The actions of THC on the intact hippocampus: a comparison of dentate and CA1 responses.

The effects of delta-9-tetrahydrocannabinol were compared on two afferent pathways of the rat hippocampal formation in urethane-anesthetized and chronically implanted awake animals. In urethane-anesthetized rats, THC produced a shift to the right in the input/output relationship without altering the relation between the measures of synaptic wave and the population spike of the CA1 response to contralateral CA3 stimulation. In contrast, the dentate response to perforant path stimulation was not altered by THC. Awake rats exhibited a similar response to THC, however, the CA1 response was less sensitive to THC and there was no change in the maximal response as there was in the anesthetized rat. Paired pulse studies demonstrated that there was a significant increase in paired pulse inhibition at 8 mg/kg in the CA1 region. There was no significant alteration of the dentate response to perforant path stimulation in the awake rat. These results suggest that alterations in limbic electrophysiology induced by THC may be mediated by structures other than the hippocampus.