Endocannabinoids modulate stress-induced suppression of hippocampal cell proliferation and activation of defensive behaviours.

The endocannabinoid system has been shown to regulate both the hypothalamic-pituitary-adrenal (HPA) axis and emotionality. The present experiment was designed to examine whether pharmacological modulation of the endocannabinoid system would affect the suppression of hippocampal cell proliferation and increase in defensive behaviours seen following exposure to predator odour (trimethylthiazoline; TMT) stress. Rats were administered either an endocannabinoid uptake inhibitor (AM404; 2 mg/kg) or a cannabinoid CB1 receptor antagonist (AM251; 5 mg/kg) 30 min prior to exposure to TMT. Exposure to TMT reduced cell proliferation in the dentate gyrus and increased the expression of defensive burying. Administration of AM404 significantly inhibited defensive burying, and attenuated the reduction in cell proliferation in response to TMT exposure. Administration of AM251 alone significantly increased cell proliferation; however, pretreatment with AM251 prevented neither the stress-induced suppression of cell proliferation nor the stress-induced increase in behavioural responses. These results support previous research demonstrating that augmentation of endocannabinoid signalling can suppress stress-responsive systems. They also suggest that endocannabinoids may play a complex role in the regulation of neurogenesis via cell proliferation in the hippocampus.