Cannabinoid receptors are responsible for the suppression of ATP-induced LTP. a Selective CB1 receptor antagonist, AM251 (500 nM), allows the induction of ATP-induced LTP in the presence of high concentration of WIN-2 (2 μM, 126.6 ± 4.2%, n = 6). b AM251 (500 nM) does not alter synaptic transmission, but yields a stronger ATP-induced LTP as compared to control LTP (149.5 ± 5.2%, n = 5)