Release-independent depression at pyramidal inputs onto specific cell targets: dual recordings in slices of rat cortex

1. Paired intracellular recordings were performed in slices of adult rat ne ocortex and hippocampus to examine presynaptic depression. A novel form of depression that occurs even in the absence of transmitter release during co nditioning activity was observed at a subset of synaptic connections. 2. In each pair studied, a pyramidal neurone was presynaptic and inputs ont o a range of morphologically identified postsynaptic target cells were anal ysed; high probability connections exhibiting the more traditional forms of release-dependent depression, as well as low probability connections exhib iting facilitation, were tested (n = 35). 3. Connections were tested with presynaptic spike pairs and trains of spike s with a range of interspike intervals. Sweeps in which the first action po tential elicited no detectable response (apparent failures of transmission) and sweeps in which the first action potential elicited large EPSPs were s elected. Second EPSPs that followed apparent failures were then compared wi th second EPSPs that followed large first EPSPs. 4. Release-independent depression was apparent when second EPSPs at brief i nterspike intervals (< 10-15 ms) were on average smaller than second EPSPs at longer interspike intervals, even following apparent failures and when t he second EPSP amplitude at these short intervals was independent of the am plitude of the first. EPSP. 5. Release-independent depression appeared selectively expressed. Depressin g inputs onto some interneurones, such as CA1 basket-like and bistratified cells, and facilitating inputs onto others, such as some fast spiking neoco rtical interneurones, exhibited this phenomenon. In contrast, depressing in puts onto 10/10 neocortical pyramids and facilitating inputs onto 7/7 orien s-lacunosum moleculare and 5/5 burst firing, sparsely spiny neocortical int erneurones did not.