Elsevier

Brain Research

Volumes 1073–1074, 16 February 2006, Pages 131-138
Brain Research

Research Report
Early chronic blockade of NR2B subunits and transient activation of NMDA receptors modulate LTP in mouse auditory cortex

https://doi.org/10.1016/j.brainres.2005.12.077Get rights and content

Abstract

In the auditory cortex, the properties of NMDA receptors depend primarily on the ratio of NR2A and NR2B subunits. NR2B subunit expression is high at the beginning of critical period and lower in adulthood. Because NMDA receptors are crucial in triggering long-term potentiation (LTP) and long-term depression, developmental or experience-dependent modification of NMDAR subunit composition is likely to influence synaptic plasticity. To examine how NMDA subunit change during postnatal development affect the adult synaptic plasticity, we employed chronic ifenprodil blockade of NR2B subunits and analyzed evoked field potentials in adult C57BL/6 mice auditory cortex (AC). We found that chronic loss of NR2B activity led to a decline in LTP magnitude in the AC of adult mice. Adding NMDA to the artificial cerebrospinal fluid (ACSF) in blocked mice had the opposite effect, producing LTP magnitudes at or exceeding those found in treated or untreated animals. These results suggest that, even in adulthood when NR2B expression is downregulated, these receptor subunits play an important role in experience-dependent plasticity of mouse auditory cortex. Blockade from P60 did not result in any decrease of LTP amplitude, suggesting that chronic block in postnatal period may permanently affect cortical circuits so that they cannot produce significant LTP in adulthood.

Introduction

There is accumulating evidence that NMDA receptors play an essential role in regulating synaptic plasticity and sensory cortical development in this postnatal period. NMDA receptors are heteromeric complexes containing both NR1 and NR2 subunits (Monyer et al., 1994). In central neurons, NMDA receptors are most abundant as NR1/NR2A and NR1/NR2B subunit combinations (Sheng et al., 1994). NR2 subunit expression is developmentally regulated in the neocortex such that NR2A expression is weak early in development and increases rapidly with age, whereas the expression of NR2B subunits is initially high and declines by adulthood (Erisir and Harris, 2003, Sheng et al., 1994, Watanabe et al., 1992). Studies on visual cortex have suggested a close relationship between the ratio of NR2A/NR2B subunits and susceptibility to experience-dependent plasticity (Kirkwood et al., 1996, Kleinschmidt et al., 1987, Philpot et al., 2001a, Quinlan et al., 1999a, Quinlan et al., 1999b). The developmental change in subunit composition results in faster excitatory postsynaptic currents (EPSCs) and lower sensitivity to NR2B-selective antagonists (Carmignoto and Vicini, 1992, Flint et al., 1997). This change is also thought to contribute to the developmental changes in NMDA-receptor-mediated plasticity at glutamatergic synapses (Philpot et al., 2001a). Crair and Malenka (1995) found that in thalamocortical synapses the time during which LTP was expressed paralleled the critical period. At the end of the critical period, a reduction on the ability to evoke LTP correlates with a decrease in NMDA-receptor-mediated synaptic currents. Because the NR1–NR2B complex generates longer excitatory postsynaptic potentials (EPSPs) than does the NR1–NR2A complex (Monyer et al., 1994), this decrease in NMDA-receptor-mediated synaptic current might be accounted for the decrease in NR2B expression.

As has been found in visual cortex, we observed previously that the expression of NR2B mRNA in mouse auditory cortex is maintained at a high level early in postnatal development (Cui et al., 2002), correlating with the beginning of the mouse critical period. We reasoned that, if NR2B is necessary for synaptic plasticity, blocking it should reduce the magnitude of long-term potentiation. Previous studies in rat auditory cortex have demonstrated that LTP of trans-synaptic field potentials could be blocked by APV, the antagonist of NMDA. To address this issue, we blocked NR2B subunit activity using ifenprodil from P13, the day when its expression is maximum (Cui et al., 2002).

Besides NR2 subunits composition change with age and experience, artificial manipulation of NMDA receptor function during the postnatal critical period also affects the development of the nervous system. Chronic blockade of superior collicular NMDA receptors during the critical period prevents normal emergence of auditory maps in the midbrain (Ingham et al., 1998). Similar results have been obtained in ferret superior colliculus (Schnupp et al., 1995) and in the neocortex of other animals (Kleinschmidt et al., 1987, Schnupp et al., 1995). Thus, we hypothesized that chronic blockade of NR2B subunits of NMDA receptors in mouse auditory cortex might undermine the experience-dependent plasticity of adult animals. A further interesting question is whether a chronic block would have a more severe effect than an acute block. Chronic blockade may permanently affect cortical circuits so that they cannot produce LTP in adulthood. Thus, we asked, are the presence of NR2B and the plasticity it produces essential for normal levels of plasticity (LTP) in adults? If this is the case, we would expect to see reduced LTP with NR2B block from P13 to adulthood and less of a reduction with NR2B block in adulthood (from P60 onward).

Based on this hypothesis, the next goal would be to investigate whether the possible undermined cortical plasticity could be reversed by acute drug application. NMDA receptor activation requires the simultaneous release of glutamate and depolarization of postsynaptic membrane. Previous studies have found that NMDA or glutamate application potentiates synaptic transmission in the hippocampus (Kauer et al., 1988), but no evidence to date has ruled out the possibility that the agonist could activate NMDA-receptor-dependent LTP in NR2B blockade animals, let alone to what extent NMDA might restore LTP.

Our results demonstrate that chronic application of ifenprodil by implantation of Elvax in mice auditory cortex from P13 but not from P60 decreased the amplitude of LTP in adult mice. Acute NMDA perfusion reversed the weakened LTP. These findings suggest that chronic blockade of NR2B subunits negatively affects the experience-dependent development of mouse auditory cortex during the critical period, leading to impaired synaptic plasticity in adult mice. The restoration of LTP by NMDA application in NR2B-blocked animals further suggests that clinical intervention may be possible in learning and memory disorders resulting from cognitive decline.

Section snippets

Effects of the NR2B antagonist ifenprodil on LTP in the auditory cortex

To determine whether NR2B subunits contribute significantly to long-term modifications at synapses, we chose ifenprodil, a noncompetitive antagonist of NR2B subunits, to block NR2B chronically during postnatal development. Animals were treated with Elvax containing ifenprodil (Elvax-Ifenprodil) or Elvax containing normal saline on P13.

Previous studies have shown that LTP of trans-synaptic population spikes in the AC can be elicited selectively by tetanic stimulation of white matter and that the

Chronic blockade of NR2B

The NMDA receptor is considered to be crucial for the synaptic plasticity that organizes the developing nervous system (Cline and Constantine-Paton, 1989, Fox et al., 1996). It has been demonstrated that the experience-dependent regulation of NR2A and NR2B subunits of the NMDA receptor can modify the induction of synaptic plasticity (Philpot et al., 2001b, Quinlan et al., 1999a, Quinlan et al., 1999b). For example, a low NR2A/NR2B ratio correlates with enhanced LTP and diminished LTD (Kirkwood

Animals

C57BL/6 mice were from the animal center of Shanghai Institute for Biological Science. Animals were reared in the normal condition in our laboratory. The brain slice preparation in the experiment was from the animal at around P73 (P13-implanted group) and P120 (P60-implanted group).

Slice preparation

Mice were anesthetized with ether. Immediately after decapitation, the brain was immersed in ice-cold ACSF (bubbled with 95% O2 and 5% CO2) for 3 min to reduce the temperature. The composition of ACSF was (in mM):

Acknowledgments

We particularly thank Dr. Sarah. L. Pallas and other members of the Pallas laboratory for critically reviewing the manuscript. We also thank Dr. Gary B. Silberstein for generously providing the Elvax. This work was supported by the National Natural Science Foundation of China (Grant No. 39870246, 90208012) and research funds from the National Doctoral Research Foundation of China (Grant No. 2000026905).

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    Current address: Graduate Program in Neurobiology and Behavior, Department of Biology, Georgia State University, Atlanta, Georgia, USA.

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