Morphine blood levels, dependence, and regulation of hippocampal subgranular zone proliferation rely on administration paradigm

Chronic morphine, administered via s.c. pellet, decreases the number of proliferating cells in the dentate gyrus subgranular zone (SGZ) in both rats and mice. This robust morphine-induced decrease could be used to better understand mechanisms regulating adult hippocampal neurogenesis, as well as to explore the relationship between neurogenesis and drug dependence, withdrawal, and relapse behaviors. Such research would benefit enormously from identifying a route of morphine administration that produces addiction-relevant blood levels of morphine, results in a high degree of dependence, translates to both rat and mouse, and is free of the behavioral confounds of s.c. pellets. Therefore, we examined a classic chronic morphine pellet paradigm (two s.c. pellets over 5 days) versus three chronic morphine injection paradigms (escalating dose i.p. injections over 2, 5, or 10 days) for their effect in adult male C57BL/6J mice. We assessed blood morphine levels, SGZ proliferation, and drug dependence as assessed by tolerance to locomotion sensitization and naloxone-precipitated withdrawal. The pellet paradigm produced high and relatively stable blood levels of morphine, a high degree of dependence, and a significant decrease in SGZ proliferation. In contrast, the three injection paradigms produced transient spikes in morphine blood levels, significantly less dependence than the pellet paradigm, and no significant decrease in SGZ proliferation. These data show that regulation of mouse SGZ proliferation requires high and relatively stable blood levels of morphine, and provide critical knowledge for the design of future studies to probe the relationship between addiction and neurogenesis.