SHORT-TERM FACILITATION EVOKED DURING BRIEF AFFERENT TETANI IS NOT ALTERED BY LONG-TERM POTENTIATION IN THE GUINEA-PIG HIPPOCAMPAL CA1 REGION

1. The aim was to examine whether long-term potentiation (LTP) had eff ects on short-term synaptic plasticity outside those predicted from it s effect on single volley-induced responses. Field recordings from the CA1 region of guinea-pig hippocampal slices were used, and shortterm plasticity was evoked by five-impulse trains of 20 and 50 Hz. 2. The f ive-impulse trains were evoked in the presence of D(-)-2-amino-5-phosp honopentanoic acid (D-AP5; 20-50 mu M), picrotoxin (100 mu M), and 2-O H-saclofen (200 mu M), and care was taken to avoid initiation of posts ynaptic spike activation. Field responses were thus considered to refl ect non-NMDA receptor-mediated activity only, and demonstrated a net f acilitation during the trains. 3. The facilitation was found, on avera ge, to be unaffected by LTP, evoked by strong afferent tetanization. T his was true also when release probability had been altered either by the adenosine agonist N-cyclohexyladenosine (CHA; 100 nM) or the antag onist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 200 nM). When examine d for individual experiments, cases with increases, or decreases, of f acilitation following LTP were observed. These deviations showed no re lation to initial release probability or to LTP magnitude, and they we re also observed in control inputs not subjected to LTP. 4. Impairment of non-NMDA receptor desensitization by cyclothiazide (30 mu M) incre ased facilitation observed during a 50 Hz, but not a 20 Hz, train. LTP had no effect on facilitation, in the presence of this drug, either d uring 20 or 50 Hz trains. 5. The results suggest that the effect of LT P in the hippocampal CA1 region on non-NMDA receptor-mediated synaptic responses to a brief afferent tetanus does not differ from that on a low-frequency, single volley-induced response. They do not support the notion that LTP is based on changes in release probability of previou sly active synapses. If LTP is based on recruitment of previously, pre -or postsynaptically, silent synapses, these synapses must have, on av erage, release characteristics similar to the previously active ones.