Tumor necrosis factor-alpha mediates one component of competitive, experience-dependent plasticity in developing visual cortex

Abstract

Rapid, experience-dependent plasticity in developing visual cortex is thought to be competitive. After monocular visual deprivation, the reduction in response of binocular neurons to one eye is matched by a corresponding increase to the other. Chronic optical imaging in mice deficient in TNFalpha reveals the normal initial loss of deprived-eye responses, but the subsequent increase in response to the open eye is absent. This mutation also blocks homeostatic synaptic scaling of mEPSCs in visual cortex in vitro, without affecting LTP. In monocular cortex, thought not to be subject to competition, responses in TNFalpha mutants are as reduced as in the binocular zone. Pharmacological inhibition of endogenous TNFalpha in wild-type mice phenocopies the knockout. These findings suggest that experience-dependent competition in developing visual cortex is the outcome of two distinct, noncompetitive processes, a loss of deprived-eye responses followed by an apparently homeostatic increase in responses dependent on TNFalpha signaling.

Fig. 1

Normal long-term potentiation (LTP) and deficient synaptic scaling in visual cortical slices prepared from TNFα knockout mice. (A) LTP is normal in Tnf−/− mice (n=6) compared to wild type mice (n=9). The peak amplitudes of field excitatory postsynaptic potentials (fEPSP) recorded in layer 3 in response to layer 4 stimulation are plotted as a function of time. Theta burst stimulation (TBS) was delivered at time 0. Insets on the right show example traces of fEPSP (averaged responses (6 traces = 1 minute) from around 1 and 2 in the graph. The smaller trace is pre-LTP (1) and the larger trace post-LTP (2). (B-D) Homeostatic synaptic scaling assayed in organotypic slice cultures of mouse visual cortex prepared from Tnf−/− and Tnf+/+ mice. The amplitudes of miniature excitatory postsynaptic currents (mEPSC) are increased after 2 days of blockade of glutamatergic excitatory transmission (using CNQX/APV) in Tnf+/+ mice but not in Tnf−/− mice (n = 10 – 13 cells per condition).

Fig. 2

Plasticity following monocular visual deprivation is reduced in Tnf−/− mice. (A) Ocular dominance index (ODI) in individual animals (circles) and group average (horizontal line) computed from responses measured by intrinsic signal optical imaging. The gray box represents the mean ± SD of baseline ODI in Tnf+/+ animals with no deprivation. ND: no deprivation, MD: 5 days of monocular deprivation starting at P26-27. Inbred C57/Bl6 wild type animals received cortical infusion of soluble TNF receptor 1 (sTR) or vehicle (veh) during the MD. ** P<0.001 and * P<0.01 vs. corresponding ND group; § P<0.05 vs. Tnf+/+ MD; † P<0.05 vs. vehicle-treated animals. The inset on the right hand illustrates the visual stimuli used to evoke intrinsic signal responses in the binocular zone. (B) Distribution of ocular dominance scores of single units recorded electrophysiologically in Tnf+/+, Tnf+/−, or Tnf−/− mice after 5 days of MD starting at P26-27 and in age-matched animals without visual deprivation (ND). Data from Tnf+/+ and Tnf+/− mice were pooled because no significant difference was detected in ODIs between these two groups. Control (Tnf+/+ and +/−) ND: 3 mice, 78 cells; control MD: 4 mice, 122 cells; Tnf−/− ND: 3 mice, 101 cells; Tnf−/− MD: 4 mice 121 cells. P values in the figure were from Fisher exact test. (C) Contralateral bias index (CBI), which measures relative responses of single neurons to the two eyes, shown for individual animals (circles) and average group values (horizontal lines) computed from single unit data presented in (B). The gray box indicates the mean ± SD of baseline CBI in control animals. **P<0.01 and *P<0.05 vs. corresponding ND group; § P<0.05 vs. MD-Tnf+/+ group. Statistical analyses in A and C were performed using one-way ANOVA with Bonferroni multiple comparisons.

Fig. 3

Repeated imaging of intrinsic signal reveals that lack of TNFα signaling impairs the delayed component of plasticity induced by MD. (A, B) Average signal amplitude of the deprived-eye response (A) and open-eye response (B) in the binocular visual area, before the lid suture (pre-MD), after 2.5 – 3 days of MD (MD3), and after 5 – 6 days of MD (MD5-6) in Tnf+/+ (n=6) and Tnf−/− (n=7) mice. All data from longitudinal measurements are in the same individual animals. (C, D) Average signal amplitude of the deprived-eye response (C) and open-eye response (D) in the binocular visual area in inbred C57Bl6 wild type mice treated with cortical infusion of soluble TNF receptor1 (sTR) (n=7) or vehicle solution (n=5), before the lid suture and after 5 days of MD. Values in A–D present mean ± S.E.M of the group. **P<0.01, *P<0.05 compared with baseline (pre-MD), repeated measure ANOVA and Bonferroni multiple comparisons. (E, F) Changes in response amplitude from baseline following MD. Fractional changes in average response to deprived eye (E) and open eye (F) were calculated from measurements presented in A–D as (post-MD – pre-MD)/(pre-MD). †P<0.05 and ‡ P<0.01, paired t-test; *P<0.05 and **P<0.01, unpaired t-test. (G) Individual ODIs (circles) and mean group values (horizontal lines). The gray box presents the mean ± SD of baseline (pre-MD) ODI in Tnf+/+ animals. **P<0.01, *P<0.05 vs. corresponding baseline. (H) Response magnitude in the monocular area to the deprived eye in Tnf+/+ animals (open bar, n=6) and Tnf−/− animals (closed bar, n=6). Values present mean changes (±S.E.M.) from the baseline in response magnitude in the monocular area. **P<0.01 compared to baseline (repeated measure ANOVA), ‡ P <0.01 between groups (t-test).