selected areas were then photographed from the selected ultrathin sections at a final magnification of 45 000×. Quantification of immunogold labelling was carried out in reference areas totalling approx. 2000 μm2 for each age. Immunoparticles identified in each reference area and present along the plasma membrane and intracellular sites in dendrites, spines and axon terminals were counted. (ii) To establish the relative abundance of GIRK1, GIRK2 and GIRK3 immunoreactivity at synaptic sites during postnatal development, quantification of immunolabelling at excitatory synapses was performed in single sections in the distal part of the stratum radiatum from 80 nm ultrathin sections obtained from Lowicryl-embedded blocks. Only synapses made by axon terminals with CA1 pyramidal cell spines were evaluated for the number of gold particles per synapse (both labelled and unlabelled) or number of gold particles per labelled synapse; labelled synapses had one or more gold particles. Synapses were only included in the analysis if the synaptic cleft was visible. (iii) To establish the density of GIRK1, GIRK2 and GIRK3 at extrasynaptic sites in dendritic spines of CA1 pyramidal cells in the adult, quantification of immunolabeling was performed from 60 μm coronal slices processed for pre-embedding immunogold in three different layers: the proximal
