Visual deprivation modifies both presynaptic glutamate release and the composition of perisynaptic/extrasynaptic NMDA receptors in adult visual cortex

Abstract

Use-dependent modifications of synapses have been well described in the developing visual cortex, but the ability for experience to modify synapses in the adult visual cortex is poorly understood. We found that 10 d of late-onset visual deprivation modifies both presynaptic and postsynaptic elements at the layer 4-->2/3 connection in the visual cortex of adult mice, and these changes differ from those observed in juveniles. Although visual deprivation in juvenile mice modifies the subunit composition and increases the current duration of synaptic NMDA receptors (NMDARs), no such effect is observed at synapses between layer 4 and layer 2/3 pyramidal neurons in adult mice. Surprisingly, visual deprivation in adult mice enhances the temporal summation of NMDAR-mediated currents induced by bursts of high-frequency stimulation. The enhanced temporal summation of NMDAR-mediated currents in deprived cortex could not be explained by a reduction in the rate of synaptic depression, because our data indicate that late-onset visual deprivation actually increases the rate of synaptic depression. Biochemical and electrophysiological evidence instead suggest that the enhanced temporal summation in adult mice could be accounted for by a change in the molecular composition of NMDARs at perisynaptic/extrasynaptic sites. Our data demonstrate that the experience-dependent modifications observed in the adult visual cortex are different from those observed during development. These differences may help to explain the unique consequences of sensory deprivation on plasticity in the developing versus mature cortex.

Figure 1.

Visual deprivation in adult mice fails to modify NMDAR EPSCs evoked by single pulses. Shown is a scatter plot of the weighted time constants (τ_w) of NMDAR-mediated EPSCs recorded from layer 2/3 pyramidal cells after layer 4 is stimulated in the visual cortex of visually deprived and control juvenile mice as well as deprived and control adult mice. Small circles represent individual data points, and larger circles represent the means ± SEM. NMDAR-mediated currents are significantly longer in the visual cortex of deprived juvenile mice as compared with controls. Visual deprivation in adult mice does not alter NMDAR-mediated current duration. Normalized traces are representative of pharmacologically isolated NMDAR EPSCs recorded at +40 mV, and an overlay of the traces (top) is included as a basis for comparisons. ^*p < 0.05.

Figure 2.

Visual deprivation in adult mice alters the temporal summation of NMDAR EPSCs evoked by burst stimulation in the visual cortex. A, Plot of the normalized and averaged amplitudes of NMDAR EPSCs evoked at 40 Hz in the adult visual cortex at a holding potential of +40 mV. Representative traces of pharmacologically isolated NMDAR EPSCs in response to 40 Hz stimulus trains are shown (dark trace represents response from pyramidal neuron in deprived mice; light trace represents response from pyramidal neuron in control mice). Stimulus artifacts were blanked for clarity. B, Same as in A, but recordings were made at a holding potential of –70 mV in ACSF containing nominal magnesium (0.1 mm MgCl₂). Error bars indicate the means ± SEM.

Figure 3.

Visual deprivation increases the rate of synaptic depression in adult mice. Shown is a plot of the AMPAR EPSC amplitudes in response to a brief 40 Hz stimulation train. Responses were normalized to the first pulse. Traces are representative AMPAR EPSCs recorded at –70 mV in cells from deprived (dark trace) and control (light trace) mice. Stimulus artifacts were blanked for clarity. Error bars indicate the means ± SEM.

Figure 4.

Visual deprivation increases the rate of neurotransmitter release in adult mice. Shown is a plot of the normalized amplitude of NMDAR EPSCs in response to repetitive stimulation in the presence of MK-801. Note that NMDAR EPSC blockade by MK-801 occurs faster in pyramidal cells from deprived adult mice than from controls. Error bars indicate the means ± SEM.

Figure 5.

Visual deprivation in juvenile mice alters the NR2A/B ratio in the PSD, but only in PNS and LSM fractions in adult mice. A, Biochemical fractionation progressively enriches NR2A, NR2B, and PSD-95 and eliminates a nonsynaptic protein, β-tubulin, invisual corticals amples from deprived and control adult mice. Samples of 10μg were loaded into each gel lane. B, Quantification of NR2A and NR2B band intensities, which were normalized to the value at 12.5μg. The inset is a representative NR2A/B immunoblot of a PSD fraction. Samples of 1–12μg were loaded into each gel lane; results from three blots were averaged. C, NR2A/B ratios were measured in PNS, LSM, and PSD fractions of visual cortices of deprived and control from both juvenile and adult mice. The values (means±SEM) are normalized to average control values. ^*p < 0.05.

Figure 6.

Minimal stimulation fails to reveal deprivation-induced differences in the temporal summation of NMDAR-mediated currents. Shown is a plot of the temporal summation of NMDAR EPSCs in deprived and control mice evoked by minimal stimulation at 40 Hz. Error bars indicate the means ± SEM.

Figure 7.

The enhanced temporal summation of NMDAR EPSCs in deprived adult cortex can be blocked by acute administration of ifenprodil, an NR2B-specific NMDAR antagonist. A, Temporal summation of NMDAR EPSCs in deprived and control mice in the presence of ifenprodil. Error bars indicate the means ±SEM. B, The average charge transfer taken from the normalized responses of the 11 pulses evoked at 40 Hz stimulation in the presence or absence of ifenprodil. ANOVA with post hoc analyses; ^*p < 0.05.