Effect of D-serine on a delayed match-to-place task for the water maze

Reduced attentional lapses in male rats following a combination treatment of low-dose D-serine and atomoxetine

BACKGROUND

Goal-directed attention involves the selective processing of behaviorally relevant sensory information. This selective processing is thought to be supported by glutamatergic and noradrenergic systems. Pharmacotherapies that simultaneously target these systems could therefore be effective treatments for impaired attention.

AIMS

We first tested an N-methyl-D-aspartate (NMDA) receptor co-agonist (D-serine) for effects on attention (processing speed and attentional lapses). NMDA receptor activation is thought to support noradrenergic effects on sensory processing; therefore, we tested a combination treatment comprising D-serine and a norepinephrine reuptake inhibitor (atomoxetine).

METHODS

D-serine was first tested in rats performing a two-choice visuospatial discrimination task. Combination treatments comprising relatively low doses of D-serine and atomoxetine were then tested in a separate group.

RESULTS

In experiment 1, D-serine reduced the skew of initiation time (IT) distributions (IT devmode) at the highest dose tested (300 mg/kg). In experiment 2, low-dose D-serine (125 mg/kg) had no effect, while low-dose atomoxetine (0.3 mg/kg) reduced IT devmode and slowed movement speed. Importantly, the combination of these relatively low doses of D-serine and atomoxetine reduced IT devmode more than either drug alone without further slowing movement speed.

CONCLUSIONS

IT devmode is thought to reflect attentional lapses; therefore, D-serine's effects on IT devmode suggest that NMDA receptors are involved in the preparatory deployment of attention. Greater effects following a combination of D-serine and atomoxetine suggest that preparatory attention can be facilitated by targeting glutamatergic and noradrenergic systems simultaneously. These results could inform the development of improved treatments for individuals with ADHD who experience abnormally high attentional lapses.

Effects of low-dose D-serine on recognition and working memory in mice

RATIONALE

D -Serine is an endogenous co-agonist of the N-methyl-D: -aspartate (NMDA) receptor and has been suggested to improve cognitive deficits in schizophrenia.

OBJECTIVES

The present study investigates the effects of treatment with D -serine in mice on tasks that require recognition learning and working memory, two cognitive domains that are impaired in schizophrenia.

METHODS

We studied the effects of various regimens of systemic administration of D -serine (50 mg/kg/day) on BALB/c mice performing object recognition, T-maze alternation, and open-field exploration tasks. For the object recognition task, we also contrasted the effects of D -serine and D -cycloserine and investigated whether D -serine could reverse alterations induced by subchronic injections of the NMDA antagonist MK-801. D -Serine levels after injections were measured by high-performance liquid chromatography.

RESULTS

In the object recognition task, pre-training treatment with D -serine or D -cycloserine significantly enhanced recognition memory 24 h after training. A single administration of D -serine 30 min (but not 6 h) after training produced similar enhancement, suggesting an effect on memory consolidation. Daily treatment with D: -serine enhanced both object recognition and T-maze performance over multiple days and improved short-term memory in MK-801-treated mice. D -Serine treatment did not alter open-field exploration. Behavioral effects were accompanied by increased levels of D -serine in the hippocampus of treated animals.

CONCLUSIONS

Our results show that treatment with D -serine can improve performance in tasks related to recognition learning and working memory, suggesting that this agent can be useful for the treatment of disorders involving declines in these cognitive domains.

D-serine treatment reduces cocaine-primed reinstatement in rats following extended access to cocaine self-administration

The most intractable feature of drug addiction is the high rate of relapse, even following extended periods of abstinence from drug-taking. Evidence suggests that allowing rats extended access to cocaine self-administration leads to behavioral characteristics in these animals that are consistent with the development of addiction in humans. In the current study, rats were allowed to self-administer cocaine over a total of 22 daily sessions, the final seven of which were long-access (LgA) sessions of 6 h duration. Assessments of reinstatement of drug-seeking behavior were made following reintroduction to the drug-taking environment and noncontingent priming with either conditioned stimulus (CS) or cocaine in both extinguished and abstinent subject groups. Three separate groups of rats were treated with either saline or D-serine (100 mg/kg i.p.) administered 2 h prior to, or immediately following, each extinction training session. Saline-treated LgA rats were resistant to the effects of extinction training to reduce noncontingent priming of reinstatement of drug-seeking behavior with either CS or cocaine. In contrast, treatment with D-serine either before or immediately following the sessions resulted in a significant enhancement in the ability of extinction training to reduce cocaine-primed reinstatement of drug-seeking behavior. These results suggest that D-serine can act to enhance the consolidation of extinction learning in LgA rats, and is therefore a promising adjunctive agent along with behavioral therapy for the treatment of cocaine addiction.

D-serine facilitates the effects of extinction to reduce cocaine-primed reinstatement of drug-seeking behavior

Male Sprague Dawley rats were allowed to self-administer cocaine (0.5 mg/kg) during 90 min sessions for a period of 15 days. On day 16, rats were either held abstinent in their home cage environment or experienced an extinction session in which the active lever had no programmed consequences. Facilitating N-methyl-D-aspartate (NMDA) receptor activity with the coagonist D-serine (100 mg/kg i.p.) before or following the extinction session significantly reduced the subsequent cocaine-primed reinstatement of drug-seeking behavior tested on day 17. D-serine significantly reduced drug-primed reinstatement only when combined with extinction, and its effectiveness when administered following the training session suggested that an enhancement of consolidation of extinction learning had occurred. In contrast, D-serine treatment did not reduce sucrose-primed reinstatement, indicating that the beneficial effects of this adjunct pharmacotherapy with extinction training were specific to an addictive substance (cocaine) and did not generalize to a natural reward (sucrose).

The effects of extinction training in reducing the reinstatement of drug-seeking behavior: involvement of NMDA receptors

Although the process of extinction has been well documented for various forms of behavioral responses, the effects of extinction on the reinstatement of drug-seeking behavior are relatively understudied. In this report, the effectiveness of an extinction training protocol to reduce primed reinstatement responses was compared with the effectiveness of an equivalent period of enforced abstinence. We found that extinction training performed in the drug taking environment significantly reduced reinstatement behavior subsequently primed by either contextual cues, conditioned cues, or cocaine infusion. The ability of extinction to reduce cocaine primed reinstatement was blocked by the systemic administration of the competitive NMDAR antagonist ((+/-)CPP, 5mg/kg i.p.) administered prior to each extinction training session. Interestingly, this pharmacological intervention had no impact on the effectiveness of extinction to reduce drug-seeking behavior primed by either contextual cues or conditioned cues. These results suggest that an extinction training experience involves multiple mechanisms that can be dissociated into nonNMDAR and NMDAR dependent components with respect to the type of reinstatement (i.e. context-, conditioned stimuli (CS)-, or drug-induced) being assessed.

N-Methyl-D-aspartate receptors as a target for improved antipsychotic agents: novel insights and clinical perspectives

RATIONALE

Activation of "co-agonist" N-methyl-D-aspartate (NMDA) and Glycine(B) sites is mandatory for the operation of NMDA receptors, which play an important role in the control of mood, cognition and motor function.

OBJECTIVES

This article outlines the complex regulation of activity at Glycine(B)/NMDA receptors by multiple classes of endogenous ligand. It also summarizes the evidence that a hypoactivity of Glycine(B)/NMDA receptors contributes to the pathogenesis of psychotic states, and that drugs which enhance activity at these sites may possess antipsychotic properties.

RESULTS

Polymorphisms in several genes known to interact with NMDA receptors are related to an altered risk for schizophrenia, and psychotic patients display changes in levels of mRNA encoding NMDA receptors, including the NR1 subunit on which Glycine(B) sites are located. Schizophrenia is also associated with an overall decrease in activity of endogenous agonists at Glycine(B)/NMDA sites, whereas levels of endogenous antagonists are elevated. NMDA receptor "open channel blockers," such as phencyclidine, are psychotomimetic in man and in rodents, and antipsychotic agents attenuate certain of their effects. Moreover, mice with genetically invalidated Glycine(B)/NMDA receptors reveal similar changes in behaviour. Finally, in initial clinical studies, Glycine(B) agonists and inhibitors of glycine reuptake have been found to potentiate the ability of "conventional" antipsychotics to improve negative and, albeit modestly, cognitive and positive symptoms. In contrast, therapeutic effects of clozapine are not reinforced, likely since clozapine itself enhances activity at NMDA receptors.

CONCLUSIONS

Reduced activity at NMDA receptors is implicated in the aetiology of schizophrenia. Correspondingly, drugs that (directly or indirectly) increase activity at Glycine(B) sites may be of use as adjuncts to other classes of antipsychotic agent. However, there is an urgent need for broader clinical evaluation of this possibility, and, to date, there is no evidence that stimulation of Glycine(B) sites alone improves psychotic states.