At birth (P0), we observed that GIRK1 was extensively expressed in the somata of principal cells in all hippocampal areas, whereas very weak labelling was detected in the dendritic layers (Fig. 4A1). At P5, GIRK1 immunoreactivity was intensely expressed in principal cell somata, and moderate labelling was detected in the dendritic layers (Fig. 4A2). During the second postnatal week (P10), immunolabelling for GIRK1 was still observed in principal cell somata but more intense labelling was found in the stratum lacunosum–moleculare of the CA1 and CA3 regions, and molecular layer of the dentate gyrus (Fig. 4A2). However, during the third postnatal week (P15), a dramatic decrease in GIRK1 immunoreactivity was detected in the principal cell layers throughout the hippocampus (Fig. 4A4). Overall, the distribution of GIRK1 in the hippocampal formation did not change from P21 to P60 (Fig. 4A5 and A6). In the CA1 region, immunolabelling for GIRK1 was strong in the stratum lacunosum–moleculare, whereas the stratum radiatum showed an uneven labelling with moderate intensity in the proximal half and high intensity in the distal half, and the stratum oriens showed
