Effect of circadian phase on memory acquisition and recall: operant conditioning vs. classical conditioning.

There have been several studies on the role of circadian clocks in the regulation of associative learning and memory processes in both vertebrate and invertebrate species. The results have been quite variable and at present it is unclear to what extent the variability observed reflects species differences or differences in methodology. Previous results have shown that following differential classical conditioning in the cockroach, Rhyparobia maderae, in an olfactory discrimination task, formation of the short-term and long-term memory is under strict circadian control. In contrast, there appeared to be no circadian regulation of the ability to recall established memories. In the present study, we show that following operant conditioning of the same species in a very similar olfactory discrimination task, there is no impact of the circadian system on either short-term or long-term memory formation. On the other hand, ability to recall established memories is strongly tied to the circadian phase of training. On the basis of these data and those previously reported for phylogenetically diverse species, it is suggested that there may be fundamental differences in the way the circadian system regulates learning and memory in classical and operant conditioning

Plots the learning index for animals trained at either CT 14 (A) or CT 2 (B) and tested 12, 24, 36, or 48 h after training.

<p>Animals tested at the same circadian time as training performed better than those tested at a phase 12 hours different from the phase of training. Numbers of animals for each time point ranged from 14 to 17 in A and 10 to 11 in B. Analysis of variance showed significant dependence of performance on the time of testing for training at both CT 2 and CT 14.</p

Plots the number of times (Mean±SEM) animals visited the vanilla odor prior to visiting peppermint as a function of the training/testing sequence when trained at CT 14 (A) or CT 2 (B).

<p>Sucrose rather than apple was offered as a reward. C, when access to the sucrose reward at peppermint was prevented during training (CT14), there was no significant change in the number of vanilla visits. P-values for the ANOVA are indicated in the figure. Bars marked with * indicate a statistically significant difference (p<0.05) when compared to the initial number of vanilla visits (Holm-Sidak post-hoc test).</p

Plots vanilla visits as a function of training/testing time when the peppermint odor was paired with the positive reinforcement of sucrose solution and the vanilla odor was paired with an accessible negative reinforcement of saline.

<p>Left panel, training at CT 2; right panel training at CT 14. P-values for the ANOVA are indicated in the figure. Bars marked with * indicate a statistically significant difference (p<0.05) when compared to the initial number of vanilla visits (Holm-Sidak post-hoc test).</p

Each panel plots the number of times (Mean±SEM) the animals visited a vanilla odor prior to visiting peppermint as a function of the training/testing time.

<p>A, animals were subjected to training sessions in the early subjective night (CT14) and were rewarded with a slice of apple when they visited the peppermint. Prior to any reward (0 min) animals exhibited a clear preference for vanilla. In subsequent trials animals showed a significant reduction in vanilla visits prior to visiting peppermint. B, Animals were subjected to two consecutive days of training at CT 14 (three trails in each session with a 5 minute inter-trial interval. There was a highly significant reduction in the number of visits to vanilla made prior to the visit to peppermint both one and two weeks later compared to the initial trial (0 min.). C, when trained at CT 2, animals exhibited a similar reduction in vanilla visits to the animals trained at CT 14. D, when access to the reward at peppermint was prevented during training (CT14), there was no significant change in the number of vanilla visits. P-values for the ANOVA are indicated in the figure. Bars marked with * indicate a statistically significant difference (p<0.05) when compared to the initial number of vanilla visits (Holm-Sidak post-hoc test).</p