The endogenous opioid system is involved in various physiological processes including neurogenesis in the dentate gyrus (DG) of the hippocampus. In the current study, we investigated the role of the mu opioid receptor (MOR-1) on DG neurogenesis and measured glucocorticoid levels following several injection paradigms to supplement the neurogenesis experiments. MOR-1 knockout (KO) mice on C57BL/6 and 129S6 backgrounds were injected with BrdU using either a single injection or two different repeated injection protocols and then sacrificed at different time points. The total number of BrdU and PCNA positive cells in the DG is significantly increased in MOR-1 KO mice compared to WT on both strains after repeated injection, but not after a single injection. Plasma corticosterone (CORT) levels increased similarly in MOR-1 KO and WT mice following both single and repeated injection, indicating that the stress response is activated following any injection protocol, but that the mechanism responsible for the increase in BrdU labeling in MOR-1 KO mice is CORT-level independent. Finally, WT 129S6 mice, independent of genotype, showed higher levels of plasma CORT compared to WT C57BL/6 mice in both non-injected controls and following injection at two separate time points; these levels were inversely correlated with low numbers of BrdU cells in the DG in 129S6 mice compared to C57BL/6 mice. In summary, these data demonstrate that loss of MOR-1 increases BrdU labeling in the DG independent of CORT levels, but only following a repeated injection, illustrating the capability of injection paradigms to influence cell proliferative responses in a genotype-dependent manner.

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