Is cholinergic activity of the caudate nucleus involved in memory?

Some highlights of research on the effects of caudate nucleus lesions over the past 200 years

This review describes experiments on the effects of caudate nucleus lesions on behavior in monkeys, cats and rats. Early work on monkeys and cats focused on the relationship of the caudate to the cortex in motor control, leading to the idea that the caudate serves to inhibit behaviors initiated by the cortex. However, investigation of this hypothesis with systematic behavioral testing in all three species did not support this idea; rather, these studies provided evidence that caudate lesions affect memory functions. Two main types of memory tasks were affected. One type involved reinforced stimulus-response (S-R) associations, the other involved spatial information, response-reinforcer contingencies, or working memory. Recent evidence, mainly from rats, suggests that the dorsolateral part of the caudoputamen is central to the processing and consolidation of memory for reinforced S-R associations, and that the more medial and anterior parts of the same structure are part of a neural circuit that (in some cases) also includes the hippocampus, and mediates relational information and certain forms of working memory. The possibility that the spatial distribution of the patch and matrix compartments within the caudoputamen underlies these regional differences is discussed.

Cholinergic activity in the insular cortex is necessary for acquisition and consolidation of contextual memory

Experiences with a high emotional content (aversive) tend to be stored as long-term memories; however, there are also contextual recollections, which form a significant part of our memories. Different research has shown that the insular cortex (IC) plays an important role during aversive memory formation, yet its role during incidental/non-aversive learning like pre-exposure contextual memory formation has received little attention. The objective of this research was to establish the role of cholinergic activity in the IC through its muscarinic receptors during the formation of inhibitory avoidance (IA) memory, as well as during pre-exposure contextual memory, using a paradigm such as latent inhibition (LI). Rats with bilateral cannulae directed into the IC were trained in the LI paradigm of IA or IA task alone. The muscarinic antagonist receptor scopolamine was infused bilaterally into the IC 5 min before the pre-exposure into the dark chamber of the IA cage, one day before the conventional IA training or during the IA training day. During the IA test, the entrance latency into the dark chamber of the IA cage was measured as an index of contextual memory. The results showed that scopolamine infused before and after IA training disrupts inhibitory avoidance memory. Also, it showed that the pre-exposed saline-infused animals (LI) had a lower entrance latency compared to the group not pre-exposed (IA). However, the group that received scopolamine into the IC before, but not after, the pre-exposure to the dark chamber, presented a similar latency to the IA group, showing a blockade of the latent inhibition of the IA. These results suggest that cholinergic activity in the insular cortex is necessary during the acquisition and consolidation of avoidance memory, but appears necessary only during the acquisition of pre-exposure non-aversive contextual memory.

Pharmacological characterization of high-fat feeding induced by opioid stimulation of the ventral striatum

Nucleus accumbens mu-opioid stimulation causes marked increases in the intake of highly palatable foods, such as a high-fat diet. However, to date there has been little examination of how other striatal neurotransmitters may mediate opioid-driven feeding of palatable foodstuffs. In the current study, free feeding rats with bilateral cannulae aimed at the nucleus accumbens received intra-accumbens pretreatment with antagonists for dopamine D-1 (SCH23390; 0 microg or 1 microg/0.5 microl/side), dopamine D-2 (raclopride; 0 microg or 2.0 microg/0.5 microl/side), AMPA (LY293558; 0 microg, 0.01 microg or 0.10 microg/0.5 microl/side), muscarinic (scopolamine 0 microg, 0.1, 1.0, or 10 microg/0.5 microl/side) or nicotinic (mecamylamine; 0 microg, 10 microg/0.5 microl/side) receptors, immediately prior to infusions of the mu-receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO; 0.25 microg/0.5 microl) or vehicle. The effects of these pretreatments on 2 hr fat intake was compared to pretreatment with a general opioid antagonist (naltrexone; 0 microg or 20 microg/0.5 microl/side). DAMGO-induced feeding was unaffected by prior antagonism of dopamine, glutamate, or nicotinic receptors. As expected, naltrexone infusions blocked DAMGO-elicited fat intake. Antagonism of muscarinic acetylcholine receptors reduced feeding in both the DAMGO and vehicle-treated conditions. In an additional experiment, cholinergic receptor stimulation alone did not affect intake of the fat diet, suggesting that nucleus accumbens cholinergic stimulation is insufficient to alter feeding of a highly palatable food. These data suggest that the feeding effects caused by striatal opioid stimulation are independent from or downstream to the actions of dopamine and glutamate signaling, and provide novel insight into the role of striatal acetylcholine on feeding behaviors.

Striatal muscarinic receptor antagonism reduces 24-h food intake in association with decreased preproenkephalin gene expression

Cholinergic interneurons of the striatum respond to motivationally relevant stimuli and are involved in appetitive learning. However, there has been relatively little inquiry into the role of striatal acetylcholine in food motivation. Here we show in rats that a single infusion of the muscarinic receptor antagonist scopolamine (0, 5.0 or 10.0 microg/0.5 microL bilaterally) potently reduced 24-h food intake following injections into either the ventral or dorsal striatum, without affecting water intake. Furthermore, muscarinic receptor blockade induced reliable and widespread reductions in striatal preproenkephalin, but not preprodynorphin, mRNA expression. These data suggest a novel role for striatal acetylcholine in modulating feeding behavior via its effects on enkephalin gene expression. As prior research indicates a critical role for striatal enkephalin in consummatory behaviors and palatability, we hypothesize that cholinergic interneurons assist in translating hypothalamic energy state signals into food-directed behaviors via their regulation of striatal opioid peptides.

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.

Nucleus Accumbens Acetylcholine Regulates Appetitive Learning and Motivation for Food via Activation of Muscarinic Receptors

These experiments tested whether nucleus accumbens muscarinic or nicotinic acetylcholine receptor activation is required for rats to learn to lever press for sucrose. Muscarinic blockade with scopolamine (1.0 microg/side or 10.0 microg/side), but not nicotinic antagonism with mecamylamine (10.0 microg/side), inhibited learning and performance when applied to the core or shell. Further experiments showed that acute accumbens scopolamine treatment increased locomotor activity and reduced sucrose consumption. However, microanalyses of behavioral events in the instrumental chamber revealed that reductions of lever press performance during muscarinic blockade were not due to gross motor dysfunction. Accumbens core scopolamine was subsequently shown to reduce the amount of work rats would expend under a progressive ratio paradigm. These novel results implicate nucleus accumbens muscarinic receptors in the modulation of appetitive learning, performance, and motivation for food.

Elevated T-maze as an animal model of memory: effects of scopolamine

The elevated T-maze (ETM) is a putative model for the assessment of anxiety and memory in rodents. This study was designed to further evaluate the utility of the ETM in the study of memory processes. We compared the performance of rats in the ETM, the water maze (WM) and the two-way avoidance task (TWA), after pretreatment with scopolamine (SCO; 0.3 or 1.2 mg/kg i.p.). In the ETM, rats were first trained to meet the criterion of remaining inside the enclosed arm for 300 seconds. Seventy-two hours after training, a retrieval test session was performed. At the lower dose, SCO impaired performance in the retrieval session on all three tasks, whereas in the training session an effect was noted only on the WM task. At the higher dose, SCO impaired the performance of rats in the training sessions for ETM and WM, but not TWA. In a fourth experiment using the elevated plus-maze, SCO showed anxiolytic-like effects at the higher dose only. In conclusion, the effects of SCO in rats submitted to the ETM were dose dependent, with the lower dose exerting a selective effect detected only on retrieval, whereas the higher dose induced motor effects that disrupted inhibitory avoidance acquisition, resulting in impaired retrieval. The results are discussed in terms of the utility of the ETM in the study of drug effects and the neurobiological mechanisms underlying anxiety, learning and memory.

Dopamine-dependent synaptic plasticity in the striatal cholinergic interneurons

The striatum, the input stage of the basal ganglia, is a critical brain structure for the learning of stimulus-response habits as well as motor, perceptual, and cognitive skills. Roles of dopamine (DA) and acetylcholine (ACh) in this form of implicit memory have long been considered essential, but the underlying cellular mechanism is still unclear. By means of patch-clamp recordings from corticostriatal slices of the mouse, we studied whether the identified striatal cholinergic interneurons undergo long-term synaptic changes after tetanic stimulation of cortico- and thalamostriatal fibers. Electrical stimulation of the fibers revealed a depolarizing and hyperpolarizing postsynaptic potential in the striatal cholinergic interneurons. The early depolarizing phase was considered to be a cortico/thalamostriatal glutamatergic EPSP, and the hyperpolarizing component was considered to be an intrastriatally evoked GABAergic IPSP. Tetanic stimulation of cortico/thalamostriatal fibers was found to induce simultaneously occurring long-term potentiation (LTP) of the EPSPs as well as the disynaptically mediated IPSPs. The induction of LTP of EPSP required a rise in intracellular Ca(2+) concentration and dopamine D(5), but not D(2) receptor activation. Ca(2+)-permeable AMPA receptors might also play a part in the LTP induction. Blockade of NMDA receptors, metabotropic glutamate receptors, or serotonin receptors had no significant effects. The long-term enhancement of the disynaptic IPSPs was caused by a long-term increase in the occurrence rate but not the amplitude of disynaptically mediated IPSP in the striatal cholinergic interneurons. This dual mechanism of synaptic plasticity may be responsible for the long-term modulation of the cortico/thalamostriatal synaptic transmission.

Dose- and time-dependent scopolamine-induced recovery of an inhibitory avoidance response after its extinction in rats

The present investigation was aimed at elucidating the dose and time dependency of scopolamine-induced recovery of inhibitory avoidance after its extinction. Two experiments were conducted: in the first, we analyzed the effects of four doses (1, 2, 4, and 8 mg/kg) of the musacrinic receptor antagonist scopolamine, on the expression of this conditioned response once it had been extinguished. Independent groups of rats were trained in a one-trial, step-through inhibitory avoidance task and submitted to daily retention (extinction) tests. After extinction had occurred, animals were injected intraperitoneally 10 min before retention testing, either with saline or scopolamine. Results show that scopolamine produced a dose-dependent recovery of the avoidance response. The second experiment was carried out in the same animals, which were now tested for retention of inhibitory avoidance at 1, 2, 3, 6, and 9 months after completion of the first experiment. All rats received counterbalanced injections of saline or scopolamine 10 min before testing at each time interval. Reliable recovery of the avoidance response was observed at the 1-month interval with a clear dose dependency while, after the second month, only the groups treated with the two higher doses continued responding. The results indicate that recovery of the extinguished response produced by muscarinic blockade follows dose- and time-dependent curves, and can be achieved long after a single training session. These data suggest that the inhibitory avoidance memory trace is retained in the brain after behavioural extinction of this response, thus supporting the view of extinction as new learning that affects the retrieval of the original memory, but does not modify its storage.

Involvement of the parafascicular nucleus of the thalamus and the cholinoreactive system of the neostriatum in controlling a food-procuring reflex in rats at different stages of learning

A model of a Skinner box food-procuring reflex in rats was used to study the relationship between the strength applied to a pedal and disruption of the parafascicular nucleus of the thalamus and microinjections of the cholinolytic scopolamine and the cholinomimetic carbachol into the neostriatum at different stages of learning. In untrained rats at the stage of learning to press strongly on the pedal without the conditioned signal being switched on (i.e., every strong press was rewarded) showed (a) a decrease in the rate of learning to press strongly and an increase in the number of weak pedal presses after bilateral lesioning of the parafascicular nucleus of the thalamus; (b) that rats with bilateral lesions of this nucleus responded to microinjections of scopolamine into the neostriatum with increases in the number of strong presses, with no change in the number of weak pedal presses, while microinjections of carbachol decreased the number of strong and increased the number of weak presses as compared with the pre-microinjection baseline. In trained rats at the stage of recovery the reflex (i.e., strong pedal presses were reinforced only during exposure to the conditioned signal), lesioning of the parafascicular nucleus of the thalamus had the effect that the time required for recovery of the reflex became dependent on the level of pre-operative training; scopolamine microinjections into the neostriatum of rats achieving high levels of correct performances of the reflex only after surgery led to sharp degradation in performance of the reflex on the day of microinjections; microinjection of carbachol into the neostriatum of rats with low post-operative levels of performance of the reflex had no effect on this measure.

D2 dopamine receptor blockade immediately post-training enhances retention in hidden and visible platform versions of the water maze

Considerable evidence shows that post-training administration of dopamine agonists can enhance memory through actions on consolidation processes, but relatively little is known regarding the effects of dopamine antagonists on consolidation. These experiments investigated the effects of post-training systemic administration of the D2 receptor antagonist sulpiride on consolidation of memory for two versions of the Morris water maze task. Rats trained in either the hidden (spatial) or visible (cued) platform version received a subcutaneous injection of sulpiride or vehicle immediately following training. Retention testing 48 hr later revealed that relative to vehicle controls, sulpiride reduced platform latencies in both task versions, suggesting that like dopamine agonists, sulpiride can also have memory-enhancing effects.

Intuition: a social cognitive neuroscience approach

This review proposes that implicit learning processes are the cognitive substrate of social intuition. This hypothesis is supported by (a) the conceptual correspondence between implicit learning and social intuition (nonverbal communication) and (b) a review of relevant neuropsychological (Huntington's and Parkinson's disease), neuroimaging, neurophysiological, and neuroanatomical data. It is concluded that the caudate and putamen, in the basal ganglia, are central components of both intuition and implicit learning, supporting the proposed relationship. Parallel, but distinct, processes of judgment and action are demonstrated at each of the social, cognitive, and neural levels of analysis. Additionally, explicit attempts to learn a sequence can interfere with implicit learning. The possible relevance of the computations of the basal ganglia to emotional appraisal, automatic evaluation, script processing, and decision making are discussed.

Effects of microinjection of scopolamine into the neostriatum of rats on performance of a food conditioned reflex at different levels of fixation

Chronic experiments performed on 32 Sprague-Dawley rats using a movement-feeding operant reflex (Skinner box) model showed that microinjection of scopolamine into the neostriatum had effects on this reflex which depended on the stage of learning. In animals with weakly fixed reflexes (prior to reaching the stage of memory consolidation), bilateral microinjection of 0.3 microgram of scopolamine into the caudate nucleus completely inhibited the reflex for a prolonged period of time. When the operant habit was well fixed, bilateral microinjection of the same doses of scopolamine into the neostriatum had no effect on the reflex. These results suggest that the neostriatum cholinergic system is critically involved in forming the motor engram. The cholinergic system of the caudate nucleus either takes no part in realizing the well-fixed conditioned reflex movement response and/or other forebrain structures are involved in the reflex, compensating for the disturbance in neostriatal cholinergic function.

Mnemonic functions of the basal ganglia

A synthesis of older and recent work on mnemonic functions of the basal ganglia in rats, monkeys and humans emphasizes a reciprocal relationship of the caudate nucleus and putamen with the cerebral cortex, which mediates the memory of consistent relationships between stimuli and responses (sometimes called habits) that often involve relationships between the individual and its environment (egocentric memory). Evidence at several levels of analysis (including neuroplastic synaptic changes, activity of single neurons, and behavioral changes caused by lesions or neurochemical manipulations) implicate dopamine release from nigro-striatal neurons in the reinforcement, or strengthening, of neural representations in the basal ganglia.

Modulation of long-term memory and extinction responses induced by growth hormone (GH) and growth hormone releasing hormone (GHRH) in rats

The purpose of this work is to study the participation of growth hormone (GH) and growth hormone releasing hormone (GHRH) in the modulation of long-term memory and the extinction response of a passive avoidance task in rats. However, the effect on memory vary according to the age of the animals due to plasma levels of either hormone being modified during the aging process. Male Wistar rats were divided according to age into two experimental blocks (young rats 3 months old and aged rats 24 months old at the start of the experiment) where each block received the same treatment. Each experimental block was then divided randomly into three groups where two were experimental and the other served as control. The animals were then submitted to a one-trial passive avoidance conditioning and tested for memory retention 24 hrs after as well as twice a week until the extinction response occurred. The control group received an isotonic saline solution and the other two groups received 0.8 U.I. of GH or 4 mcg of GHRH respectively. All substances were in a 0.08 ml volume and applied 24 hrs before training as well as 24 hrs before each retention session. The results indicate that GH and GHRH modulate long-term memory as well as the extinction response and in either case the response seems to vary with age. GH and GHRH facilitates long-term memory in young rats but not in aged rats. Finally, whereas GH delays the extinction response in both groups, GHRH retards the extinction only in aged rats.

Role of the striatal cholinergic system in the regulation of learned manipulation in rats

The experiments were performed on adult Wistar male rats trained to push with the forepaw on a fixed piston inside a narrow tube. It was found that after localized intracerebral injection of a cholinergic antagonist into the dorso-lateral (but not medial) neostriatum (i.e., the caudato-putamen) the behavioral performance requiring brief innate movements remained unchanged, but the performance requiring a prolonged pushing movement (> 50 msec) became disrupted. Micoinjection of carbacholine (0.03-3 mu g/l microliters) did not affect the performance of the acquired movements, whereas scopolamine (3 mu g/l microliters) led to the significant decrease in pushing time. We conclude that changes in the state of the dorso-lateral neostriatal cholinergic system result only in disturbances of the sensory-controlled component of a complex instrumental movement.

Modulation of short term and long term memory by steroid sexual hormones

Many studies have evidenced a functional interrelation between the nervous and endocrine systems in the modulation of mnemonic processes, and others have established the role played by certain hormones in these processes; however, few studies have dealt with the effects of sexual steroids on learning and memory. The aim of this work was to determine whether short-term and long-term memory is subject to hormonal modulation. Male Wistar rats, randomly assigned to 13 groups, 1 control and 12 experimental groups, were trained on a one-trial passive avoidance conditioning. The control group received saline solution, and each of the 12 experimental groups received a treatment consisted in one of following pharmacological doses of: 5, 10, 20, 30 mg of testosterone enanthate, 0.2, 0.4, 0.8, 1.2 mg estradiol valeriate, or 1, 2, 4, 6 mg norandrostenolone decanoate, respectively. All substances were applied 45 min before the training session. Retention of the learned response was tested 10 min (short-term memory) and 24 h (long-term memory) after the training session. Results indicate that under these experimental conditions the short-term memory is facilitated with testosterone enanthate at doses of 20 and 30 mg, the estradiol valeriate at doses of 0.4 mg and the nor-androstenolone decanoate at doses of 4 mg, whereas the long-term memory is facilitated with testosterone enanthate at doses of 30 mg, estradiol valeriate at doses of 1.2 mg and the nor-androstenolone decanoate at doses of 4 mg. The other studied doses were no effective.

Effects of central muscarinic blockade on passive avoidance: anterograde amnesia, state dependency, or both?

It was recently reported that administration of relatively high intensities of footshock (overreinforcement) during training of passive avoidance protected animals against the amnesic effect of scopolamine, injected 5 min after training. This was interpreted in terms of a lesser involvement of acetylcholine in memory consolidation. An alternative explanation was that overreinforcement accelerated the consolidation process, which could have taken place before the injection of scopolamine. To test for this possibility, male Wistar rats were injected with 4, 8, or 12 mg/kg of scopolamine, 5 min before training with low or high levels of footshock and then tested for retention of the task. Scopolamine induced the expected memory deficit after the low-intensity footshock; after overreinforcement the higher doses of scopolamine induced state dependency, while no deficits were produced with the lower dose. It was concluded that: (a) acetylcholine is indeed involved in memory consolidation of passive avoidance; (b) scopolamine interacts with high footshock levels to produce state dependency; and (c) when relatively low doses of scopolamine are used in conditions of overreinforcement, protection against scopolamine-induced amnesia becomes evident.

Modeling the integrative properties of dendrites: application to the striatal spiny neuron

The role of the striatum in the control of movements and in the processing of cortical information has received much attention in the recent years. We set out a simple biophysical model for the medium-spiny neuron (msn), the most abundant cell in striatum. This neuron receives two main kinds of inputs, namely, cortical excitatory inputs and dopaminergic inputs coming from the substantia nigra pars compacta. The msn axon impinges directly onto the globus pallidus and onto the substantia nigra pars reticulata neurons and onto striatal neurons through recurrent branches of the axon. The msn is characterized by spiny dendritic trees with a high density of spines (1 to 4 spines/microns) and the probable existence of dendritic spikes. The model predicts that the neuron can integrate excitable inputs in a linear or a nonlinear mode. In the nonlinear mode, the neuron allows the detection of simultaneous (or almost simultaneous) synaptic inputs; it facilitates either a slowing down or a speeding up of the information transfer between the synaptic input location and the soma and is sensitive to inhibiton-excitation pairing. Conversely, in the linear integrative mode, the somatic voltage is determined by a weighted summation of the synaptic inputs. Several geometrical, electrical, or temporal factors can control the switch between these behaviors: the density of excitable dendritic elements, the dendritic radius, the resistance of the spine stem, the membrane resistance, the time between excitations, and the distance between synaptic sites. Finally, the signification of this behavior is discussed in connection with the putative role of dopamine and with the striatal net organization.

Differential recovery of inhibitory avoidance learning by striatal, cortical, and mesencephalic fetal grafts

Four groups of male Wistar rats showing disrupted inhibitory avoidance conditioning due to striatal lesions were studied. Three groups received striatal, cortical, or ventral mesencephalic brain grafts and the fourth group remained as a lesioned control. Sixty days postgraft the animals were retrained in an inhibitory avoidance task. The striatal-grafted animals were the only group that significantly improved in the ability to acquire the inhibitory avoidance task. Acetylcholinesterase histochemistry revealed positive patches of cells in the striatal grafts. Cortical grafts showed less reactivity, without patches. Immunocytochemical analyses for tyrosine hydroxylase revealed positive cell reactivity in the mesencephalic grafts and few positive fibers were detected in the border between the striatal grafts and the host tissue. These results demonstrate that striatal but not cortical or mesencephalic brain grafts can promote the restoration of the ability to acquire an inhibitory avoidance task and suggest that the acetylcholine tissue content is involved in the behavioral recovery.

The participation of cholinergic systems of the nucleus accumbens in the differentiation of acoustic signals in dogs

The influence of microinjections of the choline agonist, (carbacholine 0.05-0.1 microgram), into the nucleus accumbens (Acc) of the right and left hemispheres on the realization of an instrumental defense reflex associated with the maintenance of a specific posture, and on the differentiation of acoustic signals in a defense situation, was studied in chronic experiments in five dogs. It was demonstrated that the injection of fairly small doses of carbacholine (CbCh) into the Acc of the left and right hemispheres improves the differentiation of acoustic signals in the defense situation. The introduction of fairly small doses of CbCh also improves the values of the criteria of the execution of the instrumental defense reaction associated with the maintenance of a specific posture. The changes observed are prolonged in character. Analysis of the motoric components of the instrumental responses made it possible to evaluate objectively the contribution of the motor and sensory mechanisms to those changes which were elicited by the microinjections of CbCh into the Acc, and to reach conclusion regarding an important role of the cholinergic system of the nucleus accumbens in sensory processes associated with the initiation and realization of instrumental responses to defense and differential signals, and especially in the regulation of attention to significant stimuli.

Effects of GABA antagonists on inhibitory avoidance

Experimental data indicate that GABA is involved in memory processes. However there are marked inconsistencies in the reported effects of interference with GABA synaptic activity on memory consolidation of aversively-motivated tasks. Both amnesia and improvement of performance have been reported after treatment with GABA antagonists. These contradictory effects could be explained by procedural differences in training. To test for this possibility rats were trained in passive avoidance using two levels of footshock and injected with a wide range of doses of picrotoxin and bicuculline. Picrotoxin did not modify the conditioned response while bicuculline induced amnesia only with the lower doses at both low and high footshock intensities. It was concluded that GABA is involved in memory consolidation, and that the conflicting results in the literature are indeed due, in part, to procedural differences, and also to the mode of action of these drugs.

Intrastriatal injection of choline accelerates the acquisition of positively rewarded behaviors

The prediction was made that by increasing the synthesis of striatal acetylcholine, through local injection of its precursor choline, the acquisition of a lever-pressing response in two different autoshaping situations would be accelerated. In the first experiment, choline was injected into the striatum or parietal cortex of rats immediately after dipper training; 24 h later and during 5 consecutive days the animals were submitted to an autoshaping procedure of the operant kind. In the second experiment, choline was administered to the same regions shortly after each of three classical-operant autoshaping sessions; during the next two sessions, autoshaping contingencies of the operant kind were in effect. In both experiments choline injection into the striatum induced a marked facilitation of acquisition of the conditioned responses, although cortical injection of choline produced a milder improvement only in the first experiment. These results indicate that striatal cholinergic activity is, indeed, involved in the early phases of positively reinforced learning.

A threshold for the protective effect of over-reinforced passive avoidance against scopolamine-induced amnesia

Acetylcholine-receptor blockers produce amnesia of aversively motivated behaviors. However, when animals are submitted to relatively high intensities of footshock (over-reinforcement), anticholinergic treatment does not induce memory impairments. The aim of this work was to determine whether the antiamnesic effect produced by increasing the magnitude of the negative reinforcer is gradually established or if a threshold should be reached to obtain such an effect. Wistar rats were trained in passive avoidance using 2.5, 2.6, 2.7, 2.8, 2.9 or 3.0 mA; 5 min after training they were given one systemic injection of scopolamine (8 mg/kg). An amnesic state was produced in the groups that were trained with the lower intensities (2.5-2.7 mA); with the three higher intensities near-perfect retention was evident. These results suggest that acetylcholine is critically involved in memory consolidation, and that by increasing the magnitude of the negative reinforcer, a threshold is reached where cholinergic activity of the nervous system is not necessary for the development of the consolidation process.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

The role of interactions between the cholinergic system and other neuromodulatory systems in learning and memory

Extensive evidence indicates that disruption of cholinergic function is characteristic of aging and Alzheimer's disease (AD), and experimental manipulation of the cholinergic system in laboratory animals suggests age-related cholinergic dysfunction may play an important role in cognitive deterioration associated with aging and AD. Recent research, however, suggests that cholinergic dysfunction does not provide a complete account of age-related cognitive deficits and that age-related changes in cholinergic function typically occur within the context of changes in several other neuromodulatory systems. Evidence reviewed in this paper suggests that interactions between the cholinergic system and several of these neurotransmitters and neuromodulators--including norepinephrine, dopamine, serotonin, GABA, opioid peptides, galanin, substance P, and angiotensin II--may be important in learning and memory. Thus, it is important to consider not only the independent contributions of age-related changes in neuromodulatory systems to cognitive decline, but also the contribution of interactions between these systems to the learning and memory deficits associated with aging and AD.

Is acetylcholine involved in memory consolidation of over-reinforced learning?

Systemic administration of anticholinergic drugs produces amnesia. To determine whether this effect can be prevented by increasing the magnitude of the learning experience, independent groups of rats were trained in passive avoidance, using a 3.0-mA footshock, and then injected with scopolamine (2, 4, 6, 8 or 12 mg/kg). When retention of the task was evaluated, a dose-dependent amnesic effect was found. When footshock intensity was increased to 6.0 and 9.0 mA, injections of 8 and 12 mg/kg of scopolamine did not produce memory impairments. These findings indicate that acetylcholine plays an important role in consolidation of passive avoidance, but it does not seem to be involved in memory processes when the magnitude of the negative reinforcer is increased.

Retrograde amnesia produced by intrastriatal atropine and its reversal by choline

A number of studies have shown that cholinergic blockade of the striatum produces amnesia. In the present experiment it was predicted that by increasing the synthesis of striatal acetylcholine such amnesic state would be prevented. Atropine was injected into the striatum of rats before training of passive avoidance; some of these rats were also injected, intrastriatally, with choline before testing the retention of the task. Atropine alone produced amnesia while the combination of treatments reversed this effect.

Effects of chronic lithium administration on brain weights, acetylcholinesterase activity and learning ability in rats

In order to test the effects of chronic lithium (Li) administration on learning and memory, 21 day old rats were subjected to different degrees of environmental stimulation (enriched condition, EC and impoverished condition, IC) with and without Li for 144 days. Li was administered with food (2.18 mEq/Kg weight/day). Average plasma Li concentration at the end of the experiment was 0.41 +/- 0.04 mu Eq/ml. Both Li treatment and the environmental condition showed an overall significant effect on the cortex/subcortex weight ratio and learning ability index, but not on AChE activity in occipital cortex. A similar pattern of brain Li distribution was observed in both EC-Li and IC-Li, with occipital cortex having the highest levels. Li tissue/protein/plasma ratio was higher in EC than in IC, in all the brain areas studied. Other organs (liver and kidney) did not show EC-IC differences in the tissue/protein/plasma Li ratio.

Effects of Hoe 065, a compound structurally related to inhibitors of angiotensin converting enzyme, on acetylcholine metabolism in rat brain

Hoe 065, a compound structurally related to inhibitors of angiotensin converting enzyme, caused a fall in the content of acetylcholine (ACh) in different brain areas of the rat following i.p. administration in the range 0.03-30 mg/kg. This effect occurred 0.5 h after a single injection and lasted for at least 6 h. Simultaneous administration of the choline uptake inhibitor hemicholinium-3 (HC-3) with Hoe 065 potentiated the decrease in ACh content induced by HC-3. In the same dose range Hoe 065 acutely enhanced the activity of the enzyme choline acetyltransferase as well as the capacity of the high-affinity choline uptake system which is considered as the rate-limiting step in the synthesis of ACh. Cholinesterase activity in vivo was not altered by the compound. Hoe 065 produced a concurrent elevation of brain cyclic GMP content. Taken together, these results suggest that Hoe 065 acutely increases cholinergic activity within its physiological range, probably by means of an enhanced release of ACh.

Retrograde amnesia induced by lidocaine injection into the striatum: protective effect of the negative reinforcer

A variety of manipulations which interfere with the activity of the striatum, including cholinergic blockade and spreading depression, produce amnesia. However, it has been demonstrated that with overtraining, striatal spreading depression and injections of anticholinergic drugs do not produce memory deficits in positively-rewarded tasks. In the present experiment 2% lidocaine was injected into the striatum shortly after training of passive avoidance, using three levels of footshock (0.2, 0.3, and 0.4 mA). Highly significant retention deficits were produced when the lower intensities were studied; in contrast, the animals trained with 0.4 mA showed near-perfect performance. The data show that the enhanced learning experience, which may be equivalent to overtraining, also protects against memory deficits in negatively-rewarded behaviors, and suggest that it induces a transfer of mnemonic functions from the striatum to other neural structures.

A functional hypothesis concerning the striatal matrix and patches: mediation of S-R memory and reward

Recent neurochemical and anatomical findings delineating two compartments, the patches and the matrix, in the mammalian striatum, are described. Anatomical and neurochemical aspects of manipulations affecting behaviors related to S-R memory and reward correspond in certain respects to the features of these striatal compartments. These coincidences lead to an hypothesis concerning the function of the striatal compartments and their participation in neural systems controlling both immediate and learned behaviors.

Effect of nigrostriatal dopamine depletion on the post-training, memory-improving action of amphetamine

Systemic, post-training injections of d-amphetamine improve memory in a variety of learning situations. Evidence from self-stimulation and intra-striatal injection studies suggests that this effect may be a result of the amphetamine-promoted release of endogenous dopamine from nigro-striatal neurons. In the present study this hypothesis was supported by the demonstration that destruction of these neurons with intra-nigral injections of 6-hydroxydopamine eliminate the memory-improving effect of systemic, post-training injections of amphetamine.

Potentiation of odor by taste and odor aversions in rats are regulated by cholinergic activity of dorsal hippocampus

Limbic cholinergic activity is critically involved in the retention of learned aversions tasks. The purpose of these experiments was to assess the role of cholinergic mechanisms of the dorsal hippocampus in the acquisition of both odor and potentiated odor aversions through taste aversion. Cholinergic activity was increased by physostigmine (Phys). When Phys was applied before the presentation of an odor-taste compound during acquisition, the potentiation of odor-aversion was disrupted, while taste aversion was left intact. When hippocampal cholinergic activity was reduced with the muscarinic antagonist scopolamine (Scop), enhancement of potentiated odor aversion was observed, again with no effect on taste aversion. Moreover, when Phys was applied before an odor alone it also disrupted odor avoidance in two different odor tests conditioning situations, i.e., odor was followed immediately by lithium chloride or foot shock. Neither Scop nor Phys had any effect on taste or potential odor aversions when applied to fronto-parietal cortex. These results suggest that cholinergic activity of the hippocampus is involved in the acquisition of odor aversion conditioning.