| Opioids are very effective analgesics to treat acute pain. However, the long term use of opioids is limited by anti-nociceptive tolerance and the lack of tolerance to side effects such as constipation, respiratory depression, or lethality. Since less tolerance develops to opioids' side effects than analgesia, simply increasing the dose to overcome analgesic tolerance would result in dangerous side effects such as respiratory depression or ileus of the bowl. Currently, a peripherally acting opioid receptor antagonist, methylnaltrexone, is available to treat constipation, but no therapies are available to counteract morphine respiratory depression or lethal overdose. If tolerance could be equalized between anti-nociception and respiratory depression, opioids could be made safer for chronic use.;Morphine differential tolerance was studied in this thesis since morphine is the first and prototypical opioid. In order to study morphine differential tolerance, rodent models were developed to investigate morphine's effect at the pupil, on respiratory depression, anti-nociception, and lethal overdose. With the same morphine treatment, it was shown that anti-nociception developed more tolerance than mydriasis or lethality. The Edinger-Westphal nucleus (EWN) in the midbrain was shown to have a role in mediating morphine mydriasis in the rat through microinjections of morphine and naloxone. However, mydriasis was an inefficient model and lacked clinical relevance. Literature supports pre-Botzinger complex's (preBotC) role in mediating morphine respiratory depression. Our results in MORKO mice showed that expressing MORGFP in the preBotC conferred sensitivity to morphine respiratory depression and decreased the morphine LD50. In addition to EWN and preBotC, the ventrolateral periaqueductal gray (vlPAG) was shown to mediate morphine anti-nociceptive tolerance through repeated morphine microinjections to vlPAG.;After establishing models to study morphine differential tolerance, PKC's role in modulating respiratory depression and lethality tolerance was investigated. Focus was put on PKC alpha, gamma, and epsilon due to their roles in morphine anti-nociceptive tolerance. It was shown that PKC gamma was present in similar quantities in both preBotC and vlPAG while PKC alpha and gamma quantities varied between the two nuclei. Activating either PKC alpha or gamma by expressing constitutively activated mutants in preBotC increased morphine respiratory depression and lethality tolerance. It was shown that activated PKC epsilon decreased the threshold to develop morphine tolerance, which indicated that activating PKC might increase respiratory depression and lethality tolerance by decreasing the tolerance development threshold.;In summary, this thesis developed EWN mediated mydriasis, preBotC respiratory depression/lethality, and vlPAG anti-nociception as models to study differential morphine tolerance. PKC activity was shown to be involved in mediating the different levels of morphine tolerance, and future studies could investigate additional mechanisms that also regulate morphine differential tolerance. |
