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It has come to our attention that we failed to cite a relevant study (Egertová, M., Giang, D. K., Cravatt, B. F. & Elphick, M. R. Proc. R. Soc. Lond. B 265, 2081–2085; 1998) in our Letter. Based on the immunolocalization of fatty acid amide hydrolase, which inactivates endocannabinoids, to hippocampal pyramidal cells, and the localization of the CB1 receptor to presynaptic terminals, Egertová et al. proposed that endocannabinoids might function as retrograde messengers.