Molecular mechanisms of memory in imprinting

Abstract

Converging evidence implicates the intermediate and medial mesopallium (IMM) of the domestic chick forebrain in memory for a visual imprinting stimulus. During and after imprinting training, neuronal responsiveness in the IMM to the familiar stimulus exhibits a distinct temporal profile, suggesting several memory phases. We discuss the temporal progression of learning-related biochemical changes in the IMM, relative to the start of this electrophysiological profile. c-fos gene expression increases <15 min after training onset, followed by a learning-related increase in Fos expression, in neurons immunopositive for GABA, taurine and parvalbumin (not calbindin). Approximately simultaneously or shortly after, there are increases in phosphorylation level of glutamate (AMPA) receptor subunits and in releasable neurotransmitter pools of GABA and taurine. Later, the mean area of spine synapse post-synaptic densities, N-methyl-D-aspartate receptor number and phosphorylation level of further synaptic proteins are elevated. After ∼ 15 h, learning-related changes in amounts of several synaptic proteins are observed. The results indicate progression from transient/labile to trophic synaptic modification, culminating in stable recognition memory.

Keywords: Behavioural imprinting; IMM; Learning; Memory.

Fig. 1

Position of the IMM. A, side view of the chick brain, indicating the plane of section for B. B, coronal section; Hp, hippocampus; IMM, intermediate medial mesopallium; LaM, lamina mesopallialis; M, mesopallium; VL, lateral ventricle.

Fig. 2

Fos expression in the IMM after imprinting. Number of Fos-positive nuclei per unit area (square root-transformed to normalize the data) are plotted against preference score, a measure of preference for the imprinting stimulus and thus of the strength of imprinting/learning. Nuclei were counted in a standard sampling frame placed over the IMM region in a histological section. Each point represents data from one chick. The least squares regression line has been fitted. The lower horizontal dashed line estimates the value of the ordinate corresponding to the ‘no preference’ score of 50 (characteristic of chicks showing no learning). This estimate was not significantly different from the mean value for untrained chicks, which is represented by the open circle (the error bars represent ±1 SEM, n = 16). The upper horizontal dashed line gives the estimated value of the ordinate corresponding to the maximum preference score attained in the experiment (characteristic of strongly imprinted chicks). This estimate was significantly greater than the mean value for untrained chicks. The estimates shown by the horizontal dashed lines are based on interpolation of the regression line. The thick bars on the Y axis depict ± one standard error of each estimated value.

Fig. 3

(A) Hypothetical summary of early changes, based on results described in Section 2. The changes occur within approximately the same period as the initial increase in neuronal responsiveness to a visual imprinting stimulus. Abbreviations: CaMKII, calcium/calmodulin-dependent protein kinase II; GABA, γ-aminobutyric acid; MARCKS, myristoylated alanine-rich C kinase substrate; PKCγ, protein kinase C γ isoenzyme; P-MARCKS, PKC-phosphorylated MARCKS; PSD, postsynaptic density; P-Ser831-GluA1, GluA1 subunit of AMPA glutamate receptor phosphorylated at Ser-831 by αCaMKII; P-Thr286-CaMKII, Thr286-autophosphorylated αCaMKII. (B) Approximate time-line of changes referred to in (A). The times given are those at which measurements were made; therefore the changes started to occur before these times.

Fig. 4

(A) Hypothetical summary of intermediate changes, based on results discussed in Section 3. During this period, the responsiveness of IMM neurons to a visual imprinting stimulus is unstable; uninterrupted sleep over this time establishes stable responsiveness approximately 24 h after the start of training. Abbreviations: PSD, post-synaptic density; GABA, γ-aminobutyric acid; NMDA-R, N-methyl-d-aspartate receptor. (B) Approximate time-line of changes referred to in (A). The times given are those at which measurements were made; therefore the changes started to occur before these times.

Fig. 5

Left IMM. Amount of COI plotted against amount of COII after training for 1 h. Closed symbols, uncorrected for preference score; open symbols, corrected for preference score by partial correlation. Standard major axes have been fitted respectively to the uncorrected data (solid line) and corrected data (broken line). Uncorrected data: correlation coefficient r₁₆ = 0.67, p = 0.0024; slope of fitted line 1.57 ± 0.29 SE Corrected data: partial correlation coefficient r_xy·z;15 = 0.58, P = 0.015; slope of fitted line, 1.44 ± 0.30 SE. The correlation coefficient (r) and slope of the fitted line for trained and untrained chicks combined (r₂₃ = 0.67, P < 0.001; slope = 1.37 ± 0.21) were not significantly different from the corresponding values (whether corrected or uncorrected) for the trained chicks alone. There was significantly more COI and COII in trained than in untrained chicks (COI, F_1,28 = 5.78, P = 0.023; COII, F_1,23 = 10.85, P = 0.0032).

Fig. 6

Hypothetical summary of late changes, based on results discussed in Section 4. Abbreviations: APP, amyloid precursor protein; GABA, γ-aminobutyric acid; NCAM, neuronal cell adhesion molecule; MARCKS, myristoylated alanine-rich C kinase substrate; NMDA, N-methyl-d-aspartate; PDI, protein disulphide isomerase; PSD, postsynaptic density. All measurements made ∼25 h after the start of training.