Pain is the primary reason why patients seek medical care and there is a great unmet need for the development of pain relieving medications. The treatments that are currently available either have limited efficacy or are accompanied by a multitude of unwanted side effects. However, discovering novel therapeutics for the treatment of pain has been challenging. Part of the reason for this may be that that the ways in which pain is assessed in the preclinical setting are different from the way that it is evaluated clinically in human trials. The most common method for evaluating pain in preclinical models is to measure responses to evoked stimuli. However, a change in the threshold of response to evoked pain likely does not measure whether the unpleasant component of pain has actually been reduced. The most clinically relevant question for pain is whether the treatment actually makes the patients "feel better". Here, we demonstrate that the aversiveness of pain can be captured using motivated behavior to seek pain relief. We used conditioned place preference (CPP) to establish that animals with ongoing pain will seek a context that has been paired with effective pain relief, likely as a result of negative reinforcement. These studies allowed for mechanistic investigation. Our results show that: 1) effective pain relief can be achieved by either blocking noxious peripheral input or by directly attenuating pain related unpleasantness in the brain, and 2) pain relief is rewarding and activates the reward circuitry. These studies provide a basis for development of a future platform for drug discovery for pain.

Pain is the most feared symptom of cancer and can impact patients' lives more than the cancer itself. Despite improvements in cancer prevention and detection, pain is often the first sign of cancer, with an estimated 70-75% of advanced stage cancer patients presenting with skeletal metastases. Cancer metastasis to the bone is associated with persistent pain that increases in intensity over time. Current treatments follow the World Health Organization (WHO) analgesic ladder for cancer pain management suggesting non-steroidal anti-inflammatory drugs (NSAIDs) for mild to moderate pain and opioids for moderate to severe pain. However, estimates indicate as many as 50-80% of cancer patients worldwide receive inadequate pain management. Moreover, opioid doses required for these patients are associated with adverse side effects further diminishing quality of life. Development of improved non-opioid therapies is dependent on increased understanding of mechanisms driving cancer pain and its relief. The objective of this dissertation was to characterize a rat model of cancer-induced bone pain, to develop approaches to measure both ongoing and breakthrough pain and to investigate the contribution of underlying inflammatory mechanisms to pain, bone destruction and bone remodeling. Using female Fischer F344/NhSD rats, histocompatible MAT B III mammary adenocarcinoma cells were sealed into the intramedullary space of the right rear tibia for a time course of 13 days. Ongoing pain was characterized based on the WHO 3-step ladder for pain management utilizing novel behavioral and neurochemical assays. Morphine and peripheral nerve block were sufficient to control ongoing pain, whereas NSAID treatment failed to provide pain relief. Cancer-bearing rats selectively displayed movement-induced breakthrough pain to a background of morphine-controlled ongoing pain. Furthermore, we determined that breakthrough pain is initiated, but not maintained, by peripheral afferent input from the tumor-bearing tibia using lidocaine administration prior to or following movement. For the final part of this study, we investigated the role of transient receptor potential vanilloid 1 (TRPV1) and interleukin-6 (IL-6) blockade, as these have been shown to be important mediators in animal models CIBP. Acute blockade of TRPV1 channels by AMG9810 selectively reversed inflammatory-induced pain, but failed to control evoked or ongoing CIBP. Acute blockade of interleukin-6 signaling by TB-2-081, an IL-6 receptor antagonist, successfully reversed evoke pain responses, but like AMG9810, failed to control ongoing pain. Sustained administration of TB-2-081 reversed cancer-induced tactile hypersensitivity and tumor-induced bone remodeling of the tibia. Further in vitro analysis revealed TB-2-081 functions by inhibiting the Jak/STAT cascade on both tumor cells and osteoblasts, suggesting that blockade of IL-6 signaling can effectively modulate the bone microenvironment to reduce tumor burden and pain. Combined, our data introduce a rat model of breast cancer bone metastasis, in which the underlying mechanisms of ongoing and breakthrough CIBP can be effectively studied. From this, novel therapeutic agents can be developed and investigated to help improve quality of life in patients suffering from this disease.