Acetylcholine activity in selective striatal regions supports behavioral flexibility

Daily living often requires individuals to flexibly respond to new circumstances. There is considerable evidence that the striatum is part of a larger neural network that supports flexible adaptations. Cholinergic interneurons are situated to strongly influence striatal output patterns which may enable flexible adaptations. The present experiments investigated whether acetylcholine actions in different striatal regions support behavioral flexibility by measuring acetylcholine efflux during place reversal learning. Acetylcholine efflux selectively increased in the dorsomedial striatum, but not dorsolateral or ventromedial striatum during place reversal learning. In order to modulate the M2-class of autoreceptors, administration of oxotremorine sesquifumurate (100 nM) into the dorsomedial striatum, concomitantly impaired reversal learning and an increase in acetylcholine output. These effects were reversed by the m(2) muscarinic receptor antagonist, AF-DX-116 (20 nM). The effects of oxotremorine sesquifumurate and AF-DX-116 on acetylcholine efflux were selective to behaviorally-induced changes as neither treatment affected acetylcholine output in a resting condition. In contrast to reversal learning, acetylcholine efflux in the dorsomedial striatum did not change during place acquisition. The results reveal an essential role for cholinergic activity and define its locus of control to the dorsomedial striatum in cognitive flexibility.     

Differential involvement of M1-type and M4-type muscarinic cholinergic receptors in the dorsomedial striatum in task switching

Previous experiments have demonstrated that the rat dorsomedial striatum is one brain area that plays a crucial role in learning when conditions require a shift in strategies. Further evidence indicates that muscarinic cholinergic receptors in this brain area support adaptations in behavioral responses. Unknown is whether specific muscarinic receptor subtypes in the dorsomedial striatum contribute to a flexible shift in response patterns. The present experiments investigated whether blockade of M1-type and/or M4-type cholinergic receptors in the dorsomedial striatum underlie place reversal learning. Experiment 1 investigated the effects of the M1-type muscarinic cholinergic antagonist, muscarinic-toxin 7 (MT-7) infused into the dorsomedial striatum in place acquisition and reversal learning. Experiment 2 investigated the effects of the M4-type muscarinic cholinergic antagonist, muscarinic-toxin 3 (MT-3) injected into the dorsomedial striatum in place acquisition and reversal learning. All testing occurred in a modified cross-maze across two consecutive sessions. Bilateral injections of MT-7 into the dorsomedial striatum at 1 or 2 microg, but not 0.05 microg impaired place reversal learning. Analysis of the errors revealed that MT-7 at 1 and 2 microg significantly increased regressive errors, but not perseverative errors. An injection of MT-7 2 microg into the dorsomedial striatum prior to place acquisition did not affect learning. Experiment 2 revealed that dorsomedial striatal injections of MT-3 (0.05, 1 or 2 microg) did not affect place acquisition or reversal learning. The findings suggest that activation of M1-type muscarinic cholinergic receptors in the dorsomedial striatum, but not M4-type muscarinic cholinergic receptors facilitate the flexible shifting of response patterns by maintaining or learning a new choice pattern once selected.     

The influence of NMDA receptors in the dorsomedial striatum on response reversal learning

In mammals, the dorsomedial striatum is one brain area shown to be critical for the flexible shifting of response patterns. At present, the neurochemical mechanisms that underlie learning during a shift in response patterns are unknown. The present study examined the effects of NMDA competitive antagonist, DL-2-amino-5-phosphonopentanoic acid (AP-5), injected into the dorsomedial striatum on the acquisition and reversal of a response discrimination. Male Long-Evans rats were tested across two consecutive days in a modified cross-maze. Rats received an infusion of either saline or AP-5 (5 or 25 nmol) 5 min prior to each test session. In the acquisition phase rats learned to turn in one direction (right or left) to receive a cereal reinforcement. In the reversal learning phase rats learned to turn in the opposite direction as in the acquisition phase. In both phases, criterion was achieved when a rat made 10 consecutive correct trials. Infusions of AP-5 did not impair acquisition, but impaired reversal learning of a response discrimination in a dose-dependent fashion. The reversal learning deficit induced by AP-5 resulted from reversions back to the originally learned response pattern following the initial shift. These results suggest that activation of NMDA receptors in the dorsomedial striatum are critical for the flexible shifting of response patterns by enhancing the reliable execution of a new response pattern under changing task contingencies.     

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 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.     

Impaired, spared, and enhanced ACh efflux across the hippocampus and striatum in diencephalic amnesia is dependent on task demands

Diencephalic amnesia manifests itself through a host of neurological and memory impairments. A commonly employed animal model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), results in brain lesions and impairments similar in nature and distribution to those observed in humans with Wernicke–Korsakoff syndrome (WKS). In the current investigation, 2 separate experiments were conducted in which acetylcholine (ACh) efflux was assessed in the hippocampus and striatum of PTD-treated and pair-fed (PF) control male Sprague–Dawley rats. The goal was to determine under what behavioral conditions and in which brain structures ACh efflux was spared, impaired, or adaptively enhanced. In Experiment 1, rats were assessed on a spontaneous alternation task; in Experiment 2, rats were tested on a T-maze discrimination task that could be learned via a hippocampal- or striatal-based strategy. In Experiment 1, PTD-treated rats were impaired on the spontaneous alternation task and ACh efflux in the hippocampus during testing was significantly reduced, but spared in the striatum. In Experiment 2, PTD- and PF-treated rats did not differ in the number of trials to criterion, but PTD-treated rats demonstrated greater reliance upon egocentric cues to solve the task. Furthermore, ACh efflux in the striatum was greater during maze learning in the PTD-treated animals when compared to the PF animals. These results suggest that there is behavioral and systems level plasticity that can facilitate the use of alternative strategies to solve a task following diencephalic damage and WKS.     

The differential involvement of the prelimbic and infralimbic cortices in response conflict affects behavioral flexibility in rats trained in a new automated strategy-switching task

To assess the role of the prelimbic (PL) and infralimbic (IL) cortices in mediating strategy switching, rats were trained in a new automated task in a Y-maze allowing a careful analysis of rats' behavior. In this situation, rats can only use two egocentric (Right, Left) and two visual (Light, Dark) strategies. In the first experiment, rats with PL, IL, or PL/IL lesions were compared with sham-operated rats when trained to reach a criterion of 10 consecutive correct responses with a light strategy before being trained with a response strategy (rule shifting), and finally with the reversed response strategy (reversal). In the second experiment, sham-operated and PL-lesioned rats had their first two strategy switches in the reverse order, which was followed by a second rule shifting and reversal. The results indicate that lesions did not affect initial acquisition, but impaired the first rule shifting and reversal. Thorough analyses of rats' performance indicate that lesioned rats were still able to demonstrate some behavioral flexibility but have difficulties in solving response conflicts, which in turn may affect behavioral flexibility. Both areas were differentially involved in the resolution of response conflict, with the IL involved in the choice of strategy previously known to be nonvalid, and the PL in the selection and maintenance of that strategy.     

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.     

Repeated acquisition and discrimination reversal in the squirrel monkey (Saimiri sciureus)

Repeated acquisition and discrimination reversal tasks are often used to examine behavioral relations of, respectively, learning and cognitive flexibility. Surprisingly, despite their frequent use in cognitive neuroscience and behavioral pharmacology, variables that control performance under these two tasks have not been widely studied. The present studies were conducted to directly investigate the controlling variables in nonhuman primates. Squirrel monkeys were trained with a touchscreen variant of the repeated acquisition task in which a novel pair of S⁺/S^? stimuli was presented daily. Subjects learned to discriminate the two stimuli (acquisition) and, subsequently, with the contingencies switched (reversal). Results indicate that rates of both acquisition and reversal learning increased across successive sessions, but that rate of reversal learning remained slower than acquisition learning, i.e., more trials were needed for mastery. Subsequent experiments showed this difference between the rate of learning novel discriminations and reversal was reliable for at least 5 days between acquisition and reversal and notwithstanding the interpolation of additional discriminations. Experimental analysis of the S⁺/S^? elements of the tasks revealed that the difference in the rate of learning could not be attributed to a relatively aversive quality of the S^? or to a relatively appetitive quality of the S⁺, but, rather, to contextual control by the S⁺/S^? stimulus complex. Thus, if either element (S⁺ or S^?) of the stimulus complex was replaced by a novel stimulus, the rate of acquisition approximated that expected with a novel stimulus pair. These results improve our understanding of fundamental features of discrimination acquisition and reversal.     

Lesions of the dorsolateral or dorsomedial striatum impair performance of a previously acquired simple discrimination task

Previous evidence has suggested a specific role for the dorsal striatum, especially the dorsolateral region of the dorsal striatum, in stimulus-response learning. In a previous study, we found an impairment in animals with dorsolateral striatal lesions on a simple discrimination task (CS+/CS-), thought to require the involvement of both stimulus-reward and stimulus-response learning. It is possible that the generally poor performance of dorsolateral lesioned animals on this experiment precluded adequate exposure to stimulus-reward pairings necessary for solving this task, and, thus, had little to do with stimulus-response learning. To test this hypothesis, the performance of animals with dorsolateral and dorsomedial striatal lesions was assessed on a previously acquired simple discrimination task. To independently assess the effects of each lesion on the performance of stimulus-reward learning, dorsolateral and dorsomedial lesioned animals were assessed on a previously acquired conditioned place preference task (CPP). In agreement with our earlier experiment, and the stimulus-response interpretation of dorsolateral striatal function, animals with dorsolateral striatal lesions were found to be impaired during post-lesion performance of the simple discrimination task, but not CPP learning. Additionally, dorsomedial lesioned animals were found to be impaired in performance of the simple discrimination task, but not on the CPP task. Possible explanations for the differences between the role of the dorsomedial striatum in acquisition and expression of the simple discrimination task are proposed.     

Muscarinic cholinergic receptor blockade in the rat prelimbic cortex impairs the social transmission of food preference

Previous findings demonstrate the involvement of the cholinergic NBM in the acquisition of the social transmission of food preference (STFP), a relational associative odor-guided learning task. There is also evidence that muscarinic receptors in the medial prefrontal cortex, an important NBM target area, may modulate olfactory associative memory. The present experiment determined the consequences of blocking muscarinic cholinergic receptors in a component of the medial prefrontal region (the prelimbic cortex) on the STFP task. Adult male Wistar rats were bilaterally infused with scopolamine (20 microg/site) prior to training and showed a severe impairment in the expression of the task measured in two retention sessions, both immediately and 24h after training. Local scopolamine injections in the prelimbic cortex did not affect other behavioral measures such as olfactory perception, social interaction, motivation to eat, neophobia, or exploration. Results suggest that muscarinic transmission in the prelimbic cortex is essential for the STFP, supporting the hypothesis that ACh in a specific prefrontal area is important for this naturalistic form of olfactory relational memory. Current data are discussed in the context of disruption of learning as a result of interferences in PLC functions such as behavioral flexibility, attention, and strategic planning.     

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.     

Differential effects of muscarinic receptor blockade in prelimbic cortex on acquisition and memory formation of an odor-reward task

The present experiments determined the consequences of blocking muscarinic cholinergic receptors of the prelimbic (PL) cortex in the acquisition and retention of an odor-reward associative task. Rats underwent a training test (five trials) and a 24-h retention test (two retention trials and two relearning trials). In the first experiment, rats were bilaterally infused with scopolamine (20 or 5 microg/site) prior to training. Although scopolamine rats showed acquisition equivalent to PBS-injected controls, they exhibited weakened performance in the 24-h retention test measured by number of errors. In the second experiment, rats were injected with scopolamine (20 microg/site) immediately or 1 h after training and tested 24 h later. Scopolamine rats injected immediately showed severe amnesia detected in two performance measures (errors and latencies), demonstrating deficits in retention and relearning, whereas those injected 1 h later showed good 24-h test performance, similar to controls. These results suggest that muscarinic transmission in the PL cortex is essential for early memory formation, but not for acquisition, of a rapidly learned odor discrimination task. Findings corroborate the role of acetylcholine in consolidation processes and the participation of muscarinic receptors in olfactory associative tasks.     

Specific auditory memory induced by nucleus basalis stimulation depends on intrinsic acetylcholine

Although the cholinergic system has long been implicated in the formation of memory, there had been no direct demonstration that activation of this system can actually induce specific behavioral memory. We have evaluated the "cholinergic-memory" hypothesis by pairing a tone with stimulation of the nucleus basalis (NB), which provides acetylcholine to the cerebral cortex. We found that such pairing induces behaviorally-validated auditory memory. NB-induced memory has the key features of natural memory: it is associative, highly-specific and rapidly induced. Moreover, the level of NB stimulation controls the amount of detail in memory about the tonal conditioned stimulus. While consistent with the hypothesis that properly-timed release of acetylcholine (ACh) during natural learning is sufficient to induce memory, pharmacological evidence has been lacking. This study asked whether scopolamine, a muscarinic antagonist, impairs or prevents the formation of NB-induced memory. Adult male rats were first tested for responses (disruption of ongoing respiration) to tones (1-15 kHz), constituting a pre-training behavioral frequency generalization gradient (BFGG). Then, they received a single session of 200 trials of a tone (8.00 kHz, 70 dB, 2 s) paired with electrical stimulation of the NB (100 Hz, 0.2 s). Immediately after training, they received either scopolamine (1.0 mg/kg, i.p.) or saline. Twenty-four hours later, they were tested for specific memory by obtaining post-training BFGGs. The saline group developed CS-specific memory, manifested by maximum increase in response specific to the CS frequency band. In contrast, the scopolamine group exhibited no such memory. These findings indicate that NB-induced specific associative behavioral memory requires the action of intrinsic acetylcholine at muscarinic receptors, and supports the hypothesis that natural memory formation engages the nucleus basalis and muscarinic receptors.     

Impaired discrimination learning in mice lacking the NMDA receptor NR2A subunit

N-Methyl-D-aspartate receptors (NMDARs) mediate certain forms of synaptic plasticity and learning. We used a touchscreen system to assess NR2A subunit knockout mice (KO) for (1) pairwise visual discrimination and reversal learning and (2) acquisition and extinction of an instrumental response requiring no pairwise discrimination. NR2A KO mice exhibited significantly retarded discrimination learning. Performance on reversal was impaired in NR2A KO mice during the learning phase of the task; with no evidence of heightened perseverative responses. Acquisition and extinction of an instrumental behavior requiring no pairwise discrimination was normal in NR2A KO mice. The present findings demonstrate a significant and selective deficit in discrimination learning following loss of NR2A.     

Serial reversal learning and the evolution of behavioral flexibility in three species of North American corvids (Gymnorhinus cyanocephalus, Nucifraga columbiana, Aphelocoma californica)

In serial reversal learning, subjects learn to respond differentially to 2 stimuli. When the task is fully acquired, reward contingencies are reversed, requiring the subject to relearn the altered associations. This alternation of acquisition and reversal can be repeated many times, and the ability of a species to adapt to this regimen has been considered as an indication of behavioral flexibility. Serial reversal learning of 2-choice discriminations was contrasted in 3 related species of North American corvids: pinyon jays (Gymnorhinus cyanocephalus), which are highly social; Clark's nutcrackers (Nucifraga columbiana), which are relatively solitary but specialized for spatial memory; and western scrub jays (Aphelocoma californica), which are ecological generalists. Pinyon jays displayed significantly lower error rates than did nutcrackers or scrub jays after reversal of reward contingencies for both spatial and color stimuli. The effect was most apparent in the 1st session following each reversal and did not reflect species differences in the rate of initial discrimination learning. All 3 species improved their performance over successive reversals and showed significant transfer between color and spatial tasks, suggesting a generalized learning strategy. The results are consistent with an evolutionary association between behavioral flexibility and social complexity.     

Blunted hippocampal, but not striatal, acetylcholine efflux parallels learning impairment in diencephalic-lesioned rats

A rodent model of diencephalic amnesia, pyrithiamine-induced thiamine deficiency (PTD), was used to investigate the dynamic role of hippocampal and striatal acetylcholine (ACh) efflux across acquisition of a nonmatching-to-position (NMTP) T-maze task. Changes in ACh efflux were measured in rats at different time points in the acquisition curve of the task (early=day 1, middle=day 5, and late=day 10). Overall, the control group had higher accuracy scores than the PTD group in the latter sessions of NMTP training. During the three microdialysis sampling points, all animals displayed significant increases in ACh efflux in both hippocampus and striatum, while performing the task. However, on day 10, the PTD group showed a significant behavioral impairment that paralleled their blunted hippocampal--but not striatal--ACh efflux during maze training. The results support selective diencephalic-hippocampal dysfunction in the PTD model. This diencephalic-hippocampal interaction appears to be critical for successful episodic and spatial learning/memory.     

The involvement of the orbitofrontal cortex in learning under changing task contingencies

Previous investigations examining the rat prefrontal cortex subregions in attentional-set shifting have commonly employed two-choice discriminations. To better understand how varying levels of difficulty influence the contribution of the prefrontal cortex to learning, the present studies examined the effects of orbitofrontal cortex inactivation in a two- or four-choice odor reversal learning test. Long-Evans rats were trained to dig in cups that contained distinct odors. In the two-choice odor discrimination, one odor cup was always associated with a cereal reinforcement in acquisition while the opposite odor cup was associated with a cereal reinforcement in reversal learning. In the four-choice odor discrimination, an additional two cups containing distinct odors were used that were never associated with reinforcement in acquisition or reversal learning. Bilateral infusions of the GABA-A agonist, muscimol (0.5 microg) into the orbitofrontal cortex did not impair acquisition of either the two- or four-choice discrimination task. However, muscimol infusions into the orbitofrontal cortex impaired two- and four-choice reversal learning. In the two-choice odor reversal, muscimol treatment selectively increased perseverative errors. In the four-choice odor reversal, muscimol treatment increased perseverative, regressive, as well as irrelevant errors. These findings suggest that the orbital prefrontal cortex not only enables task switching by supporting the initial inhibition of a previously relevant choice pattern, but under increasing task demands also enables the reliable execution of a new choice pattern and reduction of interference to irrelevant stimuli.     

Contributions of dorsal striatal subregions to spatial alternation behavior

Considerable evidence has shown a clear dissociation between the dorsomedial (DMS) and the dorsolateral (DLS) striatum in instrumental conditioning. In particular, DMS activity is necessary to form action-outcome associations, whereas the DLS is required for developing habitual behavior. However, few studies have investigated whether a similar dissociation exists in more complex goal-directed learning processes. The present study examined the role of the two structures in such complex learning by analyzing the effects of excitotoxic DMS and DLS lesions during the acquisition and extinction of spatial alternation behavior, in a continuous alternation T-maze task. We demonstrate that DMS and DLS lesions have opposite effects, the former impairing and the latter improving animal performance during learning and extinction. DMS lesions may impair the acquisition of spatial alternation behavior by disrupting the signal necessary to link a goal with a specific spatial sequence. In contrast, DLS lesions may accelerate goal-driven strategies by minimizing the influence of external stimuli on the response, thus increasing the impact of action-reward contingencies. Taken together, these results suggest that DMS- and DLS-mediated learning strategies develop in parallel and compete for the control of the behavioral response early in learning.     

5~6岁自闭症儿童与正常儿童的辨别反转学习研究

采用条件性辨别反转范式,考察自闭症儿童与正常儿童的反转灵活性。44名5~6岁儿童（24名自闭症儿童,20名正常儿童）参加了辨别反转学习。结果显示：⑴在习得阶段,两类儿童都能辨别两个不同的条件性刺激;⑵在晚期反转阶段,正常儿童在晚期反转阶段升高了对新的条件性厌恶刺激的主观预期值,辨别了新的条件性刺激,而自闭症儿童在晚期反转阶段对新的条件性厌恶刺激的主观预期值没有显著性变化,无法辨别新的条件性刺激。研究结果表明,5~6岁自闭症儿童反转灵活性存在缺陷。