Posttraining self-stimulation and memory: A study of some parameters

Rewarding deep brain stimulation at the medial forebrain bundle favours avoidance conditioned response in a remote memory test, hinders extinction and increases neurogenesis

Intracranial Self-Stimulation (ICSS) at the medial forebrain bundle consistently facilitates learning and memory in rats when administered post-training or when administered non-concurrent to training, but its scope regarding remote memory has not yet been studied. The present work aims to test whether the combination of these two forms of ICSS administration can cause a greater persistence of the facilitating effect on remote retention and affect neurogenesis in the dentate gyrus (DG) of the hippocampus. Rats were trained in active avoidance conditioning and tested in two retention sessions (10 and 90 days) and later extinction. Subjects received an ICSS session after each of the five avoidance acquisition sessions (post-training treatment) and half of them also received ten additional ICSS sessions during the rest period between retention tests (non-concurrent treatment). All the stimulated groups showed a higher performance in acquisition and retention sessions, but only the rats receiving both ICSS treatments showed greater resistance to extinction. Remarkably, at seven months, rats receiving the non-concurrent ICSS treatment had a greater number of DCX-positive cells in the DG as well as a higher amount of new-born cells within the granular layer compared to rats that did not receive this additional ICSS treatment. Our present findings significantly extend the temporal window of the facilitating effect of ICSS on active avoidance and demonstrate a neurogenic effect of rewarding medial forebrain bundle stimulation.

Intracranial self-stimulation facilitates memory consolidation, but not retrieval: its effects are more effective than increased training

To evaluate possible differential effects of lateral hypothalamic intracranial self-stimulation (ICSS) on memory consolidation and retrieval, independent groups of Wistar rats were trained in a single session of two-way active avoidance task (acquisition session) and tested 24 h later (retention session). The post-ICSS groups received an ICSS treatment immediately after the acquisition session, and the pre-ICSS groups received the same treatment immediately before the retention session. Because the ICSS effects on memory seem to be dependent on the initial performance level shown by the subjects, the possible influence of initial training (number of trials) on ICSS effects was also studied. Therefore, we used different control and experimental groups, which received either 30 or 50 trials in the acquisition session. Post-training ICSS facilitated the 24-h retention in both training conditions (30 and 50 trials). In contrast, pre-retention ICSS treatment did not facilitate performance in the retention test. We also observed that post-training ICSS was more effective for improving the 24-h retention than increasing the initial training from 30 to 50 trials. This findings confirm that ICSS treatment improves memory consolidation and suggest that it might not affect memory retrieval mechanisms.

Facilitation of a distributed shuttle-box conditioning with posttraining intracranial self-stimulation in old rats

Old Wistar rats (16-17 months) were trained in a two-way active avoidance task for 5 consecutive days (10 trials/day). Immediately after each training session a lateral hypothalamic intracranial self-stimulation session (ICSS group) or a sham-treatment session (Control group) was given to the animals. Long-term retention was tested 7 days after the last acquisition session. ICSS treatment led to a significant improvement in acquisition. In the long-term retention session the level of avoidance in both groups was similar to that achieved in the last acquisition session, although differences among groups failed to reach statistical significance. These results are compared with those obtained in previous experiments with young adult rats. While ICSS facilitated the process of acquisition in both young and old rats (however, it was much more powerful in young animals), further experiments are needed to elucidate whether this effect is long-lasting in old rats, as occurs in young adult subjects.

Contingent and non-contingent actions of sucrose and saccharin reinforcers: effects on taste preference and memory

Preference curves were generated by comparing 14 concentrations each of sucrose and saccharin in a 20-minute test in which rats were presented with a choice of a sweet solution and water. The most preferred concentration and one concentration above and one below the most preferred for both substances were studied further. The sucrose and saccharin solutions were contingently paired with novel flavors in a conditioned taste preference (CTP) paradigm. All of the sweet solutions enhanced the animals' subsequent conditioned taste preferences for the flavors. The lack of difference between the effects of the solutions in this paradigm suggest that they had similar rewarding values and that CTP's are established mainly on the basis of taste cues. In another experiment, post-training ingestion of sucrose solutions, injection of glucose and, to a much lesser extent, ingestion of saccharin solutions retroactively and non- contingently improved retention of a previously formed, classically conditioned association. The results indicated that this effect was mainly due to the post- ingestional effects of the sucrose solutions, although taste factors also had a slight influence. This series of experiments parallels previous findings with self-stimulation as the reinforcer. The results support the hypothesis that reinforcers have a dual action on behavior: the elicitation of affective states that, when paired with environmental stimuli, can influence future behavior towards those stimuli; and a non-contingent retroactive enhancement of retention of previously formed associations.