Mitochondrial fusion and fission proteins and the recognition memory of imprinting in domestic chicks

Visual imprinting is a learning process through which young, visually naïve animals come to recognize a visual stimulus by being exposed to it (training) and subsequently approach the stimulus in preference to others. A large body of evidence indicates that a restricted part of the forebrain, the intermediate medial mesopallium (IMM), is a memory region for visual imprinting in the domestic chick. Previous studies have shown learning-related up-regulation of several mitochondrial proteins in the IMM 24 h after training. Learning-related increases in transcription factors involved in mitochondrial biogenesis were found without significant change in mitochondrial DNA copy number, but the issue of whether mitochondrial fusion or fission processes change with learning was unresolved. The present study enquired whether proteins involved in mitochondrial fusion and fission contribute to memory following imprinting. Tissue was sampled from the left and right IMM, and the left and right posterior pole of the nidopallium (a control brain region not involved in imprinting). The amounts of the following proteins were measured by Western immunoblotting 24 h after training: mitochondrial mitofusin-1 (MTF-1, as indicator of mitochondrial fusion), membrane dynamin-related protein-1 (DRP-1, as indicator of mitochondrial fission) and cytoplasmic DRP-1. Learning-related increases in MTF-1 and DRP-1 were observed bilaterally in the IMM, but not in either side of the posterior pole of the nidopallium. Cytoplasmic DRP-1 was not changed significantly in any region studied. The results implicate increased, balanced levels of mitochondrial fusion and fission in memory formation up to 24 h after training.Supported by the Sh. Rustaveli National Science Foundation, Georgia (Grant 217592)