Differential effects of inactivation of the orbitofrontal cortex on strategy set-shifting and reversal learning

Different subregions of the rodent prefrontal cortex (PFC) mediate dissociable types of behavioral flexibility. For example, lesions of the medial or orbitofrontal (OFC) regions of the PFC impair extradimensional shifts and reversal learning, respectively, when novel stimuli are used during different phases of the task. In the present study, we assessed the effects of inactivation of the OFC on strategy set-shifting and reversal learning, using a maze based set-shifting task mediated by the medial PFC. Long–Evans rats were trained initially on a visual-cue discrimination to obtain food. On the subsequent day, rats had to shift to using a response strategy (e.g., always turn left). On Day 3 (reversal), rats were required to reverse the direction of their turn (e.g., always turn right). Infusions of the local anesthetic bupivacaine into the OFC did not impair initial visual discrimination learning, nor did it impair performance on the set-shift. In contrast, inactivation of the OFC did impair reversal learning; yet, these rats ceased using the previously acquired response rule as readily as controls. Instead, rats receiving OFC inactivations made a disproportionate number of erroneous arm entries towards the visual-cue, suggested that these animals reverted back to using the original visual-cue based strategy. These findings, in addition to previous data, further support the notion that the OFC and medial PFC play dissociable roles in reversal learning and set-shifting. Furthermore, the lack of effect of OFC inactivations on the set-shift indicates that this type of behavioral flexibility does not require cognitive operations related to reversal learning.     

大鼠注意定势转移任务模型的深入研究：种系和检测程序的影响

注意定势转移任务(attentional set-shifting task, AST)可用于特异性检测啮齿类动物前额叶皮层及其皮层下神经通路介导的认知灵活性, 是目前研究认知灵活性及其障碍神经基础的重要模型。本研究系统调查了大鼠种系和检测程序差异对AST结果的影响。通过比较Wistar和Sprague Dawley (SD)两个种系大鼠在七阶段和五阶段两种AST检测程序中的认知表现, 研究发现：(1) SD和Wistar大鼠前额叶认知功能存在差异, 后者的总体认知表现优于前者。尤其是Wistar大鼠在逆反学习阶段的达标训练次数及错误率显著低于SD大鼠, 表明Wistar大鼠具有更高的策略转换灵活性。(2)在AST测试中逆反学习和外维度定势转移是认知灵活性评价的核心指标。这两种认知转换过程分别以前期策略和注意定势建立为基础。结果显示在两种AST检测程序中Wistar和SD大鼠在逆反学习和/或外维度定势转移等复杂学习阶段的达标训练次数和错误率均高于其它简单关联学习阶段, 表明在目前实验条件下大鼠均表现出定势形成和转换困难的反应模式, 不同认知反应间的结构关系具有稳定性。这些结果提示大鼠前额叶皮质介导的认知灵活性存在种系差异, AST各阶段认知反应间的结构效度不受目前使用的大鼠种系和检测程序差异的影响, 扩展了对AST模型的认识。     

Lesions of rat infralimbic cortex enhance recovery and reinstatement of an appetitive Pavlovian response

The prefrontal cortex (PFC) has a well-established role in the inhibition of inappropriate responding, and evidence suggests that the infralimbic (IL) region of the rat medial PFC (MPFC) may be involved in some aspects of extinction of conditioned fear. MPFC lesions including, but not those sparing the IL cortex increase spontaneous recovery of extinguished conditioned fear when tested 24 h after an initial extinction session. The current experiment extended these findings by use of appetitive rather than aversive conditioning. Ten IL-lesioned and 11 sham-operated rats were trained on a Pavlovian task in which a conditioned stimulus (CS) was followed by food pellets (the unconditioned stimulus or US). IL lesions had no effect on extinction of the conditioned response (CR, magazine entries) during the first extinction session. However, the level of spontaneous recovery between the first extinction session and a second, 24 h later, was increased in IL-lesioned rats relative to sham animals. In contrast, evidence of savings measured between the extinction sessions did not differ between groups. Furthermore, reinstatement of the CR following unsignaled delivery of the US was also increased in IL-lesioned rats.     

An analysis of rat prefrontal cortex in mediating executive function

While it is acknowledged that species specific differences are an implicit condition of comparative studies, rodent models of prefrontal function serve a significant role in the acquisition of converging evidence on prefrontal function across levels of analysis and research techniques. The purpose of the present review is to examine whether the prefrontal cortex (PFC) in rats supports a variety of processes associated with executive function including working memory, temporal processing, planning (prospective coding), flexibility, rule learning, and decision making. Therefore, in this review we examined changes associated with working memory processes for spatial locations, visual objects, odors, tastes, and response domains or attributes, temporal processes including temporal order, sequence learning, prospective coding, behavioral flexibility associated with reversal learning and set shifting, paired associate learning, and decision making based on effort, time discounting, and uncertainty following damage to the PFC in rats. In addition, potential parallel processes of executive function in monkeys and humans based on several theories of subregional differentiation within the PFC will be presented. Specifically, theories based on domain or attribute specificity (Goldman-Rakic, 1996), level of processing (Petrides, 1996), rule learning based on complexity (Wise, Murray, & Gerfen, 1996), executive functions based on connectivity with other brain regions associated with top-down control (Miller & Cohen, 2001), are presented and applied to PFC function in rats with the aim of understanding subregional specificity in the rat PFC. The data suggest that there is subregional specificity within the PFC of rats, monkey and humans and there are parallel cognitive functions of the different subregions of the PFC in rats, monkeys and humans.     

Effects of acute restraint stress on set-shifting and reversal learning in male rats

Exposure to acute stress alters cognition; however, few studies have examined the effects of acute stress on executive functions such as behavioral flexibility. The goal of the present experiments was to determine the effects of acute periods of stress on two distinct forms of behavioral flexibility: set-shifting and reversal learning. Male Sprague-Dawley rats were trained and tested in an operant-chamber-based task. Some of the rats were exposed to acute restraint stress (30 min) immediately before either the set-shifting test day or the reversal learning test day. Acute stress had no effect on set-shifting, but it significantly facilitated reversal learning, as assessed by both trials to criterion and total errors. In a second experiment, the roles of glucocorticoid (GR) and mineralocorticoid receptors (MR) in the acute-stress-induced facilitation of reversal learning were examined. Systemic administration of the GR-selective antagonist RU38486 (10 mg/kg) or the MR-selective antagonist spironolactone (50 mg/kg) 30 min prior to acute stress failed to block the facilitation on reversal learning. The present results demonstrate a dissociable effect of acute stress on set-shifting and reversal learning and suggest that the facilitation of reversal learning by acute stress may be mediated by factors other than corticosterone.     

The effect of striatal dopamine depletion and the adenosine A2A antagonist KW-6002 on reversal learning in rats

This study assessed whether dopamine in the dorsomedial striatum is necessary for flexible adaptation to changes in stimulus-response contingencies. As KW-6002 (Istradefylline), an adenosine A(2A) antagonist, improves motor deficits resulting from striatal dopamine depletion, we also tested for potential ameliorative effects of KW-6002 on dopamine depletion-induced cognitive deficits. Male Lister hooded rats were presented with two bowls, discriminable by either a textured covering on the outer surface, their scent or the bowl contents (digging media) in which bait was buried. Once they had learned in which bowl food was buried, the stimulus-response contingencies were reversed. In both phases (acquisition and reversal), the criterion for learning was defined a priori as six consecutive correct trials. Following depletion of dopamine in the dorsomedial striatum, acquisition of the discriminations was intact but there was an increase in the number of trials to attain criterion performance in the reversal phases, indicating an impairment in reversal learning. KW-6002 (1mg/kg bidaily for 10 days) non-specifically increased the number of trials to criterion at all stages of the test and in both controls (sham-operated) and dopamine-depleted rats. Chronic KW-6002 treatment did not improve the reversal deficits in dopamine-depleted rats. These findings suggest that dopamine transmission in the dorsomedial striatum is critical for the flexible shifting of response patterns and the ameliorative effects of KW-6002 following depletion of dopamine in the striatum may be restricted to motor functions without relieving deficits in response-shifting flexibility.     

Visual discrimination and reversal learning in rough-necked monitor lizards (Varanus rudicollis)

Reptile learning has been studied with a variety of methods and has included numerous species. However, research on learning in lizards has generally focused on spatial memory and has been studied in only a few species. This study explored visual discrimination in two rough-necked monitors (Varanus rudicollis). Subjects were trained to discriminate between black and white stimuli. Both subjects learned an initial discrimination task as well as two reversals, with the second reversal requiring fewer sessions than the first. This reduction in trials required for reversal acquisition provides evidence for behavioral flexibility in the monitor lizard genus.     

Rule-based learning impairment in rats with lesions to the dorsal striatum.

The present study examined the effects of lesions to the dorsal striatum (DS) in Sprague-Dawley rats, when tested on the acquisition and successive shifts in the position of a goal arm in an eight-arm radial maze. In the procedure we used, rats had to retrieve the location of one baited arm among the eight arms of the maze after it had just been presented as a sample during a forced trial. After attainment of a fixed learning criterion, rats were submitted to five successive shifts in the goal location. Results showed that DS rats were able to learn the position of the goal arm during the acquisition phase as efficiently as sham-operated rats. In contrast, when the position of the goal arm was shifted, although DS rats were able to learn its new position, they made significantly more errors and required more sessions to reach criterion than sham-operated rats. These results suggested that both groups did not solve the task using the same behavioral strategy. The analysis of responses made suggested that sham-operated rats solved the task using the pairing rule between the forced and the free run (matching-to-sample rule), while DS rats solved the task using only visuo-spatial processing. These data therefore suggest that the dorsal striatum plays an important role in rule-learning ability.     

Individual differences in cognitive control: The role of psychoticism and working memory in set‐shifting

Set‐shifting refers to a process of cognitive control which is shown through flexible behavioural adaptation to changes in task parameters or demands, such as the switching of an explicit rule (extra‐dimensional rule shifting) or the reversal of a reinforcement‐contingency (reversal‐learning). Set‐shifting deficits are widely documented in specific neuropsychological disorders, but seldom investigated in relation to normally‐occurring individual differences. In a sample of healthy adults (N=78, 28% male), we demonstrate that Working Memory and trait Psychoticism have independent involvement in extra‐dimensional rule shifting as measured using an analogue of the Wisconsin Card Sorting Test. Only Psychoticism, however, was involved in reversal‐learning, as assessed using a recent modification of the Iowa Gambling Task. Individual differences in extra‐dimensional rule shifting were explained in terms of rule abstraction speed, while individual differences in reversal‐learning were explained in terms of response perseveration. These results clarify component processes in different forms of set‐shifting, and highlight the role of individual differences, especially personality, in cognitive control.     

Lesions of orbitofrontal cortex and basolateral amygdala complex disrupt acquisition of odor-guided discriminations and reversals

Recent work indicates that both orbitofrontal cortex (OFC) and the basolateral complex of the amygdala (ABL) are involved in processes by which cues are associated with predicted outcomes. To examine the respective roles of these structures in discrimination learning, rats with bilateral sham or neurotoxic lesions of either OFC or ABL were trained on a series of four 2-odor discrimination problems in a thirst-motivated go, no-go task. After acquisition of the series of odor problems, the rats were trained on serial reversals of the final odor problem. Performance on each problem was assessed by monitoring accuracy of choice behavior, and also by measuring latency to respond for fluid outcomes after odor sampling. During discrimination learning, rats in both lesioned groups had similar deficits, failing to show normal changes in response latency during learning, while at the same time exhibiting normal choice behavior relative to controls. Choice behavior was affected only during the reversal phase of training, in which OFC and ABL lesions produced distinctive deficits. Rats with ABL lesions were impaired on the first reversal (S1−/S2+), but were unimpaired at acquiring a reversal back to the original odor-outcome contigencies (S1+/S2−), whereas rats with OFC lesions were impaired on both types of reversals. These findings suggest that OFC and ABL serve partially overlapping roles in the use of incentive information that supports normal discrimination performance.     

Delay-dependent impairment of reversal learning in rats treated with trimethyltin

Recent theories of hippocampal function focus on its role in the formation of associations in the temporal domain. A reversal learning paradigm based on leverpress automaintenance was developed to vary the CS-US relationship along two independent dimensions, one temporal and one not: CS(+)-US delay and the probability of reinforcement [P(RFT)] following the CS+. Eight male hooded Long-Evans rats were trained to reverse these automaintained discriminations repeatedly, until stable performance was achieved. The neurotoxicant trimethyltin (TMT) was used to induce lesions in the CNS, including the CA3-4 region of Ammon's Horn in dorsal hippocampus. Following iv injection of 7 mg/kg TMT to half the rats, reversal learning was assessed under varying conditions of delay and P(RFT). After recovery from the acute effects of TMT (1-2 weeks), treated rats reversed normally when no delay separated the CS+ and US; with delays of 2 to 4 s, they reversed less completely within a session than did controls. Changing P(RFT) did not affect reversal learning in either group, but reduced response rates similarly in both groups. Morphological damage was quantified by measuring the length of the remaining pyramidal cell line in sections of dorsal hippocampus. The degree of behavioral impairment correlated significantly with hippocampal damage only at nonzero CS(+)-US delays. These results indicate that TMT impaired ability of rats to integrate temporal relationships between stimulus events, and are consistent with theories of hippocampal mediation of temporal associations.     

Acquisition,Extinction, Recovery,and Reversal of Different Response Sequences Under Conditional Control by Nicotine in Rats

Complex voluntary behaviors occur in sequence. Eight rats were trained in an operant procedure that used nicotine and non-drug (saline) states as interoceptive cues that signaled which of two behavioral sequences led to food reward. The distal and proximal responses in the chain were always maintained on variable interval 30-sec and fixed ratio-1 schedules, respectively, and rate differences between the responses were used as the dependent variable. Extinction and reversal training was conducted. Distal response rates were significantly greater than proximal response rates during training, testing, extinction, and reversal learning. These data suggest that (a) nicotine can establish interoceptive control over different response sequences, and (b) extinction of one response sequence may be state-dependent. The clinical relevance of extinction of complex behavioral repertoires such as drug-seeking and drug-taking behavior that are evoked by specific interoceptive cues is addressed in regard to drug abuse treatment and relapse.     

Isolation rearing effects on probabilistic learning and cognitive flexibility in rats

Isolation rearing is a neurodevelopmental manipulation that produces neurochemical, structural, and behavioral alterations in rodents that in many ways are consistent with schizophrenia. Symptoms induced by isolation rearing that mirror clinically relevant aspects of schizophrenia, such as cognitive deficits, open up the possibility of testing putative therapeutics in isolation-reared animals prior to clinical development. We investigated what effect isolation rearing would have on cognitive flexibility, a cognitive function characteristically disrupted in schizophrenia. For this purpose, we assessed cognitive flexibility using between- and within-session probabilistic reversal-learning tasks based on clinical tests. Isolation-reared rats required more sessions, though not more task trials, to acquire criterion performance in the reversal phase of the task, and were slower to adjust their task strategy after reward contingencies were switched. Isolation-reared rats also completed fewer trials and exhibited lower levels of overall activity in the probabilistic reversal-learning task than did the socially reared rats. This finding contrasted with the elevated levels of unconditioned investigatory activity and reduced levels of locomotor habituation that isolation-reared rats displayed in the behavioral pattern monitor. Finally, isolation-reared rats also exhibited sensorimotor gating deficits, reflected by decreased prepulse inhibition of the startle response, consistent with previous studies. We concluded that isolation rearing constitutes a valuable, noninvasive manipulation for modeling schizophrenia-like cognitive deficits and assessing putative therapeutics.     

Activation and role of the medial prefrontal cortex (mPFC) in extinction of ethanol-induced associative learning in mice

Although the medial prefrontal cortex (mPFC) has been shown to be integrally involved in extinction of a number of associative behaviors, its role in extinction of alcohol (ethanol)-induced associative learning has received little attention. Previous reports have provided evidence supporting a role for the mPFC in acquisition and extinction of amphetamine-induced conditioned place preference (CPP) in rats, however, it remains unknown if this region is necessary for extinction of ethanol (EtOH)-induced associative learning in mice. Using immunohistochemical analysis of phosphorylated and unphosphorylated cAMP response element-binding protein (CREB), the current set of experiments first showed that the prelimbic (PL) and infralimbic (IL) subregions of the mPFC exhibited dynamic responses in phosphorylation of CREB to a Pavlovian-conditioned, EtOH-paired cue. Interestingly, CREB phosphorylation within these regions was sensitive to manipulations of the EtOH-cue contingency-that is, the cue-induced increase of pCREB in both the PL and IL was absent following extinction. In order to confirm a functional role of the mPFC in regulating the extinction process, we then showed that electrolytic lesions of the mPFC following acquisition blocked subsequent extinction of EtOH-CPP. Together, these experiments indicate a role for the PL and IL subregions of the mPFC in processing changes of the EtOH-cue contingency, as well as in regulating extinction of EtOH-induced associative learning in mice.     

Neonatal ventral hippocampus lesions disrupt extra-dimensional shift and alter dendritic spine density in the medial prefrontal cortex of juvenile rats

Recent data showed that neonatal ventral hippocampus (VH) lesions, an approach used to model schizophrenia symptoms in rodents, produce premature deficits of working memory believed to be associated with early medial prefrontal cortex (mPFC) maldevelopment. This experiment expands the investigation of mPFC integrity in juvenile rats with neonatal VH lesions by assessing behavioral flexibility and dendritic spine density. Sixteen Sprague-Dawley male pups received bilateral microinjections of ibotenic acid in the VH or SHAM surgery on postnatal day (PND) 6. On PND 29 and 30, rats were subjected to a spatial shift task in a cross-maze; an attentional set-shifting task was then administered on two consecutive days, between PND 33 and PND 35. Rats were sacrificed at PND 36 and dendritic spine density in the mPFC was assessed using Golgi-Cox staining procedure. Results revealed impaired extra-dimensional shift in VH-lesioned rats and inconsistent reversal discrimination outcomes. Although lesioned animals displayed intact performance in the spatial shift, rates of perseverative responses were higher than normal in this task. Neonatal VH damage resulted in lower dendritic spine density in the mPFC than measured in control brains; however, no significant correlation was found between this outcome and behavioral data. Juvenile morphological and cognitive perturbations are consistent with the early emergence of mPFC anomalies following neonatal VH lesions. Results are discussed in relation with potential common mechanisms linking pre- and post-pubertal onsets of behavioral dysfunction.     

Serotonin transporter knockout rats show improved strategy set-shifting and reduced latent inhibition

Behavioral flexibility is a cognitive process depending on prefrontal areas allowing adaptive responses to environmental changes. Serotonin transporter knockout (5-HTT(-/-)) rodents show improved reversal learning in addition to orbitofrontal cortex changes. Another form of behavioral flexibility, extradimensional strategy set-shifting (EDSS), heavily depends on the medial prefrontal cortex. This region shows functional changes in 5-HTT(-/-) rodents as well. Here we subjected 5-HTT(-/-) rats and their wild-type counterparts to an EDSS paradigm and a supplementary latent inhibition task. Results indicate that 5-HTT(-/-) rats also show improved EDSS, and indicate that reduced latent inhibition may contribute as an underlying mechanism.     

Dynamic changes in acetylcholine output in the medial striatum during place reversal learning

The present studies explored the role of the medial striatum in learning when taskcontingencies change. Experiment 1 examined whether the medial striatum is involved in place reversal learning. Testing occurred in a modified cross-maze across two consecutive sessions. Injections of the local anesthetic, bupivacaine, into the medial striatum, did not impair place acquisition, but impaired place reversal learning. The reversal-learning deficit was due to an inability to maintain the new choice pattern following the initial shift. Experiment 2 determined whether changes in acetylcholine (ACh) output occur during the acquisition or reversal learning of a place discrimination. Extracellular ACh output from the medial striatum was assessed in samples collected at 6-min intervals using in vivo microdialysis during behavioral testing. ACh output did not change from basal levels during place acquisition. During reversal learning, ACh output significantly increased as rats began to learn the new choice pattern, and returned to near basal levels as a rat reliably executed the new place strategy. The present results suggest that the medial striatum may be critical for flexible adaptations involving spatial information, and that ACh actions in this area enable the shifting of choice patterns when environmental conditions change.     

The effects of hippocampal lesions on learning, memory, and reward expectancies

The hippocampus appears to be critical for the formation of certain types of memories. Hippocampal-lesioned animals fail to exhibit some spatial, contextual, and relational associations. After aspiration lesions of the hippocampus and/or cortex, male rats were allowed to recover for three weeks before being trained on a matching-to-position task. The matching-to-position task was altered to influence the type of cognitive strategies a subject would use to solve the task. The main behavioral manipulation was the reinforcement contingency assignment: Use of a differential outcomes procedure (DOP) or a nondifferential outcomes procedure (NOP). The DOP involves correlating each to-be-remembered event with a distinct reward condition via Pavlovian trace conditioning, whereas the NOP results in random reward contingency. We found that hippocampal lesions did retard learning the matching rule, regardless of the reinforcement contingency assignment. However, when delay intervals were added to the task memory performance of subjects with hippocampal lesions was dramatically impaired--if subjects were not trained with the DOP. When subjects were trained with the DOP, the hippocampal lesion had a marginal effect on delayed memory performance. These findings demonstrate two important points regarding lesions of the hippocampus: (1) hippocampal lesions have a minimal effect on the on the ability of rats to use reward information to solve a delayed discrimination task; (2) rats with hippocampal lesions have the ability to learn about reward information using Pavlovian trace conditioning procedures.     

阻断内侧前额叶皮质TrkB受体对大鼠认知和海马BDNF表达的影响

脑源性神经营养因子(brain-derived neurotrophic factor, BDNF)广泛参与了个体学习和记忆等认知功能, 通过与其酪氨酸激酶受体(tyrosine kinase, TrkB)特异性结合, 实现其多种神经生化功能。本研究观察了TrkB受体阻断剂ANA-12的慢性内侧前额叶皮质(medial prefrontal cortex, mPFC)注射对大鼠旷场行为、Morris水迷宫空间学习和逆反学习的影响。研究结果表明, mPFC的慢性BDNF阻断显著降低了大鼠在逆反学习测试中的逃离潜伏期和运动距离即增强了大鼠的逆反学习能力, 但不影响其旷场行为和水迷宫空间学习能力。同时, 慢性阻断mPFC-TrkB受体也并未导致大鼠海马BDNF蛋白含量的显著改变。这些结果提示, 对于大鼠的Morris水迷宫空间学习和逆反学习, mPFC-BDNF主要在逆反学习调节中发挥重要作用。这对于进一步探索海马和mPFC在调节个体认知功能中各自的作用及其潜在的相互关系提供了有力的证据和支持。     

Response rate, reinforcement frequency, and conditioned suppression

In the first of two experiments, periods of noise were terminated with unavoidable shock to 36 rats. The rats' continuously reinforced responding was later completely suppressed during the noise when it was introduced without shock. The rats were then assigned to nine experimental groups. Each group was exposed to different paced variable-interval schedules of reinforcement, which independently controlled response rate and reinforcement frequency. Periods of the noise were periodically superimposed on these schedules, and loss of response suppression was studied. Differences between the groups were assessed statistically. The second experiment used a steady-state design. Six rats were exposed to paced schedules which generated two alternating response rates but gave constant reinforcement frequencies, and six rats to schedules which maintained the same response rates throughout, but in which the reinforcement frequency was alternately high and low. Response suppression was studied during a pre-shock stimulus superimposed on each rat's two behavioral baselines. Both experiments suggest that (1) conditioned suppression is affected by rate of operant responding, high rates being most suppressed, and (2) the frequency of reinforcements obtained also has an effect, most suppression occurring when frequency is low.