How does cannabis affect the hippocampus and memory formation?

The hippocampus has one of the highest densities of CB1 receptors in the brain, concentrated on GABAergic interneurons and glutamatergic pyramidal cells. THC disrupts hippocampal function by impairing long-term potentiation (LTP) — the synaptic strengthening mechanism underlying memory encoding. Acutely, THC impairs encoding of new declarative memories (anterograde amnesia) while leaving retrieval of pre-existing memories largely intact. The mechanism involves CB1-mediated suppression of acetylcholine release in the hippocampus, disruption of theta oscillations (4–8Hz rhythms critical for memory encoding), and impairment of place cell firing patterns. Chronic heavy use is associated with reduced hippocampal volume (5–12% reduction in some studies), though this appears partially reversible with abstinence. CBD does not impair and may enhance hippocampal neurogenesis in animal models.