Effects of selective inhibition of N-acetylated-α-linked-acidic dipeptidase (NAALADase) on mice in learning and memory tasks

Abstract

The purpose of the present study was to examine the effects of 2-(phosphonomethyl)-pentanedioic acid (2-PMPA), a selective inhibitor of N-acetylated-α-linked-acidic dipeptidase (NAALADase, glutamate carboxypeptidase II), an enzyme catalyzing the cleavage of glutamate from the neuropeptide N-acetyl-aspartyl-glutamate (NAAG), on memory processes in mice. Long-term memory was evaluated in step-through passive avoidance task while alternation behavior, as a measure involving spatial working memory, was assessed in Y-maze task. Additionally, horizontal activity was evaluated by means of electronically monitored locomotor activity system. The mice were treated with either 2-PMPA (50, 100 and 150 mg/kg i.p.) or N-methyl-d-aspartate (NMDA) receptor antagonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) at doses of: 0.05, 0.1, 0.15 and 0.2 mg/kg i.p., as a comparator. In the passive avoidance task, the drugs were administered once before or immediately after training, and before retention test. 2-PMPA at the doses used did not affect retention of passive avoidance; however, it increased the latency to enter the dark box during the training day. In the Y-maze task, 2-PMPA (150 mg/kg i.p.) impaired spontaneous alternation and reduced locomotion while the lower dose of 100 mg/kg was ineffective. In the locomotor activity test, 2-PMPA (100 and 150 mg/kg i.p.) did not significantly affect horizontal activity. MK-801 (0.2 mg/kg i.p.) injected before training reduced retention in the passive avoidance task. In the Y-maze task, MK-801 (0.1 mg/kg i.p.) impaired alternation behavior and considerably increased locomotion in the Y-maze and locomotor activity test. These results indicate that NAALADase inhibition may impair alternation behavior.

Introduction

Accumulating data suggest that inhibition of N-acetylated-α-linked-acidic dipeptidase (NAALADase), an enzyme responsible for the hydrolysis of the neuropeptide N-acetyl-aspartyl-glutamate (NAAG) to N-acetyl-aspartate and glutamate, might be a useful strategy in the treatment of pathological conditions produced by an increase in glutamatergic transmission (Slusher et al., 1999). A selective NAALADase inhibitor, 2-(phosphonomethyl)-pentanedioic acid (2-PMPA; Jackson et al., 1996) has been shown to be neuroprotectant in animal models of ischemia (Slusher et al., 1999). During cerebral ischemia, inhibition of NAALADase might provide neuroprotection by both increasing NAAG and decreasing glutamate levels (Slusher et al., 1999). Further, it has been reported that NAALADase inhibition attenuated the development of cocaine seizure kindling (Witkin et al., 2002) and blocked the appetitive effects of cocaine in rodents (Slusher et al., 2001). Unlike N-methyl-d-aspartate (NMDA) receptor antagonists, administration of 2-PMPA does not impair learning and memory in a holeboard test (Slusher et al., 1999) and does not produce behavioral toxicity in an inverted screen test, thought to be a test for the occurrence of psychotomimetic effects (Ginski and Witkin, 1994, Witkin et al., 2002). Therefore, NAALADase inhibition may be a new therapeutic strategy without the typical behavioral side effects of glutamate receptor antagonists (Witkin et al., 2002).

The purpose of this study was to evaluate the influence of NAALADase inhibition on cognitive function by the use of two animal tests of learning and memory which are Y-maze spontaneous alternation task and passive avoidance task. In the Y-maze task, spontaneous alternation of mice is regarded as a measure involving spatial working memory (Maurice et al., 1994, Sarter et al., 1988). The step-through passive avoidance task may give information about acquisition (learning) and recall (retrieval), which are components of long-term memory (Venault et al., 1986). In the current study, mice were treated with 2-PMPA or a non-competitive NMDA receptor antagonist, (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10-imine hydrogen maleate (MK-801) (Wong et al., 1986), as a comparator. MK-801 was chosen because of its potent amnesic effects both in the Y-maze (Maurice et al., 1994, Parada-Turska and Turski, 1990) and passive avoidance task (Adriani et al., 1998, Benvenga and Spaulding, 1988).