Phosphorylation of Synaptic Proteins in Chick Forebrain: Changes with Development and Passive Avoidance Training

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Abstract
We have used synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) to study protein phosphorylation at the synapse in the developing chick forebrain and in 1‐day‐old chick forebrain following training on a passive avoidance task. Endogenous phosphorylation patterns in SPMs and PSDs prepared by extraction with n‐octylglucoside isolated from chick forebrain were investigated by labelling with [32P]ATP. The phosphoprotein components of the SPM and PSD fractions were separated using sodium dodecyl sulphate gradient polyacrylamide gel electrophoresis. Autoradiography and densitometry of the Coomassie Blue protein staining pattern revealed phosphate incorporation into several SPM components including those of molecular mass 52, 37, and 29 kilodaltons (kDa). Bands of similar molecular mass were not phosphorylated in PSD fractions. This difference in phosphorylation between SPMs and PSDs was not due to the detergent n‐octylglucoside. In a developmental study in which SPM and PSD fractions were prepared from 1‐day‐old, 14‐day‐old, and 21‐day‐old chickens, the phosphorylation patterns of SPMs were similar throughout, but striking differences occurred in PSDs, both in the level of phosphor ylation and in the components phosphorylated. A time‐course study was carried out in which phosphorylation of SPMs and PSDs from 1‐day‐old chicks trained on a passive avoidance task was compared with patterns from control chicks trained on a water‐coated bead and untrained chicks. In SPMs prepared from forebrains removed 10 mins following training, a consistent but nonsignificant decrease (‐21%) in phosphorylation of a 52 kDa band occurred in chicks with passive avoidance training compared with water‐trained and untrained chicks. After 30 min, this decrease was greater (‐34%, p < 0.05) and was significant. There was a concomitant but nonsignificant increase (+22%) in phosphorylation of a 45 kDa component in the SPM fraction. No significant changes in phosphorylation patterns were seen in PSD fractions or in SPMs and PSDs isolated from cerebella of chicks with passive avoidance training at any time.