Wheel-skill learning is dependent on practicing during the training
Experiment 1 assessed the effects of running- vs. locked-wheel training (2 days) on wheel-skill performance on days 1 and 6 after the training (). In the pre-test, the “running wheel” group showed an average of 15.4±1.5 (mean±SEM) errors (swings) per trial, the “locked wheel” group 15.5±2.9 errors. Similar pre-test error counts were obtained in experiments 2–4 (not shown). Mann-Whitney U test comparing the averaged post-training error scores of running vs. locked groups revealed a significant effect of training condition (Z=3.16, p<0.01). Furthermore, the Friedman test comparing error scores over time revealed that the training on the running wheel significantly improved the subsequent performance in the wheel-skill test (χ2=14.0, p<0.001), as these rats committed significantly fewer performance errors than before the training on both post-days 1 and 6 (p<0.05, Wilcoxon test) (). In contrast, animals that had no opportunity to practice their wheel skill (i.e., that were on the locked wheel) showed no improvement in their test performance (χ2=0.3, p>0.1) (). These findings demonstrate that running (practicing) during the training is critical for this motor-skill learning.
To further clarify the relationship between practicing during the running-wheel training and wheel-skill learning, we performed a correlation analysis using the data of vehicle-treated rats from experiments 1, 2 and 3 (2-day training) (). Our results show that, on both post-days 1 and 6, the number of errors committed during the skill test was negatively correlated with the amount of running during the training (total wheel revolutions in both sessions) (post-day 1, r=−0.61, p<0.01; post-day 6, r=−0.50, p<0.05; Spearman rank correlation; ). These findings demonstrate that wheel-skill improvement is directly related to the amount of practicing during the training.
Effects of cocaine treatment and training duration on wheel-skill learning
In experiment 2, the effects of varying the training duration (2 vs. 5 days), with and without cocaine, on wheel-skill learning were assessed on days 1, 6, 18, and 26 after the last training session (). Analysis of averaged post-training test scores showed that cocaine tended to impair skill learning, while longer training facilitated learning, but these effects did not reach statistical significance (p>0.05, Kruskal-Wallis test). Analysis of skill performance over time for individual groups confirmed these tendencies. Thus, vehicle-injected animals that trained on a running wheel for 2 days showed a significantly improved skill performance after the training compared to pre-training values (χ2=12.9, p<0.05, Friedman test), and they retained this skill for up to 26 days after the training (p<0.05) (). In contrast, in cocaine-treated rats, the effect of the 2-day training on skill performance did not reach statistical significance (χ2=9.1, p=0.059), although a trend for fewer errors after the training was seen (). After the 5-day training (), rats treated with vehicle also displayed an improved wheel skill (χ2=14.1, p<0.01). Moreover, the post hoc analysis showed that these rats reached maximal skill performance already on post-day 1 (p<0.05), whereas vehicle-treated animals that trained for only 2 days continued to improve after the first post-test (). In contrast, rats that trained under the influence of cocaine for 5 days again failed to show a significantly improved wheel skill after the training (χ2=9.0, p=0.061), but displayed a clear tendency for improvement ().
Fig. 2 Effects of training duration and cocaine on motor-skill learning. a, b The number (mean±SEM) of errors (in percent of pre-test values) before (pre) and 1, 6, 18 and 26 days after (post) a 2-day (a) or 5-day (b) training (60-min sessions) are given (more ...)
Considering the importance of practicing (running) for skill learning (see above), we assessed and compared the total number of wheel revolutions during the training between the vehicle- and cocaine-treated groups. Mann-Whitney U tests showed that cocaine-treated animals tended to run more than vehicle-treated animals during both the 2-day (Z=2.62, p<0.01; ) and the 5-day training (Z=1.72, p=0.085; ). Thus, cocaine appeared to attenuate wheel-skill learning despite enhancing the amount of running (practice) during the training.
Effects of systemic blockade of D1 receptors during the training on wheel-skill learning
Experiment 3 investigated the effects of systemic D1 receptor antagonism, with and without cocaine, on wheel-skill learning (2-day training) (). The D1 receptor antagonist SCH-23390 (0, 3, 10 μg/kg) and cocaine were administered before each training session, and the wheel skill was evaluated on days 1, 6, 18 and 26 after the training. For averaged post-training test scores, statistical analysis revealed no significant overall effects of cocaine (p>0.05, Mann-Whitney U test) or SCH-23390 (p>0.05, Kruskal-Wallis test; data not shown). However, statistical analysis of test scores over time in individual groups showed that training effects varied depending on the drug treatment. Thus, vehicle only-treated rats (SCH0) displayed a highly significant training effect (χ2=19.3, p<0.001), with an improved wheel skill present at all time points up to 26 days after the training (p<0.05) (). On the other hand, rats that received cocaine alone before each training session (SCH0+cocaine) only showed a marginal training effect (χ2=10.3, p<0.05), with wheel-skill improvement present until day 6 after the training (p<0.05).
Fig. 3 Effects of systemic D1 receptor blockade during running-wheel training on motor-skill learning. a, b, c The number (mean±SEM) of performance errors (in percent of pre-test values) before (pre) and 1, 6, 18 and 26 days after (post) a 2-day running-wheel (more ...)
The effects of SCH-23390 were dose-dependent and, when administered in conjunction with cocaine, biphasic. Given alone the D1 receptor antagonist attenuated wheel-skill learning. Thus, no significant effect of the training on skill performance was found in rats that received 3 μg/kg (SCH3; χ2=9.3, p=0.054; ) or 10 μg/kg of SCH-23390 (SCH10; χ2=3.8, p>0.05; ) before each training session, although the group receiving the lower dose did show a trend for improvement (). In contrast, animals treated with the lower dose of SCH-23390 (3 μg/kg) plus cocaine (SCH3+cocaine) displayed a highly significant training effect on wheel-skill performance (χ2=20.8, p<0.001), with an improved skill that was stable throughout the post-training period (post-days 1–26, p<0.05; ). Conversely, rats treated with the higher dose (10 μg/kg) plus cocaine (SCH10+cocaine) showed a significant training effect (χ2=13.9, p<0.01), but this effect did not last. While these rats did improve to a similar degree on days 1 and 6 after the training (p<0.05), they then lost their improved skill over time (post-days 18 and 26, p>0.05; ). Thus, when given with the lower dose of SCH-23390, cocaine improved both early (post-days 1–6) and late long-term skill memory (post-days 18–26), whereas when given with the higher dose of SCH-23390, this psychostimulant treatment only reversed the inhibition of early, but not late, long-term memory formation (). Together, these results suggest that motor-skill learning is dependent on optimal D1 receptor stimulation. Moreover, D1 receptors appear to be critical for enduring long-term skill-memory consolidation/retrieval.
The number of wheel revolutions during the training was monitored to assess possible effects of the D1 receptor antagonist on running activity. No significant differences between SCH-23390- and vehicle-treated groups were found [SCH0 vs. SCH3 vs. SCH10, 66.3±15.3 (mean±SEM) vs. 112.0±34.8 vs. 100.1±39.3; p>0.05; SCH0+cocaine vs. SCH3+cocaine vs. SCH10+cocaine, 581.7±86.1 vs. 621.8±100.2 vs. 442.4±82.8; p>0.05]. Furthermore, administration of the D1 antagonist did not change the distribution of running activity within the training sessions (data not shown).
Blockade of D1 receptors in the striatum is sufficient to inhibit skill-memory formation
In experiment 4, we investigated whether D1 receptor stimulation in the striatum is necessary for wheel-skill learning (2-day training) (). The D1 receptor antagonist SCH-23390 (0, 0.3 and 1 μg) was infused into the dorsolateral striatum () in conjunction with systemic cocaine administration. SCH-23390 had dose-dependent effects on wheel-skill learning. Statistical analysis of the averaged post-training test scores revealed a significant effect of the D1 receptor antagonist treatment (H=12.0, p<0.01, Kruskal-Wallis test) (). Thus, both rats infused with the lower dose (S0.3+cocaine; Z=2.0, p<0.05, Mann-Whitney U test) and the higher dose of SCH-23390 (S1+cocaine; Z=3.1, p<0.01) showed more performance errors after the training than the controls that received a saline infusion into the striatum (S0+cocaine). S1+cocaine animals also displayed more errors than S0.3+cocaine animals (Z=2.2, p<0.05). Statistical analysis of the test scores over time revealed that the saline controls displayed a significant training effect (χ2=12.0, p<0.01), with reduced error scores present on days 1, 13 and 27 after the training (p<0.05) (). In contrast, the group that received the lower dose of SCH-23390 (0.3 μg) showed only a trend for improved skill performance (χ2=7.1, p=0.070), with near-normal improvement on post-day 1 (). The higher dose of SCH-23390 (1 μg) completely prevented this motor learning (χ2=4.2, p>0.1; ). Again, no effect of the D1 receptor antagonist on the amount of running during the training was observed (p>0.05; ).