Infusions Of APV Research Articles

Memory trace reactivation and behavioral response during retrieval are differentially modulated by amygdalar glutamate receptors activity: interaction between amygdala and insular cortex.

The insular cortex (IC) is required for conditioned taste aversion (CTA) retrieval. However, it remains unknown which cortical neurotransmitters levels are modified upon CTA retrieval. Using in vivo microdialysis, we observed that there were clear elevations in extracellular glutamate, norepinephrine, and dopamine in and around the center of the gustatory zone of the IC during CTA retrieval. Additionally, it has been reported that the amygdala–IC interaction is highly involved in CTA memory establishment. Therefore, we evaluated the effects of infusions of an AMPA receptor antagonist (CNQX) and a NMDA receptor antagonist (APV) into the amygdala on CTA retrieval and IC neurotransmitter levels. Infusion of APV into the amygdala impaired glutamate augmentation within the IC, whereas dopamine and norepinephrine levels augmentation persisted and a reliable CTA expression was observed. Conversely, CNQX infusion into the amygdala impaired the aversion response, as well as norepinephrine and dopamine augmentations in the IC. Interestingly, CNQX infusion did not affect glutamate elevation in the IC. To evaluate the functional meaning of neurotransmitters elevations within the IC on CTA response, we infused specific antagonists for the AMPA, NMDA, D1, and β-adrenergic receptor before retrieval. Results showed that activation of AMPA, D1, and β-adrenergic receptors is necessary for CTA expression, whereas NMDA receptors are not involved in the aversion response.

Local N-methyl-d-aspartate receptor antagonism in the prefrontal cortex attenuates spatial cognitive deficits induced by gonadectomy in adult male rats

Gonadectomy in adult male rats significantly impairs spatial working memory, behavioral flexibility and other functions associated with the prefrontal cortex (PFC). However, the mechanisms through which this occurs are largely unknown. In this study, intracortical drug challenge with the selective N-methyl-d-aspartate receptor (NMDAR) antagonist D(−)-2-amino-5-phosphonopentanoic acid (APV) was combined with Barnes maze testing, gonadectomy (GDX) and hormone replacement (17β-estradiol, testosterone propionate) to explore the contributions of NMDAR-mediated activity within the PFC to hormone effects on spatial cognition in adult male rats. Previous studies have shown that Barnes maze testing reveals significant estrogen-dependent, GDX-induced deficits in spatial working memory and androgen-sensitive, GDX-induced deficits in spatial search strategy. Here we found that bilateral infusion of APV into the medial PFC prior to testing significantly improved both sets of behaviors in gonadectomized rats and significantly worsened performance measures in gonadally intact controls. In hormone-replaced cohorts, we further found that behaviors that are normally similar to controls were significantly disrupted by APV, and those that are normally similar to gonadectomized rats were rescued by intracortical APV infusion. There were, however, no residual effects of APV on retention testing conducted 24h later. Together these findings suggest that hormone regulation of NMDAR-mediated activity specifically within the PFC may be fundamental to the effects of gonadal steroids on spatial cognition in males. Our findings further identify NMDAR antagonists as potentially novel, non-steroidal means of attenuating the cognitive deficits that can accompany gonadal hormone decline in human males in aging, clinical cases of hypogonadalism and in certain neurologic and psychiatric illnesses. Accordingly, it may be important to obtain in males the kind of detailed knowledge concerning hormone effects on, for example, the channel and electrophysiological properties of NMDAR that currently exists for the female brain.

NMDA Receptors in Retrosplenial Cortex Are Necessary for Retrieval of Recent and Remote Context Fear Memory

Over time, memory retrieval is thought to transfer from the hippocampus to a distributed network of neocortical sites. Of these sites, the retrosplenial cortex (RSC) is robustly activated during retrieval of remotely acquired, emotionally valenced memories. It is unclear, however, whether RSC is specifically involved in memory storage or retrieval, and which neurotransmitter receptor mechanisms serve its function. We addressed these questions by inhibiting NMDARs in RSC via infusions of APV before tests for context fear in mice. Anterior cingulate cortex (ACC) and dorsal hippocampus (DH), which have been implicated in the retrieval of remote and recent memory, respectively, served as neuroanatomical controls. Surprisingly, infusion of APV only into RSC, but not ACC or DH, abolished retrieval of remote memory, as revealed by lack of freezing to the conditioning context. APV infused into RSC also impaired retrieval of recent memory, but had no effect on conditioning or memory storage. Within-subject experiments confirmed that the role of RSC in memory retrieval is not time limited. RSC-dependent context fear memory retrieval was mediated by NR2A, but not NR2B, subunit-containing NMDARs. Collectively, these data are the first demonstration that NMDARs in RSC are necessary for the retrieval of remote and recent memories of fear-evoking contexts. Dysfunction of RSC may thereby contribute significantly to the reexperiencing of traumatic memories in patients with posttraumatic stress disorder.

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.

Augmented Prefrontal Acetylcholine Release during Challenged Attentional Performance

Previous research has demonstrated that attentional performance depends on the integrity of the cortical cholinergic input system and that such performance is associated with increases in cortical acetylcholine (ACh) release. The present experiment tested the hypothesis that the attentional impairments produced by bilateral basal forebrain infusions of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) are associated with attenuation of performance-associated increases in ACh release. Rats were trained in a sustained attention task and equipped with three guide cannula for the bilateral infusion of the NMDA receptor antagonist APV (0, 3, 20 nmol) and for the insertion of a dialysis probe into the medial prefrontal cortex (mPFC). APV or vehicle was infused remotely following completion of the first of five blocks of trials. During the first block, attentional performance was associated with a 140% increase in ACh efflux. Infusions of APV decreased the animals' ability to detect signals and augmented the increases in ACh efflux observed prior to infusions. These data indicate a dissociation between levels of attentional performance and increases in mPFC ACh release. Augmentation of performance-associated increases in mPFC cholinergic transmission is hypothesized to mediate the increased demands on attentional 'effort' that are required to maintain performance under challenging conditions.

Amygdalar NMDA Receptors Are Critical for the Expression of Multiple Conditioned Fear Responses

There is conflicting evidence regarding the issue of whether NMDA receptors in the basolateral amygdalar complex (BLA) are critically involved in the expression of conditioned fear. This matter was addressed by infusing the rat BLA with d,l-2-amino-5-phosphonovaleric acid (APV), a competitive NMDA receptor antagonist. APV infusion into the BLA was reported to block the expression of conditioned fear when measured by freezing but not when measured by fear-potentiated startle response to a loud noise. To examine this issue further, here we used multiple indices of conditioned fear, including analgesia, 22 kHz ultrasonic vocalization (USV), defecation, and freezing. Rats with bilateral BLA cannula implants underwent fear conditioning consisting of 10 tone-footshock pairings. Before context and tone fear-retention tests, animals received intra-BLA infusions with APV (2.5 microg/side) or artificial CSF. Both tone and context tests demonstrated that the expression of conditioned freezing, USV, defecation, and analgesia were significantly impaired by intra-amygdalar infusions of APV. In a second set of experiments, intra-BLA infusions of APV markedly impaired the normal expression of postshock fear responses during training, as measured by freezing, USV, and defecation. Immediate postshock fear expression was predictive of subsequent fear retention to the tone and context when the animals were not infused. These results are consistent with the hypothesis that amygdalar NMDA receptors participate in normal synaptic transmission and therefore the overall functioning of the amygdala.

Bidirectional modulation of basal forebrain N-methyl-D-aspartate receptor function differentially affects visual attention but not visual discrimination performance.

Basal forebrain neuronal circuits, specifically the corticopetal cholinergic system, mediate attentional abilities. The effects of infusions of N-methyl-D-aspartate (NMDA) and the competitive NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) into the basal forebrain were assessed in rats trained in an operant task designed to generate measures of sustained attention performance. Control animals were trained in a cued visual discrimination task devoid of explicit demands on attentional performance, but involving similar basic operant components as the sustained attention task. The effects of intrabasalis infusions of NMDA (1, 3 and 6nmol) and APV (3, 10 and 20nmol) were tested in separate groups of animals. Infusion of neither drug affected the animals' response accuracy in the cued visual discrimination task, indicating that performance in this task remains insufficient to activate basal forebrain NMDA receptors. Infusions of APV in sustained attention task-performing animals selectively decreased the animals' ability to detect visual signals, but spared their ability to reject non-signal events. Conversely, infusions of NMDA into the basal forebrain did not affect the animals' hit rate but increased their number of false alarms, i.e. "claims" for signals in non-signal trials. The concentrations of NMDA infused into the basal forebrain did not result in neurotoxic effects as demonstrated by a separate experiment, which indicated neurodegeneration following the infusion of 30 nmol NMDA as visualized by the Fluoro-Jade method.The effects of APV correspond with the attentional consequences of other manipulations known to impair the functions of cortical cholinergic input. Conversely, the effects of NMDA infusions agree with the hypothesis that overactivity of cortical cholinergic inputs mediates an abnormal overprocessing of the stimulus situation. Basal forebrain NMDA receptor manipulations assist in determining the role of this neuronal system in cognitive processes.

Learning-associated activation of nuclear MAPK, CREB and Elk-1, along with Fos production, in the rat hippocampus after a one-trial avoidance learning: abolition by NMDA receptor blockade

It is widely accepted that the formation of long-term memory (LTM) requires neuronal gene expression, protein synthesis and the remodeling of synaptic contacts. From mollusk to mammals, the cAMP/PKA/CREB signaling pathway has been shown to play a pivotal role in the establishment of LTM. More recently, the MAPK cascade has been also involved in memory processing. Here, we provide evidence for the participation of hippocampal PKA/CREB and MAPK/Elk-1 pathways, via activation of NMDA receptors, in memory formation of a one-trial avoidance learning in rats. Learning of this task is associated with an activation of p44 and p42 MAPKs, CREB and Elk-1, along with an increase in the levels of the catalytic subunit of PKA and Fos protein in nuclear-enriched hippocampal fractions. These changes were blocked by the immediate posttraining intra-hippocampal infusion of APV, a selective blocker of glutamate NMDA receptors, which renders the animals amnesic for this task. Moreover, no changes were found in control-shocked animals. Thus, inhibitory avoidance training in the rat is associated with an increase in the protein product of an IEG, c- fos, which occurs concomitantly with the activation of nuclear MAPK, CREB and Elk-1. NMDA receptors appear to be a necessary upstream step for the activation of these intracellular cascades during learning.

Amygdalar NMDA Receptors are Critical for New Fear Learning in Previously Fear-Conditioned Rats

NMDA receptors in the amygdala seem to be critical for fear conditioning in naive rats. Recent spatial-learning studies suggest that previous learning protected animals from the amnesic effect of NMDA antagonists on new learning (of a similar behavioral task). Therefore, the present study examined whether blocking of NMDA receptors in the basolateral nucleus of the amygdala (BLA) prevents new fear learning in previously fear-conditioned rats, as measured by freezing behavior. Intra-BLA infusions of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV) completely blocked fear conditioning to a tone stimulus in animals that had previously been fear-conditioned to a light stimulus. Similar results were obtained with intra-BLA infusions of APV before contextual fear conditioning in rats that had been fear-conditioned to a different context. Additional experiments showed that intra-BLA APV infusions substantially interfere with the expression and extinction of conditioned fear to tone, light, and context stimuli. Together, these results indicate that NMDA receptors in the BLA are crucial for the encoding of new fear memories (i.e., the formation of specific conditioned stimulus-unconditioned stimulus association), the expression of conditioned fear responses, and the extinction of acquired fear.

Evidence for functional differences between entopeduncular nucleus and substantia nigra: effects of APV (DL-2-amino-5-phosphonovaleric acid) microinfusion on reaction time performance in the rat.

Overactivity of the excitatory amino acid outputs of the subthalamic nucleus (STN) has recently been found to be one of the cascade of subsequent disruptions caused by nigrostriatal dopaminergic degeneration in Parkinson's disease. The respective contribution of the excitatory glutamatergic output structures of the STN [i.e. the globus pallidus (GP), entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr)] to the control of movement is not known, however. To investigate further the function of glutamatergic transmission through NMDA receptor subtypes in these three structures, the effects of discrete local infusion of a competitive receptor antagonist, DL-2-amino-5-phosphonovaleric acid (APV), into the EP, GP and SNr were tested in rats performing a reaction time task. Bilateral infusion of APV into the different output structures of the STN differentially impaired the performance of rats trained to release a lever after the onset of a visual stimulus within a time limit to obtain a food reward. Infusion of APV (0.25 and 0.5 microgram/0.5 microliter) into the SNr was found to induce behavioural deficits characterized by a dramatic increase in the number of premature lever releases and decreased mean reaction time. In contrast, the infusion of APV at a dose of 0.25 microgram into the GP or EP was found to induce a motor initiation deficit characterized by an increased number of delayed responses (lever release after the time limit) and increased mean reaction time. At a dose of 0.5 microgram, a premature responding deficit was added to the previous motor impairment. Interestingly, when APV was infused simultaneously into the GP and SNr in the same animals, the behavioural effects tended to be similar to those observed after a single infusion into the SNr. Altogether, these results reveal that the different functional weight of the three main output pathways originating at the STN level is t.o. The behavioural deficits induced by NMDA receptor blockade in the SNr were similar to those observed previously after a neurotoxic lesion of the STN, suggesting that NMDA receptors in this structure play a major role as a functional output of the STN. Furthermore, regarding the differential effects produced by the same dose of APV in the SNr and the EP, these two structures, which are classically believed to be functionally linked should not be considered as the same functional entity in the organization of basal ganglia outflow.

Pre- and posttraining infusion of N-methyl-D-aspartate receptor antagonists into the amygdala impair memory in an inhibitory avoidance task.

Involvement of amygdaloid N-methyl-D-aspartate (NMDA) receptors in memory processes was investigated. Rats with cannulas implanted in the basolateral amygdala were trained on a 1 trial step-through inhibitory avoidance task and tested for 24-hr retention. Pretraining infusion of 2-amino-5-phosphonovaleric acid (APV) into the amygdala, but not striatum or hippocampus, produced a dose-dependent retention deficit, which was attenuated by immediate posttraining intra-amygdala infusion of NMDA. Posttraining APV infusion also caused a dose- and time-dependent retention deficit. Pretest APV infusion had no effect on performance in the retention test. Further, pre- or posttraining infusion of 5.0 micrograms APV failed to affect acquisition and retention in the Morris water maze task. These findings suggest that amygdala NMDA receptors are normally activated by aversive training and play a critical role in memory formation for affective experience.

Disruption of experience-dependent synaptic modifications in striate cortex by infusion of an NMDA receptor antagonist

To assess the possibility that NMDA receptors play a special role in visual cortical plasticity, the selective antagonist 2-amino-5-phosphonovaleric acid (APV) was continuously infused into the striate cortex of kittens as the visual environment was manipulated during the critical period. The cortex was studied using single-unit recording from sites between 3 and 6 mm from the infusion cannulae. One week of D,L-APV infusion coincident with monocular deprivation or "reverse suture" produced a concentration-dependent increase in the percentage of neurons that (1) lacked normal orientation selectivity and (2) were responsive to stimulation of the deprived eye. These effects outlasted the presence of the drug in the tissue. APV treatment also prevented the acquisition of selectivity and visual responsiveness that normally results from monocular visual experience after dark-rearing. Lasting effects of chronic APV infusion were not observed in adult striate cortex. The effects of APV on kitten striate cortex depended on the presence of the D stereoisomer as infusion of L-APV was without effect. Estimates of extracellular concentration using 3H-APV indicated that significant effects could be obtained with concentrations as low as 20 microM D,L-APV. Recordings from units during infusion indicated that visual responses were reduced by APV. Nonetheless, a normal percentage of visually responsive neurons was found at sites greater than or equal to 3 mm from the infusion cannula. There was no evidence that chronic APV infusion affected the sampling frequency of recorded neurons or disrupted cytoarchitecture at the sites further than 3 mm from the infusion cannula. Taken together, the data indicate that the effects of APV on kitten striate cortex are likely due specifically to the blockade of NMDA receptors. These data are considered in relation to several hypotheses concerning the role of NMDA receptors in the experience-dependent development of striate cortex.