EXPRESSION OF THE GABA(A) RECEPTOR GAMMA-4-SUBUNIT GENE - ANATOMICAL DISTRIBUTION OF THE CORRESPONDING MESSENGER-RNA IN THE DOMESTIC CHICK FOREBRAIN AND THE EFFECT OF IMPRINTING TRAINING

The learning process of imprinting involves morphological, electrophys iological and biochemical changes in a region of the chick (Gallus gal lus 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 syna pses and the number of N-methyl-D-aspartate (NMDA) receptor binding si tes, and changes in spontaneous and evoked electrical activity. Recent immunocytochemical and behavioural studies have suggested that inhibi tory GABAergic neurotransmission plays a role in learning. In this con text, it has previously been reported that a novel avian gamma-aminobu tyric acid (GABA) type A (GABAA) receptor gene, encoding the gamma 4 s ubunit, is highly expressed in the hyperstriatum ventrale. In this stu dy, we have used in situ hybridization to map, in detail, the expressi on of the gamma 4-subunit gene in the chick brain, and to assess the e ffect of imprinting training on the level of the corresponding transcr ipt. Our results reveal that the gamma 4-subunit mRNA has a restricted distribution, and demonstrate a highly significant, time-dependent ef fect of training on its steady-state level. At 10 h but not at 5 h aft er training there is a decrease (25-32%) in the amount of this transcr ipt in parts of the medial hyperstriatum ventrale, including the IMHV. A decrease (28-39%) is also seen in certain visual and auditory pathw ay 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-regul ation plays a role in learning.