Effects of the noncompetitive NMDA receptor antagonist MK-801 on classical eyeblink conditioning in mice

Synaptic protein interaction networks encode experience by assuming stimulus-specific and brain-region-specific states

A core network of widely expressed proteins within the glutamatergic post-synapse mediates activity-dependent synaptic plasticity throughout the brain, but the specific proteomic composition of synapses differs between brain regions. Here, we address the question, how does proteomic composition affect activity-dependent protein-protein interaction networks (PINs) downstream of synaptic activity? Using quantitative multiplex co-immunoprecipitation, we compare the PIN response of in vivo or ex vivo neurons derived from different brain regions to activation by different agonists or different forms of eyeblink conditioning. We report that PINs discriminate between incoming stimuli using differential kinetics of overlapping and non-overlapping PIN parameters. Further, these "molecular logic rules" differ by brain region. We conclude that although the PIN of the glutamatergic post-synapse is expressed widely throughout the brain, its activity-dependent dynamics show remarkable stimulus-specific and brain-region-specific diversity. This diversity may help explain the challenges in developing molecule-specific drug therapies for neurological disorders.

The effect of NMDA-R antagonist, MK-801, on neuronal mismatch along the rat auditory thalamocortical pathway

Efficient sensory processing requires that the brain maximize its response to unexpected stimuli, while suppressing responsivity to expected events. Mismatch negativity (MMN) is an auditory event-related potential that occurs when a regular pattern is interrupted by an event that violates the expected properties of the pattern. According to the predictive coding framework there are two mechanisms underlying the MMN: repetition suppression and prediction error. MMN has been found to be reduced in individuals with schizophrenia, an effect believed to be underpinned by glutamate N-methyl-d-aspartate receptor (NMDA-R) dysfunction. In the current study, we aimed to test how the NMDA-R antagonist, MK-801 in the anaesthetized rat, affected repetition suppression and prediction error processes along the auditory thalamocortical pathway. We found that low-dose systemic administration of MK-801 differentially affect thalamocortical responses, namely, increasing thalamic repetition suppression and cortical prediction error. Results demonstrate an enhancement of neuronal mismatch, also confirmed by large scale-responses. Furthermore, MK-801 produces faster and stronger dynamics of adaptation along the thalamocortical hierarchy. Clearly more research is required to understand how NMDA-R antagonism and dosage affects processes contributing to MMN. Nonetheless, because a low dose of an NMDA-R antagonist increased neuronal mismatch, the outcome has implications for schizophrenia treatment.

Experimental extinction in Pavlovian conditioning: Behavioural and neuroscience perspectives

This paper reviews the behavioural and neuroscience literatures on extinction in Pavlovian conditioning with a view towards finding possible points of contact between these two often independent lines of investigation. Recent discoveries at the behavioural level indicate (1) that conditioned stimulus (CS)–unconditioned stimulus (US) associations specific in their sensory content are fully preserved during extinction, (2) that inhibitory stimulus-response associations appear to be learned during extinction, (3) that extinction is influenced by the level of activation of the US representation during nonreinforced trials, (4) that decreases in attention can influence conditioned performance during extinction, and (5) that contexts acquire an ability to modulate learning during both conditioning and extinction. Recent discoveries at the neural systems level suggest (1) that the hippocampus is important in context-specific learning during extinction, (2) that the prefrontal cortex is possibly important in long-term memory for extinction, (3) that the basolateral amygdala may be important in sustaining attention to a CS during extinction, (4) that NMDA receptors are important either in neural plasticity during extinction or by affecting the value of the US representation during extinction, and (5) that the GABAergic system may partially mediate inhibitory learning during extinction. It is concluded that both of these levels of analysis can benefit the other in the pursuit of a more comprehensive understanding of extinction.

Trace Eyeblink Conditioning in Mice Is Dependent upon the Dorsal Medial Prefrontal Cortex, Cerebellum, and Amygdala: Behavioral Characterization and Functional Circuitry

Trace eyeblink conditioning is useful for studying the interaction of multiple brain areas in learning and memory. The goal of the current work was to determine whether trace eyeblink conditioning could be established in a mouse model in the absence of elicited startle responses and the brain circuitry that supports this learning. We show here that mice can acquire trace conditioned responses (tCRs) devoid of startle while head-restrained and permitted to freely run on a wheel. Most mice (75%) could learn with a trace interval of 250 ms. Because tCRs were not contaminated with startle-associated components, we were able to document the development and timing of tCRs in mice, as well as their long-term retention (at 7 and 14 d) and flexible expression (extinction and reacquisition). To identify the circuitry involved, we made restricted lesions of the medial prefrontal cortex (mPFC) and found that learning was prevented. Furthermore, inactivation of the cerebellum with muscimol completely abolished tCRs, demonstrating that learned responses were driven by the cerebellum. Finally, inactivation of the mPFC and amygdala in trained animals nearly abolished tCRs. Anatomical data from these critical regions showed that mPFC and amygdala both project to the rostral basilar pons and overlap with eyelid-associated pontocerebellar neurons. The data provide the first report of trace eyeblink conditioning in mice in which tCRs were driven by the cerebellum and required a localized region of mPFC for acquisition. The data further reveal a specific role for the amygdala as providing a conditioned stimulus-associated input to the cerebellum.

Cholinergic, but not NMDA, receptors in the lateral entorhinal cortex mediate acquisition in trace eyeblink conditioning

Anatomical and electrophysiological studies collectively suggest that the entorhinal cortex consists of several subregions, each of which is involved in the processing of different types of information. Consistent with this idea, we previously reported that the dorsolateral portion of the entorhinal cortex (DLE), but not the caudomedial portion, is necessary for the expression of a memory association between temporally discontiguous stimuli in trace eyeblink conditioning (Morrissey et al. (2012) J Neurosci 32:5356-5361). The present study examined whether memory acquisition depends on the DLE and what types of local neurotransmitter mechanisms are involved in memory acquisition and expression. Male Long-Evans rats experienced trace eyeblink conditioning, in which an auditory conditioned stimulus (CS) was paired with a mildly aversive electric shock to the eyelid (US) with a stimulus-free interval of 500 ms. Immediately before the conditioning, the rats received a microinfusion of neuroreactive substances into the DLE. We found that reversible inactivation of the DLE with GABAA receptor agonist, muscimol impaired memory acquisition. Furthermore, blockade of local muscarinic acetylcholine receptors (mACh) with scopolamine retarded memory acquisition while blockade of local NMDA receptors with APV had no effect. Memory expression was not impaired by either type of receptor blocker. These results suggest that the DLE is necessary for memory acquisition, and that acquisition depends on the integrity of local mACh receptor-dependent firing modulation, but not NMDA receptor-dependent synaptic plasticity.

The neural circuitry and molecular mechanisms underlying delay and trace eyeblink conditioning in mice

Classical eyeblink conditioning (EBC), a simple form of associative learning, has long been served as a model for motor learning and modulation. The neural circuitry of EBC has been studied in detail in rabbits. However, its underlying molecular mechanisms remain unclear. The advent of mouse transgenics has generated new perspectives on the studies of the neural substrates and molecular mechanisms essential for EBC. Results about EBC in mice differ in some aspects from those obtained in other mammals. Here, we review the current studies about the neural circuitry and molecular mechanisms underlying delay and trace EBC in mice. We conclude that brainstem-cerebellar circuit plays an essential role in DEC while the amygdala modulates this process, and that the medial prefrontal cortex (mPFC) as a candidate is involved in the extra-cerebellar mechanism underlying delay eyeblink conditioning (DEC) in mice. We propose the Amygdala-Cerebellum-Prefrontal Cortex-Dynamic-Conditioning Model (ACPDC model) for DEC in mice. As to trace eyeblink conditioning (TEC), the forebrain regions may play an essential role in it, whereas cerebellar cortex seems to be out of the neural circuitry in mice. Moreover, the molecular mechanisms underlying DEC and TEC in mice differ from each other. This review provides some new information and perspectives for further research on EBC.

Role of motor cortex NMDA receptors in learning-dependent synaptic plasticity of behaving mice

The primary motor cortex has an important role in the precise execution of learned motor responses. During motor learning, synaptic efficacy between sensory and primary motor cortical neurons is enhanced, possibly involving long-term potentiation and N-methyl-D-aspartate (NMDA)-specific glutamate receptor function. To investigate whether NMDA receptor in the primary motor cortex can act as a coincidence detector for activity-dependent changes in synaptic strength and associative learning, here we generate mice with deletion of the Grin1 gene, encoding the essential NMDA receptor subunit 1 (GluN1), specifically in the primary motor cortex. The loss of NMDA receptor function impairs primary motor cortex long-term potentiation in vivo. Importantly, it impairs the synaptic efficacy between the primary somatosensory and primary motor cortices and significantly reduces classically conditioned eyeblink responses. Furthermore, compared with wild-type littermates, mice lacking primary motor cortex show slower learning in Skinner-box tasks. Thus, primary motor cortex NMDA receptors are necessary for activity-dependent synaptic strengthening and associative learning.

Motor cortex NMDA receptors have a key role in the acquisition of associative memories. Hasan et al. generate mice lacking NMDA receptor activity in the motor cortex and find that this impairs LTP, strengthening of synapses between somatosensory and motor cortices, and associative 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.

d-cycloserine reverses the detrimental effects of stress on learning in females and enhances retention in males

Exposure to acute, inescapable stress produces a facilitation of subsequent classical eyeblink conditioning in male rats. The same stress exposure produces a profound deficit in classical eyeblink conditioning in females. Activation of N-methyl-d-aspartate receptors (NMDAr) is necessary for the effect of stress on learning in males while the contribution of NMDAr activation to the deficit in learning after stress is unknown. Here, we tested the influence of d-cycloserine (DCS), a positive modulator of the NMDAr, in stressed or unstressed male and female rats. Groups of males and females were exposed to an acute stressful event. One day later, they began training with four sessions of trace eyeblink conditioning. Each day before training, they were injected with DCS (15mg/kg) or saline. Females treated with DCS during training responded similarly to those that were untreated. However, those that were stressed and the next day treated with the drug during training did not express the typical learning deficit, i.e. they learned to time the CR very well. Because the drug was administered well after the stressor, these data indicate that DCS reversed the negative effects of stress on learning in females. In males, the effect of DCS was subtle, resulting in higher asymptotic responding, and enhanced retention in a drug-free retention test. Thus, as shown previously, training in the presence of an NMDA receptor agonist enhances associative learning and memory retention. In addition, it can reverse learning deficits that have already been induced.

Post-training ethanol disrupts trace conditioned fear in rats: effects of timing of ethanol, dose and trace interval duration

Ethanol has complex effects on memory performance, although hippocampus-dependent memory may be especially vulnerable to disruption by acute ethanol intoxication occurring during or shortly after a training episode. In the present experiments, the effects of post-training ethanol on delay and trace fear conditioning were examined in adolescent rats. In Experiment 1, 30-day-old Sprague-Dawley rats were given delay or trace conditioning trials in which a 10s flashing light CS was paired with a 0.5 mA shock US. For trace groups, the trace interval was 10 s. On days 31-33, animals were administered ethanol once daily (0.0 or 2.5 g/kg via intragastric intubation), and on day 34 animals were tested for CS-elicited freezing. Results showed that post-training ethanol affected the expression of trace, but had no effect on delay conditioned fear. Experiment 2 revealed that this effect was dose-dependent; doses lower than 2.5 g/kg were without effect. Experiment 3 evaluated whether proximity of ethanol to the time of training or testing was critical. Results show that ethanol administration beginning 24h after training was more detrimental to trace conditioned freezing than administration that was delayed by 48 h. Finally, in Experiment 4 animals were trained with one of three different trace intervals: 1, 3 or 10s. Results indicate that post-training administration of 2.5 g/kg ethanol disrupted trace conditioned fear in subjects trained with a 10s, but not with a 1 or 3s, trace interval. Collectively the results suggest that ethanol administration impairs post-acquisition memory processing of hippocampus-dependent trace fear conditioning.

Evoking blinks with natural stimulation and detecting them with a noninvasive optical device: a simple, inexpensive method for use with freely moving animals

Many laboratories studying eyeblinks in unanesthetized rodents use a periorbital shock to evoke the blink. The stimulus is typically delivered via a tether and usually obliterates detection of a full unconditioned response with electromyographic (EMG) recording. Here we describe the adapter we have used successfully for several years to deliver puffs of air to the cornea of freely moving rats during our studies of eyeblink conditioning. The stimulus evokes an unconditioned response that can be recorded without affecting the EMG signal. This allows a complete analysis of the unconditioned response which is important for studies examining reflex modification or the effect of drugs, genetic manipulations, or aging on the unconditioned blink reflex. We also describe an infrared reflective sensor that can be added to the tether to minimize the number of wires that need to be implanted around the eye, and which is relatively immune to electrical artifacts associated with a periorbital shock stimulus or other devices powered by alternating current. The responses recorded simultaneously by EMG wires and the optical sensor appear highly correlated and demonstrate that the optical sensor can measure responses that might otherwise be lost due to electrical interference from a shock stimulus.

Preclinical investigation of the functional effects of memantine and memantine combined with galantamine or donepezil

Combinations of drugs approved to treat Alzheimer's disease (AD) were tested in older rabbits with delay eyeblink classical conditioning, a form of associative learning severely impaired in AD. In Experiment 1 (n=49 rabbits), low doses (0.1, 0.5, 1.0, and 0.0 (vehicle) mg/kg) of memantine (Namenda) were tested. These three doses neither improved nor impaired acquisition at a statistically significant level. The 0.5 mg/kg dose had the greatest effect numerically and did not cause sensitization or habituation in explicitly unpaired controls. In Experiment 2 (n=56), doses of galantamine (Razadyne; 3.0 mg/kg) and donepezil (Aricept; 0.75 mg/kg) that had comparable magnitudes of cholinesterase inhibition were tested alone and in combination with 0.5 mg/kg memantine. Older rabbits treated with galantamine and with galantamine+memantine learned significantly better than vehicle-treated rabbits, but adding memantine did not improve learning over galantamine alone. Older rabbits treated with donepezil or a combination of memantine and donepezil did not learn significantly better than rabbits treated with vehicle. Galantamine has two mechanisms of action: mild cholinesterase inhibition and allosteric modulation of nicotinic acetylcholine receptors (nAChRs). When equated for cholinesterase inhibition, galantamine had significant efficacy in the eyeblink conditioning model system, but donepezil did not, indicating that modulation of nAChRs may be the mechanism that significantly ameliorates learning deficits in this model. In the absence of AD neuropathology in older rabbits, memantine had no efficacy alone or in combination with the other drugs.

The nicotinic agonist RJR-2403 compensates the impairment of eyeblink conditioning produced by the noncompetitive NMDA-receptor antagonist MK-801

The classical conditioning of eyelid responses using trace paradigms is a hippocampal-related model of associative learning, involving the activation of N-methyl-D-aspartate (NMDA) receptors. We have evaluated here the effects of NMDA-receptor blockage with the selective noncompetitive antagonist (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate (dizocilpine, MK-801). Mice were implanted with stimulating electrodes on the supraorbitary nerve and with recording electrodes in the ipsilateral orbicularis oculi muscle. Animals were conditioned with a trace shock-SHOCK paradigm. MK-801-injected animals (0.02 mg/kg) seemed unable to acquire this type of associative learning task, but the latency and amplitude of their unconditioned eyelid responses was not affected by drug administration. The administration of the nicotinic agonist (E)-N-methyl-4-(3-pyridinyl)-3-buten-1-amine (RJR-2403; 2 mg/kg) was able to restore completely the acquisition of the conditioned response when administered both before and after MK-801. In vitro recordings of field excitatory postsynaptic potentials (fEPSPs) evoked in the hippocampal CA1 area by the electrical stimulation of the Schaffer collateral pathway indicates that RJR-2403 application to the bath enhance the release of glutamate by a presynaptic mechanism. These findings reveal that nicotinic acetylcholine receptors enhance glutamatergic transmission in hippocampal circuits involved in the acquisition of associative learning.

Hippocampal CA3 NMDA receptors are crucial for adaptive timing of trace eyeblink conditioned response

Classical conditioning of the eyeblink reflex is a simple form of associative learning for motor responses. To examine the involvement of hippocampal CA3 NMDA receptors (NRs) in nonspatial associative memory, mice lacking an NR1 subunit selectively in adult CA3 pyramidal cells [CA3-NR1 knock-out (KO) mice] were subjected to eyeblink conditioning paradigms. Mice received paired presentations of an auditory conditioned stimulus (CS) and a periorbital shock unconditioned stimulus (US). With repeated presentation of the CS followed by the US, wild-type mice learned to blink in anticipation of the US before its onset. We first confirmed that wild-type mice require an intact hippocampus in the trace version of eyeblink conditioning in which the CS and US do not overlap, creating a stimulus-free time gap of 500 ms. Under the same condition, CA3-NR1 KO mice successfully acquired conditioned responses (CRs) during the 10 d acquisition sessions, whereas the extinction of CRs was impaired on the first day of extinction sessions. Importantly, CA3-NR1 KO mice were impaired in the formation of an adaptively timed CR during the first five trials in the daily acquisition sessions. The aberrantly timed CR was also observed in the extinction sessions in accordance with the impaired extinction of CRs. These results indicate that CA3-NR1 KO mice are unable to rapidly retrieve adaptive CR timing, suggesting that CA3 NRs play a crucial role in the memory of adaptive CR timing in trace conditioning.

NMDA receptor-dependent processes in the medial prefrontal cortex are important for acquisition and the early stage of consolidation during trace, but not delay eyeblink conditioning

Permanent lesions in the medial prefrontal cortex (mPFC) affect acquisition of conditioned responses (CRs) during trace eyeblink conditioning and retention of remotely acquired CRs. To clarify further roles of the mPFC in this type of learning, we investigated the participation of the mPFC in mnemonic processes both during and after daily conditioning using local microinfusion of the GABA(A) receptor agonist muscimol or the NMDA receptor antagonist APV into the rat mPFC. Muscimol infusions into the mPFC before daily conditioning significantly retarded CR acquisition and reduced CR expression if applied after sufficient learning. APV infusion also impaired acquisition of CRs, but not expression of well-learned CRs. When infusions were made immediately after daily conditioning, acquisition of the CR was partially impaired in both the muscimol and APV infusion groups. In contrast, rats that received muscimol infusions 3 h after daily conditioning exhibited improvement in their CR performance comparable to that of the control group. Both the pre- and post-conditioning infusion of muscimol had no effect on acquisition in the delay paradigm. These results suggest that the mPFC participates in both acquisition of a CR and the early stage of consolidation of memory in trace, but not delay eyeblink conditioning by NMDA receptor-mediated operations.

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

N-methyl-D-aspartate receptors play important roles in acquisition and expression of the eyeblink conditioned response in glutamate receptor subunit delta2 mutant mice

Classical eyeblink conditioning has been known to depend critically on the cerebellum. Apparently consistent with this, glutamate receptor subunit delta2 null mutant mice, which have serious morphological and functional deficiencies in the cerebellar cortex, are severely impaired in delay paradigm. However, these mutant mice successfully learn in trace paradigm, even in '0-trace paradigm,' in which the unconditioned stimulus starts just after the conditioned stimulus terminates. Our previous studies revealed that the hippocampus and the muscarinic acetylcholine receptors play crucial roles in 0-trace paradigm in glutamate receptor subunit delta2 null mutant mice unlike in wild-type mice, suggesting a large contribution of the forebrain to 0-trace conditioning in this type of mutant mice. In the present study, we investigated the role of N-methyl-D-aspartate receptors in 0-trace eyeblink conditioning in glutamate receptor subunit delta2 null mutant mice. Mice were injected intraperitoneally with the noncompetitive N-methyl-d-aspartate receptor antagonist (+)MK-801 (0.1mg/kg) or saline, and conditioned with 350-ms tone conditioned stimulus followed by 100-ms periorbital shock unconditioned stimulus. Glutamate receptor subunit delta2 null mutant mice that received (+)MK-801 injection exhibited a severe impairment in acquisition of the conditioned response, compared with the saline-injected glutamate receptor subunit delta2 null mutant mice. In contrast, wild-type mice were not impaired in acquisition of 0-trace conditioned response by (+)MK-801 injection. After the injection solution was changed from (+)MK-801 to saline, glutamate receptor subunit delta2 null mutant mice showed a rapid and partial recovery of performance of the conditioned response. On the other hand, when the injection solution was changed from saline to (+)MK-801, glutamate receptor subunit delta2 null mutant mice showed a marked impairment in expression of the pre-acquired conditioned response, whereas impairment of the expression was small in wild-type mice. Injection of (+)MK-801 had no significant effects on spontaneous eyeblink frequency or startle eyeblink frequency to the tone conditioned stimulus in either glutamate receptor subunit delta2 null mutant mice or wild-type mice. These results suggest that N-methyl-D-aspartate receptors play critical roles both in acquisition and expression of the conditioned response in 0-trace eyeblink conditioning in glutamate receptor subunit delta2 null mutant mice.

Dorsal hippocampus NMDA receptors differentially mediate trace and contextual fear conditioning

The dorsal hippocampus (DH) is critically involved in the acquisition and expression of trace and contextual fear conditioning. NMDA/glutamate receptor-mediated transmission is thought to be one mechanism mediating the plastic changes that support long-term memories in the DH. However, their precise involvement in acquisition and expression processes has not been defined. To examine this issue, the NMDA receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV; 10 microg/microl; 0.5 microl), was infused into the DH prior to conditioning and/or testing, using a trace fear conditioning procedure. All rats were tested for freezing to both tone and context in separate, counterbalanced sessions. The three sessions (1 training and 2 test) were separated by approximately 24 h. Using this design, it was possible to assess the role for DH NMDA receptors in the acquisition versus expression of trace and contextual fear conditioning. APV disrupted acquisition, but not expression, of contextual fear conditioning. By contrast, APV attenuated both acquisition and expression of trace fear memories. Thus, DH NMDA receptors appear to contribute to retrieval of some, but not all, fear memories.

Role of hippocampal NMDA receptors in trace eyeblink conditioning

We examined the effects of acute injections of competitive N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovaleric acid (APV) into the dorsal hippocampus on contextual fear conditioning and classical eyeblink conditioning in C57BL/6 mice. When injected 10 to 40 min before training, APV severely impaired contextual fear conditioning. Thus, APV injection under these conditions was sufficient to suppress hippocampal NMDA receptors. To investigate the role of hippocampal NMDA receptors on eyeblink conditioning, we carried out daily training of mice during 10-40 min after injection of APV. In the delay eyeblink conditioning, in which the unconditioned stimulus (US) is delayed and terminates simultaneously with the conditioned stimulus (CS), APV-injected mice acquired the conditioned responses (CRs) as well as artificial cerebrospinal fluid (aCSF)-injected control mice did. However, in the trace eyeblink conditioning, in which the CS and US were separated by a stimulus-free trace interval of 500 ms, APV-injected mice showed severe impairment in acquisition of the CR. There was no significant difference in pseudo-conditioning between APV- and aCSF-injected mice. These results provide evidence that the NMDA receptor in the dorsal hippocampus is critically involved in acquisition of the CR in long trace eyeblink conditioning.

Trace eyeblink conditioning requires the hippocampus but not autophosphorylation of alphaCaMKII in mice

Little is known about signaling mechanisms underlying temporal associative learning. Here, we show that mice with a targeted point mutation that prevents autophosphorylation of alphaCaMKII (alphaCaMKII(T286A)) learn trace eyeblink conditioning normally. This forms a sharp contrast to the severely impaired spatial learning in the water maze and contextual fear conditioning observed in alphaCaMKII(T286A) mutants. Importantly, hippocampal lesions impaired trace eyeblink conditioning in alphaCaMKII(T286A) mice, suggesting a potential role of hippocampal alphaCaMKII-independent mechanisms. These results indicate that hippocampal signaling mechanisms that underlie temporal associative learning as assessed by trace eyeblink conditioning may differ from those of spatial and contextual learning.

Brain mechanisms of extinction of the classically conditioned eyeblink response

It is well established that the cerebellum and its associated circuitry are essential for classical conditioning of the eyeblink response and other discrete motor responses (e.g., limb flexion, head turn, etc.) learned with an aversive unconditioned stimulus (US). However, brain mechanisms underlying extinction of these responses are still relatively unclear. Behavioral studies have demonstrated extinction as an active learning process distinct from acquisition. Experimental data in eyeblink conditioning suggest that plastic changes specific to extinction may play an important role in this process. Both cerebellar and hippocampal systems may be involved in extinction of these memories. The nature of this phenomenon and identification of the neural substrates necessary for extinction of originally learned responses is the topic of this review.

A biophysical model of synaptic delay learning and temporal pattern recognition in a cerebellar Purkinje cell

It has been suggested that information in the brain is encoded in temporal spike patterns which are decoded by a combination of time delays and coincidence detection. Here, we show how a multi-compartmental model of a cerebellar Purkinje cell can learn to recognise temporal parallel fibre activity patterns by adapting latencies of calcium responses after activation of metabotropic glutamate receptors (mGluRs). In each compartment of our model, the mGluR signalling cascade is represented by a set of differential equations that reflect the underlying biochemistry. Phosphorylation of the mGluRs changes the concentration of receptors which are available for activation by glutamate and thereby adjusts the time delay between mGluR stimulation and voltage response. The adaptation of a synaptic delay as opposed to a weight represents a novel non-Hebbian learning mechanism that can also implement the adaptive timing of the classically conditioned eye-blink response.

The N-methyl-D-aspartate (NMDA)-type glutamate receptor GluRepsilon2 is important for delay and trace eyeblink conditioning in mice

It has been proposed that the N-methyl-d-aspartate (NMDA)-type glutamate receptor (GluR) plays an important role in synaptic plasticity, learning, and memory. The four GluRepsilon (NR2) subunits, which constitute NMDA receptors with a GluRzeta (NR1) subunit, differ both in their expression patterns in the brain and in their functional properties. In order to specify the distinct participation of each of these subunits, we focused on the GluRepsilon2 subunits, which are expressed mainly in the forebrain. We investigated delay and trace eyeblink conditioning in GluRepsilon2 heterozygous mutant mice whose content of GluRepsilon2 protein was decreased to about half of that in wild-type mice. GluRepsilon2 mutant mice exhibited severe impairment of the attained level of conditioned response (CR) in the delay paradigm, for which the cerebellum is essential and modulation by the forebrain has been suggested. Moreover, GluRepsilon2 mutant mice showed no trend toward CR acquisition in the trace paradigm with a trace interval of 500 ms, in which the forebrain is critically involved in successful learning. On the other hand, the reduction of GluRepsilon2 proteins did not disturb any basic sensory and motor functions which might have explained the observed impairment. These results are different from those obtained with GluRepsilon1 null mutant mice, which attain a normal level of the CR but at a slower rate in the delay paradigm, and showed a severe impairment in the trace paradigm. Therefore, the NMDA receptor GluRepsilon2 plays a more critical role than the GluRepsilon1 subunit in classical eyeblink conditioning.

Evidence for disrupted NMDA receptor function in tissue plasminogen activator knockout mice

Tissue plasminogen activator (tPA), a serine protease immediate-early gene product expressed in brain areas important in learning and memory, has been shown to cleave the NR1 subunit of the NMDA receptor leading to a potentiated Ca(2+) influx. Mice lacking tPA (tPA-/- mice) have disrupted late phase-LTP in the hippocampus, possibly as a consequence of reduced Ca(2+) flux through NMDA receptors. In the present experiments, we investigated whether the NMDA antagonist dizocilpine might alter performance in tPA-/- mice in behavioural tasks shown to be sensitive to hippocampal lesions. tPA-/- mice and wild-type controls (WT) showed similar rates of acquisition and performance of a spatial working memory task (eight-arm radial maze). Dizocilpine (0.03-0.3 mg/kg, i.p.), given acutely, disrupted performance by increasing the number of errors equally across both genotypes. At asymptotic performance of a differential reinforcement of low response rate operant task (DRL), acute dizocilpine (0.03-0.3 mg/kg) impaired performance, but no differences between genotypes were observed. However, dizocilpine (0.1 mg/kg), given repeatedly during acquisition of a signalled-DRL15" task, retarded acquisition in tPA-/- but not WT mice. This treatment regime had no effect on locomotor activity in either genotype. tPA-/- mice showed no spatial learning deficits, but were more sensitive to dizocilpine during acquisition (though not expression) of a DRL task. This supports a role for tPA in modification of the NMDA receptor, although absence of tPA does not have consequences for all forms of NMDA-dependent mediated learning.

Latent inhibition disruption by MK-801 in a conditioned taste-aversion paradigm

N-Methyl-D-aspartate (NMDA) receptors appear to be involved in CS processing and memory consolidation. The present paper analyzed the effect of the non-competitive NMDA receptor antagonist Dizocilpine maleate (MK-801) on Latent Inhibition (LI)-retarded learning of a CS-US association after to-be-CS preexposures at time of testing, using Wistar rats as experimental subjects. If NMDA receptors are involved in CS processing, MK-801 administration should affect LI. In fact, previous experiments revealed that a 2.0mg/kg MK-801 dose, administered 20 h before preexposure and conditioning, abolished LI in a conditioned taste-aversion paradigm. In the present paper, MK-801 (0.2 mg/kg) was either injected after preexposure, after conditioning, or after both preexposure and conditioning stages. LI was abolished when MK-801 was injected after preexposure, but not when it was injected after conditioning. These results support the role of NMDA receptors in CS processing and memory consolidation.

Trace eyeblink conditioning in decerebrate guinea pigs

We investigated the trace eyeblink conditioning in decerebrate guinea pigs to elucidate the possible role of the cerebellum and brainstem in this hippocampus-dependent task. A 350-ms tone conditioned stimulus was paired with a 100-ms periorbital shock unconditioned stimulus with a trace interval of either 0, 100, 250 or 500 ms. Decerebrate animals readily acquired the conditioned response with a trace interval of 0 or 100 ms. Even in the paradigm with a 500-ms trace interval, which is known to depend critically on the hippocampus in all animal species examined, the decerebrate guinea pigs acquired the conditioned response, which had adaptive timing as well as in the other paradigms with a shorter trace interval. However, it took many more trials to learn in the 500-ms trace paradigm than in the shorter trace interval paradigms, and the conditioned response expression was unstable from trial to trial. When decerebrate animals were conditioned step by step with a trace interval of 100, 250 and 500 ms, sequentially, they easily acquired the adaptive conditioned response to a 500-ms trace interval. However, the frequency of conditioned responses decreased after the trace interval was shifted from 250 ms to 500 ms, which was not observed after the shift from 100 ms to 250 ms. These results suggest that the cerebellum and brainstem could maintain the 'trace' of the conditioned stimulus and associate it with the unconditioned stimulus even in the 500-ms trace paradigm, but that the forebrain might be required for facilitating and stabilizing the association.

Time-limited role of the hippocampus in the memory for trace eyeblink conditioning in mice

We examined the role of the hippocampus in memory retention after trace eyeblink conditioning in mice. After establishing the conditioned response (CR) in the trace paradigm, mice received a bilateral aspiration of the dorsal hippocampus and its overlying neocortex on the next day (1-day group) or after 4 weeks (4-week group). Control mice received a neocortical aspiration on the same schedule as the hippocampal-lesion group. After 2 weeks of recovery, these groups received additional conditioning for 3 days. Frequency of the CR of the 1-day group was as low as spontaneous values on the first day in the post-lesion session and never reached pre-surgical level during the post-lesion sessions, while that of the control group did reach pre-surgical level during the post-lesion sessions although there was a transient decline just after lesion. In contrast to the 1-day group, the 4-week-hippocampal lesion group retained the CR and showed a further increase, without significant difference from the control group. The temporal pattern of the CR also was unchanged by the hippocampal lesion 4 weeks after learning. These results suggest a time-limited role for the hippocampus in memory retention after trace conditioning in mice: the CR acquired recently requires an intact hippocampus for its retention, but the CR acquired remotely does not. This is similar to the result reported in rabbits. Therefore, the mechanism and time course of memory consolidation after trace eyeblink conditioning may be similar in mice and rabbits.