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The Journal of Neuroscience, June 10, 2009, 29(23):7413-7423; doi:10.1523/JNEUROSCI.6098-08.2009

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Neurobiology of Disease
Robust Short-Latency Perisomatic Inhibition onto Neocortical Pyramidal Cells Detected by Laser-Scanning Photostimulation

Julia Brill and John R. Huguenard

Department of Neurology and Neurological Sciences, Stanford University School of Medicine, Stanford, California 94305

Correspondence should be addressed to John R. Huguenard, Department of Neurology and Neurological Sciences, Stanford University School of Medicine, 300 Pasteur Drive, Room M016, Stanford, CA 94305. Email: john.huguenard{at}stanford.edu

Inhibitory connectivity onto neocortical pyramidal cells was mapped using LSPS (laser-scanning photostimulation/glutamate uncaging). The average onset latency of IPSCs was shorter than that of EPSCs recorded in the same cells, indicating a specific mechanism for rapid network recruitment of inhibition. The majority of strong inhibitory synaptic inputs originated within 300 µm of the recorded cell's soma, had onset latencies between 4 and 10 ms, and high amplitude [short-latency IPSCs (slIPSCs)]. slIPSCs were GABA_A receptor- mediated chloride currents that were evoked in an all-or-none manner. We tested whether slIPSCs resulted from somatic depolarization of presynaptic interneurons or from direct excitation of inhibitory presynaptic terminals via kainate receptors. Our evidence supports the former hypothesis: (1) slIPSCs had similar sensitivity to kainate and AMPA receptor blockers as electrically evoked EPSCs. (2) slIPSCs frequently had an notched rising phase suggestive of summated IPSCs resulting from repetitive firing of presynaptic neurons. (3) Latencies and interevent intervals were consistent with spike latencies and interspike intervals in fast-spiking (FS) interneurons. (4) slIPSCs were frequently evoked at spots where the recorded cell was also excited directly, but 15% of spots from which slIPSCs were evoked did not overlap with the recorded neuron's cell body. We propose that slIPSCs from FS interneurons represent a pool of powerful inhibitory signals that can be recruited by local excitation. Because of their magnitude, progressive recruitment, and short latency, slIPSCs are a effective mechanism of regulating excitability in neocortical circuits.

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Received Dec. 22, 2008; revised April 14, 2009; accepted April 30, 2009.

Correspondence should be addressed to John R. Huguenard, Department of Neurology and Neurological Sciences, Stanford University School of Medicine, 300 Pasteur Drive, Room M016, Stanford, CA 94305. Email: john.huguenard{at}stanford.edu

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