Interactions between paired-pulse facilitation, low-frequency stimulation,and behavioral stress in the pathway from hippocampal area CA1 to the subiculum: Dissociation of baseline synaptic transmission from paired-pulse facilitation and depression of the same pathway

The interaction between low-frequency stimulation (LFS; 900 stimuli at 10 A z) of field excitatory postsynaptic potentials (fEPSPs) and paired-pulse fa cilitation (PPF; 50- and 100-msec interstimulus intervals, ISIs) was invest igated in the projection from hippocampal area CA1 to the subiculum. We als o investigated whether LTD can be obtained in the subiculum using a stress protocol (previously described to induce LTD effectively in vivo in area CA 1 of the hippocampus; Xu, Anwyl, gr Rowan, 1997). Finally! we examined the interaction between the stress treatment and PPF There was no significant d ifference between fEPSP amplitude measured at 30 min post-LFS (10 Az) when compared with baseline fEPSP peak amplitude; PPF, however, increased signif icantly 30 min post-LFS when compared with PPF pre-LFS for both 50-msec and 100-msec intervals. These results indicate that there is a dissociation be tween single-pulse stimulation and paired-pulse stimulation of the CA1-subi culum pathway. The combined effect of stress and LFS produced a depression in synaptic response of 56.99% at 5 min post-LFS and 10.23% at 30 min post- LFS. Behavioral stress combined with LFS caused a significant decrease in P PF at 30 min poststimulation. At a 50-msec ISI, facilitation is minimal; at a 100-msec ISI, paired-pulse depression occurs. These data suggest that th ere possibly are previously undescribed mechanisms regulating transmission in this pathway.