Modulation of synaptic transmission at CA1 pyramidal neuron (CA1 PN)–O-LM interneuron (O-LM IN) synapses.

(A) Photomicrograph of a pair of a presynaptic CA1 pyramidal neuron and a postsynaptic O-LM interneuron filled with biocytin during recording. Inset shows putative synaptic contact at higher magnification. Inset, an expanded view corresponding to the dashed box indicates a putative synaptic contact which is 456 µm from CA1 pyramidal neuron soma. (B) Activity-dependent modulation of synaptic transmission. Paired recordings of presynaptic membrane potential (upper traces) and postsynaptic current (lower traces; 10 superimposed consecutive sweeps (gray), overlaid with the average (black)) for two different presynaptic holding potentials (red, –50 mV; black, –60 mV). Note the marked facilitation of EPSC peak amplitude. (C) Facilitation ratio (EPSC_n/EPSC₁; see Materials and Methods), plotted against stimulus number, for two different presynaptic holding potentials (red, –50 mV; black, –60 mV). Data from 11 CA1 PN–O-LM IN pairs. Error bars, SEM. Data points for the same experimental conditions are connected by lines. Asterisks indicate a significant difference. (D) Static analog modulation of synaptic transmission. Left, paired recordings of presynaptic membrane potential (upper traces) and postsynaptic current (lower traces) for single AP for two different presynaptic holding potentials (red, –50 mV; black, –60 mV). Right, postsynaptic current for a train of 5 APs (upper traces) for two different presynaptic holding potentials (red, –50 mV; black, –60 mV) and corresponding EPSC integral (lower traces), presumably proportional to the total amount of transmitter release. (E) Summary bar graph of the effects of presynaptic holding potential on peak amplitude of first EPSCs (left; n = 11) and normalized EPSC integral for a train of presynaptic stimuli (5 APs at 50 Hz; right; n = 9). Bars indicate mean ± SEM; circles denote individual experiments. Data from the same experiment were connected by lines. Note that changes in presynaptic membrane potential alter both the amplitude of first EPSCs (left) and the charge of the EPSC train (right). (F) Plot of EPSC peak amplitudes against stimulus number. Data points for the same experimental conditions are connected by lines. Black squares, –60 mV; red circles, –50 mV. Error bars, SEM. *0.01≤P<0.05, **P<0.01.