Enrichment from birth accelerates the functional and cellular development of a motor control area in the mouse

Abstract

Background: There is strong evidence that sensory experience in early life has a profound influence on the development of sensory circuits. Very little is known, however, about the role of experience in the early development of striatal networks which regulate both motor and cognitive function. To address this, we have investigated the influence of early environmental enrichment on motor development.

Methodology/principal findings: Mice were raised in standard or enriched housing from birth. For animals assessed as adults, half of the mice had their rearing condition reversed at weaning to enable the examination of the effects of pre- versus post-weaning enrichment. We found that exclusively pre-weaning enrichment significantly improved performance on the Morris water maze compared to non-enriched mice. The effects of early enrichment on the emergence of motor programs were assessed by performing behavioural tests at postnatal day 10. Enriched mice traversed a significantly larger region of the test arena in an open-field test and had improved swimming ability compared to non-enriched cohorts. A potential cellular correlate of these changes was investigated using Wisteria-floribunda agglutinin (WFA) staining to mark chondroitin-sulfate proteoglycans (CSPGs). We found that the previously reported transition of CSPG staining from striosome-associated clouds to matrix-associated perineuronal nets (PNNs) is accelerated in enriched mice.

Conclusions/significance: This is the first demonstration that the early emergence of exploratory as well as coordinated movement is sensitive to experience. These behavioural changes are correlated with an acceleration of the emergence of striatal PNNs suggesting that they may consolidate the neural circuits underlying these behaviours. Finally, we confirm that pre-weaning experience can lead to life long changes in the learning ability of mice.

Figure 1. Enrichment limited to the pre-weaning period improves performance in the Morris water maze (MWM) to a level equivalent to life-long enrichment.

Average MWM latencies across 7 training days of non-enriched mice (NN, solid black, n = 10), mice enriched from weaning (NE, solid red, n = 16), enriched from birth until weaning (EN, dashed black, n = 7) and enriched from birth into adulthood (EE, dashed red, n = 7). Mice receiving enrichment performed significantly better across the 7 training days relative to non-enriched mice (Mixed model ANOVA, F(3, 36) = 8.48, p<0.001; see text).

Figure 2. Enrichment increases exploratory behaviour in an ‘open field’ test in young mice.

Representative traces (red) of (A) non-enriched and (B) enriched P10 mice. Enriched mice typically traverse a much higher proportion of the test arena than non-enriched mice. (C) Percentage of area explored for non-enriched (‘N’, n = 17) and enriched (‘E’, n = 17) P10 mice, as obtained from the traces. The increase in the percentage of the open-field explored by enriched mice is statistically significant (t-test, p = 0.0237). *: p<0.03. (D) Total cumulative distance travelled by non-enriched and enriched mice. There was no detectable difference in absolute distance travelled between the two groups (Student's t-test, p = 0.334).

Figure 3. Pre-weaning enrichment improves swimming performance at P10.

(A, B) Representative examples of swimming behaviour of non-enriched (A) and enriched (B) mice at P10. In non-enriched mice (A), only the crown of the head and dorsal tip of the nose are above the water level (score of 2). In enriched (B) mice, the bulk of the head, including the entire ear and nose are above the water level (score of 4). The back and tail are also notably more elevated. (C) Drawings illustrating the relative position of the eyes, nose and ears used in the scoring assessment (based on the scheme of St Omer [47]). (D) Graph plotting swim scores for non-enriched (‘N’, n = 17) versus enriched (‘E’, n = 17) P10 mice. Scores were made by an observer blind to treatment group. Enriched mice scored significantly higher than non-enriched controls (Student's t-test, p<0.001). *: p<0.001.

Figure 4. Enrichment accelerates the striatal maturation as determined by the pattern of CSPG staining at P10.

(A–C) A rostral (rost.) to caudal (caud.) series showing the pattern of CSPG staining in the striatum at P10 in non-enriched mice. CSPG staining in the striatum is most prominent in diffuse clouds (some highlighted by arrows) which we have previously shown to be associated with striosomes . PNNs, characterised by more punctuate staining, can also be seen in the matrix (arrowheads) but these are present at very low density. Only one or two can be seen in sections from rostral striatum (A), though they become slightly more numerous in mid (B) and caudal (C) sections. (D–F) A similar series to that shown in A–C but for enriched mice. Here, the CSPG-associated clouds (arrows) are much less apparent. PNNs are, however, markedly more numerous and more prominent across all rostrocaudal levels compared to non-enriched mice. The inset in 4D shows the appearance of the PNNs at higher power. Note the characteristic lattice-like network of CSPGs encircling the soma and proximal dendrites. Scalebars: 500 µm in A–F, 20 µm for the Inset in D. (G–H) Graphs plotting increases in PNN (G) and decreases in CSPG cloud (H) densities in enriched mice at P10. Differences are significant (see text for details). Nine sections from 3 animals were quantified for each group. *: p<0.05, **: p<0.001.