Sensitization of locomotion following repeated ventral tegmental injections of cytisine.

Systemic injections of nicotine increase locomotion, and repeating these injections brings about a sensitization of the locomotor response. Ventral tegmental injections of the nicotinic agonist cytisine also increase locomotion. In the present study cytisine was administered repeatedly into the ventral tegmentum to determine whether sensitization of its locomotor-activating effects would develop. Four groups of animals were tested; each group received a total of six injections at a rate of one injection every 48 h. Two of these groups received injections of cytisine (10 nmol/side): one group received injections into the ventral tegmentum, and, to insure the anatomical specificity of the locomotor effect, a second group received injections dorsal to the ventral tegmentum. The remaining two groups received vehicle injections: one group received injections into the ventral tegmentum, and the other received injections into more dorsal sites. The group of animals that received injections of cytisine into the ventral tegmentum locomoted more than any other group. In addition, only with this group was a progressive increase in the locomotor response evident across test days. These findings raise the possibility that a neural substrate in the ventral tegmentum mediates the locomotor-activating and sensitizing effects associated with the systemic administration of nicotine.