Differential effects of M1 muscarinic receptor blockade and nicotinic receptor blockade in the dorsomedial striatum on response reversal learning

Arc mRNA induction in striatal efferent neurons associated with response learning

The dorsal striatum is involved in motor-response learning, but the extent to which distinct populations of striatal efferent neurons are differentially involved in such learning is unknown. Activity-regulated, cytoskeleton-associated (Arc) protein is an effector immediate-early gene implicated in synaptic plasticity. We examined arc mRNA expression in striatopallidal vs. striatonigral efferent neurons in dorsomedial and dorsolateral striatum of rats engaged in reversal learning on a T-maze motor-response task. Male Sprague-Dawley rats learned to turn right or left for 3 days. Half of the rats then underwent reversal training. The remaining rats were yoked to rats undergoing reversal training, such that they ran the same number of trials but ran them as continued-acquisition trials. Brains were removed and processed using double-label fluorescent in situ hybridization for arc and preproenkephalin (PPE) mRNA. In the reversal, but not the continued-acquisition, group there was a significant relation between the overall arc mRNA signal in dorsomedial striatum and the number of trials run, with rats reaching criterion in fewer trials having higher levels of arc mRNA expression. A similar relation was seen between the numbers of PPE(+) and PPE(-) neurons in dorsomedial striatum with cytoplasmic arc mRNA expression. Interestingly, in behaviourally activated animals significantly more PPE(-) neurons had cytoplasmic arc mRNA expression. These data suggest that Arc in both striatonigral and striatopallidal efferent neurons is involved in striatal synaptic plasticity mediating motor-response learning in the T-maze and that there is differential processing of arc mRNA in distinct subpopulations of striatal efferent neurons.

Lesions of the basal forebrain impair reversal learning but not shifting of attentional set in rats

The cholinergic neurons of the basal forebrain, which project to cortex, the thalamic reticular nucleus and the amygdala, are implicated in many aspects of attentional function, while the intrinsic neurons of the basal forebrain are implicated in learning and memory. This study compared the effects of lesions of the basal forebrain made with either the immunotoxin 192-IgG-saporin (which selectively destroys cholinergic neurons), or the non-selective excitotoxin, ibotenic acid (which destroys both cholinergic and non-cholinergic neurons) on a task which measure the acquisition and shifting of attentional set as well as the ability to learn reversals of specific stimulus-reward pairings. Rats learned to obtain food reward by digging in small bowls containing distinctive digging media that were differentially scented with distinct odours. They performed a series of two-choice discriminations, with the bait associated with either the odour or the digging medium. Rats with 192-IgG-saporin lesions of the basal forebrain were not impaired relative to control rats at any stage of the task. Rats with ibotenic acid lesions of the basal forebrain were impaired the first time stimulus-reward contingencies were reversed. They were not impaired in acquisition of new discriminations, even when an attentional-shift was required. These data are consistent with data from marmosets and so highlight the functional similarity of monkey and rodent basal forebrain. They also confirm the likely involvement of non-cholinergic neurons of the basal forebrain in reversal learning.

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.

Gestational methylazoxymethanol acetate treatment impairs select cognitive functions: parallels to schizophrenia

Gestational methylazoxymethanol acetate (MAM) exposure has been suggested to produce neural and behavioral abnormalities similar to those seen in schizophrenia. In order to assess MAM treatment as a model of schizophrenia, pregnant female rats were injected with MAM (22 mg/kg) on gestational day 17 and their offspring were assessed in adulthood on a series of cognitive tasks. The first experiment involved an attentional set-shifting task, a rodent analog of the Wisconsin card sort task. In experiment 2, animals were tested on the 5-choice serial reaction time task, a rodent analog of the continuous performance task. In the final experiment animals were assessed on a differential reinforcement of low rate of responding 20 s schedule of reinforcement (DRL-20), a task that is sensitive to changes in inhibitory control. In the first experiment, MAM-treated animals required a greater number of trials than controls to successfully learn an extradimensional shift on the set-shifting task, and had difficulties in learning to reverse a previously acquired discrimination. In contrast, MAM-treated animals showed little impairment on the 5-choice task, aside from a modest but consistent increase in premature responding. Finally, MAM exposed animals showed substantial impairments in DRL performance. Post-mortem analysis of brain tissue showed significant decreases in tissue weight in the hippocampus, parietal cortex, prefrontal cortex, and dorsal striatum of MAM-treated animals. These results support the notion that MAM treatment may simulate some aspects of schizophrenic cognition.

A convergent model for cognitive dysfunctions in Parkinson's disease: the critical dopamine-acetylcholine synaptic balance

Parkinson's disease is classically characterised as a motor neurodegenerative disorder. Motor symptoms in the disorder are secondary to an altered dopamine-acetylcholine balance due to reduced striatal dopaminergic tone and subsequent cholinergic overactivity. In the past, anticholinergic drugs were given to improve motor aspects of the disease. There is now an increasing interest in the cognitive and non-motor symptoms of Parkinson's disease and in cholinesterase-inhibitor therapy for dementia associated with Parkinson's disease. In this Personal View, we reconsider the dopamine-acetylcholine balance theory and look at recent clinical findings and the possible cooperative role of dopamine and acetylcholine in the induction and maintenance of the long-lasting changes of striatal and cortical synaptic plasticity. We also discuss a convergent versus parallel model to explain cognitive dysfunctions in Parkinson's disease according to dopamine-acetylcholine dependent alterations in synaptic plasticity.

The effect of N-methyl-D-aspartate receptor blockade on acetylcholine efflux in the dorsomedial striatum during response reversal learning

Separate experiments found that activation of N-methyl-d-aspartate (NMDA) receptors or increased acetylcholine (ACh) efflux in the rat dorsomedial striatum is critical for learning when conditions require a shift in strategies. Increasing evidence indicates that NMDA receptor activity affects cholinergic efflux in the basal ganglia. The present studies determined whether NMDA receptor blockade in the dorsomedial striatum with dl-2-amino-5-phosphonopentanoic acid (AP-5) affects dorsomedial striatal ACh output in a resting condition, as well as during response reversal learning. Experiment 1 investigated the effects of AP-5 (12.5, 25 or 50 muM) infused into the dorsomedial striatum on ACh output in a resting condition. AP-5 infusion at 25 and 50 muM led to a 20% and 40% decrease in dorsomedial striatal ACh output, respectively. AP-5 (12.5 muM) infusion did not change dorsomedial striatal ACh output from basal levels. Experiment 2 determined whether dorsomedial striatal ACh efflux increases during response reversal learning and whether AP-5, at a dose that does not affect basal levels, modifies response reversal learning and ACh efflux. Following acquisition of a response discrimination, rats had microdialysis probes bilaterally inserted into the dorsomedial striatum prior to the reversal learning test. After baseline samples, rats received a response reversal learning test for 30 min. Control rats rapidly improved in the reversal learning session while simultaneously exhibiting an approximately 40% increase in ACh output compared with baseline levels. AP-5 (12.5 muM) treatment during testing significantly impaired response reversal learning while concomitantly blocking an increase in ACh output. These findings suggest that NMDA receptor activation in the dorsomedial striatum may facilitate a shift in response patterns, in part, by increasing ACh efflux.

Acetylcholine release in the hippocampus and striatum during place and response training

These experiments examined the release of acetylcholine in the hippocampus and striatum when rats were trained, within single sessions, on place or response versions of food-rewarded mazes. Microdialysis samples of extra-cellular fluid were collected from the hippocampus and striatum at 5-min increments before, during, and after training. These samples were later analyzed for ACh content using HPLC methods. In Experiment 1, ACh release in both the hippocampus and striatum increased during training on both the place and response tasks. The magnitude of increase of training-related ACh release in the striatum was greater in rats trained on the response task than in rats trained on the place task, while the magnitude of ACh release in the hippocampus was comparable in the two tasks. Experiment 2 tested the possibility that the hippocampus was engaged and participated in learning the response task, as well as the place task, because of the availability of extra-maze cues. Rats were trained on a response version of a maze under either cue-rich or cue-poor conditions. The findings indicate that ACh release in the hippocampus increased similarly under both cue conditions, but declined during training on the cue-poor condition, when spatial processing by the hippocampus would not be suitable for solving the maze. In addition, high baseline levels of ACh release in the hippocampus predicted rapid learning in the cue-rich condition and slow learning in the cue-poor condition. These findings suggest that ACh release in the hippocampus augments response learning when extra-maze cues can be used to solve the maze but impairs response learning when extra-maze cues are not available for use in solving the maze.

Lesions of the dorsolateral striatum impair the acquisition of a simplified stimulus-response dependent conditional discrimination task

The dorsal striatum has long been thought to be important for some types of learning and memory, especially stimulus-response learning. Recently, we demonstrated that selective lesions of the dorsolateral striatum, but not dorsomedial striatum in rats, retarded the acquisition of two instrumental discrimination tasks thought to require stimulus-response learning. However, since these studies investigated the effects of dorsal striatal lesions on task acquisition, which can be confounded by differences in level of reinforcement and motor impairment caused by the lesion, the interpretation of these findings was somewhat problematic. The present experiment was designed to address these issues by assessing the effects of lesions of the dorsolateral striatum on a simplified version of the conditional discrimination task, in which the importance of reinforcement and motor factors was minimized. Animals with lesions of the dorsolateral striatum showed marked impairments in learning this task, a finding that is in agreement with the notion that the dorsolateral striatum is necessary for stimulus-response learning.

Acetylcholine actions in the dorsomedial striatum support the flexible shifting of response patterns

There is accumulating evidence that the dorsomedial striatum plays a significant role in the learning of a new response pattern and the inhibiting of old response patterns when conditions demand a shift in strategies. This paper proposes that activity of cholinergic neurons in the dorsomedial striatum is critical for enabling behavioral flexibility when there is a change in task contingencies. Recent experimental findings are provided supporting this idea. Measuring acetylcholine efflux from the dorsomedial striatum during the acquisition and reversal learning of a spatial discrimination shows that acetylcholine efflux selectively increases during reversal learning as a rat begins to learn a newly reinforced spatial location, but returns to near basal levels when a rat reliably executes the new choice pattern. Experimental findings are also described indicating that the blockade of muscarinic cholinergic receptors in the dorsomedial striatum does not impair acquisition of an egocentric response discrimination, but impairs reversal learning of an egocentric response discrimination. Based on these results, increased cholinergic activity at muscarinic receptors is part of a neurochemical process in the dorsomedial striatum that allows inhibition of a previously relevant response pattern while learning a new response pattern. In situations that demand behavioral flexibility, muscarinic cholinergic activity in the dorsomedial striatum may directly influence corticostriatal plasticity to produce changes in response patterns.