Src and Memory: A Study of Filial Imprinting and Predispositions in the Domestic Chick

Abstract

Visual imprinting is a learning process whereby young animals come to prefer a visual stimulus after exposure to it (training). The available evidence indicates that the intermediate medial mesopallium (IMM) in the domestic chick forebrain is a site of memory formation during visual imprinting. We have studied the role of Src, an important non-receptor tyrosine kinase, in memory formation. Amounts of total Src (Total-Src) and its two phosphorylated forms, tyrosine-416 (activated, 416P-Src) and tyrosine-527 (inhibited, 527P-Src), were measured 1 and 24 h after training in the IMM and in a control brain region, the posterior pole of nidopallium (PPN). One hour after training, in the left IMM, we observed a positive correlation between the amount of 527P-Src and learning strength that was attributable to learning, and there was also a positive correlation between 416P-Src and learning strength that was attributable to a predisposition to learn readily. Twenty-four hours after training, the amount of Total-Src increased with learning strength in both the left and right IMM, and amount of 527P-Src increased with learning strength only in the left IMM; both correlations were attributable to learning. A further, negative, correlation between learning strength and 416P-Src/Total-Src in the left IMM reflected a predisposition to learn. No learning-related changes were found in the PPN control region. We suggest that there are two pools of Src; one of them in an active state and reflecting a predisposition to learn, and the second one in an inhibited condition, which increases as a result of learning. These two pools may represent two or more signaling pathways, namely, one pathway downstream of Src activated by tyrosine-416 phosphorylation and another upstream of Src, keeping the enzyme in an inactivated state via phosphorylation of tyrosine-527.

Keywords: IMM; early learning; intermediate medial mesopallium; learning; memory formation; recognition memory.

Figure 1

Left IMM and right IMM, 1 and 24 h after the end of training. Standardized relative amount of Total-Src plotted against preference score. Filled circles, trained chicks. Open circles and associated bars, mean level in untrained chicks ± SEM. Vertical dashed lines, preference scores 50 (no preference/no learning) and 100 (approach only to the training stimulus /strong learning); horizontal dashed lines, y-axis intercepts for preference scores 50 and 100; gray bars on y-axis, ± SE of intercept. (A) Left IMM and (C) right IMM 1 h after the end of training. Correlations are not significant. (B) Left IMM and (D) right IMM 24 h after the end of training. For both sides, the correlations are significant and the differences between the untrained mean and the intercept at preference score 100 are also significant. The intercepts at preference score 50 are also significantly higher than the mean of untrained chicks, indicating an effect of training on Total-Src in addition to a learning-related effect.

Figure 2

Left IMM. Standardized relative amount of 416P-Src plotted against preference score 1 h (A) and 24 h (B) after the end of training. Conventions otherwise as for Figure 1A, the correlation is significant; the mean value for untrained chicks is not significantly different from either intercept on the y-axis. (B) Correlation not significant.

Figure 3

Left IMM. Standardized relative amount of 527P-Src plotted against preference score 1 h (A) and 24 h (B) after the end of training. Conventions otherwise as for Figure 1. For both times the correlations are significant. In each plot, the mean value for untrained chicks is significantly lower than the intercept at preference score 100 and not significantly different from the intercept at preference score 50.

Figure 4

Left and right IMM, 1 and 24 h after the end of training. 416P-Src/Total-Src plotted against preference score. Conventions as for Figure 1. (A,B) Left IMM; (C,D) right IMM; (A,C) 1 h after the end of training, correlations not significant. (B,D) Twenty four hours after the end of training, both correlations significant. In both left and right IMM at 24 h (B,D), the intercept at preference score 100 is significantly different from the mean of untrained chicks. In the right IMM at 24 h (D), the intercept at preference score 50 is also significantly different from the mean of untrained chicks. Data in (D) are corrected for effects of approach during training and testing, which were statistically significant (cf. Supplementary Table S4).