Separable features of visual cortical plasticity revealed by N-methyl-D-aspartate receptor 2A signaling

Abstract

How individual receptive field properties are formed in the maturing sensory neocortex remains largely unknown. The shortening of N-methyl-d-aspartate (NMDA) receptor currents by 2A subunit (NR2A) insertion has been proposed to delimit the critical period for experience-dependent refinement of circuits in visual cortex. In mice engineered to maintain prolonged NMDA responses by targeted deletion of NR2A, the sensitivity to monocular deprivation was surprisingly weakened but restricted to the typical critical period and delayed normally by dark rearing from birth. Orientation preference instead failed to mature, occluding further effects of dark rearing. Interestingly, a full ocular dominance plasticity (but not orientation bias) was selectively restored by enhanced inhibition, reflecting an imbalanced excitation in the absence of NR2A. Many of the downstream pathways involved in NMDA signaling are coupled to the receptor through a variety of protein-protein interactions and adaptor molecules. To further investigate a mechanistic dissociation of receptive field properties in the developing visual system, mice carrying a targeted disruption of the NR2A-associated 95-kDa postsynaptic density (PSD95) scaffolding protein were analyzed. Although the development and plasticity of ocular dominance was unaffected, orientation preference again failed to mature in these mice. Taken together, our results demonstrate that the cellular basis generating individual sensory response properties is separable in the developing neocortex.

Figure 1

NR2A deletion prolongs NMDA response in mouse visual cortex. (A) NR2A protein increases dramatically with eye opening (before, P10; after, P18) but well in advance of the critical period (P28) in WT animals (four mice per age normalized to P10 values). (B) Representative traces at P28 from visual cortical neurons in NR2A KO (red) or WT (black) slices. Stimulation was adjusted to evoke half-maximal monosynaptic AMPA response before isolating NMDA currents with γ-aminobutyric acid type A antagonist/6-cyano-7-nitroquinoxaline-2,3-dione (20 μM) and raising membrane potential to +40 mV. Note that AMPA currents recorded first (at −90 mV) were indistinguishable. (Scale, 50 pA, 100 ms.) (C) The decay time constant of NMDA currents is typically shortened by the peak of the critical period in WT mice (P14, six cells, and P28, seven cells; five mice each; P < 0.03). At eye opening (P14) there is no significant difference in kinetics between NR2A KO (five cells, three mice) and WT. By P28 (10 cells, seven KO mice) and beyond (eight cells, seven WT mice), NMDA currents are dramatically longer without NR2A. Adult KO currents (six cells, six mice) are no different from immature WT (P14; P = 0.15). ***, P < 0.001, KO vs. WT by age group (young P14, P14–15; critical period P28, P25–31; adult P52, P43–58). Some error bars (SEM) are smaller than symbol size. (D) Total charge transfer through NRs is enhanced at peak of the critical period in NR2A KO mice. For the same cells as described above at P28: **, P < 0.01.

Figure 2

OD plasticity is weak and restricted in NR2A KO mice. (A) Typically skewed OD histogram of WT mice (No-MD) is shifted by MD toward the open, ipsilateral eye at P25 (open circle, P25-MD; P < 0.0001 vs. no MD, χ² test) but not in adults (>P45, Adult-MD; P > 0.4 vs. no MD, χ² test). The CBI of each distribution in the upper-right corner ranges from 0 (ipsi only) to 1 (contra only). (B) NR2A KO similarly loses sensitivity to MD in adulthood (same age groups as described for A: No-MD vs. P25-MD, P < 0.002, or Adult-MD, P > 0.2, χ² test). The magnitude of shift at P25, however, is significantly weaker than in WT (P < 0.0001, χ² test).

Figure 3

E-I balance restores full plasticity to NR2A KO mice. (A) Both γ-aminobutyric acid reduction by GAD65 deletion (16) or long-lasting NMDA currents in the absence of NR2A could similarly bias cortical networks in favor of excitation. Enhancing functional inhibition (shaded region) with diazepam may restore the balance needed to detect competition during MD. (B) CBI shift from the nondeprived range (shaded region) is significantly weaker in NR2A KO (filled bars) than WT (open bars; 5 and 13 mice, respectively; ***, P < 0.0001, t test) but fully restored by diazepam (DZ) concomitant with MD during the critical period [vehicle (Veh) and diazepam, four mice each; **, P < 0.01, t test]. (C) Critical-period delay by DR regardless of NR2A expression. DR from birth (DR alone, four WT and three KO mice) retains sensitivity to brief MD in both WT (open bars) and NR2A KO (filled bars) mice (DR+MD, five WT and four KO) when the critical period is normally closed (MD alone, three WT and four KO mice) (17, 28). Shaded region, range of nondeprived, LR adults (>P45).

Figure 4

Sensory experience regulates orientation preference through NR2A. (A) The percentage of WT cells (373 cells, 13 mice) responding preferentially to bars of particular orientation is reduced by DR (90 cells, three mice) and recovers with increasing (days) reexposure to light (+4 days light, +8 days light; 156 and 115 cells, five and four mice, respectively). ***, P < 0.0005, and **, P < 0.01 vs. adult LR WT. (B) Normally reared NR2A KO mice (LR; 113 cells, four mice) exhibit poor orientation preference that occludes effects of DR (78 cells, three mice). Neither reexposure to light (DR + 30 days light; 85 cells, three mice) nor brief diazepam at P28 (DZ; 116 cells, four mice) rescues this receptive field property (Veh; 113 cells, four mice). Shaded region, range of adult WT.

Figure 5

Full OD plasticity but impaired orientation bias in PSD95 KO mice. (A) Developmental time course of PSD95 in WT mouse visual cortex with no effect of DR (four mice per age normalized to P10 values). (B) Brief MD around P28 is similarly effective in KO and WT mice (five and four mice, respectively; P = 0.8, t test). Shaded region, range of nondeprived mice (four KO and three WT; P = 0.9, t test). (C) Orientation-biased cells remain few in mature PSD95 KO mouse visual cortex compared with WT (246 and 178 cells, nine and seven mice, respectively; **, P < 0.001, t test).