Dorsal, ventral, and complete excitotoxic lesions of the hippocampus in rats failed to impair appetitive trace conditioning

Learning differentially shapes prefrontal and hippocampal activity during classical conditioning

The ability to use sensory cues to inform goal-directed actions is a critical component of behavior. To study how sounds guide anticipatory licking during classical conditioning, we employed high-density electrophysiological recordings from the hippocampal CA1 area and the prefrontal cortex (PFC) in mice. CA1 and PFC neurons undergo distinct learning-dependent changes at the single-cell level and maintain representations of cue identity at the population level. In addition, reactivation of task-related neuronal assemblies during hippocampal awake Sharp-Wave Ripples (aSWRs) changed within individual sessions in CA1 and over the course of multiple sessions in PFC. Despite both areas being highly engaged and synchronized during the task, we found no evidence for coordinated single cell or assembly activity during conditioning trials or aSWR. Taken together, our findings support the notion that persistent firing and reactivation of task-related neural activity patterns in CA1 and PFC support learning during classical conditioning.

Chronic Cyanuric Acid Exposure Depresses Hippocampal LTP but Does Not Disrupt Spatial Learning or Memory in the Morris Water Maze

Exposure to cyanuric acid (CA) causes multiple organ failure accompanied by the involvement in kinds of target proteins, which are detectable and play central roles in the CNS. The hippocampus has been identified as a brain area which was especially vulnerable in developmental condition associated with cognitive dysfunction. No studies have examined the effects of CA on hippocampal function after in vitro or in vivo treatment. Here, we aimed to examine hippocampal synaptic function and adverse behavioral effects using a rat model administered CA intraperitoneally or intrahippocampally. We found that infusion of CA induced a depression in the frequency but not the amplitude of spontaneous excitatory postsynaptic currents (sEPSCs), miniature excitatory postsynaptic currents (mEPSCs), or N-methyl-D-aspartate (NMDA)-mediated excitatory postsynaptic currents (EPSCs) of the CA1 neurons in dose-dependent pattern. Both intraperitoneal and intrahippocampal injections of CA suppressed hippocampal LTP from Schaffer collaterals to CA1 regions. Paired-pulse facilitation (PPF), a presynaptic phenomenon, was enhanced while the total and phosphorylated expression of NMDA-GluN1, NMDA-GluN2A, and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-GluA1 subunits were comparable between CA-treated and control groups. In Morris water maze test, both groups could effectively learn and retain spatial memory. Our studies provide the first evidence for the neurotoxic effect of CA and the insight into its potential mechanisms.

Training on an Appetitive Trace-Conditioning Task Increases Adult Hippocampal Neurogenesis and the Expression of Arc, Erk and CREB Proteins in the Dorsal Hippocampus

Adult hippocampal neurogenesis (AHN) plays an essential role in hippocampal-dependent memory consolidation. Increased neurogenesis enhances learning, whereas its ablation causes memory impairment. In contrast, few reports suggest that neurogenesis reduces after learning. Although the interest in exploring the role of adult neurogenesis in learning has been growing, the evidence is still limited. The role of the trace- and delay-appetitive-conditioning on AHN and its underlying mechanism are not known. The consolidation of trace-conditioned memory requires the hippocampus, but delay-conditioning does not. Moreover, the dorsal hippocampus (DH) and ventral hippocampus (VH) may have a differential role in these two conditioning paradigms. Here, we have investigated the changes in: (A) hippocampal cell proliferation and their progression towards neuronal lineage; and (B) expression of Arc, Erk1, Erk2, and CREB proteins in the DH and VH after trace- and delay-conditioning in the rat. The number of newly generated cells significantly increased in the trace-conditioned but did not change in the delay-conditioned animals compared to the control group. Similarly, the expression of Arc protein significantly increased in the DH but not in the VH after trace-conditioning. Nonetheless, it remains unaltered in the delay-conditioned group. The expression of pErk1, pErk2, and pCREB also increased in the DH after trace-conditioning. Whereas, the expression of only pErk1 pErk2 and pCREB proteins increased in the VH after delay-conditioning. Our results suggest that appetitive trace-conditioning enhances AHN. The increased DH neuronal activation and pErk1, pErk2, and pCREB in the DH may be playing an essential role in learning-induced cell-proliferation after appetitive trace-conditioning.

The hippocampus contributes to temporal duration memory in the context of event sequences: A cross-species perspective

Although a large body of research has implicated the hippocampus in the processing of memory for temporal duration, there is an exigent degree of inconsistency across studies that obfuscates the precise contributions of this structure. To shed light on this issue, the present review article surveys both historical and recent cross-species evidence emanating from a wide variety of experimental paradigms, identifying areas of convergence and divergence. We suggest that while factors such as time-scale (e.g. the length of durations involved) and the nature of memory processing (e.g. prospective vs. retrospective memory) are very helpful in the interpretation of existing data, an additional important consideration is the context in which the duration information is experienced and processed, with the hippocampus being preferentially involved in memory for durations that are embedded within a sequence of events. We consider the mechanisms that may underpin temporal duration memory and how the same mechanisms may contribute to memory for other aspects of event sequences such as temporal order.

Age-related differences in appetitive trace conditioning and novel object recognition procedures

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Longitudinal study of middle age in the rat with matched younger control cohort.

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Appetitive trace conditioning and novel object recognition tests of working memory.

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Transient between-groups working memory impairments aged 12 compared with 2 months.

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Object exploration reduced with age but working memory recovered.

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Object exploration and ITI nosepoking showed some correlation with 5-HIAA/5-HT.

Appetitive trace conditioning (TC) was examined over 6 months in younger-adult (2–8 months) and middle-aged (12–18 months) male Wistar RccHan rats, to test for early age-related impairment in working memory. Novel object recognition (NOR) was included as a comparison task, to provide a positive control in the event that the expected impairment in TC was not demonstrated. The results showed that TC improved at both ages at the 2 s but not at the 10 s trace interval. There was, however, evidence for reduced improvement from one day to the next in the middle-aged cohort tested with the 2 s trace conditioned stimulus. Moreover, within the 10 s trace, responding progressively distributed later in the trace interval, in the younger-adult but not the middle-aged cohort. Middle-aged rats showed NOR discriminative impairment at a 24 h but not at a 10 min retention interval. Object exploration was overall reduced in middle-aged rats and further reduced longitudinally. At the end of the study, assessing neurochemistry by HPLC-ED showed reduced 5-HIAA/5-HT in the dorsal striatum of the middle-aged rats and some correlations between striatal 5-HIAA/5-HT and activity parameters. Overall the results suggest that, taken in isolation, age-related impairments may be overcome by experience. This recovery in performance was seen despite the drop in activity levels in older animals, which might be expected to contribute to cognitive decline.

Infusions of scopolamine in dorsal hippocampus reduce anticipatory responding in an appetitive trace conditioning procedure

INTRODUCTION

Trace conditioning is impaired by lesions to dorsal hippocampus, as well as by treatment with the muscarinic acetylcholine antagonist scopolamine. However, the role of muscarinic receptors within hippocampus has received little attention.

METHODS

The present study examined the effects of intra-hippocampal infusion of scopolamine (30 µg/side) in an appetitive (2 vs. 10 s) trace conditioning procedure using sucrose pellets as the unconditioned stimulus (US). Locomotor activity (LMA) was examined in a different apparatus.

RESULTS

Intra-hippocampal scopolamine reduced responding to the 2 s trace conditioned stimulus (CS). Intra-hippocampal scopolamine similarly depressed responding within the inter-stimulus interval (ISI) at both 2 and 10 s trace intervals, but there was no such effect in the inter-trial interval. There was also some overall reduction in responding when the US was delivered; significant at the 10 s but not at the 2 s trace interval. A similar pattern of results to that seen in response to the CS during acquisition was shown drug-free (in the 5 s post-CS) in the extinction tests of conditioned responding. LMA was increased under scopolamine.

CONCLUSIONS

The results suggest that nonspecific changes in activity or motivation to respond for the US cannot explain the reduction in trace conditioning as measured by reduced CS responding and in the ISI. Rather, the findings of the present study point to the importance of associative aspects of the task in determining its sensitivity to the effects of scopolamine, suggesting that muscarinic receptors in the hippocampus are important modulators of short-term working memory.

Conditioning and time representation in long short-term memory networks

Dopaminergic models based on the temporal-difference learning algorithm usually do not differentiate trace from delay conditioning. Instead, they use a fixed temporal representation of elapsed time since conditioned stimulus onset. Recently, a new model was proposed in which timing is learned within a long short-term memory (LSTM) artificial neural network representing the cerebral cortex (Rivest et al. in J Comput Neurosci 28(1):107-130, 2010). In this paper, that model's ability to reproduce and explain relevant data, as well as its ability to make interesting new predictions, are evaluated. The model reveals a strikingly different temporal representation between trace and delay conditioning since trace conditioning requires working memory to remember the past conditioned stimulus while delay conditioning does not. On the other hand, the model predicts no important difference in DA responses between those two conditions when trained on one conditioning paradigm and tested on the other. The model predicts that in trace conditioning, animal timing starts with the conditioned stimulus offset as opposed to its onset. In classical conditioning, it predicts that if the conditioned stimulus does not disappear after the reward, the animal may expect a second reward. Finally, the last simulation reveals that the buildup of activity of some units in the networks can adapt to new delays by adjusting their rate of integration. Most importantly, the paper shows that it is possible, with the proposed architecture, to acquire discharge patterns similar to those observed in dopaminergic neurons and in the cerebral cortex on those tasks simply by minimizing a predictive cost function.

Towards a unified model of pavlovian conditioning: short review of trace conditioning models

There are three basic paradigms of classical conditioning: delay, trace and context conditioning where presentation of a conditioned stimulus (CS) or a context typically predicts an unconditioned stimulus (US). In delay conditioning CS and US normally coterminate, whereas in trace conditioning an interval of time exists between CS termination and US onset. The modeling of trace conditioning is a rather difficult computational problem and is a challenge to the behavior and connectionist approaches mainly due to a time gap between CS and US. To account for trace conditioning, Pavlov (Conditioned reflexes: an investigation of the physiological activity of the cerebral cortex, Oxford University Press, London, 1927) postulated the existence of a stimulus "trace" in the nervous system. Meanwhile, there exist many other options for solving this association problem. There are several excellent reviews of computational models of classical conditioning but none has thus far been devoted to trace conditioning. Eight representative models of trace conditioning aimed at building a prospective model are being reviewed below in a brief form. As a result, one of them, comprising the most important features of its predecessors, can be suggested as a real candidate for a unified model of trace conditioning.

Dorsal hippocampal lesions disrupt Pavlovian delay conditioning and conditioned-response timing

The involvement of the rat dorsal hippocampus (dhpc) in Pavlovian conditioning and timing of conditioned responding was examined in an appetitive preparation in which presentation of a relatively long, 40-s auditory conditioned stimulus (CS) was followed immediately by food delivery. Dorsal hippocampal lesions impaired Pavlovian conditioning in this task. They also produced a deficit in interval timing, replicating previous findings with short CSs. The conditioning and timing deficits observed are consistent with the findings from single-unit recording studies in other species, and suggest that the involvement of the dhpc in Pavlovian processes could be more general than is assumed by many of the current theories of hippocampal function.

Differential acetylcholine release in the prefrontal cortex and hippocampus during pavlovian trace and delay conditioning

Pavlovian trace conditioning critically depends on the medial prefrontal cortex (mPFC) and hippocampus (HPC), whereas delay conditioning does not depend on these brain structures. Given that the cholinergic basal forebrain system modulates activity in both the mPFC and HPC, it was reasoned that the level of acetylcholine (ACh) release in these regions would show distinct profiles during testing in trace and delay conditioning paradigms. To test this assumption, microdialysis probes were implanted unilaterally into the mPFC and HPC of rats that were pre-trained in appetitive trace and delay conditioning paradigms using different conditional stimuli in the two tasks. On the day of microdialysis testing, dialysate samples were collected during a quiet baseline interval before trials were initiated, and again during performance in separate blocks of trace and delay conditioning trials in each animal. ACh levels were quantified using high-performance liquid chromatography and electrochemical detection techniques. Consistent with our hypothesis, results showed that ACh release in the mPFC was greater during trace conditioning than during delay conditioning. The level of ACh released during trace conditioning in the HPC was also greater than the levels observed during delay conditioning. While ACh efflux in both the mPFC and HPC selectively increased during trace conditioning, ACh levels in the mPFC during trace conditioning testing showed the greatest increases observed. These results demonstrate a dissociation in cholinergic activation of the mPFC and HPC during performance in trace but not delay appetitive conditioning, where this cholinergic activity may contribute to attentional mechanisms, adaptive response timing, or memory consolidation necessary for successful trace conditioning.

Changing the rate and hippocampal dependence of trace eyeblink conditioning: slow learning enhances survival of new neurons

Trace eyeblink conditioning in which a conditioned stimulus and unconditioned stimulus are separated by a gap, is hippocampal dependent and can rescue new neurons in the adult dentate gyrus from death (e.g., Beylin et al., 2001; Gould et al., 1999). Tasks requiring more training trials for reliable expression of the conditioned response are most effective in enhancing survival of neurons (Waddell & Shors, 2008). To dissociate hippocampal dependence from acquisition rate, we facilitated hippocampal-dependent trace eyeblink conditioning in two ways: a shorter trace interval and signaling the intertrial interval with a post-US cue. Trace conditioning with a shorter trace interval (250ms) requires an intact hippocampus, and acquisition is faster relative to rats trained with a 500ms trace interval (e.g., Weiss et al., 1999). Using excitotoxic hippocampal lesions, we confirmed that eyeblink conditioning with the 250 or 500ms trace interval is hippocampal dependent. However, training with the post-US cue was not hippocampal dependent. The majority of lesion rats in this condition reached criterion of conditioned responding. To determine whether hippocampal dependence is sufficient to rescue adult-generated neurons in the dentate gyrus, rats were injected with BrdU and trained in one of the three trace eyeblink arrangements one week later. Of these training procedures, only the 500ms trace interval enhanced survival of new cells; acquisition of this task proceeded slowly relative to the 250ms and post-US cue conditions. These data demonstrate that rate of acquisition and not hippocampal dependence determines the impact of learning on adult neurogenesis.

A neurotensin agonist and antagonist decrease and increase activity, respectively, but do not preclude discrete cue conditioning

There is evidence to suggest that neurotensin (NT) may enhance cognitive function. For example, in aversive trace conditioning, the NT agonist PD149163 selectively increased trace conditioning (Grimond-Billa, et al., 2008). The present study, therefore, examined the role of NT in associative learning, tested using an appetitive trace conditioning procedure (0-s or 10-s inter-stimulus-interval [ISI]) with a mixed frequency noise as a conditioned stimulus (CS) and food delivery as the unconditioned stimulus (UCS). The effects of an NT agonist (PD149163, 0.125 and 0.25 mg/kg, Experiment 1) and an NT antagonist (SR142948A, 0.01 and 0.1 mg/kg, Experiment 2) were compared. To take nonspecific effects of these compounds into account, conditioning to the CS was measured as a percentage of total responding, during UCS deliveries and in the inter-trial-interval (ITI). In both experiments, associative learning to the contiguously (0-s) presented CS was demonstrated, although there was a relative reduction in this learning under 0.125 mg/kg PD149163. Counter to prediction, the only effect on trace conditioning was some overall reduction in responding to the CS in the 10-s group conditioned under 0.25 mg/kg PD149163. The NT antagonist was without any effect on appetitive conditioning. However, these NT compounds were not ineffective: decreases and increases in responding in the ITI, ISI and during UCS deliveries seen under PD149163 and SR142948A were dissociable from effects on discrete cue conditioning.

Temporal-difference reinforcement learning with distributed representations

Temporal-difference (TD) algorithms have been proposed as models of reinforcement learning (RL). We examine two issues of distributed representation in these TD algorithms: distributed representations of belief and distributed discounting factors. Distributed representation of belief allows the believed state of the world to distribute across sets of equivalent states. Distributed exponential discounting factors produce hyperbolic discounting in the behavior of the agent itself. We examine these issues in the context of a TD RL model in which state-belief is distributed over a set of exponentially-discounting "micro-Agents", each of which has a separate discounting factor (gamma). Each microAgent maintains an independent hypothesis about the state of the world, and a separate value-estimate of taking actions within that hypothesized state. The overall agent thus instantiates a flexible representation of an evolving world-state. As with other TD models, the value-error (delta) signal within the model matches dopamine signals recorded from animals in standard conditioning reward-paradigms. The distributed representation of belief provides an explanation for the decrease in dopamine at the conditioned stimulus seen in overtrained animals, for the differences between trace and delay conditioning, and for transient bursts of dopamine seen at movement initiation. Because each microAgent also includes its own exponential discounting factor, the overall agent shows hyperbolic discounting, consistent with behavioral experiments.