Afferent Input Selects NMDA Receptor Subtype to Determine the Persistency of Hippocampal LTP in Freely Behaving Mice

Abstract

The glutamatergic N-methyl-D-aspartate receptor (NMDAR) is critically involved in many forms of hippocampus-dependent memory that may be enabled by synaptic plasticity. Behavioral studies with NMDAR antagonists and NMDAR subunit (GluN2) mutants revealed distinct contributions from GluN2A- and GluN2B-containing NMDARs to rapidly and slowly acquired memory performance. Furthermore, studies of synaptic plasticity, in genetically modified mice in vitro, suggest that GluN2A and GluN2B may contribute in different ways to the induction and longevity of synaptic plasticity. In contrast to the hippocampal slice preparation, in behaving mice, the afferent frequencies that induce synaptic plasticity are very restricted and specific. In fact, it is the stimulus pattern and not variations in afferent frequency that determine the longevity of long-term potentiation (LTP) in vivo. Here, we explored the contribution of GluN2A and GluN2B to LTP of differing magnitudes and persistence in freely behaving mice. We applied differing high-frequency stimulation (HFS) patterns at 100 Hz to the hippocampal CA1 region, to induce NMDAR-dependent LTP in wild-type (WT) mice, that endured for <1 h (early (E)-LTP), (LTP, 2-4 h) or >24 h (late (L)-LTP). In GluN2A-knockout (KO) mice, E-LTP (HFS, 50 pulses) was significantly reduced in magnitude and duration, whereas LTP (HFS, 2 × 50 pulses) and L-LTP (HFS, 4 × 50 pulses) were unaffected compared to responses in WT animals. By contrast, pharmacological antagonism of GluN2B in WT had no effect on E-LTP but significantly prevented LTP. E-LTP and LTP were significantly impaired by GluN2B antagonism in GluN2A-KO mice. These data indicate that the pattern of afferent stimulation is decisive for the recruitment of distinct GluN2A and GluN2B signaling pathways that in turn determine the persistency of hippocampal LTP. Whereas brief bursts of patterned stimulation preferentially recruit GluN2A and lead to weak and short-lived forms of LTP, prolonged, more intense, afferent activation recruits GluN2B and leads to robust and persistent LTP. These unique signal-response properties of GluN2A and GluN2B enable qualitative differentiation of information encoding in hippocampal synapses.

Keywords: GluN2A; GluN2B; LTP; NMDA; hippocampus; in vivo; mouse; synaptic plasticity.

Figure 1

Basal synaptic transmission and input/output (IO) properties are equivalent in freely behaving GluN2A-knockout (KO) mice compared to wildtypes. (A) Field excitatory postsynaptic potentials (fEPSPs) evoked by test-pulse stimulation were stable and comparable in wild-type (WT) and GluN2A-KO mice over the 25 h monitoring period (“Baseline”). (B) IO properties were similar in WT and KO mice. The stimulus intensity was increased stepwise in the range of 20–125 μA. Insets: examples of fEPSPs evoked at the time-points indicated by the numbers in (A). Horizontal scale bar: 10 ms, vertical scale bar: 2 mV.

Figure 2

Early-long-term potentiation (E-LTP) is impaired, whereas LTP and late-LTP (L-LTP) are unaltered in freely behaving GluN2A-KO mice. (A) High-frequency stimulation (HFS) comprising 50 pulses at 100 Hz evokes significant E-LTP (<1 h) in WT mice. The same protocol, when given to GluN2A-KO mice, results in a significant impairment of E-LTP. (B) HFS comprising 100 Hz, given as two bursts of 50 pulses, induces robust LTP (2–4 h) in both WT and KO mice. (C) A stronger HFS that comprises 100 Hz, given as four bursts of 50 pulses induces L-LTP, (>24 h) in WT mice. KO animals respond to this protocol with L-LTP that is not significantly different to WT L-LTP. Insets: examples of fEPSPs evoked at the time-points indicated by the numbers. Horizontal scale bar: 10 ms, vertical scale bar: 2 mV.

Figure 3

Antagonism of GluN2B in freely behaving WT mice prevents LTP (2 h) but not E-LTP (<1 h), antagonism of GluN2B in GluN2A-KO mice prevents LTP (>2 h). (A) Specific antagonism of GluN2B by Ifenprodil has no effect on E-LTP induced in WT mice. (B) Ifenprodil elicits no effect on E-LTP induced in WT, whereas LTP (2 h) is prevented. (C) Ifenprodil prevents E-LTP and LTP in GluN2A KO mice. Insets: examples of fEPSPs evoked at the time-points indicated by the numbers. Horizontal scale bar: 10 ms, vertical scale bar: 2 mV.