A, graph of EPSC amplitude from a representative interneurone showing long-term potentiation (LTP) after the pairing of theta-burst stimulation (TBS) of afferents with postsynaptic depolarization (TBS + depo) delivered at time = 0 (vertical line). Ten consecutive sample traces are shown to the right for the control (top) and the 30 min period (middle). Superimposed average EPSCs (including failures; n= 128) demonstrate the increase of responses at 30 min postpairing (bottom right). In this cell, LTP results from both an increase in EPSC amplitude (potency) and a decrease in failure rate. B, graph from an untetanized interneurone indicating that test stimuli had no effect on EPSC amplitude. Consecutive traces and superimposed average EPSCs, at right, illustrate the absence of LTP in the untetanized interneurone. C, bar graph for all cells tested, showing the significant increase in amplitude of average EPSCs in tetanized interneurones and the stability of average EPSCs in the absence of tetanization. D, summary bar graphs for interneurones with LTP of average EPSCs (D1), showing that this change was accompanied by a significant decrease in failure rate (D2) and increase in amplitude (D3) of EPSCs. (*P < 0.05.)

A, average EPSCs (including failures) evoked by paired-pulse stimulation during the control (left) and 30 min postpairing (middle) periods in an interneurone showing LTP. At right, traces are scaled to the amplitude of the first responses in control. The dashed lines indicate that the amplitude of the second average EPSC is facilitated relative to the first response. However, paired-pulse facilitation was reduced after LTP. B, paired-pulse facilitation was unchanged in an untetanized cell. C, bar graph of the paired-pulse ratio for all cells tested, showing the significant decrease in paired-pulse facilitation in interneurones with LTP, but not in untetanized interneurones. D, plot of the coefficient of variation (CV⁻²) against average EPSC amplitude, both normalized to their control values. The location of data points in relation to the identity line is indicative of presynaptic changes during LTP. (*P < 0.05.)

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.)

A, confocal image of a biocytin-labelled interneurone (arrow) that showed LTP (data in D). B, immunocytochemical labelling for mGluR1a in the same tissue section. C, confocal images are merged to show mGluR1a-immunolabelling of the biocytin-filled interneurone. D, electrophysiological results showing LTP in the biocytin-filled and mGluR1a-positive interneurone illustrated in A–C.

A, graph of EPSC amplitude, sample traces and superimposed average EPSCs (including failures) showing the absence of LTP in an interneurone receiving the pairing protocol with the calcium-chelator BAPTA in the recording electrode. With time, a decrease in EPSC amplitude was noted. B, results from a cell that did not receive the pairing protocol, illustrating a similar decrease in EPSC amplitude caused by BAPTA. C, summary bar graphs for all cells tested with BAPTA (with or without the pairing protocol) and for cells given the pairing protocol with control recording solution (data from TBS + depo group shown in ). BAPTA prevented the increase in EPSC amplitude and the decrease in failure rate induced by the pairing protocol in control solution. The decrease in amplitude of average EPSCs was similar in cells with BAPTA that received or did not receive the pairing protocol. (*P < 0.05.)

A, the group II mGluR agonist DCG-IV selectively decreased the amplitude of synaptic responses showing LTP. Average EPSCs (including failures; n= 128) from control, 30 min postpairing, in 1 μm DCG-IV and after washout from representative cells showing (A1) or not showing (A2) LTP. A3, bar graph for all interneurones showing a differential effect of DCG-IV on average EPSCs from cells with and without LTP. B, selective facilitation of synaptic responses not showing LTP by the group III mGluR agonist l-AP4. Average traces (including failures; n= 128) from control, 30 min postpairing and in 100 μml-AP4 from representative cells showing (B1) or not showing (B2) LTP. B3, bar graph for all interneurones showing a significant increase in amplitude of average EPSCs by l-AP4 only for synaptic responses without LTP. (*P < 0.05.)

A, graph for all pyramidal cells tested from mGluR1 control littermates showing a long-term increase in IPSC amplitude following theta-burst stimulation (TBS; arrow). Superimposed average traces (right panel; n= 21) in a representative cell show the increase in IPSC at 30 min post-TBS. B, results for pyramidal cells from mGluR1 (–/–) transgenic mice illustrating the absence of a long-term increase in IPSC amplitude following TBS in these animals. C, summary bar graph for all groups showing the significant long-term increase in IPSC amplitude (30 min post-TBS) in cells from mGluR1 control littermates, but not in pyramidal cells from mGluR1 (–/–) mice nor in untetanized cells of mGluR1 control littermates. (*P < 0.05.)