Our prior studies have shown that the endogenous dynorphin opioid peptides, released during exposure to repeated behavioral stressors (repeated forced swim, repeated social defeat, or foot shock), strongly potentiate the rewarding effects of cocaine in mice (as measured by an enhanced conditioned place preference to cocaine) and reinstate extinguished drug seeking behavior in mice (as measured by reinstatement of extinguished cocaine conditioned place preference). We previously found that stress-induced dynorphin release activates kappa opioid receptors (KOR) that stimulate phosphorylation of p38 MAPK expressed by both astroglial cells and GABAergic neurons in brain. KOR-dependent activation of p38 MAPK was required for astrocyte activation (increased GFAP-ir and proliferation) and for the KOR-dependent potentiation of the behavioral responses to cocaine. However, a specific role of astrocyte activation in mediating the enhanced behavioral response to cocaine is not clear. In this exploratory study, we propose to use transgenic GFAP-CreERT2 mice in which Crerecombinase expression is regulated both by a tamoxifen-inducible DNA recombinase variant and by the Glial acidic fibrillary protein (GFAP) promoter. These mice will be crossed with a floxed p38???conditional knockout mouse to generate adult mice in which KOR-dependent activation of p38???may be selectively blocked in astrocytes. Using the GFAP-CreERT2/floxed p38?? CKO mice, we propose to determine: 1) whether induction of Cre expression by tamoxifen in adult mice can selectively excise the p38???gene in astrocytes, but not in neurons;2) whether the reduction in p38?? expression effectively blocks stress-induced, KOR-dependent increases in GFAP-ir expression in brain;and 3) whether astrocyte-selective reduction in p38?? expression blocks reinstatement of extinguished cocaine conditioned place preference. The proposed studies are high-risk (cell-type selective regulation of gene expression is still an art form, and a specific role for the alpha isoform of p38 MAPK is not established), but high pay-off (selective manipulation of astrocyte activation by dynorphin during stress-responses would strongly advance our understanding of the role of astrocyte activation in relapse of drug abuse in humans). The feasibility of the proposed studies is established (we have all of the mice and experimental tools in hand). Pilot funding is required because genotyping, breeding, housing, and testing of these mice in the immunohistochemical and behavioral assays proposed require an intensive effort.
This application is designed to understand the ways in which astrocytes in brain, that have been activated by stress-induced dynorphin release, contribute to addiction. Astrocytes express kappa opioid receptors and are strongly activated by stress-induced dynorphin release. Kappa receptor activation potentiates the reinforcing effects of cocaine and reinstates drug seeking behaviors in cocaine conditioned rodents. This study would identify the connection between astrocyte activation and reinstatement of extinguished cocaine place preference by assessing the effects of selectively disrupting kappa opioid signaling in astroctyes on behavior.
