Opposing actions of chronic Δ9-tetrahydrocannabinol and cannabinoid antagonists on hippocampal long-term potentiation

Memory deficits produced by marijuana arise partly via interaction of the psychoactive component, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), with cannabinoid receptors in the hippocampus. Although cannabinoids acutely reduce glutamate release and block hippocampal long- term potentiation (LTP), a potential substrate for learning and memory, the consequences of prolonged exposure to Delta(9)-THC for hippocampal function are poorly understood. Rats were injected with Delta(9)-THC (10 mg/ kg, i. p., q. d.) for 1, 3, or 7 d, and electrophysiological recordings were performed in hippocampal slices 1d after the final injection. At this time, Delta(9)-THC was undetectable in hippocampus using liquid chromatography-mass spectrometry (LC-MS). Hippocampal LTP generated using high- frequency (HFS) or theta burst stimulation was not observed in brain slices from the 7-d Delta(9)-THC-treated animals. Delta(9)-THC also blocked HFS- LTP after 3 d, but not 1 d of treatment. The complete blockade of LTP persisted for 3 d after the last Delta(9)-THC injection, and full reversal of the LTP deficit was not observed up to 14 d following Delta(9)-THC withdrawal. The cannabinoid antagonist AM251 (2 mg/kg), administered before each Delta(9)-THC injection prevented the blockade of LTP, and 7-d treatment with AM251 alone significantly increased the level of LTP. Chronic Delta(9)-THC also produced tolerance to the inhibition of synaptic GABA, but not glutamate release by the agonist WIN55,212-2. These data define consequences of repeated Delta(9)-THC exposure for synaptic plasticity in the hippocampus that may help explain memory impairments in humans following chronic marijuana use.