Ventral tegmental microinjection of Δ 9-tetrahydrocannabinol enhances ventral tegmental somatodendritic dopamine levels but not forebrain dopamine levels: evidence for local neural action by marijuana's psychoactive ingredient

Abstract

Basal extracellular levels of dopamine (DA) and its metabolites in both ventral tegmental area (VTA) and nucleus accumbens (Acb) were simultaneously monitored by in vivo brain microdialysis in laboratory rats. Microinjection of 12 μg or 24 μg Δ 9-tetrahydrocannabinol (Δ 9-THC), the psychoactive ingredient in marijuana and hashish, into the VTA produced a dose-dependent increase in somatodendritic DA levels in VTA but failed to produce any simulatenous change in extracellular DA in Acb. Direct Δ 9-THC perfusion (5×10 −5 and 2×10 −4 M) into Acb via the microdialysis probes caused a significant increase in extracellular DA levels in Acb. These findings suggest that (1) Δ 9-THC has a facilitating effect on somatodendritic DA efflux, and (2) the elevation of Acb DA levels seen in our previous studies with systemic administration of Δ 9-THC may result from local actions of Δ 9-THC at or near the DA terminal projections in Acb.