Opiates, such as morphine, decrease neurogenesis in the postnatal hippocampal subgranular zone (SGZ) by inhibiting progenitor proliferation and maturation. However, it is not known how morphine influences the growth factors and vasculature that encompass the neurogenic SGZ microenvironment. We examined morphine’s effect on pro- and anti-proliferative factors in the dentate gyrus (DG; Experiment 1) as well as the DG neurovasculature itself (Experiment 2). For Experiment 1, mice were implanted with sham or morphine pellets subcutaneously (s.c.; 0 and 48 hrs) and were decapitated 24 or 96 hrs later. One brain hemisphere was post fixed to examine proliferation by immunohistochemistry, and a DG-enriched sample was dissected from the other hemisphere to examine the neurogenic microenvironment via immunoblotting for known pro- and anti-proliferative factors. Consistent with previous results, morphine decreased the number of proliferating cells in the SGZ, as the number of Ki67-immunoreactive (IR) cells was decreased at 96 hrs. Morphine did not alter DG levels of the pro-proliferative factor BDNF, anti-proliferative factor IL1β, or their receptors TrkB and IL1R1 at either time point. However, morphine increased the pro-proliferative factor VEGF at 96 hrs. Given that VEGF is also a potent angiogenic factor, Experiment 2 examined whether the morphine-induced increase in VEGF correlated with altered DG neurovasculature. Mice were implanted with morphine pellets as in Experiment 1, and two hrs before perfusion (24 or 96 hrs) were administered bromodeoxyuridine intraperitoneally (BrdU, i.p.; 150 mg/kg). Tissue was co-stained for BrdU and the endothelial cell marker endoglin to enable examination of DG vessels and proximity of BrdU-IR cells to endoglin-IR vessels. At 96 hrs, endoglin-IR vessel area and perimeter were increased, but proximity of BrdU-IR cells to endoglin-IR vessels remained unchanged. These data suggest that following chronic morphine exposure, factors within the neurogenic microenvironment are maintained or upregulated to compensate for decreased SGZ proliferation.
Keywords: morphine, proliferation, VEGF, BDNF, neurovasculature, neurogenic niche