Edinburgh Napier University

  The learning process of imprinting involves morphological, electrophysiological and biochemical changes in a region of the chick (Gallus gallus domesticus) forebrain known as the intermediate and medial part of the hyperstriatum ventrale (IMHV). The alterations include increases in the mean length of postsynaptic density profiles of axospinous synapses and the number of N-methyl-d-aspartate (NMDA) receptor binding sites, and changes in spontaneous and evoked electrical activity. Recent immunocytochemical and behavioural studies have suggested that inhibitory GABAergic neurotransmission plays a role in learning. In this context, it has previously been reported that a novel avian γ-aminobutyric acid (GABA) type A (GABAA) receptor gene, encoding the γ4 subunit, is highly expressed in the hyperstriatum ventrale. In this study, we have used in situ hybridization to map, in detail, the expression of the γ4-subunit gene in the chick brain, and to assess the effect of imprinting training on the level of the corresponding transcript. Our results reveal that the γ4-subunit mRNA has a restricted distribution, and demonstrate a highly significant, time-dependent effect of training on its steady-state level. At 10 h but not at 5 h after training there is a decrease (25–32%) in the amount of this transcript in parts of the medial hyperstriatum ventrale, including the IMHV. A decrease (28–39%) is also seen in certain visual and auditory pathway areas but no effect was observed in other forebrain regions such as the hyperstriatum intercalatus superior (HIS). These results suggest that imprinting training leads to a time-dependent down-regulation of GABAergic transmission, and raise the possibility that this down-regulation plays a role in learning.