Morphological Correlates of Imprinting

Abstract

A particular region of the chick brain is critically involved in the storage of information acquired through the learning process of imprinting (Horn, 1981; 1985; and this volume). The region is the intermediate part of the medial hyperstriatum ventrale (IMHV). There is an increased incorporation of [¹⁴C]uracil into RNA in this region after chicks have been trained by exposing them to an artificial imprinting stimulus (Horn et al., 1979). This result extended earlier studies which demonstrated that, in chicks trained in this way, there is an increased incorporation of [³H]lysine into proteins in the forebrain roof which contains IMHV (Bateson et al., 1972). The results together suggest that exposure to an imprinting stimulus leads to a change in the metabolism of protein, possibly in protein synthesis, within IMHV. These findings are consistent with the hypothesis that information storage in the brain involves changes in synaptic connections (Tanzi, 1893; Cajal, 1911; Hebb, 1949). However, the biochemical findings provide only indirect evidence for such changes. A direct test of the hypothesis is to determine whether such changes can be demonstrated in a brain region critical for storage. We have therefore used quantitative electron microscopical techniques to investigate the effects of imprinting on the structure of synapses in the IMHV of the domestic chick.