Endogenous cannabinoid signaling is required for voluntary exercise-induced enhancement of progenitor cell proliferation in the hippocampus

Abstract

Voluntary exercise and endogenous cannabinoid activity have independently been shown to regulate hippocampal plasticity. The aim of the current study was to determine whether the endocannabinoid system is regulated by voluntary exercise and if these changes contribute to exercise-induced enhancement of cell proliferation. In Experiment 1, 8 days of free access to a running wheel increased the agonist binding site density of the cannabinoid CB(1) receptor; CB(1) receptor-mediated GTPgammaS binding; and the tissue content of the endocannabinoid anandamide in the hippocampus but not in the prefrontal cortex. In Experiment 2, the CB(1) receptor antagonist AM251 (1 mg kg(-1)) was administered daily to animals given free access to a running wheel for 8 days, after which cell proliferation in the hippocampus was examined through immunohistochemical analysis of the cell cycle protein Ki-67. Voluntary exercise increased proliferation of progenitor cells, as evidenced by the increase in the number of Ki-67 positive cells in the granule cell layer of the dentate gyrus (DG) in the hippocampus. However, this effect was abrogated by concurrent treatment with AM251, indicating that the increase in endocannabinoid signaling in the hippocampus is required for the exercise-induced increase in cell proliferation. These data demonstrate that the endocannabinoid system in the hippocampus is sensitive to environmental change and suggest that it is a mediator of experience-induced plasticity.

Figure 1. Flowchart of experimental procedures

Schematic representation of the housing/exercise protocol employed to generate tissue for examining the effects of voluntary exercise on the endocannabinois system (Experiment 1) and the role of endocannabinoid signaling in the effects of exercise on cell proliferation in the hippocampus (Experiment 2).

Figure 2. Voluntary exercise augments hippocampal endocannabinoid signaling

Eight days of voluntary exercise resulted in a significant increase in (a) CB₁ receptor binding site density in the hippocampus (n = 4-5 / condition), (b) CB₁ receptor mediated ³⁵S- GTPγS binding within the hippocampus (n = 4-5 / condition), and (c) tissue content of the endocannabinoid ligand anandamide (AEA) within the hippocampus (n = 7 / condition). There was no effect of voluntary exercise on (d) tissue content of the endocannabinoid ligand 2-arachidonoylglycerol (2-AG) in the hippocampus (n = 7 / condition), or (e) AEA content (n = 7 / condition) or (f) 2-AG content in the prefrontal cortex (n = 7 / condition). Values denoted are means ± SEM. * denotes significant differences (p < .05) between VEx animals and the control group (CON).

Figure 3. Antagonism of the cannabinoid CB₁ receptor attenuates the increase in cell proliferation in the dentate gyrus following voluntary exercise

(a) Animals which engaged in voluntary exercise (VEx) exhibited a significant increase in the total estimated number of proliferating cells expressing the endogenous cell cycle protein Ki-67 within the neurogenic region of the granule cell layer (GCL) of the dentate gyrus. This phenomenon was not seen following concurrent administration of the cannabinoid CB₁ receptor antagonist AM251 (AM; 1 mg/kg), which alone had no effect on cell proliferation. (b) Neither treatment, alone nor in combination, had any effect on the cellular expression of Ki-67 within the hilus of the dentate gyrus. Values denoted are means ± SEM; n = 5-6 / condition. * denotes significant differences (p < .05) between a treatment condition and the control group (CON) receiving vehicle injections (VEH). Representative photomicrographs (c, d, e, f) of the effects of voluntary exercise, AM251 administration (1 mg/kg), or both treatments combined on the immunoreactivity of Ki-67 (denoted by arrows) in the dentate gyrus at 10× magnification.