Morphometric studies of the rat hippocampus following chronic delta-9-tetrahydrocannabinol (THC).

Persistent behavioral effects resembling those of hippocampal brain lesions have been reported following chronic administration of marijuana or its major psychoactive constituent, delta-9-tetrahydrocannabinol (THC) to rats. We used morphometric techniques to investigate the effects of chronic THC on the anatomical integrity of the hippocampus. Rats dosed orally for 90 days with 10 to 60 mg/kg THC or vehicle were evaluated by light and electron microscopy up to 7 months after their last dose of drug. Electron micrographs revealed a striking ultrastructural appearance and statistically significant decreases in mean volume of neurons and their nuclei sampled from the hippocampal CA3 region of rats treated with the highest doses of THC. A 44% reduction in the number of synapses per unit volume was demonstrated in these same rats. Golgi impregnation studies of additional groups of rats treated with 10 or 20 mg/kg/day THC and sacrificed 2 months after their last treatment with THC revealed a reduction in the dendritic length of CA3 pyramidal neurons, despite normal appearing ultrastructure and no changes in synaptic density. The hippocampal changes reported here may constitute a morphological basis for behavioral effects after chronic exposure to marijuana.