphine and opioid peptides is likely not the result of action at some higher center in the pain pathway. Primary sensory neurons are themselves the biological substrates of morphine analgesia. The tightly controlled expression levels of a single molecule likely cause the selective inhibition of pain signals by opiates and endogenous opioid peptides.Two other opioid receptor subtypes have been identified, delta and kappa opioid receptor (DOR and KOR). Since cells did not respond to the DOR-1 agonist DPDPE in our studies, section II of this paper was an investigation of the kappa opioid receptor (KOR) on sensory neurons. Our lab and others have reported that many cells respond to dynorphin, an endogenous KOR agonist, and therefore assumed that KOR is commonly expressed by sensory neurons. Surprisingly, we found that the magnitude of the dynorphin and DAMGO responses on most cells was nearly always identical. However, few cells contained measurable levels of KOR mRNA. Using selective KOR selective antagonists, we found that dynorphin responses on nearly all sensory neurons were mediated through the MOR, and that KOR expression on sensory neurons was in fact quite rare. Dynorphin was also capable of stimulating MOR activation of potassium channels in locus coeruleus slices, a tissue completely devoid of KOR. This told us that the sensory neuron MOR is not uniquely sensitive to dynorphin. This finding further reinforces the hypothesis that MOR is the dominant opioid receptor on nociceptor sensory neurons, and that its expression is sufficient to explain morphine analgesia as well as the sensitivity of these important cells to multiple endogenous opioid agonists.