Article Figures & Data
Figures
- Figure 1.
Passive sound exposure alters frequency tuning in the aged A1. A, Representative A1 CF maps from naïve rats (top) and from rats exposed to 5-kHz pure tones during 1 week (bottom). D, dorsal; C, caudal; R, rostral; V, ventral. B, Difference in frequency tuning between naïve and exposed rats expressed as A1 percentage area and separated by CF for immature, young adult, and old adult groups. Immature group: n = 8, recorded sites = 389; YA: n = 8, recorded sites = 403; OA: n = 8, recorded sites = 382; immature-exposed: n = 8, recorded sites = 362; YA-exposed: n = 4, recorded sites = 177; OA-exposed: n = 4, recorded sites = 168. Values shown are mean, two-way ANOVA with Tukey–Kramer correction.
- Figure 2.
Restoration of inhibition stabilizes frequency representation in the aged A1. Young and old adult rats were exposed to 10-kHz pure tones for 1 week, followed immediately by exposure to 5-kHz pure tones for 1 week. A, Representative A1 CF maps from young (left) and old (right) adult rats that received sham (saline) intraperitoneal injections during the 2-week passive exposure period. B, Difference in frequency tuning between naïve and saline-treated rats expressed as A1 percentage area and separated by CF. C, Representative A1 CF map from an old adult rat that received diazepam (DZP) intraperitoneal injections during the 2-week passive exposure period. D, Difference in frequency tuning between naïve and DZP-treated rats. To investigate whether sequential exposure to pure tones would have a similar effect in immature rats, 2-week exposures were conducted starting on P10 as described in Fig. 2-1. YA-saline group: n = 4, recorded sites = 230; OA-saline: n = 4, recorded sites = 203; OA-diazepam: n = 4; recorded sites = 218. Values shown are mean, two-way ANOVA with Tukey–Kramer correction. Conventions as in Fig. 1.
- Figure 3.
Improved adaptation in the immature and aged A1 following administration of the GABAA agonist midazolam. A, Stimulation paradigm. Left, a standard (high-probability) tone was presented 80% of times. Five oddball (low-probability) tones distributed around the standard frequency (middle) were interspersed in the repetitive tone presentation (right). B, Representative normalized responses of individual A1 neurons to a standard tone (5 or 12 kHz at a repetition rate of 3 Hz) as function of tone position in the stimulus sequence. Red horizontal lines represent the average normalized firing rate in response to the standard tone during two different intervals in the stimulus sequence: early (T1, event 100–300; dashed line), and late (T2, event 900–1100; solid line). Note that adaptation is reduced in both immature (I) and old adult rats. C, Probability distribution plot of the slope of firing rate trace in response to the standard tone (interval from event no. 150–1200). Red dots denote the location of the median value for each group. Fig. 3-1 provides a summary of data related to adaptation in response to repetitive tones for all five groups. D, Frequency tuning of representative A1 neurons during T1 (dashed line) and T2 (solid line). The normalized spike rate is plotted for the standard tone (arrow) and each of the five deviant tones. Note the acute change in tuning after standard-oddball presentation in I and OA rats. E, Representative A1 activity maps depicting the change in firing rate at T2 relative to T1 (T2/T1 ratio of normalized firing rate). Warmer colors (white, yellow) denote neurons with reduced adaptation, notably in the I and OA groups. Same conventions apply for panels F–I, which show that midazolam improved adaptation and prevented changes in tuning in the immature and aged A1. Immature group: n = 8, recorded sites = 376; YA: n = 4, recorded sites = 205; OA: n = 4, recorded sites = 192; I-MDZ: n = 8, recorded sites = 346; OA-MDZ: n = 4, recorded sites = 155.
- Figure 4.
Aging and decay of training-induced A1 plasticity. Young and old adult rats were trained on a two-tone discrimination task (target tone: 10 kHz, nontarget: 5 kHz). A, Top: Experimental protocol. Bottom: Older adult rats needed on average more training sessions to reach criterion than young adult rats (D-prime ≥1; YA no. of sessions = 8.4 ± 1.1; OA = 11.9 ± 1.4, p = 0.03). B, Representative A1 characteristic frequency (CF) maps from trained young (left) and old (right) adult rats. Bolded polygons have a CF at the target tone ±0.3 octaves. Hatched polygons have a CF at the nontarget tone ±0.3 octaves. C, Difference in frequency tuning between naïve and exposed rats expressed as A1 percentage area and separated by CF. The full arrows point to the target frequency; the hatched arrows points to the nontarget frequency. D, Top: To determine the persistence of learning and training-induced A1 map plasticity, a subgroup of YA-T and two subgroups of OA-T rats were subjected to a 4-week delay after reaching criterion, followed by behavioral re-assessment and A1 mapping. Bottom: From the first session of the reassessment onwards, young adult rats performed above criterion, while old adult rats performed above criterion from the second session onwards. E, Representative A1 characteristic frequency (CF) maps from trained rats that received daily sham (saline) or diazepam (DZP) injections during the delay period. F, Difference in A1 area tuned to various frequencies between each experimental group and untrained age-matched controls. YA-T group: n = 4, recorded sites = 212; OA-T: n = 4, recorded sites = 209; YA-Tdelay: n = 4; recorded sites = 192; OA-Tdelay: n = 4; recorded sites = 203; OA-Tdelay(DZP): n = 4; recorded sites = 189. Values shown are mean ± SEM, t test, two-way ANOVA with Tukey–Kramer correction.
- Figure 5.
Impact of age on structural inhibitory elements in the auditory cortex. A, High-power microphotographs of representative sections immunolabeled for perineuronal nets (PNN) and parvalbumin (PV) from immature (I), young adult (YA), old adult, immature + diazepam treatment (IA), and old adult + diazepam treatment (OAD) rats. B, D, Group fluorescence optical density for (B) PV and (D) PNN staining for each age group (all cortical layers; green boxes represent median values). C, E, Distribution of (C) PV cell and (E) PNN intensity staining for each age group. Fig. 5-1 compares A1 GABA concentration between YA and OA rats. Cell count per field for different neuronal types and age groups are detailed in Fig. 5-2. Fig. 5-3 shows representative micrographs of PV- and SST-positive cells. A summary of the cumulative distribution of staining intensity and interindividual variability for all groups is provided in Fig. 5-4. Number of hemispheres examined: I = 12, YA = 12, OA = 12, ID = 6, OAD = 6; total cell count per group: I = 418, YA = 343, OA = 236, ID = 156, OAD = 231. Values shown are mean ± SEM. *p < 0.05 relative to YA; Kruskal–Wallis test, corrected for multiple comparisons using Tukey–Kramer test.
- Figure 6.
Proposed model of the impact of age on A1 plasticity. During periods of life characterized by a low inhibitory tone, passive exposure alters the A1 CF map. Plastic changes to the immature A1 are long lasting: as inhibition increases, the CP ends and sensory representations become stable. In contrast, plastic changes to the aged A1 are short-lived, as these cannot be consolidated due to a persistent low inhibitory tone.
Tables
Data structure Type of test Statistic and p value a Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,168) = 14.84, p < 0.001; p < 0.001 b Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 4.02, p < 0.001; p = 0.87 c Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 10.77, p < 0.001; p < 0.001 d Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 13.13, p < 0.001; p < 0.001, p = 0.35 e Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 2.69, p = 0.005; p = 1, p = 0.96 f Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 7.23, p < 0.001; p = 1, p < 0.001 g Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,132) = 14.62, p = 0; p < 0.001, p = 0.15 h Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,120) = 12.58, p = 0; p < 0.001, p = 0.1 i Nonnormal distribution Wilcoxon rank-sum test z = –4.099, p = 4.1 × 10–5 j Nonnormal distribution Wilcoxon rank-sum test z = –3.187, p = 0.0014 K Normal distribution t test t(579) = 5.64, p < 0.001 l Normal distribution t test t(395) = 3.35, p = 9 × 10–4 m Normal distribution t test t(750) = 0.75, p = 0.45 n Normal distribution t test t(408) = 0.64, p = 0.52 o Normal distribution t test t(383) = 2.55, p = 0.011 p Nonnormal distribution Wilcoxon rank-sum test z = –4.4, p = 1.1 × 10–5 q Nonnormal distribution Wilcoxon rank-sum test z = ×2.46, p = 0.013 r Normal distribution t test t(720) = 5.29, p < 0.001 s Normal distribution t test t(345) = 2.1, p = 0.03 t Normal distribution t test t(690) = 0.86, p = 0.39 u Normal distribution t test t(308) = 0.08, p = 0.94 v Normal distribution t test t(18) = 2.32, p = 0.032 w Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 13.42, p < 0.001; p = 0.018 x Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 6.57, p < 0.001; p = 0.004, p = 0.41 y Normal distribution t test t(6) = 5.02, p = 0.002 z Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 6.68, p < 0.001; p = 0.01 ab Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 1.41, p = 0.18 ac Normal distribution 2-way ANOVA; Tukey–Kramer test F(11,72) = 5.42, p < 0.001; p = 0.022 ad Normal distribution t test t(6) = 2.53, p = 0.04 ae Normal distribution t test t(6) = 3.66, p = 0.01 af Nonnormal distribution Kruskal–Wallis test; Tukey–Kramer post hoc test H(4) = 14.52, p = 0.0058; p = 0.52, p = 0.011, p = 0.96, p = 0.97 ag Nonnormal distribution Kruskal–Wallis test; Tukey–Kramer post hoc test H(4) = 83.97, p < 0.0001; p < 0.0001, p < 0.001, p = 0.96, p = 0.003 ah Nonnormal distribution Kruskal–Wallis test; Tukey–Kramer post hoc test H(4) = 13, p = 0.011; p = 0.82, p = 0.005, p = 0.99, p = 0.99 ai Nonnormal distribution Kruskal–Wallis test; Tukey–Kramer post hoc test H(4) = 17.24, p = 0.0017; p = 0.48, p = 0.04, p = 0.004, p = 0.8, p = 0.99 aj Nonnormal distribution Kruskal–Wallis test; Tukey–Kramer post hoc test H(4) = 22.06, p < 0.001; p = 0.004, p = 0.039, p = 0.99, p = 0.85
Figure 2-1
Immature rats exposed sequentially to pure tones over 2 weeks show an overrepresentation of the first tone of exposure. Rats were exposed starting at P10 to 5-kHz pure tones for 1 week, followed immediately by exposure to 10-kHz pure tones for 1 week. A, Representative A1 CF maps from rats that received sham (saline, left) or diazepam (right) intraperitoneal injections during the 2-week passive exposure period. B, Difference in frequency tuning between naïve and treated rats expressed as A1 percentage area and separated by CF. Values shown are mean ± SEM; two-way ANOVA corrected for multiple comparisons using Tukey–Kramer test. Download Figure 2-1, EPS file.
Figure 3-1
Summary of adaptation in response to repetitive tones. A, Cumulative distribution plot of responses to repetitive tones (slope of normalized response rate to the standard tone) for all experimental groups (see Fig. 3B and Fig. 4B). B, Reduced adaptation to repetitive tones (standard, circles) in immature (I) and old adult rats relative to young adults (YA). Adaptation was restored with the local administration of midazolam (asymptote of normalized response rate to standard tone; one-way ANOVA corrected for multiple comparisons with Tukey post hoc test, p = 3.8 × 10–8, F(4,1269) = 10.21; YA: 0.31 ± 0.019; I: 0.44 ± 0.017, p = 1.24 × 10–5, relative to YA; I-MDZ: 0.28 ± 0.015, p = 1.20 × 10–5, relative to I; OA: 0.43 ± 0.032, p = 5.9 × 10–4, relative to YA; OA-MDZ: 0.34 ± 0.027, p = 0.018, relative to OA). No significant differences in the overall magnitude of responses to oddballs (circles) was found between groups (asymptote of normalized response rate to oddball tones; one-way ANOVA, p = 0.29, F(4,1269) = 1.24). Both immature and aged groups showed a diminished response gap between standards and oddballs (height of gray vertical lines). This gap improved with the local administration of midazolam for immature but not old adult rats (asymptote difference between oddballs and standard; one-way ANOVA, p = 0.004, F(4,1269) = 3.84; YA, 0.30 ± 0.033; I, 0.19 ± 0.026, p = 0.057, relative to YA; OA, 0.15 ± 0.035, p = 0.0172, relative to YA; I-MDZ, 0.32 ± 0.024, p = 0.015 relative to I; OA-MDZ, 0.19 ± 0.041, p = 0.92, relative to OA; corrected for multiple comparisons). Immature group: n = 8, recorded sites = 376; YA: n = 4, recorded sites = 205; OA: n = 4, recorded sites = 192. YA group: n = 4, recorded sites = 205; I: n = 8, recorded sites = 376; OA: n = 4, recorded sites = 192; I-MDZ: n = 8, recorded sites = 346; OA-MDZ: n = 4, recorded sites = 155. Values shown are mean ± SEM. *p < 0.05. Download Figure 3-1, EPS file.
Figure 5-1
GABA concentration is reduced in the old adult A1. GABA concentration in A1 dialysate obtained (A) during silence and (B) during auditory stimulation from young adult (YA, n = 4) and old adult (OA, n = 4) rats. Values shown are mean ± SEM. *p < 0.05, **p < 0.01, t test. Download Figure 5-1, EPS file.
Figure 5-2
Interneuron cell count in A1 across the lifespan of the rat. Number of PV-, SST-, PNN-, GABA-, and Nissl-positive cells per field at P15 (n = 6), 6 months (n = 6), and 24 months (n = 6). Download Figure 5-2, DOCX file.
Figure 5-3
PV and SST expression in A1 interneurons. Representative high power confocal micrographs of (A) PV+ and (B) SST+ immunolabeled cells costained for GABA at the age intervals defined in Fig. 5-2. Download Figure 5-3, EPS file.
Figure 5-4
Restoration of PV+ and PNN staining intensity with diazepam. (A) Cumulative distribution plot and (B) individual variability of PV-labeling intensity for all experimental groups (see Fig. 5B). (C) Cumulative distribution plot and (D) individual variability of PNN-labeling intensity for all experimental groups (see Fig. 5D). Note that, although between-groups PV and PNN staining follow the same pattern; PV-staining data shows higher within-group variability. *p < 0.05, **p < 0.01, Kruskal–Wallis, corrected for multiple comparisons (Tukey–Kramer test). Download Figure 5-4, EPS file.
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