Mentor: Anna Radke, Ph.D.

The mu-opioid receptor (MOR) is a receptor in the endogenous opioid system, which is involved in the sensation of pain as well as reward and addiction. Activation of these receptors by opioids, including drugs such as fentanyl and morphine, causes pain relief, sedation and a feeling of euphoria. Repeated activation of these receptors can lead to tolerance and addiction. These receptors are found populated in the central nervous system, but are mainly found in the amygdala, nucleus accumbens, periaqueductal gray (PAG), ventral tegmental area (VTA), as well as the hypothalamus. In this study, we used mice with a genetic deletion of the MOR in cholinergic neurons to understand the role of this subpopulation of receptors. ChAT-Oprm Cre- (control) and Cre- (experimental) male and female mice were tested for locomotor sensitization to morphine.Mice were first habituated in open field chambers for 60 min. The next day, they were injected with morphine (20 mg/kg dose) and placed in the chamber for 120 min. Mice were then injected with morphine daily for 6 additional sessions. On Day 7, the locomotor test was repeated. Data from all three locomotor tests were analyzed using a three-way repeated measures ANOVA. There were differences present between sessions: morphine injection increased locomotion in both genotypes and sexes on Day 1 and Day 7 and sensitization was observed when comparing Day 1 vs. Day 7. The results suggest that MOR’s on cholinergic neurons do not have a significant effect on locomotor sensitization in mice.

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