Cross-Modal Reinstatement of Thalamocortical Plasticity Accelerates Ocular Dominance Plasticity in Adult Mice

Abstract

Plasticity of thalamocortical (TC) synapses is robust during early development and becomes limited in the adult brain. We previously reported that a short duration of deafening strengthens TC synapses in the primary visual cortex (V1) of adult mice. Here, we demonstrate that deafening restores NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) of TC synapses onto principal neurons in V1 layer 4 (L4), which is accompanied by an increase in NMDAR function. In contrast, deafening did not recover long-term depression (LTD) at TC synapses. Potentiation of TC synapses by deafening is absent in parvalbumin-positive (PV+) interneurons, resulting in an increase in feedforward excitation to inhibition (E/I) ratio. Furthermore, we found that a brief duration of deafening adult mice recovers rapid ocular dominance plasticity (ODP) mainly by accelerating potentiation of the open-eye responses. Our results suggest that cross-modal sensory deprivation promotes adult cortical plasticity by specifically recovering TC-LTP and increasing the E/I ratio.

Keywords: E/I ratio; NMDA receptor function; adult cortical plasticity; cross-modal plasticity; thalamocortical LTP; visual cortex.

Figure 1.. Deafening Specifically Recovers TC-LTP in Adult V1 L4

(A) Schematic of V1 recordings where LGN terminals expressed ChR2. LTP and LTD were induced using a pairing protocol, where postsynaptic depolarization to 0 mV and −40 mV, respectively, was paired with presynaptic stimulation with light pulses (455 nm LED; 5 ms pulse duration, 200 pulses at 1 Hz). (B) Left: low-magnification image of a V1 slice showing ChR2-YFP (green) expression in LGN axons especially in L4. Blue, DAPI. Right: high-magnification image of a recorded L4 neuron filled with biocytin (red) surrounded by ChR2-YFP-expressing LGN axons. (C) Left: TC-LTP in adult DF mice (red, n = 9 cells/5 mice, *p < 0.02 between baseline and 30 min post-pairing), but not in NR (blue, n = 9 cells/6 mice, not significant [N.S.]p = 0.9934). APV blocked LTP in the DF group (open circles, n = 6 cells/3 mice, N.S. p = 0.7564). Bottom: no significant change in input resistance (Ri). Right: example average EPSPs taken before (left) and 30 min after pairing (right). (D) Left: lack of TC-LTD in adult control (NR: blue, n = 9 cells/5 mice, N.S. p = 0.7152) and DF (red, n = 9 cells/5 mice, p = 0.3824). Bottom: no significant change in Ri. Right: example average EPSPs taken before (left) and 30 min after pairing (right). Data plotted are mean ± SEM.

Figure 2.. Regulation of NMDAR Function at TC Synapses onto Adult V1 L4 Neurons following Deafening

(A). Left: NMDA/AMPA ratio measurements for the NR and DF groups (open circles represent individual cells; NR = 0.49 ± 0.045, 4 mice; DF = 0.43 ± 0.067, 5 mice; t test, N.S. p = 0.5173). Right: averaged example traces for an NR cell (blue) and DF cell (red) normalized to the AMPAR component in NR. The AMPAR component was measured at the peak recorded at −80 mV, while the NMDAR component was measured 70 ms after the onset of the compound EPSC recorded at +40 mV. (B). Left: weighted decay time constant (Tw) for a pharmacologically isolated NMDAR current measured at +40 mV for the NR and DF groups (open circles represent individual cells; NR = 89.31 ± 11.97 ms, 4 mice; DF = 63.93 ± 6.3 ms, 3 mice; t test, N.S. p = 0.0961). Right: averaged NMDAR responses for the NR (blue) and DF (red) groups normalized to the maximum amplitudes. Bar graphs: mean ± SEM.

Figure 3.. Deafening Adults Did Not Change the Strength of TC Inputs to PV+ Interneurons and Enhanced E/I Ratio of TC Inputs to V1 L4 Neurons

(A). Schematic of a td-Tomato-expressing PV+ neuron (red) targeted in V1 L4 for LEv-Sr+² mEPSCs recordings to assess the strength of TC synapses. (B). Confocal image of a V1 slice showing LGN axons expressing ChR2-YFP (green) and PV+ neurons expressing Td-Tomato (red) and counterstained for DAPI (blue). (C). Left: example traces of LEv-Sr²⁺ mEPSCs from the NR (top) and DF (bottom) groups. Dashed gray line represents the time window used to measure spontaneous mEPSC events before LED stimulation (blue triangle, 455 nm, 5 ms duration). Solid blue line represents the time window used to measurethe post-LED events. From the measured post-LED events, pre-LED spontaneous events were mathematically subtracted (see STAR Methods) to obtain the events evoked by ChR2 activation of TC axons. Middle: average traces of calculated LEv-Sr²⁺-mEPSCs (NR, blue; DF, red). Right: comparison of average amplitude of TC LEv-Sr²⁺-mEPSCs between NR and DF (open circles represent individual cells; NR = 18.9 ± 1.7 pA, 7 mice; DF = 19.6 ± 1.65 pA, 7 mice; t test, N.S. p = 0.7893). (D). Left: schematic showing targeted V1 L4 principal neurons for E/I ratio recording where thalamic terminals (green) were stimulated with light (455 nm) to evoke a monosynaptic EPSC and a disynaptic inhibitory postsynaptic current (IPSC) responses in L4. Middle: average trace of LED evoked monosynaptic eEPSC (inward current recorded at E_GABA = —52 mV) and disynaptic eIPSC (outward current recorded at E_glu = 0 mV) from a L4 cell after light stimulation of ChR2-expressing LGN axons. Right: significant increase in average E/I ratio values after DF (open circles represent each cell; NR, 4 mice; DF, 4 mice; t test, *p = 0.0127). Bar graphs: mean ± SEM.

Figure 4.. Deafening Accelerated Ocular Dominance Plasticity in Adult V1 by Promoting Open-Eye Potentiation

(A). Outline of experimental groups. (B). Schematic of visual stimulus presented while recording V1 intrinsic signals. (C). A representative images and data from a young mouse (young short MD). Left: surface vasculature used to guide imaging in the same animal before (top) and after (bottom) MD. Scale bars, 1 mm. Middle two panels: V1 intrinsic signals recorded before (top) and after (bottom) MD for contralateral (C) and ipsilateral (I) eyes (an open circle indicates an open eye, and a closed circle indicates a closed eye; L, lateral, R, rostral). Right: histogram showing the distribution of pixels corresponding to ODI values. Note a left shift in the distribution after MD indicating that more pixels having a lower ODI (ODI = 0 indicates neurons responding equally to both eyes). (D). Deafening accelerates ODP in the adult V1. Comparison of ocular dominance index [ODI = (C − I)/(C + I)] pre- and post-MD (bars show mean + SEM, gray lines connect ODI values measured pre- and post-MD for each mouse). Short (3–4 days) MD in young mice during the critical period (gray bars) shows a significant shift in ODI toward the open eye (pre = 0.378 ± 0.036, post = 0.1534 ± 0.044; paired t test, *p = 0.0132). A week of deafening adult mice prior to a short (3- to 4-day) MD (red bars) also allows a significant shift in ODI toward the open eye (pre = 0.3164 ± 0.048, post = 0.083 ± 0.068; paired t test *p = 0.0121). The same short-duration MD (3–4 days) in normal adult mice failed to shift ODI (pre = 0.264 ± 0.023, post = 0.2389 ± 0.044; paired t test, N.S. p = 0.5892). However, a longer-duration MD (5–6 days) significantly shifted ODI in normal adults (pre = 0.2682 ± 0.035, post = 0.1411 ± 0.060; paired t test *p = 0.170). Bar graphs: mean ± SEM. (E). Comparison of average eye-specific V1 activation signal intensity through contralateral eye (square symbols with solid line) and ipsilateral eye (triangles with dashed line) pre- and post-MD. In young mice, short MD (3–4 days) significantly decreased the intensity of signal from the closed contralateral eye while it increased that from the open ipsilateral eye (contralateral: pre = 1.76 ± 0.12, post =1.41 ± 0.16, paired t test *p = 0.0204; ipsilateral: pre = 0.72 ± 0.16, post = 1.21 ± 0.35, paired t test *p = 0.0213). In adults deafened for 1 week prior to short MD (3–4 days), there was only a significant increase in the intensity of signal from the open ipsilateral eye (contralateral: pre = 1.83 ± 0.21, post = 1.68 ± 0.12, paired t test N.S. p = 0.52; ipsilateral: pre =1.02 ± 0.16, post = 1.48 ± 0.18, paired t test *p = 0.0207). In normal adults, short MD (3–4 days) did not alter the intensity of signal from either eye (contralateral: MD = 1.9 ± 0.13, MD = 1.84 ± 0.16, paired t test N.S. p = 0.6782; ipsilateral: pre = 1.12 ± 0.09, post =1.12 ± 0.10, paired t test N.S. p = 0.9168), but a longer-duration MD (5–6 days) significantly increased the intensity of signal from the open ipsilateral eye (contralateral: pre = 1.81 ± 0.14, post =1.82 ± 0.18, paired t test p = 0.9629; ipsilateral: pre =1.08 ± 0.13, post = 1.45 ± 0.11, paired t test *p = 0.0328). Data plotted are mean ± SEM.