Matthew D. McEchron

Hippocampectomy disrupts auditory trace fear conditioning and contextual fear conditioning in the rat

The hippocampus is believed to be an important structure for learning tasks that require temporal processing of information. The trace classical conditioning paradigm requires temporal processing because the conditioned stimulus (CS) and the unconditioned stimulus (US) are temporally separated by an empty trace interval. The present study sought to determine whether the hippocampus was necessary for rats to perform a classical trace fear conditioning task in which each of 10 trials consisted of an auditory tone CS (15‐s duration) followed by an empty 30‐s trace interval and then a fear‐producing floor‐shock US (0.5‐s duration). Several weeks prior to training, animals were anesthetized and given aspiration lesions of the neocortex (NEO; n = 6), hippocampus and overlying neocortex (HIPP; n = 7), or no lesions at all (control; n = 6). Approximately 24 h after trace conditioning, NEO and control animals showed a significant decrease in movement to a CS‐alone presentation that was indicative of a conditioned fear response. Animals in the HIPP group did not show conditioned fear responses to the CS alone, nor did a pseudoconditioning group (n = 7) that was trained with unpaired CSs and USs. Furthermore, all groups except the HIPP group showed conditioned fear responses to the original context in which they received shock USs. One week later, HIPP, NEO, and control animals received delay fear‐conditioning trials with no trace interval separating the CS and US. Six of seven HIPP animals could perform the delay version, but none could perform the trace version. This result suggests that the trace fear task is a reliable and useful model for examining the neural mechanisms of hippocampally dependent learning. Hippocampus 1998;8:638–646.

Cortical involvement in acquisition and extinction of trace eyeblink conditioning

Previous studies have implicated 2 cortical regions interconnected with the hippocampal formation, the retrosplenial cortex (RSC) and the medial prefrontal cortex (mPFC), as loci important for the acquisition of hippocampally dependent trace eyeblink conditioning. These loci have also been proposed to serve as long-term storage sites of task critical information. This study used lesions made prior to training to investigate the roles of the RSC, as well as the caudal and rostral subdivisions of the mPFC, in the acquisition and subsequent extinction of trace eyeblink conditioning in the rabbit. The caudal mPFC and rostral mPFC were shown to be critical for acquisition and extinction of the conditioned reflex, respectively. The data indicate that the RSC is not critical for acquisition or extinction of the trace conditioned reflex.

Single neurons in the medial prefrontal cortex of the rat exhibit tonic and phasic coding during trace fear conditioning

Trace fear conditioning is a learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single neuron activity was recorded from the prelimbic and infralimbic areas of the medial prefrontal cortex in rats during trace fear conditioning or nonassociative unpaired training. Prelimbic neurons showed learning-related increases in activity to the CS and US, whereas infralimbic neurons showed learning-related decreases in activity to these stimuli. A subset of prelimbic neurons exhibited sustained increases in activity during the trace interval. These sustained prelimbic responses may provide a bridging code that allows for overlapping representations of CS and US information within the trace fear conditioning circuit.

Hippocampal encoding of non-spatial trace conditioning

Trace eyeblink classical conditioning is a non‐spatial learning paradigm that requires an intact hippocampus. This task is hippocampus‐dependent because the auditory tone conditioned stimulus (CS) is temporally separated from the corneal airpuff unconditioned stimulus (US) by a 500‐ms trace interval. Our laboratory has performed a series of neurophysiological experiments that have examined the activity of pyramidal cells in the CA1 area of the hippocampus during trace eyeblink conditioning. We have found that the non‐spatial stimuli involved in this paradigm are encoded in the hippocampus in a logical order that is necessary for their association and the subsequent expression of behavioral learning. Although there were many profiles of single neurons responding to the CS‐US trial during training, the majority of the neurons showed an increase in activity to the airpuff‐US. Prior to learning, it appears that hippocampal cells and ensembles of cells were preferentially attending to the stimulus with immediate behavioral importance, the US. Hippocampal cells then began to respond to the associated neutral stimulus, the CS. Shortly thereafter, animals began to show increases in the behavioral expression of CRs. In some experiments, hippocampal neurons from aged animals exhibited impairments in the encoding of CS and US information. These aged animals were not able to associate these stimuli and acquire trace eyeblink CRs. Our findings along with the findings of other spatial learning studies, suggest that the hippocampus is involved in encoding information about discontiguous sets of stimuli, either spatial or nonspatial, especially early in the learning process. Hippocampus 1999;9:385–396.

Remodeling of hippocampal synapses after hippocampus-dependent associative learning

The aim of this study was to determine whether hippocampus‐dependent associative learning involves changes in the number and/or structure of hippocampal synapses. A behavioral paradigm of trace eyeblink conditioning was used. Young adult rabbits were given daily 80 trial sessions to a criterion of 80% conditioned responses in a session. During each trial, the conditioned (tone) and unconditioned (corneal airpuff) stimuli were presented with a stimulus‐free or trace interval of 500 msec. Control rabbits were pseudoconditioned by equal numbers of random presentations of the same stimuli. Brain tissue was taken for morphological analyses 24 hours after the last session. Synapses were examined in the stratum radiatum of hippocampal subfield CA1. Unbiased stereological methods were used to obtain estimates of the total number of synapses in this layer as well as the area of the postsynaptic density. The data showed that the total numbers of all synaptic contacts and various morphological subtypes of synapses did not change in conditioned animals. The area of the postsynaptic density, however, was significantly increased after conditioning in axospinous nonperforated synapses. This structural alteration may reflect an addition of signal transduction proteins (such as receptors and ion channels) and the transformation of postsynaptically silent synapses into functional ones. The findings of the present study indicate that cellular mechanisms of hippocampus‐dependent associative learning include the remodeling of existing hippocampal synapses. Further studies examining various time points along the learning curve are necessary to clarify the issue of whether these mechanisms also involve the formation of additional synaptic contacts. J. Comp. Neurol. 417:49–59, 2000. ©2000 Wiley‐Liss, Inc.

Neurotoxic lesions of the dorsal hippocampus disrupt auditory-cued trace heart rate (fear) conditioning in rabbits.

The first experiment in this study used the classical heart rate (HR) conditioning paradigm to determine if rabbits could associate an auditory conditioned stimulus (CS) and a fear‐producing shock‐unconditioned stimulus (US) separated by an empty 10‐s trace interval. Trace conditioned rabbits (n = 7) acquired significant bradycardiac conditioned HR responses on CS‐alone test trials during a single 35‐trial conditioning session. Control animals (n = 7) which received unpaired CSs and USs did not show HR conditioning. During a retention session of CS‐alone trials 24 h after the conditioning session, some trace‐conditioned animals showed conditioned HR responses immediately following CS onset (n = 3), while others showed responses appropriately timed to the US onset (n = 4) used in trace conditioning 24 h earlier. Thus, rabbits remember the duration of the long 10‐s trace interval 24 h after a single day of training. The second part of this study sought to determine if cells in the dorsal hippocampus play a role in trace HR conditioning. Rabbits were given bilateral ibotenic acid lesions in the neocortex (n = 7) or dorsal hippocampus (n = 8). During trace conditioning and retention, neocortical animals showed conditioned HR responses to the CS, whereas the hippocampal group showed no significant HR conditioning. One week after trace conditioning, the same animals received a delay HR conditioning session where no trace interval separated the CS and US. During delay conditioning, hippocampal animals showed significant conditioned HR responses to the CS that were similar to the neocortical group. Thus, the dorsal hippocampus plays a critical role in rabbit HR conditioning when the CS and US are separated by a 10‐s trace interval. This paradigm may be ideal for in vivo electrophysiological recording studies because rabbits are easily immobilized during the testing procedure, and learning occurs during a single day of training. Hippocampus 2000;10:739–751.

Perinatal nutritional iron deficiency permanently impairs hippocampus-dependent trace fear conditioning in rats

Abstract Many studies show that iron deficient (ID) children are at risk for poor cognitive development. This suggests that learning and cognitive centers in the brain, such as the hippocampus, may be compromised by developmental ID. The present study used a heart rate trace fear conditioning procedure in rats to show that perinatal nutritional ID impairs hippocampus-dependent learning. This procedure requires rats to associate a conditioned stimulus and a fearful unconditioned stimulus, which are separated by a trace interval. Rats were started on ID or control (CN) diets 10 days prior to birth, and learning was assessed on post natal day (PND)-28. The ID pups were impaired in trace fear conditioning, but an ID control group was not impaired in a non-trace basic fear conditioning procedure that does not depend on the hippocampus. Another group was switched from ID to CN diet on PND-31, and this group also showed impairments in trace fear conditioning when tested during early adulthood (i.e. PND-63). Separate control tests show that ID may produce skeletal motor deficits. The ID-induced learning impairments in this study, however, were not due to altered motor activity because learning was assessed using non-motor heart rate responses.

Single neurons in the dentate gyrus and CA1 of the hippocampus exhibit inverse patterns of encoding during trace fear conditioning.

Trace fear conditioning is a hippocampus-dependent learning task that requires the association of an auditory conditioned stimulus (CS) and a shock unconditioned stimulus (US) that are separated by a 20-s trace interval. Single-neuron activity was recorded simultaneously from the dentate gyrus (DG) and CA1 of rats during unpaired pseudoconditioning and subsequent trace fear conditioning. Single neurons in DG showed a progressive increase in learning-related activity to the CS and US across trace fear conditioning. Single neurons in CA1 showed an early increase in responding to the CS, which developed into a decrease in firing later in trace conditioning. Correlation analyses showed that DG and CA1 units exhibit inverse patterns of responding to the CS during trace fear conditioning.

Trace fear conditioning is reduced in the aging rat

Auditory trace fear conditioning is a hippocampus-dependent learning task that requires animals to associate an auditory conditioned stimulus (CS) and a fear-producing shock-unconditioned stimulus (US) that are separated by an empty 20-s trace interval. Previous studies have shown that aging impairs learning performance on hippocampus-dependent tasks. This study measured heart rate (HR) and freezing fear responses to determine if aging impairs hippocampus-dependent auditory trace fear conditioning in freely moving rats. Aging and Young rats received one long-trace fear conditioning session (10 trials). Each trial consisted of a tone-CS (5 s) and a shock-US separated by an empty 20-s trace interval. The next day rats received CS-alone retention trials. Young rats showed significantly larger HR and freezing responses on the initial CS-alone retention trials compared to the Aging rats. A control group of aging rats received fear conditioning trials with a short 1-s trace interval separating the CS and US. The Aging Short-Trace Group showed HR and freezing responses on the initial CS alone retention trials that were similar to the Young Long-Trace Group, but greater than the Aging Long-Trace Group. A second aging control group received unpaired CSs and USs, and showed no HR or freezing responses on CS-alone retention trials. These data show that HR and freezing are effective measures for detecting aging-related deficits in trace fear conditioning.

Perinatal nutritional iron deficiency reduces hippocampal synaptic transmission but does not impair short- or long-term synaptic plasticity.

Abstract Studies show that perinatal nutritional iron deficiency (ID) produces learning and memory impairments in humans and animals. This suggests that the functional physiology of learning and cognitive centers in the brain, such as the hippocampus, may be compromised by developmental ID. The present study used electrophysiological brain slice methods to examine multiple measures of hippocampal synaptic efficacy from rats that were subjected to perinatal ID diets or control (CN) diets. Measures of synaptic efficacy were obtained from the first and last synaptic regions of the hippocampal tri-synaptic loop (i.e. the dentate gyrus (DG) and CA1). Rats were placed on ID or CN diets on gestational day 11, and hippocampal brain slices were prepared between postnatal day 25 and 37. Results show that ID slices were not impaired in short-term (i.e. paired-pulse facilitation (PPF)) or long-term measures (i.e. long-term potentiation (LTP)) of synaptic plasticity in either the DG or CA1 areas. Input–output (IO) measures showed that synaptic transmission was reduced in both of these areas in the ID slices when compared with the CN slices. This suggests that ID-induced learning deficits may be the result of reductions in synaptic transmission throughout the hippocampus, and possibly in other learning and memory centers.