The effects of NMDA subunit composition on calcium influx and spike timing-dependent plasticity in striatal medium spiny neurons

Abstract

Calcium through NMDA receptors (NMDARs) is necessary for the long-term potentiation (LTP) of synaptic strength; however, NMDARs differ in several properties that can influence the amount of calcium influx into the spine. These properties, such as sensitivity to magnesium block and conductance decay kinetics, change the receptor's response to spike timing dependent plasticity (STDP) protocols, and thereby shape synaptic integration and information processing. This study investigates the role of GluN2 subunit differences on spine calcium concentration during several STDP protocols in a model of a striatal medium spiny projection neuron (MSPN). The multi-compartment, multi-channel model exhibits firing frequency, spike width, and latency to first spike similar to current clamp data from mouse dorsal striatum MSPN. We find that NMDAR-mediated calcium is dependent on GluN2 subunit type, action potential timing, duration of somatic depolarization, and number of action potentials. Furthermore, the model demonstrates that in MSPNs, GluN2A and GluN2B control which STDP intervals allow for substantial calcium elevation in spines. The model predicts that blocking GluN2B subunits would modulate the range of intervals that cause long term potentiation. We confirmed this prediction experimentally, demonstrating that blocking GluN2B in the striatum, narrows the range of STDP intervals that cause long term potentiation. This ability of the GluN2 subunit to modulate the shape of the STDP curve could underlie the role that GluN2 subunits play in learning and development.

Figure 1. Model cell shows MSPN characteristics.

A. Morphology of model MSPN (not to scale). Inset: tertiary dendrites have 11 segments each 36 µm in length. B. Comparison traces demonstrating latency to first AP in the model MSPN and an experimental whole-cell recording of a mouse MSPN. Both traces are a voltage response to current injection of 280 pA. Scale bars: 20 mV vertical, and 100 ms horizontal. C. Comparison current-voltage relationships (−500 pA to 200 pA) for the model MSPN and an experimental recording demonstrating inward rectification. Scale bars: 10 mV vertical, and 100 ms horizontal. D. current-voltage relationship for computational model compared with mean current-voltage relationship from 25 MSPNs of the mouse dorsal striatum. (See also Text S1: Supplemental Methods) Error bars ±SD.

Figure 2. STDP protocol type changes calcium influx through NMDAR.

A. Model traces showing each STDP protocol. Scale bars: 10 mV vertical, 10 ms horizontal. B. NMDAR-mediated calcium during positive (solid color line) and negative (dotted color line) Δt compared to calcium through NMDA during synaptic stimulation only (No AP, solid black line). Scale bars: 2 µM [Ca²⁺] vertical, 50 ms horizontal. C. Model calcium curves showing change in peak NMDAR calcium as a percentage of control (synapse stimulation with no AP). Spines stimulated are 40 µm from the soma. D. Example single experiments showing tLTP in response to 30 ms depolarization STDP, but not to 5 ms depolarization STDP. pre = average of baseline EPSCs prior to STDP; post = average of EPSCs around 60 minutes after STDP. Scale bars: 100 pA vertical, 20 ms horizontal. E. All experimental data points approximately 60 minutes after STDP protocol application as % of baseline. 5 ms depolarization STDP failed to induced significant tLTP when compared to 30 ms depolarization STDP. (*** = p<0.0001).

Figure 3. Calcium curves for different GluN2 subunit containing NMDARs.

Dependence of NMDAR-mediated calcium on AP timing is different for each GluN2 subunit. Normalized peak NMDAR calcium is plotted for each AP timing interval (Δt) using the 30 ms STDP protocol. Spines stimulated are 40 µm from the soma.

Figure 4. GluN2B broadens the STDP curve.

A–B. Inhibition of GluN2B restricts the time window of tLTP induction. Representative experiments illustrate the time course of synaptic efficacy changes induced by pre-post pairings in control aCSF and in the presence of ifenprodil (10 µM). Insets: averaged EPSCs before (black) or after (grey: control; blue: ifenprodil treatment) STDP. Scale bars 100 pA vertical, 10 ms horizontal. A. Example experiments showing tLTP induced by narrow AP timing intervals (0<Δt<12 ms) in control and ifenprodil conditions. B. Example experiments showing tLTP is induced by wide AP timing intervals (12<Δt<30 ms) in control conditions, but not induced when GluN2B containing NMDARs were blocked by Ifenprodil. Black arrow indicates the STDP protocol induction. C. Summary of Experimental data: Spike-timing dependent changes in synaptic efficacy estimated 60 minutes after STDP induction in control and Ifenprodil conditions. Blue shading highlights Δt shorter than 12 ms; pink shading highlights Δt between 12 ms and 30 ms. D. Bar graph of long-term synaptic efficacy changes shows that the GluN2B inhibition affects the range of Δt that permits tLTP induction. (* = p<0.05, n.s. = not significant) E. Peak NMDAR calcium curves from the model MSPN for positive Δt. All curves are normalized to the GluN2A+B no AP condition. F. Bar graph showing average NMDA calcium elevation for narrow Δt (+2 ms to +12 ms) and wide Δt. (+13 ms to +30 ms) for each NMDAR condition (GluN2A+B, GluN2A alone, and 75% GluN2A+B).

Figure 5. Distance from soma reduces dependence on AP timing.

A. Illustration of dendritic branch, showing the decay of the EPSP traveling from either the tertiary (red) or secondary (blue) dendritic spines. In all panels “tertiary” refers to the third segment of the tertiary dendrite (tert 3). Scale bars 0.5 mV vertical, 10 ms horizontal. B. Overlay of tertiary and secondary EPSPs as seen at the spine. Scale bars: 0.5 mV vertical, 10 ms horizontal. C. Overlay of tertiary and secondary EPSPs for the same stimulations in B as seen at the soma. Scale bars: 0.5 mV, 10 ms D. Spine depolarizations resulting from the back-propagating AP for the primary, secondary, and tertiary dendrites. Scale bars: 10 mV vertical, 5 ms horizontal. E. Peak NMDAR calcium curves for each GluN2 subunit on the tertiary dendrite and the secondary dendrite. Insets: NMDAR-mediated calcium traces for secondary (top) and tertiary (bottom) dendrites for positive (Δt = +11 ms, solid color), negative (Δt = −12 ms, dotted color) and no AP control conditions (solid black). Scale bars 1 µM, vertical, 10 ms horizontal.

Figure 6. The cortico-striatal and thalamo-striatal synapses onto MSPNs have different synaptic characteristics.

A. Peak NMDAR calcium curves from simulations using cortico-striatal and thalamo-striatal parameters from mouse striatum (Ding et al., 2008) and from rat striatum (Smeal et al., 2008). Cortico-striatal curve for mouse is the same as the green trace in figure 2C. B. NMDAR-mediated calcium for positive (Δt = +11 ms, solid color), negative (Δt = −12 ms, dotted color) and no AP control conditions (solid black) under cortico-striatal and thalamo-striatal conditions in mouse and rat. Scale bars: 2 µM [Ca²⁺] vertical, 50 ms horizontal. thal dist = thalamo-striatal synapses stimulated on tertiary dendritic spines, thal prox = thalamo-striatal synapses stimulated on secondary dendritic spines. cortex prox = cortico-striatal synapses stimulated on secondary dendritic spines.