A1, digital images showing dense mGluR1a-immunolabelling in stratum oriens/alveus (O/A) of the CA1 region in mGluR1 control littermates (+/+ or +/–). A2, higher power image illustrating mGluR1a-immunopositive somata (arrows) and dendritic processes in O/A. A3, graph of EPSC amplitude and sample traces (right) from a representative interneurone of a mGluR1 control littermate showing LTP after the pairing protocol. B1–B2, similar low (B1) and high (B2) power digital images showing the absence of mGluR1a-immunolabelling in mGluR1 (–/–) mice. B3, electrophysiological results from a representative interneurone of a mGluR1 (–/–) mouse indicating the absence of LTP after the pairing protocol. C1–C2, images illustrating that the hippocampal mGluR1a-immunolabelling in C57BL/6 mice was similar to that of mGluR1 littermates (A1–A2). D, bar graphs indicating the presence of LTP (increased average EPSCs, decreased failures and increased EPSCs) in mGluR1 control littermates, and the absence of LTP in mGluR1 (–/–) mice. Scale bars: 300 μm for A1, B1 and C1; 50 μm for A2, B2 and C2. Abbreviations: ori, stratum oriens; alv, alveus; pyr, stratum pyramidale; DG, dentate gyrus. (*P < 0.05.)