Acute, but not chronic, exposure to d-cycloserine facilitates extinction and modulates spontaneous recovery of a conditioned taste aversion

Scopolamine infusion in the basolateral amygdala after saccharin intake induces conditioned taste avoidance in rats

RATIONALE

Muscarinic receptor activity in the basolateral amygdala (BLA) is known to be involved in plasticity mechanisms that underlie emotional learning. The BLA is involved in the Attenuation of Neophobia, an incidental taste learning task in which a novel taste becomes familiar and recognized as safe.

OBJECTIVE

Here we assessed the role of muscarinic receptor activity in the BLA in incidental taste learning.

METHODS

Young adult male Wistar rats were bilaterally implanted with cannulas aimed at BLA. After recovery, rats were randomly assigned to either vehicle or muscarinic antagonist group, for each experiment. We tested the effect of specific and non-specific muscarinic antagonists administered either 1) 20 min before novel taste presentation; 2) immediately after novel taste presentation; 3) immediately after retrieval (the second taste presentation on Day 5 -S2-) or immediately after the fifth taste presentation on Day 8 (S5).

RESULTS

Non-specific muscarinic receptor antagonist scopolamine infused prior to novel taste, while not affecting novel taste preference, abolished AN, i.e., the increased preference observed in control animals on the second presentation. When administered after taste consumption, intra-BLA scopolamine not only prevented AN but caused a steep decrease in the taste preference on the second presentation. This scopolamine-induced taste avoidance was not dependent on taste novelty, nor did it generalize to another novel taste. Targeting putative postsynaptic muscarinic receptors with specific M1 or M3 antagonists appeared to produce a partial taste avoidance, while M2 antagonism had no effect.

CONCLUSION

These data suggest that if a salient gustatory experience is followed by muscarinic receptors antagonism in the BLA, it will be strongly and persistently avoided in the future. The study also shows that scopolamine is not just an amnesic drug, and its cognitive effects may be highly dependent on the task and the structure involved.

Behavioral Tagging: Role of Neurotransmitter Receptor Systems in Novel Object Recognition Long-Term Memory

Strong training is known to form long-term memory (LTM) as it is an inducer for both a learning tag (just like a synaptic tag/molecular tag) and plasticity-related proteins (PRPs), while weak training is an inducer of only a learning tag. However, weak training can also lead to LTM if paired with another behavioral task (open field in our study-a representative of a novel environment) around the time of PRP arrival. Weak behavioral training is a learning tag inducer, while the open field is a PRP inducer. The learning tag then captures these PRPs to form LTM. This is the basis of behavioral tagging (BT). BT is a well-known model for the evaluation of a few learning and memory forms. In this work, we examined the role of glutamate and D1/D5 (dopamine) receptors in the synthesis of a novel object recognition (NOR) tag (learning) as well as in PRP arrival, which come together to form NOR-LTM. Employing antagonists and/or agonists preceding or proceeding the open field and/or NOR training, it was revealed that the activation/stimulation of D1/D5 (dopamine) receptors and glutamatergic NMDA receptors plays a critical part in PRP arrival. We found that the activation/stimulation of NMDA receptors also contributes to the setting of the learning tag. Moreover, changes in glutamate, dopamine, and GABA neurotransmitter levels were also analyzed. These findings thus demonstrate the critical time window required for NOR-LTM formation based on the process of BT along with the role of activation/stimulation of D1/D5 (dopamine) receptors and NMDA receptors in the arrival of PRPs and learning tags for NOR-LTM formation.

GluN2B and GluN2A-containing NMDAR are differentially involved in extinction memory destabilization and restabilization during reconsolidation

Extinction memory destabilized by recall is restabilized through mTOR-dependent reconsolidation in the hippocampus, but the upstream pathways controlling these processes remain unknown. Hippocampal NMDARs drive local protein synthesis via mTOR signaling and may control active memory maintenance. We found that in adult male Wistar rats, intra dorsal-CA1 administration of the non-subunit selective NMDAR antagonist AP5 or of the GluN2A subunit-containing NMDAR antagonist TCN201 after step down inhibitory avoidance (SDIA) extinction memory recall impaired extinction memory retention and caused SDIA memory recovery. On the contrary, pre-recall administration of AP5 or of the GluN2B subunit-containing NMDAR antagonist RO25-6981 had no effect on extinction memory recall or retention per se but hindered the recovery of the avoidance response induced by post-recall intra-CA1 infusion of the mTOR inhibitor rapamycin. Our results indicate that GluN2B-containing NMDARs are necessary for extinction memory destabilization whereas GluN2A-containing NMDARs are involved in its restabilization, and suggest that pharmacological modulation of the relative activation state of these receptor subtypes around the moment of extinction memory recall may regulate the dominance of extinction memory over the original memory trace.

Intra-hippocampal D-cycloserine rescues decreased social memory, spatial learning reversal, and synaptophysin levels in aged rats

RATIONALE

Aging is characterized by a decrease in N-methyl-D-aspartate receptors (NMDARs) in the hippocampus, which might be one of the factors involved in the age-dependent cognitive decline. D-Cycloserine (DCS), a partial agonist of the NMDAR glycine recognition site, could improve memory deficits associated to neurodegenerative disorders and cognitive deficits observed in normal aging.

OBJECTIVES AND METHODS

The aim of the present study was to explore whether DCS would reverse age-dependent memory deficits and decreases in NMDA receptor subunits (GluN1, GluN2A, and GluN2B) and the presynaptic protein synaptophysin in Wistar rats. We investigated the effects of pre-training infusions of DCS (10 μg/hemisphere) in the ventral hippocampus on two hippocampal-dependent learning tasks, the social transmission of food preference (STFP), and the Morris water maze (MWM).

RESULTS

The results revealed that infusions of DCS administered before the acquisition sessions rescued deficits in the STFP retention and MWM reversal learning in old rats. DCS also significantly increased the hippocampal levels of synaptophysin in old rats, which correlated with STFP and MWM performance in all tests. Moreover, although the levels of the GluN1 subunit correlated with the MWM acquisition and reversal, DCS did not enhance the expression of such synaptic protein.

CONCLUSIONS

The present behavioral results support the role of DCS as a cognitive enhancer and suggest that enhancing the function of NMDARs and synaptic plasticity in the hippocampus may be related to improvement in social memory and spatial learning reversal in aged animals.

Effects of Augmenting N-Methyl-D-Aspartate Receptor Signaling on Working Memory and Experience-Dependent Plasticity in Schizophrenia: An Exploratory Study Using Acute d-cycloserine

Cognitive deficits in schizophrenia have been hypothesized to reflect N-methyl-D-aspartate receptor (NMDAR) dysfunction. However, the mechanisms through which the NMDAR contributes to individual cognitive functions differ. To explore how NMDAR signaling relates to specific cognitive deficits in schizophrenia, we tested the effects of enhancing NMDAR signaling on working memory and experience-dependent plasticity using d-cycloserine (DCS). Plasticity was assessed using an EEG paradigm that utilizes high-frequency visual stimulation (HFvS) to induce neural potentiation, and 2 learning tasks, the information integration (IIT) and weather prediction (WPT) tasks. Working memory was assessed using an N-back task. Forty-five schizophrenia patients were randomized to receive a single 100 mg DCS dose (SZ-DCS; n = 24) or placebo (SZ-PLC; n = 21) in a double-blind, between-groups design. Testing occurred on a single day after placebo or DCS administration; baseline values were not obtained. DCS did not affect plasticity, as indicated by similar neural potentiation, and similar IIT and WPT learning between groups. However, among patients who successfully engaged in the working memory task (ie, performed above chance), SZ-DCS (n = 17) showed superior 2-back performance compared to SZ-PLC (n = 16). Interestingly, SZ-DCS also showed larger pre-HFvS neural responses during the LTP task. Notably, this pattern of DCS effects is the opposite of those found in our prior study of healthy adults. Results are consistent with target engagement of the NMDAR by DCS, but suggest that NMDAR signaling was not translated into synaptic plasticity changes in schizophrenia. Results highlight the importance of considering how distinct NMDAR-associated processes contribute to individual cognitive deficits in schizophrenia.

New Learning and Unlearning: Strangers or Accomplices in Threat Memory Attenuation?

To achieve greatest efficacy, therapies for attenuating fear and anxiety should preclude the re-emergence of emotional responses. Of relevance to this aim, preclinical models of threat memory reduction are considered to engage one of two discrete neural processes: either establishment of a new behavioral response that competes with, and thereby temporarily interferes with the expression of, threat memory (new learning) or one that modifies and thereby disrupts threat memory (unlearning). We contend that a strict dichotomy of new learning and unlearning does not provide a compelling explanation for current data. Instead, we suggest that the evidence warrants consideration of alternative models that assume cooperation rather than competition between formation of new cellular traces and the modification of preexisting ones.

Modulatory effects of an NMDAR partial agonist in MK-801-induced memory impairment

RATIONALE

Acute administration of the N-methyl-d-aspartate (NMDA) non-competitive antagonist, MK-801, impairs novel object recognition (NOR), locomotor activity in open field (OF) and conditioned taste aversion (CTA) in rodents. NMDAR partial agonist d-cycloserine (DCS) reverses these effects in NOR and CTA via modulation of glutamatergic, cholinergic and dopaminergic systems.

OBJECTIVES AND METHODS

To test this hypothesis, we investigated the effects of DCS, a partial NMDAR agonist, on NOR memory, locomotor activity, and CTA memory in Wistar rats on NMDA-glutamate receptor antagonism by MK-801. The potential involvement of dopaminergic and cholinergic systems in improving cognitive functions was explored. MK-801-induced cognitive deficits were assessed using NOR, OF and CTA paradigms. MK-801-induced dopamine release increase in acetylcholinesterase (AChE), mono amine oxidase (MAO) activity and increase in c-fos expression were also investigated.

RESULTS

The effects caused by MK-801 (0.2 mg/kg) were inhibited by administration of the NMDA receptor agonist DCS (15 mg/kg). NOR and CTA paradigms inhibited by MK-801 were attenuated by DCS administration. Moreover, DCS also blocked the MK-801-induced abnormal increase in dopamine content, AChE activity and MAO activity. However, c-fos overexpression was controlled to some extent only.

CONCLUSIONS

Based on the NMDAR hypo function hypothesis in some neuropsychiatric disorders, our finding suggests that improving NMDAR hypo function by agonist DCS may play a significant role.

Effects of treadmill exercise on the LiCl-induced conditioned taste aversion in rats

Studies have shown that exercise can enhance learning and memory. Conditioned taste aversion (CTA) is an avoidance behavior induced by associative memory of the taste sensation for something pleasant or neutral with a negative visceral reaction caused by the coincident action of a toxic substance that is tasteless or administered systemically. We sought to measure the effects of treadmill exercise on CTA in rats by investigating the effects of exercise on acquisition, extinction and spontaneous recovery of CTA. We made two groups of rats: an exercise group that ran on a treadmill, and a control group that did not have structured exercise periods. To condition rats to disfavor a sweet taste, consumption of a 0.1% saccharin solution in place of drinking water was paired with 0.15M LiCl (2% body weight, i.p.) to induce visceral discomfort. We measured changes of saccharin consumption during acquisition and extinction of CTA. The exercise and no-exercise groups both acquired CTA to similar levels and showed maximum extinction of CTA around 6 days after acquisition. This result indicates that exercise affects neither acquisition nor extinction of CTA. However, in testing for preservation of CTA after much longer extinction periods that included exercise or not during the intervening period, exercising animals showed a significantly lower saccharin intake, irrespective of having exercised or not during the conditioning phase of the trial. This result suggests that exercise may help to preserve aversive memory (taste aversion in this example) as evidence by the significant spontaneous recovery of aversion in exercising animals.

Persistent reduction of cocaine seeking by pharmacological manipulation of adenosine A1 and A 2A receptors during extinction training in rats

RATIONALE

Adenosine receptor stimulation and blockade have been shown to modulate a variety of cocaine-related behaviors.

OBJECTIVES

These studies identify the direct effects of adenosine receptor stimulation on cocaine seeking during extinction training and the persistent effects on subsequent reinstatement to cocaine seeking.

METHODS

Rats self-administered cocaine on a fixed ratio one schedule in daily sessions over 3 weeks. Following a 1-week withdrawal, the direct effects of adenosine receptor modulation were tested by administering the adenosine A1 receptor agonist, N(6)-cyclopentyladenosine (CPA, 0.03 and 0.1 mg/kg), the adenosine A2A agonist, CGS 21680 (0.03 and 0.1 mg/kg), the presynaptic adenosine A2A receptor antagonist, SCH 442416 (0.3, 1, and 3 mg/kg), or vehicle prior to each of six daily extinction sessions. The persistent effects of adenosine receptor modulation during extinction training were subsequently tested on reinstatement to cocaine seeking induced by cues, cocaine, and the dopamine D2 receptor agonist, quinpirole.

RESULTS

All doses of CPA and CGS 21680 impaired initial extinction responding; however, only CPA treatment during extinction produced persistent impairment in subsequent cocaine- and quinpirole-induced seeking. Dissociating CPA treatment from extinction did not alter extinction responding or subsequent reinstatement. Administration of SCH 442416 had no direct effects on extinction responding but produced dose-dependent persistent impairment of cocaine- and quinpirole-induced seeking.

CONCLUSIONS

These findings demonstrate that adenosine A1 or A2A receptor stimulation directly impair extinction responding. Interestingly, adenosine A1 receptor stimulation or presynaptic adenosine A2A receptor blockade during extinction produces lasting changes in relapse susceptibility.

Chronic dietary magnesium-L-threonate speeds extinction and reduces spontaneous recovery of a conditioned taste aversion

Elevation of brain magnesium enhances synaptic plasticity and extinction of conditioned fear memories. This experiment examined the generalizability of this phenomenon by studying the effects of a novel magnesium compound, magnesium-L-threonate (MgT), on conditioned taste aversion (CTA) extinction and spontaneous recovery (SR). Adult male Sprague-Dawley rats were maintained on a 23-hour water deprivation cycle and acquired a CTA following the taste of a CS [0.3% saccharin+16 mg/ml MgT (SAC+MgT)] paired with a US [81 mg/kg (i.p.) lithium chloride (LiCl)]. Following CTA acquisition, rats drank a water+MgT solution for up to 1 hour/day over the next 31 days. For 14 additional days, some animals continued water+MgT treatment, but others drank water only to allow MgT to be eliminated from the body. We then employed 2 different extinction paradigms: (1) CS-Only (CSO), in which SAC was presented, every-other day, or (2) Explicitly Unpaired (EU), in which both SAC and LiCl were presented, but on alternate days. EU extinction procedures have been shown to speed CTA extinction and reduce spontaneous recovery of the aversion. Throughout extinction, half of the rats in each group continued to drink MgT (now in SAC or supplemental water+MgT solution), whereas the other half drank SAC only/water only until SAC drinking reached ≥90% of baseline (asymptotic extinction). Rats receiving MgT just before/during extinction drank less SAC on the first day of extinction suggesting that they had retained a stronger CTA. MgT enhanced the rate of extinction. Furthermore, the MgT-treated rats showed a relatively modest SR of the CTA 30 days later - indicating that the extinction procedure was more effective for these animals. Our data suggest that long-term dietary MgT may enhance the consolidation/retention of a CTA, speed extinction, and inhibit SR of this learned aversion.

D-cycloserine in the basolateral amygdala prevents extinction and enhances reconsolidation of odor-reward associative learning in rats

It is well established that D-cycloserine (DCS), a partial agonist of the NMDA receptor glycine site, enhances learning and memory processes. Although the effects of DCS have been especially elucidated in the extinction and reconsolidation of aversive behavioral paradigms or drug-related behaviors, they have not been clearly determined in appetitive tasks using natural reinforcers. The current study examined the effects of pre-retrieval intra-basolateral amygdala (BLA) infusions of DCS on the extinction and reconsolidation of an appetitive odor discrimination task. Rats were trained to discriminate between three odors, one of which was associated with a palatable food reward, and, 20 min prior to extinction learning (experiment 1) or reactivation (experiment 2), they received bilateral intra-BLA infusions of DCS or vehicle. In experiment 1, DCS infusion reduced the rate of extinction learning, weakened extinction retention in a post-extinction test and enhanced reacquisition of the ODT task. In experiment 2, DCS improved subsequent memory expression in the reconsolidation test performed one day after the reactivation session. Such results indicate the involvement of BLA NMDA receptors in odor-food reward associative memory and suggest that DCS may potentiate the persistence or strength of the original memory trace.