, 609 mM (closed circles) Cl-in the pipette solution. A HFS (arrow at time 0) fails to induce a long-610 lasting increase PSP amplitude when high concentration of chloride is used in the pipette 611 solution (filled circled, p = 0.4103, n = 5) but evokes normal long-term potentiation in control 612 experiments (open circles, p = 0.0047, n = 5). (C) Summary graph of experiments performed 613 using the gramicidin-perforated patch-recording configuration. HFS triggers an increase in 614 PSP amplitude, HFS (3). (B) Normalized CA2 PN SC PSPs recorded with either 7 mM (open circles) or, vol.16

, The increase in SC PSP amplitude in CA2 PNs is dependent on DOR 618 activation, Figure, vol.4

, A HFS does not trigger a long-term increase of the PSP amplitude recorded in CA2 PNs 620 in the presence of 2 ?M of the DOR competitive antagonist, vol.7, p.621

, Averaged PSP traces 627 corresponding to the time points before (a) and after (b) HFS performed in control conditions 628 (top) or in the presence of ICI 174864 (bottom) are shown on the right. (C) Application of 0.5 629 ?M of a DOR-selective agonist (DPDPE) is sufficient to induce a long lasting increase in the 630 fPSP amplitude recorded in SR of CA2 in the absence (open circles, p = 0.02, n = 5) but not 631 in the presence (filled circles, p = 0.55679, n = 6) of GABAA and GABAB receptor blockers. 632 response to a 20V stimulation of SC inputs, showing a long lasting increase in PS amplitude 658 after HFS (p=0.01, n=5). (D) Traces of whole-cell current clamp recordings in a CA2 659 pyramidal cell in response to a train of stimulations (5 pulses at 100Hz) of SC inputs, 660 illustrating how APs can be evoked in CA2 neurons after (black traces) but not before HFS 661 (grey traces). (E) Average number of APs per train at different stimulation intensities, 662 showing how a 5 pulse train at 100Hz of the SC inputs does not trigger APs, p = 0.135) but induces a normal long-term increase in the PSP magnitude in interleaved 622 control experiments (open circles, p = 0.005, n = 5). (B) Time course of normalized fPSP 623 amplitude recorded in CA2 SR showing how the application of a DOR antagonist (ICI 624 174864 or Naltrindol 0.1?M) during HFS (filled circles, vol.37, p.0