Mu opioid receptor knockout mouse: Phenotypes with implications on restless legs syndrome

Abstract

Restless legs syndrome (RLS) is characterized by an irresistible need to move the legs while sitting or lying at night with insomnia as a frequent consequence. Human RLS has been associated with abnormalities in the endogenous opioid system, the dopaminergic system, the iron regulatory system, anemia, and inflammatory and auto‐immune disorders. Our previous work indicates that mice lacking all three subtypes of opioid receptors have a phenotype similar to that of human RLS. To study the roles of each opioid receptor subtype in RLS, we first used mu opioid receptor knockout (MOR KO) mice based on our earlier studies using postmortem brain and cell culture. The KO mice showed decreased hemoglobin, hematocrit, and red blood cells (RBCs), with an appearance of microcytic RBCs indicating anemia. Together with decreased serum iron and transferrin, but increased ferritin levels, the anemia is similar to that seen with chronic inflammation in humans. A decreased serum iron level was also observed in the wildtype mice treated with an MOR antagonist. Iron was increased in the liver and spleen of the KO mice. Normal circadian variations in the dopaminergic and serotoninergic systems were absent in the KO mice. The KO mice showed hyperactivity and increased thermal sensitivity in wakefulness primarily during what would normally be the sleep phase similar to that seen in human RLS. Deficits in endogenous opioid system transmission could predispose to anemia of inflammation and loss of circadian variations in dopaminergic or serotonergic systems, thereby contributing to an RLS‐like phenotype.

Deficiency of Meis1, a transcriptional regulator, in mice and worms: Neurochemical and behavioral characterizations with implications in the restless legs syndrome

“…95% of the highly‐associated RLS conditions, defined as conditions under which RLS has shown to have a statistically higher prevalence, like multiple sclerosis and rheumatoid arthritis, are associated with inflammatory or immune alterations (Weinstock, Walters, & Paueksakon, 2012). Mu opioid receptor knockout mice show RLS‐like behavioral phenotypes accompanied by increased serum ferritin levels and anemia of inflammation (Lyu, DeAndrade, et al, 2020).…”

“…Six Meis1 KO males and seven WT males with an average age of 4 months were tested for wheel‐running activity (DeAndrade et al., 2012; Lyu, DeAndrade, et al, 2020; Lyu, Doroodchi, Sheng, et al, 2020; Lyu, Doroodchi, Xing, et al, 2020; Lyu, Xing, et al, 2019; Lyu, Xing, DeAndrade, Perez, Yokoi, et al, 2020; Lyu, Xing, DeAndrade, Perez, Zhang, et al, 2020). The sample size was determined according to (DeAndrade et al., 2012).…”

“…Blood was collected at 10:00 a.m. from eight Meis1 KO males and eight WT males with an average age of 7 months and processed as described (Lyu, DeAndrade, et al, 2019, 2020). The sample size was determined according to (Lyu et al, 2019; Lyu, DeAndrade, et al, 2020). The serum iron concentration was measured using QuantiChrom Iron Assay Kit (BioAssay Systems Inc., Catalog #: DIFE‐250, year of purchase, 2019).…”

“…The sample size was determined according to (DeAndrade et al., 2012). The experiment was conducted as before (DeAndrade et al., 2012; Lyu, DeAndrade, et al, 2019, 2020).…”

“…Other data were not normally distributed and analyzed with a generalized linear model (GENMOD, SAS University Edition). Data processing for Figure 1 was performed as (Lyu, DeAndrade, et al, 2020; Lyu, Doroodchi, Xing, et al, 2020; Lyu, Xing, et al, 2019; Lyu, Xing, DeAndrade, Perez, Yokoi, et al, 2020; Lyu, Xing, DeAndrade, Perez, Zhang, et al, 2020). Wheel‐running activity and open‐field activity were separated into the light phase and dark phase, with each phase having 12 hr.…”