Increased resting-state perfusion after repeated encoding is related to later retrieval of declarative associative memories

Electrophysiological studies in animals have shown coordinated reactivation of neuronal ensembles during a restricted time period of behavioral inactivity that immediately followed active encoding. In the present study we directly investigated off-line processing of associative memory formation in the human brain. Subjects' regional cerebral blood flow (rCBF) as a surrogate marker of neural activity during rest was measured by MR-based perfusion imaging in a sample of 14 healthy male subjects prior to (Pre2) and after (Post) extensive learning of 24 face-name associations within a selective reminding task (SR). Results demonstrated significant Post-Pre2 rCBF increases in hippocampal and temporal lobe regions, while in a control comparison of two perfusion scans with no learning task in-between (Pre2-Pre1) no differences in rCBF emerged. Post perfusion scanning was followed by a surprise cued associative recall task from which two types of correctly retrieved names were obtained: older names already correctly retrieved at least once during one of the SR blocks, and recent names acquired during the last SR block immediately prior to the Post scan. In the anterior hippocampus individual perfusion increases were correlated with both correct retrievals of older and recent names. By contrast, older but not recently learned names showed a significant correlation with perfusion increases in the left lateral temporal cortex known to be associated with long-term memory. Recent, but not older names were correlated with dopaminergic midbrain structures reported to contribute to the persistence of memory traces for novel information. Although the direct investigation of off-line memory processing did not permit concomitant experimental control, neither intentional rehearsal, nor substantial variations in subjects' states of alertness appear to contribute to present results. We suggest that the observed rCBF increases might reflect processes that possibly contribute to the long-term persistence of memory traces.     

Different neuroplasticity for task targets and distractors

Adult learning-induced sensory cortex plasticity results in enhanced action potential rates in neurons that have the most relevant information for the task, or those that respond strongly to one sensory stimulus but weakly to its comparison stimulus. Current theories suggest this plasticity is caused when target stimulus evoked activity is enhanced by reward signals from neuromodulatory nuclei. Prior work has found evidence suggestive of nonselective enhancement of neural responses, and suppression of responses to task distractors, but the differences in these effects between detection and discrimination have not been directly tested. Using cortical implants, we defined physiological responses in macaque somatosensory cortex during serial, matched, detection and discrimination tasks. Nonselective increases in neural responsiveness were observed during detection learning. Suppression of responses to task distractors was observed during discrimination learning, and this suppression was specific to cortical locations that sampled responses to the task distractor before learning. Changes in receptive field size were measured as the area of skin that had a significant response to a constant magnitude stimulus, and these areal changes paralleled changes in responsiveness. From before detection learning until after discrimination learning, the enduring changes were selective suppression of cortical locations responsive to task distractors, and nonselective enhancement of responsiveness at cortical locations selective for target and control skin sites. A comparison of observations in prior studies with the observed plasticity effects suggests that the non-selective response enhancement and selective suppression suffice to explain known plasticity phenomena in simple spatial tasks. This work suggests that differential responsiveness to task targets and distractors in primary sensory cortex for a simple spatial detection and discrimination task arise from nonselective increases in response over a broad cortical locus that includes the representation of the task target, and selective suppression of responses to the task distractor within this locus.     

Induction of amnesia induced by disturbance of memory consolidation by antagonists of glutamate or serotonin receptors will be suppressed by the protein synthesis inhibitor

We have previously found two stages of amnesia evoked by disruption of memory reconsolidation with MK-801 (NMDA glutamate receptors antagonists) application in food aversion conditioned snails. Repeated conditioning restored the food aversion at early stage of amnesia development (<10 days), whereas repeated conditioning 10 days after MK-801 application did not restore the food aversion. In present work, amnesia was induced with MK-801/reminding 24 hours after food aversion conditioning, and antiamnestic effects of NMDA receptor glycine site agonist d-cycloserine were studied at early (3rd day) or late (12th day) stages of amnesia development. D-cycloserine injection and reminding restored memory only 3 days after amnesia induction whereas d-cycloserine injection without reminding was ineffective. D-cycloserine injection and reminding as well as repeated learning 12 days after amnesia induction were also ineffective in memory restoration. Thus, for the first time, it is revealed that NMDA receptor agonist d-cycloserine influences the memory restoration processes only at early but not the later stages of amnesia development.     

Learning and Memory in Juvenile Zebrafish: What makes the Difference – Population or Rearing Environment?

The ability to learn and remember about the surrounding environment is crucial for the survival of organisms in their natural habitats. Adults of numerous fish species have been shown to display sophisticated behaviour related to learning and memory. This study deals with testing learning abilities among juveniles (10 wks old) of zebrafish (Danio rerio) through a simple task of finding food. We compared the performance of juveniles from two populations, a wild collected from a natural habitat and an aquarium‐bred purchased from a pet shop, reared in bare environment (lacking visual cues). Additionally, we also tested the effect of early habitat enrichment on the performance among juveniles of the aquarium‐bred population. The experiments involved training fishes to solve a simple maze (with a food reward at the end) and testing their memory. Learning was measured based on the time taken to complete the task (performance time) of finding food in the maze‐arena across repeated trials. Our results showed that juveniles from the two populations possessed significantly different learning abilities. There were population differences in exploratory tendency and time taken to accomplish the task. However, when memory was tested based on performance time between training and test day, individuals (from both populations) were found to be poor at memorizing learnt tasks. On the other hand, juveniles (belonging to aquarium‐bred population) reared in spatially complex environments displayed higher rates of learning and were capable of remembering learnt tasks better than their counterparts from bare environments. This study thus demonstrates the importance of rearing conditions and natural ecology in ontogenetic development of learning and memory functions among zebrafish.     

Effects of vinconate on spatial learning impairments induced by medial septal lesion in rats.

We investigated the effects of vinconate, a novel vinca alkaloid, on spatial learning deficits induced by medial septal (MS) lesion in rats. MS lesion was produced by passing an anodal DC current. In vehicle-treated MS-lesioned rats impairment of spatial learning was observed, shown by a decrease in correct choices and an increase in total errors during training in a radial arm maze task. Vinconate (10 mg/kg) treatment alleviated the decrease in correct choices and the increase in total errors induced by MS lesion. Vinconate (5 and 10 mg/kg) treatment showed a tendency to reverse the decrease in choline acetyltransferase activity in the hippocampus caused by MS lesion. The present study suggests that vinconate has an anti-amnesic effect on MS lesion-induced amnesia by ameliorating the dysfunction in cholinergic (ACh) neurons.     

Acquisition of discriminations involving ambiguous or non-ambiguous features: an evaluation of two configural learning models

The present rat experiment evaluated the validity of two formal accounts of configural learning in the framework of discrimination tasks involving the serial presentation of feature and target stimuli: Rescorla's (1973) modification of the Rescorla-Wagner model (1972) and the Pearce model (1987). The first, ambiguous feature task was of the form X-->A+, Y-->A-, X-->B-, Y-->B+, in which X and Y represent visual features, '-->' signifies a serial arrangement, A and B are auditory target stimuli, and '+' and '-' symbolise food-reinforcement and non-reinforcement, respectively. The second, non-ambiguous feature task was of the form: X-->A+, Y-->A-, X-->B+, Y-->B-. The former task was much more difficult to solve than was the latter task. The Rescorla model is able to account for the observed differences between the two tasks in learning rates and in the associative strength of feature X with more plausible parameter values than is the Pearce model. It is suggested that models acknowledging a role for both elemental and configural learning can better account for discrimination learning in discrimination tasks of the sort presented in this study than do models that exclusively allow for configural learning.     

High-performance liquid chromatography followed by peroxyoxalate chemiluminescence detection of acetylcholine and choline utilizing immobilized enzymes

A sensitive and selective method for the simultaneous determination of acetylcholine (ACh) and choline (Ch) is reported. ACh and Ch were separated on a reversed-phase column, passed through an immobilized enzymes (acetylcholine esterase and choline oxidase) column, and converted to hydrogen peroxide. The generated hydrogen peroxide was detected by the peroxyoxalate chemiluminescence reaction. The linear determination ranges were from 10 pmol to 10 nmol. The detection limit for both cholines was 1 pmol.     

Monoclonal antibodies selective for Drosophila melanogaster choline acetyltransferase

The newly developed monoclonal antibody technology was applied to the production of antibodies selective for Drosophila melanogaster choline acetyltransferase (EC 2.3.1.6). Two stable cell lines, 1C8 and 1G4, were isolated from NS-1/spleen cell hybrids by employing a choline acetyltransferase enzyme activity-screening method. Both cell lines were cloned twice and were maintained in continuous culture and as ascites tumors. Purified antibody was isolated from ascites fluids by pH elution after adsorption to Protein A-Sepharose. Both antibodies eluted from the Protein A-Sepharose as a single subclass, IgG1, and directly inhibited choline acetyltransferase activity. Scatchard analysis of titration data for choline acetyltransferase antibody-enzyme interaction generated linear curves for both antibodies: KA for 1C8 was 2.77 X 10(7) M-1 and KA for 1G4 was 0.78 X 10(7) M-1. Inclusion of the choline acetyltransferase substrate acetyl-CoA at 10 times the KM in the antibody-enzyme reaction mixture substantially reduced the level of inhibition observed with both antibodies; choline, however, exhibited no protective effect. Neither antibody reacted with choline acetyltransferase-containing extracts of vertebrates or other insect neural tissues. We conclude that the two antibodies are nonidentical, monoclonal, and highly selective for D. melanogaster choline acetyltransferase, both reacting at or near the acetyl-CoA binding region of the enzyme-active site.     

Acquisition of an olfactory learning-set in rats with lesions of the mediodorsal thalamic nucleus

Rats with lesions of the mediodorsal thalamic nucleus had goodretention of a preoperatively leamed odor detection task butmade more errors than controls in the acquisition of a seriesof two-odor discrimination problems. While controls showed rapidacquisition of a learning-set, the performance of experimentalrats was highly variable. Deficits in acquisition occurred ondifferent problem for different rats and appeared unrelatedto problem difficulty or serial position of the discrimination.Experimental rats also had a significant deficit in the acquisitionof an odor reversal task. Error scores in both the learing-setand reversal task were related to lesion size. Small lesionsconfined to the rostral third of the nucleus or asymmetricallesions which spared the central (olfactory) division of mediodorsalnucleus on one side were without effect. The results suggestthat the deficit in olfactory learning produced by destructionof the mediodorsal thalamic nucleus is complex and is not adequatelyaccounted for by stimulus novelty, problem difficulty or bypoor interproblem transfer of learning.     

Rifampin-Blood-Agar as a Selective Medium for the Isolation of Certain Anaerobic Bacteria

A selective medium which allows detection of relatively small numbers of Fusobacterium varium in fecal specimens is described. Blood-agar containing 50 mug of rifampin per ml inhibits the growth of many species of Bacteriodes and of F. fusi-forme/nucleatum but allows good growth of F. varium and most strains of F. mortiferum. Quantitative cultures of 11 fecal specimens were done on rifampin and other selective and nonselective media. F. varium was recovered in counts of 10(6) and 10(7) per gram from two specimens on rifampin only. A third specimen yielded 10(10)F. varium on several media, including rifampin. Some Eubacterium and Clostridium species also grew on rifampin, and these ordinarily were distinguished from the Fusobacterium by colony morphology. This medium is of value in fecal flora studies and should be useful with other kinds of specimens where mixtures of organisms are common.     

Dynamics of population code for working memory in the prefrontal cortex

Some neurons (delay cells) in the prefrontal cortex elevate their activities throughout the time period during which the animal is required to remember past events and prepare future behavior, suggesting that working memory is mediated by continuous neural activity. It is unknown, however, how working memory is represented within a population of prefrontal cortical neurons. We recorded from neuronal ensembles in the prefrontal cortex as rats learned a new delayed alternation task. Ensemble activities changed in parallel with behavioral learning so that they increasingly allowed correct decoding of previous and future goal choices. In well-trained rats, considerable decoding was possible based on only a few neurons and after removing continuously active delay cells. These results show that neural activity in the prefrontal cortex changes dynamically during new task learning so that working memory is robustly represented and that working memory can be mediated by sequential activation of different neural populations.     

Dietary choline augments associative memory function in Limax maximus

Previous clinical and experimental work has shown that increased dietary intake of choline elevates blood choline and brain acetylcholine levels. This change in neuronal acetylcholine concentration may augment learning and memory functions. We tested this prediction using the mollusc Limax maximus, an animal which can be readily conditioned to avoid food odors. In our experiments, initial learning of a food avoidance task was not augmented by the high choline diet. However, the duration of memory retention was prolonged. In previous studies, we have shown that intake of the choline enriched diet significantly increases blood choline and amplifies transmission at an identified cholinergic synapse in Limax. Together, these results support the involvement of cholinergic synapses in the memory retention mechanism.     

Effect of dunce and amnesiac genes on signal learning in Drosophila melanogaster: experiments with odors]

The behavior of normal Drosophila and of X-linked olfactory conditioning mutants, dunce and amnesiac, was analyzed using an olfactory search task. Normal (C-S) flies quickly learn and remember which of two odors signals the presence of food and they are capable of retaining this information for at last eight hours. Both dunce and amnesiac mutants are able to learn, whereas mutant dunce do not reach the learning level of wild type C-S flies. Also dunce flies require more than one learning trial for sizeable learning effect. Reversal learning experiments showed that normal C-S flies and amnesiac are able to switch to a new food signal in response to a new experience, while the dunce mutation inhibits the acquisition of new information in reversal learning experiments.     

Partial reversal of anticholinergic amnesia by choline chloride

Twenty young adult males were given tests of memory and mood after receiving choline chloride plus scopolamine, placebo plus scopolamine, and two placebos on separate days in randomized order. Scopolamine caused a marked impairment in performance on a verbal learning task. Pretreatment with choline caused a small but significant decrease in this impairment and partially reversed the mood changes produced by scopolamine. Although several previous studies have failed to demonstrate that choline has a significant effect on cognition when given alone, the present results indicate that choline can enhance cognition under conditions of cholinergic blockade. This is consistent with preclinical data indicating that cholinergic neurons are most sensitive to precursor availability when they are firing rapidly.     

Working memory of action: a comparative study of ability to selecting response based on previous action in New World monkeys (Saimiri sciureus and Callithrix jacchus)

Working memory of the outcome of one's own action is important for organizing and learning appropriate behaviors in a given condition. To examine whether non-human primates with different neocortical sizes show different abilities regarding working memory for action, the performance of squirrel monkeys (Saimiri sciureus) and common marmosets (Callithrix jacchus) in a kind of delayed-response task was compared. In this task, subjects were required to select a response based on short-term memory of the outcome of their own prior action, which requires working memory of action. These monkeys have a similar phylogenetic status (i.e. Ceboidea), but the size of the neocortex relative to the rest of the brain (relative size of the neocortex) is quite different. We found that both two species could attend to and remember their own actions and could select a response based on that memory. However, the performance level was higher for squirrel monkeys than for marmosets. These results suggest that non-human primates with differentially developed neocortices have different abilities regarding working memory of action.     

Rat spatial memory tasks adapted for humans: characterization in subjects with intact brain and subjects with selective medial temporal lobe thermal lesions

In the present paper we describe five tests, 3 of which were designed to be similar to tasks used with rodents. Results obtained from control subjects, patients with selective thermo-coagulation lesions to the medial temporal lobe and results from non-human primates and rodents are discussed. The tests involve memory for spatial locations acquired by moving around in a room, memory for objects subjects interacted with, or memory for objects and their locations. Two of the spatial memory tasks were designed specifically as analogs of the Morris water task and the 8-arm radial-maze tasks used with rats. The Morris water task was modeled by hiding a sensor under the carpet of a room (Invisible Sensor Task). Subjects had to learn its location by using an array of visual cues available in the room. A path integration task was developed in order to study the non-visual acquisition of a cognitive representation of the spatial location of objects. In the non-visual spatial memory task, we blindfolded subjects and led them to a room where they had to find 3 objects and remember their locations. We designed an object location task by placing 4 objects in a room that subjects observed for later recall of their locations. A recognition task, and a novelty detection task were given subsequent to the recall task. An 8-arm radial-maze was recreated by placing stands at equal distance from each other around the room, and asking subjects to visit each stand once, from a central point. A non-spatial working memory task was designed to be the non-spatial equivalent of the radial maze. Search paths recorded on the first trial of the Invisible Sensor Task, when subjects search for the target by trial and error are reported. An analysis of the search paths revealed that patients with lesions to the right or left hippocampus or parahippocampal cortex employed the same type of search strategies as normal controls did, showing similarities and differences to the search behavior recorded in rats. Interestingly, patients with lesions that included the right parahippocampal cortex were impaired relative to patients with lesions to the right hippocampus that spared the parahippocampal cortex, when recall of the sensor was tested after a 30 min delay (Bohbot et al. 1998). No differences were obtained between control subjects and patients with selective thermal lesions to the medial temporal lobe, when tested on the radial-maze, the non-spatial analogue to the radial-maze and the path integration tasks. Differences in methodological procedures, learning strategies and lesion location could account for some of the discrepant results between humans and non-human species. Patients with lesions to the right hippocampus, irrespective of whether the right parahippocampal cortex was spared or damaged, had difficulties remembering the particular configuration and identity of objects in the novelty detection of the object location task. This supports the role of the human right hippocampus for spatial memory, in this case, involving memory for the location of elements in the room; learning known to require the hippocampus in the rat.     

Right medial temporal-lobe contribution to object-location memory.

An important aspect of normal human memory, and one humans share with many other species, is the ability to remember the location of objects in their environment. There is by now strong evidence from the study of epileptic patients undergoing brain surgery that right temporal-lobe lesions that encroach extensively upon the hippocampal and parahippocampal gyrus impair the delayed, but not the immediate, recall of the location of objects within a random array. These findings have now been extended to a multiple-trial, spatial-array learning task; by including not only patients tested after unilateral anterior temporal lobectomy but also those with a selective left or right amygdalohippocampectomy, it has been shown that the deficits associated with right hippocampal lesions are not dependent upon conjoint damage to the lateral temporal neocortex. Furthermore, the fact that on the learning task no group differences were seen on Trial 1, at zero delay, strengthened the view that the impairment was in the maintenance and subsequent retrieval of information rather than in its initial encoding. These results left unresolved the question of whether the deficit was in the mediation of object-place associations or whether it could be reduced to a more general impairment in memory for location as such. Also left unanswered was the neuroanatomical question as to the relative contributions of the hippocampus and the parahippocampal gyrus to the performance of the experimental tasks. These questions were addressed in two blood-flow activation studies that made use of positron emission tomography (PET) and magnetic resonance imaging (MRI) and incorporated computerized versions of object-location and simple-location memory tasks. Taken together, the results point to a special contribution from the anterior part of the right parahippocampal gyrus, probably corresponding to the entorhinal cortex, to the retrieval of object-place associations, a result consonant with neurophysiological findings in non-human primates.     

Induction of amnesia evoked by memory reconsolidation disruption with glutamate or serotonin receptor antagonists depends on protein synthesis activation

Effects of DNQX (ionotropic AMPA/cainate glutamate receptor antagonist) and metiotepin (serotonin receptor antagonist) and cycloheximide (protein synthesis inhibitor) on long-term memory reconsolidation processes were studied in snail Helix lucorum with definite type of food aversion conditioned reflex. DNQX or cycloheximide injected immediately before reminding disrupt retrieval of the food aversion 24 hours after conditioning, and repeated trials of learning with the same food as at initial trials did not form the food aversion 2 weeks later. Metiotepin + reminding also discrupt recalling of food aversion 24 hours after conditioning, while food aversion was repaired after repeated trials 2 weeks later. Simultaneous injections of DNQX + cycloheximide or metiotepin + cycloheximide immediately before reminding do not discrupt the food aversion. We suggest that unrecovered and recovered forms of amnesia induced by AMPA/cainate glutamate receptor antagonists or serotonin receptor antagonist, respectively, need specific protein molecules de novo.     

Biotin and choline replace the growth requirement of Madin-Darby canine kidney cells for high-density lipoproteins

MDCK cells maintained on extracellular matrix (ECM)-coated dishes and exposed to Dulbecco's modified Eagle's medium (DME) supplemented with transferrin and either high-density lipoproteins (HDLs) or phosphatidyl choline (PC) liposomes have a growth rate and final cell density similar to those of cultures exposed to serum-supplemented DME. When MDCK cells are exposed to a medium consisting of a mixture (1:1) of DME and F12 medium (D/F), the addition of transferrin (10 μg/ml) alone supports cell growth and the presence of HDLs or PC liposomes is no longer required. MDCK cells exposed to D/F medium supplemented with transferrin can be passaged for more than 50 generations in total absence of serum. The F12 components that support growth in the absence of HDLs or PC liposomes are biotin (which is absent in DME) and choline (which is present in insufficient concentration in DME). Supplementation of DME with transferrin, biotin (3.6 ng/ml), and choline (10 μg/ml) allows optimal growth of MDCK cells and permits serial propagation through more than 50 generations. The growth requirement of MDCK cells for HDLs or PC liposomes can therefore be replaced by adequate concentrations of biotin and choline. The widely observed fact that a combination of DME/F12 medium is more effective than DME alone in supporting cell growth may be due in part to the lack of biotin and suboptimal choline concentration in DME.     

Premarin improves memory, prevents scopolamine-induced amnesia and increases number of basal forebrain choline acetyltransferase positive cells in middle-aged surgically menopausal rats

Conjugated equine estrogen (CEE) is the most commonly prescribed estrogen therapy, and is the estrogen used in the Women's Health Initiative study. While in-vitro studies suggest that CEE is neuroprotective, no study has evaluated CEE's effects on a cognitive battery and brain immunohistochemistry in an animal model. The current experiment tested whether CEE impacted: I) spatial learning, reference memory, working memory and long-term retention, as well as ability to handle mnemonic delay and interference challenges; and, II) the cholinergic system, via pharmacological challenge during memory testing and ChAT-immunoreactive cell counts in the basal forebrain. Middle-aged ovariectomized (Ovx) rats received chronic cyclic injections of either Oil (vehicle), CEE-Low (10 μg), CEE-Medium (20 μg) or CEE-High (30 μg) treatment. Relative to the Oil group, all three CEE groups showed less overnight forgetting on the spatial reference memory task, and the CEE-High group had enhanced platform localization during the probe trial. All CEE groups exhibited enhanced learning on the spatial working memory task, and CEE dose-dependently protected against scopolamine-induced amnesia with every rat receiving the highest CEE dose maintaining zero errors after scopolamine challenge. CEE also increased number of ChAT-immunoreactive neurons in the vertical diagonal band of the basal forebrain. Neither the ability to remember after a delay nor interference, nor long-term retention, was influenced by the CEE regimen used in this study. These findings are similar to those reported previously for 17 β-estradiol, and suggest that CEE can provide cognitive benefits on spatial learning, reference and working memory, possibly through cholinergic mechanisms.