Locomotor hyperactivity and hypoexploration after lesion of the dopaminergic-A10 area in the ventral mesencephalic tegmentum (VMT) of rats.

The lesion of the ventral mesencephalic tegmentum (VMT) and especially of the dopaminergic (DA) A10 neurons induced disturbances of spontaneous activity in the rat. Measured in different situations (circular corridor, open field, hole box), locomotor activity was dramatically and permanently increased (5 days of recording) without modifications of the circadian rhythm, although exploratory patterns of behavior were reduced. In contrast, the administration of a low dose of D-amphetamine (1 and 2 mg/kg i.p.) to naïve rats induced an increase of both locomotor and exploratory activities. A qualitative analysis of spontaneous activity of rats after VMT lesions and after administration of D-amph, revealed opposing aspects of behavior. Residual response to D-amph of VMT-lesioned rats suggest a subtotal destruction of DA-A10 neurons, the implication of an over activity of these remaining DA-A10 neurons on behavioral deficit is considered. Our results can be explained by a deficiency in attention processes.