Retrograde amnesia and cholinergic systems in the caudate-putamen complex and dorsal hippocampus of the rat

Glucocorticoid-cholinergic interactions in the dorsal striatum in memory consolidation of inhibitory avoidance training

Extensive evidence indicates that glucocorticoid hormones act in a variety of brain regions to enhance the consolidation of memory of emotionally motivated training experiences. We previously reported that corticosterone, the major glucocorticoid in the rat, administered into the dorsal striatum immediately after inhibitory avoidance training dose-dependently enhances memory consolidation of this training. There is also abundant evidence that the intrinsic cholinergic system of the dorsal striatum is importantly involved in memory consolidation of inhibitory avoidance training. However, it is presently unknown whether these two neuromodulatory systems interact within the dorsal striatum in the formation of long-term memory. To address this issue, we first investigated in male Wistar rats whether the muscarinic receptor agonist oxotremorine administered into the dorsal striatum immediately after inhibitory avoidance training enhances 48 h retention of the training. Subsequently, we examined whether an attenuation of glucocorticoid signaling by either a systemic administration of the corticosterone-synthesis inhibitor metyrapone or an intra-striatal infusion of the glucocorticoid receptor (GR) antagonist RU 38486 would block the memory enhancement induced by oxotremorine. Our findings indicate that oxotremorine dose-dependently enhanced 48 h retention latencies, but that the administration of either metyrapone or RU 38486 prevented the memory-enhancing effect of oxotremorine. In the last experiment, corticosterone was infused into the dorsal striatum together with the muscarinic receptor antagonist scopolamine immediately after inhibitory avoidance training. Scopolamine blocked the enhancing effect of corticosterone on 48 h retention performance. These findings indicate that there are mutual interactions between glucocorticoids and the striatal cholinergic system in enhancing the consolidation of memory of inhibitory avoidance training.

Glucocorticoid administration into the dorsal stratium facilitates memory consolidation of inhibitory avoidance training but not of the context or footshock components

It is well established that glucocorticoid administration into a variety of brain regions facilitates memory consolidation of fear-conditioning tasks, including inhibitory avoidance. The present findings indicate that the natural glucocorticoid corticosterone administered into the dorsal striatum (i.e., caudate nucleus) of male Wistar rats produced dose- and time-dependent enhancement of inhibitory avoidance memory consolidation. However, as assessed with a modified inhibitory avoidance procedure that took place on two sequential days to separate context training from footshock training, corticosterone administration into the dorsal striatum did not enhance memory of either the contextual or aversively motivational aspects of the task.

Learning and memory functions of the Basal Ganglia

Although the mammalian basal ganglia have long been implicated in motor behavior, it is generally recognized that the behavioral functions of this subcortical group of structures are not exclusively motoric in nature. Extensive evidence now indicates a role for the basal ganglia, in particular the dorsal striatum, in learning and memory. One prominent hypothesis is that this brain region mediates a form of learning in which stimulus-response (S-R) associations or habits are incrementally acquired. Support for this hypothesis is provided by numerous neurobehavioral studies in different mammalian species, including rats, monkeys, and humans. In rats and monkeys, localized brain lesion and pharmacological approaches have been used to examine the role of the basal ganglia in S-R learning. In humans, study of patients with neurodegenerative diseases that compromise the basal ganglia, as well as research using brain neuroimaging techniques, also provide evidence of a role for the basal ganglia in habit learning. Several of these studies have dissociated the role of the basal ganglia in S-R learning from those of a cognitive or declarative medial temporal lobe memory system that includes the hippocampus as a primary component. Evidence suggests that during learning, basal ganglia and medial temporal lobe memory systems are activated simultaneously and that in some learning situations competitive interference exists between these two systems.

Changes in rat brain muscarinic receptors after inhibitory avoidance learning

It is widely accepted that cerebral acetylcholine is necessary for learning and memory, but little is known about the type of muscarinic receptors involved in these functions. To investigate this problem, [3H]-N-methyl-scopolamine which binds to different types of muscarinic receptors, [3H]-Pirenzepine an M1 receptor antagonist, and [3H]-Oxotremorine-M which binds mainly to M2 receptors, were used as ligands to look for possible changes in muscarinic receptor density in neostriatum (NEO), hippocampus (HIP), amygdala (AMY), and temporo-parietal neocortex (CTX), after testing for retention of inhibitory avoidance, trained with high or low footshock intensities. After low reinforcement there was an M1 postsynaptic receptor up-regulation in NEO, HIP, and CTX, and an M2 presynaptic receptor down-regulation in HIP, which suggests a concerted pre- and postsynaptic cholinergic activation in this area. An up-regulation of both M1 and M2 receptors was detected in CTX of low and high footshocked animals, which indicates the presence of a cortical postsynaptic M2 receptor.

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.

Scopolamine injected into the rat amygdala impairs working memory in the double Y-maze

Recent neurochemical results suggest the hypothesis that the nucleus basalis magnocellularis (nbm) cholinergic projection to the amygdala may play a role in memory. The present study investigated the effects of intra-amygdaloid injections of the cholinergic antagonist scopolamine on working and reference memory in the double Y-maze. Rats were pretrained until working and reference memory choice accuracy stabilized to a criterion of > or = 86% correct. Bilateral cannulae were then surgically implanted in the basolateral amygdaloid complex. Rats (n = 9) received scopolamine in doses of 8.0, 24.0, and 72.0 micrograms/0.5 microliter and saline (0.5 microliter) in a counterbalanced order with retraining to criterion between injections. Intra-amygdaloid scopolamine produced a dose-dependent and differential impairment of working and reference memory. A dose of 24.0 micrograms impaired working memory without significantly affecting reference memory; doses of 8.0 micrograms and 72.0 micrograms affected neither and both types of memory, respectively. Results implicate amygdaloid acetylcholine in memory.

Effect of centrally administered atropine and pirenzepine on radial arm maze performance in the rat

The effect of two anticholinergic drugs administered intracerebroventricularly on acquisition of an 8-arm radial maze task was examined in the rat. Increasing doses of atropine (1, 4.5, 22.5, 45 micrograms/rat) and pirenzepine (4.5, 15, 60, 90 micrograms/rat) significantly impaired performance in the working-memory components of the task. For both drugs this impairment was linearly related to the log of the administered doses and log-dose-response relationship were parallel. The regression lines calculated for each parameter for both drugs were parallel to each other, thus allowing us to calculate the potency of atropine versus pirenzepine: atropine was 5.4 times more potent than pirenzepine for correct arm entries, 10 times more potent for the number of errors and 4 times more potent for the total time taken to complete the task. The relevance of M1 and M2 subtype central acetylcholine receptors in cognitive processes is discussed.

Time-dependent effects of cholinergic blockade of the striatum on memory

Rats were trained on a passive avoidance task, and a retention test was carried out 24 h later. Scopolamine was injected into the anterior striatum at one of various intervals following training: at 2 min it produced amnesia; an intermediate degree of impairment was found when given 8 min after training. With a delay of 15 min, this drug did not produce an interference with memory. These observations suggest that striatal cholinergic activity is involved in memory consolidation.

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.

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.

Retrograde amnesia induced by post-trial injection of atropine into the caudate-putamen. Protective effect of the negative reinforcer

A series of experiments was performed to test the reliability of previous reports which indicated that cholinergic blockade of the caudate-putamen produces memory deficits of passive avoidance, and to determine whether overtraining of this task protects against such deficits. In the first experiment the effects of different doses of atropine injected into the caudate-putamen of rats shortly after training were assessed, and a dose-dependent retention deficit was found. In two additional experiments it was observed that by increasing the magnitude of the negative reinforcer used in training, a protection against such retention deficit was produced. These results support the hypotheses that (a) cholinergic activity of the caudate-putamen is critically involved in memory processes that mediate passive avoidance behavior, and (b) after overtraining the control of this behavior is transferred from the striatal cholinergic system to other neurochemical systems within, or outside, the striatum.

Is cholinergic activity of the striatum involved in the acquisition of positively-motivated behaviors?

Cholinergic activity of the caudate-putamen (CPU) is crucial for the acquisition of aversively-reinforced behaviors (active and passive avoidance). To determine whether this activity is also involved in the acquisition of a positively-rewarded behavior, in the present experimental series the effects of scopolamine applications to the antero-dorsal or postero-dorsal aspects of the striatum on auto-shaping were assessed. The auto-shaping procedure that was used allowed rats to learn to bar press at their own rate. It was found that scopolamine injection into either region of the CPU produced a marked retardation in the acquisition of the conditioned behavior. These results indicate that cholinergic activity of the striatum is critically involved in the early phases of positively-reinforced learning.

Is cholinergic activity of the caudate nucleus involved in memory?

A review was made of experiments dealing with the involvement of cholinergic activity of the caudate nucleus in memory processes. Injections of acetylcholine-receptor blockers or of neurotoxins against cholinergic interneurons into the striatum produce marked impairments in acquisition and retention of instrumental tasks while injections of acetylcholine or choline into the caudate produce the opposite effect. However, after a period of overtraining cholinergic blockade or interference with neural activity of the caudate does not produce significant deficits in retention. It is concluded that striatal cholinergic activity is critically involved in memory of recent events and that long-term memory is mediated by different neurochemical systems outside the caudate nucleus.

Injections of atropine into the caudate nucleus impair the acquisition and the maintenance of passive avoidance

Two experiments were performed to test the hypotheses that cholinergic activity of the caudate nucleus (CN) is involved in the acquisition and in the maintenance of passive avoidance behavior. Rats were trained, in one trial, to avoid one of two compartments of a conditioning box and retention of the task was measured 24 hours later. Several doses of atropine were injected into the CN of independent groups of animals. In Experiment 1 the atropine was injected 2 minutes after training and in Experiment 2 it was injected 6 minutes before retention testing. In both cases a dose-dependent retention deficit was found. These results indicate that striatal cholinergic activity is indeed involved in the processes that mediate passive avoidance conditioning.

Effects of cholinergic stimulation of the caudate nucleus on active avoidance

Experiments dealing with the effects of applications of cholinergic drugs into the caudate nucleus on learned behaviors have yielded contradictory results; both improvements and deficits have been found. In the present experiment choline was injected into the CN of rats previously trained in a two-way active avoidance task. The results show that an improvement in performance can be seen when a small dose of choline is used and suggest that the reported deficits in learned performance were due to an overactivation of acetylcholine receptors.

Post-trial injection of atropine into the caudate nucleus interferes with long-term but not with short-term retention of passive avoidance

One-trial passive avoidance training was given to Wistar rats and retention of the task was measured 30 min and 24 h later. Atropine (60 micrograms) was injected into the anterior caudate nucleus 2 min after training. Excellent retention was evident 30 min after training, whereas a significant deficit in memory was found when retention was tested 24 h after training. These results suggest that blockade of cholinergic activity of the caudate nucleus induced shortly after training interferes with the consolidation of long-term memory but not with short-term memory processes.

Brain systems and long-term memory

This paper focuses mainly on those findings derived from lesion studies on the rat which help to identify ensembles of neural structures concerned with the expression of previously learned responses. At the outset, the use of the lesion method in the search for those neurological circuits underlying memory is defended. This is followed by an evaluation of neocortical and subcortical systems in long-term memory. Subsequently, a modest list of tentative functional neural "complexes" involved in the maintenance of certain classes of learned responses is given, based largely upon the author's own research. It is concluded that the key to the understanding of the neurological substrates of long-term memory lies in the identification of those subcortical sites which interact with neocortical sites in the performance of complex learned tasks. The most likely subcortical sites involved in this interaction appear to inhabit the regions of the basal ganglia, limbic midbrain area, and ventral portions of the brainstem reticular formation.

Acetylcholine metabolism in the rat hippocampus and striatum following one-trial passive training

The steady state levels of acetylcholine and choline and the synthesis of acetylcholine in the hippocampus and striatum of rat brain have been determined after one-trial passive avoidance training. Steady state levels of acetylcholine and choline were not altered, but the rate of acetylcholine synthesis was significantly elevated in the striatum but not significantly elevated in the hippocampus at one hour after training.

Time-dependent retention deficits induced by post-training injections of atropine into the caudate nucleus

Rats were trained on a one-trial passive avoidance task, and retention of the task measured 24 hr later. Atropine was injected bilaterally into the anterior caudate nuclei (ACN) of rats from independent groups at one of several intervals after training. Application of atropine 2 min after training produced a lack of retention of passive avoidance. An intermediate degree of impairment was seen when the treatment was given 3 min 45 sec after the learning experience, and interference with retention was still noted when an interval of 7 min 30 sec was studied. In contrast, no deficits were observed in groups of animals injected with atropine 15 or 30 min after training. Rats injected with atropine into the parietal cortex 2 min after training showed only a minimal reduction of retention, and a group injected with saline solution into the ACN performed as well as non-treated animals. These results suggest that there is a time-dependent process that mediates the retention of passive avoidance, and that this process requires the activation of cholinergic synapses within the anterior caudate nucleus.

Differential effects of cholinergic blockade of anterior and posterior caudate nucleus on avoidance behaviors

Groups of rats were trained in a one-trial passive avoidance task and then tested for retention 24 and 48 h later. They were also trained, in a single session, according to a one-way active avoidance paradigm. The effects of microinjections of atropine or of saline into the anterior caudate nucleus (CN) and of atropine into the posterior CN were assessed on these conditioned responses. Only those rats injected with atropine in the anterior CN showed a retention deficit in passive avoidance, while no effects on active avoidance became evident in any of the groups. These results suggest that cholinergic activity of the anterior CN is critically involved in memory consolidation of passive avoidance, but not in the processes mediating the acquisition of relatively simple active avoidance learning.

Scopolamine and KCl injections into the caudate nucleus. Overtraining-induced protection against deficits of learning

To test the hypothesis that extended training of an instrumental task prevents the performance impairments seen after cholinergic and generalized blockade of caudate-putamen complex (NC) activity in animals with a relatively low degree of training, groups of rats were trained to press a lever under a continuous reinforcement schedule for 5, 15 or 25 sessions; The effects of microinjections of scopolamine and potassium chloride into the CN were then assessed. In agreement with early studies in cats, a significant deficit in performance was produced in the animals with a low or medium degree of training, while no changes in learned behavior were seen in the overtained rats. These results show that: (a) normal neural activity of the CN is essential for performance of instrumental behavior during acquisition and early maintenance stages but not after overtraining, and (b) that after extended training the encoding necessary for performance may be transferred to another neural system outside the CN.

Improvement of learned behavior through cholinergic stimulation of the caudate nucleus

To test the hypothesis that cholinergic activity of the head of the caudate nucleus (CN) is involved in the processes underlying instrumental performance, the effects of microinjections of several doses of choline into this structure on lever pressing behavior were assessed. A dose-dependent modification of performance was found: small doses improve and large doses impair level pressing, while choline applications into the parietal cortex were without effect. The facilitatory effects were reproduced by microinjections of acetylcholine into the CN. These data further support our working hypothesis.

Effects of diethyldithiocarbamate and fusaric acid upon memory storage processes in rats

Diethyldithiocarbamate (DDC) and fusaric acid (FA), administered immediately posttraining, disrupted retention performance of rats trained on a one-trial inhibitory avoidance task. Posttraining DDC disrupted retention performance if the treatment was delayed up to at least 24 h (but not 72 h) after training. A single DDC injection also disrupted retention performance if the treatment was administered up to 10 days prior to training. The effects of DDC and FA upon several neurobiological measures were also investigated. Amnestic doses of DDC influence brain and adrenal catecholamine levels, brain electrographic activity and hippocampal Timm's staining. On the other hand, amnestic doses of FA only affected catecholamine levels. The amnestic effects of DDC were dissociated from brain seizure activity produced by DDC. The effects of DDC and FA upon retention may have been related to the drugs' effects upon catecholamine systems, but the bleaching by DDC of Timm's staining could reflect additional processes involved in DDC's effects upon retention. As opposed to the more widespread effects of DDC, FA's relative lack of effect upon other neurobiological measures more strongly supports a catecholaminergic interpretation for its effect upon retention.

Influence of post-training intrahippocampally applied oxotremorine on the consolidation of a brightness discrimination

The posttraining intrahippocampal injection of oxotremorine revealed an improvement of the retention performance in a brightness discrimination task. The oxotremorine effect seemed to be dependent on distinct variables of training and was restricted to rats exhibiting a good acquisition performance. Scopolamine impaired the retention performance of animals with few training errors. The role of hippocampal cholinergic synapses for consolidation was discussed.

Angiotensin injected into the neostriatum after learning disrupts retention performance

Angiotensin II, injected into the dorsal neostriatum of rats 5 minutes after they had learned a passive avoidance task, disrupted the retention of the task 24 hours later. Identical neostriatal injections given 22 hours after learning (2 hours before retention) were without effect on retention performance. Ventral neostriatum or posterior thalamus were ineffective sites for injection of angiotensin. Injection of thyrotropin releasing hormone or lysine-8-vasopressin into the dorsal neostriatum was ineffective. These findings indicate a possible role for endogenous angiotensin in the neostriatum on retention performance and suggest potential involvement in mnemonic processes.

Neurochemical correlate of a spatial preference in rats

Spatial (left or right) preferences were determined for rats given foot shock in a T-maze. The animals were killed, and left and right striata were assayed separately for dopamine and left and right teldiencephalic regions were assayed for norepinephrine. Dopamine content was significantly higher (by 12 percent) in the striata contralateral to rats' side preferences than in the ipsilateral striata; there was no such difference for teldiencephalic norepinephrine. The small asymmetry in striatal dopamine content is not due to any learning- or stress-related change induced by the testing procedure but is probably inherent in normal rats. Some spatial behavior appears to be the manifestation of a normal and specific difference in the activity of left and right nigrostriatal systems.