Opioid abuse in the USA has recently reached epidemic proportions, with 29,440 of the 47,055 fatalities from drug poisoning in 2014 attributable to either therapeutic opioids or heroin. Prescription opioids such as morphine, oxycodone and fentanyl as well as heroin are widely abused. The recent increase in opioid abuse is mirrored by a skyrocketing 500-1000% increase in the number of pain prescriptions over the last decade, although the abuse of therapeutic opioids is largely a result of diversion. In spite of the alarming statistics it must be acknowledged that pain requires and demands treatment. The role of pain in opioid addiction is murky; with some studies claiming addiction liability is enhanced by pain, yet others claiming that pain is protective. Technically challenging self-administration of morphine, oxycodone and remifentanil in mice (that were developed during the past funding period) will be used to assess the influence of neuropathic pain on phases of the establishment of opioid self-administration followed by cycles of withdrawal (extinction) and relapse (re- exposure). Our preliminary data suggest that neuropathic pain does not alter initial oxycodone self- administration but enhances drug-seeking during extinction and causes an enhanced escalation in drug taking during re-exposure. These findings will be expounded upon in Aims 1a and 1b using different opioid drugs and doses in self-administration protocols. The duration of time after sciatic nerve injury will be a variable since we have recently shown that pain symptoms can dissipate over time due to an increase in constitutive signaling of mu opioid receptors (MOR), yet negative affect continues to incubate over time. Given negative affect accompanies both chronic pain and opioid withdrawal, we will test in Aim1c if inhibiting neuroinflammation (published during the past funding period to be induced both by chronic opioid treatment and chronic pain), or kappa opioid receptors, (that we show in preliminary data have markedly increased function during chronic pain) will modify oxycodone reinforcement behaviors in animals with or without neuropathic pain. To identify MOR cell types and the striatal and/or habenula circuitry that are required for aspects of the opioid reward profile, Aim 2 will use conditional deletion or knock-in strategies to remove or insert MOR in striatal and extra-striatal neurons. Preliminary data shows marked differences in the self-administration profiles of these mice. For example, removing MOR from D1 neurons increases oxycodone consumption and drug seeking during extinction despite a complete absence of oxycodone-induced locomotor sensitization, while the converse is seen in mice lacking MORs on D2 neurons. Electrophysiological analyses will determine if pain alters properties of different striatal neurons following opioid self-administration. Finally, as D2 cells contribute to negative affect during withdrawal, we will use optogenetic stimulation of D2 neurons to show how these neurons modulate drug-seeking in the presence of pain. Together, Project III will assess the circuitry involved in different phases of opioid self- administration and its influence by different pain and dysphoric states.

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National Institute on Drug Abuse (NIDA)
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University of California Los Angeles
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