Article Figures & Data
Figures
- Figure 1.
Single interneuron action potentials evoke biphasic currents in postsynaptic interneurons. a, GFP-expressing neurons show electrophysiological responses typical of interneurons. Illustrated is the voltage response to a 600 ms suprathreshold, depolarizing current injection. All recorded neurons expressed GFP (green). In some recordings (n = 4), neurons were loaded with neurobiotin (red). These neurons were positive for parvalbumin (blue), confirming they are parvalbumin-expressing interneurons. b, Paired recordings from two EGFP-positive interneurons (top). Traces below show the current response in the voltage-clamped postsynaptic cell at holding potentials of −40 and −90 mV (bottom traces) to a single action potential in the presynaptic interneuron (top trace). The current–voltage relationship of the postsynaptic current is shown in the bottom panel. c, Interneuron pairs show biphasic currents. Single action potentials evoked in the presynaptic interneuron evoke two types of responses in a voltage-clamped postsynaptic interneuron (Vh, −40 mV). Traces on the left show time-locked outward current responses, whereas traces on the right show responses that are also accompanied by a delayed inward current. The average current in each case is shown in the bottom trace. The bottom panel shows overlay of averaged traces in which there were EPSC failures (thick black), EPSC successes (gray), or the average for all trials (thin black). Note the faster kinetics of the synaptic current when all trials are averaged. d, Only the early synaptic current reverses at the chloride equilibrium potential. Averaged postsynaptic responses in a voltage-clamped postsynaptic interneuron to a single action potential in the presynaptic interneuron. Note the dual component response. Hyperpolarization of the postsynaptic neuron to −90 mV reversed the initial component and increased the amplitude of the delayed inward current. The shaded region is expanded at the bottom to demonstrate the delay of the large inward component.
- Figure 2.
Delayed inward current is disynaptic and glutamatergic. a, The latency of the delayed inward current shows a larger variability. Top panels show overlaid fast-latency outward responses at a holding potential of −40 mV to a single action potential in the presynaptic neuron. The bottom panel shows overlaid delayed inward current responses recorded at a holding potential of −60 mV to an action potential in the presynaptic neuron. Averaged traces are shown below. The small inward current in the postsynaptic neuron simultaneous with the presynaptic action potentials is attributable to transmission of the action potential via a connected gap junction. The panels on the right show histogram of latencies. b, Average responses to single action potentials in the presynaptic neuron recorded at −40 mV and near the chloride reversal potentials (−60 mV). The shaded region is shown below at an expanded time base. Mean latency of the inward current was significantly longer than for the outward component. Average data for mean latencies is shown on the histogram on the right (n = 8). c, Disynaptic current is mediated by GABAergic excitation of a glutamatergic principal neuron. Disynaptic current recorded in a postsynaptic cell voltage clamped at −40 mV in response to single action potentials in the presynaptic neuron. NBQX (10 μm) selectively blocks the inward current, whereas subsequent application of bicuculline (10 μm) abolishes the remaining outward current (left panels). Application of bicuculline (10 μm) to a naive slice eliminates both components of the postsynaptic response (right panel).
- Figure 3.
Feedback excitation in parvalbumin-positive interneurons. a, Firing phenotype (top left) of an interneuron in response to a 600 ms depolarizing suprathreshold current injection. The bottom traces show superimposed single action potentials evoked by a brief (2 ms) current injection, which were followed by feedback EPSPs (arrow). b, When the neuron is voltage clamped at −70 mV, a brief (0.5 ms) voltage step to 0 mV leads to an unclamped “action current” (truncated) that results in GABA release and a feedback EPSC. Trials in which EPSCs were not observed (failures) were averaged, and individual trials of EPSCs subtracted from the failure average to reveal pure feedback EPSCs (trace at bottom of panel). c, EPSCs from feedback excitation are larger than spontaneous EPSCs and fluctuate in amplitude. Traces on the left (top panel) are individual sweeps of feedback EPSCs evoked by brief voltage steps in voltage clamp. Average EPSC is shown on the right. Bottom panels show superimposed sweeps of spontaneous EPSCs in the same neuron aligned at their peak. The average spontaneous EPSC is shown on the right. Histogram plots amplitudes of feedback EPSCs and spontaneous EPSCs. Traces at the bottom show normalized evoked and spontaneous EPSCs superimposed; note the slower rise time of the feedback EPSC. d, Mean data showing the 10–90% rise time of averaged but not individual feedback EPSCs were longer than for spontaneous EPSCs because of variability in the onset of feedback EPSCs. *p < 0.05. e, Mean data of decay time constants of feedback EPSC and spontaneous EPSC. Error bars indicate SEM.
- Figure 4.
Axoaxonic interneurons produce suprathreshold GABAergic excitation. Shown are recordings from a GFP-positive interneuron that shows feedback excitation. a, Single action potentials (APs) shown on an expanded scale have been superimposed. Action potentials are followed by a feedback EPSP (arrow). Action potential peaks have been truncated for clarity. b, Under voltage clamp (Vh, −70mV), a brief (0.5 ms) voltage step to 0 mV evokes a feedback EPSC (arrow). c, d, Examples of axonal cartridges (arrowheads) in the cell shown in a and b. e, f, Cells with feedback excitation possessed axon terminals that formed pericellular baskets (arrows) around the somata (s) of putative principal neurons. Some pericellular baskets (arrow) appear continuous with an axonal cartridge (e, arrowhead).
- Figure 5.
Excitatory interneurons drive local recurrent circuits. a, Paired recording from two EGFP-positive interneurons. A train of action potentials in the presynaptic interneuron evokes depolarizing EPSPs in both the postsynaptic and presynaptic interneurons (arrows). Application of bicuculline abolishes EPSPs in both the presynaptic and the postsynaptic neuron. Note the electrically coupled spikelets that remain in the presence of bicuculline, indicating that the two interneurons are electrically coupled. b, Single glutamatergic neuron provides input to presynaptic and postsynaptic interneurons. A paired recording from two interneurons is shown. Presynaptic cell is held in current clamp, and the postsynaptic cell is voltage clamped to −40 mV. A single action potential in the presynaptic cell evokes correlated excitation in both cells as indicated by the correlation in presynaptic and postsynaptic event latencies (graph on right). c, d, Interneuron pair in which both cells received excitation from activated principal neurons and are also connected by bidirectional GABAergic synapses (c) as well as by electrical synapses (d). c, Traces show the response in the postsynaptic neuron to an action potential in the presynaptic neuron. An action potential in neuron A evokes an outward current followed by a delayed large inward in neuron B (traces on left). An action potential in cell B evokes a GABAergic IPSC and a small glutamatergic current in cell A (traces on right). The arrows indicate voltage-clamped spikelets. d, Gap junctional coupling of connected interneurons. Hyperpolarizing current injection into one cell evokes large hyperpolarization in that neuron and a smaller hyperpolarization in the electrically coupled interneuron. The presence of the gap junctional connection is seen as a “spikelet” in the postsynaptic neuron accompanying the presynaptic action potentials in c (arrows). e, f, Feedforward or feedback excitation can drive interneurons to threshold. e, Paired recordings from an interneuron pair in which single action potentials in the presynaptic cell evokes a delayed EPSPs in the postsynaptic interneuron driving it to threshold in some cases. f, Superimposed traces from a single interneuron in which a single action potentials generates feedback excitation that evokes a second interneuron spike. The shaded area is shown on an expanded scale in the bottom panel and illustrates a feedback EPSP (arrow) after the second action potential. Schematic diagrams above each figure illustrate circuit configuration.
