Amphetamine administered to the ventral tegmental area sensitizes rats to the locomotor effects of nucleus accumbens amphetamine.

This experiment investigated whether pre-exposure to injections of amphetamine into the ventral tegmental area (VTA) sensitizes the locomotor response to a subsequent test injection of amphetamine given into the nucleus accumbens (N.Acc.). Different groups of rats received three intra-VTA injections of amphetamine (2.5 micrograms/0.5 microliters/side) or saline (0.5 microliters/side), one injection given every 3rd day, and locomotor activity was measured for 60 min. Seven to 10 days later, all animals were tested with an intra-N.Acc. challenge injection of amphetamine (0, 1.5, 2.5 or 5.0 micrograms/0.5 microliter/side) and locomotor activity was again measured. In agreement with previous reports, intra-VTA amphetamine produced no behavioral effects acutely. However, when animals were subsequently challenged with an injection of amphetamine into the N.Acc., animals pre-exposed to intra-VTA amphetamine showed significantly and dose-dependently greater levels of locomotor activity than saline pre-exposed rats. Animals pre-exposed to amphetamine in sites outside the VTA did not show sensitized responding when subsequently tested with intra-N.Acc. amphetamine. These findings support the view that the induction of behavioral sensitization to amphetamine results from an action of this drug in the cell body region of mesolimbic dopamine neurons (VTA), whereas its expression reflects an enhanced reactivity in the terminals of these neurons in the N.Acc. Implications of such findings for the molecular neurobiological basis of behavioral sensitization to amphetamine are discussed.