Effects of post-trial reinforcing vs. subreinforcing stimulation of the substantia nigra on passive avoidance...

Experiments were designed to investigate the role of post-trial reinforcing and subreinforcing stimulation of the substantia nigra on memory processing. Thirty sec post-trial reinforcing stimulation (0.2 sec on/1.8 sec off) impaired learning of response suppression in a step-down task compared to control animals as well as to animals stimulated at subthreshold current level (i.e., at 25% of the current level shown to maintain optimal self-stimulation in previously performed self-stimulation sessions). In a second small-box passive avoidance experiment, i.e., the alcove-avoidance task, opposite results were attained: Subreinforcing stimulation attenuated learning whereas neither suprathreshold stimulated animals nor control animals showed impairment of learning. The conclusions drawn from these results are as follows: Post-trial stimulation of the substantia nigra interferes with memory processing. This attenuation of learning is obviously task-dependent and can additionally be influenced by the quality of the stimulation, i.e., whether it is reinforcing or not. Possible explanations to account for these task-dependent and quality of stimulation-dependent effects of post-trial substantia nigra stimulation are discussed.