Hippocampal lesions in rats differentially affect long- and short-trace eyeblink conditioning

Impact of Fornix Lesions on Tone-Off Delay- vs Tone-On Trace- Eyeblink Conditioning in Rats

Research has shown differences in the neural mechanisms that support trace and delay eyeblink conditioning. The present experiment furthered this investigation by examining the effect of electrolytic fornix lesions on acquisition of trace and delay eyeblink conditioning in the rat. Importantly, the conditioned stimulus (CS) for trace conditioning was a standard tone-on cue, and the CS for delay conditioning was either a tone-off or tone-on CS. The results showed that fornix lesions impaired trace-, but not delay conditioning in rats trained with the tone-on CS or tone-off CS. The findings are consistent with previous studies that found trace-, but not delay eyeblink conditioning is a hippocampal dependent form of associative learning. Our results also indicate that the neural pathways for tone-off delay conditioning and tone-on trace conditioning are different, even though the structural composition of a tone-off CS and the trace conditioning interval are the same cue (i.e., the absence of sound). These findings indicate that the absence of a sensory cue (i.e., tone-off CS) and the presence of a sensory cue (i.e., tone-on CS) have equivalent associative value and effectiveness for engaging the neural pathways that support delay eyeblink conditioning.

The little brain and the seahorse: Cerebellar-hippocampal interactions

There is a growing appreciation for the cerebellum beyond its role in motor function and accumulating evidence that the cerebellum and hippocampus interact across a range of brain states and behaviors. Acute and chronic manipulations, simultaneous recordings, and imaging studies together indicate coordinated coactivation and a bidirectional functional connectivity relevant for various physiological functions, including spatiotemporal processing. This bidirectional functional connectivity is likely supported by multiple circuit paths. It is also important in temporal lobe epilepsy: the cerebellum is impacted by seizures and epilepsy, and modulation of cerebellar circuitry can be an effective strategy to inhibit hippocampal seizures. This review highlights some of the recent key hippobellum literature.

Hippocampal Interneurons are Required for Trace Eyeblink Conditioning in Mice

While the hippocampus has been implicated in supporting the association among time-separated events, the underlying cellular mechanisms have not been fully clarified. Here, we combined in vivo multi-channel recording and optogenetics to investigate the activity of hippocampal interneurons in freely-moving mice performing a trace eyeblink conditioning (tEBC) task. We found that the hippocampal interneurons exhibited conditioned stimulus (CS)-evoked sustained activity, which predicted the performance of conditioned eyeblink responses (CRs) in the early acquisition of the tEBC. Consistent with this, greater proportions of hippocampal pyramidal cells showed CS-evoked decreased activity in the early acquisition of the tEBC. Moreover, optogenetic suppression of the sustained activity in hippocampal interneurons severely impaired acquisition of the tEBC. In contrast, suppression of the sustained activity of hippocampal interneurons had no effect on the performance of well-learned CRs. Our findings highlight the role of hippocampal interneurons in the tEBC, and point to a potential cellular mechanism subserving associative learning.

A method for combining multiple-units readout of optogenetic control with natural stimulation-evoked eyeblink conditioning in freely-moving mice

A growing pool of transgenic mice expressing Cre-recombinases, together with Cre-dependent opsin viruses, provide good tools to manipulate specific neural circuits related to eyeblink conditioning (EBC). However, currently available methods do not enable to get fast and precise readout of optogenetic control when the freely-moving mice are receiving EBC training. In the current study, we describe a laser diode (LD)-optical fiber (OF)-Tetrode assembly that allows for simultaneous multiple units recording and optical stimulation. Since the numbers of various cables that require to be connected are minimized, the LD-OF-Tetrode assembly can be combined with CS-US delivery apparatus for revealing the effects of optical stimulation on EBC in freely- moving mice. Moreover, this combination of techniques can be utilized to optogenetically intervene in hippocampal neuronal activities during the post-conditioning sleep in a closed-loop manner. This novel device thus enhances our ability to explore how specific neuronal assembly contributes to associative motor memory in vivo.

Neural representations of time-linked memory

Many cognitive processes, such as episodic memory and decision making, rely on the ability to form associations between two events that occur separately in time. The formation of such temporal associations depends on neural representations of three types of information: what has been presented (trace holding), what will follow (temporal expectation), and when the following event will occur (explicit timing). The present review seeks to link these representations with firing patterns of single neurons recorded while rodents and non-human primates associate stimuli, outcomes, and motor responses over time intervals. Across these studies, two distinct firing patterns were observed in the hippocampus, neocortex, and striatum: some neurons change firing rates during or shortly after the stimulus presentation and sustain the firing rate stably or sidlingly during the subsequent intervals (tonic firings). Other neurons transiently change firing rates during a specific moment within the time intervals (phasic firings), and as a group, they form a sequential firing pattern that covers the entire interval. Clever task designs used in some of these studies collectively provide evidence that both tonic and phasic firing responses represent trace holding, temporal expectation, and explicit timing. Subsequently, we applied machine-learning based classification approaches to the two firing patterns within the same dataset collected from rat medial prefrontal cortex during trace eyeblink conditioning. This quantitative analysis revealed that phasic-firing patterns showed greater selectivity for stimulus identity and temporal position than tonic-firing patterns. Our summary illuminates distributed neural representations of temporal association in the forebrain and generates several ideas for future investigations.

A neural model of normal and abnormal learning and memory consolidation: adaptively timed conditioning, hippocampus, amnesia, neurotrophins, and consciousness

How do the hippocampus and amygdala interact with thalamocortical systems to regulate cognitive and cognitive-emotional learning? Why do lesions of thalamus, amygdala, hippocampus, and cortex have differential effects depending on the phase of learning when they occur? In particular, why is the hippocampus typically needed for trace conditioning, but not delay conditioning, and what do the exceptions reveal? Why do amygdala lesions made before or immediately after training decelerate conditioning while those made later do not? Why do thalamic or sensory cortical lesions degrade trace conditioning more than delay conditioning? Why do hippocampal lesions during trace conditioning experiments degrade recent but not temporally remote learning? Why do orbitofrontal cortical lesions degrade temporally remote but not recent or post-lesion learning? How is temporally graded amnesia caused by ablation of prefrontal cortex after memory consolidation? How are attention and consciousness linked during conditioning? How do neurotrophins, notably brain-derived neurotrophic factor (BDNF), influence memory formation and consolidation? Is there a common output path for learned performance? A neural model proposes a unified answer to these questions that overcome problems of alternative memory models.

The Anatomy and Physiology of Eyeblink Classical Conditioning

This chapter reviews the past research toward identifying the brain circuit and its computation underlying the associative memory in eyeblink classical conditioning. In the standard delay eyeblink conditioning paradigm, the conditioned stimulus (CS) and eyeblink-eliciting unconditioned stimulus (US) converge in the cerebellar cortex and interpositus nucleus (IPN) through the pontine nuclei and inferior olivary nucleus. Repeated pairings of CS and US modify synaptic weights in the cerebellar cortex and IPN, enabling IPN neurons to activate the red nucleus and generate the conditioned response (CR). In a variant of the standard paradigm, trace eyeblink conditioning, the CS and US are separated by a brief stimulus-free trace interval. Acquisition in trace eyeblink conditioning depends on several forebrain regions, including the hippocampus and medial prefrontal cortex as well as the cerebellar-brainstem circuit. Details of computations taking place in these regions remain unclear; however, recent evidence supports a view that the forebrain encodes a temporal sequence of the CS, trace interval, and US in a specific environmental context and signals the cerebellar-brainstem circuit to execute the CR when the US is likely to occur. Together, delay eyeblink conditioning represents one of the most successful cases of understanding the neural substrates of long-term memory in mammals, while trace eyeblink conditioning demonstrates its utility for uncovering detailed computations in the whole brain network underlying long-term memory.

The impact of hippocampal lesions on trace-eyeblink conditioning and forebrain-cerebellar interactions

Behavioral Neuroscience published a pivotal paper by Moyer, Deyo, and Disterhoft (1990) 25 years ago that described the impaired acquisition of trace-eyeblink conditioning in rabbits with complete removal of the hippocampus. As part of the Behavioral Neuroscience celebration commemorating the 30th anniversary of the journal, we reflect upon the impact of that study on understanding the role of the hippocampus, forebrain, and forebrain-cerebellar interactions that mediate acquisition and retention of trace-conditioned responses, and of declarative memory more globally. We discuss the expansion of the conditioning paradigm to species other than the rabbit, the heterogeneity of responses among hippocampal neurons during trace conditioning, the responsivity of hippocampal neurons following consolidation of conditioning, the role of awareness in conditioning, how blink conditioning can be used as a translational tool by assaying potential therapeutics for cognitive enhancement, how trace and delay classical conditioning may be used to investigate neurological disorders including Alzheimer's disease and schizophrenia, and how the 2 paradigms may be used to understand the relationship between declarative (explicit) and nondeclarative (implicit) memory systems.

Predictive nature of prefrontal theta oscillation on the performance of trace conditioned eyeblink responses in guinea pigs

Stimulus-evoked theta oscillations are observed in the medial prefrontal cortex (mPFC) when executing a variety of learning tasks. Here, we aimed to further determine whether spontaneous theta-band (5.0-10.0 Hz) oscillations in the mPFC predicted the subsequent behavioral performance in trace eyeblink conditioning (TEBC), in which the conditioned stimulus (CS) was separated from the unconditioned stimulus (US) by 500 ms trace interval. By recording local field potentials (LFP) signals in the guinea pigs performing the TEBC task, we found that, a higher mPFC relative theta ratio [theta/(delta+beta)] during the baseline (850-ms period prior to the onset of the CS) was predictive of higher magnitude and more adaptive timing rather than faster acquisition of trace conditioned eyeblink responses (CR). However, the prediction of baseline mPFC theta activity was time-limited to the well-learning stage. Additionally, the relative power of mPFC theta activity did not correlate with the CR performance if the trace interval between the CS and the US was shortened to 100 ms. These results suggest that the brain state in which the baseline mPFC theta activity is present or absent is detrimental for the subsequent performance of trace CRs especially when the asymptotic learning state is achieved.

Timing and expectation of reward: a neuro-computational model of the afferents to the ventral tegmental area

Neural activity in dopaminergic areas such as the ventral tegmental area is influenced by timing processes, in particular by the temporal expectation of rewards during Pavlovian conditioning. Receipt of a reward at the expected time allows to compute reward-prediction errors which can drive learning in motor or cognitive structures. Reciprocally, dopamine plays an important role in the timing of external events. Several models of the dopaminergic system exist, but the substrate of temporal learning is rather unclear. In this article, we propose a neuro-computational model of the afferent network to the ventral tegmental area, including the lateral hypothalamus, the pedunculopontine nucleus, the amygdala, the ventromedial prefrontal cortex, the ventral basal ganglia (including the nucleus accumbens and the ventral pallidum), as well as the lateral habenula and the rostromedial tegmental nucleus. Based on a plausible connectivity and realistic learning rules, this neuro-computational model reproduces several experimental observations, such as the progressive cancelation of dopaminergic bursts at reward delivery, the appearance of bursts at the onset of reward-predicting cues or the influence of reward magnitude on activity in the amygdala and ventral tegmental area. While associative learning occurs primarily in the amygdala, learning of the temporal relationship between the cue and the associated reward is implemented as a dopamine-modulated coincidence detection mechanism in the nucleus accumbens.

Modest elevation of corticosterone in preweanling rats impairs subsequent trace eyeblink conditioning during the juvenile period

The hippocampus is known to be especially sensitive to the deleterious effects of glucocorticoids. Previously, we administered exogenous corticosterone, the major stress-related glucocorticoid in rats, to young developing rats using subcutaneous pellets which produced high pharmacological levels of circulating corticosterone as well as a sex-specific learning deficit for males on a hippocampus-mediated associative learning task, trace eyeblink conditioning [1]. The present study evaluated the effects of corticosterone administered at a physiologically-relevant level by a more consistent release method, osmotic mini-pumps. Pumps were implanted subcutaneously in 15-day-old rats to deliver either corticosterone or the vehicle control (PEG) at a rate of 1 μl/h over 3 days. On Day 28, learning was assessed using trace eyeblink conditioning. The results of the present experiment revealed that a small elevation in corticosterone (11.77 μg/dl versus 6.02 μg/dl for controls) within the normal physiological range impaired learning as determined by a significantly lower percentage and amplitude of total conditioned responses (CRs) and lower amplitude of adaptive responses relative to the control group. There were no significant differences in response timing, although the corticosterone group tended to produce CRs which began and peaked a little later than controls. These findings indicate that even modest elevations of corticosterone for several days can produce later impairments on this hippocampally mediated learning task.

Why trace and delay conditioning are sometimes (but not always) hippocampal dependent: a computational model

A recurrent-network model provides a unified account of the hippocampal region in mediating the representation of temporal information in classical eyeblink conditioning. Much empirical research is consistent with a general conclusion that delay conditioning (in which the conditioned stimulus CS and unconditioned stimulus US overlap and co-terminate) is independent of the hippocampal system, while trace conditioning (in which the CS terminates before US onset) depends on the hippocampus. However, recent studies show that, under some circumstances, delay conditioning can be hippocampal-dependent and trace conditioning can be spared following hippocampal lesion. Here, we present an extension of our prior trial-level models of hippocampal function and stimulus representation that can explain these findings within a unified framework. Specifically, the current model includes adaptive recurrent collateral connections that aid in the representation of intra-trial temporal information. With this model, as in our prior models, we argue that the hippocampus is not specialized for conditioned response timing, but rather is a general-purpose system that learns to predict the next state of all stimuli given the current state of variables encoded by activity in recurrent collaterals. As such, the model correctly predicts that hippocampal involvement in classical conditioning should be critical not only when there is an intervening trace interval, but also when there is a long delay between CS onset and US onset. Our model simulates empirical data from many variants of classical conditioning, including delay and trace paradigms in which the length of the CS, the inter-stimulus interval, or the trace interval is varied. Finally, we discuss model limitations, future directions, and several novel empirical predictions of this temporal processing model of hippocampal function and learning.

Changes of synaptic ultrastructure in the guinea pig interpositus nuclei associate with response magnitude and timing after trace eyeblink conditioning

Learning-induced changes of synaptic ultrastructure have long been proposed as a mechanism that may contribute to support memory formation. Although recent studies have demonstrated that the interpositus nuclei (IN) play critical role in acquisition and retention of trace conditioned eyeblink responses (CRs), there is now limited evidence associating trace eyeblink conditioning with changes of synaptic ultrastructure in the IN. Here, we investigated this issue using a transmission electron microscope. Adult guinea pigs were randomly allocated to either a trace-paired, delay-paired, unpaired or exposure-only condition. The IN tissue was taken for morphological analysis 1h after the completion of the tenth training session. Serial section analysis of synaptic ultrastructure revealed that trace eyeblink conditioning induced increases in the thickness of excitatory PSD. Classification of the synapses into shape subtypes indicated that the increased thickness of excitatory PSD was mainly attributable to increase in the concave- and convex-shaped synapses. On the contrary, trace eyeblink conditioning resulted in decreases in the thickness of inhibitory PSD. Specifically, these significant changes of PSD thickness were limited to occur in the animals with good behavioral performance. Further analysis of correlations between the trace CR performance and synaptic ultrastructural modifications showed that the thickness of excitatory PSD within the IN correlated with the peak amplitude of trace CRs, whereas the thickness of inhibitory PSD correlated with the onset latency. The present findings suggest that trace eyeblink conditioning induces structural plasticity in the IN, which may play a crucial role in acquiring and executing adaptive eyeblink movements.

Catecholaminergic depletion in nucleus accumbens enhances trace conditioning

PURPOSE

To examine the effect of dopamine depletion in nucleus accumbens on trace conditioning; to distinguish the role of core and shell sub-regions, as far as possible.

MATERIAL/METHODS

6-hydroxydopamine was used to lesion dopamine terminals within the core and shell accumbens. Experiment 1 assessed conditioning to a tone conditioned stimulus that had previously been paired with footshock (unconditioned stimulus) at a 30s trace interval. Experiment 2 subsequently assessed contiguous conditioning (at 0s trace) using a light conditioned stimulus directly followed by the unconditioned stimulus.

RESULTS

Both sham and shell-lesioned animals showed the normal trace effect of reduced conditioning to the trace conditioned stimulus but 6-hydroxydopamine injections targeted on the core subregion of the nucleus accumbens abolished this effect and enhanced conditioning to the trace conditioned stimulus. However, the depletion produced by this lesion placement extended to the shell. In Experiment 2 (at 0s trace), there was no effect of either lesion placement as all animals showed comparable levels of conditioning to the light conditioned stimulus. Neurochemical analysis across core, shell and comparison regions showed some effects on noradrenalin as well as dopamine.

CONCLUSIONS

The pattern of changes in noradrenalin did not systematically relate to the observed behavioural changes after core injections. The pattern of changes in dopamine suggested that depletion in core mediated the increased conditioning to the trace conditioned stimulus seen in the present study. However, the comparison depletion restricted to the shell subregion was less substantial, and a role for secondarily affected brain regions cannot be excluded.

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.

Associative and non-associative blinking in classically conditioned adult rats

Over the last several years, a growing number of investigators have begun using the rat in classical eyeblink conditioning experiments, yet relatively few parametric studies have been done to examine the nature of conditioning in this species. We report here a parametric analysis of classical eyeblink conditioning in the adult rat using two conditioned stimulus (CS) modalities (light or tone) and three interstimulus intervals (ISI; 280, 580, or 880 ms). Rats trained at the shortest ISI generated the highest percentage of conditioned eyeblink responses (CRs) by the end of training. At the two longer ISIs, rats trained with the tone CS produced unusually high CR percentages over the first few acquisition sessions, relative to rats trained with the light CS. Experiment 2 assessed non-associative blink rates in response to presentations of the light or tone, in the absence of the US, at the same ISI durations used in paired conditioning. Significantly more blinks occurred with longer than shorter duration lights or tones. A higher blink rate was also recorded at all three durations during the early tone-alone sessions. The results suggest that early in classical eyeblink conditioning, rats trained with a tone CS may emit a high number of non-associative blinks, thereby inflating the CR frequency reported at this stage of training.

Eyeblink conditioning during an interstimulus interval switch in rabbits (Oryctolagus cuniculus) using picrotoxin to disrupt cerebellar cortical input to the interpositus nucleus

The role of the cerebellar cortex in eyeblink classical conditioning remains unclear. Experimental manipulations that disrupt the normal function impair learning to various degrees, and task parameters may be important factors in determining the severity of impairment. This study examined the role of cerebellar cortex in eyeblink conditioning under conditioned stimulus?unconditioned stimulus intervals known to be optimal or nonoptimal for learning. Using infusions of picrotoxin to the interpositus nucleus of the rabbit cerebellum, the authors pharmacologically disrupted input from the cerebellar cortex while training with an interstimulus interval (ISI)-switch procedure. One group of rabbits (Oryctolagus cuniculus) was 1st trained with a 250-ms ISI (optimal) and then switched to a 750-ms ISI (nonoptimal). A 2nd group was trained in the opposite order. The most striking effect was that picrotoxin-treated rabbits initially trained with a 250-ms ISI learned comparably to controls, but those initially trained with a 750-ms ISI were severely impaired. These results suggest that functional input from cerebellar cortex becomes increasingly important for the interpositus nucleus to learn delay eyeblink conditioning as the ISI departs from an optimal interval.