Imprinting. A study of cholinergic receptor sites in parts of the chick brain.

The distribution of cholinergic receptor binding sites was studied in certain areas of the chick brain, especially that part (IMHV) of hyperstriatum ventrale which has previously been implicated in the imprinting process. There was relatively little binding of [125I]-alpha-bungarotoxin in this region, as shown using an autoradiographic technique. In contrast muscarinic receptor sites, with an affinity for [3H]-quinuclidinyl benzilate (QNB), are present in relatively high concentration in a medial forebrain sample containing IMHV and also in a visual projection area, the hyperstriatum accessorium (HA). To assess the effects of imprinting on the concentration of QNB binding sites 26 young chicks were exposed to an imprinting stimulus (a pulsing red light) for 20 min. Thirteen of the chicks were returned to the dark and the other 13 were trained for a further 100 min. There was no effect of further training on specific QNB binding in HA or in the medial forebrain sample. In undertrained chicks there was a positive correlation between QNB binding in the medial forebrain sample of the left hemisphere and that in the right; there was no such relationship in overtrained birds. Mean QNB binding in the right medial forebrain sample was greater than in the left by 21%. There was no such hemispheric asymmetry in HA.