Discriminated escape learning and response to electric shock after 6-hydroxydopamine lesions of the nigro-neostriatal dopaminergic projection.

In an earlier report it was observed that bilateral stereotaxic injections of 6-hydroxydopamine (6-OHDA) into the zone compacta of the substantia nigra deficits in the acquisition of a conditioned avoidance response [8], The present experiments were designed to determine if either a generalized learning impairment or a decreased sensitivity to foot shock might be the basis for the avoidance deficit. It was found that rats subjected to bilateral 6-OHDA lesions of the substantia nigra learned a light discrimination shock escape habit in as few trials as unoperated controls. This observation indicates that the integrity of the dopaminergic nigro-neostriatal system is not essential for the formation of learned associations between sensory cues and motor responses. In a second experiment it was observed that neither the shock-induced flinch nor the jump threshold was elevated after nigral lesions, suggesting that these lesions do not decrease the aversive motivational properties of foot shock. In view of these findings, the nature of the avoidance deficit produced by substantia nigra lesions is discussed with reference to the possibility that they selectively block the initiation of voluntary motor responses. According to this hypothesis, the failure of these lesions to disrupt escape responding may be due to the fact that the unconditioned stimulus generates reflexive motor responses (flinch, jump, etc.) which are sufficient to begin the motor sequences that cannot be initiated voluntarily in response to the conditioned stimulus.