Effect of selective lesions of nucleus accumbens µ-opioid receptor-expressing cells on heroin self-administration in male and female rats: a study with novel Oprm1-Cre knock-in rats.

The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1 -Cre knock-in transgenic rat that provides cell-type specific genetic access to MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1 -Cre knock-in rats to study the involvement of nucleus accumbens (NAc) MOR-expressing cells in heroin self-administration in male and female rats.Using RNAscope, autoradiography, and fluorescence in situ hybridization chain reaction (HCR-FISH), we found no differences in Oprm1 expression in NAc, dorsal striatum (DS), and dorsal hippocampus, or MOR receptor density (except DS) or function between Oprm1 -Cre knock-in rats and wildtype littermates. HCR-FISH assay showed that iCre is highly co-expressed with Oprm1 (95-98%). There were no genotype differences in pain responses, morphine analgesia and tolerance, heroin self-administration, and relapse-related behaviors. We used the Cre-dependent vector AAV1-EF1a-Flex-taCasp3-TEVP to lesion NAc MOR-expressing cells. We found that lesions decreased acquisition of heroin self-administration in male Oprm1 -Cre rats and had a stronger inhibitory effect on the effort to self-administer heroin in female Oprm1 -Cre rats.The validation of an Oprm1 -Cre knock-in rat enables new strategies for understanding the role of MOR-expressing cells in rat models of opioid addiction, pain-related behaviors, and other opioid-mediated functions. Our initial mechanistic study indicates that lesioning NAc MOR-expressing cells had different effects on heroin self-administration in male and female rats. SIGNIFICANCE STATEMENT: The brain µ-opioid receptor (MOR) is critical for the analgesic, rewarding, and addictive effects of opioid drugs. However, in rat models of opioid-related behaviors, the circuit mechanisms of MOR-expressing cells are less known because of a lack of genetic tools to selectively manipulate them. We introduce a CRISPR-based Oprm1 -Cre knock-in transgenic rat that provides cell-type specific genetic access to brain MOR-expressing cells. After performing anatomical and behavioral validation experiments, we used the Oprm1 -Cre knock-in rats to show that lesioning nucleus accumbens MOR-expressing cells had different effects on heroin self-administration in males and females. The new Oprm1 -Cre rats can be used to study the role of brain MOR-expressing cells in animal models of opioid addiction, pain-related behaviors, and other opioid-mediated functions.