Evidence for synaptic potentiation in a cortical network during learning

An examination of the relations between hippocampal long-term potentiation, kindling, afterdischarge, and place learning in the water maze

Two approaches were used to study the relations between the acquisition of place learning in the Morris water maze and long-term potentiation (LTP), kindling, and afterdischarge (AD). In the first, the possibility of behavioral LTP in the dentate gyrus field potential evoked by stimulation of the perforant path was evaluated in rats that showed robust place learning in the water maze. There was no effect of place learning on the field potential, and field potential measures did not correlate with place learning acquisition measures. In the second approach, the effect of bilateral saturation of LTP on subsequent place learning in the water maze task, begun within 5 minutes of the last LTP session, was evaluated. The effect of kindled seizures evoked bilaterally from the perforant path, or of a single unilateral AD, on acquisition of the water maze task (begun within 10 minutes) were also evaluated. Bilateral LTP saturation did not affect place learning, and the bilateral LTP group learned as readily as controls. In contrast, the kindled and AD groups were severely impaired in their performance of the place learning task. A second day of training in the water maze without any further electrical stimulation indicated that these groups had acquired considerable information on the first day of maze training and were not distinguishable from controls on the second day of training. This indicated that the deficit in these groups on the first day of training was temporary and likely resulted from a temporary perturbation of normal brain function due to the seizures. The results indicate that bilateral saturation of LTP in the dentate gyrus does not affect place learning in the water maze. They also indicate that recent hippocampal seizures, but not kindling, disrupt place learning in this task.

Long-term potentiation in rat piriform cortex following discrimination learning

The behavioral conditions for induction of long-term potentiation (LTP) elicited by unilateral patterned electrical stimulation of the lateral olfactory tract (LOT) was studied in piriform cortex. A group of animals was trained to discriminate two natural odors while another group was trained to discriminate a patterned stimulation (bursts of 4 pulses at 100 Hz repeated at 160-ms intervals) used as an olfactory cue, versus a natural odor. Both groups were successful in the discrimination and no statistical significant difference was observed in behavioral data between these two groups on series of 5 successive daily training sessions. With animals trained to perform the task with the artificial cue, monosynaptic responses evoked by single pulse stimulation of the LOT were collected, prior to and just after each of the successive training sessions. Comparisons with behavioral data collected at the beginning and the end of a training session revealed that the population synaptic responses increased with the percentage of correct responses performed by the animals. This increase (LTP) was progressive and present only when significant discrimination between the two cues was observed. A positive correlation was found between the increase in monosynaptic responses and the level of performance. Responses elicited by control electrodes were slightly depressed at the end of the discrimination learning series. In addition, when a group of naive animals was pseudoconditioned, giving the patterned electrical stimulation for the same number of sessions but without any olfactory training, no LTP was recorded.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors regulating the magnitude of long-term potentiation induced by theta pattern stimulation

Electrical stimulation patterned after the hippocampal theta rhythm produces a robust and stable long-term potentiation (LTP) effect. Pharmacological manipulations were used in the present studies in an effort to relate characteristics of the responses occurring during theta stimulation to the magnitude of potentiation which follows it. Comparisons were made using five or ten bursts of stimulation which respectively induce sub-maximal or near maximal degrees of LTP. DPCPX, a drug that increases release by blocking adenosine A1 receptors, was used to enhance the depolarization produced by individual theta bursts. This resulted in a marked increase in the amount of stable LTP induced by five theta bursts but did not affect that resulting from ten bursts. This finding suggested that depolarization occurring during a burst response influences per burst potentiation but not the ceiling on maximum LTP. Aniracetam, a nootropic drug that enhances responses via an action on glutamate (AMPA) receptors, was used to test this conclusion. Like DPCPX, aniracetam increased the size of the burst response and enhanced the degree of LTP caused by five but not ten theta bursts. Forskolin, an activator of adenylate cyclase, was used to test the effects of blocking the hyperpolarization normally present between theta bursts on the induction of LTP. The drug augmented the degree of LTP resulting from five theta bursts and, in contrast to DPCPX and aniracetam, nearly doubled that obtained with ten bursts. Thus the drug affected both per burst potentiation and the ceiling on LTP. These results are discussed in terms of an hypothesis in which the magnitude of NMDA receptor mediated currents affects the degree of potentiation produced by individual theta bursts while the duration of the currents is related to the limit on the maximum LTP induced by a series of bursts. The possible implications of the findings for learning are also considered.

Benzodiazepines block long-term potentiation in slices of hippocampus and piriform cortex

The effects of two benzodiazepines, diazepam and triazolam, on long-term potentiation were tested in slices of hippocampus and piriform cortex. The drugs had little influence on baseline synaptic responses but both were very effective in blocking LTP elicited by theta pattern stimulation. The effects were fully reversible upon washout. Diazepam reduced the increase in burst responses that occurs during theta stimulation and thus appears to interfere with the initial triggering events for long-term potentiation. This may reflect the enhancing action of the drug on GABA-mediated inhibitory potentials. Triazolam did not detectably change the burst responses elicited by theta pattern stimulation. Experiments with slices of piriform cortex indicated that triazolam also failed to disrupt the development of long-term potentiation but instead caused the potentiation to decay back to baseline in 15-30 min. Triazolam thus seems to act on the mechanisms that stabilize long-term potentiation. These results provide a possible explanation for the amnestic effects of benzodiazepines in humans and animals and support the hypothesis that long-term potentiation contributes to memory encoding.

Variations in Synaptic Plasticity and Types of Memory in Corticohippocampal Networks

If Synaptic long-term potentiation (LTP) represents a memory storage mechanism, its induction and expression characteristics may constitute rules governing encoding and read-out of memory in cortical circuitry, The presence of variants of the LTP effect in different anatomical networks provides grounds for predictions about the types of memory operations to which potentiation contributes. Computer modeling studies incorporating the complex rules for LTP induction and the characteristics of expressed potentiation can be used to make such predictions specific. We review ttie types of synaptic plasticity found in the successive stages of the corticohippocampal pathway, and present results indicating that LTP does participate in definably different forms of memory, suggesting a classification of memory types differing somewhat from categories deduced from behavioral studies. Specifically, the results suggest that subtypes of memory operate serially, in an “assembly line” of specialized functions, each of which adds a unique aspect to the processing of memories. The effects of lesions on the encoding versus expression of memory can be interpreted from the perspective of this hypothesis.

Linear relationship between the maintenance of hippocampal long-term potentiation and retention of an associative memory

The hypothesis that the maintenance or decay of an associative memory trace after an extended retention interval is a function of the residual strength of the synapses originally strengthened during learning was examined in a classical conditioning paradigm in which high-frequency stimulation of a hippocampal input--the medial perforant path--served as a conditioned stimulus. Rats received perforant path stimulus-foot shock pairings while engaged in a previously acquired food-motivated lever-pressing task. Conditioned suppression of lever pressing was the behavioral measure of learning and retention of the association. Stimulus trains to the perforant path at an intensity above the threshold for eliciting a population spike induced long-term potentiation of synaptic transmission in the dentate gyrus. Synaptic potentials recorded extracellularly in the dentate gyrus were subsequently monitored for 31 days to examine quantitatively the decay of synaptic potentiation, a period after which retention of the learned association was assessed. All rats learned the association to a similar extent and displayed equivalent amounts of long-term potentiation by the end of conditioning. A slowly decaying function of synaptic potentiation was observed in remembering rats, i.e., rats with high retention performance after the 31-day learning-to-retention interval, while forgetting was associated with a rapid decay of long-term potentiation. Behavioral performance at the long-term memory test was linearly correlated with the amplitude of long-term potentiation maintained just prior to the retention test. The results favor the hypothesis that long-term associative memory depends, at least in part, on the maintenance of elevated synaptic strengths in the pathway activated during learning and suggest a role for the lasting component of long-term potentiation in the maintenance of memory.

Triggers and substrates of hippocampal synaptic plasticity

It is widely assumed that behavioral learning reflects adaptive properties of the neuronal networks underlying behavior. Adaptive properties of networks in turn arise from the existence of biochemical mechanisms that regulate the efficacy of synaptic transmission. Considerable progress has been made in the elucidation of the mechanisms involved in synaptic plasticity at central synapses and especially those responsible for the phenomenon of long-term potentiation (LTP) of synaptic transmission in hippocampus. While the nature and the timing requirements of the triggering steps are reasonably well known, there is still a lot of uncertainty concerning the mechanisms responsible for the long-term changes. Several biochemical processes have been proposed to play critical roles in promoting long-lasting modifications of synaptic efficacy. This review examines first the triggers that are necessary to produce LTP in the hippocampus and then the different biochemical processes that have been considered to participate in the maintenance of LTP. Finally, we examine the relationships between LTP and behavioral learning.

Kindling-induced potentiation in the piriform cortex

At intensities sufficient to induce epileptiform afterdischarges, repeated electrical stimulation of limbic structures can lead to the development of permanent increases in the strength of the epileptiform response (kindling). Field potentials evoked by pulse stimulation are also increased in amplitude in a number of forebrain pathways following kindling. This kindling-induced potentiation effect is similar in many respects to the 'long-term potentiation' (LTP) effect which is produced by non-epileptogenic stimulation. There are, however, some interesting differences. For example, kindling-induced potentiation can far outlast LTP. In these experiments, we attempted to determine the longevity of the kindling-induced potentiation of the response evoked in the piriform cortex by olfactory bulb stimulation, following olfactory bulb kindling. This system was targeted because both the olfactory bulb and the piriform cortex are highly reactive kindling sites. In addition, we used the paired pulse technique to monitor facilitation and inhibition in this system. Kindling was found to induce a potentiation in the piriform field potential that lasted for at least 3 months (the period of the experiment) with little or no decay. Kindling also produced a decrease in paired pulse facilitation. In some animals the net facilitation was changed to a net depression. These results are consistent with the interpretation that kindling produces an increase in recurrent inhibition in the piriform cortex. The paired pulse measures, however, returned to near baseline levels over the 3-month test period.

Higher olfactory processes: perceptual learning and memory

The past year has seen several important findings emerge from studies of higher olfactory processes. The identification of synaptic long-term potentiation in the olfactory cortex, induced via repetitive burst stimulation at the theta rhythm, and physiological activity patterns associated with learning, some of which mimic long-term potentiation induction patterns, have suggested relationships between rhythmic activity, behavioral learning and synaptic plasticity. In addition, the construction of computational models of the olfactory bulb and cortex have generated testable behavioral and physiological predictions which have been supported by experimental evidence.

Posttetanic and frequency potentiation in slices of rat olfactory cortex

The electrical tetanization of the lateral olfactory tract at a frequency of 30/sec for 15 sec elicited the development of posttetanic potentiation of populational EPSP and IPSP in surviving slices of rat olfactory cortex. The stimulation of the lateral olfactory tract by series of stimuli at a constant frequency of 10/sec and with intervals of 4-5 sec between series facilitates the emergence of the phenomenon of frequency potentiation. The data obtained indicate that such forms of functional plasticity as posttetanic and frequency potentiation are manifested in the pyriform cortex.

Effects of the novel NMDA antagonist, NPC 12626, on long-term potentiation, learning and memory

NPC 12626 (2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid), a newly developed drug which crosses the blood-brain barrier, is a competitive antagonist of N-methyl-D-aspartate receptors. In Experiment I, the effects of NPC 12626 on perforant path - dentate gyrus LTP were tested. NPC 12626 (100 mg/kg, i.p.), injected 150 min prior to tetanization, prevented potentiation of the EPSP slope and population spike amplitude. EPSP-spike potentiation was also prevented. Post-tetanus administration was ineffective. In Experiment II, mice were injected with NPC 12626 (35 mg/kg, i.p.) or saline 35 min prior to spontaneous alternation testing. NPC 12626 significantly decreased alternation rates, but did not affect turn bias or the mean delay between arm entries. This pattern of results may reflect impaired learning or memory. In Experiment III, mice were tested on an inhibitory avoidance task. NPC 12626 (35 mg/kg, i.p.), administered before but not after training, significantly impaired performance. When the drug was administered before training as well as before testing, performance was similarly impaired, indicating that the observed deficits were not attributable to state-dependent learning. Pre-test injections were ineffective. Overall, these results support the hypothesis that some forms of learning require the participation of NMDA receptors and that this participation is largely limited to acquisition processes. In addition, these results point to the utility of peripherally administered NPC 12626 as a tool with which to examine the involvement of NMDA receptors in LTP and learning.

Characterization and anatomical distribution of selective long-term potentiation in the olfactory forebrain

High-frequency stimulation of the granule cell layer of the olfactory bulb (OB) has previously been shown to result in a form of long-term potentiation in the piriform cortex (PC) that is selective to late components of the potential evoked in the PC58. This phenomenon was explored in male Long-Evans rats with chronically implanted electrodes by recording potentials evoked in the OB and in various sites in the ipsilateral and contralateral PC before and after repeated high-frequency stimulation of the OB. Recordings at all sites exhibited a gradually developing potentiation that was selective to late components of the evoked potential. In the OB and ipsilateral PC this potentiation had an overt long-term component that lasted for days, and all sites exhibited a latent potentiation that enabled the reestablishment of substantial levels of potentiation by mild patterns of stimulation that had no effect in control animals. No potentiation of the population EPSP representing input from the lateral olfactory tract to the PC was seen. Available evidence concerning the neuronal elements activated by the stimulation and the neuronal events likely to underlie the potentiated components of the evoked potentials suggests that this potentiation may represent an enhancement of inhibitory interactions within the PC and between the PC and OB.

Evaluation of memory dysfunction following experimental brain injury using the Morris water maze

Memory dysfunction, a common clinical feature of traumatic brain injury (TBI), is thought to be related to secondary damage of key anatomic structures in the brain, including the hippocampus. In the present study, we have characterized and evaluated a novel experimental paradigm using the Morris water maze (MWM) technique, to measure post-TBI memory retention after lateral (parasagittal) fluid percussion (FP) brain injury in rats. Male Sprague-Dawley rats (n = 37) received a total of 20 training trials over 2 days in the MWM. Two and a half hours after the last training trial, the animals received FP brain injury of moderate severity (2.3 atmospheres, n = 12), high severity (2.6 atm, n = 13), or no injury (n = 12). Forty-two hours after FP brain injury, we observed a highly sufficient memory dysfunction in animals from both injury groups compared to the uninjured group (p less than 0.001). The degree of memory dysfunction was found to be directly related to the severity of injury, with the high severity group scoring significantly worse than the moderately injured group (p = 0.15). In addition, hippocampal cell loss was observed after brain injury, but only unilaterally. These data suggest that lateral FP brain injury causes memory dysfunction possibly related to concurrent hippocampal cell loss and that posttraumatic memory deficits may be sensitively quantitated using the memory testing paradigm described.

Evidence for the Involvement of Rat Olfactory Bulb in Processes Supporting Long-Term Olfactory Memory

Current advances in the neurobiology of learning and memory suggest the existence of experience-induced plasticity in sensorial pathways conveying relevant information to higher integrative brain structures. For instance, olfactory learning is known to induce long-lasting modifications of neural activity at the level of the first relay structure of the olfactory system, the olfactory bulb. The observed forms of plasticity depend on the action exerted during learning by ascending neuromodulatory systems, such as the noradrenergic (NA) system originating from the locus ceruleus. This study was aimed at investigating the importance of olfactory bulb plasticity in learning and retention of an olfactory task. In a daily training schedule animals had to learn to use multi-site electrical stimulation patterns of the olfactory bulb as discriminative cues for choosing between a palatable and a nonpalatable solution. We first examined the effects of a continuous intrabulbar infusion of propranolol (a beta-NA receptor antagonist) carried out during the learning period. We found that this treatment neither impaired the retention of a previously learned task nor the learning of a new task. However, the animals presented a severe deficit in long-term retention (>5 days) of the task learned under perfusion. Unexpectedly, this effect cannot be ascribed to a selective blockade of beta-NA receptors since infusion of the drug vehicle (saline-ascorbate) produced exactly the same deficit while a saline solution remained without effect. A final experiment showed that the selective deficit in long-term retention was not observed when the infusion of the saline-ascorbate solution started on the day following completion of learning. Taken together, these results suggest that ascorbate-sensitive neural processes occurring within the olfactory bulb during learning are of functional importance for long-term storage of olfactory information.

NMDA-dependent induction of long-term potentiation in afferent and association fiber systems of piriform cortex in vitro

Long-term potentiation (LTP) was demonstrated in a slice preparation of piriform (olfactory) cortex. LTP could be reliably induced in both afferent and association fiber pathways. The magnitude of the observed potentiation was greater in the association fiber pathway. 2-Amino-5-phosphonovalerate (APV) blocked induction of LTP in both pathways, indicating that N-methyl-D-aspartate (NMDA) receptor activation is required for induction.

Stable depression of potentiated synaptic responses in the hippocampus with 1-5 Hz stimulation

Adult rats with two chronic stimulating electrodes in the Schaffer collateral/commissural system of the hippocampus and one recording electrode in the stratum radiatum (apical dendrites) of field CA1 were administered high-frequency stimulation (10 brief bursts at theta frequency) to produce long-term potentiation (LTP). 'Low frequency' stimulation (100 pulses at 1 Hz alone or followed by 250 pulses at 5 Hz) delivered 5-15 min later had no effect on LTP in 18% of the rats, caused a transient reversal in 18% of the group, but produced an apparent reversal of LTP for the remainder of a 1 h test session in 64% of the animals. LTP did not recover in animals tested 24 h later, at which point a second episode of high-frequency stimulation but without subsequent low-frequency stimulation was administered. This produced an LTP effect that persisted for a 1 h test session in 94% of the cases and that was still present in 86% of the animals tested 24 h later. Low-frequency stimulation applied prior to induction of LTP had no lasting effects on evoked responses not did it affect responses to a control stimulating electrode in those cases in which it reversed LTP. Possible implications of these results for hypotheses concerning the substrates of LTP and mechanisms of forgetting are discussed.

Anoxia reveals a vulnerable period in the development of long-term potentiation

Transient anoxia occurring 1-2 min after high-frequency stimulation selectively prevented the stable expression of long-term potentiation (LTP). Anoxia occurring after this brief vulnerable period did not reverse LTP. Experiments on the duration of anoxia necessary to block LTP expression indicated that simply reducing synaptic transmission was insufficient but that membrane depolarization was not required. The effects of anoxia on LTP were blocked by antagonists of A1 adenosine receptors. It is concluded that LTP develops in about one minute and that the chemistries operating in this period are easily disrupted by an event triggered by adenosine receptors.

Simulation of paleocortex performs hierarchical clustering

Simulations were performed of layers I and II of olfactory paleocortex, as connected to its primary input structure, olfactory bulb. Induction of synaptic long-term potentiation by means of repetitive sampling of inputs caused the simulation to organize encodings of learned cues into a hierarchical memory that uncovered statistical relationships in the cue environment, corresponding to the performance of hierarchical clustering by the biological network. Simplification led to characterization of those parts of the network responsible for the mechanism, resulting in a novel, efficient algorithm for hierarchical clustering. The hypothesis is put forward that these corticobulbar networks and circuitry of similar design in other brain regions contain computational elements sufficient to construct perceptual hierarchies for use in recognizing environmental cues.

Long-term potentiation of monosynaptic EPSPs in rat piriform cortex in vitro

Induction of long-term potentiation (LTP) by burst stimulation patterned after the limbic system theta rhythm was studied in slices of piriform cortex. Monosynaptic responses were evoked by stimulation of afferent fibers of the lateral olfactory tract (LOT) or the intrinsic associational (ASSN) feedback system. LTP was difficult to elicit at LOT synapses in the presence of 2.5 mM extracellular Mg2+, and when it was induced potentiation increased for 20-30 min after burst stimulation before stabilizing. The probability of inducing LTP was increased when the extracellular Mg2+ concentration was reduced to 50 microM. In ASSN synapses LTP developed in about 1 min after burst stimulation and then remained stable. ASSN system LTP was more readily induced in slices from caudal than rostral piriform. Induction of LTP at both LOT and ASSN synapses was blocked by D-2-amino-5-phosphonopentanoate, indicating that NMDA receptor activation was required. Neither system exhibited the decremental short-term potentiation effect observed after burst stimulation of inputs to the CA1 field of hippocampus.

The nature and causes of hippocampal long-term potentiation

One of the most fascinating features of the hippocampus is its capacity for plasticity. Long-term potentiation (LTP), a stable facilitation of synaptic potentials after high-frequency synaptic activity, is very prominent in hippocampus and is a leading candidate memory storage mechanism. Here, we discuss the nature and causes of LTP and relate them to endogenous rhythmic neuronal activity patterns and their potential roles in memory. Anatomical studies indicate that LTP is accompanied by postsynaptic structural modifications while pharmacological studies strongly suggest that LTP is not due to an increase in presynaptic transmitter release. In field CA1, LTP induction appears to be triggered by a postsynaptic influx of calcium through NMDA receptor-linked channels. Possible roles of several calcium-sensitive enzyme systems in LTP are discussed and it is argued that activation of a calcium-dependent protease (calpain) could produce the structural changes linked to LTP. Rhythmic bursting activity is highly effective in inducing LTP and it is argued that the endogenous hippocampal theta rhythm plays a role in LTP induction in vivo. Finally, studies indicate that LTP and certain types of memory share a common pharmacology and the use of electrical brain stimulation as a sensory cue suggests that LTP develops when the significance of that cue is learned.

The protease inhibitor leupeptin interferes with the development of LTP in hippocampal slices

The effect of leupeptin, an inhibitor of thiol-proteases, was tested on the induction of long-term potentiation (LTP) in field CA1 of hippocampal slices. Two h of drug application did not produce substantial changes while a greater than 3-h application caused a sizeable reduction in the degree of LTP induced. Leupeptin had no obvious effects on the facilitation of postsynaptic responses occurring within or between the short high frequency bursts used to induce LTP, suggesting that the drug acted on cellular chemistries occurring after the initial physiological events that normally trigger LTP. These results are consistent with the hypothesis that a calcium-activated thiol protease (calpain) is involved in the induction of LTP.

Olfactory cortex: model circuit for study of associative memory?

The piriform (olfactory) cortex is a phylogenetically old type of cerebral cortex with parallels in its organization to the architecture of certain 'neural network' models for distributed pattern recognition and association. These features, in combination with unique structural characteristics that facilitate experimental study, make the piriform cortex a potentially good model for analysis of associative (content-addressable) memory processes.

Effects of two ACTH analogs on successive odor discrimination learning in rats

Olfactory discrimination learning has been shown to provide a powerful tool to investigate the mechanisms involved in the formation, storage and retrieval of information in rodent CNS. In the present study we tested the effects of two ACTH analogs, which were previously reported to influence the processes of learning and memory, on various olfactory learning tasks. The ACTH(4-9) analog HOE 427 produced an apparent increase in storage of olfactory information as shown by the difficulty experienced by the animals to rapidly reverse their behavioral responses to previously learned odors. Similarly, the ACTH (4-9) analog ORG 2766 appears to enhance the storage of olfactory information when administered either before or after the learning trials. These data are consistent with the notion that ACTH and related analogs facilitate performance in a variety of learning tasks. In addition, our results suggest possible mechanisms by which some neuroactive peptides might modulate learning and memory processes in the CNS.

Derivation of Encoding Characteristics of Layer II Cerebral Cortex

Computer simulations of layers I and II of pirifonn (olfactory) cortex indicate that this biological network can generate a series of distinct output responses to individual stimuli, such that different responses encode different levels of information about a stimulus. In particular, after learning a set of stimuli modeled after distinct groups of odors, the simulated network's initial response to a cue indicates only its group or category, whereas subsequent responses to the same stimulus successively subdivide the group into increasingly specific encoding of the individual cue. These sequences of responses amount to an automated organization of perceptual memories according to both their similarities and differences, facilitating transfer of learned information to novel stimuli without loss of specific information about exceptions. Human recognition performance robustly exhibits such multiple levels: a given object can be identified as a vehicle, as an automobile, or as a Mustang. The findings reported here suggest that a function as apparently complex as hierarchical recognition memory, which seems suggestive of higher ‘cognitive’ processes, may be a fundamental intrinsic property of the operation of this single cortical cell layer in response to naturally-occurring inputs to the structure. We offer the hypothesis that the network function of superficial cerebral conical layers may simultaneously acquire and hierarchically organize information about the similarities and differences among perceived stimuli. Experimental manipulation of the simulation has generated hypotheses of direct links between the values of specific biological features and particular attributes of behavior, generating testable physiological and behavioral predictions.

Linear relation between the magnitude of long-term potentiation in the dentate gyrus and associative learning in the rat. A demonstration using commissural inhibition and local infusion of an N-methyl-D-aspartate receptor antagonist

Field potentials were recorded in the dentate gyrus of freely-moving rats in a classical conditioning paradigm in which high-frequency stimulation of the perforant path served as a conditioned stimulus. Paired or unpaired perforant path stimulus-footshock presentations were given to animals engaged in a previously acquired food-motivated lever-pressing task. Conditioned suppression of lever-pressing was the behavioural measure of conditioning. Perforant path stimulus trains at an intensity above spike threshold induced long-term potentiation of synaptic transmission in the dentate gyrus. In this condition, animals learned the perforant path stimulus-shock association. Three strategies were employed to block the induction or reduce the magnitude of long-term potentiation induced by the conditioned stimulus: (1) reduction of the intensity of the stimulus below the spike threshold resulted in no long-term potentiation and a failure by the animals to learn the perforant path stimulus-shock association; (2) inhibitory modulation of long-term potentiation by high-frequency activation of commissural input to the dentate gyrus resulted in learning deficits; (3) chronic infusion of DL-2-amino-5-phosphonovalerate, a selective antagonist of the N-methyl-D-aspartate subtype of glutamate receptor, blocked the induction of long-term potentiation and prevented associative learning. A highly significant linear relation emerged from a correlational analysis between the magnitude of the change in synaptic efficacy at the activated synapses and the amount the animals learned about the perforant path stimulus-shock association. The results presented in this paper are consistent with the hypothesis that associative learning depends on the development of lasting changes in synaptic function. We propose that the activation of N-methyl-D-aspartate receptors in the dentate gyrus is involved in this process and that the more change in synaptic efficacy is produced in the activated network, the more the animals learn.

Role of dorsomedial thalamic nucleus and piriform cortex in processing olfactory information

Four experiments were conducted to characterize the role of primary and secondary olfactory projection areas (piriform cortex and dorsomedial thalamic nucleus (DMN] in olfactory information processing. Rats had to learn to discriminate between odors that were simultaneously released from different arms of an automated olfactory maze. When standard training conditions were used, damage of the DMN severely impaired both preoperatively trained and naive animals in acquiring an odor discrimination set (i.e. in most problems no learning was demonstrated). An additional group of DMN animals that received 4 times the standard amount of daily trials was unable to acquire the first two problems but successfully solved the third and all subsequent discriminations. Analysis of performance patterns suggested that destruction of the DMN initially leads to a strong procedural impairment that can be overcome by extensive training. After solving the third problem the animals with DMN damage required much less training to reach the learning criterion but generally made more errors than controls. Transfer of savings rarely occurred when a problem was repeated. Whether this secondary learning deficit observed in later discriminations is due to a specific effect of the lesion on the encoding of olfactory cues and thus on memory formation, or due to a disturbance in the regulation of emotional factors such as motivation, arousal, and attention is discussed. Lesions of the thalamus that spared the DMN had no effect on learning or retention of olfactory discriminations. Animals with ablations of the piriform cortex only acquired odor discriminations if they had been trained in the olfactory maze before the lesion. Moreover, their performance depended on the odor quality: they had great difficulty learning complex cues consisting of several odorants and learned simple odors virtually identical to control rats. The results indicate that an intact piriform cortex is needed to acquire the procedures involved to perform an olfactory discrimination task as well as to build neural representations of olfactory cues.