Rats' use of hierarchical organization in serial pattern learning.

Many organisms show aptitude for learning and performing patterned sequences. However, we do not yet have a complete account of how they accomplish this. One of the most successful is Restle's hierarchical model, which supposes organisms represent sequences using the simplest form available through using hierarchies to organize the sequences' elements. In two experiments, we evaluated if Restle's model accurately accounted for rats' patterned sequence learning. First, we compared rats' learning of a runs pattern with three levels of hierarchical organization with no violations of pattern structure to learning of the same pattern with one violation of pattern structure in the pattern's second half. Restle's model predicts rats should have more difficulty learning the violation pattern and that rats learning the pattern with the violation in the second half of the pattern should attempt to reflect that violation to the first half. The results indicated that rats had more difficulty learning the violation pattern and that inserting a violation in the second half of the pattern led rats to make significantly more errors on the second half of the pattern relative to the first half of the pattern. Rats did not insert the violation from the second half of the pattern into the first half of the pattern. In a second experiment, we continued to explore if Restle's model accurately accounted for rats' pattern learning using more complex patterns. The present results of both experiments and prior work with humans were inconsistent with Restle's model, suggesting more work is needed to develop a model that accurately accounts for rats' learning of complex patterned sequences.

Sex differences in serial pattern learning in mice.

Numerous investigators have examined the hypothesis that males and females learn or perform differentially on various tasks. However, many of the behavioural investigations with nonhuman animals (e.g., rats) have used paradigms that do not permit the exploration of complex learning and memory between the sexes. To this end, we explored the ability of male versus female mice to learn three different patterns in succession in three separate experiments: single alternation (e.g., right, left, right, left), double alternation (e.g., right, right, left, left), and runs (e.g., 123, 234, 345, 456, 567, 678, 781, 812, where digits represent locations within a circular array in the counterclockwise direction). We hypothesized that sex differences, if they existed, would be most likely to appear as the pattern to be learned became more complex (required more rules to capture how elements relate to one another). The results indicated that mice can learn all three pattern types, but learning was more difficult as pattern complexity increased. Males learned the runs pattern significantly more quickly than females did; no significant differences were found between males and females for acquisition of the single-alternation or double-alternation patterns. These results suggest that sex differences in serial pattern learning within rodents are not unique to rats and are more likely to be seen during acquisition of more complex patterns.

Adolescent exposure to fluoxetine impairs serial pattern learning in the serial multiple choice (SMC) task in adult rats.

The effects of chronic adolescent fluoxetine (FLX, Prozac®) exposure on adult cognition are largely unknown. We used a serial multiple choice (SMC) task to characterize the effects of adolescent FLX exposure on rat serial pattern learning in adulthood. Male rats were exposed to either 1.0, 2.0, or 4.0 mg/kg/day FLX for five consecutive days each week for five weeks during adolescence, followed by a 35-day drug-free period. As adults, the rats were trained in a task that required them to learn a highly structured sequential pattern of responses in an octagonal chamber for water reinforcement. In a transfer phase, the terminal element of the pattern was replaced by a violation element that was inconsistent with previously learned pattern structure. Results indicated that adolescent FLX exposure caused differential learning deficits for different types of elements in the serial pattern. Adolescent exposure to 1.0 or 4.0 mg/kg/day FLX, but not 2.0 mg/kg/day FLX, impaired chunk-boundary element learning, which is known to be mediated by stimulus-response (S-R) learning. All three doses of FLX impaired violation element learning, which is known to be mediated by multiple-cue learning. FLX did not impair within-chunk element learning, which is known to be mediated by rule-learning mechanisms. The results indicate that adolescent FLX exposure produced multiple cognitive impairments that were detectable in adulthood long after drug exposure ended.

Adolescent exposure to methylphenidate impairs serial pattern learning in the serial multiple choice (SMC) task in adult rats.

The long-term effects of adolescent exposure to methylphenidate (MPD) on adult cognitive capacity are largely unknown. We utilized a serial multiple choice (SMC) task, which is a sequential learning paradigm for studying complex learning, to observe the effects of methylphenidate exposure during adolescence on later serial pattern acquisition during adulthood. Following 20.0mg/kg/day MPD or saline exposure for 5 days/week for 5 weeks during adolescence, male rats were trained to produce a highly structured serial response pattern in an octagonal operant chamber for water reinforcement as adults. During a transfer phase, a violation to the previously-learned pattern structure was introduced as the last element of the sequential pattern. Results indicated that while rats in both groups were able to learn the training and transfer patterns, adolescent exposure to MPD impaired learning for some aspects of pattern learning in the training phase which are learned using discrimination learning or serial position learning. In contrast adolescent exposure to MPD had no effect on other aspects of pattern learning which have been shown to tap into rule learning mechanisms. Additionally, adolescent MPD exposure impaired learning for the violation element in the transfer phase. This indicates a deficit in multi-item learning previously shown to be responsible for violation element learning. Thus, these results clearly show that adolescent MPD produced multiple cognitive impairments in male rats that persisted into adulthood long after MPD exposure ended.

Central cholinergic involvement in sequential behavior: impairments of performance by atropine in a serial multiple choice task for rats.

Two experiments examined whether muscarinic cholinergic systems play a role in rats' ability to perform well-learned highly-structured serial response patterns, particularly focusing on rats' performance on pattern elements learned by encoding rules versus by acquisition of stimulus-response (S-R) associations. Rats performed serial patterns of responses in a serial multiple choice task in an 8-lever circular array for hypothalamic brain-stimulation reward. Two experiments examined the effects of atropine, a centrally-acting muscarinic cholinergic receptor antagonist, on rats' ability to perform pattern elements where responses were controlled by rules versus elements, such as rule-inconsistent "violation elements" and elements following "phrasing cues," where responses were controlled by associative cues. In Experiment 1, 3-element chunks of both patterns were signaled by pauses that served as phrasing cues before chunk-boundary elements, but one pattern also included a violation element that was inconsistent with pattern structure. Once rats reached a high criterion of performance, the drug challenge was intraperitoneal injection of a single dose of 50 mg/kg atropine sulfate. Atropine impaired performance on elements learned by S-R learning, namely, chunk-boundary elements and the violation element, but had no effect on performance of rule-based within-chunk elements. In Experiment 2, patterns were phrased and unphrased perfect patterns (i.e., without violation elements). To control for peripheral effects of atropine, rats were treated with a series of doses of either centrally-acting atropine or peripherally-acting atropine methyl nitrate (AMN), which does not cross the blood-brain barrier. Once rats reached a high criterion, the drug challenges were on alternate days in the order 50, 25, and 100 mg/kg of either atropine sulfate or AMN. Atropine, but not AMN, impaired performance in the phrased perfect pattern for pattern elements where S-R associations were important for performance, namely, chunk-boundary elements. However, in the structurally more ambiguous unphrased perfect pattern where rats had fewer cues and presumably relied more on S-R associations throughout, atropine impaired performance on all pattern elements. Thus, intact muscarinic cholinergic systems were shown to be necessary for discriminative control previously established by S-R learning, but were not necessary for rule-based serial pattern performance.

Sex differences in adult cognitive deficits after adolescent nicotine exposure in rats.

This study was designed to determine whether deficits in adult serial pattern learning caused by adolescent nicotine exposure persist as impairments in asymptotic performance, whether adolescent nicotine exposure differentially retards learning about pattern elements that are inconsistent with "perfect" pattern structure, and whether there are sex differences in rats' response to adolescent nicotine exposure as assessed by a serial multiple choice task. The current study replicated the results of our initial report (Fountain et al., 2008) using this task by showing that adolescent nicotine exposure (1.0mg/kg/day nicotine for 35days) produced a specific cognitive impairment in male rats that persisted into adulthood at least a month after adolescent nicotine exposure ended. In addition, sex differences were observed even in controls, with additional evidence that adolescent nicotine exposure significantly impaired learning relative to same-sex controls for chunk boundary elements in males and for violation elements in females. All nicotine-induced impairments were overcome by additional training so that groups did not differ at asymptote. An examination of the types of errors rats made indicated that adolescent nicotine exposure slowed learning without affecting rats' cognitive strategy in the task. This data pattern suggests that exposure to nicotine in adolescence may have impaired different aspects of adult stimulus-response discrimination learning processes in males and females, but left abstract rule learning processes relatively spared in both sexes. These effects converge with other findings in the field and reinforce the concern that adolescent nicotine exposure poses an important threat to cognitive capacity in adulthood.

Adolescent exposure to nicotine impairs adult serial pattern learning in rats.

In the present study investigating the effects of adolescent nicotine exposure on adult serial pattern learning, adolescent rats received daily i.p. injections of either 1.0 mg/kg nicotine or saline for 5 days per week for 5 weeks beginning on postnatal day 25 (P25), then were allowed 35 days drug free. Rats then began training on P95 as adults on a 24-element serial pattern composed of eight 3-element chunks. Adolescent exposure to 1.0 mg/kg nicotine produced persistent retardation of learning for the first element of each 3-element chunk of the pattern, that is, for chunk boundary elements, and transient retardation of learning for elements 2 and 3 of each chunk of the pattern, that is, for the within-chunk elements. Deficits at chunk boundaries were interpreted as deficits of phrasing cue discrimination learning whereas deficits for learning responses for elements within-chunks (elements 2 and 3 of chunks) were interpreted as deficits of rule learning. These results indicate that the effects of adolescent nicotine exposure on adult learning and cognitive capacity deserve further scrutiny.

Determinants of phrasing effects in rat serial pattern learning.

Two experiments investigated how brief pauses introduced into serial patterns as phrasing cues would affect pattern learning in rats. In Experiment 1, a 24-element pattern consisted of eight 3-element chunks, whereas a 20-element pattern consisted of four 5-element chunks. In both patterns, 3.0-s temporal pauses placed at chunk boundaries (synchronous phrasing cues) facilitated learning compared to no phrasing. Cues "out of sync" with pattern structure (asynchronous phrasing cues) facilitated learning for the 24-element pattern and retarded learning for the 20-element pattern. Evidence suggested that in the latter case, 3.0-s pauses served as "blank" trials that induced rats to "skip" to the next serial position in sequence. In Experiment 2, shorter 0.5-s pauses served as phrasing cues in the 20-element pattern of Experiment 1. Synchronous short cues facilitated learning, whereas asynchronous phrasing cues had no effect. Furthermore, removal of synchronous cues produced deficits in performance on formerly cued trials, whereas removal of asynchronous cues had no effect. The results of Experiment 2 support the notion that in both experiments phrasing cues served as discriminative cues and indirectly suggest that rats are concurrently sensitive to pattern element cues, extra-sequence cues (such as phrasing cues), and to the relative timing of sequential events.

Long-term, low-level adolescent nicotine exposure produces dose-dependent changes in cocaine sensitivity and reward in adult mice.

Cigarette smoking by adolescents is a strong predictor of future drug use, abuse, and dependence. While this "gateway drug effect" is assumed to be related to psychosocial factors, data from our laboratory suggests that adolescent nicotine use may permanently disrupt reward systems through changes in dopamine receptor function. Behavioral pharmacological methods known to be indirectly (motor activity) and directly (conditioned-place-preference) related to drug reinforcement were used to examine changes in cocaine sensitivity. Testing was performed on adult mice that were exposed to nicotine (0.3, 1.0, and 3.0 mg/kg, SC, M-F, b.i.d.) or saline during adolescence (postnatal days 25-57). Prior to testing, subjects had a 28 day drug-free, time-off period. After acclimation to the testing apparatus, the locomotor effects (30 min, 30 cm traveled) of cocaine (5, 10, and 20 mg/kg, IP) were measured daily; cocaine tests were preceded and followed by saline control tests. Following the acute dose-response curve, mice received saline followed by 5 days of 20.0 mg/kg cocaine. Thereafter, mice underwent condition-place-preference testing. A pre-test was performed to determine compartment preference (i.e., no injection, 20 min test). Cocaine (10 mg/kg, IP) was paired with the subjects non-preferred side and saline with the other. Conditioning sessions were conducted for 8 days with the order of drug/saline injections counter-balanced across subjects. A drug-free, post-test occurred on the day following the final conditioning session. A dose-dependent relationship between adolescent nicotine exposure and cocaine reward was noted in the adult mice across both test conditions. Subjects exposed to nicotine showed an increased response to cocaine's motor activating effects and a decreased response to cocaine's rewarding effects. A follow-up study was undertaken to evaluate dopamine D1, D2, and D3 receptor function in adult mice exposed to the highest dose of nicotine from the first study. While both interesting and revealing, the results of motor activity tests with dopamine agonist only approached significance. Further research will be required to more fully examine the mechanism of action for the observed changes in cocaine reward. In summary, this is the first study to demonstrate a dose-response relationship between adolescent nicotine exposure and changes in cocaine reward and sensitivity during adulthood.