Regular paperThe N-methyl-d-aspartate receptor modulator GLYX-13 enhances learning and memory, in young adult and learning impaired aging rats
Introduction
The glutamate N-methyl-d-aspartate receptor (NMDAR) plays a critical role in learning and memory. The NMDAR is critical for the induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic strength, forms of activity-dependent synaptic plasticity believed to be associated with learning and memory formation (Martin et al., 2000, Rison and Stanton, 1996, Stanton, 1996). NMDAR activation has been suggested to be critical for the acquisition of hippocampal-dependent learning tasks such as trace eye blink conditioning (tEBC; Weiss et al., 1999a, Weiss et al., 1999b), because NMDAR antagonists MK-801 and PCP prevent the acquisition of tEBC (Thompson and Disterhoft, 1997a) and the NMDAR glycine site partial agonist d-cycloserine (DCS) facilitates acquisition of tEBC (Thompson and Disterhoft, 1997b). Mice with point mutations in the NMDAR glycine binding site that greatly reduce NMDAR function show severe deficits in learning in the MWM, and learning is rescued by administration of the NMDAR glycine site partial agonist d-serine (Ballard et al., 2002, Kew et al., 2000).
There are age related decreases in NMDAR protein expression and function in humans and other animals. Receptor binding studies have shown an age related decrease in NMDAR binding in the frontal cortex and hippocampus of mice (Magnusson and Cotman, 1993), rats (Tamaru et al., 1991), rhesus monkeys, (Wenk et al., 1991), and humans (Piggott et al., 1992). In addition, hippocampal protein levels of the NMDAR obligatory NR1 subunit that contains the glycine binding sites (Lynch and Guttmann, 2001), decrease as a consequence of aging in mice (Magnusson et al., 2002), rats (Eckles-Smith et al., 2000), and rhesus monkeys (Gazzaley et al., 1996).
NMDAR alterations have also been reported in Alzheimer's disease (AD). Levels of NMDAR receptor binding (Jansen et al., 1990, Ułas et al., 1992), NR1 protein expression (Hynd et al., 2004, Mishizen-Eberz et al., 2004) and gene expression (Mishizen-Eberz et al., 2004) are all decreased in the cortex and hippocampus of AD patients compared to aged-matched controls.
Cognitive deficits in aging correlate with NMDAR deficits. Protein levels of NR1 (Adams et al., 2001) and NMDAR binding (Davis et al., 1993) in rats both correlate positively with learning acquisition rates in the MWM. Moreover, old rats are more sensitive than young adult rats to the memory impairing effect of the NMDA open channel blocker MK-801 (Ingram et al., 1992).
GLYX-13 is a recently developed tetrapeptide (Thr-Pro-Pro-Thr) that acts as a NMDAR receptor partial agonist at the glycine site with therapeutic potential as a cognitive enhancer (Moskal et al., 2005). GLYX-13 readily crosses the blood brain barrier and has been shown to increase both Schaffer collateral-CA1 LTP in vitro, and learning and memory in 3-month-old rats in a trace eye blink conditioning paradigm (Moskal et al., 2005). GLYX-13 administration in concert with a learning task has also been shown to elevate gene expression of hippocampal NR1 subunit in 3-month-old rats (Moskal et al., 2007).
In the present study, we examined the effect of GLYX-13 on learning and memory in the MWM, and t-maze tasks in young adult (3 months old) and aged F1 hybrid Fischer 344 X Brown Norway rats (FBNF1). We also examined the effect of GLYX-13 on tEBC in aging rats in order to compare their performance to that of young adult rats reported previously (Moskal et al., 2005).
Twenty-seven months of age was chosen because a study examining four different ages of adult rats (from 6 to 35 months), found that half of the 27-month-old group exhibited impairment of the trace eye blink conditioning task (Knuttinen et al., 2001). Since GLYX-13 increases learning and memory in hippocampally dependent tasks (Moskal et al., 2005), and enhances hippocampal LTP (Zhang et al., 2008), we also tested the hypothesis that the effects of GLYX-13 on both learning and synaptic plasticity might be greater in aged animals. Thus, we examined the effects of GLYX-13 on long-term potentiation (LTP) and depression (LTD) at Schaffer collateral-CA1 synapses in hippocampal slices from young adult (2-month-old) and aged (24-month-old) Fischer 344 rats.
Section snippets
Animals and housing
Male F1 hybrid Fischer 344 X Brown Norway (FBNF1) specific pathogen-free rats were used in these studies. Two groups of FBNF1 were used; young adult (3 months of age) and old (27–32 months of age). Animals were purchased from the National Institute of Aging Colony Maintained at Harlan (USA). By 32 months of age these animals have approximately a 50% mortality (Turturro et al., 1999). Animals were equipped with subdural femoral vascular access ports (Access Technologies, Chicago). Patency was
Trace eye blink conditioning
As illustrated in Fig. 1, GLYX-13 (1.0 mg/kg, i.v.) enhanced tEBC learning in the aged rats, indexed by percent adaptive CRs across the 10 test days compared to vehicle treated animals (main effect of drug − (F(1,23) = 14.4, P < .001). In both vehicle-treated and GLYX-13-treated 27–32 months old FBNF1 rats, performance improved across test days (F(9,23) = 16.0, P < .0001). GLYX-13-treated animals required 90% fewer trials to learn the task (8 out of 10 consecutive trials with an adaptive CRs) compared to
Discussion
GLYX-13 enhanced learning in all three hippocampus-dependent learning tasks (tEBC, MWM, T-Maze) in both young adult and age impaired old animals. In the MWM in which a clear age related deficit in learning was seen, GLYX-13 enhancement of learning was greater in old compared to young adult rats. These data corroborate and extend our previous report that GLYX-13 enhanced hippocampally dependent tEBC in vivo in young adult animals (Moskal et al., 2005). The previous tEBC study suggested that the
Acknowledgments
This research was supported by MH071932 to J.R.M., NS004421 to P.K.S., and AG08796 to J.F.D. We thank the Northwestern University Behavioral Phenotyping Core for its assistance.
Disclosure statements
1.
(a) Joseph Moskal is the inventor of the Glyxin family of neuropeptides of which GLYX-13 is one. He is a co-founder and stock holder in Naurex, Inc. Naurex, Inc. owns the Intellectual Property for GLYX-13. There are no financial, personal or other relationships with other people or organizations
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