The roles of BDNF, pCREB and Wnt3a in the latent period preceding activation of progenitor cell mitosis in the adult dentate gyrus by fluoxetine

Abstract

The formation of new neurons continues into adult life in the dentate gyrus of the rat hippocampus, as in many other species. Neurogenesis itself turns out to be highly labile, and is regulated by a number of factors. One of these is the serotoninergic system: treatment with drugs (such as the SSRI fluoxetine) markedly stimulates mitosis in the progenitor cells of the dentate gyrus. But this process has one remarkable feature: it takes at least 14 days of continuous treatment to be effective. This is despite the fact that the pharmacological action of fluoxetine occurs within an hour or so of first administration. This paper explores the role of BDNF in this process, using the effect of a Trk antagonist (K252a) on the labelling of progenitor cells with the mitosis marker Ki67 and the associated expression of pCREB and Wnt3a. These experiments show that (i) Fluoxetine increased Ki67 counts, as well as pCREB and Wnt3a expression in the dentate gyrus. The action of fluoxetine on the progenitor cells and on pCREB (but not Wnt3a) depends upon Trk receptor activation, since it was prevented by icv infusion of K252a. (ii) These receptors are required for both the first 7 days of fluoxetine action, during which no apparent change in progenitor mitosis occurs, as well as the second 7 days. Increased pCREB was always associated with progenitor cell mitosis, but Wnt3a expression may be necessary but not sufficient for increased progenitor cell proliferation. These results shed new light on the action of fluoxetine on neurogenesis in the adult dentate gyrus, and have both clinical and experimental interest.

Figure 1. The effect of K252a icv on the mitosis rates (Ki67) of progenitor cells in the dentate gyrus.

A: Mean number per section after 14 days treatment with either fluoxetine (10 mg/kg/day) sc, K252a icv, or both compared to controls (saline sc and icv). Values are means ± SEM. C,D: Photomicrographs of the expression of pCREB and Wnt3a after the four treatments. Bar represents a distance of 100 µm. B; BDNF mRNA in the dentate gyrus following the same treatment. Values are mean ±SEM. *P<0.05, **p<0.001 compared to control.

Figure 2. The effect of either 7 or 14 days treatment with fluoxetine (10 mg/kg/day).

A: Ki67 cell counts, B: BDNF mRNA expression, C: pCREB D: Wnt3a. E,F: in situ hybridization images of BDNF mRNA. Values as in Figure 1. Bar represents 100 µm. Values are mean ±SEM.**p<0.001 compared to control. Representative sections are shown from fluoxetine-treated animals only, since there is virtually no expression in controls.

Figure 3. Effect of treatment with K252a icv for either the first or second 7 days of a 14 day fluoxetine (10 mg/kg/day) treatment.

A: Ki67-labelled cell counts; B: Wnt3a expression in the dentate gyrus. Values as in Figure 1. Bar represents 100 µm.